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[Jittraporn Saengkaew](https://orcid.org/0000-0002-8285-8152), Emiko Mizuki, [Shoichi Matsuda](https://orcid.org/0000-0002-0640-3404)

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[Performance evaluation of lithium metal rechargeable batteries with a lithium excess cation-disordered rocksalt based positive electrode under high mass loading and lean electrolyte conditions](https://mdr.nims.go.jp/datasets/ab76e1ee-42e8-4259-8cfe-3fb4d411ad79)

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Performance evaluation of lithium metal rechargeable batteries with a lithium excess cation-disordered rocksalt based positive electrode under high mass loading and lean electrolyte conditions248 |  Energy Adv., 2024, 3, 248–254 © 2024 The Author(s). Published by the Royal Society of ChemistryCite this: Energy Adv., 2024,3, 248Performance evaluation of lithium metalrechargeable batteries with a lithium excesscation-disordered rocksalt based positiveelectrode under high mass loading and leanelectrolyte conditions†Jittraporn Saengkaew,a Emiko Mizukia and Shoichi Matsuda *abcAlthough lithium excess cation-disordered rock salt (DRX) metal oxides have been identified aspromising candidates for positive-electrode materials, their actual potential remains unclear becauseprevious studies have used inappropriate technological parameters, such as low mass loadings orexcessive amounts of electrolyte. In this study, Li2RuO3/Li2SO4 was selected as the model DRX material,and its performance was investigated under cell-level high-energy-density conditions. A highly-mass-loaded positive electrode (30 mg cm�2) with an active material ratio exceeding 96% was fabricated bysuppression of the gelation of slurry solution during the electrode preparation process, which isachieved by proper control of the particle size of Li2RuO3/Li2SO4. Notably, using a protected lithiummetal electrode setup, superior capacity of the Li2RuO3/Li2SO4 electrode over 180 mA h g�1 wasachieved over the 80th cycle under high mass loading and lean electrolyte conditions. The resultsobtained in the present study reveal the potential of the DRX based positive electrode for realizingsuperior performance even under practical cell conditions.IntroductionIn recent years, there has been a constantly growing demandfor rechargeable energy-storage devices with high energy den-sity. Lithium–metal-based (LMB) rechargeable batteries haveattracted researchers’ attention owing to their potential toachieve energy densities considerably higher than those ofconventional lithium-ion batteries. In recent studies, LMBrechargeable batteries with cell-level energy densities exceeding350 W h kg�1 have been developed using NMC811-basedpositive electrodes.1,2 To achieve high energy density at the celllevel, appropriate technological parameters should be adopted.For instance, a high-mass-loading positive electrode can beutilized to achieve high areal capacity, while the use of a leanelectrolyte and thin lithium foil can minimize the weight of cellcomponents. In addition, to realize an LMB with a cell-levelenergy density exceeding 500 W h kg�1, utilizing a high-energy-density positive electrode is crucial.3Among the positive electrode materials, lithium-rich layeredoxide materials with the formula Li1+xM1�xO2 (M = transitionmetal) have gained significant attention as active materials dueto their ability to achieve high capacities (over 250 mA h g�1)because of their cumulative cationic redox and anionic latticeoxygen redox reactions.4,5 In particular, there has been growinginterest in the class of lithium-excess cation-disordered rocksalt (DRX) metal oxides due to their potential for exhibitingsuperior capacity over 300 mA h g�1.6,7 However, in moststudies, battery performance was evaluated under