Structural and Magnetic Nature for Fully Delithiated LixNiO2: Comparative Study betweenChemically and Electrochemically Prepared Samples

Kazuhiko Mukai,*,† Jun Sugiyama,† Yutaka Ikedo,†,| Yoshifumi Aoki,† Daniel Andreica,‡,§ andAlex Amato‡

Toyota Central Research and DeVelopment Laboratories, Inc., 41-1 Yokomichi, Nagakute,Aichi 480-1192, Japan, Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institut,Villigen CH-5232, Switzerland, and Faculty of Physics, Babes-Bolyai UniVersity, 400084 Cluj-Napoca, Romania

ReceiVed: January 26, 2010; ReVised Manuscript ReceiVed: March 7, 2010

Structural and magnetic properties of two LixNiO2 samples with x e 0.1 have been studied by a powderX-ray diffraction, magnetic susceptibility (�), and muon-spin rotation/relaxation (µSR) measurements. Oneof the two samples was prepared by an electrochemical (EC) reaction in a nonaqueous lithium cell, whereasthe other sample by a chemical (C) reaction in a HNO3 aqueous solution. Although the crystal structure ofboth C-Li0.01NiO2 and EC-Li0.10NiO2 samples were assigned as a mixture of a cubic close-packed phase anda hexagonal close-packed phase, the effective magnetic moment (µeff) of Ni ions for C-Li0.01NiO2 was estimatedas µeff ) 1.43 µB and was very close to that for EC-Li0.5NiO2 (µeff ) 1.39 µB). This implies that the vacanttetrahedral or octahedral sites in C-Li0.01NiO2 are partially occupied by H+ ions. Actually, the zero-field µSRtime spectrum in the paramagnetic state for C-Li0.01NiO2 exhibited a large relaxation compared to those forEC-Li0.5NiO2 and EC-Li0.10NiO2. Furthermore, a pryrolysis gas chromatography/mass spectroscopy analysisconfirmed the existence of H+ ions in the C-Li0.01NiO2 crystalline lattice. The actual composition of C-Li0.01NiO2

is, thus, determined to be H∼0.5Li0.01NiO2.

Introduction

The lithium insertion materials, for which Li+ ions are insertedinto (or extracted from) a rigid matrix without destruction of aframework structure (so-called topotactic), have been heavilyinvestigated by electrochemists and battery researchers, becauseof their practical application to Li ion batteries (LIB).1 Althougha nonaqueous electrolyte is currently used in the commercialLIB, it is widely recognized that the acid treating with anaqueous solution provides a fully delithiated phase. Hunterreported2 that a fully delithiated LixMn2O4 (λ-MnO2) is producedby digesting LiMn2O4 in an acid solution with the followingdisproportionation reaction

This is because the acid treating in a sufficient H+ solutioncorresponds to the voltage of 1.23 V vs the standard hydrogenelectrode (SHE) at pH ) 03 and eventually is equivalent to theelectrochemical charging up to ∼4.2 V vs Li+/Li in a nonaque-ous solution. The fully delithiated LixNiO2 is also obtained bya chemical reaction in an aqueous solution of acid, as in thecase for LixMn2O4.2 Actually, Arai et al.4,5 prepared LixNiO2

with x e 0.1 using the reaction of LiNiO2 with a sulfuric acidand reported that the electrochemical discharge curve (lithiated

process) for the chemically delithiated LixNiO2 compound withx ) 0.1 almost trace that for the pristine LiNiO2. This indicatesthat the chemically delithiated process for LixNiO2 is almostthe same to the electrochemically process, although the dispro-portionation reaction described in eq 1 always involves adissolution from a particle, and consequently, the solid-liquidinterface is renewed during the reaction.

In our previous paper,6 the magnetic nature for the fullydelithiated LixMn2O4 and LixNiO2 samples, which are preparedby both electrochemical (EC) and chemical (C) reactions, wasexamined by a magnetic susceptibility (�) measurement. Al-though the magnetism for C-Li0.07Mn2O4 is almost identical tothat for EC-Li0.03Mn2O4, the magnetism for C-Li0.01NiO2 is verydifferent from that for EC-Li0.05NiO2. That is, the magnitude of� for C-Li0.01NiO2 is considerably large, if we assume that Ni4+

ions are nonmagnetic (t62g), as in the case for EC-Li0.05NiO2.

