A sacrificial additive agent mediated strategy for high performance sodium ion capacitor

A sacrificial additive agent mediated strategy for high performance sodium ion capacitor | 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 Research Article A sacrificial additive agent mediated strategy for high performance sodium ion capacitor Zhaowen Huang, Xiaolin Li, Yang Hu, Jingbo Liu, Manlan Guo, Fenglin Wang, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7278220/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Sodium-ion capacitors (SICs) endow extremely potential in electrochemical energy storage system owning to the high energy-power characteristics elemental abundance and environmental friendliness. However, the irreversible sodium loss caused by the consumed Na + for the side reactions and formation of solid-electrolyte interphase (SEI) inevitable lead to the degradation of Na + storage capability for SICs. Herein, an effective pre-sodiation strategy is purposed by using the sodium oxalate (Na₂C₂O₄) as sacrificial additive agent to compensate the reduction of Na + for SICs during the charge-discharge process. The Na₂C₂O₄ possesses highly electrochemical stability at the high potential of 3.6 V and exhibits superior Na + diffusion coefficient than that of active carbon, evidenced by the galvanostatic intermittent titration technique test. Moreover, the as-prepared Na₂C₂O₄ shows intimate interface contact with active carbon derived from the formation of C-O bond between the Na₂C₂O₄ and active carbon, enhancing the structural stability of electrode and insertion-extraction reversibility of Na + upon cycling. Thus, the NICs assembled by pre-sodiation activate carbon cathode and hard carbon anode displays impressive capacitance retention of 95.7% after 13600 cycles and excellent rate ability (high energy density of 59 Wh kg -1 at the power density of 1500 W kg -1 ). The underlying mechanisms for the excellent electrochemical performance of the assembled SICs are systematic investigated by ex-situ scanning electron microscope and galvanostatic intermittent titration technique. This work paves a way for the development of the pre-sodiation techniques and provide valid guidance for the future research directions of advanced energy storage and transfer equipment. Sodium-ion capacitors Pre-sodiation Electrochemical Stability C-O bond Sacrificial Additive Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Nowadays, sodium ions capacitors (SICs) technologies have aroused lots of attentions owning to their advantages of low cost, abundant resources for production, and suitability for large-scale energy storage than that of traditional electrochemical energy storage device [ 1 , 2 ]. Moreover, the SICs combine the advantages of sodium ion battery (high energy density) and supercapacitors (high power density), which shows promising potential in portable electronics, electric vehicle and large-scale energy storage [ 3 , 4 ]. Nevertheless, the SICs suffer from kinetic mismatch between battery-type anodes and capacitor-type cathodes, which leads to the rapid degradation of electrochemical performance and can-not meet the ever-increasing demands of multifarious applications of high energy density [ 5 ]. Thus, tremendous efforts have been employed to explore suitable electrode material to further improve the energy storage capability of SICs especially for the battery-type anodes materials, which suffers from huge volume effect and poor reaction kinetics upon (de)sodiation due to the larger radius of Na + (1.02 Å) than that of Li + (0.76 Å) [ 6 ]. Worse still, the SICs are subjected to the reduction of specific capacity results from the Na + loss or dissolution on the battery-type anodes side, which inevitable cause the degradation of Na + storage ability [ 7 ]. Thus, it is of vital importance to explore a valid method to compensate the lessen of Na + during the charge/discharge process of SICs for the improvement of electrochemical performance. Pre-sodiation strategies are validly way to accommodate the decrease of irreversible capacity, increase operating voltage and keep the electrolyte concentration, consequently improve the Na + storage capability of SICs [ 8 , 9 ]. The pre-sodiation represents that pre-doping of Na + into active materials to offer enough sodium resource and avoid the reduction of Na + storage Capaciability [ 10 ]. Generally, there are three methods to pre-doping the sodium into electrode: 1) the operation with Na metal by means of electrochemical method or directly contact with Na metal; 2) the usage of Na-based alternatives more secure than that of sodium metal; 3) the introduction of Na-rich additives into the anode [ 11 , 12 ]. Among them, the usage of Na-based alternatives is the most widely and effective pre-sodiation method due to its mild and uniform reaction in liquid and scale-up industrial applications [ 13 ]. Moreover, these alternatives (such as Na-naphthaline, sodium biphenyl, etc) always endow intimate interface contact with active carbon due to the emergence of C-C bond at local region of the whole electrode, rising the initial coulombic efficiency and energy density for the assembled SICs [ 14 ]. Although the pre-sodiation course could be effectively achieved by alternatives of Na metal, the products after de-sodiated process will turn to be electrochemical inertia and dead in the active carbon part, which greatly reduced the specific energy densities of SICs upon cycling [ 15 ]. Currently, the sacrificial organic salts can irreversibly provide sodium cations to the anode during an initial operando charging step without any negative effects in SICs system [ 16 ]. However, the sacrificial organic salts with intimate interface contact among active carbon, electrolyte and current collector, are still lacking. In this work, we propose a facile aqueous solvent reaction strategy to synthesize the sacrificial organic salts of sodium oxalate (Na 2 C 2 O 4 ) to employ as pre-sodiation reagents in SICs. The Na 2 C 2 O 4 endows intimate interface contact with active carbon and extremely stable electrochemical performance upon (de)sodiation of electrode. The structure and composition of the as-prepared Na 2 C 2 O 4 are systematically characterized and analyzed. The electrochemical performance of the NICs coupled with Na 2 C 2 O 4 -active carbon//hard carbon pouch cell are detailed investigated. Results and discussion 2.1 Characterization of squarate Na salt The crystal structure of sodium oxalate powders was prepared by solvent method and characterized by X-ray diffraction is shown in Fig. 1 a. The diffraction peaks centered at 2θ = 15.6°, 17.0°, 21.6°, 25.4°, 26.1°, 29.8°, 30.4°, 31.3°, 33.1°, 33.8°, 37.1°, 38.4°, 40.5°, 41.1°, 43.4°, 43.8°, 44.9°, 45.9°, 47.7°, 52.4°, 52.9°, 