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Wei Yu, Zhaohan Shen, Takeharu Yoshii, Shinichiroh Iwamura, [Manai Ono](https://orcid.org/0000-0003-4406-4113), [Shoichi Matsuda](https://orcid.org/0000-0002-0640-3404), Makoto Aoki, Toshihiro Kondo, Shin R. Mukai, Shuji Nakanishi, Hirotomo Nishihara

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[Hierarchically Porous and Minimally Stacked Graphene Cathodes for High‐Performance Lithium–Oxygen Batteries](https://mdr.nims.go.jp/datasets/7c63aa99-16fe-4553-8458-cf4a9aedf24a)

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Hierarchically Porous and Minimally Stacked Graphene Cathodes for High‐Performance Lithium–Oxygen BatteriesRESEARCH ARTICLEwww.advenergymat.deHierarchically Porous and Minimally Stacked GrapheneCathodes for High-Performance Lithium–Oxygen BatteriesWei Yu,* Zhaohan Shen, Takeharu Yoshii, Shinichiroh Iwamura, Manai Ono,Shoichi Matsuda, Makoto Aoki, Toshihiro Kondo, Shin R. Mukai, Shuji Nakanishi,and Hirotomo Nishihara*Although lithium–oxygen batteries have attracted attention due to theirextremely high energy densities, rational design, and critical evaluation ofhigh-energy-density cathode for practical Li–O2 batteries is still urgentlyneeded. Herein, the multiscale, angstrom-to-millimeter, precisely controllablesynthesis of binder-free cathodes with minimally stacked graphene free fromedge sites is demonstrated. The proposed Li–O2 battery, based on ahierarchically porous cathode with a practical mass loading of >4.0 mg cm−2,simultaneously exhibits an unprecedented specific areal (>30.0 mAh cm−2),mass (>6300 mAh g−1), and volumetric (>480 mAh cm−3) capacities. Thebattery displays the optimal energy density of 793 Wh kg−1 criticallynormalized to the total mass of all active materials including electrolytes andeven discharge products Li2O2. Comprehensive in situ characterizationsdemonstrate a unique discharge mechanism in hierarchical pores whichcontributes to competitive battery performance. Superior rate performance ina current density range of 0.1 to 0.8 mA cm−2 and long-cycle stability (>260cycles) at a current density of 0.4 mA cm−2, outperforming state-of-the-artcarbon cathodes. This study yields insight into next-generation carboncathodes, not only for use in practical Li–O2 batteries, but also in othermetal–gas batteries with high energy densities.W. Yu, S. Iwamura, H. NishiharaAdvanced Institute for Materials Research (WPI-AIMR)Tohoku UniversitySendai 980-8577, JapanE-mail: yu.wei.a3@tohoku.ac.jp; hirotomo.nishihara.b1@tohoku.ac.jpZ. Shen, T. Yoshii, H. NishiharaInstitute of Multidisciplinary Research for Advanced MaterialsTohoku UniversitySendai 980-8577, JapanS. Iwamura3DC Inc.Sendai 980-8577, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/aenm.202303055© 2023 The Authors. Advanced Energy Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivs License,which permits use and distribution in any medium, provided the originalwork is properly cited, the use is non-commercial and no modificationsor adaptations are made.DOI: 10.1002/aenm.2023030551. IntroductionNext-generation energy storage andconversion technologies are urgentlyrequired to satisfy development goalsvia large-scale power grids, electricvehicles, and portable electronics.[1]Lithium–oxygen (Li–O2) batteries attractconsiderable attention because of theirhigh theoretical energy densities.[2] ALi–O2 battery typically comprises a Limetal anode, liquid/solid electrolyte,and porous cathode. Several types ofdischarge products exist, such as lithiumsuperoxide (LiO2),[3] lithium hydroxide(LiOH),[4] and lithium oxide (Li2O),[5]but the most typical discharge product innon-aqueous Li–O2 batteries is lithiumperoxide (Li2O2). An extremely highenergy density (3500 Wh kg−1) maybe re-alized via the oxygen reduction/evolutionreactions corresponding to Li2O2 forma-tion/decomposition during dischargingand charging.[6] However, the sluggishM. Ono, S. MatsudaCenter for Green Research on Energy and Environmental MaterialsNational Institute for Material ScienceTsukuba, Ibaraki 305-0044, JapanS. MatsudaNIMS-SoftBank Advanced Technologies Development CenterNational Institute for Material ScienceTsukuba, Ibaraki 305-0044, JapanM. Aoki, T. KondoGraduate School of Humanities and SciencesOchanomizu UniversityTokyo 112-8610, JapanS. R. MukaiFaculty of EngineeringHokkaido UniversitySapporo 060-6828, JapanS. NakanishiResearch Center for Solar Energy ChemistryGraduate School of Engineering ScienceOsaka UniversityToyonaka, Osaka 560-8531, JapanAdv. Energy Mater. 2024, 14, 2303055 2303055 (1 of 10) © 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbHhttp://www.advenergymat.demailto:yu.wei.a3@tohoku.ac.jpmailto:hirotomo.nishihara.b1@tohoku.ac.jphttps://doi.org/10.1002/aenm.202303055http://creativecommons.org/licenses/by-nc-nd/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Faenm.202303055&domain=pdf&date_stamp=2023-11-10www.advancedsciencenews.com www.advenergymat.dekinetics of the formation/decomposition of electrically insu-lated Li2O2 result in a large overpotential, low capacity, andlimited cycle life.