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[Toshihiko Mandai](https://orcid.org/0000-0002-2403-7794), Umi Tanaka, Shin Kimura

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[Electrode Engineering Study Toward High‐Energy‐Density Sodium‐Ion Battery Fabrication](https://mdr.nims.go.jp/datasets/359c5f2f-a299-4c03-8da8-4bb80aea548d)

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Electrode Engineering Study Toward High-Energy-Density Sodium-Ion Battery FabricationElectrode Engineering Study Toward High-Energy-DensitySodium-Ion Battery FabricationToshihiko Mandai,* Umi Tanaka, and Shin Kimura1. IntroductionElectrochemical energy storage technolo-gies, such as lithium-ion batteries (LIBs),have rapidly evolved alongside industrialand technological advancements, becom-ing indispensable devices in our daily lives.However, this progress has been accompa-nied by environmental challenges, includ-ing global warming, stemming from therecent industrial revolution. To mitigateenvironmental degradation and fostera sustainable future society, there is apressing need to transition from currenttechnologies and further innovate inenergy storage.Lithium, a crucial component in LIBmanufacturing, is often considered a scarcemineral resource. However, based oncurrent battery production scales, estimatessuggest that lithium resources on Earth areabundant enough to last over 200 years[1]without depletion. Therefore, lithiumresources themselves are not expected topose a significant bottleneck in large-scalebattery production. Nonetheless, concerns persist regardingstable supply and costs due to resource concentration in specificregions. Moreover, the process of converting lithium raw mate-rials containing LiCl and Li2CO3 into metal resources, essentialfor battery production, generates substantial CO2 emissions andwaste,[2] highlighting the urgent need to reduce environmentalimpacts. Against this backdrop, the development of energystorage technologies utilizing resources unaffected by con-straints and with minimal environmental footprints is eagerlyanticipated.Sodium-ion batteries (SIBs) are promising energy storagetechnologies for auxiliary power supply in electric devices andgrid-scale applications, thanks to their relatively wide operatingtemperature range and low material costs due to the abundanceof sodium resources.[3,4] Extensive research has been conductedon electrode and electrolyte materials to achieve SIBs withcompetitive or superior performance compared to currentLIBs. Despite the challenges inherent in surpassing LIBs’ energystorage performance with SIBs, specific materials tailored forsodium-ion storage have been developed with the help ofadvanced characterization techniques and computationalscience.[5–8] However, the energy density of SIBs is often evalu-ated based solely on the capacities and voltages of active materialsin half-cell configurations, disregarding practical considerationsfor full-cell configurations and engineering aspects. LimitedT. MandaiFunctional Electrolyte Synthesis TeamResearch Center for Energy and Environmental MaterialsNational Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanE-mail: MANDAI.Toshihiko@nims.go.jpT. Mandai, U. TanakaCenter for Advanced Battery CollaborationResearch Center for Energy and Environmental MaterialsNational Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanS. KimuraBattery Research PlatformResearch Center for Energy and Environmental MaterialsNational Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/aesr.202400059.© 2024 The Authors. Advanced Energy and Sustainability Research pub-lished by Wiley-VCH GmbH. This is an open access article under the termsof the Creative Commons Attribution License, which permits use,distribution and reproduction in any medium, provided the originalwork is properly cited.DOI: 10.1002/aesr.202400059Sodium-ion batteries (SIBs) are emerging as promising energy storagetechnologies, particularly for grid-scale applications, due to their low materialcosts stemming from abundant natural resources. Meeting the increasingdemand for higher energy density requires the development of innovativeelectrode and electrolyte materials, along with advanced analytical and fabricationtechniques. However, the energy density of SIBs is often evaluated based solelyon the capacities and cell voltages of active materials in half-cell configurations,neglecting engineering considerations for full-cell configurations. This studyinvestigates the effects of electrode composition and the balance in capacitiesbetween positive and negative electrodes (N/P ratio) on the performance offull-cell configurations, using Na3V2(PO4)3 (NVP) and hard carbon (HC) asrepresentative electrode materials. Through a systematic analysis, an optimalcomposition for NVP and HC electrodes is proposed, considering areal capacityand capacity retention during full-cell operations. Additionally, the importance ofbalancing the N/P ratio and the necessity of presodiation techniques to achievehigh-energy-density SIBs are underscored. Overall, this work sheds light on keyfactors influencing the performance of SIBs and provides insights into strategiesfor enhancing their energy density and operational efficiency.RESEARCH ARTICLEwww.