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[Yasuhiro Domi](https://orcid.org/0000-0003-3983-2202), [Hiroyuki Usui](https://orcid.org/0000-0002-1156-0340), Takumi Okasaka, [Kei Nishikawa](https://orcid.org/0000-0002-7718-7606), [Hiroki Sakaguchi](https://orcid.org/0000-0002-4125-7182)

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[Silicon-Based Nanocomposite Anodes with Excellent Cycle Life for Lithium-Ion Batteries Achieved by the Synergistic Effect of Two Silicides](https://mdr.nims.go.jp/datasets/3263b914-28b9-402b-b5ae-461b6c63b17e)

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Silicon-Based Nanocomposite Anodes with Excellent Cycle Life for Lithium-Ion Batteries Achieved by the Synergistic Effect of Two SilicidesJournal of TheElectrochemical Society     OPEN ACCESSSilicon-Based Nanocomposite Anodes withExcellent Cycle Life for Lithium-Ion BatteriesAchieved by the Synergistic Effect of Two SilicidesTo cite this article: Yasuhiro Domi et al 2024 J. Electrochem. Soc. 171 080506 View the article online for updates and enhancements.You may also likeThe effect of impurity kinds and content onphase constituent and the microstructureof LaSi alloyShuang Wang, Dehong Chen, JiaminZhong et al.-Anode properties of LaSi2/Si compositethick-film electrodes for lithium secondarybatteriesHiroki Sakaguchi, Takahisa Iida, MamoruItoh et al.-Electrochemical Synthesis of LaSi2 from aNaCl-Naf-LaF3-K2SiF6 Melt at 1023 KSvetlana Kochetova, Ruslan N Savchuk,Alexandr Dmitrievich Pisanenko et al.-This content was downloaded from IP address 144.213.253.16 on 04/12/2024 at 09:47https://doi.org/10.1149/1945-7111/ad69c6/article/10.1088/2053-1591/acb529/article/10.1088/2053-1591/acb529/article/10.1088/2053-1591/acb529/article/10.1088/1757-8981/1/1/012030/article/10.1088/1757-8981/1/1/012030/article/10.1088/1757-8981/1/1/012030/article/10.1088/1757-8981/1/1/012030/article/10.1149/MA2016-02/47/3501/article/10.1149/MA2016-02/47/3501/article/10.1149/MA2016-02/47/3501/article/10.1149/MA2016-02/47/3501/article/10.1149/MA2016-02/47/3501/article/10.1149/MA2016-02/47/3501/article/10.1149/MA2016-02/47/3501/article/10.1149/MA2016-02/47/3501https://pagead2.googlesyndication.com/pcs/click?xai=AKAOjsszayc9-wijAnOrtFGEnJuljTFWAT69GekbkBjYLDwSyZ0LGfs5WlW8_AVxS_zQCHQSzvHje240KU6q0geLFv3KIe79vTOs_brGm4MLWQFTQW-Gli_IkeqatBSqFH8dWHvrExtWZvsxLMwL8vv6nXTym75OZoJPuIAKrRQ01Kx1RzWRwgNyXfPEFaiPC-BTf1SSW4YI_NkKJnJ83XWRr9bZlJbwgZplX06EhB1VKhjGMWGI2j9o0FPYTzps4n0B6QUObrUffxitwQKQdiX1xZq6xSqhWdFVXePClVuYQM8Jj_Nw_B7kAKNFw0G4PZPytfempcmNEIcf2BF4gRLiW0G1K4wK0GQeoOxld0BCO9IC&sig=Cg0ArKJSzMAhTPGv7Q_-&fbs_aeid=%5Bgw_fbsaeid%5D&adurl=https://www.el-cell.com/products/test-cells/electrochemical-dilatometer/ecd-4-nano/%3Fmtm_campaign%3Diop%2520pdf%2520advert%26mtm_kwd%3Decd-4-nano%26mtm_source%3Dpdf%26mtm_cid%3D2024Silicon-Based Nanocomposite Anodes with Excellent Cycle Life forLithium-Ion Batteries Achieved by the Synergistic Effect of TwoSilicidesYasuhiro Domi,1,2,z Hiroyuki Usui,1,2 Takumi Okasaka,1,3 Kei Nishikawa,4 andHiroki Sakaguchi1,2,z1Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, Minami 4-101, Koyama-cho, Tottori 680-8552, Japan2Center for Research on Green Sustainable Chemistry, Tottori University, Koyama-cho, Tottori 680-8552, Japan3Department of Engineering, Graduate School of Sustainability Science, Tottori University, Koyama-cho, Tottori 680-8552,Japan4Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS),Tsukuba 305-0044, JapanNanocomposite electrodes comprising LaSi2 and Si exhibit satisfactory charge–discharge cycling performances but their capacityis degraded after repeated cycles. A metallographic structure, in which the Si phase was finely dispersed in the LaSi2 matrix phase,was formed before cycling. The elastic LaSi2 relieved Si-generated stress and suppressed electrode disintegration. Contrarily, theLaSi2 phase in the metallographic structure was surrounded by the Si matrix phase after cycling. The positional relationshipbetween the two phases was reversed, and LaSi2 could not relieve the stress. For a nanocomposite electrode containing CrSi2,which exhibits stiffness to withstand the Si-generated stress, the structural changes were suppressed after cycling, resulting in goodcycling stability. Here, we considered that the addition of stiff silicides as a third phase to the LaSi2/Si composite could improve thecycle life. Thus, this study prepared nanocomposite electrodes containing elastic LaSi2, stiff MSi2 (where M = Cr, Mo, Nb, Ta, Ti,or W), and elemental Si and investigated their electrochemical performances. Reaction behaviors, such as the metallographicstructure, electrode thickness, and phase transition, were also clarified. The LaSi2/NbSi2/Si electrode exhibited the best cycle lifewithout changes in its metallographic structure owing to the synergistic effect of stiff and elastic silicides.© 2024 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open accessarticle distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/1945-7111/ad69c6]Manuscript submitted April 2, 2024; revised manuscript received July 5, 2024. Published August 7, 2024.Supplementary material for this article is available onlineHigh-performance rechargeable batteries are essential for theestablishment of a decarbonized society.1–4 Owing to their highenergy densities, lithium-ion batteries (LIBs) are widely used aspower sources in mobile electronic devices and electric vehicles(EVs). As a power source for EVs, the performance of LIBs, interms of high energy density, long life, safety, and fast charge–-discharge, requires improvement. Si is preferred as a next-generationactive material for LIB anodes because its theoretical capacity isapproximately 10 times that of the currently used graphite electrode(crystalline Li3.75Si: 3580 mA h g–1; LiC6: 372 mA h g–1).5–8 Thus,Si-based anodes are highly desirable; however, the large volumechange associated with Si lithiation (charging) and delithiation(discharging) hinders its practical application. The volume expan-sion ratio of Si to the crystalline