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Structural Phase Transition from Rhombohedral (R3̅m) to Monoclinic (C2/m) Symmetry in Lithium Overstoichiometric Li1+δCo1−δO2−δ Kazuhiko Mukai,* Yoshihiro Kishida, Hiroshi Nozaki, and Kazuhiko Dohmae Toyota Central Research and Development Laboratories, Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan S Supporting Information *

ABSTRACT: Stoichiometric lithium cobalt oxide LiCoO2 is known to exhibit several structural phase transitions with x in LixCoO2 at ambient temperature (T); e.g., an initial rhombohedral (R3̅m) phase transforms into a monoclinic (C2/m) phase at x ∼ 0.5. In contrast, lithium overstoichiometric (Li)3b[LiδCo1−δ]3aO2−δ with δ ≥ ∼0.02, where δ is the Li+ ions at the 3a (Co) site, maintains the R3m ̅ symmetry until x ∼ 0.5 in Lix(LiδCo1−δ)O2−δ at ambient T, and this is the reason why such material has been widely used in commercial lithium ion batteries. We performed X-ray diffraction measurements in the T range between 100 and 300 K for the lithium overstoichiometric Lix(Li0.02Co0.98)O1.98 samples with x = 1, 0.56, and 0.51 to understand the factors that govern the structural changes in Lix(LiδCo1−δ)O2−δ with δ ≥ 0. Both x = 0.56 and 0.51 samples exhibit a structural phase transition from the high-T R3̅m phase to the low-T C2/m phase at 250 K (=Ts1). Furthermore, these samples indicate another structural phase transition at 170 K (=Ts2); although their crystal structures still have the C2/m symmetry, the degree of monoclinic distortion starts to decrease below Ts2, associated with a magnetic anomaly and a freezing of the Li+ ions at the 3b site. Because the two structural phase transitions of Ts1 (=330 K) and Ts2 (=150 K) are also observed for the stoichiometric LixCoO2 compound with x ∼ 0.5, the C2/m phase in Lix(LiδCo1−δ)O2−δ is found to appear in the limited x and T ranges. The characteristics and possible origin of Ts1 and Ts2 for both stoichiometric LixCoO2 and lithium overstoichiometric Lix(Li0.02Co0.98)O1.98 samples are discussed. KEYWORDS: lithium ion batteries, lithium insertion materials, layered structure, distortion, magnetic susceptibility, phase diagram

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Although the structural change at 0.95 ≥ x ≥ 0.75 is attributed to an insulator−metal transition by electrical7 and Li nuclear magnetic resonance (NMR) measurements,4 the origin of the structural change between the R3̅m and C2/m phases at x ∼ 0.5 is still not fully understood. Reimers et al.6,7 proposed that the Li+ ions and the vacancies are ordered in straight lines in the Li layer at x = 0.5, and they predicted that the x range in the ordered (C2/m) phase increases with a decrease in temperature (T) from ∼333 K. Indeed, Shao-Horn et al.9 showed the existence of superlattice spots for the Li0.5CoO2 compound by an electron diffraction (ED) analysis at 100 K. They claimed that these spots are due to in-plane and/or intraplane ordering between the Li+ ions and vacancies in a 1: 1 ratio.9 However, this model should be carefully confirmed for the following reasons. As described by Ohzuku and Ueda,3 the x value in the C2/m phase ranges between 0.56 and 0.51; therefore, it is difficult to imagine the 1:1 ordering between the Li+ ions and vacancies. Furthermore, Shao-Horn et al.7 observed √3 × √3 type superlattice spots in Li0.5CoO2, indicating the coexistence of 1:2 or 2:1 ordering between the Li+ ions and vacancies.

INTRODUCTION The science and technology of today’s lithium ion batteries (LIBs) are a result of the discovery by Mizushima et al.1 that Li+ ions can be topotactically extracted from and inserted into lithium cobalt oxide LiCoO2. Although numerous transition metal oxides and phosphates such as LiMn2O4, LiNiO2, and LiFePO4 have been reported to exhibit electrochemical reactivity as a positive electrode material for LIBs,2 LiCoO2 is still widely employed in commercial LIBs. This is probably because of its moderate operating voltage of ∼3.9 V versus Li+/ Li, 1,3 high electrical conductivity, 4 and ease of mass production.5 The crystal structure of stoichiometric LiCoO2 belongs to an α-NaFeO2-type framework with an R3̅m space group, in which Li+ ions and Co3+ ions occupy the octahedral 3b and 3a sites, respectively, in an ABCABC packing of O2− layers. When the delithiation reaction proceeds to form LixCoO2, the crystal structure of LixCoO2 varies with x. An initial rhombohedral (R3̅m) phase transforms into a mixture of two R3̅m phases at 0.95 ≥ x ≥ 0.75; a monoclinic (C2/m) phase appears around x = 0.5, and finally, the R3̅m phase is observed again below x ∼ 0.5.3−6 Because such structural changes induce a fracture of the LiCoO2 particle and/or an isolation from electrical conducting additives, extensive experimental3,5−9 and theoretical10 studies have been devoted to the understanding of the factors that govern the structural changes in LixCoO2. © XXXX American Chemical Society

