# Fileset

[93_25-00015.pdf](https://mdr.nims.go.jp/filesets/cfe68821-321b-4116-b4be-0a35463e0e23/download)

## Creator

[Yasuhiro DOMI](https://orcid.org/0000-0003-3983-2202), [Hiroyuki USUI](https://orcid.org/0000-0002-1156-0340), Takumi ANDO, Ryuto TANAKA, [Kazuma GOTOH](https://orcid.org/0000-0002-8197-5701), [Takeo HOSHI](https://orcid.org/0000-0002-2487-5245), [Kei NISHIKAWA](https://orcid.org/0000-0002-7718-7606), [Hiroki SAKAGUCHI](https://orcid.org/0000-0002-4125-7182)

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Electrochemical Lithiation Mechanism of Nickel Silicide Electrode](https://mdr.nims.go.jp/datasets/2bfa4d4e-e7bf-4def-9d01-c811b019bf85)

## Fulltext

Electrochemical Lithiation Mechanism of Nickel Silicide ElectrodeArticle Electrochemistry, 93(3), 037009 (2025)Electrochemical Lithiation Mechanism of Nickel Silicide ElectrodeYasuhiro DOMI,a,c,*,§ Hiroyuki USUI,a,c,§ Takumi ANDO,b,c Ryuto TANAKA,b,c,§§Kazuma GOTOH,d,§ Takeo HOSHI,e Kei NISHIKAWA,f,§ and Hiroki SAKAGUCHIa,c,*,§§§a Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University,4-101 Minami, Koyama-cho, Tottori 680-8552, Japanb Course of Chemistry and Biotechnology, Department of Engineering, Graduate School of Sustainability Science, Tottori University,4-101 Minami, Koyama-cho, Tottori 680-8552, Japanc Center for Research on Green Sustainable Chemistry, Tottori University, 4-101 Minami, Koyama-cho, Tottori 680-8552, Japand Center for Nano Materials and Technology, Japan Advance Institute of Science and Technology,1-1 Asahidai, Nomi, Ishikawa 923-1292, Japane Plasma Quantum Process Unit, National Institute for Fusion Science, 322-6 Oroshi, Toki-shi, Gifu 509-5292, Japanf Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS),1-1 Namiki, Tsukuba 305-0044, Japan*Corresponding authors: domi@tottori-u.ac.jp (Y. D.), Tel/Fax: +81-857-31-5249, sakaguch@tottori-u.ac.jp (H. S.), Tel/Fax: +81-857-31-5265ABSTRACTPure silicide electrodes have attracted attention as a promisinganode material in lithium-ion batteries using certain ionic liquidelectrolytes. However, the reaction mechanisms of silicide elec-trodes, particularly the lithiation sites in the crystal lattice and thereaction sites (bulk versus surface), remain unclear. Here, weinvestigated the electrochemical lithiation mechanism of a nickelsilicide (NiSi2) electrode. X-ray diffraction, transmission electronmicroscopy, and other techniques revealed that NiSi2 phase did notseparate and no lithiation of Si generated from NiSi2 occurred. Incontrast, 7Li magic angle spinning nuclear magnetic resonancedemonstrated that stable deposition–dissolution of Li metal did notoccur on the NiSi2 electrode, and electrochemical lithiation of NiSi2proceeded. Additionally, we investigated the lithiation sites usingcomputational chemistry. The peak positions in the nuclear magnetic resonance spectra differed from those predicted using the calculatedvalence electron numbers. This resulted from an increase in conduction electrons near the Fermi energy associated with the amount of Listored in the NiSi2 crystal lattice, followed by a Knight shift.© The Author(s) 2025. Published by ECSJ. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY,https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium provided the original work is properly cited. [DOI:10.5796/electrochemistry.25-00015].Keywords : Reaction Mechanism, Lithiation Site, Ionic-liquid Electrolyte, Knight Shift1. IntroductionSilicon (Si) has attracted attention as a promising anode materialin lithium-ion batteries (LIBs) with gravimetric high-energy densitybecause of its high theoretical capacity of 3600mAh g¹1 in thecrystalline Li3.75Si phase at room temperature.1–4 The developmentof high-performance LIBs is essential to realize a decarbonizedsociety, and the practical application of Si anodes is gainingmomentum.5–8 However, the poor cycling stability caused by largevolume changes in Si during alloying (charge) and dealloying(discharge) with lithium (Li) prevents its practical application. Wepreviously investigated how to improve the anode properties of pureSi electrodes by doping impurities into Si,9,10 fabricating compositeswith materials that compensate for the Si defects,11–16 andprelithiation.17,18 Additionally, we have developed Si-specific inter-face observation methods.19–21In contrast, we investigated the feasibility of pure silicide (MSix,M: transition metal) electrodes, which are Si compounds withmetals. MSix electrodes have not received much attention becausethey have not shown good cycling performance in conventionalorganic liquid electrolytes.11 We also discovered that the MSixelectrodes achieved very high capacity and excellent cyclingstability in a particular ionic liquid electrolyte of 1mol dm¹3 (M)Li bis(fluorosulfonyl)amide (LiFSA) in N-methyl-N-propylpyrroli-dinium bis(fluorosulfonyl)amide (Py13-FSA).22 Furthermore, mostMSix have high density, therefore, high volumetric reversiblecapacity can be expected.23The electrochemical lithiation mechanism of some MSixelectrodes