a low elec-trode mass loading and/or with a cell containing excess electro-lyte, thereby limiting the actual cell level energy density.Although there is growing interest for the practical implemen-tation of LMBs equipped with DRX materials, the actualpotential of DRX for application in such high-energy-densityLMBs at the cell level remains unexplored.In the present study, we selected Li2RuO3/Li2SO4 as themodel DRX system due to the following two reasons. (i) Thiselectrode was recently reported to exhibit a capacity of over300 mA h g�1,7,8 although the performance was evaluated at lowa Research Center for Energy and Environmental Materials (GREEN), NationalInstitute for Material Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044,Japan. E-mail: matsuda.shoichi@nims.go.jpb Center for Advanced Battery Collaboration, Center for Green Research on Energyand Environmental Materials, National Institute for Materials Science (NIMS), 1-1Namiki, Tsukuba, Ibaraki 305-0044, Japanc NIMS-SoftBank Advanced Technologies Development Center, National Institute forMaterials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ya00281kReceived 15th June 2023,Accepted 7th December 2023DOI: 10.1039/d3ya00281krsc.li/energy-advancesEnergyAdvancesPAPEROpen Access Article. Published on 03 January 2024. Downloaded on 6/19/2024 4:00:28 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttps://orcid.org/0000-0002-0640-3404http://crossmark.crossref.org/dialog/?doi=10.1039/d3ya00281k&domain=pdf&date_stamp=2024-01-02https://doi.org/10.1039/d3ya00281khttps://doi.org/10.1039/d3ya00281khttps://rsc.li/energy-advanceshttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3ya00281khttps://pubs.rsc.org/en/journals/journal/YAhttps://pubs.rsc.org/en/journals/journal/YA?issueid=YA003001© 2024 The Author(s). Published by the Royal Society of Chemistry Energy Adv., 2024, 3, 248–254 |  249mass loading conditions (o10 mg cm�2). (ii) Ru based materialsare known to exhibit superior electrical conductivity comparedwith Mn based materials. The high electrical conductivity of activematerials is beneficial for decreasing the conductive additives inthe electrode, which results in the improvement of cell level energydensity, especially in the case of high mass loading conditions.Using the Li2RuO3/Li2SO4 positive electrode as the model DRXsystem, we revealed that proper control of the particle size of activematerials is a crucial factor for preparing a high mass loadingelectrode (420 mg cm�2). As a result, we successfully prepared ahigh-mass-loading electrode with an active material ratio of over96%, while avoiding the undesired gelation of the slurry solution.This resulted in a lithium–metal-based battery cell with a cell-levelenergy density of more than 500 W h kg�1.ExperimentalSynthesis of Li2RuO3/Li2SO4 powdersA series of (Li2RuO3)1�x/(Li2SO4)x powders (x = 0.10, 0.16, 0.20,0.24, and 0.34) were synthesized by a mechanochemistry synth-esis, using appropriate amounts of crystalline Li2RuO3 andLi2SO4. Crystalline Li2RuO3 was prepared from a 10% excessof Li2CO3 (99.99%; Kojundo Chemical Laboratory Co., Ltd) andRuO2 (Kojundo Chemical Laboratory Co., Ltd) via a solid-statesynthesis. The mixture was preheated in an alumina crucible at900 1C for 12 h and then calcined at 1100 1C for 12 h undernitrogen and oxygen flows at a heating rate of 5 1C min�1. Theobtained sample was ground homogeneously to obtain a well-crystallized Li2RuO3 powder material. Li2SO4�H2O (KojundoChemical Laboratory Co., Ltd) was heated at 300 1C for 3 hunder an Ar atmosphere to obtain a Li2SO4 crystal powder. Forthe synthesis of Li2RuO3/Li2SO4 positive electrode materials,stoichiometric amounts of Li2RuO3, and Li2SO4 were mixed andhomogenized by using a mortar and pestle. The mixture wasball milled for 50 h in an 80 mL zirconia pot with 5 mmdiameter