53.7° and 54.6° are associated with the reflections of (011), (002), (012), (100), (020), (11 − 1), (10 − 2), (102), (11 − 2), (112), (014), (11 − 3), (12 − 2), (122), (032), (024), (11 − 4), (015), (130), (11 − 5), (006), (115) and (21 − 1) of sodium oxalate (ICSD No. 154355) [ 17 ]. Raman spectroscopy was further used to confirm the component of synthesized sample. The main strong peaks located at 1803 cm − 1 , 1644 cm − 1 , 1606 cm − 1 , 1127 cm − 1 , 729 cm − 1 , 651 cm − 1 and 307 cm − 1 in Fig. 1 b are indexed to v (C = O), v (C = C), v (C = C)/(C = O), v (C-C), ring breathing, δ ring and δ (C = O) vibrational modes of sodium oxalate, respectively (Fig S1 and table S1 , supporting information) [ 18 ]. Moreover, the positions of the peaks in the Raman spectra for sodium oxalate do not include the Raman peak positions of acid 3,4-dihydroxy-3-cyclobutene-1,2-dione and sodium carbonate powders, indicating that the mild chemical synthetic method could be benefit of the removing impurities from the synthesis process. The irreversibility of sodium extraction from the sodium oxalate-activated carbon composite electrode in 1 mol L − 1 NaClO 4 in EC/PC (vol. 1:1) electrolyte was investigated by cyclic voltammogram and chronopotentiometry vs. metallic sodium counter/reference electrode in the potential range from ca. 2.5 V (open-circuit voltage) to 4.1 V vs. Na/Na + (Fig. 2 a and 2 b). The N 2 adsorption-desorption isotherms and pore size distribution of the activated carbon is characterized (Fig S2, supporting information). During the first cyclic voltammogram anodic scan (Fig. 2 a), the peak at 3.8 V vs. Na/Na + was observed, which is related to sodium extraction cause. However, there was no peak in the first cathodic scan, indicating that sodium ion is almost extracted wholly from sodium oxalate after the first oxidation scan. The oxidation capacity of sodium oxalate calculated from the integral inter-area from 3.6 V to 4.1 V vs. Na/Na + is 326 mAh g − 1 . Subsequently, the 2nd cyclic voltammogram shows a wavelet ranging from 3.4 V to 4.1 V vs. Na/Na + , due to the small amount of residual sodium extraction from the remaining sodium oxalate after the first anodization. The wave becomes weak during the fourth cathodic scan, which is linked to the capacitive behavior of the existing activated carbon in the electrode evidenced by the rectangular cyclic voltammogram. The first chronopotentiometry curve of the sodium oxalate-activated carbon composite electrode in Fig. 2 b shows long platform of 3.6 V vs. Na/Na + , corresponding to the oxidation peak observed in the first cyclic voltammogram (Fig. 2 a). Before 3.6 V vs. Na/Na + , there is a linear slope related to capacitive behavior of activated carbon in the electrode. Thereafter, the sodium oxalate-activated carbon composite electrode displays an extraction platform of 344 mAh g − 1 at 3.6 V vs. Na/Na + and a sloppy region up from 3.9 V vs. Na/Na + to the limit potential of 4.1 V vs. Na/Na + . For the second chronopotentiometry curve, the negligible extraction capacity at 3.6 V vs. Na/Na + is correlated to the complete extraction of sodium from the first oxidation process. 2.2 Mechanism of sodium ion extraction from di-sodium squarate and oxidation of di-sodium squarate The kinetic properties of sodium ion removal were investigated by the galvanostatic intermittent titration technique (GITT). The cell with the sodium oxalate-activated carbon counter electrode and sodium counter/reference electrode was charged/discharged every half hour and rested for another half an hour (Fig. 3 a). Before 3.6 V vs. Na/Na + during the charging-discharging process, the potential changes rapidly. When oxidizing di-sodium squarate, the platform appears jagged. When the electrode material shows capacitive characteristics (before 3.6 V vs. Na/Na + charging and during discharge), the sodium ion diffusion coefficient is in a gentle range (Fig. 3 b), ranging from 1.44 × 10 − 7 cm 2 s − 1 to 9.71 × 10 − 8 cm 2 s − 1 from 2.72 × 10 − 8 cm 2 s − 1 to 1.04 × 10 − 7 cm 2 s − 1 . When di-sodium squarate is oxidized, the sodium ion diffusion coefficient presents a U-shaped range from 4.56 × 10 − 11 cm 2 s − 1 to 1.83 × 10 − 9 cm 2 s − 1 . Therefore, it can be concluded that the diffusion coefficient of sodium ion is lower in battery performance than that in capacitance performance. Since the electro-oxidation process of di-sodium squarate occurs in a single charging process, we apply the in-situ electrochemical mass spectrometry technology to measure the gas pressure generation and the composition of gas species during this process. From the OCP to the 3.6 V vs. Na/Na + , there is almost no pressure change with linearly increasing potential in Fig. 3 c. When the di-sodium squarate electro-oxidation plateaus, the gas pressure gradually increases linearly to the maximum value until the potential of 3.8 V vs. Na/Na + . At a capacity of 136 mAh g − 1 , the concentration of CO increases slightly while the solubility of CO 2 decreases to a minimum due to the reaction of sodium with CO 2 . Then, the concentrations of CO and CO 2 expressly increase. Finally, when the potential is between 3.8 V and 4.1 V vs. Na/Na + , the pressure of the gas is almost constant. Ex-situ Raman was used to examine the effect of the oxidized products of sodium oxalate on the electrodes. In Fig. 3 d, the original sodium oxalate-activated carbon electrode includes peaks of δ ring at 651 cm − 1 , ring breathing at 717 cm − 1 and v (C-C) at 1118 cm − 1 for di-sodium squarate, and D peak (1333 cm − 1 ) and G peak (1598 cm − 1 ) for carbon material. However, all Raman peaks of sodium oxalate disappear in the oxidized electrode, leaving only D and G bands of carbons. Ex-situ scanning electron microscope and energy-dispersive X-ray spectroscopy for analyzing electrode elements were conducted to test morphology and element distribution of initial and oxidized di-sodium quarate-AC electrodes. The sodium oxalate and activated carbon are uniformly distributed in the initial electrode (Fig. 4 a). C, O, F, Na elements are present in the electrode (Fig. 4 b), and a large number of pores appeared after the electrode oxidation by sodium oxalate (Fig. 4 c). As evidenced in Fig. 4 d, Na element is entirely removed after the sodium oxalate-activated carbon electrode oxidation. 