Furthermore, the effective magnetic moment (µeff) forC-Li0.01NiO2 () 1.43 µB) is twice the size of µeff for EC-Li0.05NiO2 () 0.71 µB). This means that the average valence ofNi ions (Vave,Ni) is not simply determined by the Li/Ni ratio forC-LixNiO2.4

According to past structural analyses on LixMn2O4,2 theoxygen stacking sequence for LixMn2O4 is maintained in a cubicclose-packed (CCP) structure down to x ≈ 0. On the other hand,that for both C-LixNiO2

4,5 and EC-LixNiO27-9 partially changes

from the CCP structure to a hexagonal close-packed (HCP)structure below x ≈ 0.1. Here, the proton insertion materialssuch as NiOOH and CoOOH have the HCP structure with aABAB stacking sequence, while the lithium insertion materialssuch as LiNiO2 (NiOOLi) and LiCoO2 (CoOOLi) have the CCPstructure with a ABCABC stacking sequence. Therefore, sucha structural variation from CCP to HCP leads to the questionwhether H+ ions are inserted into the C-Li0.01NiO2 sample. Inother words, there is a possibility that the inserted H+ ions

* To whom correspondence should be addressed. E-mail: e1089@mosk.tytlabs.co.jp. Phone: +81-561-71-7698. Fax: +81-561-63-6137.

† Toyota Central Research and Development Laboratories, Inc.‡ Paul Scherrer Institut.§ Babes-Bolyai University.| Present address: Muon Science Laboratory, Institute of Materials

Structure Science, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan.

4LiMn2O4 + 8H+ f 6λ-MnO2 + 2Mn2+ + 4Li+ +4H2O (1)

J. Phys. Chem. C 2010, 114, 8626–86328626

10.1021/jp1007818 2010 American Chemical SocietyPublished on Web 04/16/2010

increase µeff for C-Li0.01NiO2, although Arai et al.5 proposed a“proton-free model” for C-LixNiO2 by an X-ray photoelectronspectroscopy (XPS) analysis. The existence of H+ ions is usuallydetected by a solid state 1H NMR. However, albeit notimpossible, it is very difficult to determine the content of H+

ions, because the 1H nucleus (I ) 1/2) gives a broad NMR powerpattern in hertz due to a dipolar coupling to the paramegnets.10

Therefore, we have investigated the structural and magneticnature for both EC-LixNiO2 and C-Li0.01NiO2 samples by X-raydiffraction (XRD), �, thermogravimetry (TG), pyrolysis gaschromatography/mass spectrometry (Py-GC/MS) measurementsto clarify whether H+ ions are inserted into the C-Li0.01NiO2

sample. Furthermore, we have performed muon-spin rotation/relaxation (µSR) measurements because µSR is a powerfultechnique to detect local magnetic fields caused by nuclear andelectronic origin and corresponds to the volume fraction of themagnetic phases in the sample.11 Indeed, Ariza et al.12 reportedthat zero-field µSR spectrum for C-Li[Li1/3Mn5/3]O4 (H+-MnO2)shows a larger relaxation rate than that for C-LiMn2O4 at 100K and speculated the presence of H+ ions in the C-Li[Li1/3Mn5/3]-O4 sample.

Experimental Methods

Two different samples of LiNiO2 (Lot A and B) weresynthesized by a conventional solid-state reaction techniqueusing reagent grade LiNO3 (Li(OH) ·H2O for Lot B) and NiCO3

(NiO for Lot B) powders. The reaction mixture was pressedinto a pellet of 23 mm diameter and ∼5 mm thickness and thenheated at 650 °C in oxygen flow for 12 h. The obtained powderwas crushed, pressed into a pellet again, and finally fired at750 °C in an oxygen flow for 12 h. The products werecharacterized by a powder XRD (RINT-2200, Rigaku Co. Ltd.,Japan) analysis and an electrochemical charge/discharge test.

The LixNiO2 samples with x < 1 were prepared by twodifferent methods; an electrochemical reaction in a nonaqueouslithium cell and a chemical reaction, as previously reported.2,4,5

For the electrochemical reaction, the pressed LiNiO2 powder(Lot A and B) and a Li-metal sheet were used as a workingand counter electrode, respectively. The electrolyte was 1 MLiPF6 dissolved in ethylene carbonate (EC)/diethyl carbonate(DMC) (3:7 volume ratio) solution. To avoid the signals fromconducting additives and a binder, the electrodes were entirelymade from LiNiO2 powder. The Li/LiNiO2 cells were operatedwith a rate of 0.057 mA · cm-2 (constant current mode) at 298K (25 °C). For the chemical reaction, 2 g of powder of LiNiO2

(Lot A) was immersed in 100 mL of 1 M HNO3 and then stirredat room temperature for 24 h. The initial molar ratio of H+/LiNiO2 was ∼4.9. Such large ratio was selected so as tocomplete the following disproportionation reaction

The product formed after the reaction was filtered and driedat 40 °C in an air-oven in order to avoid the decomposition ofthe sample. The Li/Ni ratio was determined by an inductivelycoupled plasma (ICP) atomic emission spectral (AES) analysis(CIROS 120, Rigaku Co. Ltd., Japan) and was found to be 0.01.TG was performed using a thermal analyzer (TGA-50, ShimadzuCo. Ltd., Japan). Py-GC/MS (Py-2010D, Frontier Lab Co. Ltd.,Japan) was used to confirm the existence of H+ ions.