Received: April 16, 2013 Revised: June 23, 2013

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To clarify the structural nature of the C2/m phase at x ∼ 0.5, we adopted various approaches: magnetic susceptibility (χ),11,12 muon-spin rotation and relaxation (μSR),13,14 and low-T X-ray diffraction (XRD)15 measurements. We have chosen to do this because magnetic measurements, especially at low temperatures, are very sensitive with respect to the local structure of the material, which can lead us to an in-depth understanding of the structural changes within LixCoO2. Recently, we found a structural phase transition for the C2/m phase with x ∼ 0.5 coupled with a magnetic anomaly.12,14,15 As T decreased from 300 K, one of the lattice parameters in the C2/m phase, βm, monotonically increased as T decreased to 150 K and then rapidly increased with a further decrease in T.15 This indicated that the x range in the C2/m phase decreases with a decrease in T. In this study, we performed low-T XRD measurements for the lithium overstoichiometric Li1.05CoO2 sample that was synthesized using an excess Li:Co ratio (y) of 1.05. Levasseur et al.16,17 proposed that the chemical formula of the lithium overstoichiometric LiyCoO2 with y > 1 is represented as Li1+δCo1−δO2−δ, where δ is the number of Li+ ions at the 3a (Co) site or the oxygen deficiency. Hence, the chemical formula of the Li1.05CoO2 sample is rewritten as Li1+δCo1−δO2−δ with δ ∼ 0.02 by using the relation y = (1 + δ)/(1 − δ). Hereafter, we use the formula Li1+δCo1−δO2−δ for this work, i.e., Li1.02Co0.98O1.98, to satisfy both site occupancy and charge neutrality conditions. Although lithium overstoichiometric Li1+δCo1−δO2−δ with δ ≥ ∼0.02 is believed to maintain the R3̅m symmetry until x ∼ 0.5 in Lix(LiδCo1−δ)O2−δ,16,17 we reveal a structural phase transition from R3̅m to C2/m symmetry below 250 K, as in the case for the stoichiometric LixCoO2.

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V, respectively. The sample was removed from the lithium cell in an Ar-filled glovebox just before the XRD measurements were conducted and was sealed within a borosilicate glass capillary tube (W. Müller Glas Technik) with a 0.3 mm diameter. To stabilize the T of the sample upon reaching the desired T, we maintained the T for 3 min. A continuous N2-flow device (CGD-1, Rigaku Co. Ltd.) with a precision of ±0.1 K controlled the T of the sample during the XRD measurements. The wavelength of the X-ray was determined to be 0.70000(1) Å by the XRD measurements for the NIST CeO2 standard (674a). The chemical composition for the delithiated Lix(Li1.02Co0.98)O1.98 samples was examined by inductively coupled plasma (ICP) atomic-emission spectral (AES) analysis (CIROS, Rigaku Co. Ltd.). Rietveld analyses were performed with RIETAN2000.18 χ was measured by a superconducting quantum interference device (SQUID) magnetometer (MPMS, Quantum Design) in the T range between 5 and 400 K under a magnetic field (H) of 10 kOe. The delithiated Lix(Li0.02Co0.98)O1.98 sample with x = 0.51 was prepared by an electrochemical reaction and was removed from the cell in an Arfilled glovebox just before the χ measurements. Details of the experimental setup for the χ measurements are described elsewhere.11−14

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RESULTS AND DISCUSSION Sample Characterization. To characterize the Li1.02Co0.98O1.98 sample, we undertook electrochemical and χ measurements. Figure 1 shows the charge and discharge curves

EXPERIMENTAL SECTION

A powder sample of lithium overstoichiometric Li1.02Co0.98O1.98 was synthesized by a conventional solid-state reaction technique reported previously.11−15 First, Co3O4 powder was prepared by heating CoO powder (99.7%, Kojyundo Chemical Lab. Co. Ltd.) at 1023 K for 12 h in the air. Then, the reaction mixture consisting of Li2CO3 (99%, Wako Pure Chemical Industries, Ltd.) and Co3O4 with a Li:Co molar ratio of 1.05 was mixed in a mortar with a pestle and then pressed into a pellet (23 mm diameter and ∼5 mm thickness). The pellet was heated at 1173 K for 12 h in the air and then cooled to room temperature at a rate of 5 K/min. Scanning electron microscopy (SEM) (S-3600N, Hitachi High-Technologies Co. Ltd.) observations indicated that the average size of the primary Li1.02Co0.98O1.98 particles is approximately 10 μm. The electrochemical reactivity of Li1.02Co0.98O1.98 was examined in a nonaqueous lithium cell. Polyvinylidene fluoride (PVdF) dissolved in an N-methyl-2-pyrrolidone solution was used as a binder when preparing the electrode. The black viscous slurry consisting of 88 wt % Li1.02Co0.98O1.98, 6 wt % acetylene black (AB), and 6 wt % PVdF was cast on an aluminum foil using a blade. The electrode (φ = 16 mm) was dried under vacuum at 393 K for 12 h. The lithium metal sheet pressed on a stainless steel plate (φ = 19 mm) was used as the counter electrode. Two sheets of porous polyethylene membrane (TonenGeneral Sekiyu K. K.) were used as the separator. The electrolyte was 1 M LiPF6 dissolved in an ethylene carbonate (EC)/dimethyl carbonate (DEC) (1:1 volume ratio) solution (Kishida Chemical Co. Ltd.). The charge and discharge test was performed in the voltage range between 3 and 4.2 V with a current density of 0.15 mA/cm2 at 298 K. XRD measurements were conducted in the T range between 300 and 100 K upon cooling, using the BL19B2 beamline at the SPring-8 synchrotron radiation facility. The delithiated Lix(Li1.02Co0.98)O1.98 samples with x = 0.56 and 0.51 were prepared by an electrochemical reaction in the constant current mode by charging up to 4.10 and 4.15