has been reported; lithiation of Si resulting from thephase separation of MSix occurred.24–26 However, the lithiation-§ECSJ Active Member§§ECSJ Student Member§§§ECSJ FellowY. Domi orcid.org/0000-0003-3983-2202H. Usui orcid.org/0000-0002-1156-0340K. Gotoh orcid.org/0000-0002-8197-5701T. Hoshi orcid.org/0000-0002-2487-5245K. Nishikawa orcid.org/0000-0002-7718-7606H. Sakaguchi orcid.org/0000-0002-4125-7182ElectrochemistryThe Electrochemical Society of Japan https://doi.org/10.5796/electrochemistry.25-00015https://doi.org/10.50892/data.electrochemistry.28490150Received: January 21, 2025Accepted: February 24, 2025Published online: February 27, 2025Issued: March 29, 20251https://orcid.org/0000-0003-3983-2202https://orcid.org/0000-0002-1156-0340https://orcid.org/0000-0002-8197-5701https://orcid.org/0000-0002-2487-5245https://orcid.org/0000-0002-7718-7606https://orcid.org/0000-0002-4125-7182https://creativecommons.org/licenses/by/4.0/https://doi.org/10.5796/electrochemistry.25-00015https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://orcid.org/0000-0003-3983-2202https://orcid.org/0000-0003-3983-2202https://orcid.org/0000-0003-3983-2202https://orcid.org/0000-0002-1156-0340https://orcid.org/0000-0002-1156-0340https://orcid.org/0000-0002-1156-0340https://orcid.org/0000-0002-8197-5701https://orcid.org/0000-0002-8197-5701https://orcid.org/0000-0002-8197-5701https://orcid.org/0000-0002-2487-5245https://orcid.org/0000-0002-2487-5245https://orcid.org/0000-0002-2487-5245https://orcid.org/0000-0002-7718-7606https://orcid.org/0000-0002-7718-7606https://orcid.org/0000-0002-7718-7606https://orcid.org/0000-0002-4125-7182https://orcid.org/0000-0002-4125-7182https://orcid.org/0000-0002-4125-7182https://doi.org/10.5796/electrochemistry.25-00015https://doi.org/10.5796/electrochemistry.25-00015https://doi.org/10.50892/data.electrochemistry.28490150https://doi.org/10.50892/data.electrochemistry.28490150delithiation of MSix itself is expected to occur based on thedissolution enthalpy, rather than the phase separation of MSix. Forexample, the dissolution enthalpy of Fe-Si, Ni-Si, and La-Si is ¹67,¹86, and ¹285 kJmol¹1, respectively.27 Larger negative value ofthe enthalpy indicates that these silicides are thermodynamicallystable. In contrast, the dissolution enthalpy of Si-Li has beenreported to be ¹51 kJmol¹1.27 Because Si has a higher affinity fortransition metals than for Li, it is inferred that MSix is less likely tobe decomposed during charge-discharge cycling.We previously investigated the reaction behavior of an ironsilicide (FeSi2) electrode using X-ray diffraction (XRD), Ramanspectroscopy, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and solid-state 7Li magicangle spinning (MAS) nuclear magnetic resonance (NMR).28 Weconcluded that the FeSi2 electrode alloyed with Li; nevertheless,lithiation sites in the FeSi2 crystal lattice and reaction locations (bulkor surface) remained unclear. It is also unclear whether the reactionbehavior is identical when different M is used. In the present study,the electrochemical lithiation mechanism of a nickel silicide (NiSi2)electrode in 1M LiFSA/Py13-FSAwas investigated using the aboveanalytical methods. Additionally, we attempted to identify thelithiation sites using density functional theory for first-principlescalculations. We predicted the NMR peak positions based on thecalculated valence electron numbers and discussed the Knight shift,which occurs when conduction electrons near the Fermi energyincrease according to the quantity of Li stored in the NiSi2 crystallattice.2. Experimental2.1 Sample preparation and electrode fabricationNiSi2 was prepared by a mechanical alloying (MA) method. Amixture of silicon (99.9%, FUJIFILM Wako Pure Chemical Corp.,Ltd.) and nickel (99.9%, Nilaco Corp.) powders was put in a ZrO2container together with ZrO2 balls. Dry Ar gas was filled thecontainer. The Si/Ni molar ratio was 2.0 and the weight ratio of themixture to the balls was approximately 1 : 15. High-energyplanetary ball mill apparatus (P-6, Fritch) was used for the MAprocess performing with a rotary speed of 380 rpm at roomtemperature for 20 h. The crystal structure of prepared powder wasverified by XRD (Ultima IV, Rigaku) with Cu-KA radiation andobtained XRD pattern was identified using the Inorganic CrystalStructure Database (ICSD).An NiSi2 electrode was fabricated by slurry coating method for7Li MAS NMR and XRD measurements. Acetylene black (AB),styrene-butadiene rubber (SBR), and carboxymethyl cellulose(CMC) were used as the conductive agent, binder, and thickener,respectively. The ratio of NiSi2/AB/SBR/CMC was 70/15/5/10wt%. 