Operando Neutron Imaging-guided Gradient Design of Li-ion Solid Conductor for Extremely High Mass-loading Cathodes

Operando Neutron Imaging-guided Gradient Design of Li-ion Solid Conductor for Extremely High Mass-loading Cathodes | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Operando Neutron Imaging-guided Gradient Design of Li-ion Solid Conductor for Extremely High Mass-loading Cathodes Hongli Zhu, Tongtai Ji, Yuxuan Zhang, James Torres, Jean Bilheux, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4511100/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 18 Aug, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract High mass-loading cathodes are crucial for achieving high energy density in all-solid-state batteries from lab scale to industry. However, as mass-loading increases, electrochemical performance is significantly compromised due to sluggish kinetics. Operando neutron imaging of a high mass-loading NMC 811 cathode of 33 mg/cm 2 (5.0 mAh/cm 2 , 180 µm thick) reveals the lithiation prioritization of the cathode active material (CAM) from the solid electrolyte layer to the current collector side. In addition to the tortuosity, another key limitation to ion transfer in the cathode arises from the mismatch between the uniform distribution of the solid electrolyte (catholyte) in the conventional composite cathode and the non-uniform Li + flux generated by the Faraday reaction of CAMs. Therefore, a novel design with a gradient in the catholyte concentration is engineered to match the Li + flux distribution, aiming to eliminate the ion transfer obstacle. This innovative approach demonstrates enhanced rate performance, even with ultra-high mass-loading cathodes. A LiCoO 2 composite cathode with 100 mg/cm 2 ultra-high mass-loading exhibited an areal capacity of 10.4 mAh/cm 2 at a current density of 2.25 mA/cm 2 . This work demonstrated an effective gradient design to optimize ion transport in high mass-loading cathodes to overcome the kinetic barrier and achieve high battery performance. Physical sciences/Energy science and technology/Energy storage/Batteries Physical sciences/Materials science/Materials for energy and catalysis/Batteries All-solid-state battery Operando neutron imaging High mass-loading Reaction Kinetic Ion transfer Li+ flux Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction All-solid-state batteries (ASSBs) are among the most promising next-generation energy storage technologies, offering the potential for ultra-high energy density and significantly enhanced safety by replacing flammable organic liquid electrolytes (LEs) with nonflammable inorganic solid electrolytes (SEs). 1 To improve energy density, many researchers have focused on developing electrode materials with high specific capacity or high potential. However, the cell level energy density is much lower than that of the material level because of the substantial content of inactive materials, such as the battery case, current collectors, solid electrolytes, and other components that do not directly contribute to capacity. Therefore, increasing the mass-loading of the cathode is considered to be one of the most effective strategies for enhancing the proposed high energy density at the cell level. 2 Achieving excellent electrochemical performance in high mass-loading cathodes is significantly more challenging than in thin cathodes. In the thick electrode, the higher areal current density under the same C-rate leads to a much larger ohmic polarization across the entire cell, and the increased tortuosity and longer diffusion paths result in sluggish ionic and electronic transport kinetics. 3 – 5 The specific mechanism of the decay of the rate performance in thick electrodes is more complicated, especially for the ion transfer. In LEs, the transport of Li + is based on the combination of electric field-induced migration and salt concentration gradient-induced diffusion. 6 Under the high rate, a large Li salt concentration gradient in the liquid phase will be formed and further hinder the Li + diffusion, and the Li + depletion in the LE will cause the underutilization of the cathode active materials (CAMs). 7 , 8 However, for the inorganic SE used in ASSBs, anions are fixed, and Li + is the only ion that can move with a Li + transference number around 1.0. Thus, there is no Li + gradient or concertation change in SEs. Moreover, an increasing number of SEs demonstrate high ionic conductivities, sometimes surpassing those of LEs. 9 – 12 Therefore, ASSBs are supposed to have improved rate capabilities. 13 Although impressive rate performance (> 40C) has been achieved in ASSBs, the cathode mass-loading is usually low (< 5 mg/cm 2 ). 14,15 The rate performances of high mass-loading cathode ASSBs still fall short of expectations. In the composite cathode of ASSBs, the Li + ion transfer paths are formed solely by the solid-solid contact between the CAM and SE partials. Compared to LE-based batteries, thick cathodes in ASSBs exhibit much higher tortuosity and lower effective ionic conductivity. 16 , 17 The size, proportion, and arrangement of the CAM, SE, and electron-conductive additive particles must be optimized carefully. 18 Inspired by the LE-based batteries, low ionic tortuosity design has been introduced to the ASSBs by mixing different sizes of SE particles to improve the rate performance. 19 Nevertheless, despite the absence of Li + concentration gradients in SEs, recent studies have still observed non-uniform reactions of CAMs in ASSBs. 20 , 21 Given the unique physical properties of SEs compared to LEs, the impact of these properties on the performance of thick electrodes in ASSBs remains unclear. Therefore, it is crucial to investigate kinetic transfer in high mass-loading cathodes using direct operando visualization techniques and provide insights for designing thick cathodes with uniform and fast ion transport in all-solid-state systems. Operando and in-situ investigation with sufficient temporal and spatial resolution is significant to understanding the kinetics in ASSBs, especially by tracking the Li behavior. Because of the nature of the ASSBs, conventional characterization methods, such as electron-beam and optical-light-based methods with limited transmission length, can only observe the surface behavior of the ASSBs. 22 X-ray-based methods make it difficult to detect Li directly due to its low X-ray attenuation coefficient of Li. 23 In contrast, neutron imaging shows unique advantages for tracking Li transport on account of the high visibility of Li for both ionic and metallic states. 22 – 26 In addition, neutron imaging also has great Li isotope contrast, so it can be used to track Li + diffusion in the solid electrolyte. 27 Consequently, neutron imaging is a powerful operando characterization method for ASSBs. Herein, to identify the limit factor of the electrochemical reaction in thick cathodes in ASSBs, operando neutron imaging was successfully conducted to visualize the reaction gradient over the whole thick cathode with a mass-loading of 33 mg/cm 2 (180 µm thick). By performing image calculation, the spatial inhomogeneous reaction of CAMs was observed. Combined with electrochemical analysis, the mismatch of the non-uniform Li + flux with the homogeneous SE distribution in the thick cathode is identified as the main cause of poor electrochemical performance. Inspired by the visualized electrochemical reaction gradient within the thick cathode, a three-layer cathode with a catholyte content gradient was strategically designed to promote fast Li + transport and uniform reaction throughout the entire thick cathode. By tailoring the catholyte content to match the Li + flux at different depths within the cathode, this innovative design aims to alleviate the kinetic limitations observed in high mass-loading all-solid-state battery cathodes, ultimately leading to enhanced electrochemical performance. As a result, ultra-high mass-loading cathodes, 100 mg/cm 2 with theoretical areal capacities of 15.0 mAh/cm 2 for LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NMC 811) and 11.25 mAh/cm 2 for LiCoO 2 (LCO), with three-layer design showed significantly improved rate performances compared to that of the thick cathode without SE gradient under the same CAMs content. Results and discussion Visualizing Reaction Kinetics in High Mass-Loading Cathodes through Operando Neutron Imaging Low mass-loading cathodes (< 5 mg/cm 2 or 1.0 mAh/cm 2 ) are commonly employed in research to investigate the material-level capabilities and usually demonstrate excellent rate performance. However, as the cathode mass-loading increases, poor kinetics emerge as a significant issue. Ion transfer, electron transfer, interface resistance, and ion diffusion in the CAM are the most common factors contributing to kinetic limitations in composite cathodes (Fig. 1 a). The ion and electron transfer will further affect the reaction prioritization and uniformity of CAM on the electrode level. Despite previous works demonstrating ultra-thick and ultra-high areal capacity cathodes (> 10 mAh/cm 2 ), this capacity was obtained at an extremely low rate (0.025C), even when utilizing a highly ion-conductive SE with a conductivity of 32 mS/cm. 9 Faster cycling of high mass-loading cathodes in ASSBs without compromising rate performance is crucial for real-world applications. However, the primary bottleneck for achieving high mass-loading cathodes in ASSBs remains unclear, making efforts to investigate cathode mass-loading essential. To unravel the reaction kinetic limitation, operando neutron imaging was carried out to visualize the reaction homogeneity within a thick cathode during charge and discharge processes (Supplementary Fig. 1). Figure. 1b illustrates the mechanism and setup of operando neutron imaging for ASSBs. The operando cell, featuring a high mass-loading cathode (33 mg/cm 2 and 5.0 mAh/cm 2 ), was positioned vertically in front of the detector, aligning the layer interfaces parallel to the neutron beam. Given the distinct neutron attenuation coefficients of the materials in the ASSB (Supplementary Table 1), the intensity of the transmitted neutron beam varies after passing through the battery. This variance facilitates easy differentiation between the cathode, SE, and anode layers based on the gray level, achieved by normalizing the neutron transmission intensity (Tr) from zero (no transmission) to one (full transmission). Darker areas correspond to lower neutron transmission, indicating that the material has a higher neutron attenuation coefficient. In the neutron image, the three layers- the In- 6 Li anode, SE, and the thick cathode- could be clearly recognized from top to bottom (Fig. 1 b). Subsequently, the battery underwent charging and discharging processes while simultaneously collecting electrochemical impedance spectroscopy (EIS) data (Supplementary Fig. 2) and operando neutron imaging data. To amplify the neutron transmission change of the cell during charge and discharge, the operando images were further calculated by dividing the image at the pristine state to get the transmission change ratio, Tr t /Tr 0 (Fig. 1 c). The pseudo color was applied to present the value of Tr t /Tr 0 in the neutron image (Fig. 1 d-f). For the area with limited transmission change (Tr t /Tr 0 ≈ 1), the color remains green. The colors toward red and blue indicating the decrease (Tr t /Tr 0 1) of the neutron transmission, respectively. The pseudo-color video presenting the transmission change over the charge and discharge process is in Supplementary Movie 1. Considering the cylindrical shape of the operando cell, which leads to different transmission lengths of the neutron beam over the cross-section and further affects the neutron transmission, we narrowed the sampling region to the center area (1.2 mm) of the cell with the transmission length close to the cell diameter (4 mm) to have the most reliable data (Supplementary Fig. 3). To better track the transmission changes over time, the average transmission change ratio of the sampling region of the operando battery is plotted in Fig. 2 a as a function of time, and the corresponding charge/discharge profile is shown in Fig. 2 b. Figure 2 c and 2 d provides detailed 2D images at specific states of charge (SoC) and depth of discharge (DoD). We first focused on the behaviors at the whole cell level. During the charging process, the colors of the cathode layer gradually changed to blue. This is because the CAM underwent a delithiation process, resulting in a decrease in the neutron attenuation coefficient (Supplementary Table 1). The Li + flux was directed toward the anode side during charging, but there was almost no transmission change in the SE layer, proving that there was no Li + concentration change in the SE layer, since Li + in both CAM and SE are natural Li. A small amount of red color was observed at the interface between the SE and the In- 6 Li anode, caused by the self-diffusion of 6 Li + from the anode layer to the SE layer by replacing the natural Li + in SE. 28 For the In- 6 Li anode layer, due to the high neutron attenuation coefficients of In and 6 Li, the neutron transmission remained almost zero throughout the process under the current setup, resulting in no observable transmission change. During the discharge