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (1 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbHmailto:MANDAI.Toshihiko@nims.go.jphttps://doi.org/10.1002/aesr.202400059http://creativecommons.org/licenses/by/4.0/http://www.advenergysustres.comreports exist on the engineering aspects of fabricating SIB fullcells, such as experimental conditions for slurry preparation,[9,10]loading amounts of active materials,[11,12] and balancing capaci-ties between positive and negative electrodes (N/P ratio).[13,14]This oversight can lead to an overestimation of SIBs’ potential.This study systematically investigates the effects of electrodecomposition and the N/P ratio on the energy storage perfor-mance of full-cell configurations, using Na3V2(PO4)3 (NVP)and hard carbon (HC) as positive and negative electrodes, respec-tively, aided by an energy density calculator. The results of thesystematic survey using model systems confirm that carefulbalancing of the N/P ratio is crucial for enhancing the energydensity of SIBs. Additionally, two presodiation approaches areadopted to fabricate SIB full cells with a 100Wh kg�1 energydensity class, mitigating unbalanced N/P ratios resulting fromthe consumption of reactive Naþ during initial stabilizationprocesses.2. Results and Discussions2.1. Binder, Conductive Support, and Electrolyte Study onHalf-Cell ConfigurationVarious binder polymers are utilized in the production ofcomposite electrodes, taking into account factors such as cost-effectiveness, eco-friendliness, processability, and battery perfor-mance. The physical properties of binder polymers significantlyimpact the resulting battery performance. Polyvinylidenedifluoride (PVdF), a fluorocarbon-based polymer, is widely used,particularly for positive electrodes in commercial LIBs, due to itsnumerous advantageous properties.[15] Given its adhesivenessand excellent electrochemical stability, PVdF was chosen asthe binder polymer for the NVP electrodes in this study.Figure 1 illustrates the typical charge–discharge profiles of [Na| base electrolyte | NVP] half cells employing low (L) and high(H) molecular weight PVdF, L-PVdF and H-PVdF, polymersas binders. The composite working electrodes consisted ofNVP, acetylene black (AB), and PVdF in a weight ratio of85:5:10. These two composite NVP electrodes are subsequentlyreferred to as NVPAB/L-PVdF and NVPAB/H-PVdF. As depicted inFigure 1, the choice of binder significantly affects the charge–discharge performance. NVPAB/H-PVdF exhibited higher revers-ible capacities over 100 cycles, while capacities rapidly decreasedfor NVPAB/L-PVdF. A similar trend was observed for comparative[Na || NVP] cells using relatively coarse NVP particles anddifferent electrolytes (Figure S3, Supporting Information).To comprehend the potential causes of the observed differen-ces with different binders, the physical structure of the compos-ite electrodes was analyzed via scanning electron microscopy(SEM). Unfortunately, it was challenging to identify decisivedifferences in their physical structure, as depicted in Figure 2.It can be inferred that, concerning homogeneity, the active mate-rials and conductive carbons are relatively well homogenized forNVPAB/H-PVdF, whereas these components are somewhat aggre-gated in parts for NVPAB/L-PVdF. Such aggregation may impedeelectron and ion conduction in the composite electrodes,potentially leading to inferior charge–discharge performance.Due to the superior performance of electrodes with H-PVdF,Figure 1. a,b) Galvanostatic charge–discharge cycling profiles and the corresponding Coulombic efficiency of [Na || NVP] half cells employingNVPAB/L-PVdF and c,d) NVPAB/H-PVdF electrodes.www.advancedsciencenews.com www.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (2 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbH 26999412, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aesr.202400059 by National Institute For, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergysustres.comall subsequent studies utilized this binder for the NVP electro-des, unless otherwise specified.The impact of conductive support on enhancing the energystorage performance of electrodes was investigated in this study.By partially replacing AB with carbon nanotube (CNT), the rela-tive amount of PVdF binder could be significantly reduced,resulting in NVP electrodes (NVPCNT/H-PVdF) with a weight ratioof NVP:carbon:binder= 92:4:4. These electrodes exhibitedcomparable performance to those containing higher amountsof binder (Figure 1b and 3), attributed to the fiber propertiesof CNT facilitating the folding of active materials.Additionally, spaces between NVP particles were observed inNVPCNT/H-PVdF (Figure 3), likely contributing to enhancedpenetration of electrolyte solutions throughout the compositeelectrodes. This positively affected electrode performance byimproving ion transport characteristics. Fabricating electrodeswith higher relative content of active materials and lower contentof other components is beneficial for achieving energy andpower-balanced batteries, avoiding thick electrodes due to exces-sive amounts of conductive supports and binders.Figure 2. a–f ) Cross-sectional SEM images and corresponding EDX mapping profiles of NVPAB/L-PVdF and g–l) NVPAB/H-PVdF electrodes.Figure 3. a) Galvanostatic charge–discharge cycling profiles and b) the corresponding Coulombic efficiency of [Na || NVP] half cell employing aNVPCNT/H-PVdF electrode. c–e) Cross-sectional SEM images of NVPCNT/H-PVdF electrodes.www.advancedsciencenews.com www.