2.3 Electrochemical properties of sodium ion capacitors The assembly process of the sodium-ion capacitor is shown in the experimental section (Supporting Information). The charge/discharge curve of HC (hard carbon) and different electrode capacity ratio are tested, which displayed ultra-high initial coulomb efficiency of 83.3% for HC (Fig S3 and Table S2). Di-sodium squarate was oxidized at a current of C/20 until the device was charged to 3.8 V (Fig. 5 a). Meanwhile, the di-sodium squarate-AC cathode and HC anode reached 3.824 V vs. Na/Na + and 0.024 V vs. Na/Na + , respectively. It can be anticipated that the di-sodium squarate component no longer contributes to the energy storage mechanism during the cycling of the sodium-ion capacitor. The sodium extraction capacity of di-sodium squarate withdrawing from the linear part before 3.5 V vs. Na/Na + is about 333 mAh g − 1 from the charging platform, which is close to the theoretical capacity of di-sodium squarate (339 mAh g − 1 ), which proves that almost all the sodium is extracted from di-sodium squarate. During charge and discharge process in the voltage range of 2.2 V to 3.8 V, the voltage curve of the sodium-ion capacitor is a typical linear curve, the positive electrode potential is a line parallel to the device voltage, and the HC negative electrode potential is a horizontal line (Fig. 5 b). Importantly, it can be seen from the graph that the minimum potential of the HC electrode is higher than 0.01 V vs. Na/Na + , which completely excludes any risk of sodium plating. After 13,600 cycles, the sodium-ion capacitor shows excellent capacitance of 37 F g − 1 (corresponding to a capacitance retention of 95.7%) and the energy efficiency remains at the level of 89% (Fig. 5 c). The specific power of di-sodium squarate-AC//HC sodium-ion capacitors is 50 W kg − 1 , 100 W kg − 1 , 200 W kg − 1 , 300 W kg − 1 , 500 W kg − 1 , 700 W kg − 1 , 1000 W kg − 1 , 1500 W kg − 1 . The corresponding specific energies are 66.2 Wh kg − 1 , 65.8 Wh kg − 1 , 65.7 Wh kg − 1 , 65.1 Wh kg − 1 , 63.9 Wh kg − 1 , 62.8 Wh kg − 1 , 61.2 Wh kg − 1 , 59.0 Wh kg − 1 , respectively. The obtained specific energy values are about 4 times higher than those of conventional supercapacitors (Fig. 5 d and Table 1 ). To explain this, we need to find the answer according to the formula of specific energy output \(\:{E}_{m}=\frac{1}{2}{C}_{m}\left({U}_{max}^{2}-{U}_{min}^{2}\right)\) . According to published literature, the specific capacitance of hybrid capacitors is twice than that of EDLC. Considering the output voltage of NIC and EDLC, it can be concluded that the output energy density of NIC is four times that of EDLC. Moreover, the electrochemical performance of NIC is superior than that of reported literature [ 19 – 25 ], resulting from the impressive insertion-extraction reversibility and excellent transmission rate of Na + in electrode (Supplementary Materials, Table S3). The Electrochemical impedance spectroscopy (EIS) of the sodium ion capacitor is tested at the stage of before cycling and after 500 cycles, which indicates that the sodium ion capacitor displays neglectable changes of charge transfer impedance after 500 loops, illustrating the well interface stability between electrode and electrolyte (Fig S4) and which is also confirmed by the changes of equivalent internal resistance (ESR) and Charge transfer resistance (EDR) (Fig S5). Table 1 Comparable specific energies of the NIC and EDLC at different specific powers. P (W kg − 1 ) 50 100 200 300 500 700 1000 1500 E (Wh kg − 1 ) for NIC 66.2 65.8 65.7 65.1 63.9 62.8 61.2 59.0 E (Wh kg − 1 ) for EDLC 19.2 19.3 19.0 18.6 17.8 17.0 15.9 14.5 E ratio 3.45 3.41 3.46 3.50 3.59 3.69 3.85 4.07 Conclusion In total, the functional sodium oxalate (Na₂C₂O₄) was prepared by a mild chemical method, which acts as a sacrificial additive agent to improve the electrochemical performance of sodium ions capacitor (SICs). The introduction of Na₂C₂O₄ in active carbon can effectively compensate the reduction of Na + during charge-discharge process and improve the cycle life of SICs. Moreover, the functional Na₂C₂O₄ and active carbon show intimate interfacial contact resulting from the emergence of the C-O bond, which further enhance the reversibility of insertion-extraction process for Na + upon cycling cause and improve the stability of the electrode. Thus, the assembled NICs displays excellent capacity retention ratio of 95.7% after 13600 loops and excellent rate ability (high energy density of 59 Wh kg − 1 at the power density of 1500 W kg − 1 ). The electrochemical performance enhancement mechanism is deeply investigated and evidenced by ex-situ scanning electron microscope and galvanostatic intermittent titration technique. Overall, the proposed strategy offers new insights into pre-sodiation techniques and give the meaningful guidance for the development of sodium ions capacitors. Declarations Author contributions Zhaowen Huang: Conceptualization, Methodology development, Main investigation, Writing-Original Draft preparation; Xiaolin Li : Experimental work, Data validation; Yang Hu : Data acquisition, Formal analysis; Jingbo Liu : Resources provision, Writing-Review&Editing; Manlan Guo : Electrochemistry tests; Fenglin Wang : Diagram drawing; Xianjie Yang : Material synthesis; Longwen Chen : Materials characterization; Youpeng Li : Supervision, Project administration, Funding acquisition, Writing - Review & Editing (final approval). Declaration of Competing Interest There are no conflicts to declare. Supplementary information description The supplementary information consists of the “Experimental Section”, Figures and Table about the electrochemical performance comparison of NIC with other reported anode materials in NICs. Acknowledgments This work was supported by the Basic and Applied Basic Research Foundation of Guangdong Province (2020A1515110273, 2020A1515110146, 2022A1515140005, 2022A1515110146), National Natural Science Foundation of China (62101534); Guangdong Provincial Department of Education Key Area R&D Program (2022ZDZC1078); Innovative Research Group Project of Higher Education Institutions in Guangdong Province (2022KCXTD066); GuangDong Engineering Technology Research Center (2022E075); References P. Cai, K. Zou, X. Deng, B. Wang, M. Zheng, L. Li, H. Hou, G. Zou, X. Ji, (2021) Comprehensive Understanding of Sodium-Ion Capacitors: Definition, Mechanisms, Configurations, Materials, Key Technologies, and Future Developments, Adv. Energy Mater. 11(16) 2003804. T. Wang, Y. Li, Z. Chen, Q. Liu, J. Lang, L. Wu, W. Dong, Z. Ju, H. Li, X. Zhang, G. Yu. 