Li3.75Si phase can be as high as280%, resulting in high stress and strain in active materials.5,9 Wepreviously reported that the expansion ratio considerably exceeded280% owing to the formation of voids and cracks in the activematerial during charge–discharge tests.10 Furthermore, Si exhibitshigh electrical resistivity, low initial Coulombic efficiency (CE), anda low Li+ diffusion coefficient, which limit its application.11–13Several approaches have been employed to address the afore-mentioned limitations. They include the synthesis of nanostructuredSi materials (i.e., nanoparticles, nanowires, and nanorods) to relieveSi-generated stress,14–19 Si coating using conductive materials toreduce the electrical resistivity of Si,20,21 doping of Si withimpurities (e.g., phosphorus and boron) to improve its electricalconductivity and alter its phase transition behavior,22–27 andprelithiation of Si to enhance the initial CE.28–30 Furthermore, weproposed futuristic Si-based active materials, e.g., binary silicide/Sicomposites, to address the limitations of Si.31–34 In particular, theLaSi2/Si electrode exhibited the best cyclability among severalbinary silicide/Si composites. In addition, the electrochemicalperformance of the ternary silicide/Si composite electrode wassuperior to that of the binary silicide/Si electrode.35–38 We estab-lished that composite materials should possess mechanical propertiesto accommodate Si-induced stress, low electrical resistivity, ade-quate reactivity with Li+, and high thermodynamic stability thatdoes not deteriorate even after repeated charge–dischargecycling.13,31,32,34It is essential to understand the degradation mechanism of theLaSi2/Si electrode and mitigate it to improve electrochemicalperformance. Prior to charge–discharge cycling, a metallographicstructure was formed, in which Si phases with a diameter of severalhundred nanometers were finely dispersed in LaSi2 matrix phases.39This metallographic structure resulted in good cycling stabilitybecause the elastic properties of LaSi2 mitigated the Si-generatedstresses and suppressed electrode disintegration. Contrarily, prior tothe capacity degradation, the microstructure changed into a structurein which the LaSi2 phase was finely dispersed in the Si matrix phase,i.e., the positional relationship between the two phases was reversed.The elastic LaSi2 matrix ruptured and reduced in size, owing to theSi-generated stress. Thus, it was concluded that the LaSi2 phasecould no longer relieve the Si-generated stress and that the LaSi2/Sielectrode deteriorated as a result.Composites containing CrSi2, instead of elastic silicide exhibit ametallographic structure, in which the Si phase surrounds the silicidephase, preventing microstructural changes even after charge–-discharge cycles (Figs. S1 and S2).37,40 CrSi2/Si exhibits stiffnessto resist the Si-generated stress. Consequently, the CrSi2/Si electrodeexhibited good cycle stability. This implies that the addition of CrSi2as a third phase to a LaSi2/Si electrode can improve the cycle lifebecause microstructure inversion can be suppressed and LaSi2 canrelieve the Si-generated stress. In addition, Si is known to soften dueto lithiation, and the introduction of the CrSi2 as a support frame-work is essential to suppress electrode disintegration due to thezE-mail: domi@tottori-u.ac.jp; sakaguch@tottori-u.ac.jpJournal of The Electrochemical Society, 2024 171 080506https://orcid.org/0000-0003-3983-2202https://orcid.org/0000-0002-1156-0340https://orcid.org/0000-0002-7718-7606https://orcid.org/0000-0002-4125-7182http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/https://doi.org/10.1149/1945-7111/ad69c6https://doi.org/10.1149/1945-7111/ad69c6https://doi.org/10.1149/1945-7111/ad69c6mailto:domi@tottori-u.ac.jpmailto:sakaguch@tottori-u.ac.jphttps://crossmark.crossref.org/dialog/?doi=10.1149/1945-7111/ad69c6&domain=pdf&date_stamp=2024-08-07softening. This study prepared nanocomposite electrodes comprisingelastic LaSi2, stiff CrSi2, and elemental Si and investigated theirelectrochemical performance. Reaction behaviors such as the me-tallographic structure, electrode thickness, and amount of Li-richphase (x = 2.00–3.75 in LixSi) formed, were clarified. The effect ofstiffness on the electrochemical performance was also investigated.Although the effects of the Si grain size on the microstructuralstability and electrochemical performance of Si-based compositeelectrodes have been reported,41 the differences in the mechanicalfunctions of Si alloys have not been examined, as far as we know.This study closes that knowledge gap by clarifying the effects ofvarious silicide functions on the charge–discharge characteristics ofSi-based electrodes.Experimental MethodsSynthesis of LaSi2/MSi2/Si composites.—A Si-based nanocom-posite comprising two functional silicides and elemental Si wassynthesized through a mechanical alloying (MA) method. Weconsidered LaSi2 and CrSi2 to be elastic and stiff silicides,respectively. A thin La shot (Santoku Corp.) was fabricated usinga press machine, and fine chips were obtained using a nipper. Weplaced a mixture of elemental La chips, Cr powder (Nilaco Corp.),and Si powder (FUJIFILM Wako Pure Chemical Corporation, Ltd.;preparation weight ratio of 35/35/30 for the LaSi2/CrSi2/Si compo-site) with ZrO2 balls in a zirconia pod. The pod was filled with dryAr gas, and the Cr and Si powders were used as received. The weightratio of the balls to the sample was approximately 15:1. A high-energy planetary ball mill (P-6, Fritsch) was used for the MAprocess, which was performed at a rotary speed of 380 rpm and 30 °C for 25 h. We stopped the operation of the ball mill after 5 and 10 hof MA and stirred the samples. Other LaSi2/MSi2/Si (35/35/30 wt%;M = Mo, Nb, Ta, Ti, or W) nanocomposites were synthesized. Thetotal MA times of LaSi2/MoSi2/Si, LaSi2/NbSi2/Si, LaSi2/TaSi2/Si,LaSi2/TiSi2/Si, and LaSi2/WSi2/Si were 20, 30, 30, 50, and 50 h,respectively. X-ray diffraction (XRD; Ultima IV, Rigaku) wasperformed at 5- or 10-h intervals after the beginning of the MAtreatment. We determined that we had the target sample when thepeak assigned to the two silicides appeared and the peak assigned tothe raw materials disappeared. No peaks attributable to the impurityphase were observed. Therefore, the total MA time for each activematerial differed. The procedures were performed in an Ar-filledglove box (Miwa MFG, DBO-2.5LNKP-TS) with a dew point below–100 °C and an O2 content below 1 ppm.Electrode preparation.