zirconia balls (250 balls) of a planetary ball miller(Pulverisette 6; Fritsch) at different rotating speeds (600 rpm)and then ground well. During sample preparation, the sampleswere not exposed to the ambient atmosphere.Characterization of the Li2RuO3/Li2SO4 powderThe particle size of the powder-based sample was measured byusing a laser scattering particle size distribution analyzer (LA-950V2, HORIBA). Field-emission SEM (FE-SEM, S-4800, Hitachi)and X-ray diffraction (XRD; SmartLab, Rigaku) were used tocharacterize the powder-based samples.Preparation of the Li2RuO3/Li2SO4 positive electrodeA slurry of Li2RuO3/Li2SO4 active materials (94 wt%), acetyleneblack (Denka Black HS100; DENKA Co.; 3 wt%), and thepolyvinylidene fluoride (PVDF; KUREHA Co.; 3 wt%) binderdissolved in N-methyl-1,2-pyrrolidone (NMP; Super Dehydrated;FUJIFILM Wako Pure Chemical Co.) was coated onto an alu-minum (Al) current collector (a thickness of 10 mm). The NMPsolvent was removed by heating at 230 1C in a nitrogenatmosphere for 30 minutes, and the electrode sheets wereobtained. The loading amount of the active materials was about30 mg cm�2.Electrochemical measurements1 M Lithium bis(fluorosulfonyl)imide (LiFSI; Kishida Chemical Co.,Ltd, purity 4 99.0%) dissolved in sulfolane (Kishida Chemical Co.,Ltd, purity 4 99%) and 4M LiFSI in 1,2-dimethoxyethane (DME;Kishida Chemical Co., Ltd, purity 4 99.5%) were used as electro-lytes. A 100 mm thick lithium foil attached with a 12 mm thickcopper current collector and a 20 mm thick lithium foil attachedwith a 2 mm thick copper current collector were used as the negativeelectrode for cell A and cell B, respectively. For fabrication of cell A,the positive electrode (20 mm � 20 mm), a PO separator (20 mmthickness, 22 mm� 22 mm), a ceramic separator (90 mm thickness,24 mm � 24 mm), a PO separator (22 mm � 22 mm) and thenegative electrode (42 mm � 32 mm) were stacked. In such aconfiguration, the lithium metal was completely sealed by aceramic-based solid-state separator and laminated film, whichcompletely isolated the electrolyte on the positive and negativeelectrode sides. 60 mg of 1M LiFSI in suflolane electrolyte(15 mg cm�2) was injected for the positive electrode side and10 mg of the 4M LiFSI in the DME electrolyte (2.5 mg cm�2) wasinjected for the negative electrode side. For fabrication of cell B, thepositive electrode (40 mm � 30 mm), separator (46 mm � 36 mm)and the negative electrode (42 mm� 32 mm) were stacked inside alaminated film and sides of the stack were sealed. 120 mg of theelectrolyte (10 mg cm�2) was injected before sealing the cell undervacuum. All the cells were fabricated inside a dry room (dew pointo�50 1C) and electrolyte injection was carried out inside a fumehood (dew point o�85 1C). Charge and discharge of the cells werecarried out with Hokuto Denko HJ1001SD8. All the cells were cycledat a constant current density of 0.6 mA cm�2 in the voltage range of2–4.2 V.In situ MS analysisFor on-line MS analysis, the generated gases were directed tothe MS detector by the Canon Anelva Quadrupole Mass Spectro-meter M-401GA-DM equipped with a capillary tube (internaldiameter: 0.05 mm, length: 7 m). After discharge, the test cellwas purged with excess He (50 mL min�1) for 1 min to removethe remaining O2. He as a carrier gas was flown at 5 mL min�1and maintained for 2 h before charge. The measurement wascarried out at 100 mV applied voltage, in the m/z range from 11to 110 under ambient conditions.XCT analysisX-ray CT analyses of the pouch cells were carried out using aXradia 520 Versa (ZEISS, Germany) instrument, where thesource voltage and power were 140 kV and 10 W, respectively.The cells after certain charge/discharge cycles were mounted onthe sample holder, and the cell was rotated 3601 for 4501 scanswith an exposure time of 10 s. The pixel resolution