Electron-beam sintering achieves dual-phase transparent solid-state electrolytes for batteries

Electron-beam sintering achieves dual-phase transparent solid-state electrolytes for batteries | 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 Electron-beam sintering achieves dual-phase transparent solid-state electrolytes for batteries Kai Chen, Zhanhui Jia, Weijiang Xue, Hao Shen, Ruohong Ke, Sujia Yan, and 12 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8091488/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Amorphous–crystalline dual-phase grain structure often enables materials with unique performance. This is especially the case for the garnet-type solid-state electrolyte Li 7 La 3 Zr 2 O 12 (LLZO), which is a promising candidate for hybrid and all-solid-state lithium batteries. A major synthesis challenge, however, is that amorphous LLZO is difficult to retain via conventional sintering and even Flash Joule heating. Using electron beam focused to sub-millimeter scale with point-by-point scanning, we achieved highly localized heating to very high temperature followed by ultrafast quenching, enabling the direct fabrication of uniformly fine-grained amorphous–crystalline dual-phase LLZO. Fast ion transport offered by the crystalline grains, together with the amorphous shell that promotes a more homogeneous space-charge distribution at grain boundaries, markedly enhance the critical current density, cycling stability, and optical transparency. Our scalable fabrication route offers unparalleled tunability in electrochemical performance, and also has broad applicability for other functional ceramic systems requiring optimized electrical and optical properties. Physical sciences/Materials science/Materials for energy and catalysis/Batteries Physical sciences/Energy science and technology/Energy storage/Batteries Non-equilibrium electron-beam sintering amorphous–crystalline dual-phase grain structure transparent solid-state electrolytes lithium dendrite suppression grain-boundary amorphous nano-shell Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Materials featuring an amorphous–crystalline dual-phase structure offer unique opportunities for enhanced functionality and tunable performance. For example, encasing nanograins with amorphous intergranular shells in Mg-based alloys has been shown to break conventional strength limits. 1 , 2 In silicon nitride, an amorphous second phase at grain boundaries markedly improves fracture toughness. 3 The dual-phase strategy would be particularly useful for solid-state electrolytes (SSEs) for the following reason. Garnet-type SSEs, particularly those based on Li 7 La 3 Zr 2 O 12 (LLZO), have emerged as promising ceramic candidates for all-solid-state lithium batteries, owing to their kinetic stability against lithium and wide electrochemical window (0–6 V), which enables compatibility with high-voltage cathode materials and pure lithium anodes. 4 – 7 However, challenges remain, particularly in mitigating lithium dendrite growth, which tends to propagate along the grain boundaries 8 of the ceramic electrolytes. This issue is accentuated in conventional sintered crystalline LLZO (Fig. 1 a), 9, 10 as the oxygen vacancies at grain boundaries narrows the bandgap and enhances electronic conductivity, thereby increasing the likelihood of lithium metal deposition in these regions. 9 , 10 Additionally, segregation of dopants, especially Ta, can modify the grain-boundary space charge, thereby hampering the short-circuit endurance of the electrolyte. 11 This problem can be circumvented by an amorphous grain-boundary phase, which suppresses grain-boundary electronic conductivity to offer better resistance to lithium dendrite growth, improving cycling stability. Amorphous LLZO can offer a higher lithium-ion transference number \(\:{t}_{+}^{0}\) , very low electronic conductivity (around 10 −14 S/cm), and improved interfacial mechanical compliance with a more homogeneous space-charge distribution, all of which promote uniform Li plating and stripping. 12 Inspired by this, a thin amorphous LLZO shell was used to enclose the LLZO pellet, enabling critical current densities up to 3.2 mA/cm 2 . 13−16 However, the ionic conductivity of amorphous LLZO, approximately 10 −7 S/cm, is so far several orders lower than that of its crystalline counterpart (up to 10 −3 S/cm). 17 Motivated by the possibility to have LLZO either in amorphous or crystalline phase based on Time-Temperature-Transformation diagram 15 , we propose to synthesize, for the first time, amorphous–crystalline dual-phase grain structure in LLZO, in which crystalline LLZO grains are conformally coated with an amorphous LLZO shell, thereby synergistically combining the advantages of both phases. Recent simulations also indicate that grain-boundary amorphization enhances the strain-accommodation capacity and disperses localized stress concentrations. Concurrently, the amorphous layer homogenizes the interfacial charge-density distribution, diminishing reduction hotspots and thereby fostering more uniform lithium deposition. 18 A critical challenge, however, is that the amorphous phase of LLZO is difficult to preserve. The main challenge lies in LLZO’s intrinsically poor glass-forming ability as a non-Zachariasen glass, making it difficult to obtain an amorphous phase using conventional sintering, 12 which requires long dwell times at temperatures above 1100°C. 19–21 Sequential decomposition synthesis (SDS) 22 enables the low-temperature (~ 500°C) formation of amorphous LLZO thin films, and subsequent post-synthesis annealing drives nucleation and densification to yield an amorphous–crystalline dual-phase LLZO. However, because this approach operates at relatively low temperatures, the tetragonal phase is often preferentially stabilized, hindering the simultaneous retention of an amorphous-crystalline core–shell structure and the high-ionic-conductivity cubic LLZO phase. All the above highlights the need for an innovative sintering method that directly constructs an amorphous–crystalline dual-phase LLZO. We propose to use electron-beam local melting of LLZO at very high temperatures, followed by hyperquenching to kinetically trap the amorphous phase at grain boundaries, which can further evolve into amorphous shells encapsulating the grains. This synthesis route (Fig. 1 c) gives much higher quench rates (> 10 6 K/s) than Flash Joule Heating (Fig. 1 b), 23 which can only reach cooling rates on the order of 10 2 K/s 24,25 . Specifically, the heat source is localized by focusing the electron beam to sub-millimeter in diameter, to impart only a brief and intense thermal pulse. The heat is dissipated rapidly to the surrounding matrix (large heat sink), once the beam moves away. This effectively realizes ultrafast cooling ( hyperquenching ), with a reported cooling rate as high as > 10 6 K/s such as in laser beam additive manufacturing 26 . This high spatiotemporal resolution method establishes a new sintering window, simultaneously enabling densification, precise control of grain growth, and retention of an interfacial amorphous structure. Compared to laser-based techniques, electron-beam (e-beam) heating offers improved energy efficiency due to reduced reflection losses and allows faster beam scanning speeds. This synthesis route enabled us to fabricate Ta-doped LLZO (LLZTO) with uniform fine grains, minimal compositional variation, high optical transparency, an optimized bandgap, and a metastable hyperquenched dual-phase grain architecture consisting of crystalline cores enclosed by amorphous nano-shells. This proof-of-concept experiment demonstrates unprecedented properties of LLZO for battery applications. In general, the level of control achieved via our approach paves a new pathway for the fabrication of amorphous-crystalline dual-phase architected materials with enhanced structural and functional tunability. Results and Discussion E-beam-sintered high-quality LLZTO pellets are labeled as LLZTO-1, LLZTO-3, LLZTO-5, and LLZTO-10, corresponding to four experimental batches with total heating time of 1, 3, 5, and 10 min, respectively. LLZTO sintered in tube furnace is labeled LLZTO-TF, representing the standard slow cooling rate processing. Remarkably, LLZTO-10 exhibits significantly higher transparency, evidenced by the sharper and better distinct gear edge of the logo beneath it (Fig. 2 a). In contrast, LLZTO-TF shows much reduced light transmission, appearing more opaque and scattering light more strongly. In the X-ray diffraction (XRD) pattern of LLZTO-1 (Figure S1 a), impurity reflections attributable to La 2 Zr 2 O 7 are evident, likely arising from residual precursors and incomplete phase formation. By contrast, LLZTO-3 through LLZTO-10 (Figure S1 b–d) exhibit sharp, well-defined peaks, indicative of high crystallinity. The lattice constant from Rietveld refinement was 1.29 nm (Figure S1 ). High-resolution 3D surface topography imaging achieved by atom force microscope (AFM) confirms nanoscale surface smoothness, with LLZTO-10 exhibiting a significantly smoother surface than LLZTO-TF (Fig. 2 b). The root-mean-square roughness (R q ) were quantified as 26.0 nm, 14.5 nm, 7.97 nm, 24.3 nm for LLZTO-3, LLZTO-5, LLZTO-10 and LLZTO-TF, respectively. This comparison suggests the finest pores and tightest grain-to-grain contact in LLZTO-10 among all specimens. Figure 2 c presents the grain sizes of LLZTO-10 and LLZTO-TF, combined with the cross-sectional microstructure of these two specimens. The grains in the e-beam-sintered LLZTO are much finer and more uniform than those conventionally sintered (see also Figure S2). The measured relative densities for LLZTO-3, LLZTO-5, LLZTO-10, and LLZTO-TF are 90.2%, 91.9%, 93.6%, and 91.7%, respectively. To resolve the local grain-boundary structure, cryogenic transmission electron microscopy (cryo-TEM) specimens were prepared (from LLZTO-5 as a representative) using a cryogenic focused ion beam (cryo-FIB). To minimize beam damage during observation, all imaging was conducted under cryo-TEM conditions. In the bright-field images, besides well-ordered