—We prepared LaSi2/MSi2/Si (M = Cr,Mo, Nb, Ta, Ti, or W) electrodes using the gas-deposition (GD)method.42,43 This method does not require a binder or conductiveadditive; the electrode can only include the active material andcurrent collector. This allows for the accurate evaluation of the basicelectrochemical properties of the active material. Using GD method,the raw material powder was aerosolized with carrier gas in theconduit pipe and ejected from the nozzle at nearly the speed of soundagainst the substrate. The collision impact between the particles andsubstrate fractured and plastically deformed the particles. Thesurface of the particles created by the fracture faced the surface ofother particles. Owing to the high-impact energy, the interdiffusionof atoms occurred at the interface. Thus, at room temperature, theparticles strongly adhered to each other, resulting in mechanicaldurability and moderate electrical conductivity. This phenomenon iscalled “room-temperature impact consolidation.” 44,45 Here, theLaSi2/CrSi2/Si nanocomposite was deposited on a current collectorsubstrate of Cu foil (thickness = 20 μm). The detailed GD conditionshave been reported.10 The weight of the deposited nanocompositesamples was 100 ± 5 μg (elemental Si = approximately 30 μg). Theminimum weighing capacity of the electronic balance used in thisstudy was 1 μg; thus, the error in weight measurement wasapproximately 1%. The deposition amount per unit area was 0.509and 0.153 mg cm–2 for nanocomposite and pure Si electrodes,respectively (the deposited area was 0.196 cm2). For comparison,we fabricated a pure Si electrode through the GD method with adeposition weight of approximately 30 μg after subjecting thepurchased Si powder at 380 rpm for 5 min. Figure S3 shows theparticle-size distribution of the milled Si powder.Coin-cell assembly and charge–discharge testing.—The pre-pared GD electrode was mounted in a 2032-type coin cell as theworking electrode. A Li metal sheet (99.90%; thickness, 1 mm; RareMetallic Co., Ltd.) and glass fiber filter (Whatman GF/A) were usedas the counter electrode and separator, respectively. We used1 mol dm–3 (M) lithium bis(fluorosulfonyl)amide (LiFSA KishidaChemical Co., Ltd.) dissolved in N-methyl-N-propylpyrrolidiniumbis(fluorosulfonyl)amide (Py13-FSA; Kanto Chemical Co., Inc.) as anionic liquid electrolyte. High safety is required when handling high-energy-density LIBs incorporating Si-based anodes. Nonflammableelectrolytes are essential for improving battery safety. We haveinvestigated ionic liquids as electrolyte solvents because of theirsuperior physicochemical properties46–49 and reported that Si-basedelectrodes exhibit better electrochemical performances and ensureincreased safety in certain ionic liquid electrolytes compared withtypical organic liquid electrolytes.10,26,29,39 The cell assembly andelectrolyte preparation were performed in the Ar-filled glove box.Galvanostatic charge–discharge tests were conducted using anelectrochemical measurement system (HJ-1001SM8A, HokutoDenko Co., Ltd.) at 30 °C. To form a good surface film on theelectrode, which should exhibit high ionic and low electronicconductivities, precycling was performed. The electrode wascharged from an open circuit voltage to 0.500 V at 0.1 C, maintainedat 0.500 V for 12 h, and subsequently discharged to 2.000 V at 0.1 C(1 C: 3600 mA g–1).50 The potential range in the main tests was0.005–2.000 V with a charging capacity limit of 1000 mA h g(Si)–1.It is unlikely that all of the theoretical capacity of the Si electrode(3600 mA h g–1) will be needed, considering the actual capacity ofcathodes. Therefore, we limited the charging capacity to 1000 mA hg(Si)–1, which is 3 times actual capacity of the graphite electrodecurrently used. Additionally, 1000 mA h g(Si)–1 corresponded to0.153 mA h cm(active material)–2. The limitation was performed bycontrolling the charging time (approximately 17 min), whereas thedischarging time was unlimited. The lithiation and delithiation wereconducted in constant current mode. Here, the capacity of thesilicides was ignored; the silicide-only and Si-only electrodes storedLi,10,34,51,52 whereas the silicide in the silicide/Si composite elec-trodes did not exhibit charge–discharge capacity.13Characterization.—The crystal structures of the prepared pow-ders were analyzed by XRD. Al foil was used as the internalstandard instead of Cu foil because the peaks of CrSi2 overlappedwith those of Cu. The XRD patterns were identified using theInorganic Crystal Structure Database. The weight ratio of eachcomposite sample was determined using an X-ray fluorescence(XRF) spectrometer (EDX-720, Shimadzu Corp.). The particle-sizedistribution of the synthesized powder was measured in an aqueoussolution of 0.5 wt% sodium hexametaphosphate (SALD-2300,Shimadzu Corp.).The cell was disassembled after the charge–discharge tests in theAr-filled glove box. The dismounted electrode was washed withpropylene carbonate and diethyl carbonate (Kishida Chemical Co.,Ltd.) before drying. Thereafter, the electrode was introduced into theobservation chamber of a field-emission scanning electron micro-scope (JSM-IT800, JEOL Co., Ltd.) or a scanning transmissionelectron microscope (JEM-ARM200F, JEOL Co., Ltd.) under non-atmospheric exposure using a transfer vessel. An electrode cross-section was prepared using a cross-section polisher (IB-19520CCP,JEOL Co., Ltd.) for scanning electron microscopy (SEM) and afocused ion beam (FIB) scanning electron microscope (SMF2000,Hitachi High-Tech Science Corp.) for transmission electron micro-scopy (TEM). SEM was performed with an acceleration voltage anda working distance of 10 kV and 10 mm, respectively. Energy-Journal of The Electrochemical Society, 2024 171 080506dispersive X-ray spectroscopy (EDS) was performed at 20 kV. Acarbon layer was deposited on the electrode surface to protect it fromdamage using the Ga-ion beam of the FIB process. TEM wasperformed at 200 kV, and the ED spectra were recorded with 10scans. Soft X-ray emission spectroscopy (SXES) was performedusing a SXE spectrometer (SS-94000SXES) attached to the JSM-IT800 system. The EDS maps were superimposed using theScanning Probe Image Processor (SPIP; ver. 5.1.1, Image metrologyA/S). The black areas in the EDS maps of Cr and La were convertedinto white and superimposed. The yellow and blue colors of Cr andLa, respectively, remained unchanged.Results and DiscussionCharacterization of synthesized powder.