process, the Li + flux was directed toward the cathode side. As the CAMs underwent lithiation, the color of the cathode layer gradually turned back from blue to green after discharge. For the SE layer, based on the data from the charging process, Li + flux did not affect the Li + concentration and neutron attenuation coefficient of the SE layer. Therefore, in Fig. 2 a, the enlargement of the red color region in SE during discharge was contributed by 6 Li + diffusing into the SE along with the Li + flux. Because 6 Li + replaced the natural Li in the SE, the neutron attenuation coefficient of SE significantly increased (Supplementary Table 1). A clear front of 6 Li + proceeding through the SE from the anode side to the cathode side was observed, which can also be seen in Fig. 2 d as well as Supplementary Movie 2. The operando neutron imaging successfully visualized the Li + diffusion in ASSBs from the anode to the cathode side through the SE layer during discharge based on the 6 Li isotope. The cathode layer was further magnified to investigate the delithiation uniformity of the CAM along the vertical direction (Fig. 2 e). The thickness of the composite cathode layer with a mass loading of 33 mg/cm 2 (5.0 mAh/cm 2 ) is around 180 µm. At the start of the discharge, it was hard to observe the transmission change because of the limited concentration change. After 3 hours, with a capacity of around 0.4 mAh/cm 2 , the region close to the SE side started to turn blue. As mentioned above, this indicates the delithiation of the CAM. The cathode close to the current collector remained unchanged. As the charging time increased, a gradual movement of the reaction associated with the CAM delithiation was observed, progressing from the SE side toward the current collector side. After around 16 hours, the entire cathode turned blue with a specific charge capacity of 175 mAh/g (4.4 mAh/cm 2 ) with the voltage reaching 3.7 V (4.3 V vs Li/Li + ). A similar trend was observed for the discharge process (Supplementary Fig. 4). The CAM close to the SE side first turned back to green, indicating that the lithiation process also started from the SE side. A relatively thinner cathode (16.7 mg/cm 2 ) was also characterized under the operando neutron imaging (Supplementary Fig. 5). The reaction inhomogeneity is not obvious in the thin cathode (Supplementary Fig. 6). Although both the thick and thin cathode cells were cycled under the same C-rate, the thin cathode cell shows small overpotential on account of the smaller areal current density. Moreover, the color change in the SE layer due to the 6 Li + diffusion in the discharge process was also much lighter in the thin cathode, reflecting the smaller Li + flux and corresponding with the smaller current density. Identifying the Key Factors Limiting Ion Transport in Thick Cathodes The reaction uniformity of the thick electrode is highly related to the reaction kinetics, especially the transport of charge carriers (both for ions and electrons). In the early studies of the composite cathode used in ASSBs, considering the decomposition of the SE when contacting the high surface area carbon, electron conductive additives (mainly carbon) were usually avoided in the composite cathode. 29 , 30 The electron conductive pathway only relies on the contact between CAM particles. Therefore, the restriction of electron transport rises with the increase in SE content because of the contact loss between CAM particles. 17 Recently, one-dimensional carbon fibers have been proven to be ideal electron-conductive additives in ASSBs due to their low surface area and good electrical conductivity. According to our test (Supplementary Fig. 7), the decomposition of SE is negligible in this composite cathode when using one-dimensional carbon fibers as the electron-conductive additive. Therefore, the electron-conductive additives were applied in our composite cathode. Our operando neutron imaging data reveals that the delithiation/lithiation processes of CAM were nonuniform within the thick cathode. Most of the Li + ions were first reacted in the area near the SE layer and hard to reach the CAM close to the current collector, indicating that the electrochemical reaction kinetics is primarily restricted by Li + ion transport along the long pathway within the thick electrode, rather than by electron transfer. The ion transfer limitation led to slow kinetics, necessitating a lower current density and a longer time for Li + ions to transfer to the area near the current collector side and react with the CAM. If a high current or C-rate is applied, the CAM within the thick cathode cannot be sufficiently reacted. Therefore, it is crucial to purposefully regulate the Li-ion transport within the thick cathode to ensure optimal rate performance. To further diagnose the ion transport limitation within the thick cathode, electrochemical performances were examined with different cathode mass-loadings of 3, 10, and 30 mg/cm 2 at 60 ℃. In-Li symmetric cells were also studied to evaluate the In-Li anode behavior and determine whether the anode contributes to the battery performance limitations. The symmetric cells maintained stable performance with limited overpotential (< 100 mV), even at high current densities up to 15.0 mA/cm 2 (Supplementary Figs. 8 and 9). This result proved that the anode is not the main performance barrier in our ASSBs system. A low mass-loading (3 mg/cm 2 ) cathode with high SE content (33.0 wt%) and sufficient carbon additives (2.0 wt% of carbon nanofibers) was tested to explore the intrinsic rate performance on the material level of NMC 811. The cell shows a specific discharge capacity of 199 mAh/g at C/10, approaching the theoretical capacity of NMC 811 (Fig. 3 a). At C/2, 1C, and 2C, the capacities were 174, 160, and 141 mAh/g, individually, representing the rate capacity of our NMC 811 at the material level under the condition with minimized limitations of the ion and electron transfer from the electrode level. We further evaluated the rate performance for our normal cathode (75.0 wt% NMC with 1.5 wt % of carbon nanofibers) with mass-loadings of 10 and 30 mg/cm 2 . When using a mass-loading of 10 mg/cm 2 with an areal capacity of 1.5 mAh/cm 2 , there is no obvious decay of the rate performance, with the discharge capacities of 194, 170, 153, and 134 mAh/g at C/10, C/2, 1C, and 2C, individually (Fig. 3 b). However, when the mass loading increased to 30 mg/cm 2 with an areal capacity of 4.5 mAh/cm 2 , the rate performance significantly decreased with the discharge capacities of 152, 140, 109, 80, and 49 mAh/g at C/10, C/2, 1C, and 2C, individually ( Fig. 3 c), which are only 76%, 63%, 50% and 35% of them achieved in the 3mg/cm 2 cell, respectively. The great rate performance of the 10 mg/cm 2 cathode was due to the short ion pathway within the thin electrodes, whereas the cell with 30 mg/cm 2 cathode showed much worse rate performance, suggesting the insufficient reaction of the cathode materials within the thick electrode because of sluggish ion transport along the long ion pathway. The reaction homogeneity at the electrode level can also be characterized by the dQ/dV analysis. 31 A uniform reaction of the CAM (NMC 811) on the electrode level can present a similar dQ/dV curve of the CAM shown on the material level, which has four clear peaks representing the sequential intercalation reactions of NMC 811. 32 As shown in Fig. 3 d, the dQ/dV curve for the 3 mg/cm 2 cathode at the low rate (C/10) corresponds to the behavior of the CAM at the material level. The four peaks can be observed even at 1C. When the rate increases to 2C, the shape of the dQ/dV curve changes to a rounded rectangle. Since the cathode is thin, this behavior mainly contributes to the rate capacity of CAM on the material level. Using these dQ/dV curves as the baseline behavior of our CAM, we further analyzed the effects of the increase in mass-loading. For the cathodes with 10 and 30 mg/cm 2 mass-loadings (Fig. 3 e, f), both cells show four pairs of peaks indicating uniform and sequential multiphase transitions under a low rate of C/10 or C/20. With the increase in the rate, the change of the dQ/dV curves of the 10 mg/cm 2 mass-loading cathode is similar to our baseline. However, for the cathode with 30 mg/cm 2 mass-loading, the trend of the dQ/dV curves varies with the increase of the C-rate. Even at C/10, four pairs of peaks are difficult to observe. At C/5, there is a new peak emerging at 3.4 V (Fig. 3 f), which is a new peak but not a shift of peak two because it cannot be simply corrected by the IR drop. 31 The shape of the dQ/dV curve for oxidation transforms into a semi-isosceles triangle (yellow line in Fig. 3 f). When the rate increases to C/2, the shape of the dQ/dV curve of the thick cathode becomes a right triangle (blue line in Fig. 3 f), which is significantly different from that of the thin cathode of 3 and 10 mg/cm 2 . The sequential intercalation reactions of NMC 811 were completely unobservable. Consequently, the strange dQ/dV curves indicate a huge reaction inhomogeneity in the thick cathode of 30 mg/cm 2 , especially at high rates, which is consistent with the findings from the operando neutron imaging. While the electrochemical results and operando neutron imaging provide valuable insights into the reaction inhomogeneity and performance limitations in thick cathodes, it is essential to understand the underlying factors that contribute to these issues. In the conventional discussion, ionic tortuosity (τ) is considered a key parameter of the kinetics of the cathode, which controls Li + diffusion and transportation in the electrodes. 21 , 33 Tortuosity is defined as the fraction of the shortest pathway through a structure (Δl) and the Euclidean distance between the starting and end points of that pathway (Δx) (Fig. 3 g). τ could be calculated using the equation (S1). The effective ion conductivity (σ i,eff ) in the cathode was usually measured by AC impedance-based techniques or polarization-interrupt method (DC method) with electron blocking electrolyte (Fig. 3 g) and further simulated based on the transmission line model. 18 In this case, the Faraday reaction of the CAM was ignored. For the thin cathode, these methods are acceptable. However, in the thick cathode, the Li + flux will be dramatically different from the current collector side to the SE side because of the accumulation of Li + flux generated from the Faraday reaction of the CAM with the increase of the thickness, as shown in Fig. 3 h. The CAM not only acts as a resistor element affecting the ion and electron transfer but also provides or consumes ions and electrons through the Faraday reaction. Only using tortuosity to evaluate the ion transfer is insufficient to explain the degradation of the rate performance in the thick cathode. The effective ion conductivity cannot adequately represent the resistance of ion transfer under the dramatically different Li + flux across the electrode thickness. A large Li + flux accumulation at the SE side without a corresponding ion transfer channel provision becomes a serious impediment to Li + transport. Different from the liquid electrolyte, there is no Li + concertation gradient in the SE. Therefore, appropriate allocation of SE gradient with Li + flux over the thick cathode will effectively improve the rate performance. Gradient Design in Ultra-high Mass-loading Cathode for Enhancing Rate Performance Inspired by the findings from the operando neutron imaging and the above-mentioned point of Li + flux generated from the Faraday reaction, we designed a three-layer cathode with gradient ion transport channels for realizing homogenous electrochemical reaction and improved rate performance in the thick cathode (Fig. 4 a and 4 b). The overall CAM content in the three-layer cathode was kept at 75.0 wt% which is the same as the non-gradient thick cathode. From the SE layer side to the current collector side, layer A is the fastest Li + transfer layer with 65.0 wt.% of CAM, 33.0 wt.% of SE, and 2.0 wt% of carbon nanofibers. The highest content of SE is designed to bear the largest Li + flux close to the SE layer. More carbon additives were also used to prevent the electron isolation of CAM due to the high SE content. Layer B is composed of 75.0 wt.% of CAM, 23.5 wt.% of SE, and 1.5 wt.% of carbon nanofibers, the same as the original composite cathode. Layer C is a high energy density layer with the highest CAM content (85.0 wt.%) and lowest SE content (14.0 wt.%). A compare group with a reversed sequence of the three layers was also prepared, as well as the control group with only one composition of 75.0 wt.% of CAM, 23.5 wt.% of SE, and 1.5 wt.