Figure 1. Charge and discharge curves of the Li/Li1.02Co0.98O1.98 cell operated in the voltage range between 3 and 4.2 V with a current density of 0.15 mA/cm2 at 298 K. The red lines indicate the charge curves for the low-temperature XRD measurements. The x value in Lix(Li0.02Co0.98)O1.98 was calculated by comparing the observed charge capacity and the theoretical capacity of 277.70 mA/g, assuming that the Li+ ions at the 3b (Li) site participate in the electrochemical reaction and one-electron transfer only with a formula weight of Li1.02Co0.98O1.98.

of the Li/Li1.02Co0.98O1.98 cell operated with a current density of 0.15 mA/cm2 at 298 K. As the charge (discharge) reaction proceeds, the cell voltage almost monotonically increases (decreases) with an increase in capacity. The rechargeable capacity in the voltage range between 3.0 and 4.2 V is ∼142 mAh/g. It is widely accepted that stoichiometric LiCoO2 shows a subtle change in the charge and discharge curves at ∼4.1 V because of the structural change between the R3̅m and C2/m phases, 3 , 5 , 6 whereas the lithium overstoichiometric Li1+δCo1−δO2−δ with δ ≥ ∼0.02 shows featureless charge and discharge curves at ∼4.1 V.5,16,17 Therefore, our sample can be identified as lithium overstoichiometric Li1+δCo1−δO2−δ. Levasseur et al.17 proposed that the ionic distribution of the lithium overstoichiometric Li1+δCo1−δO2−δ is represented as (Li)3b[LiδCo1−3δ3+(LS)Co2δ3+(IS))]3aO2−δ, where LS is the lowspin state with S = 0 (t2g6) and IS is the intermediate-spin state with S = 1 (t2g5eg1). The Li+ ions at the 3a site (δ) generate the B

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formation of the square-based Co3+O5 pyramid and oxygen deficiency (δ).17 Because the amount of Co3+ ions in the IS state (or oxygen deficiency) can be evaluated by the effective magnetic moment (μeff) of Co ions,17 the observed μeff and theoretical μeff (μtheo eff ) are compared. Figure 2 shows the T

Figure 3. Results of the Rietveld analyses for the Lix(Li0.02Co0.98)O1.98 samples with x values of (a) 1, (b) 0.56, and (c) 0.51 at 300 K. The enlarged XRD peak around 2θ = 20° corresponds to the 104 diffraction line in the hexagonal symmetry.

Figure 2. Temperature (T) dependence of the magnetic susceptibility (χ) for the lithium overstoichiometric Li1.02Co0.98O1.98 sample measured in the field-cooling (FC) mode with H = 10 kOe. The solid line represents the fitting result with eq 1.

dependence of χ for the Li1.02Co0.98O1.98 sample measured in the field-cooling (FC) mode with H = 10 kOe. As T decreases from 400 K, χ gradually increases as T decreases to ∼50 K and then rapidly increases with a further decrease in T, suggesting that there are localized moments produced within the sample. In a paramagnetic state, the Curie−Weiss equation is described as χ=

Nμeff 2 3kB(T − Θp)

+ χ0

symmetry, in which the Li+ ions occupy both 3b and 3a sites and the Co ions occupy the 3a site. The occupancies (g) for the Li, Co, and O atoms are fixed for all the samples. The lattice parameters in the hexagonal setting are calculated to have the following values: ah = 2.8137(1) Å and ch = 14.040(1) Å. For the x = 0.56 and 0.51 samples, the XRD patterns can also be assigned to a single phase of the layered structure with R3̅m symmetry. This is consistent with previous studies,5,16,17 which show that lithium overstoichiometric Lix(LiδCo1−δ)O2−δ with δ ≥ ∼0.02 maintains a single phase of the R3̅m symmetry until x ∼ 0.5. As x decreases from 1 to 0.51, the length of the ah axis decreases to 2.8108(1) Å, while that of the ch axis increases to 14.392(1) Å. The former is caused by the oxidation of the Co3+ ions, while the latter is due to the increase in the degree of electrostatic repulsion between adjacent O2− layers. As seen from Table 1, the reliability factors such as Rwp and S for the delithiated samples are relatively larger than those for the x = 1 sample. This is probably due to the background from the AB and PVdF in the electrode. Actually, the reliability factors were T-independent over the whole T range measured. Figure 4a shows the T dependence of the XRD patterns for the x = 0.51 sample. Enlarged XRD patterns are also shown in panels b and c to clarify the change in the XRD pattern with T. As T decreases from 300 K, the 003 diffraction line around 2θ = 8.4° simply shifts to a higher diffraction angle, suggesting that the length of the ch axis decreases with a decrease in T because of thermal contraction (Figure 4b). However, the intensity of the 104 diffraction line around 2θ = 8.4° rapidly decreases in the T range from 250 to 200 K. Furthermore, its diffraction line separates into two diffraction lines represented as P1 and P2 (Figure 4b). The I(P1)/I(P2) integrated peak intensity ratio is ∼2 at ≤200 K. As shown in Figure 4c, the 110 diffraction line shows no significant change except for its peak position, while the 202 and 204 diffraction lines clearly separate into two diffraction lines. Because such variations in the XRD pattern are also reported for stoichiometric LixCoO2 with x ∼ 0.5,6 the XRD patterns at ≤200 K can be assigned to the C2/m symmetry. Hence, the x = 0.51 sample is found to exhibit a structural phase transition from R3̅m to C2/m symmetry below 250 K, whereas lithium overstoichiometric Lix(LiδCo1−δ)O2−δ