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University","correspondingAuthor":false,"prefix":"","firstName":"Xiaolin","middleName":"","lastName":"Li","suffix":""},{"id":498319495,"identity":"6ab9c72a-171f-4289-897b-5fbabc925fdd","order_by":2,"name":"Yang Hu","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yang","middleName":"","lastName":"Hu","suffix":""},{"id":498319496,"identity":"e671b2c1-063e-4c9f-8b90-e8a6b060f7de","order_by":3,"name":"Jingbo Liu","email":"","orcid":"","institution":"Shunde Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Jingbo","middleName":"","lastName":"Liu","suffix":""},{"id":498319497,"identity":"c004bb13-a124-4b48-9404-9e4f9230e6f1","order_by":4,"name":"Manlan Guo","email":"","orcid":"","institution":"Shunde Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Manlan","middleName":"","lastName":"Guo","suffix":""},{"id":498319498,"identity":"32c6a959-680d-40ca-84e7-057a9f086ec9","order_by":5,"name":"Fenglin Wang","email":"","orcid":"","institution":"Shunde Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Fenglin","middleName":"","lastName":"Wang","suffix":""},{"id":498319499,"identity":"49413589-22c7-4e68-b434-c762db912976","order_by":6,"name":"Xianjie Yang","email":"","orcid":"","institution":"Shunde Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Xianjie","middleName":"","lastName":"Yang","suffix":""},{"id":498319500,"identity":"0b265b2c-6b7b-4093-bf66-b77aaf036f74","order_by":7,"name":"Longwen Chen","email":"","orcid":"","institution":"Shunde Polytechnic University","correspondingAuthor":false,"prefix":"","firstName":"Longwen","middleName":"","lastName":"Chen","suffix":""},{"id":498319501,"identity":"a5ebb448-87df-4d6b-aae2-0272e9d631ef","order_by":8,"name":"Youpeng Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAApUlEQVRIiWNgGAWjYLCCBwY2PPz8DaRoSTBIk5GccYAkLQyHbQwaEohUbXC8x+xBQsF5HgOGA4wfPuYQo+XMGXODBIPbPObMDcySM7cRo+VGjpkESItlwwE2Zl4StJzjMTiQQJqWAyRokTxzrBzol2QeyRkHm4nzC9/x5m0PPvyxs+fnbz744SMxWhQOMLBBmYwNRKgHAvkGuJZRMApGwSgYBTgAAMk0NkCUPRGJAAAAAElFTkSuQmCC","orcid":"","institution":"Shunde Polytechnic University","correspondingAuthor":true,"prefix":"","firstName":"Youpeng","middleName":"","lastName":"Li","suffix":""}],"badges":[],"createdAt":"2025-08-02 12:23:32","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7278220/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7278220/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":88788200,"identity":"37477820-0bf2-4251-8e23-90e48af8cb67","added_by":"auto","created_at":"2025-08-11 12:11:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":237918,"visible":true,"origin":"","legend":"\u003cp\u003e(a) X-ray diffractograms and (b) Raman spectrum of di-lithium squarate powders.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-7278220/v1/28722a6b43eed30729bf742a.png"},{"id":88788202,"identity":"b2a85ba2-672b-4308-8683-5336b02e8cbc","added_by":"auto","created_at":"2025-08-11 12:11:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":234576,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Cyclic voltammograms, derived capacity-potential curves and (b) chronopotentiometry curves of di-sodium squarate-AC electrode.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-7278220/v1/41d020b5412dd037dcbeafbb.png"},{"id":88788201,"identity":"bdf5e8a5-6893-434e-bac7-2428c6f449a2","added_by":"auto","created_at":"2025-08-11 12:11:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":236993,"visible":true,"origin":"","legend":"\u003cp\u003e(a) GITT curves, (b) Na\u003csup\u003e+\u003c/sup\u003e diffusion coefficients (D\u003csub\u003eNa+\u003c/sub\u003e), (c) electrochemical mass spectrometry and (d) \u003cem\u003eex-situ\u003c/em\u003e Raman of di-sodium squarate-AC electrode.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-7278220/v1/f1026132f6fa596cae52b8c9.png"},{"id":88788203,"identity":"e1836f40-d5f2-4703-b332-fc6de1926000","added_by":"auto","created_at":"2025-08-11 12:11:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":449799,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Scanning electron micrograph and (b) energy-dispersive X-ray spectrum of the fresh di-sodium squarate-AC electrode. (c) Scanning electron micrograph and (d) energy-dispersive X-ray spectrum of oxidized di-sodium squarate-AC electrode.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-7278220/v1/77807708b1509c90ae332c06.png"},{"id":88788209,"identity":"31442afa-daf8-4b30-b6cf-830a08d73f87","added_by":"auto","created_at":"2025-08-11 12:11:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":373716,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Prematallization of HC negative electrode from di-sodium squarate. (b) Chronopotentiometry curve, (c) cycle lifespan and (d) E-P plots of the Na-ion capacitors.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-7278220/v1/6ab23c16b6387c4f23f5b82d.png"},{"id":90255202,"identity":"dede6c8f-f35c-44cd-97cc-84aa33c25911","added_by":"auto","created_at":"2025-08-31 10:46:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2192603,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7278220/v1/15425963-1e4a-43a2-8179-5c4a7939f2ec.pdf"},{"id":88788205,"identity":"b9f83203-5949-4bb5-b537-c3e41ec15842","added_by":"auto","created_at":"2025-08-11 12:11:23","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":533938,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-7278220/v1/6555a9a27c018a9317a1334f.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"A sacrificial additive agent mediated strategy for high performance sodium ion capacitor","fulltext":[{"header":"Introduction","content":"\u003cp\u003eNowadays, sodium ions capacitors (SICs) technologies have aroused lots of attentions owning to their advantages of low cost, abundant resources for production, and suitability for large-scale energy storage than that of traditional electrochemical energy storage device [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Moreover, the SICs combine the advantages of sodium ion battery (high energy density) and supercapacitors (high power density), which shows promising potential in portable electronics, electric vehicle and large-scale energy storage [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Nevertheless, the SICs suffer from kinetic mismatch between battery-type anodes and capacitor-type cathodes, which leads to the rapid degradation of electrochemical performance and can-not meet the ever-increasing demands of multifarious applications of high energy density [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Thus, tremendous efforts have been employed to explore suitable electrode material to further improve the energy storage capability of SICs especially for the battery-type anodes materials, which suffers from huge volume effect and poor reaction kinetics upon (de)sodiation due to the larger radius of Na\u003csup\u003e+\u003c/sup\u003e (1.02 \u0026Aring;) than that of Li\u003csup\u003e+\u003c/sup\u003e (0.76 \u0026Aring;) [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Worse still, the SICs are subjected to the reduction of specific capacity results from the Na\u003csup\u003e+\u003c/sup\u003e loss or dissolution on the battery-type anodes side, which inevitable cause the degradation of Na\u003csup\u003e+\u003c/sup\u003e storage ability [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Thus, it is of vital importance to explore a valid method to compensate the lessen of Na\u003csup\u003e+\u003c/sup\u003e during the charge/discharge process of SICs for the improvement of electrochemical performance.