—Figure 1 shows theXRD patterns of the prepared powders. Figure 1a shows peaksassigned to CrSi2, LaSi2, and elemental Si, indicating the successfulfabrication of the LaSi2/CrSi2/Si nanocomposite despite the single-pod synthesis. For comparison, CrSi2/Si and LaSi2/Si compositesamples were prepared (Figs. 1b and 1c), and their crystal structuresare shown in Fig. S4. Table I lists the weight ratios of thecomposites. Their volume ratios were obtained using the measuredweight ratios, which were close to the preparation values. Slightvariations were observed owing to XRF measurement errors. Thecorrect weight ratio was obtained by inductively coupled plasmaatomic emission spectroscopy. The sample was mixed with hydro-fluoric acid to dissolve Si. However, a white precipitate (probablyLaF3) was formed if the sample contained La, and the elementalconcentration would not be accurately determined.Figure 2 shows the particle-size distribution of the compositesamples measured in an aqueous solution of 0.5 wt% sodiumhexametaphosphate, along with the D10, D50, and D90 values.Figure 3 shows the SEM images of these composite powders. Theresults showed that the secondary particles of LaSi2/CrSi2/Si werelarger than those of CrSi2/Si and LaSi2/Si. In addition, the primaryparticles of each composite were within the submicron range.Charge–discharge behavior of LaSi2/CrSi2/Si nanocompositeelectrode.—Figure 4 shows the 1st, 2nd, 100th, 600th, and 1200thcharge–discharge curves of the nanocomposite electrodes and thecurve of the cycle where the CE reached 99.0%. The pure Si electrodeexhibited potential plateaus at approximately 0.1 and 0.4 V on thecharge and discharge curves, attributable to the lithiation anddelithiation of Si, respectively. The LaSi2/CrSi2/Si and LaSi2/Sinanocomposite electrodes exhibited the same lithiation behavior asthe pure Si electrode on the charge side and a 0.1 V higher potentialplateau on the discharge side during the first and second cycles. Thereare two reasons for this difference. First, an overpotential may haveoccurred; if so, the plateau potential on the charge side should belower than that of the Si electrode. However, this was unlikelybecause the potentials were practically identical. Second, the capacitywas attributed to the silicide in the composite. We reported that puresilicide electrodes exhibited higher lithiation–delithiation plateausand/or slopes than Si electrodes.51–53 However, the silicide phase inthe composite electrode hardly stored Li (although it may havefunctioned as a Li-diffusion path).13,54 Table S1 lists Li+ diffusioncoefficients (DLi+) of Si and certain silicides estimated using agalvanostatic intermittent titration technique.10 The DLi+ of thesilicides was an order of magnitude higher than that of Si.Assuming that CrSi2 and LaSi2 stored Li in the LaSi2/CrSi2/Si andLaSi2/Si electrodes, the plateau potential on the charge side shouldhave exceeded than that of the Si electrode; however, they werepractically identical. Thus, the difference in the aforementionedreaction behavior had not been clarified at this stage. However, theSi environment may have had a certain influence, as described below.Prior to 100th cycle, the CrSi2/Si electrode exhibited a highpotential plateau and slope on the charge and discharge sides,respectively. We assumed that the CrSi2 phase in the compositeelectrode stored and released Li, although the plateau and slopepotentials decreased as the cycle proceeded (Figs. 4a–4d). After1200 cycles, the behavior of the CrSi2/Si electrode was practicallyidentical to that of the Si electrode (Fig. 4e). It is possible that a partof the electrode collapsed and the active material was no longerinvolved in the charge–discharge process. However, it was unlikelythat only CrSi2 was electrically isolated and not simultaneouslystoring Li. The metallographic structure of the CrSi2/Si electrode didnot change before and after 1200 cycles (Fig. S1). However, changesin the physicochemical and/or mechanical properties of CrSi2 wereunclear from the image. The properties that were prominent duringthe charge–discharge cycling might have affected the curve. FigureS5 shows the CEs of the electrodes during the initial cycles. Theprecycling was similar for all the electrodes. However, the pure Sielectrode exhibited the highest initial CE, and the three compositeelectrodes exhibited lower CEs owing to the additional electrolytedecomposition on the surface of the silicide with high electricalconductivity. The CEs of the four electrodes reached 99.0% at the20th cycle (Fig. 4f).Figure 1. XRD patterns of powders synthesized from (a) elemental Cr, La,and Si by MA for 25 h, (b) elemental Cr and Si by MA for 40 h, and (c)elemental La and Si by MA for 5 h. The increase in the baseline in (c) wasattributed to the Kapton film.Table I. Weight and volume ratios of LaSi2/CrSi2/Si, CrSi2/Si, andLaSi2/Si nanocomposite powders.CompositePreparation ratio/wt%Measured ratio/wt%Volume ratio/vol%LaSi2/CrSi2/Si 35/35/30 39/27/33 29/19/52CrSi2 70/30 74/26 57/43LaSi2 70/30 60/40 41/59*Weight ratio was investigated by XRF.