(1)

where N is the number density of Co ions, kB is Boltzmann’s constant, T is the absolute temperature, Θp is the Weiss temperature, and χ0 is the T-independent susceptibility. Using eq 1 in the T range between 100 and 400 K, we obtain a μeff of 0.45(2) μB and a Θp of −52(2) K for the Li1.02Co0.98O1.98 sample. The μ etfhf e o for lithium overstoichiometric Li1.02Co0.98O1.98 is calculated as 0.63 μB, assuming that all the excess Li ions (0.02) occupy the 3a site and that the gyromagnetic factor is 2. Because the observed μeff is almost comparable to μtheo eff , we used the IS model for the Rietveld analyses; the ionic distribution of the Li1.02Co0.98O1.98 sample is (Li)3b[Li0.02Co0.98]3aO1.98. As seen Figure 1, the charge curves for the low-T XRD measurements (red lines) trace those for the initial charge and discharge test, confirming that the delithiation reaction was successfully achieved for all the samples. If we assume that the Li+ ions at the 3b site participate in only the electrochemical reaction, the x values in Lix(Li1.02Co0.98)O1.98 are calculated to be 0.56 and 0.51 using their charge capacities. In fact, the x values determined by the ICP-AES analyses were 0.56 and 0.53, being almost consistent with results of the electrochemical measurements. The open circuit voltage just before the XRD measurements was 4.086 V for x = 0.56 and 4.134 V for x = 0.51. X-ray Diffraction Patterns below Ambient Temperature. Figure 3 shows the results of the Rietveld analyses for the Lix(Li1.02Co0.98)O1.98 samples with x values of (a) 1, (b) 0.56, and (c) 0.51 at 300 K. The structural parameters are listed in Table 1. The XRD pattern for the x = 1 sample can be assigned to a single phase of the layered structure with R3̅m C

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Table 1. Structural Parameters for the Lix(Li0.02Co0.98)O1.98 Samples with x = 1, 0.56, and 0.51 at 300 K Determined by Rietveld Analysis sample

a

space group

x=1

R3̅m

x = 0.56

R3m ̅

x = 0.51

R3̅m

atom

occupancy g

Wyckoff position

x

y

Li1 3b 1.0 0 0 Co 3a 0.98 0 0 Li2 3a 0.02 0 0 O 6c 0.99 0 0 ah = 2.8137(1) Å, ch = 14.040(1) Å, Rwp = 4.29, RB = 2.45, and S = 1.08 Li1 3b 0.56 0 0 Co 3a 0.98 0 0 Li2 3a 0.02 0 0 O 6c 0.99 0 0 ah = 2.8108(1) Å, ch = 14.397(1) Å, Rwp = 7.60, RB = 4.94, and S = 2.14 Li1 3b 0.51 0 0 Co 3a 0.98 0 0 Li2 3a 0.02 0 0 O 6c 0.99 0 0 ah = 2.8063(1) Å, ch = 14.392(1) Å, Rwp = 8.02, RB = 4.11, and S = 2.38

z

Bisoa (Å2)

1/2 0 0 0.260(1)

1.6(1) 0.8(1) 0.8(1) 1.0(1)

1/2 0 0 0.266(1)

3.8(3) 1.3(1) 1.3(1) 1.8(1)

1/2 0 0 0.265(1)

4.4(4) 1.2(1) 1.2(1) 1.5(1)

Constraint: Biso(Li2) = Biso(Co).

Figure 5. Results of the Rietveld analyses for the Lix(Li0.02Co0.98)O1.98 samples with x values of (a) 1, (b) 0.56, and (c) 0.51 at 100 K. The crystal structure for the x = 1 sample is assigned to a single phase with rhombohedral (R3̅m) symmetry, while those for the x = 0.56 and 0.51 samples are assigned to a single phase with monoclinic (C2/m) symmetry.

Figure 4. (a) Temperature (T) dependence of the whole XRD pattern for the Lix(Li0.02Co0.98)O1.98 sample with x = 0.51. XRD measurements were performed in the T range between 100 and 300 K while the sample was being cooled. The enlarged XRD patterns at 8.2° ≤ 2θ ≤ 20.3° and 28° ≤ 2θ ≤ 36° are also shown in panels b and c, respectively. The Miller indices of the XRD peaks are given in the hexagonal symmetry, because the crystal structure is in the rhombohedral (R3̅m) phase at 300 K.

Table 2. For the x = 0.51 sample, the calculated XRD data slightly deviate from the observed XRD data at around 2θ = 20.1°, suggesting that the P1:P2 ratio is not exactly 2. This is probably due to a stacking fault along the cm axis and/or the existence of the Co ions at the interstitial (tetrahedral) site. Shao-Horn et al.9 reported that the ED patterns for the stoichiometric LixCoO2 compound with x = 0.5 show the superlattice spots, because of the 1:1 and/or 2:1 ordering between the Li+ ions and vacancies at 100 K. Moreover, they assigned its crystal structure to either the P2/m or Pm space group.9 According to the simulated XRD patterns for the Li0.5CoO2 compound (Figure S3 of the Supporting Information), the superlattice diffraction lines should be observed in the 2θ range between 10° and 15° when the Li+ ions and vacancies are ordered in the P2/m space group. However, any superlattice diffraction lines are not observed for both x = 0.56 and 0.51 samples, even for the XRD measurements at the synchrotron radiation facility. This is probably because the intensity of such

with δ ≥ ∼0.02 is believed to maintain the R3m ̅ symmetry until x ∼ 0.5.16,17 For the x = 1 sample, there is no significant change in the XRD patterns as T decreases to 100 K, indicating the absence of a structural phase transition across the whole T range measured (Figure S1 of the Supporting Information). However, the x = 0.56 sample shows a structural phase transition from R3̅m to C2/m symmetry below 250 K, as in the case for the x = 0.51 sample (Figure S2 of the Supporting Information). Figure 5 shows the results of the Rietveld analyses for the Lix(Li0.02Co0.98)O1.98 samples with x values of (a) 1, (b) 0.56, and (c) 0.51 at 100 K. The structural parameters are listed in D