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (3 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbH 26999412, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aesr.202400059 by National Institute For, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergysustres.comFor hard carbon (HC)-negative electrodes, aqueous mixturesof carboxymethyl cellulose (CMC) and styrene-butadiene rubber(SBR) are commonly used to create composite electrodes. Inthese mixtures, SBR acts as a binder while CMC serves as a thick-ener to regulate slurry viscosity.[15] Similar to graphite electrodesin LIBs, a conductive support is necessary for HC electrodes, pos-sibly due to the insufficient electronic conductivity of HC powderitself. HC electrodes without AB (HCAB-free) did not perform wellin [Na || HC] cells, whereas those incorporating AB exhibitedsome reversible capacities, as shown in Figure 4 and S4a,Supporting Information, respectively. The effect of binder com-position in HC-negative electrodes was also examined. Althoughsome activation process is necessary, and the initial deliverablecapacities after the activation process are inferior for HC electro-des containing 3 wt% of CMC-SBR mixture (HC3%CMC-SBR) com-pared to those with 5 wt% (HC5%CMC-SBR), the former electrodesdemonstrated better capacity retention over 100 cycles (Figure 4).The systematic comparative experiments clearly reveal theelectrolyte-dependent charge–discharge characteristics of [Na ||HC] and [Na || NVP] half cells. The [Na || HC] half cells utilizingthe carbonate-based base electrolyte exhibited unexpectedly lowcapacities (Figure 4). Previous reports on the sodiationmechanism of HC indicate that the slope region in the chargingprofiles, ≈1.0–0.2 V versus Naþ/Na, is attributed to surfaceadsorption and/or intercalation of Naþ on/into the defectiveHC structure, while the plateau region around 0.02–0.01 V ver-sus Naþ/Na arises from clustering of Na(0) in (pseudo-)closedpores.[16,17] Based on this mechanism, the observed results withthe base electrolyte suggest no or limited Na(0) clustering.Conversely, cells with the ether-based counterpart, NaPF6/G2 (G2;diglyme), delivered reasonable HC capacities (≈240mAh g�1)under the same experimental conditions, albeit with irregularlyfluctuating profiles presumably due to microshorts (Figure S5,Supporting Information). The reductive stability of the solventsin these electrolytes may explain the observed differences.Carbonates are known to be susceptible to reduction, leading tosuccessive decomposition of carbonate molecules upon contactwith highly reductive Na metal during charge–discharge cycling.Additionally, stable solid electrolyte interface (SEI) formation onthe surface of Na metal is challenging in conventional carbonate-based electrolytes.[18,19] This unstable interface can cause signifi-cant polarization of Na plating/stripping at Na metal counter elec-trodes, resulting in limited or no utilization of micropores forNa(0) accommodation in HC electrodes. While micropores canbe utilized for Naþ accommodation by adjusting the experimentalcut-off potential, changing the potential from þ0.01 to �0.05 Vversus Na metal may increase the risk of short-circuit due toNa metal deposition on the surface of HC electrodes (Figure S6,Supporting Information). In contrast, stable Na plating/strippingis reportedly possible in ether-based electrolytes due to theirgreater stability against Nametal.[20,21] This enhanced stability con-tributes to the relatively stable cycling of [Na || HC] cells (Figure S5,Supporting Information). The stability of the electrolyte against Nametal counter electrodes also significantly affects the observed per-formances of [Na || NVP] cells (Figure 1 and S3, SupportingInformation). It is important to note that ethereal solvents areincompatible with most positive electrodes of SIBs due to theiranodic limits lying around 3.8 V versus Na.[22] The charge curvesFigure 4. a,b) Galvanostatic charge–discharge cycling profiles and the corresponding Coulombic efficiency of [Na || HC] half cells employing HC5%CMC-SBRand c,d) HC3%CMC-SBR electrodes.www.advancedsciencenews.com www.