crystalline grains, distinct amorphous phases are visible at triple junctions (thickness > 30 nm) and along narrow grain boundaries (thickness < 10 nm), confirming the existence of an amorphous nano-shell encasing the grains (Fig. 2 d and e). Clear diffuse diffraction patterns further confirm the presence of amorphous regions. In the first trial batches of e-beam-sintered samples, microcracks were frequently observed on the surface facing the e-beam (Fig. 2 f, left). As revealed by X-ray computed tomography (XCT) scans (Fig. 2 f, right), the microcracks could reach several hundred microns in depth. As the e-beam irradiation time was lengthened from 3 min to 10 min, the microcracks became narrower from 10–20 µm to less than 10 µm, and the crack area was reduced by more than half (Figure S3a and S3b). However, microcracks were not fully eliminated due to the high-temperature gradient caused by localized heating on one side and the free-standing sintering method, which does not constrain shrinkage. To address this problem, the sample was placed in between a graphite felt on top and a stainless-steel plate underneath. A significant temperature gradient develops within the 1-mm-thick sample, as visually confirmed by color gradient changes (Fig. 2 g, left). Replacing the graphite felt with graphite paper as the top cover layer reduces the temperature gradient but significantly lowers the maximum achievable temperature (Figure S4). To balance temperature uniformity and sintering efficiency, a 0.1-mm-thick graphite paper cover was adopted in combination with graphite felt beneath the sample, ensuring a synergy between controlled temperature gradient and sufficient sintering temperature (Fig. 2 g, right). With this optimization, crack-free LLZTO pellets were obtained, as confirmed by XCT inspection (Fig. 2 h, left). Under light illumination, the LLZTO sample appears exceptionally bright on both the front and rear surfaces (Fig. 2 h, right panel and Figure S5). As a brief summary, the e-beam-sintered LLZTO samples exhibit minimal grain coarsening, fewer and smaller pores, and a crack-free structure, along with consistent dimensions. Remarkably, owing to the improved microstructure, the sintered LLZTO pellets displayed a glossy translucent white appearance compared to conventional sintering (see SI Video 1, thick pellet), even at thicknesses exceeding 1 mm, as shown in Figure S6. After annealing at 1000°C, a fully transparent LLZO pellet with a thickness of 1 mm was obtained without noticeable grain growth (Figure S7). Furthermore, e-beam sintering can be used to fabricate multiple pellets in a single run, limited only by the size of the chamber, demonstrating excellent scalability (Figure S8). Electrochemical properties of sintered LLZTO The measured ionic conductivities of LLZTO-3, LLZTO-5, LLZTO-10, and LLZTO-TF were 8.5 × 10 − 4 S/cm, 9.0 × 10 − 4 S/cm, 1.04 × 10 − 3 S/cm, and 6.2 × 10 − 4 S/cm, respectively (Fig. 3 a), consistently higher than those of their tube furnace sintered counterparts and approaching the highest values reported for all SSEs in the literature. 21 , 27 , 28 In addition, as shown in Fig. 3 b, e-beam-sintered LLZTO specimens exhibit significantly lower electronic conductivity (on the order of 10 − 9 S/cm) than the conventionally sintered LLZTO-TF (1.88 × 10 − 8 S/cm). LLZTO-5 and LLZTO-TF, exhibiting comparable impedance spectra, were selected for electrochemical performance studies (Figure S9). Analysis of the Nyquist plots shows that, for both LLZTO-5 and LLZTO-TF cells, the major impedance contribution arises from the Li/LLZTO interfaces. However, the critical current density (CCD) of LLZTO-5 is 1.6 mA/cm² (Fig. 3 c), twice that of LLZTO-TF (Fig. 3 d). Upon cycling with a fixed capacity of 0.1 mAh/cm 2 for each plating/stripping process at 60°C, LLZTO-5 achieves a CCD of 4.2 mA/cm², while LLZTO-TF reaches only 1.4 mA/cm² (Figure S10). Prior to the short-circuit failure, the cell with LLZTO-5 experienced a sharp increase in overpotential, which did not occur with the one containing LLZTO-TF. This difference in failure mode possibly suggests that interfacial degradation via lithium dendrite penetration encountered greater resistance in the e-beam-sintered LLZTO-5. A similar inference can be drawn from the post-mortem cross-sectional analysis. In Fig. 3 e and 3 f, the leftmost panels display optical images after 100 h of constant current cycling at a current density of 0.1 mA/cm², the middle optical images show the cross-sections following CCD cycling failure (see Figure S11 for detailed test data), whereas the rightmost back scatter electron (BSE)-SEM images reveal localized lithium infiltration at the failure sites observed in the corresponding optical images. Lithium penetration is much more severe in LLZTO-TF, due to its grain boundary morphology and local uprise of electronic conductivity at grain boundaries when compared to its crystalline bulk. In stark contrast, LLZTO-5 possesses a denser grain boundary network and lowered electronic conductivity (Fig. 3 b), both hindering lithium dendrite nucleation and growth. 29 In long-term cycling, cells with LLZTO-5 demonstrate greater stability at 0.1 mAh/cm² compared to those with LLZTO-TF, as shown in Fig. 3 g. Additionally, cells with LLZTO-5 sustained cycling at a higher current density over an extended period (Fig. 3 h, 0.25 mAh/cm²). The optical transparency of e-beam-sintered LLZTO The transparency of LLZTO is a convenient and effective indicator for evaluating its microstructure uniformity, defect density, and chemical purity. Achieving high transparency in isotropic crystals like garnet LLZTO requires: (i) minimal population and size of residual pores, (ii) high chemical purity to eliminate light-absorbing impurities and color centers, (iii) tight grain boundaries or large grains to reduce light scattering, and (iv) minimal point defects, such as oxygen vacancies. 30 – 33 Transparent LLZTO specimens with single-crystalline or coarsened microstructures were obtained before; 30, 32 however, both structures exhibit drawbacks in terms of cost, chemical stability (easier Li 2 CO 3 formation), and mechanical properties (loose grain boundaries and poor resistance to deformation). 20 , 25 , 28 Our e-beam-sintered LLZTO specimens have refined and homogeneous grain size, leading to uniformly distributed ultra-fine (1 µm or smaller) pores. Full transparency can be achieved via post-annealing (Figure S7) without causing any grain growth. Such highly transparent LLZTO has potential applications across a wide range, such as the study of lithium dendrite nucleation and penetration behaviors and new-type scintillators. 34 The e-beam sintering is also expected to produce other ceramics that demand both fine grain structure and high transparency. Buffer layer applied in e-beam sintering E-beam irradiation damage is a long-standing concern, resulting in severe surface damage, lithium loss, and impurities (Figures S12 - S14). Besides direct lattice disruption, 35 powder splashing has also been reported in e-beam based sintering/melting technologies 36 . Monte Carlo simulations 37 indicate that a 60 kV electron beam can penetrate to a depth of approximately 15 µm in densified LLZTO (Figure S15). The resulting e-beam damage is effectively suppressed by inserting a 0.1 mm graphite-paper buffer layer, which provides an attenuation depth of about 50 µm (Figure S16). Furthermore, scanning TEM energy dispersive spectroscopy (STEM-EDS) mapping confirmed a uniform spatial distribution of La, Zr, Ta, and O elements within the e-beam-sintered LLZTO, and no significant element enrichment around the internal pores (Fig. 4 a). Suppressed lithium loss and electron transport The critical issues in the high-temperature sintering of garnet-type SSEs include the control of lithium loss and electron transport. 38 For e-beam-sintered LLZTO, inductively coupled plasma atomic emission spectroscopy (ICP-AES) reveals that the lithium content is about 5 mol% higher than that in the conventional muffle furnace sintered ones (Fig. 4 b), even though a mother powder bed was employed during the conventional tube furnace sintering process to compensate for lithium loss. Oxygen vacancies in LLZTO are closely linked to its electronic conductivity as they can act as electron donors. In conventionally sintered garnet-type SSEs, 39, 40 prolonged thermal treatment often results in lattice oxygen loss, and hence widespread presence of oxygen vacancies and unwanted enhanced electron transport. To uncover the reason for the almost one order of magnitude lowered electronic conductivity of e-beam-sintered LLZTO specimens compared to conventional ones (Fig. 3 b), synchrotron XAS comparative analysis (Fig. 4 c and S17) was conducted. From the Zr-O absorption peak intensity significantly reduced oxygen vacancy concentration was observed in the e-beam-sintered LLZTO. This is further supported by EPR and UV-vis measurements (Fig. 4 d and S18), which confirmed a reduction in paramagnetic defect centers. Interestingly, the band gap is widened as a result, which further favors the transparency enhancement (Fig. 4 e). Fast sintering kinetics As shown in Fig. 5 a, cross-sectional SEM analysis reveals that with increasing heating time, the LLZTO microstructure evolves from incomplete sintering (LLZTO-1) to a fully densified structure (LLZTO-3, -5, and − 10). Upon closer examination, the uniform grains appear well-faceted with little rounding degrees, characteristic of highly densified ceramics. 41 Grain coalescence is evident in LLZTO-10, suggesting active grain boundary migration (early stage of grain coarsening). 