of XCTimaging was 3.385 mm.Paper Energy AdvancesOpen Access Article. Published on 03 January 2024. Downloaded on 6/19/2024 4:00:28 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3ya00281k250 |  Energy Adv., 2024, 3, 248–254 © 2024 The Author(s). Published by the Royal Society of ChemistryResults and discussionFirst, we considered how energy density of LMBs is affected bythe technological parameters, such as the electrolyte amountand the mass loading of the positive electrode. The parametersof the LMB components used in our simulation are listed inTables S1 (ESI†), and the simulated energy densities of LMBsare listed in Table S2 (ESI†). Here, we set the capacity of thepositive electrode to 300 mA h g�1 and an average dischargevoltage of 3.1 V as the model case of the DRX material. Here, theporosity of the positive electrode was set as 33% and the electro-lyte amount was estimated for fully filling the pore volume in thepositive electrode and separator. When a glass fiber separator(thickness = 100 mm, porosity = 91%) was employed, the cell levelenergy density was less than 100 W h kg�1 even when themass loading of the active material in the positive electrode was10 mg cm�2. Although the amount of electrolyte in the positiveelectrode is small (1.7 mg cm�2), a large amount of electrolyte(up to 24 mg cm�2) was required to fully fill the pore space in theseparator. As a result, the electrolyte accounted for 50% ofthe total weight (Fig. 1a), which resulted in low energy density.When a polyolefin-based separator (thickness = 20 mm, porosity =46%) commonly used in commercial lithium-ion batteries wasemployed, the energy density was 230 W h kg�1, as the electrolyteloading decreased to 0.9 mg cm�2 (Fig. 1b). When the massloading increases from 10 to 30 mg cm�2, the energy densityexceeded over 500 W h kg�1 (Fig. 1c). Under such high energydensity conditions, the positive electrode accounted for more than60% of the total weight. We also performed similar energy densitysimulations for the NMC811 based positive electrode materialunder the same conditions as those used for the case of DRX.The results revealed that cell level energy density of the LMBequipped with the NMC based positive electrode is lower than500 W h kg�1 (Table S3, ESI†), revealing the importance of usinghigh-capacity positive electrode materials for realizing high energydensity LMBs.The results of the above simulations revealed the impor-tance of decreasing the electrolyte amount and increasingthe mass loading of the positive electrode for maximizing theenergy density of LMBs. In particular, preparation of a highmass loading DRX-based positive electrode is crucial.A series of (Li2RuO3)1�x/(Li2SO4)x powders (x = 0.10, 0.16,0.20, 0.24, and 0.34) were synthesized via a mechanochemicalmethod by mixing Li2RuO3 and Li2SO4 in appropriate propor-tions. Li2RuO3 was synthesized using a solid-state method,9while Li2SO4 was dried at 300 1C before use. Details of thesynthesis procedure can be found in the Experimental section.The XRD patterns of the samples are shown in Fig. S4 (ESI†),which indicate the presence of clear peaks corresponding to theNaCl-type cation-disordered phase, with no clear peaks assign-able to the precursor compound of Li2RuO3 or Li2SO4.Fig. 2 shows the SEM images of a series of Li2RuO3/Li2SO4samples. For the pristine Li2RuO3 sample, 10 um sized particlescan be seen (Fig. 2a). In contrast, the size of mechanically treatedLi2RuO3 was hundred nanometer. Notably, SEM observationsrevealed that an increase in the Li2SO4 concentration was asso-ciated with an increase in the Li2RuO3/Li2SO4 particle size (Fig. 2).The particle size distribution was also evaluated, and Fig. S5 (ESI†)shows a histogram of the particle size distributions of pristineLi2RuO3 and a series of Li2RuO3/Li2SO4 samples. The averageparticle