% of carbon nanofibers. The rate capabilities of the three different cathodes were compared to evaluate the superiority of our gradient design (Fig. 4 c). The results show that the rate performance of the cell with the three-layer cathode outperforms that of the cell without a gradient design, even when tested under a very low current rate of C/20. The superiority of the gradient cathode is further exaggerated when the current rate increased from C/20 to high rates of C/10, C/5, C/2, 1C, and 2C (9.0 mA/cm 2 ) with 112%, 119%, 120%, 135%, 155%, and 171% of the capacities obtained from the conventional cathode, individually. To further verify that the designed gradient ion transport channels can benefit the rate performance for the thick cathode, a reversed gradient cathode was examined, and it shows a much worse rate performance than that of the non-gradient cathode. Although the initial discharge capacity of the reversed three-layer cathode is very close to the original single-layer cathode at the low rate (C/20), its capacity decays dramatically as the rate increases. There is almost no capacity obtained when the rate is higher than C/2. We further compared the charge/discharge profiles of these three groups to dig into the fundamentals of the three-layer design. Under the lowest rate we tested (C/20) with a current density of 0.225 mA/cm 2 (Fig. 4 d), the three-layer cathode exhibited a noticeably smaller overpotential and larger capacity. As the C-rate increased, the advantage became more pronounced (Fig. 4 e, f). The dQ/dV analysis can provide more detailed insights into kinetics. As shown in Fig. 4 g, the dQ/dV curves at C/20 of the three-layer cathode and the conventional cathode are very similar, with four pairs of peaks corresponding to the sequential intercalation reactions of NMC 811. However, for the reversed three-layer cathode, in addition to the shift of the peaks due to the larger overpotential, one pair of peaks at high voltage (around 3.6 V vs In-Li/Li + or 4.2 V vs Li/Li + ) is missing due to the sluggish kinetics. Specifically, the low SE content of the top layer creates significant resistance for the Li + transfer into or out of the cathode, resulting in a rapid voltage rise to the cut-off voltage without the last phase transition (H2 → H3) of NMC 811. When the rate increases to C/10 (Fig. 4 h), the cell with the three-layer cathode still clearly shows the four pairs of peaks for the sequential intercalation reactions, indicating a homogeneity reaction throughout the cathode thickness. However, for the conventional and reversed cathodes, the four pairs of peaks become blurred, wider, or even disappear caused by reaction inhomogeneity in the electrode level during the charge and discharge process. 31 Overall, the three-layer cathode demonstrates better rate performance with more homogenous delithiation and lithiation reactions across the entire cathode. Although the three-layer cathode and reversed groups have the same tortuosity, they show dramatically different performances. The result clearly proved that the appropriate allocation of SE content to match the Li + flux over the whole thick cathode is an effective way to improve homogenous electrochemical reactions and the rate performance of the thick cathode. To further explore the performance of our three-layer design cathode, we introduced another CAM, LiCoO 2 (LCO). LCO shows much better C-rate performance on the material level (Supplementary Fig. 10). In the dQ/dV curves for the low mass-loading (3 mg/cm 2 ) LCO cell, there is almost no shift of the reaction peaks even increasing to 2C (Supplementary Fig. 11). Therefore, LCO cathode can better exhibit the improvement of the ion transfer on the electrode level. The thick LCO cathode (30 mg/cm 2 ) with the three-layer design shows almost no capacity decay even when increasing the rate to 2.5C (8.44 mA/cm 2 ) with the areal capacity over 3.0 mAh/cm 2 (Fig. 5 a and 5 b). In contrast, dramatic decay of the rate performance was observed for the ASSB with the reversed three-layer LCO cathode ( Fig. 5 c). Since there is no rate limitation from the material level, and the interface between SE and CAM is identical, the remarkable difference between two cells primarily relies on the different ion transport kinetics on the electrode level, which further highlights the significance of aligning the arrangement of the SE content with Li + flux to the ion transport in the thick cathode. The ultrahigh mass-loading cathodes (100 mg/cm 2 ) with the theoretical capacity of 15.0 and 11.25 mAh/cm 2 for NMC 811 and LCO as CAM were also studied (Fig. 5 d-i). The ultrahigh mass-loading NMC 811 cathode with the three-layer design exhibited around 189 and 170 mAh/g specific capacity for the first charge and discharge, equivalent to 14.25 and 12.75 mAh/cm 2 areal capacity under the current density of 0.38 mA/cm 2 . Even under the current densities of 1.5 and 3.0 mA/cm 2 , the cell can still obtain the capacities of 9.9 and 7.9 mAh/cm 2 , which are 1.5 and 2.5 times better than the conventional cell, respectively. The ultrahigh mass loading LCO cathode with the three-layer design exhibited even better rate performance, achieving an areal capacity of 10.4 mAh/cm 2 at the current density of 2.25 mA/cm 2 . All of them show obvious improvement rate performance compared with the traditional one-layer cathode further, proving the importance of aligning the SE arrangement with the Li + flux on the thick electrode. With the increase in current density over 10 mA/cm 2 for the 30 mg/cm 2 cathode or over 5 mA/cm 2 for the 100 mg/cm 2 cathode, all cells met an unnormal failure (Supplementary Figs. 12 and 13). The capacity suddenly decreases within a few cycles. For example, the cell with 30 mg/cm 2 LCO cathode cycled stably at 2.5 C with the current density of 8.44 mA/cm 2 (Supplementary Fig. 14). However, when the current increased to 10.13 mA/cm 2 , the overpotential suddenly increased even over the cutoff voltage. The constant current (CC) charge period disappeared, and the capacity was obtained by the constant voltage (CV) charge process. This phenomenon cannot be simply explained as the ohmic resistance because it is not linearly related to the current density if we compare it with the increase of the overpotential from 2C (6.75 mA/cm 2 ) to 2.5C (8.44 mA/cm 2 ) (Supplementary Fig. 15). The unnormal fail is also not due to the rate performance of the CAM since both materials met the same issue under the similar current density. Therefore, the issue is still related to the ion transfer in the thick cathode, and there should be a critical current density for the thick cathode based on the SE content and distribution. Since the Li + flux gradually increased with the thickness, but our cells only provide a three-level gradient, there are still mismatches of SE component and Li + flux on the smaller scale. A smoother and more delicate arrangement of the SE will further increase the critical current density and benefit the rate performance of the thick cathode. In summary, this work successfully visualized the lithium reaction gradients in an all-solid-state battery with a high mass-loading (33 mg/cm 2 ) NMC811 cathode using operando neutron imaging. The results confirmed the inhomogeneous lithiation of CAM in the thick cathode, with a lithiation gradient from the solid electrolyte (SE) layer side to the current collector side. The electrochemical evaluations of the ASSBs with different cathode mass-loadings of 3, 10, and 30 mg/cm 2 further validated the inhomogeneous lithiation of CAM in the thick cathode, especially at high rates. Based on the study, ion transfer was identified as the key limitation causing kinetic issues in the thick cathode. We pioneered the concept of "Li + flux" and its effect on ion transfer in the thick cathode of all-solid-state batteries. Due to the Faraday reaction of the cathode active materials, which consume or generate Li + flux, the Li + flux across the ion conductor (SE), catholyte, in the cathode accumulate in terms of the thickness of the cathode. The mismatch between the Li + flux and ion transfer channel causes a huge obstacle for the Li + transport in the thick cathode. To address the ion transfer limitation arising from the significant variation in Li + flux from the SE layer to the current collector side in the thick cathode, a tailored arrangement of the catholyte in the composite cathodes was designed and studied, resulting in significantly improved rate performances (171% of the capacity obtained in the conventional cathode at the current density of 9.0 mA/cm 2 for 30 mg/cm 2 NMC 811 cathode). The effectiveness of this gradient design was further demonstrated in ultrahigh mass-loading cathodes (100 mg/cm 2 ), which achieved areal capacities of 10.4 mAh/cm 2 at a current density of 2.25 mA/cm 2 with LCO cathode and 9.9 mAh/cm 2 at the current density of 1.50 mA/cm 2 for NMC cathode. We also observed a critical current density threshold in the thick electrodes, beyond which an abnormal capacity drop occurs attributed to the mismatch between the catholyte and Li + flux at smaller length scales. This work highlights the importance of understanding and optimizing ion transport in high mass-loading cathodes for the development of high-performance all-solid-state batteries. The insights gained from operando neutron imaging and the demonstrated effectiveness of the gradient design provide valuable inspiration for future advancements in high mass-loading all-solid-state battery fast charge technology. Methods Materials preparation : Solid electrolyte, Li 5.4 PS 4.4 Cl 1.6 , was prepared by high-energy ball milling with an annealing process. 34 Li 2 S (Sigma-Aldrich, 99.98%), P 2 S 5 (Sigma-Aldrich, 99%), and LiCl (Sigma-Aldrich, 99%) were stoichiometrically mixed through a ball milling for 10 h at 500 rpm. After that, the mixture was annealed at 510°C for 2 h. The cathode active materials (CAMs), Single-crystal NMC 811 (Nanoramic Inc., USA) and Lithium cobalt oxide (Sigma-Aldrich, 99.8%), were coated with Li 2 SiO x through a wet chemical method to stabilize their interface between the SE. 35 Graphitized carbon nanofibers (US Research Nanomaterials Inc., 99.9%) were used as the electronic conductive additive because of its high electrical conductivity (> 100 s/cm) and relatively low surface area (18 m 2 /g). The composite cathodes with different weight percents (65%, 75%, and 85%) of CAMs were prepared by mixing Li 2 SiO x coated CAMs, SE, and carbon additive with the following formula: 65.0% of CAM: 33.0% of SE: 2.0% of C, 75.0% of CAM: 23.5% of SE: 1.5% of C, and 85.0% of CAM: 14.0% of SE: 1.0% of C. Operando Neutron imaging : The operando neutron imaging was taken on the Multimodal Advanced Radiography Station (MARS), HFIR beamline CG-1D, at Oak Ridge National Laboratory. The detector-to-pinhole distance was 6.59 m. The pinhole diameter was 11 mm. The cells used for operando neutron imaging were assembled in the argon-filled glovebox (O 2 < 0.1 ppm, H 2 O < 0.1 ppm). 15 mg of SE were prepressed by 50 MPa within the quartz tube with a diameter of 4 mm. Then, the cathode powder was cast onto one side of the SE pallet with a pressure of 300 MPa. One piece of In and one piece of 6 Li metal (Cambridge Isotope Laboratories, Inc., 95%) were sequentially attached to the other side of the SE pallet. An aluminum pillar and a stainless steel pillar were used as the current collector for the cathode side and anode side respectively. A stacking pressure of 50 MPa was applied to the cell by an aluminum framework. All gaps on the cell were further sealed by sealing grease and hot melt glue. For the operando neutron imaging collecting, the cell was placed in front of a scientific Complementary Metal-Oxide-Semiconductor (sCMOS) camera system (Zyla5.5, Andor Technology plc. Belfast, UL) with a 20 µm thick Gd2O2S : Tb scintillator screen. The exposure time for each image was 5 minutes under the neutron beam with a wavelength range from 0.8 to 6 Å and a peak flux of 2.2 ×10 6 n cm − 2 s − 1 at 2.6 Å. Open beam and dark field images were collected before and after the operando neutron imaging test. The ambient temperature was around 20°C. The neutron imaging Jupyter Notebook developed by ORNL and Fiji-image software were used for the data processing of the operando neutron images. All-solid-state Batteries Assembling and Electrochemical tests The ASSBs were assembled by the cold pressing method in the argon-filled glovebox (O 2 < 0.1 ppm, H 2 O < 0.1 ppm). 100 mg/cm 2 of SE was first pre-pressed into the homemade PEEK cell with a diameter of 1/2 inch (12.7 mm) under 50 MPa. Different amounts of cathode materials were further cast on one side of the SE pellet. For the multi-layer cathode, the cathode materials were cast layer by layer with a pre-press pressure of 10 MPa. The cathode and SE were further densified under the pressure of 300 MPa by two steel pillars. One piece of In foil and Li foil were sequentially attached to the other side of the SE pellet. An aluminum foil and a copper foil were used as the current collectors for the cathode and anode separately. A stacking pressure of 50 MPa was applied to the cell by a stainless-steel framework during cycling. For the battery test at 60°C, all batteries were placed in the gravity convection oven (Fisher scientific) and cycled by a battery cycler system (LAND Electronic Co., Ltd). Declarations ACKNOWLEDGEMENTS This research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. H.Z. acknowledges the primary support of the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under award number DESC0024528. AUTHOR CONTRIBUTIONS H.Z. supervised and supported this study. T.J., Y.Z., and J.T. conducted the operando neutron imaging experiment; J.B., Y.Z., J.T., and T.J. processed the operando neutron imaging data; T.J., J.B., Y.Z., and J.T. analyzed the operando neutron imaging data; T.J. performed electrochemical experiments. T.J. wrote the original draft of the manuscript; H.Z., T.J., J.W., J.B., Y.Z., and J.T. modified the manuscript; All authors have read and agreed to the published version of the manuscript. DECLARATION OF INTERESTS The authors declare no competing interests. References Randau, S. et al. Benchmarking the performance of all-solid-state lithium batteries. Nature Energy 5 , 259-270 (2020). Heubner, C. et al. From lithium‐metal toward anode‐free solid‐state batteries: current developments, issues, and challenges. Advanced Functional Materials 31 , 2106608 (2021). Bielefeld, A., Weber, D. A. & Janek, J. r. Microstructural modeling of composite cathodes for all-solid-state batteries. The journal of physical chemistry C 123 , 1626-1634 (2018). Wang, J. et al. High-areal-capacity thick cathode with vertically-aligned micro-channels for advanced lithium ion batteries. Energy Storage Materials 39 , 287-293 (2021). Yang, X., Doyle-Davis, K., Gao, X. & Sun, X. Recent progress and perspectives on designing high-performance thick electrodes for all-solid-state lithium batteries. ETransportation 11 , 100152 (2022). Vargas‐Barbosa, N. M. & Roling, B. Dynamic ion correlations in solid and liquid electrolytes: how do they affect charge and mass transport? ChemElectroChem 7 , 367-385 (2020). Shkrob, I. A., Okasinski, J. S., Rodrigues, M.