\u003c/p\u003e\u003cp\u003ePre-sodiation strategies are validly way to accommodate the decrease of irreversible capacity, increase operating voltage and keep the electrolyte concentration, consequently improve the Na\u003csup\u003e+\u003c/sup\u003e storage capability of SICs [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The pre-sodiation represents that pre-doping of Na\u003csup\u003e+\u003c/sup\u003e into active materials to offer enough sodium resource and avoid the reduction of Na\u003csup\u003e+\u003c/sup\u003e storage Capaciability [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Generally, there are three methods to pre-doping the sodium into electrode: 1) the operation with Na metal by means of electrochemical method or directly contact with Na metal; 2) the usage of Na-based alternatives more secure than that of sodium metal; 3) the introduction of Na-rich additives into the anode [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Among them, the usage of Na-based alternatives is the most widely and effective pre-sodiation method due to its mild and uniform reaction in liquid and scale-up industrial applications [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Moreover, these alternatives (such as Na-naphthaline, sodium biphenyl, etc) always endow intimate interface contact with active carbon due to the emergence of C-C bond at local region of the whole electrode, rising the initial coulombic efficiency and energy density for the assembled SICs [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAlthough the pre-sodiation course could be effectively achieved by alternatives of Na metal, the products after de-sodiated process will turn to be electrochemical inertia and dead in the active carbon part, which greatly reduced the specific energy densities of SICs upon cycling [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Currently, the sacrificial organic salts can irreversibly provide sodium cations to the anode during an initial operando charging step without any negative effects in SICs system [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. However, the sacrificial organic salts with intimate interface contact among active carbon, electrolyte and current collector, are still lacking.\u003c/p\u003e\u003cp\u003eIn this work, we propose a facile aqueous solvent reaction strategy to synthesize the sacrificial organic salts of sodium oxalate (Na\u003csub\u003e2\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) to employ as pre-sodiation reagents in SICs. The Na\u003csub\u003e2\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e endows intimate interface contact with active carbon and extremely stable electrochemical performance upon (de)sodiation of electrode. The structure and composition of the as-prepared Na\u003csub\u003e2\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e are systematically characterized and analyzed. The electrochemical performance of the NICs coupled with Na\u003csub\u003e2\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-active carbon//hard carbon pouch cell are detailed investigated.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Characterization of squarate Na salt\u003c/h2\u003e\u003cp\u003eThe crystal structure of sodium oxalate powders was prepared by solvent method and characterized by X-ray diffraction is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. The diffraction peaks centered at 2θ\u0026thinsp;=\u0026thinsp;15.6\u0026deg;, 17.0\u0026deg;, 21.6\u0026deg;, 25.4\u0026deg;, 26.1\u0026deg;, 29.8\u0026deg;, 30.4\u0026deg;, 31.3\u0026deg;, 33.1\u0026deg;, 33.8\u0026deg;, 37.1\u0026deg;, 38.4\u0026deg;, 40.5\u0026deg;, 41.1\u0026deg;, 43.4\u0026deg;, 43.8\u0026deg;, 44.9\u0026deg;, 45.9\u0026deg;, 47.7\u0026deg;, 52.4\u0026deg;, 52.9\u0026deg;, 53.7\u0026deg; and 54.6\u0026deg; are associated with the reflections of (011), (002), (012), (100), (020), (11\u0026thinsp;\u0026minus;\u0026thinsp;1), (10\u0026thinsp;\u0026minus;\u0026thinsp;2), (102), (11\u0026thinsp;\u0026minus;\u0026thinsp;2), (112), (014), (11\u0026thinsp;\u0026minus;\u0026thinsp;3), (12\u0026thinsp;\u0026minus;\u0026thinsp;2), (122), (032), (024), (11\u0026thinsp;\u0026minus;\u0026thinsp;4), (015), (130), (11\u0026thinsp;\u0026minus;\u0026thinsp;5), (006), (115) and (21\u0026thinsp;\u0026minus;\u0026thinsp;1) of sodium oxalate (ICSD No. 154355) [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eRaman spectroscopy was further used to confirm the component of synthesized sample. The main strong peaks located at 1803 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1644 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1606 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1127 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 729 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 651 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 307 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb are indexed to \u003cem\u003ev\u003c/em\u003e (C\u0026thinsp;=\u0026thinsp;O), \u003cem\u003ev\u003c/em\u003e (C\u0026thinsp;=\u0026thinsp;C), \u003cem\u003ev\u003c/em\u003e (C\u0026thinsp;=\u0026thinsp;C)/(C\u0026thinsp;=\u0026thinsp;O), \u003cem\u003ev\u003c/em\u003e (C-C), ring breathing, δ ring and δ (C\u0026thinsp;=\u0026thinsp;O) vibrational modes of sodium oxalate, respectively (Fig \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, supporting information) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Moreover, the positions of the peaks in the Raman spectra for sodium oxalate do not include the Raman peak positions of acid 3,4-dihydroxy-3-cyclobutene-1,2-dione and sodium carbonate powders, indicating that the mild chemical synthetic method could be benefit of the removing impurities from the synthesis process.\u003c/p\u003e\u003cp\u003eThe irreversibility of sodium extraction from the sodium oxalate-activated carbon composite electrode in 1 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e NaClO\u003csub\u003e4\u003c/sub\u003e in EC/PC (vol. 1:1) electrolyte was investigated by cyclic voltammogram and chronopotentiometry vs. metallic sodium counter/reference electrode in the potential range from ca. 2.5 V (open-circuit voltage) to 4.1 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). The N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherms and pore size distribution of the activated carbon is characterized (Fig S2, supporting information). During the first cyclic voltammogram anodic scan (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea), the peak at 3.8 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e was observed, which is related to sodium extraction cause. However, there was no peak in the first cathodic scan, indicating that sodium ion is almost extracted wholly from sodium oxalate after the first oxidation scan. The oxidation capacity of sodium oxalate calculated from the integral inter-area from 3.6 V to 4.1 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e is 326 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Subsequently, the 2nd cyclic voltammogram shows a wavelet ranging from 3.4 V to 4.1 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e, due to the small amount of residual sodium extraction from the remaining sodium oxalate after the first anodization. The wave becomes weak during the fourth cathodic scan, which is linked to the capacitive behavior of the existing activated carbon in the electrode evidenced by the rectangular cyclic voltammogram.