[7] Recently, significant efforts were directedtoward investigating the reaction mechanism.[8] Considerableprogress has been reported in reducing the overpotential andextending the battery life by using protected Li anodes,[9] stableelectrolytes,[10] efficient solid/soluble catalysts,[11] and robustporous cathodes.[12] However, few studies focus on energydensity improvements using practical cathodes with largemass loadings (>4.0 mg cm−2),[13] a crucial aspect in com-mercializing Li–O2 batteries with high device energy densities(>500 Wh kg−1).[14]As the formation, storage, and decomposition of Li2O2 onthe cathode directly determine the battery’s specific capacity anddischarge/charge potential of the battery, a well-designed cath-ode is crucial in a practical Li–O2 battery.[15] Carbon materi-als are commonly used as cathodes in Li–O2 batteries owing totheir relatively large surface areas, high conductivities, and abun-dant pores. However, the areal densities of carbon cathodes inmost previous studies are <1.0 mg cm−2.[16] Even if the batteryreaches a very high mass capacity of >10 000 mAh g−1, the ac-tual areal capacity is still less than the target areal capacity of12 mAh cm−2, which is required for a low cost per usable en-ergy (<$100 per kWh).[13b,14] A thick carbon cathode (>500 μm)with a high areal loading (>4 mg cm−2) is necessary for a highareal capacity (>12 mAh cm−2). However, the specific mass ca-pacity is inversely related to the thickness of the carbon cathodebecause of the significant internal resistance and poor Li+ and O2mass transportation.[17] An ultra-thick cathode (>1 cm) generallyexhibits a low volumetric capacity (<200 mAh cm−3),[18] whichis rarely discussed in previous studies. The challenge in prepar-ing carbon cathodes with satisfactory performances is due to thestrong anisotropy of graphene, which is the primary unit of car-bon materials hindering precise structural control over a widesize scale from angstroms to sub-millimeters.[19] Therefore, tomeet the criteria of an ideal carbon (Figure 1a) for a high en-ergy density Li–O2 battery, the following characteristics shouldbe achieved: 1) Minimally stacked graphene walls, which providemore active sites and higher specific capacity in grams; 2) Hier-archical structure, in which the micro/mesopores promote themass transfer of Li+ and the nucleation of intermediates (LiO2),and the macropores benefit the mass transfer of O2 and thegrowth of discharge products; 3) Free of carbon edge sites andorganic binder, which improves the stability of carbon cathodes.Herein, we propose a new concept for ideal carbon cathodedesign: a hierarchical porous membrane composing graphenewalls without stacking or carbon edge sites prepared via chemicalvapor deposition (CVD), followed by high-temperature anneal-ing. We fabricated a prototype cathode by integrating graphenemesosponge (GMS)[20] into a binder-free sheet, with the prod-uct denoted GMS-sheet. The multiscale-controllable GMS-sheetwith an areal mass (5.0 mg cm−2) exhibited extremely highS. NakanishiInnovative Catalysis Science DivisionInstitute for Open and Transdisciplinary Research Initiatives (ICS-OTRI)Osaka UniversitySuita, Osaka 565-0871, Japanareal, mass, and volumetric capacities in Li–O2 batteries, cor-responding to a remarkable energy density of 793 Wh kg−1based on all active materials. Moreover, the cycle performancewas stable under a current density of 0.4 mA cm−2 and alimited capacity of 4.0 mAh cm−2. Therefore, the GMS-sheetis a promising cathode for high-energy-density Li–O2 batter-ies and other sustainable energy storage devices, such asNa, K, and Zn–air batteries for application in a post Li-ionbattery era.[21]2. Results and Discussion2.1. Synthesizing and Optimizing Hierarchically Porous CarbonCathodesFigure 1b illustrates the synthesis of a hierarchically porousGMS-sheet cathode with graphene walls without significantstacking or edge sites. Al2O3 powder with an average parti-cle size of 7 nm (Figure S1, Supporting Information) is pel-letized using a graphite mold (Figure S2, Supporting Infor-mation) at 100 N (0.75 MPa) for 5 s (Figure 1c). Then, as-prepared white Al2O3-sheets (ϕ 13.0 mm, Figure 1d, Left) aresubjected to CVD, using CH4 as the carbon source, at 900 °Cto coat the surfaces of the Al2O3 nanoparticles with an ex-tremely thin carbon layer.[20] The graphene stacking can be pre-cisely controlled by the specific catalysis of oxygen vacanciesgenerated on the surface of the oxide template at the initial insitu catalyst-activation step.