-T. F. & Abraham, D. P. Electrochemical Modeling and Experimental Verification of Lithiation Gradients in Oxide Cathodes of Lithium-Ion Cells. 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Editors’ choice—quantifying the impact of charge transport bottlenecks in composite cathodes of all-solid-state batteries. Journal of The Electrochemical Society 168 , 040537 (2021). Wang, Y. & Li, X. Fast Kinetics Design for Solid‐State Battery Device. Advanced Materials , 2309306 (2024). Davis, A. L. et al. Rate limitations in composite solid-state battery electrodes: revealing heterogeneity with operando microscopy. ACS Energy Letters 6 , 2993-3003 (2021). Stavola, A. M. et al. Lithiation Gradients and Tortuosity Factors in Thick NMC111-Argyrodite Solid-State Cathodes. ACS Energy Letters 8 , 1273-1280 (2023). Cao, D., Zhang, Y., Ji, T. & Zhu, H. In operando neutron imaging characterizations of all-solid-state batteries. MRS Bulletin 48 , 1257-1268, doi:10.1557/s43577-023-00611-7 (2023). Ziesche, R. F., Kardjilov, N., Kockelmann, W., Brett, D. J. & Shearing, P. R. Neutron imaging of lithium batteries. Joule 6 , 35-52 (2022). Cao, D. et al. Li Dynamics in Mixed Ionic-Electronic Conducting Interlayer of All-Solid-State Li-metal Batteries. Nano Letters (2024). Cao, D. et al. Nondestructively Visualizing and Understanding the Mechano‐Electro‐chemical Origins of “Soft Short” and “Creeping” in All‐Solid‐State Batteries. Advanced Functional Materials 33 , 2307998 (2023). Bradbury, R. et al. Visualizing Reaction Fronts and Transport Limitations in Solid‐State Li–S Batteries via Operando Neutron Imaging. Advanced Energy Materials 13 , 2203426 (2023). Bradbury, R. et al. Visualizing Lithium Ion Transport in Solid‐State Li–S Batteries Using 6Li Contrast Enhanced Neutron Imaging. Advanced Functional Materials 33 , 2302619 (2023). Lodding, A., Mundy, J. & Ott, A. Isotope inter‐diffusion and self‐diffusion in solid lithium metal. physica status solidi 38 , 559-569 (1970). Xiao, Y. et al. Understanding interface stability in solid-state batteries. Nature Reviews Materials 5 , 105-126, doi:10.1038/s41578-019-0157-5 (2020). Banerjee, A., Wang, X., Fang, C., Wu, E. A. & Meng, Y. S. Interfaces and interphases in all-solid-state batteries with inorganic solid electrolytes. Chemical reviews 120 , 6878-6933 (2020). Kim, J. et al. Electrochemical profiling method for diagnosis of inhomogeneous reactions in lithium-ion batteries. Cell Reports Physical Science 4 (2023). Noh, H.-J., Youn, S., Yoon, C. S. & Sun, Y.-K. Comparison of the structural and electrochemical properties of layered Li [NixCoyMnz] O2 (x= 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) cathode material for lithium-ion batteries. Journal of power sources 233 , 121-130 (2013). Chen, H. et al. Tortuosity effects in lithium-metal host anodes. Joule 4 , 938-952 (2020). Cao, D. et al. Enhancing Lithium Stripping Efficiency in Anode-Free Solid-State Batteries through Self-Regulated Internal Pressure. Nano Letters 23 , 9392-9398 (2023). Cao, D. et al. Unveiling the mechanical and electrochemical evolution of nanosilicon composite anodes in sulfide‐based all‐solid‐state batteries. Advanced Energy Materials 13 , 2203969 (2023). Additional Declarations There is NO Competing Interest. Supplementary Files SupportingmaterialV7NC.docx SupplementaryMovie1.avi Supplementary Movie 1 Cite Share Download PDF Status: Published Journal Publication published 18 Aug, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4511100","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":310082629,"identity":"f4629ac5-5962-47a7-a709-cc407a1ffbd5","order_by":0,"name":"Hongli Zhu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAo0lEQVRIiWNgGAWjYBACAwYGNgaGCghHggQtZ0jWwthGihZzBvZnj3nn3ZEzOMB88DYPMVosGxjSjXm3PTM2OMCWbE2UFoMDDMekebcdTtxwgMdMmkgtjG3SvHMO1284wP+NWC3MbNK8DYcTDA7wsBGp5TAbm+ScY4cNZx5mM7acQ5SW4+3PJN7UHJbnO9788MYbYrQwMGMwRsEoGAWjYBRQDgB0UywEmM7UCQAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0003-1733-4333","institution":"Northeastern University","correspondingAuthor":true,"prefix":"","firstName":"Hongli","middleName":"","lastName":"Zhu","suffix":""},{"id":310082630,"identity":"80615030-3aaf-4ded-9717-1fef704a12c9","order_by":1,"name":"Tongtai Ji","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Tongtai","middleName":"","lastName":"Ji","suffix":""},{"id":310082631,"identity":"25d17094-82af-4641-8d76-9953b173d9fd","order_by":2,"name":"Yuxuan Zhang","email":"","orcid":"https://orcid.org/0000-0002-0083-1408","institution":"Oak Ridge National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Yuxuan","middleName":"","lastName":"Zhang","suffix":""},{"id":310082632,"identity":"94ead63d-b3c2-4479-a7fe-985c50e4911e","order_by":3,"name":"James Torres","email":"","orcid":"","institution":"Oak Ridge National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"James","middleName":"","lastName":"Torres","suffix":""},{"id":310082633,"identity":"9381a180-7414-4d0f-8021-d94d7ba3eb21","order_by":4,"name":"Jean Bilheux","email":"","orcid":"","institution":"Oak Ridge National Laboratory","correspondingAuthor":false,"prefix":"","firstName":"Jean","middleName":"","lastName":"Bilheux","suffix":""},{"id":310082634,"identity":"16f50cee-6f78-4259-90f2-fe9d19a691ed","order_by":5,"name":"Jiwei Wang","email":"","orcid":"","institution":"Northeastern University","correspondingAuthor":false,"prefix":"","firstName":"Jiwei","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2024-05-31 21:30:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4511100/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4511100/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-62518-y","type":"published","date":"2025-08-18T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":57863479,"identity":"8ed434f1-b93a-496a-837c-178ae6f8230f","added_by":"auto","created_at":"2024-06-06 15:17:36","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":312779,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic\u003c/strong\u003e \u003cstrong\u003eof\u003c/strong\u003e \u003cstrong\u003edifferent mass-loading cathodes in ASSBs and the setup of \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eoperando\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e neutron imaging.\u003c/strong\u003e \u0026nbsp;\u003cstrong\u003ea\u003c/strong\u003e Configurations with different mass-loading of the cathode and the influence factors for the kinetic issues in the thick cathode. \u003cstrong\u003eb\u003c/strong\u003e Schematic of the experimental setup of \u003cem\u003eoperando\u003c/em\u003e neutron imaging. \u003cstrong\u003ec\u003c/strong\u003e Schematic of the image calculation process. The results at different states: \u003cstrong\u003ed\u003c/strong\u003e before charge, \u003cstrong\u003ee\u003c/strong\u003e fully charge, and \u003cstrong\u003ef\u003c/strong\u003e fully discharge, presented by the pseudo color images\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4511100/v1/bd7ee834f07035b6766f9975.png"},{"id":57863481,"identity":"dc24649b-3695-42d4-b7b3-bb5b03ca3b06","added_by":"auto","created_at":"2024-06-06 15:17:37","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":842603,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eOperando\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e neutron imaging of the thick cathode in ASSBs\u003c/strong\u003e. \u003cstrong\u003ea\u003c/strong\u003e The average neutron transmission change ratio (Tr\u003csub\u003et\u003c/sub\u003e/Tr\u003csub\u003e0\u003c/sub\u003e) of the \u003cem\u003eoperando\u003c/em\u003e neutron imaging cell as a function of time, and \u003cstrong\u003eb\u003c/strong\u003e the corresponding charge/discharge curve; Pseudo color 2D images of the \u003cem\u003eoperando\u003c/em\u003e cell at different \u003cstrong\u003ec\u003c/strong\u003e state of charge (SoC) and \u003cstrong\u003ed\u003c/strong\u003e depth of discharge (DoD) \u003cstrong\u003ee\u003c/strong\u003e Zoomed-in neutron transmission change (Tr\u003csub\u003et\u003c/sub\u003e/Tr\u003csub\u003e0\u003c/sub\u003e) for the thick cathode layer during charging process.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4511100/v1/8c1c37e5cfe69c1c16c329d7.png"},{"id":57864033,"identity":"f00cca00-8e77-41d6-adf0-626b0acd2244","added_by":"auto","created_at":"2024-06-06 15:25:37","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":307566,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical investigation of the ASSBs with different mass-loading cathode\u003c/strong\u003e. Charge and discharge profiles of the ASSBs with different mass-loading cathode of \u003cstrong\u003ea \u003c/strong\u003e3 mg/cm\u003csup\u003e2\u003c/sup\u003e, \u003cstrong\u003eb\u003c/strong\u003e 10 mg/cm\u003csup\u003e2\u003c/sup\u003e, and \u003cstrong\u003ec\u003c/strong\u003e 30 mg/cm\u003csup\u003e2\u003c/sup\u003e; and the corresponding dQ/dV curves of the the ASSBs with different mass-loading cathode of \u003cstrong\u003ed\u003c/strong\u003e 3mg/cm\u003csup\u003e2\u003c/sup\u003e, \u003cstrong\u003ee\u003c/strong\u003e 10 mg/cm\u003csup\u003e2\u003c/sup\u003e, and \u003cstrong\u003ef\u003c/strong\u003e 30 mg/cm\u003csup\u003e2\u003c/sup\u003e. \u003cstrong\u003eg\u003c/strong\u003e Schematic of the setup for measuring the effective ion-conductivity of the composite cathode with the electron block electrode and the geometric definition of tortuosity, \u003cstrong\u003eh\u003c/strong\u003e Schematic of the Li\u003csup\u003e+\u003c/sup\u003e flux crossing the thick cathode with Faraday reaction during charge and discharge.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4511100/v1/f419ce7ef598e53f7ecb4eb5.png"},{"id":57863485,"identity":"503225d1-c67d-4284-82d6-7d2a904310e2","added_by":"auto","created_at":"2024-06-06 15:17:37","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":390373,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic and evaluation of the three-layer cathode with gradient design.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Schematic of the three-layer cathode with gradient design and \u003cstrong\u003eb\u003c/strong\u003e the design details of each layer corresponding to the Li\u003csup\u003e+\u003c/sup\u003e flux over the thickness of the cathode; \u003cstrong\u003ec\u003c/strong\u003e Comparison of the capacities at different rate of the three-layer cathode, reversed three-layer cathode, and the control group without the gradient design; The comparison of charge and discharge curves at \u003cstrong\u003ed\u003c/strong\u003e C/20, \u003cstrong\u003ee\u003c/strong\u003e C/10, and \u003cstrong\u003ef\u003c/strong\u003e 1C, and the dQ/dV curves at \u003cstrong\u003eg\u003c/strong\u003e C/20 and \u003cstrong\u003eh\u003c/strong\u003e C/10\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4511100/v1/4c26c2a560b1f9070ce4c705.png"},{"id":57864773,"identity":"db08b98a-9f11-471c-b9f6-687aaec1efb1","added_by":"auto","created_at":"2024-06-06 15:33:37","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":341260,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical performances of the high mass-loading cathodes with the three-layer gradient design. \u003c/strong\u003eComparison of the rate performances of ASSBs \u003cstrong\u003ea\u003c/strong\u003e 30 mg/cm\u003csup\u003e2\u003c/sup\u003e high mass-loading of LCO cathode with three-layer design and reversed three-layer cathode, and the specific charge/discharge profiles of \u003cstrong\u003eb\u003c/strong\u003e the three-layer cathode and \u003cstrong\u003ec\u003c/strong\u003e the reversed three-layer cathode;\u0026nbsp; \u003cstrong\u003ed\u003c/strong\u003e Comparison of the rate performances of ASSBs with three-layer cathode and conventional one-layer cathode with 100 mg/cm\u003csup\u003e2\u003c/sup\u003e ultra high mass-loading of NMC 811 cathode, and the charge/discharge profiles of batteries with \u003cstrong\u003ee\u003c/strong\u003e three-layer cathode and \u003cstrong\u003ef\u003c/strong\u003e conventional one layer cathode. \u003cstrong\u003eg\u003c/strong\u003e Comparison of the rate performances of ASSBs with three-layer cathode and traditional one-layer cathode with 100 mg/cm\u003csup\u003e2\u003c/sup\u003e ultra high mass-loading of LCO cathode, and the charge/discharge profiles of batteries with \u003cstrong\u003eh\u003c/strong\u003e three-layer cathode and \u003cstrong\u003ei\u003c/strong\u003e conventional one-layer cathode.