42 Grain size and density analysis (Fig. 5 b) reveal a narrow size distribution with an average grain size of approximately 1 µm, only 2–4 times larger than the initial powder size (Figure S19). The densities of e-beam-sintered LLZTO increases gradually with heating time. LLZTO-1 exhibits a density close to that of the green body (approximately 50% − 60%). Subsequent electron beam heating for 2 min significantly increases the density to ~ 90% in LLZTO-3, and for 9 min to ~ 94% in LLZTO-10. Achieving such rapid densification within minutes is noteworthy, in the absence of applied electric fields, mechanical pressure, or sintering aids. 41 , 43 – 47 This effective densification is facilitated by point-focused heating that results in very high sintering temperatures. Based on the parameters used for beam scanning, the dwell time of the e-beam on a single pellet per scan is approximately 6.36 ms (Figure S20), and under a total heating duration of 3 min, the cumulative beam exposure per pellet was ~ 2 s. By analogy to 2D heating techniques such as Joule heating, it is reasonable to infer that densification in the e-beam process resulted from a local temperature exceeding 1500 o C, significantly higher than that in the conventional tube furnace. There is likely localized surface melting, significantly enhancing diffusion and sintering kinetics. As observed in Fig. 2 d and 2 e, the thickness of the amorphous phase varies with grain-boundary character. Triple junctions allow partially molten LLZO to fill in and consequently yield thicker amorphous films after hyperquenching. As shown in Fig. 5 b, from LLZTO-3 to LLZTO-5, the density increases significantly, while grain growth remains limited. The latter is realized because, compared to the bulk heating in conventional sintering or Joule heating (Figure S21), e-beam sintering offers an ultrafast cooling rate (hyperquenching). 48 The temperature drops to room temperature within a very short time, inhibiting grain coarsening, which is further suppressed by the amorphous boundary separating the neighboring grains. The preserved fine-grained microstructure is beneficial for the electrochemical performance. LLZTO thin films with a thickness of 50–70 µm, targeting the requirements of industrial applications, have also been successfully fabricated using the e-beam-sintering approach, while preserving the dual-phase core-shell grain structure. SEM reveals a glassy, partially sintered phase between adjacent grains, and elemental mapping confirms a uniform distribution of the LLZTO constituents within the intergranular regions (Fig. 5 c). Subsequent annealing at 1000 o C for 30 min promotes crystallization of the amorphous phase, resulting in larger and well-faceted LLZTO grains (Fig. 5 d). According to the previous simulation, an amorphous grain boundary helps to suppress lithium dendrite penetration by homogenizing charge distribution and relieving mechanical strain during lithium insertion. 18 E-beam sintering is so far the only easy-to-access approach to achieve such a highly desired structure. More interestingly, a theoretical analysis once predicted a disadvantage of the grain boundary amorphization for its reduction of ionic conductivity. This drawback did not arise in our e-beam-sintered samples, thanks to the ultrathin thickness of the amorphous nano-shells. Figure 6 schematically summarizes the kinetics of e-beam sintering. In the initial stage, as the e-beam is turned on, the raw powders undergo localized surface melting, which accelerates mass transport and promotes the sintering into a dense microstructure. In the second stage when the e-beam is turned off, hyperquenching preserves the amorphous structure of the interfacial boundary region between the grains. As a result, uniform fine faceted grains are formed with the crystalline cores and amorphous nano-shells. The grains faceted with low-energy crystallographic planes offer a low driving force for grain growth, thus disfavoring coarsening. 49 Note here that the cyclic beam on and off process also results in thermal accumulation; therefore, if the sintering time is excessively long, grain growth may eventually take off once the local driving force for grain boundary migration exceeds the critical energy ΔG c . Conclusion Grain boundaries in polycrystalline SSEs, especially in LLZO, often exhibit inhomogeneous space-charge distributions that bias local Li deposition and impede reliable operation. We show that an amorphous–crystalline dual-phase LLZTO grain structure integrates the complementary advantages of both phases: the crystalline core preserves high ionic conductivity, while the amorphous nano-shell improves interfacial compliance and promotes more uniform Li plating/stripping, thereby raising the critical current density and enhancing cycling stability. This work highlights e-beam sintering as a high-spatiotemporal nonequilibrium processing route that overcomes the difficulty of preserving amorphous LLZO and, to our knowledge, enables for the first time a bulk LLZO comprising crystalline grains conformally coated by an amorphous LLZO nano-shell. The process involves local surface melting followed by hyperquenching, which kinetically freezes the amorphous phase while limiting grain growth. The resulting microstructures display uniform fine grains, suppressed microcracking, and markedly reduced lithium loss. As such, the the dual-phase LLZTO exhibits high optical transparency, increased CCD and improved cycling stability relative to conventionally sintered counterparts. These property gains are consistent with a lower concentration of oxygen vacancies and a widened band gap, together with more homogeneous interfacial charge distributions. Beyond LLZO, the ability of our synthesis approach to tailor grain structure and oxygen-defect chemistry from the micro- to atomic scale opens opportunities to optimize electrical, ionic, catalytic, magnetic, and optical responses across advanced oxides. Electron-beam sintering offers practical advantages, including localized point-source heating, ultrafast heating and quenching, free-standing processing, and compatibility with scalable, high-throughput manufacturing. Notably, the unique thermal pathway also facilitates targeted co-doping of the amorphized outer shell, enabling unprecedented control over local space-charge potentials via the wider amorphous grain-boundary region. Together, these capabilities provide a versatile platform for grain-boundary engineering and expand the design space for functional ceramic systems. Methods E-beam sintering of LLZTO pellets Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 powder (D 50 = 500 nm) purchased from MTI Corporation was made into pellets using a die at the pressure of 75 MPa in a drying room. The diameter of each LLZTO green body was 13 mm, and the thickness was around 1.6 mm. All green bodies were wrapped up in different ways to improve the sintered microstructure, and the optimal approach was determined to be covering with graphite paper (0.1 mm thick) on top (towards the incident electron beam) and graphitic felt (0.28 mm thick) underneath. Then the wrapped green bodies were transferred to an electron beam melting (e-beamM) system (Y150, manufactured by Xi’an Sailong AM Technologies Co., Ltd). Programmed sintering was performed at the vacuum condition ≤ 1×10 − 1 Pa using the e-beam with an acceleration voltage of 60 keV and scanning velocity of 17.6 m/s. The heating program was set as follows: raster scanned for heating for 10 s, followed by a 10-second pause, repeating this cycle three times; after that, continuous heating was applied. For the three cycles during the first minute, the e-beam current was set at 10 mA, 20 mA, and 30 mA, respectively, with the final continuous heating reaching 40 mA. In the comparative experiment, the total sintering time for LLZTO was set to 1 min, 3 min, 5 min, and 10 min. LLZTO thin films prepared by e-beam A slurry containing Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) powder was formulated and processed via tape casting to fabricate thin films. The cast films, initially 150–200 µm thick, were dried and subsequently subjected to binder removal in a muffle furnace. After debinding, the resulting LLZTO films were sandwiched between graphite papers, following the same sample configuration used for pellet specimens. E-beam heating was then applied using a stepwise current ramping protocol. The specimen was sequentially irradiated by the e-beam at 10, 20, and 30 mA for 10 s each, with 10-second intervals between steps, followed by continuous irradiation at 40 mA for 1 min. The resulting LLZTO thin films had a final thickness of 50–70 µm. LLZTO pellets prepared by conventional method The LLZTO green bodies for conventional synthesis were fabricated by the same approach as those for e-beam sintering. The green bodies were placed in alumina crucibles, covered with mother power to compensate for lithium loss, and sintered at 1150°C for 6 h in Ar atmosphere in a tube furnace. Electrochemical tests Electrochemical tests of the Li/LLZTO/Li symmetric cells were conducted on a PARSTAT MC multichannel workstation. The LLZTO pellets were finely polished down to 850 µm thick, yielding a mirrorlike surface. Li foils with a diameter of 8 mm and thickness of 600 µm were placed on both sides of the LLZTO and assembled into 2032-type coin cells inside a glovebox. The obtained coin cells were annealed at 150°C for 10 min to improve the interface contact between LLZTO with Li. Electrochemical impedance spectra (EIS) was obtained with 10 mV AC amplitude over a frequency range of 0.1 Hz to 1 MHz. Galvanostatic cycling and CCD tests were conducted using a Neware BTS 4000 at 60℃. Ionic conductivity of LLZTO was measured via AC impedance spectroscopy, with Pt electrodes sputtered onto both sides of the pellet to act as blocking electrodes. Electronic conductivity was determined through DC polarization by applying a constant voltage of 1 V for 1800 s at room temperature. Materials characterization To reveal the crystal structure of the synthesized materials, X-ray diffraction (XRD) patterns were captured using a Bruker D8 ADVANCE diffractometer, which scanned across angles from 10° to 80° using Cu Kα radiation. The resulting data were analyzed through Rietveld refinement with Profex software. To examine the surface topography, LLZTO pellets was thermal etched to visualize the grain structure in a tube furnace with Ar atmosphere at 1150°C for 5 minutes followed by furnace cooling, and then observed in a Hitachi SU8230 scanning