[22] As carbon-coated Al2O3 can beconsidered a composite of carbon mesosponge (CMS)[20] andAl2O3, the resulting black samples (Figure 1d, Middle) are de-noted CMS-Al2O3-sheets. As we pelletized the Al2O3 nanopar-ticles to form a 3D framework on the millimeter scale, thegraphene-wall framework loaded on the Al2O3 nanoparticlesis also spread over the millimeter scale. The thicknesses andporosities of the Al2O3-sheets could be easily controlled viapelletization forces in the range 50–10000 N (Figure S3 andTable S1, Supporting Information). The carbon loading onthe CMS-Al2O3-sheets, as measured via thermogravimetry (TG,Figure S4, Supporting Information), is ≈28 wt.%, correspond-ing to an average graphene-stacking number of 2.8.[20] Pure-carbon CMS-sheets are prepared by removing the Al2O3 tem-plate via HF etching, based on the observation of only broadcarbon 002 and 10 peaks in the X-ray diffraction (XRD) pat-tern (Figure S5, Supporting Information). Finally, a binder-freeGMS-sheet (Figure 1d, Right) is fabricated via high-temperatureannealing (1800 °C) to remove the edge sites and improve thequalities of the graphene frameworks (Figure S6, SupportingInformation).The structure of the GMS-sheets can be controlled at theangstrom (graphene-stacking number and carbon edge sites),nanometer (spherical mesopores derived from Al2O3 nanopar-ticles), micrometer (microporosity controlled by a pelletizationforce), and millimeter (thickness and diameter of a GMS-sheet)scales (Figure 1b). Accordingly, the crucial synthetic parame-ters are the pelletization force (x [N]), amount of Al2O3 tem-plate (y [mg]), and duration of CVD (z [h]), and the GMS-sheetssynthesized under different conditions are denoted as xN-ymg-zh-GMS-sheets. As shown in the cross-sectional scanning elec-tron microscopy (SEM) image (Figure 2a), the 100N-20mg-4.0h-Adv. Energy Mater. 2024, 14, 2303055 2303055 (2 of 10) © 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202303055 by National Institute For, Wiley Online Library on [18/06/2024]. 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.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deFigure 1. Schematic diagram and synthesis of the ideal carbon cathode. a) Radar chart of ideal carbon cathode requirements and state-of-the-art achieve-ments. b) Schematic illustration of GMS-sheet synthesis process. c) Schematic of the Al2O3 pelletization process. A graphite mold with an inner diameterof 13.2 mm and two graphite rods with a diameter of 13.0 mm were used. d) Digital photos of Al2O3-sheets before CVD, CMS-Al2O3-sheets after CVD,and a free-standing GMS-sheet after the HF and high-temperature treatments.Figure 2. Structural controllability of the GMS-sheet. a—c) Micrographs of the 100N-20mg-4.0h-GMS-sheet. Cross–sectional SEM images at a) lowand b) high magnifications. c) A TEM image. d) Total porosities of GMS-sheets synthesized under different pelletization forces varied from 50 to10 000 N. The same Al2O3 amount (20 mg) and duration of CVD (4.0 h) were used in preparing the GMS-sheets. e) Relationship between the CVDdurations and average graphene-stacking numbers of the GMS-sheets prepared using different amounts of Al2O3. f) Specific surface areas and VN2values of GMS-sheets shown in e. VN2 is the sum of the micro- and mesopore volumes calculated from P/P0 = 0.96, as shown in Figure S12 (SupportingInformation).Adv. Energy Mater. 2024, 14, 2303055 2303055 (3 of 10) © 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202303055 by National Institute For, Wiley Online Library on [18/06/2024]. 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.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deGMS-sheet is a uniform self-standing membrane with a thick-ness of ≈435 μm. The thickness, which depends on x [N] andy [mg], is controllable between 127 and 637 μm (Figures S7and S8, Supporting Information). The high-magnification SEMimage (Figure 2b) shows the presence of macropores betweenthe aggregates of the spherical nanoparticles. In comparison,the transmission electron microscopy (TEM) image (Figure 2c)shows a nanobubble-like mesoporous framework derived fromthe Al2O3 template (Figure S1, Supporting Information). The de-veloped meso- and macropores can also be confirmed based onN2 ad-/desorption isotherms (Figure S9, Supporting Informa-tion), showing a large hysteresis and the successive N2 uptakeabove P/P0 = 0.9. The xN-20mg-4.0h-GMS-sheets display simi-lar N2 adsorption isotherms at P/P0<0.8, mesoporous size dis-tributions of <10 nm, and the Brunauer–Emmett–Teller (BET)specific surface areas (SSAs) of ≈1200 m2 g−1 (Figure S10 andTable S2, Supporting Information). These indicate uniform car-bon coatings formed on the Al2O3 nanoparticles via catalyticCVD without severe restriction of CH4 diffusion within thetemplate pellets, regardless of the pellet densities. The totalporosity (the sum of the micro-, meso-, and macroporosities)of the xN-20mg-4.0h-GMS-sheet, as calculated based on theareal density and thickness, is inversely related to the pelleti-zation force (Figure 2d). This suggested that the pelletizationforce may mainly control the macroporosity, which cannot be de-tected via N2 adsorption measurements. However, the mechan-ical strength of the GMS-sheets weaken when the pelletizationforce is smaller. As the 50 N-20 mg-4.0 h-GMS-sheet is not suf-ficiently rigid, we used the optimal pelletization force of 100 Nin the subsequent experiments and further optimized the otherparameters.Generally, a carbon cathode with a large SSA may providemore active sites for Li2O2 formation, which is favorable in re-alizing a large capacity.