size of the layered Li2RuO3 precursor was found tobe 9 mm, whereas the mechanochemically treated Li2RuO3 witha cation disordered rock-salt structure had an average particlesize of less than 1 mm. With increasing Li2SO4 concentration, anincrease in the average particle size was observed. For sampleswith Li2SO4 ratios of 0.10 and 0.16, the particle sizes were in therange of 0.1 to 0.3 mm, while in samples with Li2SO4 ratios of 0.20,0.24, and 0.34, most particles were larger than 0.3 mm.Fig. 1 Weight fractions of LMB components calculated using the parameters listed in Table S1 (ESI†). (a) Glass fiber separator, a mass loading of 10 mgcm�2, (b) polyolefin separator, a mass loading of 10 mg cm�2, and (c) polyolefin separator, a mass loading of 30 mg cm�2.Energy Advances PaperOpen Access Article. Published on 03 January 2024. Downloaded on 6/19/2024 4:00:28 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3ya00281k© 2024 The Author(s). Published by the Royal Society of Chemistry Energy Adv., 2024, 3, 248–254 |  251Next, high-mass-loading electrodes were fabricated usingprepared Li2RuO3/Li2SO4 samples. A slurry solution was pre-pared by mixing 94.4 wt% Li2RuO3/Li2SO4 powder, 0.4 wt%CB, 0.2 wt% CNT, and 5 wt% PVDF binders. For the caseof Li2RuO3/Li2SO4 with Li2SO4 ratios of 0.2, 0.24, and 0.34,uniform electrodes with mass loadings over 20 mg cm�2 weresuccessfully prepared (Fig. 3a). Fig. S6 (ESI†) displays a repre-sentative cross-sectional SEM image of a Li2RuO3/Li2SO4 elec-trode, showing a uniform distribution of O, F, S, and Ru inthe electrode, indicating that the active material, conductivecarbon, and binder were well mixed (Fig. 3b). In contrast, slurrygelation occurred in the case of Li2RuO3/Li2SO4 with Li2SO4ratios of 0, 0.1, and 0.16 (Fig. 3c). Considering the fact that nogelation occurred during the slurry preparation process usingLi2RuO3/Li2SO4 with Li2SO4 ratios of 0.2, 0.24, and 0.34, thegelation observed in Li2RuO3/Li2SO4 with Li2SO4 ratios of 0, 0.1,and 0.16 was originated from their high surface area character-istics due to their smaller particle sizes (o0.3 mm).To evaluate the battery performance, electrochemical cellswere fabricated using Li2RuO3/Li2SO4 electrodes with Li2SO4ratios of 0.2 and 0.34. To accurately evaluate the performance ofthe positive electrode, we used a relatively thick 100 mm lithiumfoil as the negative electrode. In addition, a ceramic-basedsolid-state separator sandwiched between two pieces of a PO-based separator was used to prevent undesired side reactions atthe lithium metal electrode. The lithium metal was completelysealed by a ceramic-based solid-state separator and a laminatedfilm, which completely isolated the electrolyte on the positiveand negative electrode sides. A solution of 4 M LiFSI dissolvedin DME was used as the electrolyte on the negative electrodeside due to its compatibility with the lithium metal electrode.10Furthermore, 1 M LiFSI in sulfolane was selected as the positiveelectrolyte because of its high oxidative stability. The details ofcell components are described in Table S7 (ESI†). The ratio of theelectrolyte weight to areal capacity (E/C, g A�1 h�1) is used as anempirical parameter of the electrolyte amount in the field ofLiBs. For the cell A fabricated in the present study, the value of E/C was 2.95 g A�1 h�1. Such a low value of E/C was realized byadopting a high mass loading positive electrode (20 mg cm�2)and a thin separator (20 mm thickness).Fig. 4 shows the charge/discharge profile of a cell withLi2RuO3/Li2SO4 (x = 0.20) as the positive electrode. The cellshowed a gradual increase in voltage from 3.3 V to 4.0 V (black curvein Fig. 4a) during the first charging process, eventually reaching thecutoff voltage of 4.2 V with a capacity of 260 mA h g�1. Uponswitching to discharge, the voltage initially remained