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (4 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbH 26999412, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aesr.202400059 by National Institute For, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergysustres.comof [Na || NVPCNT/H-PVdF] cells with the NaPF6/G2 electrolyteindeed indicate electrolyte decomposition associated with charg-ing (Figure S3, Supporting Information).In summary, comprehensive studies on the half-cell configu-ration can provide guidance for the favorable compositionof composite electrodes. However, the incompatibility ofelectrolytes against Na metal counter electrodes can lead tomisconceptions regarding the obtained experimental results.Fluoroethylene carbonate (FEC) is well-known as an SEI-formingadditive for electrode materials, including carbonous andmetallic negative electrodes. The incorporation of FEC in typicalNaPF6/carbonate-based electrolytes can significantly enhance Naplating/stripping efficiencies.[23] The integration of FEC issomewhat effective in enhancing the cycling stability of [Na ||NVPCNT/H-PVdF] half cells (Figure S7, Supporting Information).Unfortunately, the integration of FEC also results in fluctuationsin charge–discharge performance, possibly due to the crossoverof decomposition products. To further optimize both NVP andHC electrodes without the contribution of ambiguous impuritiesand relevant misconceptions arising from electrolyte additives,the electrochemical performances will be studied under thefull-cell configuration with carbonate-based base electrolytes.2.2. Full-Cell Fabrication and Optimization of CompositeElectrodesThe battery performance of the primary full cells, [HC || NVP],was evaluated using coin-type cells. Figure 5 summarizes thecharge–discharge cycling profiles of the full cells with three dif-ferent compositions in composite electrodes. To minimize theinfluence of the balance in capacities of the positive and negativeelectrodes, the N/P ratio was fixed at ≈1.70–1.73 among the cells.Similar to the performance of the corresponding half cellsdescribed above, the composition of each positive and negativeelectrode significantly impacts the full cell performance.Regarding the positive electrodes, the cells with the optimalNVPCNT/H-PVdF electrodes showed superior capacities from bothgravimetric and areal perspectives. These results further demon-strate the beneficial effect of CNT on enhancing the energy den-sity of batteries. Conversely, the absence of AB in HC electrodesresulted in significantly inferior performance. It is worth notingthat the corresponding half cells did not function at all(Figure S4, Supporting Information). This discrepancy in thereactivity of HC electrodes may arise from the cell configuration,as relatively deep polarization during charging can activate theAB-free HC electrodes. Indeed, the charging voltage of the cor-responding full cell was lower than that of the control experiment(Figure 5a,c). It is worth noting here that, as well as the cyclingperformance of half-cell configurations (Figure 4), the fullcells using the HC3%CMC-SBR showed better capacity retentionthan those using the HC5%CMC-SBR (Figure S8, SupportingInformation). Based on the results of the primary full-cellconfigurations, the optimal compositions of each positive andnegative electrode are proposed to be NVP:carbon:binder= 92:4:4 (NVPCNT/H-PVdF) and HC:AB:CMC-SBR= 92:5:1.5:1.5(HC3%CMC-SBR), respectively. This configuration also performswell with a different electrolyte formulation (Figure S9,Supporting Information).2.3. Factors Affecting Full-Cell PerformanceIt is crucial to minimize the difference in capacities betweenpositive and negative electrodes to achieve high-energy densitybatteries, with the ideal N/P ratio being unity. By preciselycontrolling the N/P ratio, overlooked aspects for practical batterymaterialization, not addressed in studies on half-cell configura-tions, can be discussed. To avoid redundancy, the optimalelectrodes mentioned earlier are hereby referred to simply asNVP and HC.The impact of the N/P ratio on full-cell performance is evidentin the cycling results of full cells with different N/P ratios, asshown in Figure 6. As the N/P ratio increases, initial dischargecapacities decrease, although corresponding charging capacitiesbased on the mass of NVP remain almost the same. Subsequentcycles inherit the performance of the initial cycle, with relativelyinferior performance observed for full cells fabricated underhigher N/P ratios. These results suggest the consumption ofreactive Naþ from the NVP cathodes by side reactions duringthe initial charging process. The charging capacity retentionbetween the 1st and 2nd cycles, or simply initial Coulombicefficiency, also strongly indicates consumption of reactive Naþduring the 1st charging. Additionally, an extra plateau around0.8 V versus Na is observed in the initial discharge process for[Na || HC] cells (Figure 4), indicating the formation of the SEIon negative electrodes upon electrolyte decomposition.