[15] Thus, we attempted to increase theSSA by decreasing the duration of CVD, which is a key syn-thetic parameter in determining the average graphene-stackingnumber. As shown in Figure 2e,f, a CVD duration of 2 h pro-vides a high BET SSA of ≈2000 m2 g−1. The corresponding aver-age graphene-stacking number is 1.5, based on the TG analysis(Figure S11, Supporting Information). Such a BET SSA is com-parable to that of high-performance activated carbon,[20] whereasthe ultra-high pore volume (>4.0 cm3 g−1) is distinct. Moreover,the areal densities and the thicknesses of the GMS-sheets canbe increased (Figure S8, Supporting Information) while main-taining identical micro-/mesoporosity (Figure 2f, Figure S12,Supporting Information), demonstrating the flexibility of thestructure control from the nano- to the micrometer scale. Wehave developed an advanced temperature-programmed desorp-tion (TPD) analysis up to 1800 °C that can accurately quan-tify the number of edge sites in carbon materials using the to-tal amount of H2O, CO, CO2, and H2 as an index.[23] Basedon the edge-site-free property of the GMS-sheet, as confirmedvia TPD analysis (Figure S13, Supporting Information), thehigh SSA of the GMS-sheet is mainly due to the graphenebasal planes rather than the edge sites, which destabilize thecarbon material in electrochemical systems. [24] The edge-site-free GMS-sheet with a high SSA may thus exhibit a superiorstability.2.2. Specific Capacities and Energy Densities of the GMS-SheetsCyclic voltammetry (CV) was performed with a stepwise expan-sion of the upper limit potential from 3.2 to 4.8 V (vs Li/Li+) in0.5 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dis-solved in tetraethylene glycol dimethyl ether (TEGDME), denoted500 electrolyte, to evaluate the electrochemical stability of theGMS-sheet. We examined the oxidation onset potential at which anoticeable current increase is observed. As shown in Figure S14(Supporting Information) the GMS-sheet exhibits an onset po-tential of up to 4.6 V, which is higher than those of the CMS-sheetand a reference carbon nanotube sheet, indicating the superiorelectrochemical stability of the edge-site-free GMS-sheet.[24b]Full-discharge–charge tests were performed in 0.5 mLiTFSI, 0.5 m lithium nitrate (LiNO3), and 0.1 M 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) dissolved in TEGDME(denoted 551 electrolyte) using 2032-coin cells with componentsas shown in Figure S15 (Supporting Information). LiNO3 ef-fectively protects the Li metal anode,[25] and TEMPO acts asa redox mediator to reduce the charge potential.[11a,26] TwelveGMS-sheets synthesized under different conditions (Table S2,Supporting Information) were evaluated under a current densityof 0.4 mA cm−2 with cut-off potentials of 2.0 and 4.5 V (vs Li/Li+).All current densities were calculated based on the geometricarea of GMS-sheet cathode (ϕ 12.5 mm, 1.23 cm2). Note thata current density of 0.4 mAh cm−2 is a relatively high valuecompared to previous work, although a much higher currentdensity can be used in Li-ion batteries. The Li–O2 batteriesdisplay a discharge plateau at 2.65 V, as shown in Figure 3a,b.The initial specific capacities of Li–O2 batteries were calculatedby normalizing the full-discharge capacities to the geomet-ric area and mass of the GMS-sheets (Table S3 and Note S2,Supporting Information). With a decrease in the pelletizationforce, the respective areal (Figure 3a) and mass (Figure S16a,Supporting Information) capacities increase significantly to>20.0 mAh cm−2 and >4500 mAh g−1 due to the increase intotal porosity. Based on the data obtained for Nos. 8, 9, and 10,the smaller the graphene-stacking number, the larger the masscapacity (Figure S16b, Supporting Information), whereas theareal capacity remains almost identical (Figure 3b). Conversely,the larger the GMS-sheet cathode thickness, the larger was theareal discharge capacity (Figure 3b), whereas the mass dischargecapacity remains almost identical (Figure S16b, Supporting In-formation). Figure 3c shows the relationship between the massand areal capacities, which is controlled by three parameters:the pelletization force (porosity), amount of Al2O3 template(thickness), and duration of CVD (carbon layer). Notably, the100N-30mg-2.0h-GMS-sheet (No. 12), as a practically heavycarbon cathode (> 4.0 mg cm−2), displays ultra-high areal(>30.0 mAh cm−2) and mass (>6200 mAh g−1) capacities at acurrent density of 0.4 mA cm−2, which affords the best perfor-mance compared to those reported in previous studies usingother types of binder-free carbon cathodes (Figure 3d; TablesS3 and S4, Supporting Information).[10c,19a,27] We note that thecompetitive discharge capacities of GMS-sheets were achievedwithout the assistance of discharge catalysts. The addition of dis-charge catalysts, for example, 2,5-Di-tert-butyl-1,4-benzoquinone(DBBQ), may further improve the capacities.[27a,28]Adv. Energy Mater. 2024, 14, 2303055 2303055 (4 of 10) © 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202303055 by National Institute For, Wiley Online Library on [18/06/2024]. 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.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deFigure 3. Full-discharge–charge measurements of GMS-sheets. Galvanostatic full-discharge–charge curves of Li–O2 batteries based on GMS-sheetssynthesized via various a) pelletization forces and b) Al2O3 amounts and durations of CVD. All the batteries were tested under a current density of0.4 mA cm−2 and cut-off potentials of 2.0 and 4.5 V (vs Li/Li+) with a 551 electrolyte. c) Mass and areal full-discharge capacities of twelve GMS-sheetsshown in (a) and (b). d), Comparison of specific full-discharge mass and areal capacities with other representative published data. References with anareal weight of >1.0 mg cm−2 were selected.Besides, volumetric energy density is also a critical perfor-mance parameter, particularly for practical batteries. In thisstudy, a volumetric capacity of >450 mAh cm−3 could be obtainedin high-porosity GMS-sheets (>95%) possibly due to their hier-archically porous structures (Figure S17a, Supporting Informa-tion). Besides, the battery with the 100N-30mg-2.0h-GMS-sheet(No. 12) displays a specific energy density of up to 793 Wh kg−1(Figure S17a, Supporting Information) normalized to the totalmass of all the active materials, including the Li metal, liquidelectrolyte, GMS-sheet cathode, and discharge products (Li2O2)on the cathode.[13b,c] As summarized in Figure S17b and Table S4(Supporting Information), GMS-sheets overperform the reportedcarbon materials, including a gas diffusion layer (GDL),[10c,27a,29]wood-derived carbon,[18,30] commercial carbon powder (mainlycarbon black),[19a,27b] CNT,[17a,19b,27c,d] and graphene.[17b,31] Dueto its superior performance in terms of specific energy densityand mass, areal, and volumetric capacities, angstrom to sub-millimeter synthesis-controllable GMS-sheet is one of the mostpromising carbon cathodes for practical Li–O2 batteries.As to the charge process, a charge plateau below 3.7 V corre-sponds to the redox potential of TEMPO.[11a,27a] A sudden poten-tial drop occurs at the end of charging (No. 12, Figure 3b), indi-cating a short circuit caused by the penetration of Li dendritesthrough the glass fiber separator in the first cycle owing to theultra-high areal charge capacity (>30.0 mAh cm−2).[32] Conven-tional Li–O2 batteries are based on the balanced performancesand stabilities of their components, including cathodes, sepa-rators, electrolytes, and anodes. The extremely high capacity ofthe 100N-30mg-2.0h-GMS-sheet causes the unexpected deterio-ration of other battery components in the typical configuration.This implies that the GMS-sheet significantly overperforms andthe development of the other battery components is required tofully utilize the potential of the GMS-sheet.2.3. Discharge–Charge MechanismTo investigate the discharge–charge mechanism, GMS-sheets(100N-25mg-2.5h) at different discharge/charge stages were char-acterized via comprehensive ex situ and in situ techniques(Figure 4). As shown in Figure 4a, four Li–O2 batteries werestopped after discharge for 10 h (10h-D), discharge for 30 h (30h-D), full-discharge to the lower cut-off potential (F-D), and full-discharge–charge to the upper cut-off potential (F-D-C), respec-tively. The GMS-sheet cathode is then removed from the bat-tery and thoroughly washed with 1,2-dimethoxyethane (DME) tocompletely remove the electrolyte before further characterization(Figure S18, Supporting Information). The XRD patterns of theGMS-sheets after the battery tests reveal that the formation anddecomposition of Li2O2 dominates the discharge–charge pro-cess (Figure S19, Supporting Information). Time-of-flight sec-ondary ion mass spectrometry (TOF-SIMS) was used to obtainthe depth distribution data of Li2O2 (LiO− signals) within the vari-ous GMS-sheets after discharge–charge (Figure 4b). The signal inthe 10h-D sample exhibits no clear change during the tests, witha consistently low intensity, which is possibly due to the uniformAdv. Energy Mater. 2024, 14, 2303055 2303055 (5 of 10) © 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202303055 by National Institute For, Wiley Online Library on [18/06/2024]. 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.