Direct current � measurements were carried out using asuperconducting quantum interference device magnetometer

(MPMS, Quantum Design) in the temperature (T) range between5 and 350 K under the magnetic field H e 10 kOe. Electronspin resonance (ESR) spectra were recorded by a ESP300E(Bruker) spectrometer in the temperature range between 100and 300 K. The gyromagnetic ratio of Ni ions (g) wasdetermined with respect to a MnO/MgO standard. µSR experi-ments were performed at the Paul Scherrer Institut, Switzerland.Here, we wish to describe the features of µSR briefly. Muon isa spin 1/2 particle with a gyromagnetic ratio γµ/2π ) 13.554kHz/Oe. When polarized muons are implanted into a material,the muon-spin processes by the local magnetic field of thematerial. The unstable muons soon decay into positrons (themuon lifetime is 2.2 µs). The decay positron is emittedpreferentially along the muons pin direction. By collectingseveral million positrons as a function of the evolution time,one can construct the time dependence of the muon-spinpolarization [A0P(t)], which reflects the magnitude of themagnetic field at the muon site. Zero-field (ZF) µSR is a verysensitive method for detecting weak internal magnetism thatarises due to ordered magnetic moments or random fields thatare static or fluctuating with time. Transverse-field (TF) µSRinvolves the application of an external magnetic field perpen-dicular to the initial direction of the muon-spin polarization.More details about µSR technique are found in elsewhere.11

For the µSR measurements, the powder LixNiO2 samples werepressed into a disk of about 15 mm diameter and 1 mm thicknessand subsequently placed into a fork-type low background sampleholder. The LixNiO2 powders were removed from the cells in aHe-filled glovebox just before the µSR measurements. Theabove procedure is essentially the same to that of our recentµSR works on LixNiO2

13,14 and LixCoO2.15,16

Results

Electrochemical and Structural Properties. The XRDanalysis shows that both LiNiO2 samples (Lot A and B) have alayered structure with space group of R3jm, in which Li+ andNi3+ ions are located at the 3b and 3a sites, respectively. Thelattice parameters in hexagonal setting are calculated as ah )2.8749(1) Å, ch ) 14.2033(5) Å for Lot A and ah ) 2.8785(1)Å, ch ) 14.1940(1) Å for Lot B. It is widely accepted that adisordered rocksalt-type LiNiO2 (Fm3jm) domain, which iselectrochemically inactive, is easily formed in the LiNiO2 (R3jm)phase.17 This is because the LiNiO2 (Fm3jm) domain isthermodynamically stable at high temperatures above 750 °C.The rocksalt-type LiNiO2 (Fm3jm) domain is also known toprevent a structural change from the CCP to HCP structure.5,9

According to a Rietveld analysis using RIETAN-2000,18 theamount of Ni ions at the 3b site (z in Li1-zNi1+zO2) is estimatedas z e 0.03 for both samples.

Figure 1 shows the charge curves of Li/LiNiO2 cells for the� and µSR measurements. The charge curves for all the samplesexhibit three plateaus around 3.67, 4.02, and 4.20 V, indicatingthat the crystal structure of LixNiO2 varies as a function of x;as x decreases from 1, a rhombohedral (R3jm) phase is stabledown to x ) 0.75, whereas a monoclinic (C2/m) phase in the xrange between 0.75 and ∼0.45. Then, a rhombohedral phaseappears again with 0.45 g x > 0.25, and finally two rhombo-hedral phases coexist with 0.25 g x > ∼ 0.1.17 The electro-chemical properties for the present LiNiO2 (Lots A and B) areessentially the same to the result for the nearly stoichiometricLiNiO2.17 Here, the initial charge curve of the cell using theelectrode mix, which consists of 88 mass % LiNiO2, 6 mass %acetylene black, and 6 mass % PVdF dispersed in N-methyl-2-pyrrolidone, is also shown for comparison. In spite of the

2LiNi3+O2 + 4H+ f Ni4+O2 + Ni2+ + 2Li+ +2H2O (2)

Structure and Magnetism for LixNiO2 J. Phys. Chem. C, Vol. 114, No. 18, 2010 8627

absence of conducting additives and binder in the positiveelectrode, the charge curves for all the samples are almost thesame to the curve for the electrode mix. The Li-extractedreaction is, therefore, successfully achieved for all the LixNiO2

samples. The Li/Ni ratios were examined by the ICP-AESanalysis after the � and µSR measurements. In this paper, weuse the Li/Ni ratios determined by the ICP-AES analysis.