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe first chronopotentiometry curve of the sodium oxalate-activated carbon composite electrode in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb shows long platform of 3.6 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e, corresponding to the oxidation peak observed in the first cyclic voltammogram (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). Before 3.6 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e, there is a linear slope related to capacitive behavior of activated carbon in the electrode. Thereafter, the sodium oxalate-activated carbon composite electrode displays an extraction platform of 344 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 3.6 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e and a sloppy region up from 3.9 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e to the limit potential of 4.1 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e. For the second chronopotentiometry curve, the negligible extraction capacity at 3.6 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e is correlated to the complete extraction of sodium from the first oxidation process.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Mechanism of sodium ion extraction from di-sodium squarate and oxidation of di-sodium squarate\u003c/h2\u003e\u003cp\u003eThe kinetic properties of sodium ion removal were investigated by the galvanostatic intermittent titration technique (GITT). The cell with the sodium oxalate-activated carbon counter electrode and sodium counter/reference electrode was charged/discharged every half hour and rested for another half an hour (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Before 3.6 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e during the charging-discharging process, the potential changes rapidly. When oxidizing di-sodium squarate, the platform appears jagged. When the electrode material shows capacitive characteristics (before 3.6 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e charging and during discharge), the sodium ion diffusion coefficient is in a gentle range (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), ranging from 1.44 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 9.71 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e from 2.72 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 1.04 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. When di-sodium squarate is oxidized, the sodium ion diffusion coefficient presents a U-shaped range from 4.56 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;11\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 1.83 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Therefore, it can be concluded that the diffusion coefficient of sodium ion is lower in battery performance than that in capacitance performance.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSince the electro-oxidation process of di-sodium squarate occurs in a single charging process, we apply the \u003cem\u003ein-situ\u003c/em\u003e electrochemical mass spectrometry technology to measure the gas pressure generation and the composition of gas species during this process. From the OCP to the 3.6 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e, there is almost no pressure change with linearly increasing potential in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec. When the di-sodium squarate electro-oxidation plateaus, the gas pressure gradually increases linearly to the maximum value until the potential of 3.8 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e. At a capacity of 136 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the concentration of CO increases slightly while the solubility of CO\u003csub\u003e2\u003c/sub\u003e decreases to a minimum due to the reaction of sodium with CO\u003csub\u003e2\u003c/sub\u003e. Then, the concentrations of CO and CO\u003csub\u003e2\u003c/sub\u003e expressly increase. Finally, when the potential is between 3.8 V and 4.1 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e, the pressure of the gas is almost constant.\u003c/p\u003e\u003cp\u003e\u003cem\u003eEx-situ\u003c/em\u003e Raman was used to examine the effect of the oxidized products of sodium oxalate on the electrodes. In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, the original sodium oxalate-activated carbon electrode includes peaks of δ ring at 651 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, ring breathing at 717 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and \u003cem\u003ev\u003c/em\u003e (C-C) at 1118 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for di-sodium squarate, and D peak (1333 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and G peak (1598 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) for carbon material. However, all Raman peaks of sodium oxalate disappear in the oxidized electrode, leaving only D and G bands of carbons.