**Volume ratio was estimated using the weight ratio and density. Thedensities of LaSi2, CrSi2, and Si were 5.14, 5.02, and 2.33 g cm–3,respectively.Journal of The Electrochemical Society, 2024 171 080506Figure 2. Particle-size distribution of (a) LaSi2/CrSi2/Si, (b) CrSi2/Si, and (c) LaSi2/Si powders measured in an aqueous solution of 0.5 wt% sodiumhexametaphosphate.Figure 3. SEM images of (a) LaSi2/CrSi2/Si, (b) CrSi2/Si, and (c) LaSi2/Si powders.Figure 4. Charge–discharge curves of LaSi2/CrSi2/Si, CrSi2/Si, LaSi2/Si, and Si electrodes at the (a) 1st, (b) 2nd, (c) 100th, (d) 600th, and (e) 1200th cycles and(f) the cycle where the Coulombic efficiency reached 99.0%.Journal of The Electrochemical Society, 2024 171 080506Figure 5 shows the cycle lives of the LaSi2/CrSi2/Si, CrSi2/Si,LaSi2/Si, and pure Si electrodes under a charge capacity limitation of1000 mA h g(Si)–1. The CE data are also shown. The threenanocomposite electrodes exhibited a longer cycle life than thepure Si electrode. The CrSi2/LaSi2/Si electrode exhibited the longestlife among the composite electrodes. It is believed that charging anddischarging causes repeated expansion and construction of the Si-based electrodes, increasing the amount of Si that cannot collectcurrent. When the Si that can collect current could no longer carry acapacity of 1000 mA h g(Si)–1, the discharge capacity would fade.The difference between the cycle lives of the electrodes wasapproximately 100 cycles, and a reproducibility within a maximumof 50 cycles was confirmed for each electrode (Fig. S6). The cyclelife significantly improved when silicides of different stiffnesses(hardnesses) were employed instead of CrSi2, as described below.TEM revealed whether the metallographic structure changed be-cause of the addition of CrSi2 or not. The high-rate performance ofthe LaSi2/CrSi2/Si electrode was virtually the same as that of theLaSi2/Si and CrSi2/Si electrodes (Fig. S7), indicating that theelectrical conductivities of silicides were almost identical.Changes in metallographic structure before and after charge-–discharge cycles.—Figure 6 shows a bright-field (BF) TEM image,a high-resolution (HR) BF-TEM image, and the corresponding EDSmaps of a LaSi2/CrSi2/Si nanocomposite particle prior to charge–-discharge cycling. Figure S8 shows the corresponding selected areaelectron diffraction (SAED) and d-spacings based on the SAEDanalysis. The lattice fringe analysis in the HR BF-TEM image(Fig. 6b) revealed that the CrSi2, LaSi2, and elemental Si werehomogeneously mixed at the nanoscale level. The existence of thesephases is confirmed in Fig. S8. However, it is difficult to infer thetype of metallographic structure formed by the silicide and elementalSi from Figs. 6a–6e. Thus, we determined the structure by super-imposing the EDS maps of Cr and La (Fig. 6f) and observed that theSi phase (with a diameter of several tens of nanometers) wassurrounded by CrSi2 and LaSi2.Figure 7 shows the BF-TEM and HR BF-TEM images, corre-sponding EDS maps, and superimposed image of the LaSi2/CrSi2/Sinanocomposite electrode after the 1200th cycle. Figure S9 shows theresults after the 600th cycle. The Si phase was highly dispersed inthe silicide matrix phase. Although microstructural inversion onlyoccurred for the LaSi2/Si electrode,39 the addition of CrSi2 to theLaSi2/Si composite suppressed the changes in the metallographicstructure after long-term cycling. The stiff CrSi2 is known tofunction as a support framework, improving the structural stabilityof the whole electrode and preventing microstructural inversion.Thus, the superior cycle life was achieved because of the synergisticeffect of LaSi2 and CrSi2. The elasticity of LaSi2 relaxed the Si-generated stress, and the stiffness of CrSi2 withstood the Si-generated stress and maintained the metallographic structure. Theparticle size of the elemental Si remained unchanged before and afterthe charge–discharge cycling. Although it is well known thatcrystalline Si (c-Si) changes into amorphous Si (a-Si) after charge-–discharge cycles, the presence of c-Si was confirmed after the long-term cycle test. Considering that not all the c-Si in the electrodestored Li with the charge capacity limitation of 1000 mA h g–1,unreacted Si microcrystals were observed (Fig. 7).Figure S10 shows low-magnification TEM images of thecomposite electrodes before and after charge–discharge cycling.The microstructural changes in LaSi2/Si and the maintenance of theLaSi2/CrSi2/Si and CrSi2/Si microstructures after 1200 cycles wereobserved even at a low magnification. For the LaSi2/CrSi2/Sielectrode, a drastic microstructural change (i.e., a reversal betweenthe silicide and Si phases) occurred after 1700 cycles, wherecapacity fading was observed. Contrarily, for the LaSi2/Si electrode,microstructural changes were observed after 600 cycles, earlier thanwhere the capacity degradation occurred. These results highlightvarious mechanisms for the degradation of silicide/Si compositeelectrodes. Microstructural changes should not necessarily degradecapacity (subsequently discussed).Change in electrode thickness and phase transition duringcharge–discharge cycling.—Figure 8 shows the cross-sectionalSEM images of the CrSi2/Si, LaSi2/Si, and LaSi2/CrSi2/Si electrodesbefore and after charge–discharge cycling. The thickness of theactive material layer was approximately 3 μm prior to cycling.Differences in thickness were observed at the 600th cycle. Thethickness of the LaSi2/Si electrode reached 23 μm at the 1200thcycle, and the increase in thickness of the CrSi2/Si electrode wassuppressed to approximately half of that of the LaSi2/Si electrode.The thickness of the LaSi2/CrSi2/Si electrode was suppressed toapproximately one-third that of the LaSi2/Si electrode.Figure 9a shows the cycle dependence of the relative thickness(t/t0) of the CrSi2/Si, LaSi2/Si, and LaSi2/CrSi2/Si electrodes. Thecorresponding thicknesses are shown in Fig. S11. The pure Sielectrode maintained a t/t0 of approximately 1.5 over 300 cycles;however, the t/t0 reached 8.3 at the 600th cycle prior to capacityfading.10 The t/t0 of the LaSi2/Si electrode rapidly