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4511100/v1/39ca76989447b2ffa047a5da.png"},{"id":89354300,"identity":"1ca9a80f-e35e-46c1-9668-9815480971f0","added_by":"auto","created_at":"2025-08-19 07:05:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3131600,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4511100/v1/b3638276-d484-4e4f-b1f7-096fdf4e5537.pdf"},{"id":57865494,"identity":"66071619-1c79-4aec-bd1d-862f163026c9","added_by":"auto","created_at":"2024-06-06 15:41:37","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1826811,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"SupportingmaterialV7NC.docx","url":"https://assets-eu.researchsquare.com/files/rs-4511100/v1/a5b7b4564462548e971efeb3.docx"},{"id":57863486,"identity":"ffcd7c45-270e-4b91-872b-e255a0c88a69","added_by":"auto","created_at":"2024-06-06 15:17:37","extension":"avi","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":9026760,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Movie 1\u003c/p\u003e","description":"","filename":"SupplementaryMovie1.avi","url":"https://assets-eu.researchsquare.com/files/rs-4511100/v1/9efa7d5cc8f06af852331e62.avi"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Operando Neutron Imaging-guided Gradient Design of Li-ion Solid Conductor for Extremely High Mass-loading Cathodes","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAll-solid-state batteries (ASSBs) are among the most promising next-generation energy storage technologies, offering the potential for ultra-high energy density and significantly enhanced safety by replacing flammable organic liquid electrolytes (LEs) with nonflammable inorganic solid electrolytes (SEs).\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e To improve energy density, many researchers have focused on developing electrode materials with high specific capacity or high potential. However, the cell level energy density is much lower than that of the material level because of the substantial content of inactive materials, such as the battery case, current collectors, solid electrolytes, and other components that do not directly contribute to capacity. Therefore, increasing the mass-loading of the cathode is considered to be one of the most effective strategies for enhancing the proposed high energy density at the cell level.\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eAchieving excellent electrochemical performance in high mass-loading cathodes is significantly more challenging than in thin cathodes. In the thick electrode, the higher areal current density under the same C-rate leads to a much larger ohmic polarization across the entire cell, and the increased tortuosity and longer diffusion paths result in sluggish ionic and electronic transport kinetics.\u003csup\u003e\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e The specific mechanism of the decay of the rate performance in thick electrodes is more complicated, especially for the ion transfer. In LEs, the transport of Li\u003csup\u003e+\u003c/sup\u003e is based on the combination of electric field-induced migration and salt concentration gradient-induced diffusion.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e Under the high rate, a large Li salt concentration gradient in the liquid phase will be formed and further hinder the Li\u003csup\u003e+\u003c/sup\u003e diffusion, and the Li\u003csup\u003e+\u003c/sup\u003e depletion in the LE will cause the underutilization of the cathode active materials (CAMs).\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e However, for the inorganic SE used in ASSBs, anions are fixed, and Li\u003csup\u003e+\u003c/sup\u003e is the only ion that can move with a Li\u003csup\u003e+\u003c/sup\u003e transference number around 1.0. Thus, there is no Li\u003csup\u003e+\u003c/sup\u003e gradient or concertation change in SEs. Moreover, an increasing number of SEs demonstrate high ionic conductivities, sometimes surpassing those of LEs.\u003csup\u003e\u003cspan additionalcitationids=\"CR10 CR11\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e Therefore, ASSBs are supposed to have improved rate capabilities.\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e Although impressive rate performance (\u0026gt;\u0026thinsp;40C) has been achieved in ASSBs, the cathode mass-loading is usually low (\u0026lt;\u0026thinsp;5 mg/cm\u003csup\u003e2\u003c/sup\u003e).\u003csup\u003e14,15\u003c/sup\u003e The rate performances of high mass-loading cathode ASSBs still fall short of expectations.\u003c/p\u003e \u003cp\u003eIn the composite cathode of ASSBs, the Li\u003csup\u003e+\u003c/sup\u003e ion transfer paths are formed solely by the solid-solid contact between the CAM and SE partials. Compared to LE-based batteries, thick cathodes in ASSBs exhibit much higher tortuosity and lower effective ionic conductivity.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e The size, proportion, and arrangement of the CAM, SE, and electron-conductive additive particles must be optimized carefully.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e Inspired by the LE-based batteries, low ionic tortuosity design has been introduced to the ASSBs by mixing different sizes of SE particles to improve the rate performance.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Nevertheless, despite the absence of Li\u003csup\u003e+\u003c/sup\u003e concentration gradients in SEs, recent studies have still observed non-uniform reactions of CAMs in ASSBs.\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e Given the unique physical properties of SEs compared to LEs, the impact of these properties on the performance of thick electrodes in ASSBs remains unclear. Therefore, it is crucial to investigate kinetic transfer in high mass-loading cathodes using direct \u003cem\u003eoperando\u003c/em\u003e visualization techniques and provide insights for designing thick cathodes with uniform and fast ion transport in all-solid-state systems.\u003c/p\u003e \u003cp\u003e \u003cem\u003eOperando\u003c/em\u003e and \u003cem\u003ein-situ\u003c/em\u003e investigation with sufficient temporal and spatial resolution is significant to understanding the kinetics in ASSBs, especially by tracking the Li behavior. Because of the nature of the ASSBs, conventional characterization methods, such as electron-beam and optical-light-based methods with limited transmission length, can only observe the surface behavior of the ASSBs.\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e X-ray-based methods make it difficult to detect Li directly due to its low X-ray attenuation coefficient of Li.\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e In contrast, neutron imaging shows unique advantages for tracking Li transport on account of the high visibility of Li for both ionic and metallic states.\u003csup\u003e\u003cspan additionalcitationids=\"CR23 CR24 CR25\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e In addition, neutron imaging also has great Li isotope contrast, so it can be used to track Li\u003csup\u003e+\u003c/sup\u003e diffusion in the solid electrolyte.\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e Consequently, neutron imaging is a powerful \u003cem\u003eoperando\u003c/em\u003e characterization method for ASSBs.\u003c/p\u003e \u003cp\u003eHerein, to identify the limit factor of the electrochemical reaction in thick cathodes in ASSBs, \u003cem\u003eoperando\u003c/em\u003e neutron imaging was successfully conducted to visualize the reaction gradient over the whole thick cathode with a mass-loading of 33 mg/cm\u003csup\u003e2\u003c/sup\u003e (180 \u0026micro;m thick). By performing image calculation, the spatial inhomogeneous reaction of CAMs was observed. Combined with electrochemical analysis, the mismatch of the non-uniform Li\u003csup\u003e+\u003c/sup\u003e flux with the homogeneous SE distribution in the thick cathode is identified as the main cause of poor electrochemical performance. Inspired by the visualized electrochemical reaction gradient within the thick cathode, a three-layer cathode with a catholyte content gradient was strategically designed to promote fast Li\u003csup\u003e+\u003c/sup\u003e transport and uniform reaction throughout the entire thick cathode. By tailoring the catholyte content to match the Li\u003csup\u003e+\u003c/sup\u003e flux at different depths within the cathode, this innovative design aims to alleviate the kinetic limitations observed in high mass-loading all-solid-state battery cathodes, ultimately leading to enhanced electrochemical performance. As a result, ultra-high mass-loading cathodes, 100 mg/cm\u003csup\u003e2\u003c/sup\u003e with theoretical areal capacities of 15.0 mAh/cm\u003csup\u003e2\u003c/sup\u003e for LiNi\u003csub\u003e0.8\u003c/sub\u003eCo\u003csub\u003e0.1\u003c/sub\u003eMn\u003csub\u003e0.1\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (NMC 811) and 11.25 mAh/cm\u003csup\u003e2\u003c/sup\u003e for LiCoO\u003csub\u003e2\u003c/sub\u003e (LCO), with three-layer design showed significantly improved rate performances compared to that of the thick cathode without SE gradient under the same CAMs content.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eVisualizing Reaction Kinetics in High Mass-Loading Cathodes through Operando Neutron Imaging\u003c/h2\u003e \u003cp\u003eLow mass-loading cathodes (\u0026lt;\u0026thinsp;5 mg/cm\u003csup\u003e2\u003c/sup\u003e or 1.0 mAh/cm\u003csup\u003e2\u003c/sup\u003e) are commonly employed in research to investigate the material-level capabilities and usually demonstrate excellent rate performance. However, as the cathode mass-loading increases, poor kinetics emerge as a significant issue. Ion transfer, electron transfer, interface resistance, and ion diffusion in the CAM are the most common factors contributing to kinetic limitations in composite cathodes (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). The ion and electron transfer will further affect the reaction prioritization and uniformity of CAM on the electrode level. Despite previous works demonstrating ultra-thick and ultra-high areal capacity cathodes (\u0026gt;\u0026thinsp;10 mAh/cm\u003csup\u003e2\u003c/sup\u003e), this capacity was obtained at an extremely low rate (0.025C), even when utilizing a highly ion-conductive SE with a conductivity of 32 mS/cm.\u003csup\u003e9\u003c/sup\u003e Faster cycling of high mass-loading cathodes in ASSBs without compromising rate performance is crucial for real-world applications. However, the primary bottleneck for achieving high mass-loading cathodes in ASSBs remains unclear, making efforts to investigate cathode mass-loading essential.\u003c/p\u003e \u003cp\u003eTo unravel the reaction kinetic limitation, \u003cem\u003eoperando\u003c/em\u003e neutron imaging was carried out to visualize the reaction homogeneity within a thick cathode during charge and discharge processes (Supplementary Fig.\u0026nbsp;1). Figure. 1b illustrates the mechanism and setup of \u003cem\u003eoperando\u003c/em\u003e neutron imaging for ASSBs. The \u003cem\u003eoperando\u003c/em\u003e cell, featuring a high mass-loading cathode (33 mg/cm\u003csup\u003e2\u003c/sup\u003e and 5.0 mAh/cm\u003csup\u003e2\u003c/sup\u003e), was positioned vertically in front of the detector, aligning the layer interfaces parallel to the neutron beam. Given the distinct neutron attenuation coefficients of the materials in the ASSB (Supplementary Table\u0026nbsp;1), the intensity of the transmitted neutron beam varies after passing through the battery. This variance facilitates easy differentiation between the cathode, SE, and anode layers based on the gray level, achieved by normalizing the neutron transmission intensity (Tr) from zero (no transmission) to one (full transmission). Darker areas correspond to lower neutron transmission, indicating that the material has a higher neutron attenuation coefficient. In the neutron image, the three layers- the In-\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003eLi anode, SE, and the thick cathode- could be clearly recognized from top to bottom (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Subsequently, the battery underwent charging and discharging processes while simultaneously collecting electrochemical impedance spectroscopy (EIS) data (Supplementary Fig.\u0026nbsp;2) and \u003cem\u003eoperando\u003c/em\u003e neutron imaging data.