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deFigure 4. Mechanistic study of Li2O2 formation and decomposition in GMS-sheets. a) Galvanostatic discharge–charge curves of GMS-sheets withvarious cut-off conditions. A current density of 0.4 mA cm−2 was used for all four Li–O2 batteries and the batteries were stopped after 10 h of discharge(10h-D), 30 h of discharge (30h-D), full-discharge to a cut-off potential of 2.3 V (F-D), and full-discharge–charge with a cut-off potentials of 2.3 and4.5 V (F-D-C). GMS-sheets after discharge–charge were removed from the batteries stopped at certain conditions and washed three times with DME.b) Depth profiles of LiO−-signal in GMS-sheets after discharge—charge analyzed via TOF-SIMS. c) Comparison of Li2O2 occupation in micro-, meso-,and macropores of GMS-sheets, calculated from Figure S20a (Supporting Information). d) Discharge–charge curves of GMS-sheets and 2D color filledcontour plot of in situ XRD patterns during the battery test under a current density of 0.4 mA cm−2 and a limited time of 10 h. e) Potential and gasevolution profiles during isotope DEMS test using GMS-sheet as a cathode. The battery was first discharged under isotope 18O2 atmosphere for 5 hfollowed by another (5 h) discharge under 16O2 atmosphere with a current density of 0.4 mA cm−2. f) SEM images of the gas/electrode interfaces offour GMS-sheets after discharge–charge as shown in Figure 4a.formation of Li2O2 around the abundant mesopores of the GMS-sheet at the early stage of the discharge. The gradually decreas-ing signals in the 30h-D and F-D samples with sputtering timeindicate the accumulation of Li2O2 on the exterior of the GMS-sheet,[33] which suggests a different Li2O2 formation mechanismin the macropores at the later stage. As evidenced by the N2 ad-/desorption isotherms (Figure S20, Supporting Information), theGMS-sheets maintain their hierarchically porous structures evenafter discharge, but the micro-/mesopores decrease during dis-charge and then recover after the charge. As shown in Figure 4cand Table S6 (Supporting Information), most of the Li2O2 (92%)is formed in the micro-/mesopores at the beginning of the dis-charge (10h-D), but >50% of the Li2O2 occupies the macroporeswith increasing discharge time (30h-D and F-D). Remarkably, theratios of the percentages of Li2O2 in the micro- and mesopores ofthe GMS-sheets are almost the same (1:2) at different dischargestages, which may be related to the uniform nucleation of Li2O2on the surface of the graphene framework.[34]In situ XRD of the GMS-sheet cathode during discharge andcharge were performed in a Li–O2 battery using the 0.5 m LiTFSI,0.5 m LiNO3, and 0.2 m lithium bromide (LiBr) dissolved inTEGDME (denoted 552 electrolyte), where the dominant reac-tions are still the formation and decomposition of Li2O2 (FigureS21, Supporting Information). As shown in the contour plot(Figure 4d), clear diffraction signals representing the (100) and(101) facets of Li2O2 are observed after 4 h, corresponding to adischarge capacity of 1.6 mAh cm−2. These signals are no longerobserved after charging for 6 h (4 h to the end). To further clar-ify the mechanism of Li2O2 decomposition on the GMS-sheet,differential electrochemical mass spectrometry (DEMS) was per-formed using 18O2/16O2 isotope (Figure 4e). Discharge is initiallyperformed in an 18O2 atmosphere for 5 h and then in a 16O2Adv. Energy Mater. 2024, 14, 2303055 2303055 (6 of 10) © 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202303055 by National Institute For, Wiley Online Library on [18/06/2024]. 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.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deFigure 5. Cycle stabilities of GMS-sheets in Li–O2 batteries. a) Long-cycle stability of GMS-sheet-based Li–O2 batteries using 500 and 551 elec-trolytes at a respective current density and limited capacity of 0.2 mA cm−2 and 0.5 mAh cm−2. b) Comparison of current densities and cy-cle stability with other representative published data. The reference numbers and detailed parameters in b are listed in Table S7 (Support-ing Information). c) The 1st galvanostatic discharge–charge curves and d) Galvanostatic discharge–charge curves of a Li–O2 battery contain-ing a solid electrolyte. All the batteries in c and d were evaluated at a respective current density and limited capacity of 0.4 mA cm−2 and4.0 mAh cm−2.atmosphere for another 5 h at a current density of 0.4 mA cm−2.The second supplied oxygen, 16O2, is detected as the O2 atthe beginning of charging, suggesting that Li2O2 decomposi-tion possibly occurs at the Li2O2/electrolyte interface.