electron microscope (SEM), which is equipped with Bruker QUANTAX FlatQUAD energy-dispersive X-ray spectroscopy (EDS) detector for elemental distribution study. For more microstructural details at the atomic level, transmission electron microscopy (TEM) was used. To overcome the limitation of e-beam irradiation damage caused by conventional TEM sample preparation and observation, the TEM specimens were lifted out and thinned down to less than 100 nm in a cryogenic focused ion beam (cryo-FIB) system (Thermo Scientific, Helios G5) at -150°C. Afterwards, an aberration-corrected FEI Krios G3i TEM was employed for low-dose high-resolution TEM (HRTEM) characterization, which was operating at 300 kV and equipped with a Falcon 3 direct electron detector. An automated liquid nitrogen filling system maintained the sample chamber and lens barrel at approximately − 192°C. The distribution of pores and cracks in the LLZTO pellets was visualized by a Zeiss Xradia 610 Versa X-ray computed tomography (XCT) system. The X-ray energy was 140 keV, achieving a voxel resolution of about 1 × 1 × 1 µm 3 . Over 1000 slices were collected per sample and reconstructed using Avizo software (FEI). The Li content of LLZTO was measured via inductively coupled plasma atomic emission spectroscopy (ICP-AES, Agilent 5110). Samples were digested in diluted aqua regia and hydrofluoric acid. UV-Vis spectra were collected using a PerkinElmer Lambda 950 spectrophotometer over the wavelength range of 200–800 nm. Electron paramagnetic resonance (EPR) spectra were measured on a Bruker A300 EPR spectrometer at room temperature. Synchrotron X-ray absorption spectroscopy (XAS) experiments were conducted on TPS 21A beamline at Taiwan Photon Source (TPS). Atom force microscope (AFM) was carried out using a Bruker Dimension ICON; samples were sequentially ground with 220, 800, 1500, and 2000 grit sandpaper and polished with 1 µm diamond suspension. Declarations Acknowledgements: This work was supported by the National Natural Science Foundation of China (Grant Nos. 22409160, W2411048), the National Key Research and Development Program of China (2023YFB12002). The authors thank the Instrumental Analysis Center of Xi’an Jiaotong University for the assistance with CT characterization. This work was supported by the Assistant Secretary for Energy, Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. References Ding W, Tao Q, Liu C, Chen G, Yoo S, Cai W, Cao P, Jia B, Wu H, Zhang D, Zhu H, Zhang L, Qu X, Zou J, Qin M (2025) Lean design of a strong and ductile dual-phase titanium-oxygen alloy. 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Supplementary Files Video1.mp4 Video 1 ManuscriptEBLLZTOSIclean.docx Electron-beam sintering achieves dual-phase transparent solid-state electrolytes for batteries Cite Share Download PDF Status: Under Review 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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10:22:01","extension":"png","order_by":31,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":91882,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/1c7ba815264dedf175727d8a.png"},{"id":98767348,"identity":"cfad56f5-e1dc-4c42-a9d9-cc70550929f1","added_by":"auto","created_at":"2025-12-22 10:22:01","extension":"png","order_by":32,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":157715,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/5b3329092115619c785d6e06.png"},{"id":98767342,"identity":"8f7e020f-f471-4c7d-80ae-54a634035707","added_by":"auto","created_at":"2025-12-22 10:22:01","extension":"png","order_by":33,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":69185,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/0ce25e354006b292de8149a9.png"},{"id":98780434,"identity":"935dbec4-a97c-4ba5-8645-539a2490f582","added_by":"auto","created_at":"2025-12-22 12:31:20","extension":"xml","order_by":34,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":138120,"visible":true,"origin":"","legend":"","description":"","filename":"NCOMMS2596033T0structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/808ab792e344e1853eca41a6.xml"},{"id":98779097,"identity":"08b4491a-a532-40ab-b7c8-ef41904e691d","added_by":"auto","created_at":"2025-12-22 12:29:57","extension":"html","order_by":35,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":151259,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/97ab6f99c18d561f760c4d3c.html"},{"id":98767305,"identity":"0d49fd46-b9c1-4de5-a2d3-a952d77adb91","added_by":"auto","created_at":"2025-12-22 10:22:00","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":3990887,"visible":true,"origin":"","legend":"\u003cp\u003eIllustrations of various heating methods, in the order of increasing cooling rate. (a) Conventional sintering exhibits limited control over microstructure and lattice integrity. (b) High temporal resolution methods effectively suppress grain growth. (c) High spatiotemporal resolution methods enable grain melting and rapid quenching, facilitating the retention of amorphous phases.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/724e95182265277eb6401559.png"},{"id":98779900,"identity":"16c0e29a-42e5-4fb3-a07d-a5abd093ecbd","added_by":"auto","created_at":"2025-12-22 12:30:55","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":11570421,"visible":true,"origin":"","legend":"\u003cp\u003eE-beam sintering of LLZTO. The relative transparency (a) of e-beam sintered and tube furnace sintered LLZTO. Both LLZTO pellets have a thickness of ~200 µm and a well-polished surface. AFM scan (b) of e-beam and tube furnace sintered LLZTO surface. Histogram of grain size distribution (c) of e-beam-sintered and tube furnace sintered LLZTO pellets, with two SEM cross-sectional insets. High-resolution TEM (HRTEM) image acquired using cryo-TEM of a triple grain boundary junction (d) in LLZTO-5. The inset in the lower right corner shows the selected area electron diffraction pattern from the amorphous region within the triple junction. HRTEM image of grain boundary (e) in close proximity to the triple junction. Optical photo (f) and X-ray computed tomography (XCT) reconstructed volume of LLZTO with microcracks. Temperature profile from the finite element analysis showing (g) the thermal gradient. The XCT reconstructed volume and optical photo (h) of the LLZTO after thermal gradient improvement.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/81224ab102802dd76c2638b2.png"},{"id":98767311,"identity":"5475f45f-563c-451d-81b3-65240a5732a0","added_by":"auto","created_at":"2025-12-22 10:22:00","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":5611608,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical properties of LLZTO in symmetrical cells. Ionic conductivity (a) and electronic conductivity (b) measurements on LLZTO pellets. CCD comparison (c, d) of LLZTO-5 and LLZTO-TF cells. Ex-situ optical and cross-sectional BSE-SEM images (e, f) of fractured LLZTO-5 and LLZTO-TF samples, taken before and after the occurrence of electrical short-circuiting. Long cycle test (g) of cells containing LLZTO-5 and LLZTO-TF at 0.2 mA/cm\u003csup\u003e2\u003c/sup\u003e. Long cycle test (h) of LLZTO-5 cell at 0.5 mA/cm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/b9af2631d57a5960725ea95b.png"},{"id":98779563,"identity":"9427df3b-91e7-4dfa-9aae-61cd9848396b","added_by":"auto","created_at":"2025-12-22 12:30:27","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":4238027,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelated evolution of elements, defects, and band structure in LLZTO. STEM-EDS mapping (a) of LLZTO-10. (b) Li content of different LLZTO samples sintered by e-beam heating and a conventional furnace heating method. (c) R-space Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra of the Zr \u003cem\u003eK\u003c/em\u003e-edge X-ray absorption spectra (XAS), (d) electron paramagnetic resonance (EPR) spectra and (e) Tauc plots of UV–vis spectra of the LLZTO pellets.\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/43ad1d3eb2e1be0987502b83.png"},{"id":98779695,"identity":"ffa38fd5-cb1b-4e56-9936-e5db12758da3","added_by":"auto","created_at":"2025-12-22 12:30:36","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":9915187,"visible":true,"origin":"","legend":"\u003cp\u003eSintering kinetics of e-beam-sintered LLZTO. The SEM images (a) of the cross section of LLZTO pellets with different sintering times. Comparison of grain size and density (b) of LLZTO pellets. The cross-sectional SEM images(c) and EDS spectra (d) of e-beam-sintered LLZTO film. The cross-sectional SEM images (e) of e-beam-sintered LLZTO films after annealing.\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/2a67ff7265749ca1ca1c3d32.png"},{"id":98767312,"identity":"9e38346e-091f-499c-99d7-5242fa171bad","added_by":"auto","created_at":"2025-12-22 10:22:00","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":4256336,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of e-beam sintering kinetics.\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/f4fbb6b7b9b8d607ef11ef5f.png"},{"id":99306928,"identity":"ba2257b5-a039-4e73-813d-1d764b208b39","added_by":"auto","created_at":"2025-12-31 16:04:24","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":39454869,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/e0c2f5d1-2467-4d95-bf2d-0f72e92605eb.pdf"},{"id":98767304,"identity":"fc24c03d-07f0-4d30-abde-9f8ea1c05b5d","added_by":"auto","created_at":"2025-12-22 10:22:00","extension":"mp4","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":666189,"visible":true,"origin":"","legend":"Video 1","description":"","filename":"Video1.mp4","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/db4fab25292b3410a8fc7735.mp4"},{"id":98779964,"identity":"4e1b324d-f0d2-4c21-8a09-aefdd085df79","added_by":"auto","created_at":"2025-12-22 12:30:57","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":8132384,"visible":true,"origin":"","legend":"Electron-beam sintering achieves dual-phase transparent solid-state electrolytes for batteries","description":"","filename":"ManuscriptEBLLZTOSIclean.docx","url":"https://assets-eu.researchsquare.com/files/rs-8091488/v1/e74112865d520002035e8ed3.