\u003c/p\u003e\u003cp\u003e\u003cem\u003eEx-situ\u003c/em\u003e scanning electron microscope and energy-dispersive X-ray spectroscopy for analyzing electrode elements were conducted to test morphology and element distribution of initial and oxidized di-sodium quarate-AC electrodes. The sodium oxalate and activated carbon are uniformly distributed in the initial electrode (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). C, O, F, Na elements are present in the electrode (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), and a large number of pores appeared after the electrode oxidation by sodium oxalate (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). As evidenced in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, Na element is entirely removed after the sodium oxalate-activated carbon electrode oxidation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Electrochemical properties of sodium ion capacitors\u003c/h2\u003e\u003cp\u003eThe assembly process of the sodium-ion capacitor is shown in the experimental section (Supporting Information). The charge/discharge curve of HC (hard carbon) and different electrode capacity ratio are tested, which displayed ultra-high initial coulomb efficiency of 83.3% for HC (Fig S3 and Table S2). Di-sodium squarate was oxidized at a current of C/20 until the device was charged to 3.8 V (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Meanwhile, the di-sodium squarate-AC cathode and HC anode reached 3.824 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e and 0.024 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e, respectively. It can be anticipated that the di-sodium squarate component no longer contributes to the energy storage mechanism during the cycling of the sodium-ion capacitor. The sodium extraction capacity of di-sodium squarate withdrawing from the linear part before 3.5 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e is about 333 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e from the charging platform, which is close to the theoretical capacity of di-sodium squarate (339 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), which proves that almost all the sodium is extracted from di-sodium squarate. During charge and discharge process in the voltage range of 2.2 V to 3.8 V, the voltage curve of the sodium-ion capacitor is a typical linear curve, the positive electrode potential is a line parallel to the device voltage, and the HC negative electrode potential is a horizontal line (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). Importantly, it can be seen from the graph that the minimum potential of the HC electrode is higher than 0.01 V vs. Na/Na\u003csup\u003e+\u003c/sup\u003e, which completely excludes any risk of sodium plating. After 13,600 cycles, the sodium-ion capacitor shows excellent capacitance of 37 F g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (corresponding to a capacitance retention of 95.7%) and the energy efficiency remains at the level of 89% (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). The specific power of di-sodium squarate-AC//HC sodium-ion capacitors is 50 W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 100 W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 200 W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 300 W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 500 W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 700 W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1000 W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1500 W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The corresponding specific energies are 66.2 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 65.8 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 65.7 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 65.1 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 63.9 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 62.8 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 61.2 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 59.0 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. The obtained specific energy values are about 4 times higher than those of conventional supercapacitors (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). To explain this, we need to find the answer according to the formula of specific energy output \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{E}_{m}=\\frac{1}{2}{C}_{m}\\left({U}_{max}^{2}-{U}_{min}^{2}\\right)\\)\u003c/span\u003e\u003c/span\u003e. According to published literature, the specific capacitance of hybrid capacitors is twice than that of EDLC. Considering the output voltage of NIC and EDLC, it can be concluded that the output energy density of NIC is four times that of EDLC. Moreover, the electrochemical performance of NIC is superior than that of reported literature [\u003cspan additionalcitationids=\"CR20 CR21 CR22 CR23 CR24\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], resulting from the impressive insertion-extraction reversibility and excellent transmission rate of Na\u003csup\u003e+\u003c/sup\u003e in electrode (Supplementary Materials, Table S3). The Electrochemical impedance spectroscopy (EIS) of the sodium ion capacitor is tested at the stage of before cycling and after 500 cycles, which indicates that the sodium ion capacitor displays neglectable changes of charge transfer impedance after 500 loops, illustrating the well interface stability between electrode and electrolyte (Fig S4) and which is also confirmed by the changes of equivalent internal resistance (ESR) and Charge transfer resistance (EDR) (Fig S5).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eComparable specific energies of the NIC and EDLC at different specific powers.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"9\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eP (W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e200\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e500\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003e700\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003e1000\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c9\"\u003e\u003cp\u003e1500\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eE (Wh kg\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u0026minus;\u0026thinsp;1\u003c/b\u003e\u003c/sup\u003e\u003cb\u003e) for NIC\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e66.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e65.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e65.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e65.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e63.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e62.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e61.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e59.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eE (Wh kg\u003c/b\u003e\u003csup\u003e\u003cb\u003e\u0026minus;\u0026thinsp;1\u003c/b\u003e\u003c/sup\u003e\u003cb\u003e) for EDLC\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e19.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e19.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e19.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e18.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e17.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e17.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e15.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e14.