4 g of deionized water was used as a dispersing agent for1 g of the mixture. The prepared slurry was coated on a coppercurrent collector and was dried at 120 °C. In contrast, an NiSi2 gas-deposition (GD) electrode was prepared for TEM and Ramanmicroscopy measurements. The GD electrode was used instead ofthe slurry electrode to investigate changes in the active materialitself in TEM observation, and because of the possibility that CMC-derived peaks might appear at 500 cm¹1, where Si-derived peaksappear in the Raman spectra.29 The GD method requires noconductive agent or binder. He gas (99.9999%) was used as a carriergas and the deposited amount of NiSi2 on the current collector wasapproximately 30µg. Other detail conditions of GD method weredescribed in our previous paper.192.2 Cell assembly and charge–discharge testingA 2032-type coin cell was assembled using the NiSi2 electrode, aglass fiber filter (Whatman GF/A), and a Li metal sheet (99.90%,thickness: 1mm, Rare Metallic Co., Ltd.) as the working electrode,the separator, and the counter electrode, respectively. We used 1MLiFSA/Py13-FSA as an ionic-liquid electrolyte. Cell assembly andelectrolyte preparation were conducted in an Ar-filled glovebox(Miwa MFG, DBO-2.5LNKP-TS) with an oxygen content less than1 ppm and a dew point below ¹90 °C.Galvanostatic charge–discharge testing was performed usingan electrochemical measurement system (HJ-1001SM8A or HJ-1001SD8, MEIDEN HOKUTO, Co., Ltd.) in the potential rangebetween 0.005 and 2.000V vs. Li+/Li at 30 °C. The current densitywas set at 50mAg¹1. After the testing of the cell using slurryelectrode for 4 cycles, the cell was charged to adjust the state ofcharge (SOC) at the fifth cycle for each point of the 7Li MAS NMRmeasurement. The potential was maintained at each point in Fig. S1afor 12 h. The charge capacity at the first cycle was higher than thatafter the second cycle due to the formation of a surface film, whereasthe charge–discharge capacity and the corresponding curve werealmost the same after the second cycle (Fig. S1b). The testing of thecell using GD electrode was carried out for 5 cycles. The Coulombicefficiency of the slurry electrode was higher than that of the GDelectrode, and hence, a side reaction of AB with Li+ should not occur.2.3 7Li MAS NMR measurement and TEM observationThe coin cell was disassembled in the Ar-filled glovebox after theabove–mentioned charge–discharge testing. The slurry electrodewas washed thoroughly with diethyl carbonate (DEC, KishidaChemical Co., Ltd.) and was dried. The active material layerincluding NiSi2, AB, SBR, and CMC was exfoliated from Cusubstrate using a spatula. The resulting powder was packed in a3.2mmº sample tube for the NMR measurement. 7Li MAS NMRspectra of NiSi2 at each SOC were obtained using an NMR system(11.7 T magnet, DD2 Agilent Technologies Inc.) at a MASfrequency of 14 kHz. The chemical shifts were referenced to a1M aqueous LiCl solution. The peak fitting of NMR spectra wasperformed by Origin Pro 8.5.0J (LightStone) software.Removed NiSi2 GD electrode from the coin cell was washed withpropylene carbonate and DEC before drying. The electrode wassliced into thin sections using focused ion beam scanning electronmicroscopy (FIB-SEM, SMF2000, Hitach High-Tech ScienceCorp.). The electrode surface was coated with carbon to protect itfrom damage by the Ga ion beam. The sliced electrode was notexposed to the atmosphere until it was introduced into the chamberof the TEM instrument (JEM-ARM200F, JEOL, Co., Ltd.) using atransfer vessel. TEM observation was performed with accelerationvoltage of 200KV and energy-dispersive X-ray spectroscopy (EDS)was carried out with 10 scans.2.4 First-principles calculationWe used a commercially available Advance/PHASE softwarepackage (Advance Soft Corp.) using plane wave expansion andpseudopotentials based on density functional theory for first-principles calculation. Projector augmented wave (PAW) andgeneralized gradient approximation (GGA) were utilized ascalculation method and exchange–correlation energy function. Weused the k-space integrations of 5 © 5 © 5 k-point mesh. We alsoused a cut-off energy of 340 eV for wave function (total density ofstate: total DOS) and 3060 eV for charge density (Bader analysis),which corresponds to 12.5 and 112.5 Hartree, respectively(1 Hartree equals to 27.2114 eV). The valence electron number ofLi was estimated by the Bader analysis.3. Results and Discussion3.1 Possible electrochemical lithiation mechanisms of theNiSi2 electrodeFigure 1 shows a schematic diagram of the possible electrochem-ical lithiation mechanisms for NiSi2 electrodes. In the first mechan-Electrochemistry, 93(3), 037009 (2025)2ism, the NiSi2 phase separates into Si and Ni and/or Si-poor nickelsilicides (NiSi, Ni2Si, and Ni3Si) phases, generating Si alloys with Li(Fig. 1a). The following reactions are expected to occur:NiSi2 þ xLiþ þ xe� ! Niþ 2LixSi ð1ÞNiSi2 þ xLiþ þ xe� ! NiSiþ LixSi ð2Þ2NiSi2 þ xLiþ þ xe� ! Ni2Siþ 3LixSi ð3Þ3NiSi2 þ xLiþ þ xe� ! Ni3Siþ 5LixSi ð4ÞIn the second possible mechanism, deposition and dissolution of Limetal occur on the NiSi2 electrode (Fig. 1b). Because the charge–discharge cycle stability of the electrode in 1M Py13-FSA is good,Li dendrites should not be produced. The NiSi2 electrode may act asa foothold for stable Li deposition–dissolution. In the third possiblemechanism, the NiSi2 itself alloys and dealloys with Li as follows(Fig. 1c):NiSi2 þ xLiþ þ xe� ! LixNiSi2 ð5ÞIn the fourth possible mechanism, the bulk of the NiSi2 particle isnot lithiated and only the surface alloys with Li (Fig. 1d). This isa new hypothesis that arose while investigating the above threepossibilities. Herein, each of these four mechanisms was examined.3.2 Lithiation of Si generated from NiSi2 phase separationThe active material of NiSi2 was successfully synthesized using amechanical alloying method and characterized using XRD, Ramanspectroscopy, field-emission scanning electron microscopy (FE–SEM), and a particle size distribution analyzer (Figs. S2 and S3). Toverify whether the NiSi2 phase separates into Si and Ni and/or Si-poor nickel silicides (Fig. 1a), we investigated the change in theNiSi2 crystal structure during charge–discharge cycling. Figure 2adisplays the XRD pattern of the fully lithiated NiSi2 electrode duringthe fifth cycle. For comparison, the result of the pristine NiSi2electrode is also shown. All peaks except those derived from thecopper substrate and the protective film were attributed to NiSi2. Nonew peaks derived from Si, Ni, Si-poor nickel silicides (NiSi, Ni2Si,and Ni3Si), or lithium silicides (LixSi) appeared. The crystallinity ofSi decreases after charge–discharge cycling, i.e., crystalline Si (c-Si)changes to amorphous Si (a-Si).19,30 Because XRD can not detect a-Si, Raman spectroscopy, which can detect Si of any crystallinity,Figure 1. Schematic diagram of possible electrochemical lithiation mechanisms for NiSi2 electrodes: (a) lithiation of Si generated fromNiSi2, (b) stable deposition of Li metal on the NiSi2, (c) alloying of NiSi2 with Li, and (d) lithiation of only the NiSi2 surface.20 30 40 50 60Intensity / a.u.2 (Cu-K) / deg.NiSi2●Cu (substrate)□● 3 1 1● 2 2 0● 1 1 1□ 1 1 1□ 2 0 0Protective film〇〇〇Before cyclingLithiation state(after the 5th cycle)Si, ICSD:00-027-1402Cu, ICSD:00-006-0696Ni, ICSD:00-004-0850NiSi2, ICSD:01-089-7095a450 500 550Raman shift / cm-1Intensity / a. u.Before cyclinga-Si c-SiDelithiation state(after the 5th cycle)b45 46 47 48 49 50Intensity / a.u.2 (Cu-K) / deg.Figure 2. (a) XRD patterns of the pristine NiSi2 and lithiated NiSi2 electrode after the fifth charge–discharge cycle and (b) Raman spectra ofthe NiSi2 electrode before and after the fifth cycle with those of a-Si and c-Si.Electrochemistry, 93(3), 037009 (2025)3was used to investigate whether elemental Si was generated fromNiSi2 (Fig. 2b). No Si of any crystallinity was detected, indicatingthat phase separation of NiSi2 did not occur during charge–dischargecycling. However, if the NiSi2 phase separates into Si and Ni and/orSi-poor NiSix (x = 1, 1/2, and 1/3) phases at the extreme surfaceand ultrafine Si particles (diameter ¯ 100 nm) are formed, Ramanspectroscopy cannot detect the particles because of the definiteresolution. Hence, we used TEM to observe a cross-section of theNiSi2 electrode after the charge–discharge cycling.Figure 3a shows a TEM image of the delithiated NiSi2 electrodeafter the fifth charge–discharge cycle. The corresponding EDS mapsof Ni and Si, the selective area electron diffraction (SAED) pattern,and the obtained d-spacing values are also shown (Figs. 3b–3d,respectively). Ni and Si elements were uniformly distributed, and noultrafine Si particles appeared. The SAED and d-spacing resultsshowed that a Si phase was not formed, and the active material layerremained NiSi2 after cycling. Figures 2 and 3 show that no phaseseparation of NiSi2 occurred and a pure Si phase was not generated;thus, the electrochemical lithiation mechanism of the NiSi2 electrodecould not be explained by the first hypothesis (Fig. 1a).3.3 Li Deposition on NiSi2Figure 4 provides the 7Li MAS NMR spectrum of the fullylithiated NiSi2 electrode during the fifth cycle. Peaks with asteriskswere assigned to spinning sidebands, which is outside the scope ofthis discussion. Li metal peak, including Li dendrites, appeared atapproximately 260 ppm,31,32 such peak was not detected. The samephenomenon was previously confirmed on a FeSi2 electrode.28 Incontrast, main peaks at approximately 0 ppm were confirmed, asdiscussed below. Therefore, the NiSi2 electrode did not serve as afoundation for stable Li deposition–dissolution; thus, the elec-trochemical lithiation mechanism of the NiSi2 electrode could not beexplained by the second hypothesis (Fig. 1b).3.4 Lithiation and delithiation of NiSi2 itselfFigure 5a provides the 7Li MAS NMR spectra of theNiSi2 electrodes at each state of charge (SOC) during the fifthcycle. At an SOC of 0%, a peak defined as Peak A appeared atapproximately 0 ppm. Peak A at 0% SOC could not be attributed toLi stored in the NiSi2 because the electrode potential was too high(Fig. S1). Therefore, Peak Awas attributed to either Li salt (LiFSA)in the electrolyte (LiFSA shows extremely clear NMR peak atapproximately ¹1 ppm) or Li contained in the surface film formedon the NiSi2 electrode by reductive decomposition of the electrolyte.Because the NiSi2 electrode had been carefully cleaned with DEC,no LiFSA was expected