[16]Such SEI can suppress further electrolyte decomposition, thusstabilizing charge–discharge performances. Unfortunately, SEIformation consumes reactive Naþ, consequently leading to poorFigure 5. a) Galvanostatic charge–discharge cycling profiles of [HC3%CMC-SBR || NVPCNT/H-PVdF], b) [HC3%CMC-SBR || NVPAB/H-PVdF], and c) [HCAB-free|| NVPCNT/H-PVdF].www.advancedsciencenews.com www.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (5 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbH 26999412, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aesr.202400059 by National Institute For, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergysustres.comenergy storage performances. It is worth noting that Na metalelectrodes can supply extra Naþ, making the undesired capacitydecay observed in full-cell configurations difficult to observe intheir half-cell counterparts. Moreover, a high N/P ratio inducessloping of the voltage plateaus during both charging and dis-charging, limiting the enhancement of battery energy density.From the above results, it can be anticipated that an electrolyteadditive is less effective in improving battery performance in thepresent case. Despite the remarkable performance improvementupon FEC integration for Na metal plating/stripping,[23] increas-ing the amount of electrolyte additive leads to worse cycling per-formance, even at the same sufficiently low N/P ratio of 1.25(Figure S10, Supporting Information). As SEI formation associ-ated with FEC decomposition involves the consumption of reac-tive Naþ in the cells, excessive FEC additive leads to inferiorperformance. X-ray photoelectron spectroscopy (XPS) analysison cycled NVP electrodes further supports the formation of thickdecomposition products on the surface (Figure S11, SupportingInformation). The relative peak intensity of Na–F compounds inF 1s spectra for HC electrodes becomes larger with the amount ofthe FEC additive.[24] These observations strongly suggest that the(electro)chemical stability of SEI on HC electrodes generated inthe additive-free conventional base electrolytes is sufficientlyhigh, and the reported instability of SEIs originating fromconventional carbonate-based electrolytes would be induced bythe crossover of harmful compounds from Na metal counterelectrodes due to half-cell configurations.[18,19]Even at minimal N/P ratios, the deliverable practical capacitiesof the [HC || NVP] full cells were at most ≈80mAh g�1 based onthe mass of NVP, corresponding to ≈70% of the theoreticalcapacity. Using the energy density calculator established byUe et al.[25] the potential energy density of these full cells wascalculated to be 73Wh kg�1, considering experimental parame-ters of composite electrodes, mass loading of NVP (≈5mg cm�2),N/P ratio (1.05), and electrolyte/capacity ratio (5.0 g Ah�1).Increasing the practical capacities up to 105mAh g�1 couldresult in an energy density of 100Wh kg�1. As the loss of capacitymainly arises from the consumption of reactive Naþ by SEI for-mation during the initial charging process, presodiation wasapplied to compensate for the loss of Naþ. Both electrochemicaland chemical presodiation were adopted in this study.Electrochemical presodiation on HC electrodes involved pre-charging the [HC || NVP] full cells, disassembling the cells,and reassembling them using the charged HC and freshNVP. The N/P ratio for the precharging and subsequentcharge–discharge measurements were controlled to be ≈1.1, esti-mated based on the mass loading and capacity of pristine electro-des. Calculations of energy density of full cells were summarizedin Figure S12 and Table S1, Supporting Information.With an increasing depth of charge for the precharging pro-cesses (DOPC), the deliverable capacities of the 1st dischargeprocesses increased from 75 to 105mAh g�1 at 0% and 100%DOPC, respectively (Figure 7). The charge–discharge profilesalso changed with the degree of DOPC, and flat initial reactionplateaus were observed for cells precharged at 100% DOPC.However, a high degree of DOPC simultaneously led to inferiorcapacity retention for subsequent cycles, although the Coulombicefficiencies remained almost unity over 200 cycles, irrespective ofFigure 6. Galvanostatic charge–discharge cycling profiles of [HC || NVP] full cells with the N/P at a) 2.59, b) 1.73, and c) 1.05. d) Discharge capacities at1st and 100th cycles and initial Coulombic efficiency.www.advancedsciencenews.com www.