[35] In ad-dition, the total O2 evolution amount is close to the theoreti-cal line of the two-electron transfer reaction throughout charg-ing, indicating the efficient decomposition of Li2O2 within theGMS-sheet.SEM images show the morphological changes of Li2O2 on thesurface (Figure 4f) and inside the GMS-sheet (Figure S22, Sup-porting Information) during the discharge–charge. As shown inFigure 4f, the average particle size of Li2O2 on the outer sur-face (close to air) gradually increases throughout discharging,from <100 nm (10h-D) to 300 (30h-D) and then to 800 nm (F-D). In comparison, the Li2O2 formed inside the GMS-sheets dis-plays a limited size of 300 nm owing to spatial confinement(Figure S22a, Supporting Information). Based on the charac-terization results (Figure 4b–f), a possible mechanism is pro-posed, as shown in Figure S22b (Supporting Information). Inshort, the discharge process follows a mechanism of nucleationwithin the micro-/mesopores and growth in the macropores,and the charge process corresponds to the typical decomposi-tion of Li2O2 by redox mediators. Therefore, the excellent bat-tery performance is strongly related to the unique discharge–charge mechanism in a GMS-sheet with a hierarchically porousstructure.2.4. Cycle StabilityThe stability of Li–O2 batteries using GMS-sheets (100N-25mg-4.0h) with an areal density of 6.5 mg cm−2 was first examinedin the 500 and 551 electrolytes using a medium current densityof 0.2 mA cm−2 and a limited capacity of 0.5 mAh cm−2. Thenumber of cycles is defined as the maximum number of cyclesthat the discharge process can contribute to a limited capacityof 0.5 mAh cm−2 before the potential reaches a cut-off potentialof 2.0 V (V vs Li/Li+). As shown in Figure 5a, the GMS-sheet ex-hibits a superior cycle stability (185 cycles) in the 500 electrolytes,even without solid/soluble catalysts. The cycle life of the GMS-sheet increases to 216 cycles (>1080 h, Figure S23, SupportingInformation) in the 551 electrolytes. In addition to the excellentoxidation resistance of the GMS-sheet (Figure S14, SupportingInformation), the charging process could be much cleaner by re-ducing the charging overpotential using redox mediators. This issupported by the increased O2 evolution amount close to the the-oretical line of 2-electron transfer and the suppressed CO2 evo-lution during DEMS test using the 551-electrolytes (Figure S24,Supporting Information). Moreover, the GMS-sheet shows an ex-cellent rate performance at a current density of 0.1–0.8 mA cm−2in the 551-electrolyte (Figure S25, Supporting Information). TheGMS-sheet shows 260 cycles at 0.4 mA cm−2 and 185 cycles at0.8 mA cm−2 in the 551-electrolytes (Figure S26, Supporting In-formation), demonstrating the excellent rate performance andAdv. Energy Mater. 2024, 14, 2303055 2303055 (7 of 10) © 2023 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 2, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202303055 by National Institute For, Wiley Online Library on [18/06/2024]. 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.advenergymat.dewww.advancedsciencenews.com www.advenergymat.desuperior cycle stability. Note that despite the upper limited po-tential of 4.8 V (V vs Li/Li+) being set, the charge potential is<4.6 V (V vs Li/Li+) except for the last few cycles, which is a rela-tively safe potential for GMS-sheets (Figure S14a, Supporting In-formation). We compared the cycle stabilities of the GMS-sheetand representative carbon cathodes reported in previous studies(Table S7, Supporting Information).[18b,24b,27a,29–31,36] As summa-rized in Figure 5b, without any solid or soluble catalysts (solidcircles), the binder-free GMS-sheet outperforms state-of-the-artcarbon cathodes.[24b] Similar to previous reports, the cycle life wasenhanced by adding a mediator (TEMPO in this study, dash cir-cles in Figure 5b). Interestingly, adding the mediator at a lowercurrent of 0.2 mA cm−2 does not significantly improve batterycycle stability (from 185 to 216 cycles), as opposed to the casewith a higher current density of 0.4 mA cm−2 (from 130 to 260cycles). This result indicates the necessity of both a stable GMS-sheet cathode and a mediator when operating at a high currentdensity.Finally, the cycle stability of GMS-sheets was estimated in Li–O2 batteries with a current density of 0.4 mA cm−2 and a lim-ited capacity of 4.0 mAh cm−2 (615 mAh g−1). We note that thecapacity of 4.0 mAh cm−2 is much higher than conventionalvalues (<1.0 mAh cm−2) used as limited capacities in previ-ous work.[13b] As shown in Figure 5c, four high-porosity GMS-sheets exhibit similar high-potential discharge plateau (2.75 V)and low-potential charge plateau (3.5 V), corresponding to ahigh energy efficiency of 78%.[13b] All the GMS-sheets workedwell during the first 10 cycles (Figure S27, Supporting Informa-tion), but during the following cycles, all batteries experienced asudden potential drop during the charge process, possibly ow-ing to precipitous short circuit caused by Li dendrite.