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Electron-beam sintering achieves dual-phase transparent solid-state electrolytes for batteries","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMaterials featuring an amorphous\u0026ndash;crystalline dual-phase structure offer unique opportunities for enhanced functionality and tunable performance. For example, encasing nanograins with amorphous intergranular shells in Mg-based alloys has been shown to break conventional strength limits.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e In silicon nitride, an amorphous second phase at grain boundaries markedly improves fracture toughness.\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e The dual-phase strategy would be particularly useful for solid-state electrolytes (SSEs) for the following reason. Garnet-type SSEs, particularly those based on Li\u003csub\u003e7\u003c/sub\u003eLa\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e (LLZO), have emerged as promising ceramic candidates for all-solid-state lithium batteries, owing to their kinetic stability against lithium and wide electrochemical window (0\u0026ndash;6 V), which enables compatibility with high-voltage cathode materials and pure lithium anodes.\u003csup\u003e\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e However, challenges remain, particularly in mitigating lithium dendrite growth, which tends to propagate along the grain boundaries\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e of the ceramic electrolytes. This issue is accentuated in conventional sintered crystalline LLZO (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea),\u003csup\u003e9, 10\u003c/sup\u003e as the oxygen vacancies at grain boundaries narrows the bandgap and enhances electronic conductivity, thereby increasing the likelihood of lithium metal deposition in these regions.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e Additionally, segregation of dopants, especially Ta, can modify the grain-boundary space charge, thereby hampering the short-circuit endurance of the electrolyte.\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e This problem can be circumvented by an amorphous grain-boundary phase, which suppresses grain-boundary electronic conductivity to offer better resistance to lithium dendrite growth, improving cycling stability. Amorphous LLZO can offer a higher lithium-ion transference number \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{t}_{+}^{0}\\)\u003c/span\u003e\u003c/span\u003e, very low electronic conductivity (around 10\u003csup\u003e\u0026minus;14\u003c/sup\u003e S/cm), and improved interfacial mechanical compliance with a more homogeneous space-charge distribution, all of which promote uniform Li plating and stripping.\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e Inspired by this, a thin amorphous LLZO shell was used to enclose the LLZO pellet, enabling critical current densities up to 3.2 mA/cm\u003csup\u003e2\u003c/sup\u003e.\u003csup\u003e13\u0026minus;16\u003c/sup\u003e However, the ionic conductivity of amorphous LLZO, approximately 10\u003csup\u003e\u0026minus;7\u003c/sup\u003e S/cm, is so far several orders lower than that of its crystalline counterpart (up to 10\u003csup\u003e\u0026minus;3\u003c/sup\u003e S/cm).\u003csup\u003e17\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eMotivated by the possibility to have LLZO either in amorphous or crystalline phase based on Time-Temperature-Transformation diagram\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, we propose to synthesize, for the first time, amorphous\u0026ndash;crystalline dual-phase grain structure in LLZO, in which crystalline LLZO grains are conformally coated with an amorphous LLZO shell, thereby synergistically combining the advantages of both phases. Recent simulations also indicate that grain-boundary amorphization enhances the strain-accommodation capacity and disperses localized stress concentrations. Concurrently, the amorphous layer homogenizes the interfacial charge-density distribution, diminishing reduction hotspots and thereby fostering more uniform lithium deposition.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eA critical challenge, however, is that the amorphous phase of LLZO is difficult to preserve. The main challenge lies in LLZO\u0026rsquo;s intrinsically poor glass-forming ability as a non-Zachariasen glass, making it difficult to obtain an amorphous phase using conventional sintering,\u003csup\u003e12\u003c/sup\u003e which requires long dwell times at temperatures above 1100\u0026deg;C.\u003csup\u003e19\u0026ndash;21\u003c/sup\u003e Sequential decomposition synthesis (SDS)\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e enables the low-temperature (~\u0026thinsp;500\u0026deg;C) formation of amorphous LLZO thin films, and subsequent post-synthesis annealing drives nucleation and densification to yield an amorphous\u0026ndash;crystalline dual-phase LLZO. However, because this approach operates at relatively low temperatures, the tetragonal phase is often preferentially stabilized, hindering the simultaneous retention of an amorphous-crystalline core\u0026ndash;shell structure and the high-ionic-conductivity cubic LLZO phase.\u003c/p\u003e \u003cp\u003e \u003cb\u003eAll the above highlights the need for an innovative sintering method that directly constructs an amorphous\u0026ndash;crystalline dual-phase LLZO.\u003c/b\u003e We propose to use electron-beam local melting of LLZO at very high temperatures, followed by hyperquenching to kinetically trap the amorphous phase at grain boundaries, which can further evolve into amorphous shells encapsulating the grains. This synthesis route (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec) gives much higher quench rates (\u0026gt;\u0026thinsp;10\u003csup\u003e6\u003c/sup\u003e K/s) than Flash Joule Heating (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb),\u003csup\u003e23\u003c/sup\u003e which can only reach cooling rates on the order of 10\u003csup\u003e2\u003c/sup\u003e K/s \u003csup\u003e24,25\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eSpecifically, the heat source is localized by focusing the electron beam to sub-millimeter in diameter, to impart only a brief and intense thermal pulse. The heat is dissipated rapidly to the surrounding matrix (large heat sink), once the beam moves away. This effectively realizes ultrafast cooling (\u003cb\u003ehyperquenching\u003c/b\u003e), with a reported cooling rate as high as \u0026gt;\u0026thinsp;10\u003csup\u003e6\u003c/sup\u003e K/s such as in laser beam additive manufacturing\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. This \u003cb\u003ehigh spatiotemporal resolution\u003c/b\u003e method establishes a new sintering window, simultaneously enabling densification, precise control of grain growth, and retention of an interfacial amorphous structure. Compared to laser-based techniques, electron-beam (e-beam) heating offers improved energy efficiency due to reduced reflection losses and allows faster beam scanning speeds. This synthesis route enabled us to fabricate Ta-doped LLZO (LLZTO) with uniform fine grains, minimal compositional variation, high optical transparency, an optimized bandgap, and a metastable hyperquenched dual-phase grain architecture consisting of crystalline cores enclosed by amorphous nano-shells. This proof-of-concept experiment demonstrates unprecedented properties of LLZO for battery applications. In general, the level of control achieved via our approach paves a new pathway for the fabrication of amorphous-crystalline dual-phase architected materials with enhanced structural and functional tunability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003eE-beam-sintered high-quality LLZTO pellets are labeled as LLZTO-1, LLZTO-3, LLZTO-5, and LLZTO-10, corresponding to four experimental batches with total heating time of 1, 3, 5, and 10 min, respectively. LLZTO sintered in tube furnace is labeled LLZTO-TF, representing the standard slow cooling rate processing. Remarkably, LLZTO-10 exhibits significantly higher transparency, evidenced by the sharper and better distinct gear edge of the logo beneath it (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). In contrast, LLZTO-TF shows much reduced light transmission, appearing more opaque and scattering light more strongly. In the X-ray diffraction (XRD) pattern of LLZTO-1 (Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ea), impurity reflections attributable to La\u003csub\u003e2\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e are evident, likely arising from residual precursors and incomplete phase formation. By contrast, LLZTO-3 through LLZTO-10 (Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003eb\u0026ndash;d) exhibit sharp, well-defined peaks, indicative of high crystallinity. The lattice constant from Rietveld refinement was 1.29 nm (Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). High-resolution 3D surface topography imaging achieved by atom force microscope (AFM) confirms nanoscale surface smoothness, with LLZTO-10 exhibiting a significantly smoother surface than LLZTO-TF (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). The root-mean-square roughness (R\u003csub\u003eq\u003c/sub\u003e) were quantified as 26.0 nm, 14.5 nm, 7.97 nm, 24.3 nm for LLZTO-3, LLZTO-5, LLZTO-10 and LLZTO-TF, respectively. This comparison suggests the finest pores and tightest grain-to-grain contact in LLZTO-10 among all specimens. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec presents the grain sizes of LLZTO-10 and LLZTO-TF, combined with the cross-sectional microstructure of these two specimens. The grains in the e-beam-sintered LLZTO are much finer and more uniform than those conventionally sintered (see also Figure S2). The measured relative densities for LLZTO-3, LLZTO-5, LLZTO-10, and LLZTO-TF are 90.2%, 91.9%, 93.6%, and 91.7%, respectively.