around4.0 V and then decreased gradually, reaching the cutoff voltageof 2.0 V with a capacity of 300 mA h g�1. This voltage profile isconsistent with those reported in the literature. In situ MSanalysis during the charging process revealed that the oxygenevolution started at a capacity of 225 mA h g�1, correspondingto a charging voltage of 4.0 V (black curve in Fig. 4b). Incontrast, no significant CO2 evolution was observed (blackcurve in Fig. 4c). The cell with Li2RuO3/Li2SO4 (x = 0.34)Fig. 2 SEM-EDS images of (a) and (b) pristine Li2RuO3 and (c)–(h) Li2RuO3/Li2SO4 samples. Scale bars are (a) 10 mm and (b)–(h) 1 mm.Fig. 3 (a) Photographic image of the electrode of Li2RuO3/Li2SO4 with a Li2SO4 ratio of 0.24 and with a mass loading of 20 mg cm�2. (b) and (c)Photographic image of slurry solution of Li2RuO3/Li2SO4 with Li2SO4 ratios of (b) 0.24 and (c) 0.1.Paper Energy AdvancesOpen Access Article. Published on 03 January 2024. Downloaded on 6/19/2024 4:00:28 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3ya00281k252 |  Energy Adv., 2024, 3, 248–254 © 2024 The Author(s). Published by the Royal Society of Chemistryexhibited a slightly higher charging voltage than the cell withLi2RuO3/Li2SO4 (x = 0.2) (blue curve in Fig. 4a). During dis-charge, the voltage profile was similar to that observed forLi2RuO3/Li2SO4 (x = 0.2), but the cell exhibited a dischargecapacity of 260 mA h g�1. In situ MS analysis of the gasgenerated during charging revealed that O2 generation startedat a capacity of 210 mA h g�1, corresponding to a chargingvoltage of 4.0 V. The generation of CO2 began at a similar time.After the first charge/discharge process, both cells were sub-jected to repeated cycling. Fig. 4d shows the voltage profileduring the second charge/discharge process, revealing that thecell exhibited a discharge capacity of 270 mA h g�1 for bothLi2RuO3/Li2SO4 (x = 0.2) and Li2RuO3/Li2SO4 (x = 0.34). More-over, no significant O2 or CO2 generation was observed (blackcurves in Fig. 4b and d), indicating limited side reactions in thissystem. These results clearly revealed that Li2RuO3/Li2SO4 (x =0.2) exhibited higher capacity in the charging process and alsogenerated less gas compared with Li2RuO3/Li2SO4 (x = 0.34).Therefore, in the following, we examined the details of Li2RuO3/Li2SO4 (x = 0.2).We conducted an extended long-cycle test on an electroche-mical cell (named cell A). As shown in Fig. 5a, the capacitygradually decreased over repeated cycles. Fig. 5c displays thedischarge capacity plotted against the cycle life. During the10th cycle, the capacity decreased rapidly. Subsequently, therewas a gradual decrease in the capacity with the progress of thecycles, until it reached 180 mA h g�1 for the 30th cycle, afterwhich the capacity remained stable even up to the 80th cycle.Fig. 4 (a) and (c) Charge/discharge profile for 1st and 2nd cycle; (b) and (d) generated gas profile in the (b)1st and (d) 2nd cycles in an electrochemicalcell equipped with Li2RuO3/Li2SO4 samples (black curve: x = 0.2 and blue curve: x = 0.34) as positive electrodes.Fig. 5 (a) and (b) Charge/discharge profile of the electrochemical cell equipped with the Li2RuO3/Li2SO4 sample (x = 0.2) as the positive electrode: (a)cell A and (b) cell B. (c) Discharge capacity plotted against the cycle number for cell A (black curve) and cell B (red curve).Energy Advances PaperOpen Access Article. Published on 03 January 2024. Downloaded on 6/19/2024 4:00:28 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3ya00281k© 2024 The Author(s). Published by the Royal Society of Chemistry Energy Adv., 2024, 3, 248–254 |  253These results indicate that Li2RuO3/Li2SO4 (x = 0.2) exhibitedsuperior performance even under high mass loading and leanelectrolyte conditions.Subsequently, a high-energy-density battery cell (cell B) wasfabricated