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eE ratio\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e\u003cb\u003e3.45\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003e3.41\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e3.46\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003e3.50\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003e3.59\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003e3.69\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e\u003cb\u003e3.85\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e\u003cb\u003e4.07\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn total, the functional sodium oxalate (Na₂C₂O₄) was prepared by a mild chemical method, which acts as a sacrificial additive agent to improve the electrochemical performance of sodium ions capacitor (SICs). The introduction of Na₂C₂O₄ in active carbon can effectively compensate the reduction of Na\u003csup\u003e+\u003c/sup\u003e during charge-discharge process and improve the cycle life of SICs. Moreover, the functional Na₂C₂O₄ and active carbon show intimate interfacial contact resulting from the emergence of the C-O bond, which further enhance the reversibility of insertion-extraction process for Na\u003csup\u003e+\u003c/sup\u003e upon cycling cause and improve the stability of the electrode. Thus, the assembled NICs displays excellent capacity retention ratio of 95.7% after 13600 loops and excellent rate ability (high energy density of 59 Wh kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the power density of 1500 W kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The electrochemical performance enhancement mechanism is deeply investigated and evidenced by \u003cem\u003eex-situ\u003c/em\u003e scanning electron microscope and galvanostatic intermittent titration technique. Overall, the proposed strategy offers new insights into pre-sodiation techniques and give the meaningful guidance for the development of sodium ions capacitors.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eZhaowen Huang:\u003c/strong\u003e Conceptualization, Methodology development, Main investigation, Writing-Original Draft preparation; \u003cstrong\u003eXiaolin Li\u003c/strong\u003e: Experimental work, Data validation; \u003cstrong\u003eYang Hu\u003c/strong\u003e: Data acquisition, Formal analysis; \u003cstrong\u003eJingbo Liu\u003c/strong\u003e: Resources provision, Writing-Review\u0026amp;Editing; \u003cstrong\u003eManlan Guo\u003c/strong\u003e: Electrochemistry tests; \u003cstrong\u003eFenglin Wang\u003c/strong\u003e: Diagram drawing; \u003cstrong\u003eXianjie Yang\u003c/strong\u003e: Material synthesis; \u003cstrong\u003eLongwen Chen\u003c/strong\u003e: Materials characterization; \u003cstrong\u003eYoupeng Li\u003c/strong\u003e: Supervision, Project administration, Funding acquisition, Writing - Review \u0026amp; Editing (final approval).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere are no conflicts to declare.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information description\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe supplementary information consists of the \u0026ldquo;Experimental Section\u0026rdquo;, Figures and Table about the electrochemical performance comparison of NIC with other reported anode materials in NICs.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Basic and Applied Basic Research Foundation of Guangdong Province (2020A1515110273, 2020A1515110146, 2022A1515140005, 2022A1515110146), National Natural Science Foundation of China (62101534); Guangdong Provincial Department of Education Key Area R\u0026amp;D Program (2022ZDZC1078); Innovative Research Group Project of Higher Education Institutions in Guangdong Province (2022KCXTD066); GuangDong Engineering Technology Research Center (2022E075);\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eP. 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Mater. 30 1908309.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"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":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Sodium-ion capacitors, Pre-sodiation, Electrochemical Stability, C-O bond, Sacrificial Additive","lastPublishedDoi":"10.21203/rs.3.rs-7278220/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7278220/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSodium-ion capacitors (SICs) endow extremely potential in electrochemical energy storage system owning to the high energy-power characteristics elemental abundance and environmental friendliness. However, the irreversible sodium loss caused by the consumed Na\u003csup\u003e+\u003c/sup\u003e for the side reactions and formation of solid-electrolyte interphase (SEI) inevitable lead to the degradation of Na\u003csup\u003e+\u003c/sup\u003e storage capability for SICs. Herein, an effective pre-sodiation strategy is purposed by using the sodium oxalate (Na₂C₂O₄) as sacrificial additive agent to compensate the reduction of Na\u003csup\u003e+\u003c/sup\u003e for SICs during the charge-discharge process. The Na₂C₂O₄ possesses highly electrochemical stability at the high potential of 3.6 V and exhibits superior Na\u003csup\u003e+\u003c/sup\u003e diffusion coefficient than that of active carbon, evidenced by the galvanostatic intermittent titration technique test. Moreover, the as-prepared Na₂C₂O₄ shows intimate interface contact with active carbon derived from the formation of C-O bond between the Na₂C₂O₄ and active carbon, enhancing the structural stability of electrode and insertion-extraction reversibility of Na\u003csup\u003e+\u003c/sup\u003e upon cycling. Thus, the NICs assembled by pre-sodiation activate carbon cathode and hard carbon anode displays impressive capacitance retention of 95.7% after 13600 cycles and excellent rate ability (high energy density of 59 Wh kg\u003csup\u003e-1\u003c/sup\u003e at the power density of 1500 W kg\u003csup\u003e-1\u003c/sup\u003e). The underlying mechanisms for the excellent electrochemical performance of the assembled SICs are systematic investigated by \u003cem\u003eex-situ\u003c/em\u003e scanning electron microscope and galvanostatic intermittent titration technique. This work paves a way for the development of the pre-sodiation techniques and provide valid guidance for the future research directions of advanced energy storage and transfer equipment.\u003c/p\u003e","manuscriptTitle":"A sacrificial additive agent mediated strategy for high performance sodium ion capacitor","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-11 12:11:18","doi":"10.21203/rs.3.rs-7278220/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a91b7639-a013-4453-b7fd-3c3033af88fc","owner":[],"postedDate":"August 11th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-08-31T10:38:30+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-11 12:11:18","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7278220","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7278220","identity":"rs-7278220","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}