\u003c/p\u003e \u003cp\u003eTo resolve the local grain-boundary structure, cryogenic transmission electron microscopy (cryo-TEM) specimens were prepared (from LLZTO-5 as a representative) using a cryogenic focused ion beam (cryo-FIB). To minimize beam damage during observation, all imaging was conducted under cryo-TEM conditions. In the bright-field images, besides well-ordered crystalline grains, distinct amorphous phases are visible at triple junctions (thickness\u0026thinsp;\u0026gt;\u0026thinsp;30 nm) and along narrow grain boundaries (thickness\u0026thinsp;\u0026lt;\u0026thinsp;10 nm), confirming the existence of an amorphous nano-shell encasing the grains (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed and e). Clear diffuse diffraction patterns further confirm the presence of amorphous regions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the first trial batches of e-beam-sintered samples, microcracks were frequently observed on the surface facing the e-beam (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef, left). As revealed by X-ray computed tomography (XCT) scans (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef, right), the microcracks could reach several hundred microns in depth. As the e-beam irradiation time was lengthened from 3 min to 10 min, the microcracks became narrower from 10\u0026ndash;20 \u0026micro;m to less than 10 \u0026micro;m, and the crack area was reduced by more than half (Figure S3a and S3b). However, microcracks were not fully eliminated due to the high-temperature gradient caused by localized heating on one side and the free-standing sintering method, which does not constrain shrinkage. To address this problem, the sample was placed in between a graphite felt on top and a stainless-steel plate underneath. A significant temperature gradient develops within the 1-mm-thick sample, as visually confirmed by color gradient changes (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg, left). Replacing the graphite felt with graphite paper as the top cover layer reduces the temperature gradient but significantly lowers the maximum achievable temperature (Figure S4). To balance temperature uniformity and sintering efficiency, a 0.1-mm-thick graphite paper cover was adopted in combination with graphite felt beneath the sample, ensuring a synergy between controlled temperature gradient and sufficient sintering temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg, right). With this optimization, crack-free LLZTO pellets were obtained, as confirmed by XCT inspection (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh, left). Under light illumination, the LLZTO sample appears exceptionally bright on both the front and rear surfaces (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh, right panel and Figure S5).\u003c/p\u003e \u003cp\u003eAs a brief summary, the e-beam-sintered LLZTO samples exhibit minimal grain coarsening, fewer and smaller pores, and a crack-free structure, along with consistent dimensions. Remarkably, owing to the improved microstructure, the sintered LLZTO pellets displayed a glossy translucent white appearance compared to conventional sintering (see SI Video 1, thick pellet), even at thicknesses exceeding 1 mm, as shown in Figure S6. After annealing at 1000\u0026deg;C, a fully transparent LLZO pellet with a thickness of 1 mm was obtained without noticeable grain growth (Figure S7). Furthermore, e-beam sintering can be used to fabricate multiple pellets in a single run, limited only by the size of the chamber, demonstrating excellent scalability (Figure S8).\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eElectrochemical properties of sintered LLZTO\u003c/h2\u003e \u003cp\u003eThe measured ionic conductivities of LLZTO-3, LLZTO-5, LLZTO-10, and LLZTO-TF were 8.5 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e S/cm, 9.0 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e S/cm, 1.04 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e S/cm, and 6.2 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e S/cm, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), consistently higher than those of their tube furnace sintered counterparts and approaching the highest values reported for all SSEs in the literature.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e In addition, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, e-beam-sintered LLZTO specimens exhibit significantly lower electronic conductivity (on the order of 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e S/cm) than the conventionally sintered LLZTO-TF (1.88 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e S/cm).\u003c/p\u003e \u003cp\u003eLLZTO-5 and LLZTO-TF, exhibiting comparable impedance spectra, were selected for electrochemical performance studies (Figure S9). Analysis of the Nyquist plots shows that, for both LLZTO-5 and LLZTO-TF cells, the major impedance contribution arises from the Li/LLZTO interfaces. However, the critical current density (CCD) of LLZTO-5 is 1.6 mA/cm\u0026sup2; (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), twice that of LLZTO-TF (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). Upon cycling with a fixed capacity of 0.1 mAh/cm\u003csup\u003e2\u003c/sup\u003e for each plating/stripping process at 60\u0026deg;C, LLZTO-5 achieves a CCD of 4.2 mA/cm\u0026sup2;, while LLZTO-TF reaches only 1.4 mA/cm\u0026sup2; (Figure S10). Prior to the short-circuit failure, the cell with LLZTO-5 experienced a sharp increase in overpotential, which did not occur with the one containing LLZTO-TF. This difference in failure mode possibly suggests that interfacial degradation via lithium dendrite penetration encountered greater resistance in the e-beam-sintered LLZTO-5. A similar inference can be drawn from the post-mortem cross-sectional analysis. In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef, the leftmost panels display optical images after 100 h of constant current cycling at a current density of 0.1 mA/cm\u0026sup2;, the middle optical images show the cross-sections following CCD cycling failure (see Figure S11 for detailed test data), whereas the rightmost back scatter electron (BSE)-SEM images reveal localized lithium infiltration at the failure sites observed in the corresponding optical images. Lithium penetration is much more severe in LLZTO-TF, due to its grain boundary morphology and local uprise of electronic conductivity at grain boundaries when compared to its crystalline bulk. In stark contrast, LLZTO-5 possesses a denser grain boundary network and lowered electronic conductivity (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), both hindering lithium dendrite nucleation and growth.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e In long-term cycling, cells with LLZTO-5 demonstrate greater stability at 0.1 mAh/cm\u0026sup2; compared to those with LLZTO-TF, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg. Additionally, cells with LLZTO-5 sustained cycling at a higher current density over an extended period (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh, 0.25 mAh/cm\u0026sup2;).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eThe optical transparency of e-beam-sintered LLZTO\u003c/h3\u003e\n\u003cp\u003eThe transparency of LLZTO is a convenient and effective indicator for evaluating its microstructure uniformity, defect density, and chemical purity. Achieving high transparency in isotropic crystals like garnet LLZTO requires: (i) minimal population and size of residual pores, (ii) high chemical purity to eliminate light-absorbing impurities and color centers, (iii) tight grain boundaries or large grains to reduce light scattering, and (iv) minimal point defects, such as oxygen vacancies.\u003csup\u003e\u003cspan additionalcitationids=\"CR31 CR32\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e Transparent LLZTO specimens with single-crystalline or coarsened microstructures were obtained before;\u003csup\u003e30, 32\u003c/sup\u003e however, both structures exhibit drawbacks in terms of cost, chemical stability (easier Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e formation), and mechanical properties (loose grain boundaries and poor resistance to deformation).\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Our e-beam-sintered LLZTO specimens have refined and homogeneous grain size, leading to uniformly distributed ultra-fine (1 \u0026micro;m or smaller) pores. Full transparency can be achieved via post-annealing (Figure S7) without causing any grain growth. Such highly transparent LLZTO has potential applications across a wide range, such as the study of lithium dendrite nucleation and penetration behaviors and new-type scintillators.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e The e-beam sintering is also expected to produce other ceramics that demand both fine grain structure and high transparency.\u003c/p\u003e\n\u003ch3\u003eBuffer layer applied in e-beam sintering\u003c/h3\u003e\n\u003cp\u003eE-beam irradiation damage is a long-standing concern, resulting in severe surface damage, lithium loss, and impurities (Figures S12 - S14). Besides direct lattice disruption,\u003csup\u003e35\u003c/sup\u003e powder splashing has also been reported in e-beam based sintering/melting technologies\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Monte Carlo simulations\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e indicate that a 60 kV electron beam can penetrate to a depth of approximately 15 \u0026micro;m in densified LLZTO (Figure S15). The resulting e-beam damage is effectively suppressed by inserting a 0.1 mm graphite-paper buffer layer, which provides an attenuation depth of about 50 \u0026micro;m (Figure S16). Furthermore, scanning TEM energy dispersive spectroscopy (STEM-EDS) mapping confirmed a uniform spatial distribution of La, Zr, Ta, and O elements within the e-beam-sintered LLZTO, and no significant element enrichment around the internal pores (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea).\u003c/p\u003e\n\u003ch3\u003eSuppressed lithium loss and electron transport\u003c/h3\u003e\n\u003cp\u003eThe critical issues in the high-temperature sintering of garnet-type SSEs include the control of lithium loss and electron transport.