Figure 2a shows the XRD pattern for the highest delithiatedsample by an electrochemical reaction (EC-Li0.10NiO2), andFigure 2b does that by a chemical reaction (C-Li0.01NiO2). TheLi/Ni ratio for the electrochemically prepared sample wasestimated as 0.10 by the ICP-AES analysis. As reportedpreviously,5,9 a single-phase was not obtained for both samples,and consequently, a detailed structural analysis by a Rietveldrefinement cannot be performed. As seen in the 2θ rangebetween 16 and 22°, there are at least four phases in bothsamples, that is, E1, E2, E3, and E4 phases for the EC-Li0.10NiO2

sample, and C1, C2, C3, and C4 phases for the C-Li0.01NiO2

sample. The d-value for each phase is calculated as 4.84, 4.70,4.48, and 4.38 Å for the E1, E2, E3, and E4 phases and 7.36,

4.68, 4.43, 4.36 Å for the C1, C2, C3, and C4 phases,respectively, by using the relation d ) λ/2 sin θ. The latticeparameters of ah and ch axes for the E3 and C3 phases werecalculated by a least-squares method by using more than 11diffraction lines, while those for the E1, E4, and C4 phases by5 diffraction lines. Here, the crystal structure of the fullydelithiated LixNiO2 was studied by Croguennec et al. using EC-LixNiO2 samples,7-9 and Arai et al. using C-LixNiO2 samples4,5

(see Table 1). Although the lattice parameters for the E4 andC4 phases are ambiguous due to the lack of multiple diffractionlines, on the basis of the ch axis length, the E3 (E4) phase isalmost identical to the R3 (H4) phase,7-9 while the C3 (C4)phase corresponds to their B2 (B3) phase.5 Therefore, it isconsidered that the E3 and C3 phases belong to the CCPstructure with a ABCABC stacking sequence, whereas the E4and C4 phases the HCP structure with a ABAB stackingsequence. Since the XRD peaks for the E3 (C3) phase are mostintense among those of four phases, the majority of LixNiO2 isassigned to be a CCP structure for both samples.

It is difficult to determine the crystal structure and the latticeparameters of ah and ch axes for the E2, C1, and C2 phases,because the diffraction lines above 2θ ) 20° were not seen forthese samples. To our knowledge, the C1 phase, having thelargest d-value with 7.36 Å (2θ ) 12.01°), has never beenreported in LixNiO2. The large d-value implies that a smallamount of H2O molecule would be inserted into the NiO2

interlayer only for the C-LixNiO2 sample, as in the case forγ-NiOOH (d-value ≈ 7 Å).19

Macroscopic Magnetism by � Measurements. Figure 3shows the T dependence of (a) � and (b) �-1 for the EC-LixNiO2

(open circles) and C-Li0.01NiO2 (closed circle) samples. � wasmeasured in field-cooling (FC) mode with H ) 10 kOe. TheNi3+ ions in LiNiO2 are known to be in a low-spin state witht6

2ge1g (S ) 1/2).20 Actually, the �(T) curve for the x ) 1 sample

exhibits a rapid increase below ∼100 K, indicating the presenceof localized moments of the Ni3+ ions. For the EC-LixNiO2

samples with x < 1, the magnitude of � at low T decreases withdecreasing x. This means that the amount of nonmagnetic Ni4+

ions with t62g (S ) 0) increases with decreasing x. On the other

hand, the �(T) curve for C-Li0.01NiO2 locates between those forEC-Li0.88NiO2 and EC-Li0.75NiO2, in contrast to the fact thatthe Li+ ions are almost fully delithiated. To speculate the Vave,Ni,we attempted to fit the �(T) curve with a Curie-Weiss formulain the T range between 200 and 350 K

Figure 1. The charge curves of Li/LiNiO2 cells operated at 298 K(25 °C) for the � and µSR measurements. To avoid the signals fromother components, the electrodes for � and µSR measurements wereentirely made from LiNiO2 powder. The charge curve of the cell usingan electrode mix, which consists of 88 mass % LiNiO2, 6 mass %conductive carbon, and 6 mass % binder, is also shown for comparison.The applied current density of the cells for � (or µSR) and electrodemix was 0.057 and 0.17 mA · cm-2, respectively. Note that the Li/Niratio determined by ICP-AES analysis is in good agreement with theresult of an electrochemical reaction (EC).

Figure 2. XRD patterns of the fully delithiated LixNiO2 samples: (a)EC-Li0.10NiO2 and (b) C-Li0.01NiO2. The enlarged XRD patterns in the2θ range between 16 and 22° are also shown in the left side. Fourphases for (a) are denoted as E1, E2, E3, and E4, while those for (b)are denoted as C1, C2, C3, and C4, respectively. The open circle, closedcircle, and closed triangle indicate the E1, E4, and C4 phases,respectively.