by utilizing suitable technological parameters, (i)increasing the mass loading of positive electrode (30 mg cm�2),(ii) removing ceramic-based separator, and (iii) replacing thinlithium foil (20 mm thickness). As result, the E/C value of cell Bwas 1.28 g A�1 h�1. Details of the fabricated cells are listed inTable S1 (ESI†). Fig. 5b shows the repeated charge/dischargeprofiles. During the initial charging, cell B exhibited acapacity of 220 mA h g�1, which was lower than that of cell A(260 mA h g�1). However, during the overall charging process,cell B exhibited a full capacity of 260 mA h g�1 with an averagedischarge voltage of 3.1 V. As a result, the energy density at thecell level reached 512 W h kg�1. Cell B exhibited a smallerdischarge capacity compared to cell A. It is believed that thiscould be attributed to the limited amount of electrolyte presentin the positive electrode. Upon repeating the charge/dischargecycles, the capacity of cell B further decreased. In the 20th cycle,cell B exhibited a discharge capacity of 230 mA h g�1 with anaverage discharge voltage of 3.1 V, resulting in a cell-levelenergy density of 400 W h kg�1. Subsequently, the dischargecapacity decreased linearly as the cycles progressed (redplots in Fig. 5c). Although the capacity of cell A was stable at180 mA h g�1 after the 40th cycle, the capacity of cell B keptdecreasing even after 40 cycles. In the voltage profile of cell B, asudden voltage drop was observed at the end of the 40thdischarge cycle, which was significantly different from that ofcell A. It should be noted that the average discharge voltage ofboth cell A and cell B was around 3.1–3.0 V thoughout the cycles.We performed the SEM analysis of the positive electrodesafter the 40th cycle of the cell. No clear difference was observedbetween the electrodes before and after the cycle test in bothcells A and cell B (Fig. S8, ESI†). The results of XRD analysisrevealed that intensity of peaks corresponding to the NaCl-typecation-disordered phase decreased with the progress of thecycle (Fig. S9, ESI†), suggesting the deterioration of positiveelectrode materials.In order to explain the poor capacity retention of cell B, thedifference in the cell configuration between cells A and B mustbe considered. The observed severe capacity fading phenom-enon in cell B could be attributed to the degradation of thelithium metal electrode. Thus, we conducted an analysis of thelithium metal electrode post cycling. However, during the celldisassembly process, the electrode easily collapsed and couldnot be analyzed using conventional ex situ techniques such asSEM and XRD. Hence, we performed XCT analysis of the cell,which is a representative non-destructive analytical techniquefor monitoring the structural changes in the electrode.11 Fig. 6ashows the side-view XCT image of cell B under the preparedconditions. There can be seen the Li2RuO3/Li2SO4-based posi-tive electrode with a thickness of 150 mm and a lithium metalnegative electrode with a thickness of 20 mm. Fig. 6b shows theXCT image of the cell after the 10th cycle, revealing a largevolume change in the lithium metal electrode. Although thethickness of the Li2RuO3/Li2SO4-based positive electrode didnot change, that of the lithium–metal-based negative electrodeexceeded 120 mm, which is six times greater than the initialthickness (20 mm). After the 20th cycle, the thickness ofthe lithium–metal electrode reached 150 mm. Fig. 6c showsthe cross-sectional XCT image of the lithium electrode after the20th cycle. Dense lithium metal remained in the region close tothe copper foil, which is shown in Fig. 6d and e. On the otherhand, a porous structure was detected in the central area and inthe region close to the separator side (Fig. 6f and g). A similarFig. 