increased after600 cycles, where the inversion of the metallographic structure wasobserved.39 Although the t/t0 of the CrSi2/Si electrode rapidlyincreased after 600 cycles, no microstructural inversion was ob-served (Figs. S1 and S2). We previously reported that the t/t0 of theLaSi2/Si electrode reached approximately 3 during the first 20cycles.39 This was higher than that obtained in the present studyowing to the different conditions of the charge–discharge cyclingtest. Here, precycling was performed to form a good surface film,and the cycling test was performed at 1.0 C. However, in theprevious study, precycling was not performed, and the cycling testwas performed at 0.4 C. Thus, the t/t0 varies across studies. Althoughthe increase in the t/t0 of the CrSi2/Si and LaSi2/Si electrodes wassuppressed at the 600th cycle, the t/t0 reached approximately 6.8 atthe 1200th cycle.The t/t0 of the LaSi2/CrSi2/Si electrode remained unchangedduring the first 200 cycles and was a lower than those of theCrSi2/Si, LaSi2/Si, and Si electrodes over 1200 cycles. Thecombination of the two silicides resulted in different electrodethicknesses despite having the same Li storage contents.Previously, we reported that the expansion of Si-based electrodesis dependent on the amount of the Li-rich phase formed, which has alarge expansion ratio of 280%, and Li distribution in the Silayer.10,27 The former was studied based on a differential capacity(dQ/dV) plot obtained by differentiating the discharge curve withrespect to potential (voltage), and the latter was investigated usingour SXES-based analysis method.Figure 5. Cycle lives of LaSi2/CrSi2/Si, CrSi2/Si, LaSi2/Si, and Si electrodesin 1 M LiFSA/Py13-FSA with a charge capacity limitation of 1000 mA h g–1at 1 C per unit weight of elemental Si.Journal of The Electrochemical Society, 2024 171 080506Figure S12 shows the SXE spectra of the pure Si and LaSi2electrodes prior to charge–discharge cycling. In the Si spectrum, apeak at approximately 90 eV was attributed to a low-lying 3 s state.In contrast, a peak and a broad shoulder at approximately 92 and96 eV, respectively, were attributed to the p components of the sp3hybrid orbital. In the LaSi2 spectrum of, a peak at approximately80 eV was assigned to La, and two peaks around 90 and 98 eV wereattributed to Si. The spatial resolution of SXES was approximately1 μm, and SXES revealed that the silicides and elemental Si phaseswere mixed at the nanoscale level. Although we ignored thecapacities of CrSi2 and LaSi2, Li can pass through silicides. Thus,the SXE spectrum of the silicides may have changed after thecharge–discharge tests. Consequently, it was difficult to clarify thedistribution of Li in the nanocomposite electrodes by SXES. Thus,we investigated the amount of the Li-rich phase formed using thedQ/dV plot of the electrode.Figure 6. (a) BF-TEM and (b) HR BF-TEM images of LaSi2/CrSi2/Si composite powder prior to charge–discharge cycling and corresponding EDS maps for (c)Cr, (d) La, and (e) Si. (f) Overlaid view of Cr (yellow) and La (blue); the black areas were inverted into white areas, which denote elemental Si.Journal of The Electrochemical Society, 2024 171 080506Figure 9b shows the change in the amount of the amorphous Li-rich (a-Li-rich) phase formed with the cycle number estimated fromthe dQ/dV plot (Figs. S13–S16). As the crystalline Li-rich phase israrely formed electrochemically, we investigated the amount of thea-Li-rich phase. The amount formed on the composite electrodeswas lower than that on the pure Si electrode. The differences in theamounts of the a-Li-rich phase, despite having the same Li contents,indicated that the Li distribution was heterogeneous in the activematerial layer. Owing to the higher electronic and Li-ion conductiv-ities of the silicides compared with those of pure Si, we assumed thatLi was distributed throughout the entire composite electrode and thatthe formation of the a-Li-rich phase was suppressed.Prior to the 600th cycle, the largest amount of the a-Li-rich phasewas formed on the LaSi2/Si electrode, followed by theLaSi2/CrSi2/Si and CrSi2/Si electrodes. The CrSi2 phase was finelydispersed in the Si matrix (Fig. S1). However, there might have beenFigure 7. (a) BF-TEM and (b) HR BF-TEM images of LaSi2/CrSi2/Si composite electrode after the 1200th cycle and corresponding EDS maps for (c) Cr, (d)La, and (e) Si. (f) Overlaid view of Cr (yellow) and La (blue); the black areas were inverted into white areas, which denote elemental Si.Journal of The Electrochemical Society, 2024 171 080506microscopic areas where stiff CrSi2 surrounded Si because CrSi2was more abundant in terms of the volume ratio (Table I). In suchareas, even if Si stored Li, it could not expand outward because ofthe presence of stiff CrSi2. Thus, Li could not remain close to the Sisurface, and the a-Li-rich phase could not form but had to moveinside the Si. Thus, the a-Li-rich phase on the CrSi2/Si electrode wasconsidered to be the smallest. Owing to such an effect, the amount ofthe a-Li-rich phase formed on the LaSi2/CrSi2/Si electrode waslower than that in the LaSi2/Si electrode.After 600 cycles, the slopes of the LaSi2/Si and CrSi2/Sielectrodes steepened, whereas that of the LaSi2/CrSi2/Si electroderemained unchanged (Fig. 9b). The metallographic change inLaSi2/Si, which occurred after the 600th cycle, rendered it impos-sible to relax the Si-generated stress,39 resulting in a rapid increasein the t/t0 (Fig. 9a). Fine cracks were observed in the CrSi2/Sielectrode layer after 600 cycles (Fig. S10f). This was probablybecause CrSi2 could not withstand the Si-generated stress owing tothe repeated charge–discharge cycles. The amount of the a-Li-richphase formed increased because the active materials were electri-cally isolated owing to the occurrence of cracks, and Li was stored inthe remaining Si, leading to a rapid increase in the t/t0. The slope ofthe LaSi2/CrSi2/Si electrode remained unchanged because themetallographic structure did not change and there was no electricalisolation.Effect of differences in silicide stiffness on cycle life.