TABLE 1: Lattice Parametersa of ah and ch Axes for theEC-Li0.10NiO2 and C-Li0.01NiO2 Samples

lattice parameter/Å

sample composition method phase ah chb packing

this work Li0.10NiO2 nonaqueous E1 2.833 14.407 CCPE3 2.824 13.352 CCPE4 2.825 13.051 HCP

Li0.01NiO2 aqueous C3 2.817 13.274 CCPC4 2.818 13.042 HCP

Croguennecet al.8,9

Ni1.02O2 nonaqueous R3 2.815 13.363 CCP

R4 2.815 13.039 HCPArai et al.5 Li0.04NiO2 aqueous B1 2.817 13.820 CCP

B2 2.819 13.318 CCPB3 2.818 13.101 HCP

a The lattice parameters for the E3 and C3 phases were calculatedby a least-squares method by using more than 11 diffraction lines,while those for the E1, E4, and C4 phases by 5 diffraction lines.b The length of the ch axis for the E4, C4, H4, and B3 phases wasmultiplied by 3 for comparison.

8628 J. Phys. Chem. C, Vol. 114, No. 18, 2010 Mukai et al.

where kB is the Boltzmann’s constant, T is the absolute T, Θp

is the Weiss temperature, N is the number density of Ni ions,µeff is the effective magnetic moment of Ni ions, and �0 is theT-independent susceptibility. Figure 4 shows the x dependencesof (a) Θp, (b) µeff, and (c) �0 for the EC-LixNiO2 and C-Li0.01NiO2

samples. The Θp(x) curve for the EC-LixNiO2 sample exhibitsa broad minimum around x ) 0.5 (Θp ) 12 K). Since Θp ) 45K for the C-Li0.01NiO2 sample, the predominant interactionbetween the Ni moments is still ferromagnetic (FM). As seenin Figure 4b, µeff for EC-LixNiO2 decreases monotonously withdecreasing x. The solid line in Figure 4b represents the predictedµeff

pre for LixNiO2, assuming that Ni3+ ions are in a low-spinstate with t6

2ge1g (S ) 1/2), Ni4+ ions with t6

2g (S ) 0), and g )2.13. Here, the g factor was estimated from the ESR spectrafor LiNiO2 using the following relation

where h is the Planck constant, νr is the resonance frequency,µB is the Bohr magneton, and Hr is the resonance field. Althoughthe obtained µeff is slightly larger than µeff

pre, as in the case forLiNixCo1-xO2,21 the x dependence of µeff for EC-LixNiO2 isquantitatively explained by the decrease in the number densityof magnetic Ni3+ ions. In other words, the electrochemicallydelithiated process for LixNiO2 is roughly understood by theoxidation reaction of Ni3+ (S ) 1/2) f Ni4+ (S ) 0) + e-.

As reported previously,6 µeff () 1.43 µB) for the C-Li0.01NiO2

sample is considerably large compared with µeff for EC-Li0.10NiO2 and is comparable to µeff for EC-Li0.50NiO2. Thisindicates the presence of larger number of the Ni3+ ions in theC-Li0.01NiO2 sample than the prediction from the Li/Ni ratio.To clarify the magnetic nature of the C-Li0.01NiO2 sample, aµSR experiment was performed.

Microscopic Magnetism by µSR Measurements. Figure 5shows the T dependence of the normalized weak TF asymmetry

(NATF) for (a) EC-LixNiO2 with x ) 0.60, 050, and 0.10 and (b)C-Li0.01NiO2 samples obtained in an applied magnetic field (HwTF

)) 30 Oe. Here, NATF is defined by NATF ) ATF/ATF, max ) ATF/∼0.25 and is roughly proportional to the volume fraction ofparamagnetic (PM) phases in the sample. In other words, whenNATF ) 1, the whole sample is in a PM state, but, when NATF )0, the whole sample is in a magnetic phase, such as FM,antiferromagnetic, ferrimagnetic, or spin-glass-like phase. For

Figure 3. Magnetic susceptibility (a) � and (b) �-1 for the EC-LixNiO2

(open circles) and C-Li0.01NiO2 (closed circle) samples. � was measuredin FC mode with H ) 10 kOe.

� )Nµeff

2

3kB(T - Θp)+ �0 (3)

hVr ) gµBHr (4)Figure 4. Variation of (a) Weiss temperature (Θp), (b) effectivemagnetic moment (µeff), and (c) temperature (T)-independent suscep-tibility (�0) for the EC-LixNiO2 (open circles) and C-Li0.01NiO2 (closedcircle) samples. µeff and Θp were estimated by fitting the �(T) curvesin the T range between 200 and 350 K using eq 3. �0 was used forfitting due to a convex �(T) curve particularly at x e 0.75. The solidline in (b) is the predicted µeff

pre for LixNiO2 using the assumption thatNi3+ are in the low-spin state with S ) 1/2, Ni4+ ions also in the low-spin state with S ) 0 and g ) 2.13.