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (6 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbH 26999412, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aesr.202400059 by National Institute For, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergysustres.comthe degree of DOPC (Figure 7a–e). The capacity retention of cellsprecharged at 100%-DOPC was only 67.3% after 500 cycles,while those at 50%, 25%, and 0% DOPC were over 77.7%,85.6%, and 97.4% for 500 cycles, respectively. This worse capac-ity retention rate for the deeply precharged cells is possibly due tothe overcharging of HC electrodes and resulting in undesiredside reactions, such as deterioration of the electrolyte andelectrode by deposited Na metal. Based on the N/P ratios ofthe cells, the HC electrodes precharged at 100%, 50%, and25% DOPC would be overcharged by ≈70%, 30%, and 10% oftheir practical capacity, respectively, in the following measure-ments (Figure 7f ). The overcharged HC electrodes indeed exhib-ited unstable discharge–charge behavior (Figure S6, SupportingInformation). To avoid such situations, the N/P ratios for bothprecharging and subsequent charge–discharge measurementswere carefully modulated. Furthermore, to minimize theinhomogeneity in confining pressure during measurements,pouch-type cells were fabricated, and a confining pressure of112 kPa was applied to the cells. By considering the N/P ratios,especially during subsequent charge–discharge measurements,the pouch-type [HC || NVP] full cells achieved balanced perform-ances with respect to apparent capacities, capacity retention, andpractical energy density upon modulating the N/P ratio to be 2.2,which substantially corresponds to 1.2 for the subsequentcharge–discharge cycles. This primitive optimum cell showednegligible irreversible capacity between the 1st charge and dis-charge and delivered stable discharge capacity over 500 cycles(Figure 8a,b). The formation of stable SEI during the prechargingprocess and the utilization of ≈45% of the full capacity of theprecharged HC negative electrode jointly contributed to theexceptionally stable cycling performance of the present optimumcell. The practical energy density of that cell is however estimatedFigure 7. Galvanostatic precharging and subsequent charge–discharge cycling profiles of [HC || NVP] full cells with a different DOPC; a) 0%, b) 25%,c) 50%, and d) 100%. e) Discharge capacities at 1st and 500th cycles, initial Coulombic efficiency, and capacity retention after 500 cycles. f ) Degree ofovercharging of HC electrodes.www.advancedsciencenews.com www.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (7 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbH 26999412, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aesr.202400059 by National Institute For, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergysustres.comto be 85Wh kg�1, still less than 100Wh kg�1, certainly due to arelatively high N/P ratio (Figure S12 and Table S1, SupportingInformation). The significant loss in the energy density was com-pensated by other engineering approaches. Upon increasing themass loading of the electrodes, from 5 to 11mg cm�2 for NVP,the resulting energy density can reach 105Wh kg�1 even at theN/P ratio of 2 (Figure S12 and Table S1, SupportingInformation). The stable charge–discharge performance of thewell-designated pouch-type cell has successfully been demon-strated (Figure 8c,d).Although electrochemical presodiation can give significantinsights for full-cell fabrication, this method complicates themanufacturing of SIBs, leading to high production costs, thusis unadoptable to practical operation. The present systems alsorequire a relatively high N/P ratio to stabilize HC electrodes.Decreasing the N/P ratio from 2.2 to 1.05 in the same [HC ||NVP] setup can increase the energy density to 120Wh kg�1.Chemical presodiation and/or inclusion of sacrificial additiveshave been proposed as alternatives.[26–29] Chemical presodiationon HC electrodes can lead to instability of the resulting electro-des against the ambient atmosphere due to the accommodationof reactive Naþ and Na(0) species in their structure.[28,29]Conversely, chemical presodiation on NVP cathodes is expectedto be more accessible owing to the potential chemical stability ofthe presodiated NVP, Na4V2(PO4)3.[26] The chemically-sodiatedNVP coupled with HC electrodes at N/P= 1.2 showed anFigure 8. a,c) Galvanostatic precharging and subsequent charge–discharge cycling profiles and b,d) the corresponding Coulombic efficiency ofpouch-type [HC || NVP] full cells. Mass loading of NVP; (a,b) 5 mg cm�2; (c,d) 11 mg cm�2.Figure 9. a) Galvanostatic charge–discharge cycling profiles and b) the corresponding Coulombic efficiency of pouch-type [HC || presodiated NVP]full cells.www.advancedsciencenews.com www.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (8 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbH 26999412, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aesr.202400059 by National Institute For, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergysustres.comadditional charge plateau at around 1.2 V, attributable to Naþextraction from the Na4V2(PO4)3 structure,[26] and a substantiallylarge initial charging capacity was obtained (Figure 9). The fullcell delivered an initial discharge capacity of 96mAh g�1, and thedeliverable capacities were stabilized at around 100mAh g�1 forsubsequent cycles. This observation strongly implies that the sta-ble SEI was formed on the HC electrode by the consumption ofextra Naþ from Na4V2(PO4)3 and most of the reactive Naþ ionsseem to be preserved in the electrodes. Although the mass load-ing of the above cell is ≈5mg cm�2, hence optimization