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Table 2. Structural Parameters for the Lix(Li0.02Co0.98)O1.98 Samples with x = 1, 0.56, and 0.51 at 100 K Determined by Rietveld Analysis sample x=1

x = 0.56

x = 0.51

a

space group

atom

Wyckoff position

occupancy g

x

y

R3̅m

z

Li1 3b 1.0 0 0 1/2 Co 3a 0.98 0 0 0 Li2 3a 0.02 0 0 0 O 6c 0.99 0 0 0.260(1) ah = 2.8106(1) Å, ch = 14.009(1) Å, Rwp = 4.96, RB = 2.26, and S = 1.26 C2/m Li1 2d 0.56 0 1/2 1/2 Co 2a 0.98 0 0 0 Li2 2a 0.02 0 0 0 O 4i 0.99 0.728(1) 0 0.200(1) am = 4.8687(1) Å, bm = 2.8103(1) Å, cm = 5.0266(1) Å, βm = 108.71(1)°, Rwp = 7.57, RB = 5.20, and S = 2.18 C2/m Li1 2d 0.51 0 1/2 1/2 Co 2a 0.98 0 0 0 Li2 2a 0.02 0 0 0 O 4i 0.99 0.725(1) 0 0.202(1) am = 4.8604(1) Å, bm = 2.8061(1) Å, cm = 5.0223(2) Å, βm = 108.56(1)°, Rwp = 8.22, RB = 3.32, and S = 2.42

Bisoa (Å2) 1.1(1) 0.8(1) 0.8(1) 0.9(1) 2.7(3) 1.2(1) 1.2(1) 1.5(1) 3.0(4) 1.2(1) 1.2(1) 1.5(1)

Constraint: Biso(Li2) = Biso(Co).

between the hexagonal and monoclinic settings. As shown in panels a and b of Figure 6, the hexagonal unit cell can be

superlattice diffraction lines are very weak (>0.1% of the maximal diffraction line) because of the small atomic scattering factor of the Li+ ions. Furthermore, the background from the AB and PVdF in the electrode hinders the observation of the superlattice diffraction lines. Because the ED patterns correspond to the microscopic crystal structure, the absence of the superlattice diffraction lines in the present XRD patterns is attributed to the small atomic scattering factor of the Li+ atoms, the background from the additives, and the spatial resolution of the XRD measurements. The XRD patterns for the x = 0.56 and 0.51 samples are identified as a single phase with C2/m symmetry, in which the Li+ ions occupy both 2d and 2a sites and the Co ions occupy the 2a site. The structural transition for the x = 0.56 sample indicates that the x range in the C2/m phase for lithium overstoichiometric Lix(LiδCo1−δ)O2−δ also deviates from 0.5. For the x = 0.56 and 0.51 samples, the isotropic atomic displacement parameters of the Li+ ions [Biso(Li)] at the regular (3b or 2d) site are 2.7(3)−4.4(4) Å2, which are relatively large compared to that for the x = 1 sample (Tables 1 and 2). This indicates the large thermal vibration and/or the dynamic positional disorder of the Li+ ions. The large atomic displacement parameter is also reported for hydrogen storage material Ca(BH4)219 and oxygen ion conductors Ce0.8R0.2O1.920 with R = La, Nd, or Sm. In the case of Ce0.8R0.2O1.9, the larger B(O) provides the larger ionic conductivity of the O2− ions (σO2−), although the activation energy for the displacement to a neighboring site also affects σO2−.20 According to our previous XRD measurements of the stoichiometric LixCoO2 compounds with 1 ≥ x ≥ 0.49,14 the x dependence of Biso(Li) at 300 K exhibits the maximum around the C2/m phase. That is, as x decreases from 1, Biso(Li) gradually increases, then reaches a maximum at x = 0.56, and finally decreases with a further decrease in x (Figure S4 of the Supporting Information). Although the x dependence of Biso(Li) for lithium overstoichiometric Lix(Li0.02Co0.98)O1.98 is currently unknown, further XRD measurements of Lix(Li0.02Co0.98)O1.98 would clarify the relation between Biso(Li) and σLi+ (or diffusivity of the Li+ ions). Temperature Dependence of the Lattice Parameters. Before describing the results of the T dependence of the lattice parameters, we briefly mention the structural relationship

Figure 6. Schematic illustration of the crystal structure in the (a) hexagonal phase in the basal (inter) plane, (b) hexagonal phase along the ch axis (intraplane), (c) monoclinic phase in the basal plane, and (d) monoclinic phase along the cm axis. There are two parameters that characterize a monoclinic distortion from the hexagonal phase, am/bm and Δβ (= βh − βm), which correspond to the interplane and intraplane distortion, respectively. When am/bm = √3 and Δβ = 0 (a and b), the lattice parameters of the hexagonal setting, ah and ch, are converted to those of the monoclinic setting, am, bm, cm, and βm, by using eq 3. In contrast, when am/bm ≠ √3, Δβ ≠ 0, or both, the hexagonal phase is transformed into the monoclinic phase.