to remain on the surface. Additionally, Li-containing compounds (i.e., lithium hydroxide and lithium fluoride),which were components of the surface film, show NMR peaks atapproximately 0 ppm.33–35 Hence, Peak A at 0% SOC was assignedto Li in the surface film.At SOCs from 20% to 40%, the intensity of Peak A increased(Figs. 5a and 5b), although the peak position did not change(Fig. 5c). The Coulombic efficiency was as high as 97.7% duringthe fifth cycle (Fig. S4); thus the increase in Peak A at SOCsbetween 20% and 40% was not attributed to the growth of thesurface film. Grey et al. reported that various Li–Si alloy phases(LixSi, x = 1.71, 2.33, 3.25, and 3.75) generate 7Li NMR peaksbetween 18.0 ppm and 6.0 ppm, which were assigned to Li nearsmall Si clusters and isolated Si.36,37 The peak centered at 18.0 ppmshifted toward a lower chemical shift (higher magnetic field) as xincreased. The peak position of Peak A did not correspond to theabove Li–Si alloys; hence, Peak A between 20% and 40% SOCwas assigned to Li stored in NiSi2.Figure 3. (a) TEM image of delithiated NiSi2 after the fifth cycle and the corresponding EDS maps of (b) Ni and (c) Si. (d) SAED patternwith the corresponding d-spacings are also shown.Figure 4. 7Li MAS NMR spectrum of NiSi2 at an SOC of 100%during the fifth cycle. Magnified views at approximately 0 ppm andapproximately 260 ppm are also shown.Electrochemistry, 93(3), 037009 (2025)4At a high SOC of 60%, a shoulder appeared at approximately15 ppm, which was defined as Peak B. Peak B increased with theincrease in SOC and shifted toward a lower chemical shift. Theappearance of both peaks denoted two possibilities: (i) Li was storedin two different forms (i.e., Si and NiSi), or (ii) Li existed in twodifferent chemical environments within the same material (NiSi2).Because the Li2.33Si phase produces an NMR peak at approximately16.5 ppm,36 Peak B could arise from this phase. However, Figs. 2and 3 revealed the absence of Si before and after charge–dischargecycling. Thus, the occurrence of Peaks A and B indicated thatLi existed in two different chemical environments within NiSi2.Consequently, the increase in the peak area between SOCs of 20%and 100% can be attributed to the lithiation of the NiSi2 electrodeaccording to Eq. 5; thus, the electrochemical lithiation mechanismof the NiSi2 electrode can be explained by the third hypothesis(Fig. 1c). The details of the lithiation sites are described in the nextsection.When LiyNiSi2 forms after cycling, the XRD peaks of NiSi2 shifttoward a lower angle because of NiSi2 lattice expansion caused byLi storage. It was also confirmed that the NiSi2 lattice volumeexpanded with an increase in the amount of Li stored in the NiSi2crystal lattice by first-principles calculations (Table S1, the crystalstructure was optimized); however, such a peak shift was notobserved (Fig. 2a inset). It is possible that the bulk of NiSi2 did notstore Li and only the surface was alloyed with Li, but we did nothave a method to distinguish the reaction point (bulk or surface) ofthe metal silicide. Thus, the fourth hypothesis (Fig. 1d) remains apossibility. Although the y value in LiyNiSi2 was estimated to be 2.1for the capacity of 500mAhg¹1, the y value becomes extremelylarge when only the surface is alloyed with Li.3.5 Lithiation site of NiSi2We investigated the lithiation site of NiSi2 using first-principlescalculations. Figure 6 shows the optimized crystal structure ofLiyNiSi2 for y = 0.5 (SOC: 20%), 1.0 (SOC: 40%), and 2.25 (SOC:90%). All the Li sites at SOCs of 20% and 40% were equivalent tothe position of (x, y, z) = (0.5, 0.5, 0.5) and denoted by purple ballsin Fig. 6, whereas those at an SOC of 90% were classified into twoinequivalent sites denoted by purple and blue balls in Fig. 6. Thesites of the blue balls were equivalent to the position of (x, y,z) = (0.28, 0.5, 0.5). Although we also attempted to optimize thestructures of y = 1.5 (SOC: 60%) and 2.0 (SOC: 80%), thecalculation did not converge. The valence electron numbers of the Lisites were calculated using Bader analysis (Table 1). The NMRspectra at SOCs of 20% and 40% exhibited only one peak (Peak Ain Fig. 5), whereas those at SOCs of 60%–100% showed two peaks(Peak A and B in Fig. 5). These results indicate that all of the Lisites at SOCs of 20% and 40% were equivalent, whereas those atSOCs of 60%–100% were divided into two equivalent sites, whichcorresponded to the optimized structures in Fig. 6.The valence electron number of Li atom at SOCs of 20% and40% was 0.20 (Table 1). In contrast, the valence electron number ofLi atom at 90% SOC was 0.16 and 0.29. The number 0.16, which isclose to the original value of 0.20, was expected to belong toPeak A, and the number 0.29 was expected to belong to Peak B.Figure 5b shows that the peak area ratio at 100% SOC was almostidentical to the ratio of Li atoms. However, when the valenceelectron number was larger, the peak appeared at a higher magneticfield (lower chemical shift) in the NMR spectra, contradicting theresult in Fig. 