\u003c/p\u003e \u003cp\u003eTo amplify the neutron transmission change of the cell during charge and discharge, the \u003cem\u003eoperando\u003c/em\u003e images were further calculated by dividing the image at the pristine state to get the transmission change ratio, Tr\u003csub\u003et\u003c/sub\u003e/Tr\u003csub\u003e0\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). The pseudo color was applied to present the value of Tr\u003csub\u003et\u003c/sub\u003e/Tr\u003csub\u003e0\u003c/sub\u003e in the neutron image (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed-f). For the area with limited transmission change (Tr\u003csub\u003et\u003c/sub\u003e/Tr\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;1), the color remains green. The colors toward red and blue indicating the decrease (Tr\u003csub\u003et\u003c/sub\u003e/Tr\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;1) and increase (Tr\u003csub\u003et\u003c/sub\u003e/Tr\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;1) of the neutron transmission, respectively. The pseudo-color video presenting the transmission change over the charge and discharge process is in Supplementary Movie 1. Considering the cylindrical shape of the \u003cem\u003eoperando\u003c/em\u003e cell, which leads to different transmission lengths of the neutron beam over the cross-section and further affects the neutron transmission, we narrowed the sampling region to the center area (1.2 mm) of the cell with the transmission length close to the cell diameter (4 mm) to have the most reliable data (Supplementary Fig.\u0026nbsp;3).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo better track the transmission changes over time, the average transmission change ratio of the sampling region of the \u003cem\u003eoperando\u003c/em\u003e battery is plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea as a function of time, and the corresponding charge/discharge profile is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed provides detailed 2D images at specific states of charge (SoC) and depth of discharge (DoD). We first focused on the behaviors at the whole cell level. During the charging process, the colors of the cathode layer gradually changed to blue. This is because the CAM underwent a delithiation process, resulting in a decrease in the neutron attenuation coefficient (Supplementary Table\u0026nbsp;1). The Li\u003csup\u003e+\u003c/sup\u003e flux was directed toward the anode side during charging, but there was almost no transmission change in the SE layer, proving that there was no Li\u003csup\u003e+\u003c/sup\u003e concentration change in the SE layer, since Li\u003csup\u003e+\u003c/sup\u003e in both CAM and SE are natural Li. A small amount of red color was observed at the interface between the SE and the In-\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003eLi anode, caused by the self-diffusion of \u003csup\u003e6\u003c/sup\u003eLi\u003csup\u003e+\u003c/sup\u003e from the anode layer to the SE layer by replacing the natural Li\u003csup\u003e+\u003c/sup\u003e in SE.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e For the In-\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003eLi anode layer, due to the high neutron attenuation coefficients of In and \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003eLi, the neutron transmission remained almost zero throughout the process under the current setup, resulting in no observable transmission change.\u003c/p\u003e \u003cp\u003eDuring the discharge process, the Li\u003csup\u003e+\u003c/sup\u003e flux was directed toward the cathode side. As the CAMs underwent lithiation, the color of the cathode layer gradually turned back from blue to green after discharge. For the SE layer, based on the data from the charging process, Li\u003csup\u003e+\u003c/sup\u003e flux did not affect the Li\u003csup\u003e+\u003c/sup\u003e concentration and neutron attenuation coefficient of the SE layer. Therefore, in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, the enlargement of the red color region in SE during discharge was contributed by \u003csup\u003e6\u003c/sup\u003eLi\u003csup\u003e+\u003c/sup\u003e diffusing into the SE along with the Li\u003csup\u003e+\u003c/sup\u003e flux. Because \u003csup\u003e6\u003c/sup\u003eLi\u003csup\u003e+\u003c/sup\u003e replaced the natural Li in the SE, the neutron attenuation coefficient of SE significantly increased (Supplementary Table\u0026nbsp;1). A clear front of \u003csup\u003e6\u003c/sup\u003eLi\u003csup\u003e+\u003c/sup\u003e proceeding through the SE from the anode side to the cathode side was observed, which can also be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed as well as Supplementary Movie 2. The \u003cem\u003eoperando\u003c/em\u003e neutron imaging successfully visualized the Li\u003csup\u003e+\u003c/sup\u003e diffusion in ASSBs from the anode to the cathode side through the SE layer during discharge based on the \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003eLi isotope.\u003c/p\u003e \u003cp\u003eThe cathode layer was further magnified to investigate the delithiation uniformity of the CAM along the vertical direction (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). The thickness of the composite cathode layer with a mass loading of 33 mg/cm\u003csup\u003e2\u003c/sup\u003e (5.0 mAh/cm\u003csup\u003e2\u003c/sup\u003e) is around 180 \u0026micro;m. At the start of the discharge, it was hard to observe the transmission change because of the limited concentration change. After 3 hours, with a capacity of around 0.4 mAh/cm\u003csup\u003e2\u003c/sup\u003e, the region close to the SE side started to turn blue. As mentioned above, this indicates the delithiation of the CAM. The cathode close to the current collector remained unchanged. As the charging time increased, a gradual movement of the reaction associated with the CAM delithiation was observed, progressing from the SE side toward the current collector side. After around 16 hours, the entire cathode turned blue with a specific charge capacity of 175 mAh/g (4.4 mAh/cm\u003csup\u003e2\u003c/sup\u003e) with the voltage reaching 3.7 V (4.3 V vs Li/Li\u003csup\u003e+\u003c/sup\u003e). A similar trend was observed for the discharge process (Supplementary Fig.\u0026nbsp;4). The CAM close to the SE side first turned back to green, indicating that the lithiation process also started from the SE side.\u003c/p\u003e \u003cp\u003eA relatively thinner cathode (16.7 mg/cm\u003csup\u003e2\u003c/sup\u003e) was also characterized under the \u003cem\u003eoperando\u003c/em\u003e neutron imaging (Supplementary Fig.\u0026nbsp;5). The reaction inhomogeneity is not obvious in the thin cathode (Supplementary Fig.\u0026nbsp;6). Although both the thick and thin cathode cells were cycled under the same C-rate, the thin cathode cell shows small overpotential on account of the smaller areal current density. Moreover, the color change in the SE layer due to the \u003csup\u003e6\u003c/sup\u003eLi\u003csup\u003e+\u003c/sup\u003e diffusion in the discharge process was also much lighter in the thin cathode, reflecting the smaller Li\u003csup\u003e+\u003c/sup\u003e flux and corresponding with the smaller current density.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eIdentifying the Key Factors Limiting Ion Transport in Thick Cathodes\u003c/h2\u003e \u003cp\u003eThe reaction uniformity of the thick electrode is highly related to the reaction kinetics, especially the transport of charge carriers (both for ions and electrons). In the early studies of the composite cathode used in ASSBs, considering the decomposition of the SE when contacting the high surface area carbon, electron conductive additives (mainly carbon) were usually avoided in the composite cathode.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e The electron conductive pathway only relies on the contact between CAM particles. Therefore, the restriction of electron transport rises with the increase in SE content because of the contact loss between CAM particles.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e Recently, one-dimensional carbon fibers have been proven to be ideal electron-conductive additives in ASSBs due to their low surface area and good electrical conductivity. According to our test (Supplementary Fig.\u0026nbsp;7), the decomposition of SE is negligible in this composite cathode when using one-dimensional carbon fibers as the electron-conductive additive. Therefore, the electron-conductive additives were applied in our composite cathode.\u003c/p\u003e \u003cp\u003eOur \u003cem\u003eoperando\u003c/em\u003e neutron imaging data reveals that the delithiation/lithiation processes of CAM were nonuniform within the thick cathode. Most of the Li\u003csup\u003e+\u003c/sup\u003e ions were first reacted in the area near the SE layer and hard to reach the CAM close to the current collector, indicating that the electrochemical reaction kinetics is primarily restricted by Li\u003csup\u003e+\u003c/sup\u003e ion transport along the long pathway within the thick electrode, rather than by electron transfer. The ion transfer limitation led to slow kinetics, necessitating a lower current density and a longer time for Li\u003csup\u003e+\u003c/sup\u003e ions to transfer to the area near the current collector side and react with the CAM. If a high current or C-rate is applied, the CAM within the thick cathode cannot be sufficiently reacted. Therefore, it is crucial to purposefully regulate the Li-ion transport within the thick cathode to ensure optimal rate performance.\u003c/p\u003e \u003cp\u003eTo further diagnose the ion transport limitation within the thick cathode, electrochemical performances were examined with different cathode mass-loadings of 3, 10, and 30 mg/cm\u003csup\u003e2\u003c/sup\u003e at 60 ℃. In-Li symmetric cells were also studied to evaluate the In-Li anode behavior and determine whether the anode contributes to the battery performance limitations. The symmetric cells maintained stable performance with limited overpotential (\u0026lt;\u0026thinsp;100 mV), even at high current densities up to 15.0 mA/cm\u003csup\u003e2\u003c/sup\u003e (Supplementary Figs.\u0026nbsp;8 and 9). This result proved that the anode is not the main performance barrier in our ASSBs system. A low mass-loading (3 mg/cm\u003csup\u003e2\u003c/sup\u003e) cathode with high SE content (33.0 wt%) and sufficient carbon additives (2.0 wt% of carbon nanofibers) was tested to explore the intrinsic rate performance on the material level of NMC 811. The cell shows a specific discharge capacity of 199 mAh/g at C/10, approaching the theoretical capacity of NMC 811 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). At C/2, 1C, and 2C, the capacities were 174, 160, and 141 mAh/g, individually, representing the rate capacity of our NMC 811 at the material level under the condition with minimized limitations of the ion and electron transfer from the electrode level.\u003c/p\u003e \u003cp\u003eWe further evaluated the rate performance for our normal cathode (75.0 wt% NMC with 1.5 wt % of carbon nanofibers) with mass-loadings of 10 and 30 mg/cm\u003csup\u003e2\u003c/sup\u003e. When using a mass-loading of 10 mg/cm\u003csup\u003e2\u003c/sup\u003e with an areal capacity of 1.5 mAh/cm\u003csup\u003e2\u003c/sup\u003e, there is no obvious decay of the rate performance, with the discharge capacities of 194, 170, 153, and 134 mAh/g at C/10, C/2, 1C, and 2C, individually (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). However, when the mass loading increased to 30 mg/cm\u003csup\u003e2\u003c/sup\u003e with an areal capacity of 4.5 mAh/cm\u003csup\u003e2\u003c/sup\u003e, the rate performance significantly decreased with the discharge capacities of 152, 140, 109, 80, and 49 mAh/g at C/10, C/2, 1C, and 2C, individually \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), which are only 76%, 63%, 50% and 35% of them achieved in the 3mg/cm\u003csup\u003e2\u003c/sup\u003e cell, respectively. The great rate performance of the 10 mg/cm\u003csup\u003e2\u003c/sup\u003e cathode was due to the short ion pathway within the thin electrodes, whereas the cell with 30 mg/cm\u003csup\u003e2\u003c/sup\u003e cathode showed much worse rate performance, suggesting the insufficient reaction of the cathode materials within the thick electrode because of sluggish ion transport along the long ion pathway.\u003c/p\u003e \u003cp\u003eThe reaction homogeneity at the electrode level can also be characterized by the dQ/dV analysis.\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e A uniform reaction of the CAM (NMC 811) on the electrode level can present a similar dQ/dV curve of the CAM shown on the material level, which has four clear peaks representing the sequential intercalation reactions of NMC 811.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, the dQ/dV curve for the 3 mg/cm\u003csup\u003e2\u003c/sup\u003e cathode at the low rate (C/10) corresponds to the behavior of the CAM at the material level. The four peaks can be observed even at 1C. When the rate increases to 2C, the shape of the dQ/dV curve changes to a rounded rectangle. Since the cathode is thin, this behavior mainly contributes to the rate capacity of CAM on the material level. Using these dQ/dV curves as the baseline behavior of our CAM, we further analyzed the effects of the increase in mass-loading. For the cathodes with 10 and 30 mg/cm\u003csup\u003e2\u003c/sup\u003e mass-loadings (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee, f), both cells show four pairs of peaks indicating uniform and sequential multiphase transitions under a low rate of C/10 or C/20. With the increase in the rate, the change of the dQ/dV curves of the 10 mg/cm\u003csup\u003e2\u003c/sup\u003e mass-loading cathode is similar to our baseline. However, for the cathode with 30 mg/cm\u003csup\u003e2\u003c/sup\u003e mass-loading, the trend of the dQ/dV curves varies with the increase of the C-rate. Even at C/10, four pairs of peaks are difficult to observe. At C/5, there is a new peak emerging at 3.4 V (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef), which is a new peak but not a shift of peak two because it cannot be simply corrected by the IR drop.