represented by the monoclinic unit cell using the following relationship: ⎛ 2 0 2/3⎞ ⎜ ⎟ (abc)m = (abc)h ⎜1 1 1/3 ⎟ ⎜ ⎟ ⎝ 0 0 1/3 ⎠

(2)

where h and m represent the hexagonal and monoclinic settings, respectively. Hexagonal lattice parameters ah and ch E

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are, hence, converted to monoclinic lattice parameters of am, bm, cm, and βm as follows: am =

As shown in Figure 7d, the x = 0.56 and 0.51 samples indicate a complicated T dependence of βm, although βm for the x = 1 sample gradually increases with a decrease in T. That is, as T decreases from 300 K, βm for the x = 0.56 (0.51) sample slightly increases to 108.72(1)° [108.70(1)°] at 250 K, then decreases to 108.67(1)° [108.50(1)°] at 180 K, and finally monotonically increases with a further decrease in T. This suggests that at least two structural phase transitions exist in the x = 0.56 and 0.51 samples at 250 and 180 K. To clarify such structural changes with T, the parameters for characterizing monoclinic distortion are defined as follows.14 Panels c and d of Figure 6 show the schematic illustration of the crystal structure in the C2/m phase. There are two parameters that characterize a monoclinic distortion from the hexagonal symmetry. One is the in-plane distortion that is defined as am/bm. As shown in Figure 6c, the deviation from the value of √3 gives rise to a structural change from the hexagonal to monoclinic symmetry. Figure 8a shows the T dependence for the Lix(Li1.02Co0.98)O1.98

3 × ah

bm = ah cm =

ch 3 sin βm

βm = 180° − tan−1

ch 3 × ah

(3)

Figure 7 shows the T dependence of the lattice parameters of (a) am, (b) bm, (c) cm, and (d) βm for the Lix(Li1.02Co0.98)O1.98

Figure 8. Temperature (T) dependence of the parameters that characterize a monoclinic distortion for the lithium overstoichiometric Lix(Li0.02Co0.98)O1.98 (OST-LCO) samples with x = 0.56 and 0.51: (a) interplane distortion am/bm and (b) intraplane distortion Δβ. The am/ bm and Δβ values for the stoichiometric LixCoO2 (ST-LCO) compound with x = 0.53 from ref 14 are also shown for comparison. There are two structural phase transitions denoted as Ts1 and Ts2 for both ST-LCO and OST-LCO. To clarify the Ts2 for OST-LCO, the enlarged T dependence of Δβ is shown in panel c.

Figure 7. Temperature dependence of lattice parameters (a) am, (b) bm, (c) cm, and (d) βm for the Lix(Li0.02Co0.98)O1.98 samples with x = 1, 0.56, and 0.51. Although the x = 1 sample (over the whole measured T range) and the x = 0.56 and 0.51 samples (>250 K) are in the rhombohedral phase, their lattice parameters have been converted into the monoclinic setting for comparison.

samples with x = 1, 0.56, and 0.51. For all the samples, the lattice parameters of am and bm remain almost constant down to 100 K (Figure 7a,b). The average coefficients of linear thermal expansion (αL) along the bm axis (or am axis) are estimated to have the following values: αL = 5.7(1) × 10−6 K−1 for x = 1, αL = 8.7(3) × 10−7 K−1 for x = 0.56, and αL = 2.2(1) × 10−6 K−1 for x = 0.51. On the other hand, lattice parameter cm monotonically decreases with a decrease in T: αL = 1.1(1) × 10−5 K−1 for x = 1, αL = 3.9(1) × 10−5 K−1 for x = 0.56, and αL = 4.3(1) × 10−5 K−1 for x = 0.51 (Figure 7c). The large difference in αL between the bm axis (or am axis) and cm axis is also observed for the stoichiometric LixCoO2 compounds with x ∼ 0.5.14 This indicates that the distance between the adjacent CoO2 plane is easily decreased by T in comparison with that between the nearest neighboring Co ions. In addition, this is a typical characteristic of the layered lithium insertion materials, because the αL value for cubic spinel structure Li[LixMn2−x]O4 is ∼7 × 10−6 K−1 in the T range between 100 and 400 K.21

samples with x = 0.56 and 0.51 together with the previous result for the stoichiometric LixCoO2 compound with x = 0.53.14 The am/bm values for all the samples are almost close to √3 over the whole measured T range, suggesting that the distribution of the Co (Li+) ions maintains the hexagonal symmetry. This is significantly different from the LixNiO2 compound with 0.75 ≥ x ≥ 0.45.22 The am/bm value at room temperature is ∼1.76, which is probably due to a cooperative Jahn−Teller effect induced by the Ni3+ ions in the LS state with S = 1/2 (t2g6eg1).22 Because the in-plane distortion is negligibly small for the x = 0.56 and 0.51 samples, the other distortion parameter, the degree of intraplane distortion (Δβ), is calculated by ⎡ ⎛ 3cm sin βm ⎞⎤ Δβ = βm − βh = βm − ⎢180° − tan−1⎜ ⎟⎥ ⎢⎣ am ⎠⎥⎦ ⎝ F

(4)