withrespect to loading amount and N/P ratio is ongoing, the energydensity of the present primal full cell with chemically presodiatedNVP also reaches ≈100Wh kg�1 (Figure S12 and Table S1,Supporting Information).3. ConclusionThis work demonstrates how the engineering aspects of batter-ies, such as the composition of electrodes and N/P ratio, affectthe performance of full cells and highlights the importance ofadopting positive and negative electrodes with well-balancedcapacities to achieve high-energy density practical SIBs. Uponcomparative survey, the optimum composition of NVP andHC electrodes with respect to the areal capacity and capacityretention under full-cell operations was proposed. The systematicstudy on the N/P ratio also clarifies the significant detrimentaleffect of the consumption of reactive Naþ by SEI formationon the energy density of full cells. The full cells that adoptedeither appropriate electrochemical precharging of HC electrodescombined with the well-controlled N/P ratio or chemical preso-diation on NVP electrodes achieved a practical energy density of100Wh kg�1 despite employing primitive electrode materials.Improvements in capacities and working voltages of electrodematerials are straightforward approaches to enhance the energydensity of batteries. A practical energy density of 150Wh kg�1 ispotentially achievable by adopting prospective positive electrodeswith stable capacities of 120mAh g�1 at a working voltage of3.5 V. In contrast, for the present [HC || NVP] systems, increas-ing the mass loading of NVP to>19mg cm�2, which is the samelevel as positive electrodes of commercialized LIBs, will also leadto an energy density of 150Wh kg�1. Development in both activematerials and techniques of thick electrode fabrication will pavethe way for high-energy-density SIB materialization.4. Experimental SectionMaterials: The positive electrode material, Na3V2(PO4)3 (NVP), wasprocured from Kojundo Chemical Laboratory CO., Ltd., or MTICorporation and utilized without further treatment. SEM analysis revealeddifferences in particle morphology between the two NVP samples, withfine irregular and coarse secondary particles observed for the formerand the latter, respectively (Figure S1, Supporting Information).Primarily, the finer NVP powder was utilized unless stated otherwise inthis study. HC powder, designated KURANODE (Type 1), was generouslyprovided by Kuraray. Prior to use, the HC powder underwent vacuum heat-ing at 200 °C for a minimum of 3 days to eliminate surface-adsorbed mois-ture. AB (Denka Black, Denka) and a carbon nanotube/fluoropolymerdispersion (CNT; NEOFLON VTD-475 N, Daikin Industries, LTD.) wereemployed as conductive supports. PVdF powders with varying molecularweights, low (L-PVdF, #1100) and high (H-PVdF, #7500), were obtainedfrom Kureha. The PVdF binder solution for the NVP electrodes was pre-pared by dissolving a certain amount of predried PVdF powder (≈12 wt%)in anhydrous N-methylpyrrolidone (NMP; Kanto Chemical CO., Inc.)under a dry air atmosphere with a controlled dew point of <�76 °C(Dry chamber; Soda Kogyo). For the HC electrodes, a suitable combina-tion of an aqueous dispersion of sodium CMC(CMC2000, Daicel Miraizu)and SBR (TRD104A, ENEOS Material) was utilized as a binder.Composite NVP and HC electrodes were prepared under dry and ambi-ent atmospheres, respectively, using the appropriate conductive supportsand binder. The resulting NVP and HC slurries were applied onto analuminum current collector and dried at 80 °C for 24 h. The resultant com-posite sheet electrodes were compressed using a roller pressing machine(Eager Corporation) to enhance electrical conductivity. A standard electro-lyte solution of 1 mol dm�3 NaPF6/EC-DEC (1:1) (EC: ethylene carbonate,DEC: diethyl carbonate) served as the base electrolyte. Additionally, forcomparison, two solutions of 1 mol dm�3 NaPF6/diglyme (G2; for electro-chemistry, Kanto Chemical CO., Inc.) and 0.5mol dm�3 Na[B(HFIP)4]/EC-DEC (1:1) were prepared. Na[B(HFIP)4] was synthesized following areported procedure.[30] The impact of a conventional electrolyte additive,FEC (Merck), on battery performance was also investigated by adding apredetermined amount of FEC to the base electrolyte within an Ar-filledglove box. Polyolefin-based separators were employed for all charge–discharge measurements.Charge–Discharge Measurements: Charge–discharge cycling tests wereconducted using both 2032-type coin cells and pouch-type cells withNVP and HC electrodes. Both half-cell and full-cell configurations wereutilized in the tests with coin-type cells. For half cells, Na metal foils(≈200 μm thick) were employed as counter electrodes, while NVP andHC were paired together for full-cell configuration. To minimize contami-nation from side reactions between Na metal and atmospheric impurities,all half cells were assembled in an Ar-filled glovebox with controlled levelsof H2O and O2 (<1 ppm). Conversely, both coin and pouch-type full cellswere assembled under a dry air atmosphere (dew point ≈�40 °C). Theeffect