6 (a)–(c) X-ray CT images of cell B; (a) as prepared, (b) after the 10th cycle, and (c) after the 20th cycle in the cross-sectional direction; (d)–(g) X-rayCT images of cell B after the 20th cycle in the surface direction. Scale bars are 100 mm.Paper Energy AdvancesOpen Access Article. Published on 03 January 2024. Downloaded on 6/19/2024 4:00:28 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d3ya00281k254 |  Energy Adv., 2024, 3, 248–254 © 2024 The Author(s). Published by the Royal Society of Chemistryporous Li electrode has been reported in the literature.11However, such porous characteristics can accelerate the for-mation of electrochemically isolated dead lithium, resulting ina poor reaction efficiency at the lithium negative electrode.Moreover, the electrolyte is absorbed into the porous lithiumelectrode, resulting in a shortage of electrolytes in the entirecell. Further detailed analyses of the electrodes in high-energy-density lithium–metal-based rechargeable battery cells are cur-rently underway in our laboratory.ConclusionsIn this study, we used a Li2RuO3/Li2SO4-based positive elec-trode as the model material for lithium–metal-based recharge-able batteries. By controlling the particle size of Li2RuO3/Li2SO4by varying the Li2SO4 ratio, a high-mass-loading electrode withan active material ratio of over 96% was successfully preparedby suppressing undesired gelation of the slurry solution.Notably, using a protected lithium metal electrode setup, super-ior capacity of the Li2RuO3/Li2SO4 electrode over 180 mA h g�1was achieved over the 80th cycle under high mass loading andlean electrolyte conditions. We also fabricated a high-energy-density battery cell, exhibiting energy density over 500 W h kg�1and a stable charge/discharge reaction. However, as cyclingprogressed, the cell capacity decreased rapidly, causing theenergy density of the cell to drop to 400 W h kg�1 by the 20thcycle. Non-destructive XCT analysis revealed a significantvolume expansion of the lithium electrode, which is consideredto be the primary reason for the capacity fading. Importantly,not the reaction at the DRX positive electrode, but the reactionat the lithium metal negative electrode is the bottleneck pro-cess for achieving a prolonged cycle life. Similar problems canexist for the case in which Mn-based DRX electrode materialswere used, which is more attractive from a practical point ofview. The results obtained in the present study highlight theimportance of suppressing the significant volume change oflithium–metal electrodes, as well as the development of DRXmaterials, in order to achieve a prolonged cycle life in highenergy density rechargeable batteries.Author contributionsS. M. supervised the project. J. S. and E. M. performed experi-ments. All authors contributed for data analysis and discus-sion. J. S. and S. M. wrote the manuscript.Conflicts of interestThere are no conflicts to declare.AcknowledgementsA part of this work was carried out at the SoftBank-NIMSAdvanced Technologies Development Center as a joint researchbetween NIMS and SoftBank Corp. This work also receivedsupport from the National Institute for Materials Science(NIMS) Battery Research Platform.Notes and references1 C. Niu, H. Lee, S. Chen, Q. Li, J. Du, W. Xu, J.-G. Zhang,M. S. Whittingham, J. Xiao and J. Liu, High-energy lithiummetal pouch cells with limited anode swelling and longstable cycles, Nat. Energy, 2019, 4(7), 551–559.2 C. Niu, D. Liu, J. A. Lochala, C. S. Anderson, X. Cao,M. E. Gross, W. Xu, J. G. Zhang, M. S. Whittingham,J. Xiao and J. Liu, Balancing interfacial reactions to achievelong cycle life in high-energy lithium metal batteries, Nat.Energy, 2021, 6(7), 723–732.3 J. Liu, Z. Bao, Y. Cui, E. J. Dufek, J. B. Goodenough, P. Khalifah,Q. Li, B. Y. Liaw, P. Liu, A. Manthiram, Y. S. Meng,V. R. Subramanian, M. F. 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