Figure 5. Temperature dependence of the normalized wTF asymmetry(NATF) for the (a) EC-LixNiO2 with x ) 0.60, 050, and 0.10 and (b)C-Li0.01NiO2 samples. The applied magnetic field was HwTF ) 30 Oe.The solid lines are a guide to the eye. Tm is the transition temperatureat which NATF ) 0.5. Mi symbolizes the magnetic phase, and PM theparamagnetic phase.

Structure and Magnetism for LixNiO2 J. Phys. Chem. C, Vol. 114, No. 18, 2010 8629

the EC-Li0.10NiO2 sample, as T decreases from 50 K, NATF ∼ 1down to ∼20 K, then slightly decreases by ∼0.1 with furtherlowering T. This is consistent with the fact that the majority ofthe Ni ions is in a nonmagnetic 4+ state with S ) 0. On theother hand, as T decreases from 30 K the NATF (T) curve forthe EC-LixNiO2 samples with x ) 0.60 (0.50) exhibits a steplikedecrease from 1 to 0 at 10 K (4 K), demonstrating the existenceof a sharp magnetic transition. The magnetic transition T (Tm)was determined as the T, at which NATF ) 0.5. Since the NATF(T)curve reaches almost 0 below the vicinity of Tm, the wholesample enters into the magnetic phase below Tm. For theC-Li0.01NiO2 sample, as T decreases from 50 K, the NATF(T)curve shows two step-like drops at ∼15 K and below 10 K,although NATF ) 0.32 even at 1.8 K. This indicates that; (i) themagnetic Ni ions still exist in the sample as expected fromFigure 4(b), (ii) there are at least two different magnetic phasesdenoted as M1 and M2 in the sample, and (iii) 1/3 of the sampleis still PM even at 1.8 K.

Figure 6 shows the ZF-µSR time spectra for the (a) EC-LixNiO2 samples with x ) 0.5 and 0.10 and (b) C-Li0.01NiO2

sample. Note that all the samples are in the PM state (see partsa and b of Figure 5). In the PM state, muon-spins should bedepolarized by the randomly oriented nuclear magnetism, notby the electronic magnetism. The ZF-µSR spectrum is, thus,fitted by a dynamic Kubo-Toyabe function GDGKT(t, ∆, ν)22

where A0 is the empirical maximum muon decay asymmetry,AKT and AM are the asymmetries of Kubo-Toyabe (paramagnetic)and residual magnetic phases, ∆ is the static width of the localfrequencies at the disordered sites, ν is the field distributionrate, and λM is the relaxation rate. When ν ) 0, GDGKT(t, ∆, ν)is the static Gaussian Kubo-Toyabe function GKT

zz (t, ∆) givenby

The ZF-µSR spectrum for EC-Li0.10NiO2 shows a slowrelaxation, indicating that muon-spins are mainly depolarizedby the residual nuclear magnetism of 6Li and 7Li. On the otherhand, the ZF-µSR spectrum for the C-Li0.01NiO2 sample showsa large relaxation. The ∆, which corresponds to the internalmagnetic field, is calculated as 0.075(2) µs-1 at 12.5 K for theEC-Li0.10NiO2 sample, while that is calculated as 0.355(3) µs-1

at 15 K for the C-Li0.01NiO2 sample. Here, the C-Li0.01NiO2

sample is prepared by digesting the LiNiO2 into a HNO3 solutionand is almost fully delithiated. This implies that, even if weconsider the effect of the Ni moment of C-Li0.01NiO2 (µeff forC-Li0.01NiO2 is ∼1.4 µB, which is comparable to µeff for EC-Li0.50NiO2), there is the other origin for the large relaxation rateonly for the C-Li0.01NiO2 sample. Since the nuclear magneticmoment caused by both O and Ni nuclei is negligibly small,the large relaxation rate for the C-Li0.01NiO2 sample is mostlikely due to the nuclear magnetic moment of the 1H nucleus.In other words, µSR result suggests the presence of H+ ions inthe C-Li0.01NiO2 sample, although the past work for C-LixNiO2

proposed the proton-free model.4,5 Here, we wish to emphasizethat µSR is a bulk probe, i.e., µSR signals roughly correspondto the volume fraction of the magnetic phases in the sample.

Thermal Analysis. To confirm the absence/existence ofoxygen deficiency and H+ ions, TG and Py-GC/MS analyseswere carried out. Figure 7a shows the TG curve for theC-Li0.01NiO2 sample. As T increases from ambient T, the TGcurve shows a slight decrease above ∼380 K, then exhibits adrastic decrease around 473 K, and finally keeps nearly constantvalue until 873 K. Here, after the acid reaction, the C-Li0.01NiO2

sample was dried only at 40 °C in an air oven in order to avoidthe decomposition of the sample. Thus, the decrease around380 K is most likely to the absorbed and/or adsorbed water ofthe sample. The weight loss at 1273 K [m(1273 K) - m(300K)] is ∼82.3%, which well agrees with the expected change(17.6%) by the following oxygen evolution reaction

This suggests the absence of the oxygen deficiency in theC-Li0.01NiO2 sample, as reported previously.4,5 Note that it is

Figure 6. ZF µSR time spectra for the (a) EC-LixNiO2 samples withx ) 0.50 and 0.10, and (b) C-Li0.01NiO2 sample. The solid lines arefitting results using eq 5.