5a (Peak B with a larger valence electron number mustappear on the right side of Peak A).It has been reported that LixSi phases with low lithiation (x =1.71, 2.33, 3.25, and 3.75) are semiconductors, whereas Li4.20Sionly occurs as a metallic phase. With an increase in conductionelectrons near the Fermi level, the NMR peak of Li4.20Si appears at alower magnetic field (higher chemical shift) than other LixSi phases,which is influenced by large Knight shifts.36 Although NiSi2 ismetallic, Peak B could appear at a lower magnetic field than Peak Abecause of an increase in the conduction electrons. To estimate thechange in the number of conduction electrons as y increases inLiyNiSi2, we investigated the charge transfer resistance usingelectrochemical impedance spectroscopy. However, there was nochange in resistance as the SOC increased. Therefore, we calculatedthe total density of states (total DOS) and attempted to determine thenumber of conduction electrons near the Fermi level.3.6 DOS analysis of LiyNiSi2Figure S5 shows the total DOS of LiyNiSi2 (y = 0, 0.5, 1.0, 2.25,and 2.5) at 298K. When the absolute temperature (T ) of the systemis sufficiently ²0K, the thermal energy excites electrons at levelsbelow the Fermi energy (EF), resulting in electrons above the EF. Insuch cases, the energy distribution function of electrons ( f (E)) isdetermined according to the Fermi–Dirac distribution function givenby0510150 20 40 60 80 100Chemical shift / ppmState of charge / %cPeak APeak B05001000Peak area of NMR spectraExperimental■▲Peak APeak B●b-2002040Chemical Shift / ppmLiFSA0%20%40%60%80%SOC=100%a Peak A(ca. 0 ppm)Peak B(ca. 12 ppm)Figure 5. (a) 7Li MAS NMR spectra of NiSi2 at various SOCsduring the fifth cycle in 1M LiFSA/Py13-FSA at 50mAg¹1 afterpre-cycling. Correlation between the SOC and the (b) peak area and(c) chemical shift. Peak fitting data are shown as red and blue linesin part (a).Figure 6. Crystal structure of LiyNiSi2 (y = 0.5, 1.0, and 2.25).Table 1. Number of valence electrons and Li atoms in LiyNiSi2(y = 0.5, 1.0, and 2.25).y in LiyNiSi2 SOCValence electronnumbers of LiNumberof Li atomsBall colorin Fig. 60.5 20 0.20 2 Purple1.0 40 0.20 4 Purple2.25 900.16 3 Purple0.28 6 BlueElectrochemistry, 93(3), 037009 (2025)5f ðEÞ ¼ 1expE � EFkBT� �þ 1ð6Þwhere kB is the Boltzmann constant.38 The electronic states in theenergy range of EF ¹ 0.25 eV < E < EF + 0.25 eV were used toestimate the number of conduction electrons (Fig. S5). The resultswere 1.81, 2.40, 2.96, 4.84, and 5.49 for y = 0, 0.5, 1.0, 2.25, and2.5 in LiyNiSi2, respectively (Fig. 7), confirming that the number ofconduction electrons was an increasing function of y.Knight shifts in the 7Li NMR of Li4.20Si have only been reportedamong different LixSi phases. We confirmed the total DOS ofdifferent LixSi phases and compared them with that of LiyNiSi2.Figure S6 provides the DOS of LixSi (x = 0, 2.33, 3.25, 3.75, and4.20) at 298K, which was consistent with the trend found in theprevious report.39 The numbers of conduction electrons wereestimated to be 0.20, 1.59, 3.47, 6.17, and 9.76 for x = 0, 2.33,3.25, 3.75, and 4.20, respectively (Fig. 7). As in LiyNiSi2, thenumber of conduction electrons increased with an increase in the xvalue. Although Knight shifts have not previously been confirmed inLi3.75Si phase,36,37 the number of conduction electrons was relativelyhigh and the Li3.75Si seemed to be a metallic phase (Fig. S6d).However, the various LixSi (x = 0–3.75) phases have been reportedto be semiconductors.36,40–42 In contrast, because NiSi2 wasoriginally a metallic phase, it was assumed that a Knight shiftoccurred at an SOC of >60%.Although Peak B must appear at a higher magnetic field (lowerchemical shift) than Peak A based on the valence electron numbers,it appeared at a lower magnetic field (higher chemical shift). Thiswas caused by an increase in the number of conduction electronsnear the Fermi level and the occurrence of Knight shifts. Therefore,Peaks A and B were assigned valence electron numbers of 0.16 and0.29, respectively. Additionally, the Li storage sites of Peaks A andB corresponded to the purple and blue atomic positions in Fig. 6. Aspreviously reported, the FeSi2 electrode at 20% SOC produced ashoulder derived from Peak B at approximately 11 ppm during thefifth cycle, and the intensity of Peak B increased with an increase inSOC.28 For the NiSi2 electrode, the lithiation site arising fromPeak B is expected to be slightly different because the positions ofPeak B and the SOCs at which Peak B appeared differed betweenNiSi2 and FeSi2.4. ConclusionsWe clarified the electrochemical lithiation mechanism of an NiSi2electrode. Four possible mechanisms were suggested, and eachpossibility was investigated in detail using XRD, Raman spec-troscopy, TEM, EDS, and NMR. The results showed that there wasno NiSi2 phase separation and a pure Si phase was not generated;thus, lithiation of Si generated from NiSi2 did not occur. Addition-ally, stable Li metal deposition–dissolution did not occur on theNiSi2 