\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e The shape of the dQ/dV curve for oxidation transforms into a semi-isosceles triangle (yellow line in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef). When the rate increases to C/2, the shape of the dQ/dV curve of the thick cathode becomes a right triangle (blue line in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef), which is significantly different from that of the thin cathode of 3 and 10 mg/cm\u003csup\u003e2\u003c/sup\u003e. The sequential intercalation reactions of NMC 811 were completely unobservable. Consequently, the strange dQ/dV curves indicate a huge reaction inhomogeneity in the thick cathode of 30 mg/cm\u003csup\u003e2\u003c/sup\u003e, especially at high rates, which is consistent with the findings from the \u003cem\u003eoperando\u003c/em\u003e neutron imaging.\u003c/p\u003e \u003cp\u003eWhile the electrochemical results and \u003cem\u003eoperando\u003c/em\u003e neutron imaging provide valuable insights into the reaction inhomogeneity and performance limitations in thick cathodes, it is essential to understand the underlying factors that contribute to these issues. In the conventional discussion, ionic tortuosity (τ) is considered a key parameter of the kinetics of the cathode, which controls Li\u003csup\u003e+\u003c/sup\u003e diffusion and transportation in the electrodes.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e Tortuosity is defined as the fraction of the shortest pathway through a structure (Δl) and the Euclidean distance between the starting and end points of that pathway (Δx) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg). τ could be calculated using the equation (S1). The effective ion conductivity (σ\u003csub\u003ei,eff\u003c/sub\u003e) in the cathode was usually measured by AC impedance-based techniques or polarization-interrupt method (DC method) with electron blocking electrolyte (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg) and further simulated based on the transmission line model.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e In this case, the Faraday reaction of the CAM was ignored. For the thin cathode, these methods are acceptable. However, in the thick cathode, the Li\u003csup\u003e+\u003c/sup\u003e flux will be dramatically different from the current collector side to the SE side because of the accumulation of Li\u003csup\u003e+\u003c/sup\u003e flux generated from the Faraday reaction of the CAM with the increase of the thickness, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh. The CAM not only acts as a resistor element affecting the ion and electron transfer but also provides or consumes ions and electrons through the Faraday reaction. Only using tortuosity to evaluate the ion transfer is insufficient to explain the degradation of the rate performance in the thick cathode. The effective ion conductivity cannot adequately represent the resistance of ion transfer under the dramatically different Li\u003csup\u003e+\u003c/sup\u003e flux across the electrode thickness. A large Li\u003csup\u003e+\u003c/sup\u003e flux accumulation at the SE side without a corresponding ion transfer channel provision becomes a serious impediment to Li\u003csup\u003e+\u003c/sup\u003e transport. Different from the liquid electrolyte, there is no Li\u003csup\u003e+\u003c/sup\u003e concertation gradient in the SE. Therefore, appropriate allocation of SE gradient with Li\u003csup\u003e+\u003c/sup\u003e flux over the thick cathode will effectively improve the rate performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eGradient Design in Ultra-high Mass-loading Cathode for Enhancing Rate Performance\u003c/h2\u003e \u003cp\u003eInspired by the findings from the \u003cem\u003eoperando\u003c/em\u003e neutron imaging and the above-mentioned point of Li\u003csup\u003e+\u003c/sup\u003e flux generated from the Faraday reaction, we designed a three-layer cathode with gradient ion transport channels for realizing homogenous electrochemical reaction and improved rate performance in the thick cathode (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). The overall CAM content in the three-layer cathode was kept at 75.0 wt% which is the same as the non-gradient thick cathode. From the SE layer side to the current collector side, layer A is the fastest Li\u003csup\u003e+\u003c/sup\u003e transfer layer with 65.0 wt.% of CAM, 33.0 wt.% of SE, and 2.0 wt% of carbon nanofibers. The highest content of SE is designed to bear the largest Li\u003csup\u003e+\u003c/sup\u003e flux close to the SE layer. More carbon additives were also used to prevent the electron isolation of CAM due to the high SE content. Layer B is composed of 75.0 wt.% of CAM, 23.5 wt.% of SE, and 1.5 wt.% of carbon nanofibers, the same as the original composite cathode. Layer C is a high energy density layer with the highest CAM content (85.0 wt.%) and lowest SE content (14.0 wt.%). A compare group with a reversed sequence of the three layers was also prepared, as well as the control group with only one composition of 75.0 wt.% of CAM, 23.5 wt.% of SE, and 1.5 wt.% of carbon nanofibers.\u003c/p\u003e \u003cp\u003eThe rate capabilities of the three different cathodes were compared to evaluate the superiority of our gradient design (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). The results show that the rate performance of the cell with the three-layer cathode outperforms that of the cell without a gradient design, even when tested under a very low current rate of C/20. The superiority of the gradient cathode is further exaggerated when the current rate increased from C/20 to high rates of C/10, C/5, C/2, 1C, and 2C (9.0 mA/cm\u003csup\u003e2\u003c/sup\u003e) with 112%, 119%, 120%, 135%, 155%, and 171% of the capacities obtained from the conventional cathode, individually. To further verify that the designed gradient ion transport channels can benefit the rate performance for the thick cathode, a reversed gradient cathode was examined, and it shows a much worse rate performance than that of the non-gradient cathode. Although the initial discharge capacity of the reversed three-layer cathode is very close to the original single-layer cathode at the low rate (C/20), its capacity decays dramatically as the rate increases. There is almost no capacity obtained when the rate is higher than C/2.\u003c/p\u003e \u003cp\u003eWe further compared the charge/discharge profiles of these three groups to dig into the fundamentals of the three-layer design. Under the lowest rate we tested (C/20) with a current density of 0.225 mA/cm\u003csup\u003e2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), the three-layer cathode exhibited a noticeably smaller overpotential and larger capacity. As the C-rate increased, the advantage became more pronounced (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, f). The dQ/dV analysis can provide more detailed insights into kinetics. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg, the dQ/dV curves at C/20 of the three-layer cathode and the conventional cathode are very similar, with four pairs of peaks corresponding to the sequential intercalation reactions of NMC 811. However, for the reversed three-layer cathode, in addition to the shift of the peaks due to the larger overpotential, one pair of peaks at high voltage (around 3.6 V vs In-Li/Li\u003csup\u003e+\u003c/sup\u003e or 4.2 V vs Li/Li\u003csup\u003e+\u003c/sup\u003e) is missing due to the sluggish kinetics. Specifically, the low SE content of the top layer creates significant resistance for the Li\u003csup\u003e+\u003c/sup\u003e transfer into or out of the cathode, resulting in a rapid voltage rise to the cut-off voltage without the last phase transition (H2 \u0026rarr; H3) of NMC 811. When the rate increases to C/10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh), the cell with the three-layer cathode still clearly shows the four pairs of peaks for the sequential intercalation reactions, indicating a homogeneity reaction throughout the cathode thickness. However, for the conventional and reversed cathodes, the four pairs of peaks become blurred, wider, or even disappear caused by reaction inhomogeneity in the electrode level during the charge and discharge process.\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e Overall, the three-layer cathode demonstrates better rate performance with more homogenous delithiation and lithiation reactions across the entire cathode. Although the three-layer cathode and reversed groups have the same tortuosity, they show dramatically different performances. The result clearly proved that the appropriate allocation of SE content to match the Li\u003csup\u003e+\u003c/sup\u003e flux over the whole thick cathode is an effective way to improve homogenous electrochemical reactions and the rate performance of the thick cathode.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further explore the performance of our three-layer design cathode, we introduced another CAM, LiCoO\u003csub\u003e2\u003c/sub\u003e (LCO). LCO shows much better C-rate performance on the material level (Supplementary Fig.\u0026nbsp;10). In the dQ/dV curves for the low mass-loading (3 mg/cm\u003csup\u003e2\u003c/sup\u003e) LCO cell, there is almost no shift of the reaction peaks even increasing to 2C (Supplementary Fig.\u0026nbsp;11). Therefore, LCO cathode can better exhibit the improvement of the ion transfer on the electrode level. The thick LCO cathode (30 mg/cm\u003csup\u003e2\u003c/sup\u003e) with the three-layer design shows almost no capacity decay even when increasing the rate to 2.5C (8.44 mA/cm\u003csup\u003e2\u003c/sup\u003e) with the areal capacity over 3.0 mAh/cm\u003csup\u003e2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). In contrast, dramatic decay of the rate performance was observed for the ASSB with the reversed three-layer LCO cathode \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). Since there is no rate limitation from the material level, and the interface between SE and CAM is identical, the remarkable difference between two cells primarily relies on the different ion transport kinetics on the electrode level, which further highlights the significance of aligning the arrangement of the SE content with Li\u003csup\u003e+\u003c/sup\u003e flux to the ion transport in the thick cathode.\u003c/p\u003e \u003cp\u003eThe ultrahigh mass-loading cathodes (100 mg/cm\u003csup\u003e2\u003c/sup\u003e) with the theoretical capacity of 15.0 and 11.25 mAh/cm\u003csup\u003e2\u003c/sup\u003e for NMC 811 and LCO as CAM were also studied (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed-i). The ultrahigh mass-loading NMC 811 cathode with the three-layer design exhibited around 189 and 170 mAh/g specific capacity for the first charge and discharge, equivalent to 14.25 and 12.75 mAh/cm\u003csup\u003e2\u003c/sup\u003e areal capacity under the current density of 0.38 mA/cm\u003csup\u003e2\u003c/sup\u003e. Even under the current densities of 1.5 and 3.0 mA/cm\u003csup\u003e2\u003c/sup\u003e, the cell can still obtain the capacities of 9.9 and 7.9 mAh/cm\u003csup\u003e2\u003c/sup\u003e, which are 1.5 and 2.5 times better than the conventional cell, respectively. The ultrahigh mass loading LCO cathode with the three-layer design exhibited even better rate performance, achieving an areal capacity of 10.4 mAh/cm\u003csup\u003e2\u003c/sup\u003e at the current density of 2.25 mA/cm\u003csup\u003e2\u003c/sup\u003e. All of them show obvious improvement rate performance compared with the traditional one-layer cathode further, proving the importance of aligning the SE arrangement with the Li\u003csup\u003e+\u003c/sup\u003e flux on the thick electrode.\u003c/p\u003e \u003cp\u003eWith the increase in current density over 10 mA/cm\u003csup\u003e2\u003c/sup\u003e for the 30 mg/cm\u003csup\u003e2\u003c/sup\u003e cathode or over 5 mA/cm\u003csup\u003e2\u003c/sup\u003e for the 100 mg/cm\u003csup\u003e2\u003c/sup\u003e cathode, all cells met an unnormal failure (Supplementary Figs.\u0026nbsp;12 and 13). The capacity suddenly decreases within a few cycles. For example, the cell with 30 mg/cm\u003csup\u003e2\u003c/sup\u003e LCO cathode cycled stably at 2.5 C with the current density of 8.44 mA/cm\u003csup\u003e2\u003c/sup\u003e (Supplementary Fig.\u0026nbsp;14). However, when the current increased to 10.13 mA/cm\u003csup\u003e2\u003c/sup\u003e, the overpotential suddenly increased even over the cutoff voltage. The constant current (CC) charge period disappeared, and the capacity was obtained by the constant voltage (CV) charge process. This phenomenon cannot be simply explained as the ohmic resistance because it is not linearly related to the current density if we compare it with the increase of the overpotential from 2C (6.75 mA/cm\u003csup\u003e2\u003c/sup\u003e) to 2.5C (8.44 mA/cm\u003csup\u003e2\u003c/sup\u003e) (Supplementary Fig.