—Theintroduction of CrSi2, which exhibits stiffness, into LaSi2/Sinanocomposites, where microstructural changes occur, suppressedchanges in the metallographic structure and improved the charge–-discharge cycle life. This study investigated the effect of differentsilicide stiffnesses on the cycle life. Figure 10a shows the cycle livesof the LaSi2/MSi2/Si (where M = Cr, Mo, Nb, Ta, Ti, or W)electrodes with a charge capacity limit of 1000 mA h g(Si)–1.Furthermore, the Vickers hardness of MSi2 was demonstrated.55The XRD pattern, particle-size distribution, and high-magnificationSEM results of LaSi2/MSi2/Si are shown in Figs. S17–S19,respectively. It can be seen that the LaSi2/MSi2/Si nanocompositeswere successfully prepared, the D50 value was in the range of 3 to5 μm, and each phase was uniformly mixed. The weight ratios of thecomposites are presented in Table S2, and their volume ratios wereobtained using the measured weight ratios. Many of the synthesizedpowders had almost the same weight ratios as the preparation values,whereas the weight ratios of the Ta-containing samples(LaSi2/TaSi2/Si and TaSi2/Si) significantly differed from the initialvalues. This was because the characteristic X-rays of Ta (Mα1 line:1.71 keV) and Si (Kα line: 1.74 keV) overlapped and could not beaccurately measured. In addition, MoSi2/Si could not be synthesizedby MA under the present conditions. Consequently, α-MoSi2 andβ-MoSi2 were formed. Figures S20–S24 show the charge–dischargecurves of the LaSi2/MSi2/Si (M = Cr, Mo, Nb, Ta, Ti, or W)electrodes. For comparison, the results of the MSi2/Si, LaSi2/Si, andpure Si electrodes were considered, and the phenomena shown inFig. 4 were identified.The cycle life was the shortest when WSi2, with the higheststiffness (highest Vickers hardness), was added. The cycle lifeimproved with a decrease in the stiffness of MSi2 (low Vickershardness). Superior cycling performance was achieved when NbSi2was added, maintaining a reversible capacity of 1000 mA h g(Si)–1for approximately 1800 cycles. Contrarily, the cycle life decreasedwhen TiSi2, which exhibits the lowest stiffness, was used.Figure 10b shows the correlation between the Vickers hardness ofthe stiff silicide and the cycle life of nanocomposite electrodesFigure 8. Cross-sectional SEM images of (a–c) CrSi2/Si, (d–f) LaSi2/Si, and (g–i) LaSi2/CrSi2/Si electrodes. The images were obtained before (a, d, and g) andafter 600 (b, e, and h) and 1200 cycles (c, f, and i).Journal of The Electrochemical Society, 2024 171 080506containing LaSi2, silicides with varying stiffnesses, and elemental Si.The cycle life increased with a decrease in the Vickers hardness, andthe longest cycle life was obtained at approximately 9 GPa.Conversely, the cycle life decreased with a decrease in hardness.This trend was observed when LaSi2 was adopted as the elasticsilicide, and the same trend may not have been observed with otherelastic silicides. Additionally, Fig. S25 shows the relationshipbetween the weight of active material and the number of cyclesbefore the discharge capacity begins to drop below 1000 mA h g–1.The lower the amount of active material deposited, the less stress isgenerated in the active material layer, which can improve the cyclelife. However, no such correlation was observed.Figure 11 compares the cycle lives of the LaSi2/MSi2/Si,MSi2/Si,LaSi2/Si, and pure Si electrodes. The capacity degradation of theLaSi2/WSi2/Si electrode occurred between cycles where the capacitydegradation of the WSi2/Si and LaSi2/Si electrodes was confirmed.Therefore, no synergistic effect of stiff WSi2 and elastic LaSi2 wasachieved. For the LaSi2/MoSi2/Si electrode, we could not determineif the synergistic effect was achieved because MoSi2/Si could not besynthesized. Contrarily, the cycle lives of the LaSi2/TaSi2/Si,LaSi2/TiSi2/Si, and LaSi2/NbSi2/Si electrodes were superior to thoseof theMSi2/Si and LaSi2/Si electrodes, indicating that the synergisticeffect was achieved. Although the NbSi2/Si electrode exhibited acycle life comparable to that of the pure Si electrode, the bestperformance was obtained by combining it with LaSi2. Thus, a highsynergistic effect was obtained for the LaSi2/NbSi2/Si electrode.Reaction behaviors of LaSi2/MSi2/Si electrodes.—Figure 12shows the change in the amount of the a-Li-rich phase of theLaSi2/MSi2/Si nanocomposite electrodes (M = Cr, Mo, Nb, Ta, Ti,or W) formed over the cycle. Although the difference in the amountswas not confirmed before the 600th cycle, the amount began to varyafter 900 cycles. After 1200 cycles, more a-Li-rich phase wasformed on the LaSi2/WSi2/Si and LaSi2/TaSi2/Si electrodes, withrelatively poor cycle lives. However, less a-Li-rich phase wasformed on the LaSi2/MoSi2/Si and LaSi2/NbSi2/Si electrodes. Thelatter two electrodes exhibited superior performance owing tosuppressed Si expansion and electrode collapse. Although theperformance of the LaSi2/CrSi2/Si and LaSi2/TiSi2/Si electrodesdiffered, the amounts of a-Li-rich phase formed were practically thesame, suggesting a difference in Li distribution.Figure S26 shows the variation in the amounts of the a-Li-richphase formed on the LaSi2/MSi2/Si, MSi2/Si, LaSi2/Si, and pure Si(M = Cr, Mo, Nb, Ta, Ti, or W) electrodes with the cycle number.The a-Li-rich phase of the LaSi2/MSi2/Si electrode was less than thatof the LaSi2/Si electrode in all the cycles regardless of the element ofM. Although the a-Li-rich phase of the MSi2/Si electrode was lessthan that of the LaSi2/MSi2/Si electrode early in the cycles, itexceeded those of the LaSi2/MSi2/Si and LaSi2/Si electrodes afterFigure 9. Changes in the (a) relative thickness (t/t0) and (b) the amount ofthe a-Li-rich phase of each electrode with cycle number. t0 and t representthe thickness before and after cycling, respectively. The value of t wasinvestigated in the lithiation state. The formed amount indicates the peak areaof the a-Li-rich phase estimated by peak fitting each dQ/dV plot (Figs.S13–S16).Figure 10. (a) Cycle lives of LaSi2/MSi2/Si