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Here as shown in Figure 6d, βh is the angle between the am and cm axes in the hexagonal phase. Figure 9b shows the T

ions in a regular Li layer (3b) and O2− ions (dLi−O) for the Lix(Li1.02Co0.98)O1.98 sample with x = 0.51. At ≥250 K, the LiO6 octahedra have Oh symmetry with the six equivalent bonds with dLi−O = 2.153(1) Å. This dLi−O is almost consistent with the calculated dLi−O (2.16 Å) using Shannon’s ionic radii (r);23 rLi+ = 0.76 Å for CN = 6 and rO2− = 1.40 Å, where CN is the coordination number. However, at 200 K, they are deformed to D4h symmetry with four long bonds with dLi−O = 2.167(2) Å and two short bonds with dLi−O = 2.101(3) Å, because of the structural phase transition from R3̅m to C2/m symmetry. As T decreases from 200 K, the lengths of two short bonds linearly increase with a decrease in T, while the four long bonds are almost T-independent. This suggests that D4h symmetry approaches Oh symmetry with a decrease in T. As shown in Figure 9b, the bond distance between the Co ions and O2− ions (dCo−O) also separates into two bonds at 200 K. This is because of the deformation from Oh to D4h symmetry, but the T dependence of dCo−O is opposite that of dLi−O. That is, the six equivalent bonds with dCo−O = 1.893(1) Å separate into the two long bonds with dCo−O = 1.938(3) Å and the four short bonds with dCo−O = 1.876(1) Å at 200 K. The former dCo−O is comparable to the calculated dCo−O (1.945 Å) when the Co3+ ions are in the LS state (rCo3+ = 0.545 Å for CN = 6).23 Below 200 K, the lengths of the two long bonds linearly decrease with a decrease in T, while the lengths of the four short bonds are almost T-independent. Note that the dLi−O and dCo−O values are determined by the oxygen position at the 6c site (0, 0, z) in R3̅m symmetry or 4i site (x, 0, z) in C2/m symmetry, because the positions for the Li+ ions and Co ions are fixed in these symmetries (see Tables 1 and 2). On the basis of these results regarding the T dependence of dLi−O and dCo−O, the crystal structure in the C2/m phase is shown in Figure 10. When the

Figure 9. Temperature dependence of (a) the bond length between the Li+ ions at the regular Li layer (3b site) and O2− ions (dLi−O) and (b) the bond length between the Co ions and O2− ions (dCo−O) for the lithium overstoichiometric LixCoO2 sample with x = 0.51. Because of the structural phase transition from rhombohedral (R3̅m) to monoclinic (C2/m) symmetry, the LiO6 (CoO6) octahedra become deformed from Oh to D4h symmetry.

dependence of Δβ for the x = 0.56 and 0.51 samples together with the previous result for the stoichiometric LixCoO2 compound with x = 0.53.14 As T decreases from 300 K, Δβ for the lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 samples starts to increase from 0 at 250 K (=Ts1). This corresponds to the structural phase transition from R3̅m to C2/m symmetry. Then, Δβ reaches a maximum (=Δβmax) around 170 K (=Ts2) with a Δβmax of 0.12(1)° for x = 0.56 and a Δβmax of 0.25(1)° for x = 0.51 and gradually decreases with a further decrease in T. The decrease in Δβ is better understood by the enlarged T dependence of Δβ as shown in Figure 8c. This indicates that the monoclinic phase is getting close to the hexagonal phase again below 170 K. As shown in Figure 8b, the stoichiometric LixCoO2 compound with x = 0.53 exhibits a T dependence of Δβ similar to those for the lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 samples, although Ts1 = 330 K, Ts2 = 140 K, and Δβmax = 0.75(1)° for stoichiometric LixCoO2. Thus, the decrease in Δβ below Ts2, in other words, the decrease in the degree of the distortion from the hexagonal phase below Ts2, is a characteristic for both stoichiometric LixCoO2 and lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 samples. The difference between LixNiO2 and Lix(Li1.02Co0.98)O1.98 (or LixCoO2) should again be emphasized. For the LixNiO2 compound with 0.75 ≥ x ≥ 0.45, the Δβ value is relatively small, and the direction of the intraplane distortion is opposite to that for LixCoO2 (Δβ = −0.2°).9,14 Two Successive Structural Phase Transitions in Lix(Li1.02Co0.98)O1.98. According to the T dependence of Δβ, there are two successive structural phase transitions at Ts1 = 250 K and Ts2 = 170 K in the lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 samples with x = 0.56 and 0.51. In this section, we discuss a characteristic of such structural phase transitions in more detail. Structural phase transition Ts1 unambiguously corresponds to the structural change from R3̅m to C2/m symmetry. Figure 9a shows the T dependence of the bond distance between the Li+

Figure 10. Schematic illustration showing the crystal structure for the lithium overstoichiometric Lix(Li0.02Co0.98)O1.98 sample with x = 0.51 in a monoclinic (C2/m) structure. When the crystal structure is transformed from rhombohedral (R3̅m) to monoclinic (C2/m) symmetry below 250 K, the LiO2 layer shrinks because of the decrease in the length of the bond between the Li+ ions and O2− ions. The CoO2 layer expands because of the increase in the length of the bond between the Co ions and O2− ions.

crystal structure is transformed from R3̅m to C2/m symmetry, the LiO2 layer shrinks because of the decrease in the lengths of the two bonds between the Li+ ions and O2− ions, while the CoO2 layer expands because of the increase in the lengths of the two bonds between the Co ions and O2− ions. This indicates that the change in the distance between the LiO2 and CoO2 layers plays a significant role in the structural phase transition from R3m ̅ to C2/m symmetry. As shown in panels b and c of Figure 8, the T dependence of Δβ clearly reveals the existence of Ts2 for the stoichiometric LixCoO2 compound at 140 K and lithium overstoichiometric G

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Lix(Li1.02Co0.98)O1.98 samples at 170 K, respectively. To clarify the characteristic of Ts2, χ measurements were performed for the delithiated LixCoO2 samples. First, the T dependence of χ for the stoichiometric LixCoO2 compound with x = 0.53 is displayed in Figure 11a. The χ data were taken from ref 15. As

Figure 12. Temperature (T) dependence of the (a) magnetic susceptibility (χ), (b) the slope of χ vs T, and (c) the isotropic atomic displacement factor of the Li+ ions at the regular Li layer [Biso(Li)] for the lithium overstoichiometric Lix(Li0.02Co0.98)O1.98 sample with x = 0.51.