electrode. Although the reaction point (bulk or surface) of themetal silicide remains unclear, electrochemical lithiation of the NiSi2itself proceeded. Furthermore, we determined the lithiation site inNiSi2 using NMR analysis and first-principles calculations. At alower SOC, there was only one lithiation site equivalent to (x, y, z) =(0.5, 0.5, 0.5). At a higher SOC, Li atoms were stored at another sitein the NiSi2 crystal lattice, which was equivalent to (x, y, z) = (0.28,0.5, 0.5). The position of Peak B in the NMR spectra could not beexplained by the valence electron numbers. However, the number ofconduction electrons was examined based on the total DOS resultsand it was found that Knight shifts occurred. Therefore, the lithiationsites of Peaks A and B were elucidated.AcknowledgmentsTEM and EDS were performed at the NIMS Battery ResearchPlatform. The authors thank Mr. K. Imanishi at Tottori Universityfor his assistance with the XRD analysis. This study was partiallysupported by the Japan Society for the Promotion of Science(JSPS) KAKENHI (Grant numbers JP24K08565, JP20H00399,JP23K26758, JP23K04535, and JP23K18465) and NIMS JointResearch Hub Program.CRediT Authorship Contribution StatementYasuhiro Domi: Conceptualization (Lead), Formal analysis (Equal), Fundingacquisition (Equal), Writing – original draft (Lead), Writing – review & editing(Lead)Hiroyuki Usui: Conceptualization (Lead), Funding acquisition (Supporting),Methodology (Supporting), Writing – original draft (Equal), Writing – review &editing (Equal)Takumi Ando: Data curation (Lead), Formal analysis (Equal), Investigation (Lead),Writing – original draft (Equal), Writing – review & editing (Supporting)Ryuto Tanaka: Data curation (Equal), Investigation (Equal), Methodology (Supporting),Writing – original draft (Supporting), Writing – review & editing (Equal)Kazuma Gotoh: Conceptualization (Supporting), Data curation (Lead), Investigation(Equal), Methodology (Equal), Writing – review & editing (Supporting)Takeo Hoshi: Conceptualization (Supporting), Data curation (Supporting), Investigation(Supporting), Methodology (Equal), Writing – review & editing (Equal)Kei Nishikawa: Data curation (Supporting), Investigation (Supporting), Methodology(Supporting), Writing – review & editing (Supporting)Hiroki Sakaguchi: Conceptualization (Equal), Funding acquisition (Lead),Methodology (Equal), Writing – original draft (Equal), Writing – review &editing (Supporting)Data Availability StatementThe data that support the findings of this study are openly available under the termsof the designated Creative Commons License in J-STAGE Data listed in D1 ofReferences.Conflict of InterestThe authors declare no conflict of interest in the manuscript.FundingJapan Society for the Promotion of Science: JP24K08565Japan Society for the Promotion of Science: JP20H00399Japan Society for the Promotion of Science: JP23K04535Japan Society for the Promotion of Science: JP23K18465Japan Society for the Promotion of Science: JP23K26758ReferencesD1. Y. Domi, H. Usui, T. Ando, R. Tanaka, K. Gotoh, T. Hoshi, K. Nishikawa, and02468100 1 2 3 4 5Number of conduction electrons near the Fermi levelx or y in LixSi or LiyNiSi2LiyNiSi2LixSiFigure 7. Change in number of conduction electrons near theFermi level with an increase in x or y in LixSi or LiyNiSi2,respectively.Electrochemistry, 93(3), 037009 (2025)6H. Sakaguchi, J-STAGE Data, https://doi.org/10.50892/data.electrochemistry.28490150, (2025).1. S. C. Lai, J. Electrochem. Soc., 123, 1196 (1976).2. C. J. Wen and R. A. Huggins, J. Solid State Chem., 37, 271 (1981).3. M. N. Obrovac and L. Christensen, Electrochem. Solid-State Lett., 7, A93 (2004).4. M. N. Obrovac and L. J. Krause, J. Electrochem. Soc., 154, A103 (2007).5. B. Liang, Y. Liu, and Y. Xu, J. Power Sources, 267, 469 (2014).6. X. Zuo, J. Zhu, P. Müller-Buschbaum, and Y.-J. Cheng, Nano Energy, 31, 113(2017).7. P. Li, H. Kim, S. T. Myung, and Y.-K. Sun, Energy Storage Mater., 35, 550 (2021).8. H. Zhao, H. Zuo, J. Wang, and S. Jiao, J. Energy Storage, 98, 113125 (2024).9. Y. Domi, H. Usui, M. Shimizu, Y. Kakimoto, and H. Sakaguchi, ACS Appl. Mater.Interfaces, 8, 7125 (2016).10. S. Yodoya, Y. Domi, H. Usui, and H. Sakaguchi, ChemistrySelect, 4, 1375 (2019).11. H. Sakaguchi, T. Iida, M. Itoh, N. Shibamura, and T. Hirono, IOP Conf. Ser.:Mater. Sci. Eng., 1, 012030 (2009).12. H. Usui, K. Maebara, K. Nakai, and H. Sakaguchi, Int. J. Electrochem. Sci., 6,2246 (2011).13. Y. Domi, H. Usui, H. Itoh, and H. Sakaguchi, J. Alloys Compd., 695, 2035 (2017).14. Y. Domi, H. Usui, A. Ueno, Y. Shindo, H. Mizuguchi, T. Komura, T. Nokami, T.Itoh, and H. Sakaguchi, J. Electrochem. Soc., 167, 040512 (2020).15. Y. Domi, H. Usui, E. Nakabayashi, Y. Kimura, and H. Sakaguchi, ACS Appl.Energy Mater., 3, 7438 (2020).16. Y. Domi, H. Usui, T. Okasaka, K. Nishikawa, and H. Sakaguchi, J. Electrochem.Soc., 171, 080506 (2024).17. Y. Domi, H. Usui, D. Iwanari, and H. Sakaguchi, J. Electrochem. Soc., 164,A1651 (2017).18. Y. Domi, H. Usui, N. Ieuji, K. Nishikawa, and H. Sakaguchi, ACS Appl. Mater.Interfaces, 13, 3816 (2021).19. Y. Domi, H. Usui, K. Yamaguchi, S. Yodoya, and H. Sakaguchi, ACS Appl. Mater.Interfaces, 11, 2950 (2019).20. Y. Domi, H. Usui, A. Ando, K. Nishikawa, and H. Sakaguchi, ACS Appl. EnergyMater., 3, 8619 (2020).21. Y. Domi, H. Usui, K. Nishikawa, and H. Sakaguchi, ACS Appl. Nano Mater., 4,8473 (2021).22. Y. Domi, H. Usui, R. Takaishi, and H. Sakaguchi, ChemElectroChem, 6, 581(2019).23. Y. Domi, H. Usui, T. Ando, and H. Sakaguchi, Mater. Adv., 3, 6231 (2022).24. G. X. Wang, L. Sun, D. H. Bradhurst, S. Zhong, S. X. Dou, and H. K. Liu,J. Power Sources, 88, 278 (2000).25. Y.