A0PZF(t) ) AKTGDGKT(t, ∆, V) + AMexp(-λMt) (5)

GzzKT(t, ∆) ) 1/3 + 2/3(1 - ∆2t2)exp(∆2t2/2) (6)

Figure 7. (a) Thermogravimetric curve and (b) Py-GC/MS spectrumfor the C-Li0.01NiO2 sample. Heating rates are 10 K ·min-1 for (a) and20 K ·min-1 for (b).

NiO2 f NiO + 1/2O2 (7)

8630 J. Phys. Chem. C, Vol. 114, No. 18, 2010 Mukai et al.

difficult to distinguish the absence/existence of H+ ions in theC-Li0.01NiO2 sample by the TG curve, since the weight loss iscalculated as ∼81.5% even for the HNiO2 (NiOOH) composition.

On the other hand, the two spectra, which are identified asthe O2+ (mass/charge ) 32) and H2O+ (mass/charge ) 18) ionsby the mass spectrum at 522 K are clearly observed in the Py-GC/MS spectrum (see Figure 7b). This unambiguously dem-onstrates the presence of H+ ions in the C-Li0.01NiO2 sample,being consistent with the result by µSR measurements. Here, itshould be noted that the trend for the H2O+ spectrum is almostthe similar to that for the O2+ spectrum; i.e., both spectra showtwo peaks at ∼490 and 535 K. This indicates that the structurallybonded and/or intercalated water exists in the C-Li0.01NiO2

crystalline lattice. Indeed, the TG curve for the Ni(OH)2

compound showed two weight loss regions in the T ranges of323-363 K and 453-723 K, which are attributed to thedehydration reactions of absorbed/adsorbed water and structur-ally bonded/intercalated water, respectively.23 Therefore, thedecomposition reaction for the C-Li0.01NiO2 sample is repre-sented by

Discussion

According to the � measurements, µeff for the C-Li0.01NiO2

sample was estimated as µeff ) 1.43 µB and was almost thesame to that for the EC-Li0.5NiO2 sample (µeff ) 1.39 µB),indicating the decrease in Vave,Ni from the tetravalent state(Ni4+O2). Furthermore, µSR measurements and Py-GC/MSanalysis clearly demonstrated the existence of H+ ions in theC-Li0.01NiO2 crystalline lattice. Note that we prepared theC-Li0.07Mn2O4 sample by drying at 60 °C in an air-oven afterthe acid reaction.6 However, � measurements indicated that µeff

() 3.85 µB) for the C-Li0.07Mn2O4 sample is very close to that(3.80 µB) for the EC-Li0.03Mn2O4 sample.6 Therefore, it is mostlikely that not the absorbed and/or adsorbed water but thestructurally bonded and/or intercalated water (H+ ions) increasesthe µeff for the C-Li0.01NiO2 sample. As seen in Figure 4b, µeff

for the EC-LixNiO2 samples decreases with decreasing x asexpected. The total amount of H+ ions in the C-Li0.01NiO2

sample is, thus, evaluated as ∼0.5, i.e., H∼0.5Li0.01NiO2, by usingthe linear relationship between µeff and x in the EC-LixNiO2

sample. In other words, the vacant tetrahedral or octahedral sitesin C-Li0.01NiO2 are partially occupied by H+ ions. In thiscalculation, we assume that both Ni3+ and Ni4+ ions are in thelow-spin state, since Ni ions occupy the octahedral site in bothCCP and HCP structure. Actually, the g-factor (2.10) forC-Li0.01NiO2 is almost temperature-independent above 100 K,as for LiNiO2 (g ) 2.13). Moreover, Demorgues et al.24 showedby extended X-ray absorption fine structure that the Ni3+O6

octahedra in both �-NiOOH and γ-NiOOH (specificallyH0.20Na0.10K0.20Ni0.70Co0.30O2 ·0.5H2O) exhibit a Jahn-Tellerdistortion. This supports that the Ni3+ ions in the C-Li0.01NiO2

sample are also in a low-spin state with t62ge1

g (S ) 1/2). Theresults by wTF-µSR measurements also support the aboveconsideration. That is, as seen in Figure 5b, three differentmagnetic phases of M1, M2, and PM are found from the Tdependence of NATF, indicating the inhomogeneity of the sample.