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e For e-beam-sintered LLZTO, inductively coupled plasma atomic emission spectroscopy (ICP-AES) reveals that the lithium content is about 5 mol% higher than that in the conventional muffle furnace sintered ones (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), even though a mother powder bed was employed during the conventional tube furnace sintering process to compensate for lithium loss.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOxygen vacancies in LLZTO are closely linked to its electronic conductivity as they can act as electron donors. In conventionally sintered garnet-type SSEs,\u003csup\u003e39, 40\u003c/sup\u003e prolonged thermal treatment often results in lattice oxygen loss, and hence widespread presence of oxygen vacancies and unwanted enhanced electron transport. To uncover the reason for the almost one order of magnitude lowered electronic conductivity of e-beam-sintered LLZTO specimens compared to conventional ones (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), synchrotron XAS comparative analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and S17) was conducted. From the Zr-O absorption peak intensity significantly reduced oxygen vacancy concentration was observed in the e-beam-sintered LLZTO. This is further supported by EPR and UV-vis measurements (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed and S18), which confirmed a reduction in paramagnetic defect centers. Interestingly, the band gap is widened as a result, which further favors the transparency enhancement (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee).\u003c/p\u003e\n\u003ch3\u003eFast sintering kinetics\u003c/h3\u003e\n\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, cross-sectional SEM analysis reveals that with increasing heating time, the LLZTO microstructure evolves from incomplete sintering (LLZTO-1) to a fully densified structure (LLZTO-3, -5, and \u0026minus;\u0026thinsp;10). Upon closer examination, the uniform grains appear well-faceted with little rounding degrees, characteristic of highly densified ceramics.\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e Grain coalescence is evident in LLZTO-10, suggesting active grain boundary migration (early stage of grain coarsening).\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e Grain size and density analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) reveal a narrow size distribution with an average grain size of approximately 1 \u0026micro;m, only 2\u0026ndash;4 times larger than the initial powder size (Figure S19). The densities of e-beam-sintered LLZTO increases gradually with heating time. LLZTO-1 exhibits a density close to that of the green body (approximately 50% \u0026minus;\u0026thinsp;60%). Subsequent electron beam heating for 2 min significantly increases the density to ~\u0026thinsp;90% in LLZTO-3, and for 9 min to ~\u0026thinsp;94% in LLZTO-10. Achieving such rapid densification within minutes is noteworthy, in the absence of applied electric fields, mechanical pressure, or sintering aids.\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan additionalcitationids=\"CR44 CR45 CR46\" citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e This effective densification is facilitated by point-focused heating that results in very high sintering temperatures. Based on the parameters used for beam scanning, the dwell time of the e-beam on a single pellet per scan is approximately 6.36 ms (Figure S20), and under a total heating duration of 3 min, the cumulative beam exposure per pellet was ~\u0026thinsp;2 s. By analogy to 2D heating techniques such as Joule heating, it is reasonable to infer that densification in the e-beam process resulted from a local temperature exceeding 1500 \u003csup\u003eo\u003c/sup\u003eC, significantly higher than that in the conventional tube furnace. There is likely localized surface melting, significantly enhancing diffusion and sintering kinetics. As observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee, the thickness of the amorphous phase varies with grain-boundary character. Triple junctions allow partially molten LLZO to fill in and consequently yield thicker amorphous films after hyperquenching. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, from LLZTO-3 to LLZTO-5, the density increases significantly, while grain growth remains limited. The latter is realized because, compared to the bulk heating in conventional sintering or Joule heating (Figure S21), e-beam sintering offers an ultrafast cooling rate (hyperquenching).\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e The temperature drops to room temperature within a very short time, inhibiting grain coarsening, which is further suppressed by the amorphous boundary separating the neighboring grains. The preserved fine-grained microstructure is beneficial for the electrochemical performance.\u003c/p\u003e \u003cp\u003eLLZTO thin films with a thickness of 50\u0026ndash;70 \u0026micro;m, targeting the requirements of industrial applications, have also been successfully fabricated using the e-beam-sintering approach, while preserving the dual-phase core-shell grain structure. SEM reveals a glassy, partially sintered phase between adjacent grains, and elemental mapping confirms a uniform distribution of the LLZTO constituents within the intergranular regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). Subsequent annealing at 1000 \u003csup\u003eo\u003c/sup\u003eC for 30 min promotes crystallization of the amorphous phase, resulting in larger and well-faceted LLZTO grains (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003eAccording to the previous simulation, an amorphous grain boundary helps to suppress lithium dendrite penetration by homogenizing charge distribution and relieving mechanical strain during lithium insertion.\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e E-beam sintering is so far the only easy-to-access approach to achieve such a highly desired structure. More interestingly, a theoretical analysis once predicted a disadvantage of the grain boundary amorphization for its reduction of ionic conductivity. This drawback did not arise in our e-beam-sintered samples, thanks to the ultrathin thickness of the amorphous nano-shells.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e schematically summarizes the kinetics of e-beam sintering. In the initial stage, as the e-beam is turned on, the raw powders undergo localized surface melting, which accelerates mass transport and promotes the sintering into a dense microstructure. In the second stage when the e-beam is turned off, hyperquenching preserves the amorphous structure of the interfacial boundary region between the grains. As a result, uniform fine faceted grains are formed with the crystalline cores and amorphous nano-shells. The grains faceted with low-energy crystallographic planes offer a low driving force for grain growth, thus disfavoring coarsening.\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e Note here that the cyclic beam on and off process also results in thermal accumulation; therefore, if the sintering time is excessively long, grain growth may eventually take off once the local driving force for grain boundary migration exceeds the critical energy ΔG\u003csub\u003ec\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eGrain boundaries in polycrystalline SSEs, especially in LLZO, often exhibit inhomogeneous space-charge distributions that bias local Li deposition and impede reliable operation. We show that an amorphous–crystalline dual-phase LLZTO grain structure integrates the complementary advantages of both phases: the crystalline core preserves high ionic conductivity, while the amorphous nano-shell improves interfacial compliance and promotes more uniform Li plating/stripping, thereby raising the critical current density and enhancing cycling stability.\u003c/p\u003e \u003cp\u003eThis work highlights e-beam sintering as a high-spatiotemporal nonequilibrium processing route that overcomes the difficulty of preserving amorphous LLZO and, to our knowledge, enables for the first time a bulk LLZO comprising crystalline grains conformally coated by an amorphous LLZO nano-shell. The process involves local surface melting followed by hyperquenching, which kinetically freezes the amorphous phase while limiting grain growth. The resulting microstructures display uniform fine grains, suppressed microcracking, and markedly reduced lithium loss. As such, the the dual-phase LLZTO exhibits high optical transparency, increased CCD and improved cycling stability relative to conventionally sintered counterparts. These property gains are consistent with a lower concentration of oxygen vacancies and a widened band gap, together with more homogeneous interfacial charge distributions.\u003c/p\u003e \u003cp\u003eBeyond LLZO, the ability of our synthesis approach to tailor grain structure and oxygen-defect chemistry from the micro- to atomic scale opens opportunities to optimize electrical, ionic, catalytic, magnetic, and optical responses across advanced oxides. Electron-beam sintering offers practical advantages, including localized point-source heating, ultrafast heating and quenching, free-standing processing, and compatibility with scalable, high-throughput manufacturing. Notably, the unique thermal pathway also facilitates targeted co-doping of the amorphized outer shell, enabling unprecedented control over local space-charge potentials via the wider amorphous grain-boundary region. Together, these capabilities provide a versatile platform for grain-boundary engineering and expand the design space for functional ceramic systems.