composite electrodes (M = Cr, Mo, Nb, Ta, Ti, or W) in 1 M LiFSA/Py13-FSA with a charge capacity limitation of1000 mA h g–1 at 1 C per unit weight of elemental Si. The values in parentheses denote the Vickers hardness of MSi2. (b) Correlation between the Vickershardness of stiff silicide and the cycle lives of composite electrodes containing various stiff silicides, LaSi2, and elemental Si.Journal of The Electrochemical Society, 2024 171 080506cycling (M =W, Ta, or Nb). The final amount of the a-Li-rich phaseformed on the TiSi2/Si and CrSi2/Si electrodes was comparable tothat formed on the LaSi2/Si electrode.Figure S27 shows the overlaid views of the EDS maps of theLaSi2/WSi2/Si, LaSi2/CrSi2/Si, and LaSi2/MoSi2/Si electrodes. Priorto charge–discharge cycling, the Si phase (with a diameter of severaltens of nanometers) was surrounded by a stiff silicide (WSi2, CrSi2,or MoSi2) and LaSi2. This indicated that its metallographic structurevirtually remained the same, regardless of the stiff silicide. After the600th and 1200th cycles, no microstructural inversion was observed.No differences were observed in the microstructure of the electrodes;thus, we obtained low-magnification BF-TEM images (Fig. 13).After 600 cycles, the LaSi2/MoSi2/Si electrode with a relatively longcycle life exhibited more cracks than the LaSi2/CrSi2/Si electrodewith a medium cycle life (similar to the LaSi2/WSi2/Si electrodewith the shortest cycle life). After 1200 cycles, both electrodesexhibited the further formation of fine cracks. No significant crackswere observed on the LaSi2/CrSi2/Si electrode until the 1200thcycle. After 1700 cycles, when the capacity fading of theLaSi2/CrSi2/Si electrode was observed, many fine cracks wereobserved. Three electrodes maintained a discharge capacity of1000 mA h g(Si)–1 up to 1200 cycles, implying that the aforemen-tioned cracks were not directly related to capacity decay. Figure 13show that other mechanical properties, such as fatigue life andfracture toughness, may have influenced electrode collapse.Figure 11. Cycle lives of LaSi2/MSi2/Si composite electrodes, where M = (a) W, (b) Ta, (c) Ti, (d) Mo, and (e) Nb in 1 M LiFSA/Py13-FSA with a chargecapacity limitation of 1000 mA h g–1 at 1 C per unit weight of elemental Si. The results of MSi2/Si, LaSi2/Si, and pure Si electrodes are shown.Figure 12. Changes in the amount of a-Li-rich phase formed on theLaSi2/MSi2/Si composite electrodes (M = Cr, Mo, Nb, Ta, Ti, or W) withthe cycle number. The results for the pure Si electrode are illustrated. Theamount formed indicates the peak area of the a-Li-rich phase estimated bypeak fitting each dQ/dV plot.Journal of The Electrochemical Society, 2024 171 080506ConclusionsTo further improve the charge–discharge cycle life of LaSi2/Sinanocomposite anodes for LIBs, we added stiff CrSi2 to LaSi2/Si.The cycle life of the LaSi2/CrSi2/Si electrode was longer than thoseof the CrSi2/Si and LaSi2/Si electrodes. Although the microstructuralinversion of the LaSi2/Si electrode was observed, the addition ofCrSi2 suppressed the change in the metallographic structure aftercycling. Stiff CrSi2 functioned as a support framework, improvingthe structural stability of the electrode and preventing changes in themetallographic structure. Consequently, a superior cycle life wasachieved owing to the synergistic effect of LaSi2 and CrSi2; theformer exhibited elasticity to relax the Si-generated stress, and thelatter exhibited stiffness to withstand the Si-generated stress andmaintain the metallographic structure. We investigated the cyclelives of the prepared LaSi2/MSi2 (whereM =Mo, Nb, Ta, Ti, or W)/Si nanocomposite electrodes. The cycle life improved as the Vickershardness of M decreased, and the longest cycle life (a reversiblecapacity of 1000 mA h g(Si)–1 for 1800 cycles) was obtained usingNbSi2 as the stiff silicide. Although the LaSi2/CrSi2/Si,LaSi2/TaSi2/Si, LaSi2/TiSi2/Si, and LaSi2/NbSi2/Si electrodes ex-hibited the synergistic effect of elastic LaSi2 and stiff MSi2, no clearsynergistic effect was observed when other stiff silicides (M = Moand W) were used. The trend was observed when LaSi2 was used asthe elastic silicide, and it is unclear whether the same trend might nothave been obtained in combination with other elastic silicides. Basedon the dQ/dV plots, the a-Li-rich phase formed on the LaSi2/WSi2/Siand TaSi2/LaSi2/Si electrodes, with relatively poor cycle lives, wasmore than that on the LaSi2/MoSi2/Si and LaSi2/NbSi2/Si electrodes,with relatively superior performance. Consequently, the latter twoelectrodes exhibited longer cycle livesowing to suppressed Siexpansion and electrode collapse. Other mechanical properties, e.g., fatigue life and fracture toughness, may have influenced electrodecollapse. This study provides insights into the synergistic effects ofvarious silicide functions on the charge–discharge cycle life ofbatteries.AcknowledgmentsTEM was performed at NIMS Battery Research Platform. Theauthors thank Dr Y. Kimura of the Daido Steel Co. Ltd. for hissupport in measuring the Vickers hardness (TiSi2). Moreover, theauthors thank Ms. N. Hino, Mr T. Yamaga, and R. Tanaka of theTottori University for their assistance with image analysis. Themanuscript was written with contributions from all authors. Thisstudy was partially supported by the Japan Society for the Promotionof Science (JSPS) KAKENHI (Grant numbers JP24K08565,JP20H00399, and JP23K26758) and NIMS Joint Research HubProgram.ORCIDYasuhiro Domi https://orcid.org/0000-0003-3983-2202Hiroyuki Usui https://orcid.org/0000-0002-1156-0340Kei Nishikawa https://orcid.org/0000-0002-7718-7606Hiroki Sakaguchi https://orcid.org/0000-0002-4125-7182References1. J. M. Tarascon and M. Armand, “Issues and challenges facing rechargeable lithiumbatteries.” Nature, 414, 359 (2001).2. M. Armand and J. M. Tarascon, “Building better batteries.” Nature, 451, 652(2008).3. M. S. Whittingham, “Lithium batteries and cathode materials.” Chem. Rev., 104,4271 (2004).4. J. B. Goodenough and Y. 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