The volume fraction of the M1, M2, and PM phases isapproximately 40, 30, and 30%, respectively. Since Tm of theM1 (M2) phase is almost the same to that for the EC-Li0.60NiO2

(EC-Li0.50NiO2) sample, the Vave,Ni for the C-Li0.01NiO2 sampleis roughly estimated as ∼3.6 (4 - 0.6 × 0.4 - 0.5 × 0.3).This is very consistent with the result of the � measurements(Vave, Ni ∼ 3.5).

As described in the introduction, Arai et al.4,5 proposed theproton-free model for the chemically delithiated LixNiO2

compound with x e 0.1. Although the origin of the differencebetween the present and past results is currently unknown, theamount of Ni ions in the Li layer (z) would participate in sucha difference, since the Li1-zNi1+zO2 compound with z ) 0.07was reported to keep CCP structure even for the fully delithiatedstate.7 Moreover, they employed the X-ray photoelectronspectroscopy (XPS) analysis for studying the oxidation state ofNi ions and concluded the proton-free.5 However, it is thoughtto be difficult to detect the H+ ions in a crystalline lattice bythe surface probe such as XPS and Fourier transform infraredspectroscopy (FT-IR) analyses.

XRD measurements indicated that the C-Li0.01NiO2 samplehas at least four phases with different d-values (see Figure 2b).Although the total amount of H+ ions in the C-Li0.01NiO2 sampleis estimated as ∼0.5, the location of H+ ions in the phase andlattice is currently unclear. We expect that further neutrondiffraction measurement provides crucial information on thelocation of the H+ ions. It should be noted that the major phaseof C-Li0.01NiO2 is the CCP structure (C3 phase) and not theHCP structure (C4 phase). Croguennec et al. reported that theEC-LixNiO2 phase in the HCP structure (H4) is thermodynami-cally unstable and slowly converts into a new LixNiO2 phase inthe CCP structure (R3′) with decreasing the distance of NiO2

interlayer.7-9 Since the interlayer distance of the C3 phase rangesat the middle of the C4 (or E4) and E3 phases, the C4 phase inthe HCP structure is thought to transform into the C3 phase inthe CCP structure.

Finally, we wish to comment on the possibility of theexistence of H+ ions in the EC-Li0.10NiO2 sample, because anelectrochemical charging at high voltages above 4.2 V vs Li+/Li sometimes generates H+ ions resulting from the decomposi-tion of a nonaqueous electrolyte. Indeed, Robertson and Brucereported25 that the abnormal capacity around 4.5 V of anonaqueous Li/Li2MnO3 cell is attributed to an exchangereaction between Li+ and H+ ions. For the EC-LixNiO2 samplesdescribed here, the values of µeff decreases with decreasing x,as expected (see Figure 4b). Also, the temperature dependenceof NATF shows that ∼90% of the sample is in the PM state evenat 1.8 K (see Figure 5a). The proton insertion reaction, hence,does not occur in the present EC-Li0.10NiO2 samples.

Conclusion

The magnetic and thermal analyses clearly showed that H+

ions exist in the C-Li0.01NiO2 crystalline lattice. That is, theexpected composition of C-Li0.01NiO2 is represented asH∼0.5Li0.01NiO2. Although the mechanism of proton insertionfor LixNiO2 is still unclear, we expect that further studies onLixNiO2 and NiOOH provide crucial information on the differ-ences and similarities of electrochemical reaction betweennonaqueous and aqueous solutions. We also emphasize that thedetection of H+ ions in the actual LIB is significantly importantfor optimizing the cycle and storage performances at elevatedtemperatures above 55 °C. The magnetic susceptibility andmuon-spin rotation/relaxation measurements are found to be apowerful technique for studying such subjects.

380-470 K

HxLi0.01NiO2.yH2O f HxLi0.01NiO2 + yH2O

(8)

470-870 KHxLi0.01NiO2 f Li0.01NiO + x/2H2O + (1 - x/4)O2

(9)

Structure and Magnetism for LixNiO2 J. Phys. Chem. C, Vol. 114, No. 18, 2010 8631

Acknowledgment. This work was performed at the SwissMuon Source, Paul Scherrer Institut (PSI), Villigen, Switzerland.We thank the staff of PSI for help with the µSR experiments.We also appreciate T. Ohzuku, K. Ariyoshi, and S. Kohno ofOsaka City University for preparation of LiNiO2 and discussionthrough this work. We wish to thank Y. Kondo of TCRDL forICP-AES analysis and M. Yamamoto of TCRDL for Py-GC/MS analysis. K.M., Y.I., and J.S. are partially supported by theKEK-MSL Inter-University Program for Overseas Muon Facili-ties. This work is supported by Grant-in-Aid for ScientificResearch (B), 1934107, MEXT, Japan.

Supporting Information Available: XRD patterns in logscale for the EC-Li0.10NiO2 and C-Li0.01NiO2 samples. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

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