[32] TheLi dendrite problem is alleviated by introducing an additionalpolypropylene (PP) membrane on the Li anode side, and the bat-tery life is extended to 16 cycles (Figure S27, Supporting Infor-mation). Hence, the GMS-sheets are sufficiently stable as high-performance cathodes, and other factors cause battery death.To further improve the cycle performance, a solid electrolyte(lithium-ion conducting glass-ceramics, LICGC, Ohara) was in-troduced into the battery to inhibit the shuttle effect, whereinthe byproducts formed at the cathode migrate to the Li anodeand induce its deterioration.[37] Consequently, stable cycling upto 30 cycles is achieved (Figure 5d), which is one of the best cycleperformances using a current density of 0.4 mA cm−2 and largelimited capacity of 4.0 mAh cm−2 .[17a,19a,27b,c] The stable cyclingperformance (19 cycles) of the Li–O2 battery based on the solidelectrolyte and the 552-electrolyte also proves the stability of theGMS-sheet (Figure S28, Supporting Information), and the ma-jor cause of battery death is not the GMS-sheet cathode but otherbattery components, including the electrolyte and anode. Thus,Li–O2 batteries with improved cyclability can be constructed bycombining a GMS-sheet cathode with other advanced techniquesand materials.[10b,38] For perspective, the GMS sheet with a hier-archical porous structure can potentially be used in other metal–gas batteries, especially in nonaqueous systems, given the hy-drophobicity of the GMS sheet (Figure S29, Supporting Informa-tion). For example, a nonaqueous Na–O2 battery based on a 100N-25mg-2.5h-GMS-sheet cathode also works well in a limited capac-ity cycle and shows lower overpotential in the first cycle comparedto a CNT-film cathode (Figure S30, Supporting Information).3. ConclusionWe report synthesis of a binder-free GMS-sheet cathode with acontrollable size on the angstrom to submillimeter scale and ahierarchical porous graphene-wall structure for practical Li–O2batteries with high energy densities. By optimizing the pelletiza-tion force, template amount, and duration of CVD, high-porosity(>95%) GMS-sheets with high surface area of >2000 m2 g−1and large pore volume up to 4.54 cm3 g−1 were synthesized.Using these GMS-sheets as practical cathodes (>4.0 mg cm−2),Li–O2 batteries simultaneously exhibited an ultra-large areal(>30.0 mAh cm−2), mass (>6300 mAh g−1), and volumetric(>480 mAh cm−3) capacities. The Li–O2 battery with the opti-mized GMS-sheet cathode displayed the optimal energy densitiesof 793 (normalized by all active materials, electrolytes, and Li2O2)and 2609 Wh kg−1 (normalized by GMS-sheet and Li2O2). Com-prehensive characterization demonstrated that the high specificcapacity of the GMS-sheet was attributable to its precisely con-trolled hierarchical structure. The outstanding rate performanceand cycle stability tested at a current density of 0.4 mA cm−2) andlimited capacity of 4.0 mAh cm−2 prove that the GMS-sheet isa promising carbon cathode for practical Li–O2 batteries. Thisstudy demonstrates that a proper structural design of carboncathode may overcome most of the major issues of Li–O2 batterycathodes. The optimized carbon cathode is no longer a bottleneckof Li–O2 batteries in terms of energy density and cyclability. Inthe future, large-scale synthesis of the hierarchical high-porosityGMS-sheet cathode with a practical pouch-cell size should pro-mote the commercialization of practical Li–O2 batteries with highenergy density.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was supported by the JST ALCA-SPRING, Japan (grant no. JPM-JAL1301), the JSPS KAKENHI (grant nos. 22K14757 and 21K14490), andthe JST SICORP Grant no. JPMJSC2112.Conflict of InterestThe authors declare no competing interests.Author ContributionsW.Y. and H.N. conceived the idea, designed the experiments, and wrote themanuscript, and H.N. directed the project. W.Y. and Z.H.S. synthesized theGMS-sheets. Z.H.S. performed the TEM tests. W.Y. conducted the batterytests, the SEM tests, the N2 adsorption/desorption tests, the TPD testsand the Raman tests. W.Y. and T.Y. and A.A. performed the TOF-SIMS.W.Y., S.M., and S.N. performed DEMS tests and battery tests with a solidelectrolyte. M.A. and T.K. performed in situ XRD tests. W.Y., S.I., and S.R.M.conducted BET tests. All authors discussed and analyzed the data.Data Availability StatementThe data that support the findings of this study are available from the cor-responding author upon reasonable request.Adv. Energy Mater. 2024, 14, 2303055 2303055 (8 of 10) © 2023 The Authors. 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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.advenergymat.de Hierarchically Porous and Minimally Stacked Graphene Cathodes for High-Performance Lithium9040�Oxygen Batteries 1. Introduction 2. Results and Discussion 2.1. Synthesizing and Optimizing Hierarchically Porous Carbon Cathodes 2.2. Specific Capacities and Energy Densities of the GMS-Sheets 2.3. Discharge9040�Charge Mechanism 2.4. Cycle Stability 3. Conclusion Supporting Information Acknowledgements Conflict of Interest Author Contributions Data Availability Statement Keywords