of capacity balance between negative and positive electrodes, knownas the N/P ratio, was examined by considering the amount of active mate-rials and the practical reversible capacity of NVP (117mAh g�1) and HC(300mAh g�1). Electrochemical precharging on HC electrodes was prelim-inarily carried out by charging the [HC || NVP] coin-type full cells to differ-ent depths of charge, followed by disassembling the charged cells,replacing the charged NVP with pristine NVP while leaving other compo-nents unchanged, reassembling the cells, and restarting charge–dischargemeasurements. For the pouch-type cell configuration, the electrochemicalprecharging was carried out by charging the [HC || NVP for precharging ||NVP for cycling] full cells to 100% of depths of charge using NVP for pre-charging, followed by disassembling the charged cells, removing the NVPfor precharging and relevant appurtenance (separator, tab, and tab-lead),reassembling the cells, and restarting charge–discharge measurementswith the [precharged HC || NVP for cycling] cells. The configuration ofpouch-type cells for electrochemical precharging is illustrated in Figure S2,Supporting Information. Chemically predoped NVP composite cathodeswere prepared by soaking the composite in a 0.05mol dm�3 phenazine-Na/G1 solution for 120 s, following literature protocols.[26] Charge–discharge tests were performed at a 1C rate (120mA gNVP�1) based onthe mass of active materials for half-cell configurations and based onthe mass of NVP for full-cell configurations, unless otherwise specified.Characterization: Composite electrodes were observed using SEM(JSM-7800F, JEOL) and subsequently characterized by energy-dispersiveX-ray (EDX) spectroscopy. Electrodes for SEM observations were proc-essed using a cross-sectional polisher (CP; IB-09020CP, JEOL) to obtainsmooth cross-sectional views. The surface composition of NVP andHC electrodes before and after cycling was examined usingXPS(VersaProbe II, ULVACPHI, Japan). All cycled samples were washedwith anhydrous DEC to remove residual electrolyte, dried under high vac-uum at ambient temperature, placed in an airtight chamber, and trans-ferred for XPS analysis without exposure to air. XPS measurementswere conducted with an Al Kα X-ray source under a base pressure ofwww.advancedsciencenews.com www.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (9 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbH 26999412, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aesr.202400059 by National Institute For, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergysustres.com6.7� 10�8 Pa. The binding energy of the obtained spectra was calibratedusing the C 1s peak from sp2-hybridized carbon at 284.5 eV as a reference.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work received financial support from the NEXT Center of InnovationProgram (COI-NEXT, grant no. JPMJPF2016) of the Japan Science andTechnology Agency and a Grant-in-Aid for Scientific Research(KAKENHI, JP21K05263) from the Japan Society for the Promotion ofScience. The authors appreciate the support received for SEM observa-tions and XPS measurements at the NIMS Battery Research Platform.T.M. also thanks Prof. Shinichi Komaba and Dr. Kei Kubota for their kindadvice on battery fabrication.Conflict of InterestThe authors declare no conflict of interest.Author ContributionsT.M.: Conceptualization, data curation, formal analysis, fundingacquisition, investigation, project administration, resources, validation,writing–original draft and review and editing. U.T.: Data curation, formalanalysis, investigation. S.K.: Dara curation, investigation, formal analysis.Data Availability StatementThe data that support the findings of this study are available from thecorresponding author upon reasonable request.Keywordscapacity ratio, electrodes, full-cell configurations, sodium-ion batteriesReceived: February 24, 2024Revised: April 16, 2024Published online: May 19, 2024[1] United States Geological Survey, Lithium. 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Li, Angew.Chem., Int. Ed. 2023, 62, e202214717.[29] M. Sathiya, J. Thomas, D. Batuk, V. Pimenta, R. Gopalan,J.-M. Tarascon, Chem. Mater. 2017, 29, 5948.[30] T. Mandai, H. Naya, M. Hyuma, J. Phys. Chem. C 2023, 127, 7987.www.advancedsciencenews.com www.advenergysustres.comAdv. Energy Sustainability Res. 2024, 5, 2400059 2400059 (10 of 10) © 2024 The Authors. Advanced Energy and Sustainability Researchpublished by Wiley-VCH GmbH 26999412, 2024, 9, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aesr.202400059 by National Institute For, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergysustres.com Electrode Engineering Study Toward High-Energy-Density Sodium-Ion Battery Fabrication 1. Introduction 2. Results and Discussions 2.1. Binder, Conductive Support, and Electrolyte Study on Half-Cell Configuration 2.2. Full-Cell Fabrication and Optimization of Composite Electrodes 2.3. Factors Affecting Full-Cell Performance 3. Conclusion 4. Experimental Section