\u0026nbsp;15). The unnormal fail is also not due to the rate performance of the CAM since both materials met the same issue under the similar current density. Therefore, the issue is still related to the ion transfer in the thick cathode, and there should be a critical current density for the thick cathode based on the SE content and distribution. Since the Li\u003csup\u003e+\u003c/sup\u003e flux gradually increased with the thickness, but our cells only provide a three-level gradient, there are still mismatches of SE component and Li\u003csup\u003e+\u003c/sup\u003e flux on the smaller scale. A smoother and more delicate arrangement of the SE will further increase the critical current density and benefit the rate performance of the thick cathode.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn summary, this work successfully visualized the lithium reaction gradients in an all-solid-state battery with a high mass-loading (33 mg/cm\u003csup\u003e2\u003c/sup\u003e) NMC811 cathode using \u003cem\u003eoperando\u003c/em\u003e neutron imaging. The results confirmed the inhomogeneous lithiation of CAM in the thick cathode, with a lithiation gradient from the solid electrolyte (SE) layer side to the current collector side. The electrochemical evaluations of the ASSBs with different cathode mass-loadings of 3, 10, and 30 mg/cm\u003csup\u003e2\u003c/sup\u003e further validated the inhomogeneous lithiation of CAM in the thick cathode, especially at high rates. Based on the study, ion transfer was identified as the key limitation causing kinetic issues in the thick cathode. We pioneered the concept of \"Li\u003csup\u003e+\u003c/sup\u003e flux\" and its effect on ion transfer in the thick cathode of all-solid-state batteries. Due to the Faraday reaction of the cathode active materials, which consume or generate Li\u003csup\u003e+\u003c/sup\u003e flux, the Li\u003csup\u003e+\u003c/sup\u003e flux across the ion conductor (SE), catholyte, in the cathode accumulate in terms of the thickness of the cathode. The mismatch between the Li\u003csup\u003e+\u003c/sup\u003e flux and ion transfer channel causes a huge obstacle for the Li\u003csup\u003e+\u003c/sup\u003e transport in the thick cathode.\u003c/p\u003e \u003cp\u003eTo address the ion transfer limitation arising from the significant variation in Li\u003csup\u003e+\u003c/sup\u003e flux from the SE layer to the current collector side in the thick cathode, a tailored arrangement of the catholyte in the composite cathodes was designed and studied, resulting in significantly improved rate performances (171% of the capacity obtained in the conventional cathode at the current density of 9.0 mA/cm\u003csup\u003e2\u003c/sup\u003e for 30 mg/cm\u003csup\u003e2\u003c/sup\u003e NMC 811 cathode). The effectiveness of this gradient design was further demonstrated in ultrahigh mass-loading cathodes (100 mg/cm\u003csup\u003e2\u003c/sup\u003e), which achieved areal capacities of 10.4 mAh/cm\u003csup\u003e2\u003c/sup\u003e at a current density of 2.25 mA/cm\u003csup\u003e2\u003c/sup\u003e with LCO cathode and 9.9 mAh/cm\u003csup\u003e2\u003c/sup\u003e at the current density of 1.50 mA/cm\u003csup\u003e2\u003c/sup\u003e for NMC cathode. We also observed a critical current density threshold in the thick electrodes, beyond which an abnormal capacity drop occurs attributed to the mismatch between the catholyte and Li\u003csup\u003e+\u003c/sup\u003e flux at smaller length scales. This work highlights the importance of understanding and optimizing ion transport in high mass-loading cathodes for the development of high-performance all-solid-state batteries. The insights gained from \u003cem\u003eoperando\u003c/em\u003e neutron imaging and the demonstrated effectiveness of the gradient design provide valuable inspiration for future advancements in high mass-loading all-solid-state battery fast charge technology.\u003c/p\u003e \u003c/div\u003e"},{"header":"Methods","content":"\u003cp\u003e \u003cem\u003eMaterials preparation\u003c/em\u003e: Solid electrolyte, Li\u003csub\u003e5.4\u003c/sub\u003ePS\u003csub\u003e4.4\u003c/sub\u003eCl\u003csub\u003e1.6\u003c/sub\u003e, was prepared by high-energy ball milling with an annealing process.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e Li\u003csub\u003e2\u003c/sub\u003eS (Sigma-Aldrich, 99.98%), P\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e5\u003c/sub\u003e (Sigma-Aldrich, 99%), and LiCl (Sigma-Aldrich, 99%) were stoichiometrically mixed through a ball milling for 10 h at 500 rpm. After that, the mixture was annealed at 510\u0026deg;C for 2 h. The cathode active materials (CAMs), Single-crystal NMC 811 (Nanoramic Inc., USA) and Lithium cobalt oxide (Sigma-Aldrich, 99.8%), were coated with Li\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003ex\u003c/sub\u003e through a wet chemical method to stabilize their interface between the SE.\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e Graphitized carbon nanofibers (US Research Nanomaterials Inc., 99.9%) were used as the electronic conductive additive because of its high electrical conductivity (\u0026gt;\u0026thinsp;100 s/cm) and relatively low surface area (18 m\u003csup\u003e2\u003c/sup\u003e/g). The composite cathodes with different weight percents (65%, 75%, and 85%) of CAMs were prepared by mixing Li\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003ex\u003c/sub\u003e coated CAMs, SE, and carbon additive with the following formula: 65.0% of CAM: 33.0% of SE: 2.0% of C, 75.0% of CAM: 23.5% of SE: 1.5% of C, and 85.0% of CAM: 14.0% of SE: 1.0% of C.\u003c/p\u003e \u003cp\u003e \u003cem\u003eOperando Neutron imaging\u003c/em\u003e: The \u003cem\u003eoperando\u003c/em\u003e neutron imaging was taken on the Multimodal Advanced Radiography Station (MARS), HFIR beamline CG-1D, at Oak Ridge National Laboratory. The detector-to-pinhole distance was 6.59 m. The pinhole diameter was 11 mm. The cells used for \u003cem\u003eoperando\u003c/em\u003e neutron imaging were assembled in the argon-filled glovebox (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.1 ppm, H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;\u0026lt;\u0026thinsp;0.1 ppm). 15 mg of SE were prepressed by 50 MPa within the quartz tube with a diameter of 4 mm. Then, the cathode powder was cast onto one side of the SE pallet with a pressure of 300 MPa. One piece of In and one piece of \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003eLi metal (Cambridge Isotope Laboratories, Inc., 95%) were sequentially attached to the other side of the SE pallet. An aluminum pillar and a stainless steel pillar were used as the current collector for the cathode side and anode side respectively. A stacking pressure of 50 MPa was applied to the cell by an aluminum framework. All gaps on the cell were further sealed by sealing grease and hot melt glue. For the \u003cem\u003eoperando\u003c/em\u003e neutron imaging collecting, the cell was placed in front of a scientific Complementary Metal-Oxide-Semiconductor (sCMOS) camera system (Zyla5.5, Andor Technology plc. Belfast, UL) with a 20 \u0026micro;m thick Gd2O2S : Tb scintillator screen. The exposure time for each image was 5 minutes under the neutron beam with a wavelength range from 0.8 to 6 \u0026Aring; and a peak flux of 2.2 \u0026times;10\u003csup\u003e6\u003c/sup\u003e n cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 2.6 \u0026Aring;. Open beam and dark field images were collected before and after the \u003cem\u003eoperando\u003c/em\u003e neutron imaging test. The ambient temperature was around 20\u0026deg;C. The neutron imaging Jupyter Notebook developed by ORNL and Fiji-image software were used for the data processing of the \u003cem\u003eoperando\u003c/em\u003e neutron images.\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eAll-solid-state Batteries Assembling and Electrochemical tests\u003c/strong\u003e \u003cp\u003eThe ASSBs were assembled by the cold pressing method in the argon-filled glovebox (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.1 ppm, H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;\u0026lt;\u0026thinsp;0.1 ppm). 100 mg/cm\u003csup\u003e2\u003c/sup\u003e of SE was first pre-pressed into the homemade PEEK cell with a diameter of 1/2 inch (12.7 mm) under 50 MPa. Different amounts of cathode materials were further cast on one side of the SE pellet. For the multi-layer cathode, the cathode materials were cast layer by layer with a pre-press pressure of 10 MPa. The cathode and SE were further densified under the pressure of 300 MPa by two steel pillars. One piece of In foil and Li foil were sequentially attached to the other side of the SE pellet. An aluminum foil and a copper foil were used as the current collectors for the cathode and anode separately. A stacking pressure of 50 MPa was applied to the cell by a stainless-steel framework during cycling. For the battery test at 60\u0026deg;C, all batteries were placed in the gravity convection oven (Fisher scientific) and cycled by a battery cycler system (LAND Electronic Co., Ltd).\u003c/p\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGEMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. H.Z. acknowledges the primary support of the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under award number DESC0024528.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAUTHOR CONTRIBUTIONS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eH.Z. supervised and supported this study. T.J., Y.Z., and J.T. conducted the operando neutron imaging experiment; J.B., Y.Z., J.T., and T.J. processed the operando neutron imaging data; T.J., J.B., Y.Z., and J.T. analyzed the operando neutron imaging data; T.J. performed electrochemical experiments. T.J. wrote the original draft of the manuscript; H.Z., T.J., J.W., J.B., Y.Z., and J.T. modified the manuscript; All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDECLARATION OF INTERESTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eRandau, S.\u003cem\u003e\u0026nbsp;et al.\u003c/em\u003e Benchmarking the performance of all-solid-state lithium batteries. \u003cem\u003eNature Energy\u003c/em\u003e \u003cstrong\u003e5\u003c/strong\u003e, 259-270 (2020).\u003c/li\u003e\n \u003cli\u003eHeubner, C.\u003cem\u003e\u0026nbsp;et al.\u003c/em\u003e From lithium‐metal toward anode‐free solid‐state batteries: current developments, issues, and challenges. \u003cem\u003eAdvanced Functional Materials\u003c/em\u003e \u003cstrong\u003e31\u003c/strong\u003e, 2106608 (2021).\u003c/li\u003e\n \u003cli\u003eBielefeld, A., Weber, D. 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However, as mass-loading increases, electrochemical performance is significantly compromised due to sluggish kinetics. \u003cem\u003eOperando\u003c/em\u003e neutron imaging of a high mass-loading NMC 811 cathode of 33 mg/cm\u003csup\u003e2\u003c/sup\u003e (5.0 mAh/cm\u003csup\u003e2\u003c/sup\u003e, 180 \u0026micro;m thick) reveals the lithiation prioritization of the cathode active material (CAM) from the solid electrolyte layer to the current collector side. In addition to the tortuosity, another key limitation to ion transfer in the cathode arises from the mismatch between the uniform distribution of the solid electrolyte (catholyte) in the conventional composite cathode and the non-uniform Li\u003csup\u003e+\u003c/sup\u003e flux generated by the Faraday reaction of CAMs. Therefore, a novel design with a gradient in the catholyte concentration is engineered to match the Li\u003csup\u003e+\u003c/sup\u003e flux distribution, aiming to eliminate the ion transfer obstacle. This innovative approach demonstrates enhanced rate performance, even with ultra-high mass-loading cathodes. A LiCoO\u003csub\u003e2\u003c/sub\u003e composite cathode with 100 mg/cm\u003csup\u003e2\u003c/sup\u003e ultra-high mass-loading exhibited an areal capacity of 10.4 mAh/cm\u003csup\u003e2\u003c/sup\u003e at a current density of 2.25 mA/cm\u003csup\u003e2\u003c/sup\u003e. This work demonstrated an effective gradient design to optimize ion transport in high mass-loading cathodes to overcome the kinetic barrier and achieve high battery performance.\u003c/p\u003e","manuscriptTitle":"Operando Neutron Imaging-guided Gradient Design of Li-ion Solid Conductor for Extremely High Mass-loading Cathodes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-06 15:17:32","doi":"10.21203/rs.3.rs-4511100/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"6b818a5b-99a6-4053-bcd7-f819969a987a","owner":[],"postedDate":"June 6th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":32766295,"name":"Physical sciences/Energy science and technology/Energy storage/Batteries"},{"id":32766296,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Batteries"}],"tags":[],"updatedAt":"2025-08-19T07:05:31+00:00","versionOfRecord":{"articleIdentity":"rs-4511100","link":"https://doi.org/10.1038/s41467-025-62518-y","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-08-18 04:00:00","publishedOnDateReadable":"August 18th, 2025"},"versionCreatedAt":"2024-06-06 15:17:32","video":"","vorDoi":"10.1038/s41467-025-62518-y","vorDoiUrl":"https://doi.org/10.1038/s41467-025-62518-y","workflowStages":[]},"version":"v1","identity":"rs-4511100","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4511100","identity":"rs-4511100","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}