-N. Zhou, W.-J. Li, H.-J. Chen, C. Liu, L. Zhang, and Z. Fu, Electrochem.Commun., 13, 546 (2011).26. Z. Du, T. D. Hatchard, P. Bissonnette, R. A. Dunlap, and M. N. Obrovac,J. Electrochem. Soc., 163, A2456 (2016).27. A. K. Niessen, P. R. de Boer, R. Boom, P. F. de Châtel, W. C. M. Mattens, andA. R. Miedema, Calphad, 7, 51 (1983).28. Y. Domi, H. Usui, K. Sugimoto, K. Gotoh, K. Nishikawa, and H. Sakaguchi, ACSOmega, 5, 22631 (2020).29. H. A. Ambjörnsson, K. Schenzel, and U. Germgård, BioResources, 8, 1918(2013).30. M. Shimizu, H. Usui, T. Suzumura, and H. Sakaguchi, J. Phys. Chem. C, 119,2975 (2015).31. O. Pecher, J. Carretero-González, K. J. Griffith, and C. P. Grey, Chem. Mater., 29,213 (2017).32. K. Gotoh, T. Yamakami, I. Nishimura, H. Kometani, H. Ando, K. Hashi, T.Shimizu, and H. Ishida, J. Mater. Chem. A, 8, 14472 (2020).33. B. M. Meyer, N. Leifer, S. Sakamoto, S. G. Greenbaum, and C. P. Grey,Electrochem. Solid-State Lett., 8, A145 (2005).34. A. Budi, A. Basile, G. Opletal, A. F. Hollenkamp, A. S. Best, R. J. Rees, A. I.Bhatt, A. P. O’Mullane, and S. P. Russo, J. Phys. Chem. C, 116, 19789 (2012).35. D. M. Piper, T. Evans, K. Leung, T. Watkins, J. Olson, S. C. Kim, S. S. Han, V.Bhat, K. H. Oh, D. A. Buttry, and S.-H. Lee, Nat. Commun., 6, 6230 (2015).36. B. Key, R. Bhattacharyya, M. Morcrette, V. Seznéc, J. M. Tarascon, and C. P.Grey, J. Am. Chem. Soc., 131, 9239 (2009).37. B. Key, M. Morcrette, J. M. Tarascon, and C. P. Grey, J. Am. Chem. Soc., 133, 503(2011).38. P. H. Chavanis and J. Sommeria, Mon. Not. R. Astron. Soc., 296, 569 (1998).39. V. L. Chevrier, J. W. Zwanziger, and J. R. Dahn, J. Alloys Compd., 496, 25 (2010).40. R. Nesper, Prog. Solid State Chem., 20, 1 (1990).41. H. G. von Schnering, R. Nesper, J. Curda, and K. F. Tebbe, Angew. Chem., 92,1070 (1980).42. R. Nesper and H. G. von Schnering, J. Solid State Chem., 70, 48 (1987).Electrochemistry, 93(3), 037009 (2025)7https://doi.org/10.50892/data.electrochemistry.28490150https://doi.org/10.50892/data.electrochemistry.28490150https://doi.org/10.1149/1.2133033https://doi.org/10.1016/0022-4596(81)90487-4https://doi.org/10.1149/1.1652421https://doi.org/10.1149/1.2402112https://doi.org/10.1016/j.jpowsour.2014.05.096https://doi.org/10.1016/j.nanoen.2016.11.013https://doi.org/10.1016/j.nanoen.2016.11.013https://doi.org/10.1016/j.ensm.2020.11.028https://doi.org/10.1016/j.est.2024.113125https://doi.org/10.1021/acsami.6b00386https://doi.org/10.1021/acsami.6b00386https://doi.org/10.1002/slct.201803282https://doi.org/10.1088/1757-8981/1/1/012030https://doi.org/10.1088/1757-8981/1/1/012030https://doi.org/10.1016/S1452-3981(23)18181-8https://doi.org/10.1016/S1452-3981(23)18181-8https://doi.org/10.1016/j.jallcom.2016.11.041https://doi.org/10.1149/1945-7111/ab743fhttps://doi.org/10.1021/acsaem.0c00846https://doi.org/10.1021/acsaem.0c00846https://doi.org/10.1149/1945-7111/ad69c6https://doi.org/10.1149/1945-7111/ad69c6https://doi.org/10.1149/2.1361707jeshttps://doi.org/10.1149/2.1361707jeshttps://doi.org/10.1021/acsami.0c17552https://doi.org/10.1021/acsami.0c17552https://doi.org/10.1021/acsami.8b17123https://doi.org/10.1021/acsami.8b17123https://doi.org/10.1021/acsaem.0c01238https://doi.org/10.1021/acsaem.0c01238https://doi.org/10.1021/acsanm.1c01765https://doi.org/10.1021/acsanm.1c01765https://doi.org/10.1002/celc.201801088https://doi.org/10.1002/celc.201801088https://doi.org/10.1039/D2MA00171Chttps://doi.org/10.1016/S0378-7753(00)00385-2https://doi.org/10.1016/j.elecom.2011.03.006https://doi.org/10.1016/j.elecom.2011.03.006https://doi.org/10.1149/2.0061613jeshttps://doi.org/10.1016/0364-5916(83)90030-5https://doi.org/10.1021/acsomega.0c03357https://doi.org/10.1021/acsomega.0c03357https://doi.org/10.15376/biores.8.2.1918-1932https://doi.org/10.15376/biores.8.2.1918-1932https://doi.org/10.1021/jp5121965https://doi.org/10.1021/jp5121965https://doi.org/10.1021/acs.chemmater.6b03183https://doi.org/10.1021/acs.chemmater.6b03183https://doi.org/10.1039/D0TA04005Chttps://doi.org/10.1149/1.1854117https://doi.org/10.1021/jp304581ghttps://doi.org/10.1038/ncomms7230https://doi.org/10.1021/ja8086278https://doi.org/10.1021/ja108085dhttps://doi.org/10.1021/ja108085dhttps://doi.org/10.1046/j.1365-8711.1998.01414.xhttps://doi.org/10.1016/j.jallcom.2010.01.142https://doi.org/10.1016/0079-6786(90)90006-2https://doi.org/10.1002/ange.19800921235https://doi.org/10.1002/ange.19800921235https://doi.org/10.1016/0022-4596(87)90176-9