\u003c/p\u003e \n\n\n\n "},{"header":"Methods","content":"\u003ch3\u003eE-beam sintering of LLZTO pellets\u003c/h3\u003e\u003cp\u003eLi\u003csub\u003e6.4\u003c/sub\u003eLa\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e1.4\u003c/sub\u003eTa\u003csub\u003e0.6\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e powder (D\u003csub\u003e50\u003c/sub\u003e = 500 nm) purchased from MTI Corporation was made into pellets using a die at the pressure of 75 MPa in a drying room. The diameter of each LLZTO green body was 13 mm, and the thickness was around 1.6 mm. All green bodies were wrapped up in different ways to improve the sintered microstructure, and the optimal approach was determined to be covering with graphite paper (0.1 mm thick) on top (towards the incident electron beam) and graphitic felt (0.28 mm thick) underneath. Then the wrapped green bodies were transferred to an electron beam melting (e-beamM) system (Y150, manufactured by Xi’an Sailong AM Technologies Co., Ltd). Programmed sintering was performed at the vacuum condition ≤ 1×10\u003csup\u003e− 1\u003c/sup\u003e Pa using the e-beam with an acceleration voltage of 60 keV and scanning velocity of 17.6 m/s. The heating program was set as follows: raster scanned for heating for 10 s, followed by a 10-second pause, repeating this cycle three times; after that, continuous heating was applied. For the three cycles during the first minute, the e-beam current was set at 10 mA, 20 mA, and 30 mA, respectively, with the final continuous heating reaching 40 mA. In the comparative experiment, the total sintering time for LLZTO was set to 1 min, 3 min, 5 min, and 10 min.\u003c/p\u003e\u003ch3\u003eLLZTO thin films prepared by e-beam\u003c/h3\u003e\u003cp\u003eA slurry containing Li\u003csub\u003e6.4\u003c/sub\u003eLa\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e1.4\u003c/sub\u003eTa\u003csub\u003e0.6\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e (LLZTO) powder was formulated and processed via tape casting to fabricate thin films. The cast films, initially 150–200 µm thick, were dried and subsequently subjected to binder removal in a muffle furnace. After debinding, the resulting LLZTO films were sandwiched between graphite papers, following the same sample configuration used for pellet specimens. E-beam heating was then applied using a stepwise current ramping protocol. The specimen was sequentially irradiated by the e-beam at 10, 20, and 30 mA for 10 s each, with 10-second intervals between steps, followed by continuous irradiation at 40 mA for 1 min. The resulting LLZTO thin films had a final thickness of 50–70 µm.\u003c/p\u003e\u003ch2\u003eLLZTO pellets prepared by conventional method\u003c/h2\u003e\u003cp\u003eThe LLZTO green bodies for conventional synthesis were fabricated by the same approach as those for e-beam sintering. The green bodies were placed in alumina crucibles, covered with mother power to compensate for lithium loss, and sintered at 1150°C for 6 h in Ar atmosphere in a tube furnace.\u003c/p\u003e\u003ch2\u003eElectrochemical tests\u003c/h2\u003e\u003cp\u003eElectrochemical tests of the Li/LLZTO/Li symmetric cells were conducted on a PARSTAT MC multichannel workstation. The LLZTO pellets were finely polished down to 850 µm thick, yielding a mirrorlike surface. Li foils with a diameter of 8 mm and thickness of 600 µm were placed on both sides of the LLZTO and assembled into 2032-type coin cells inside a glovebox. The obtained coin cells were annealed at 150°C for 10 min to improve the interface contact between LLZTO with Li. Electrochemical impedance spectra (EIS) was obtained with 10 mV AC amplitude over a frequency range of 0.1 Hz to 1 MHz. Galvanostatic cycling and CCD tests were conducted using a Neware BTS 4000 at 60℃. Ionic conductivity of LLZTO was measured via AC impedance spectroscopy, with Pt electrodes sputtered onto both sides of the pellet to act as blocking electrodes. Electronic conductivity was determined through DC polarization by applying a constant voltage of 1 V for 1800 s at room temperature.\u003c/p\u003e\u003ch2\u003eMaterials characterization\u003c/h2\u003e\u003cp\u003eTo reveal the crystal structure of the synthesized materials, X-ray diffraction (XRD) patterns were captured using a Bruker D8 ADVANCE diffractometer, which scanned across angles from 10° to 80° using Cu Kα radiation. The resulting data were analyzed through Rietveld refinement with Profex software. To examine the surface topography, LLZTO pellets was thermal etched to visualize the grain structure in a tube furnace with Ar atmosphere at 1150°C for 5 minutes followed by furnace cooling, and then observed in a Hitachi SU8230 scanning electron microscope (SEM), which is equipped with Bruker QUANTAX FlatQUAD energy-dispersive X-ray spectroscopy (EDS) detector for elemental distribution study.\u003c/p\u003e\u003cp\u003eFor more microstructural details at the atomic level, transmission electron microscopy (TEM) was used. To overcome the limitation of e-beam irradiation damage caused by conventional TEM sample preparation and observation, the TEM specimens were lifted out and thinned down to less than 100 nm in a cryogenic focused ion beam (cryo-FIB) system (Thermo Scientific, Helios G5) at -150°C. Afterwards, an aberration-corrected FEI Krios G3i TEM was employed for low-dose high-resolution TEM (HRTEM) characterization, which was operating at 300 kV and equipped with a Falcon 3 direct electron detector. An automated liquid nitrogen filling system maintained the sample chamber and lens barrel at approximately − 192°C.\u003c/p\u003e\u003cp\u003eThe distribution of pores and cracks in the LLZTO pellets was visualized by a Zeiss Xradia 610 Versa X-ray computed tomography (XCT) system. The X-ray energy was 140 keV, achieving a voxel resolution of about 1 × 1 × 1 µm\u003csup\u003e3\u003c/sup\u003e. Over 1000 slices were collected per sample and reconstructed using Avizo software (FEI).\u003c/p\u003e\u003cp\u003eThe Li content of LLZTO was measured via inductively coupled plasma atomic emission spectroscopy (ICP-AES, Agilent 5110). Samples were digested in diluted aqua regia and hydrofluoric acid. UV-Vis spectra were collected using a PerkinElmer Lambda 950 spectrophotometer over the wavelength range of 200–800 nm. Electron paramagnetic resonance (EPR) spectra were measured on a Bruker A300 EPR spectrometer at room temperature. Synchrotron X-ray absorption spectroscopy (XAS) experiments were conducted on TPS 21A beamline at Taiwan Photon Source (TPS). Atom force microscope (AFM) was carried out using a Bruker Dimension ICON; samples were sequentially ground with 220, 800, 1500, and 2000 grit sandpaper and polished with 1 µm diamond suspension.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements:\u003c/h2\u003e \u003cp\u003eThis work was supported by the National Natural Science Foundation of China (Grant Nos. 22409160, W2411048), the National Key Research and Development Program of China (2023YFB12002). The authors thank the Instrumental Analysis Center of Xi\u0026rsquo;an Jiaotong University for the assistance with CT characterization. This work was supported by the Assistant Secretary for Energy, Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eDing W, Tao Q, Liu C, Chen G, Yoo S, Cai W, Cao P, Jia B, Wu H, Zhang D, Zhu H, Zhang L, Qu X, Zou J, Qin M (2025) Lean design of a strong and ductile dual-phase titanium-oxygen alloy. Nat Mater 24(4):506\u0026ndash;512\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWu G, Chan KC, Zhu L, Sun L, Lu J (2017) Dual-phase nanostructuring as a route to high-strength magnesium alloys. Nature 545(7652):80\u0026ndash;83\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHampshire S, Pomeroy MJ (2012) Grain boundary glasses in silicon nitride: A review of chemistry, properties and crystallisation. 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J Am Ceram Soc 86(12):2228\u0026ndash;2230\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Non-equilibrium electron-beam sintering, amorphous–crystalline dual-phase grain structure, transparent solid-state electrolytes, lithium dendrite suppression, grain-boundary amorphous nano-shell","lastPublishedDoi":"10.21203/rs.3.rs-8091488/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8091488/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAmorphous\u0026ndash;crystalline dual-phase grain structure often enables materials with unique performance. This is especially the case for the garnet-type solid-state electrolyte Li\u003csub\u003e7\u003c/sub\u003eLa\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e (LLZO), which is a promising candidate for hybrid and all-solid-state lithium batteries. A major synthesis challenge, however, is that amorphous LLZO is difficult to retain via conventional sintering and even Flash Joule heating. Using electron beam focused to sub-millimeter scale with point-by-point scanning, we achieved highly localized heating to very high temperature followed by ultrafast quenching, enabling the direct fabrication of uniformly fine-grained amorphous\u0026ndash;crystalline dual-phase LLZO. Fast ion transport offered by the crystalline grains, together with the amorphous shell that promotes a more homogeneous space-charge distribution at grain boundaries, markedly enhance the critical current density, cycling stability, and optical transparency. Our scalable fabrication route offers unparalleled tunability in electrochemical performance, and also has broad applicability for other functional ceramic systems requiring optimized electrical and optical properties.\u003c/p\u003e","manuscriptTitle":"Electron-beam sintering achieves dual-phase transparent solid-state electrolytes for batteries","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-22 10:21:55","doi":"10.21203/rs.3.rs-8091488/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":"181c41aa-b358-4d40-ab50-8eb0f587e282","owner":[],"postedDate":"December 22nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":59856614,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Batteries"},{"id":59856615,"name":"Physical sciences/Energy science and technology/Energy storage/Batteries"}],"tags":[],"updatedAt":"2026-02-06T04:05:34+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-22 10:21:55","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8091488","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8091488","identity":"rs-8091488","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}