Figure 11. Temperature dependence of the (a) magnetic susceptibility (χ) and (b) isotropic atomic displacement factor of the Li+ ions [Biso(Li)] of the stoichiometric LixCoO2 sample with x = 0.53. The χ value was taken from ref 15, and the Biso(Li) value was obtained by the XRD measurements in ref 14.

12c, the Biso(Li) value shows a maximum at 180 K. The Ts2 for the overstoichiometric sample is, thus, attributed to the freezing/diffusion of the Li+ ions and/or the order−disorder transition of the Li+ ions, although the changes in χ and Biso(Li) are obscured in comparison with those for the stoichiometric LixCoO2 compound. Structural Phase Diagram of Lix(LiδCo1−δ)O2−δ. Figure 13 shows the structural phase diagram of the lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 samples with x = 1, 0.56, and 0.51 together with the result for stoichiometric LixCoO2.14 The lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 samples with x = 0.56 and 0.51 indicated two successive structural phase transitions at Ts1 = 250 K and Ts2 = 170 K,

T decreases from 300 K, χ maintains an almost constant value (∼0.50 × 10−3 emu/mol) as T decreases to ∼160 K, suggesting a Pauli-paramagnetic behavior. Then, χ decreases to ∼0.40 × 10−3 emu/mol around 150 K, and finally, it rapidly increases with a further decrease in T. According to our μSR measurements on the stoichiometric LixCoO2 compounds with x ≤ 0.95,12,13,15 the whole volume of the sample is in a paramagnetic state above 20 K. Furthermore, field distribution width Δ, which is one of the μSR parameters that reflect the local nuclear magnetism of 7Li (3.25 μN), rapidly decreases at 150 K for the stoichiometric LixCoO2 compound with x = 0.53.15 Therefore, the magnetic anomaly around 150 K is not attributed to a typical magnetic transition such as a ferromagnetic, ferrimagnetic, or spin-glass-like transition, but to a freezing/diffusion of the Li+ ions and/or an order−disorder transition of the Li+ ions. Indeed, as shown in Figure 11b, the T dependence of Biso(Li) exhibits a maximum at ∼150 K. That is, as T decreases from 200 K, the Biso(Li) value for the stoichiometric LixCoO2 compound with x = 0.53 linearly increases as T decreases to 160 K, then rapidly decreases at 140 K, and finally levels off with a further decrease in T. Considering the change in χ, Δ, and Biso(Li), the Ts2 (140 K) for the stoichiometric LixCoO2 compound is thought to be caused by the freezing/diffusion of the Li+ ions or the order− disorder transition of the Li+ ions. This is consistent with the result that the superlattice ED patterns in Li0.5CoO2 are observed at 100 K, not room temperature.9 Moreover, 7Li NMR measurements on Li0.6 CoO 2 support the above consideration; the NMR line width ΔW versus T curve rapidly decreases around 150 K by motional narrowing because of Li+ diffusion.24 Panels a and b of Figure 12 show the T dependence of χ and the slope of χ versus T (dχ/dT), respectively, for the lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 sample with x = 0.51. The magnetic anomaly is observed as for the case of the stoichiometric LixCoO2 compound with x = 0.53, but its temperature is around 170 K. Furthermore, as seen in Figure

Figure 13. Structural phase diagram of Lix(LiδCo1−δ)O2−δ (LCO) determined by low-temperature XRD measurements. Both stoichiometric (ST-) and lithium overstoichiometric (OST-) LCO exhibit two structural phase transitions denoted as Ts1 and Ts2; Ts1 = 330 K and Ts2 = 150 K for ST-LCO (red circles), and Ts1 = 250 K and Ts2 = 170 K for OST-LCO (blue circles). Ts1 corresponds to a phase transition from rhombohedral (R3̅m) to monoclinic (C2/m) symmetry, while Ts2 corresponds to a freezing/diffusion of the Li+ ions and/or an order−disorder transition of the Li+ ions. Note that the degree of the monoclinic distortion from the hexagonal phase starts to decrease below Ts2, although the crystal structure still is in the C2/m phase down to 100 K. The structural phase diagram below 90 K is currently unknown because of the limitation of the cooling system (liquid nitrogen). H

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where the former is ∼80 K lower and the latter is ∼30 K higher than that for the stoichiometric LixCoO2 with x = 0.56 and 0.53.14 On the basis of the characteristics of Ts1 and Ts2 (Figures 9−12), the difference in Ts1 (or Ts2) between the lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 and stoichiometric LixCoO2 samples would be explained by the presence of the Li ions at the 3a (Co) site, δ. On the one hand, δ stabilizes the CoO6 octahedra with Oh symmetry at lower T, but on the other hand, it disturbs the diffusion of the Li+ ions at higher temperatures. The T dependence of Δβ, dLi−O, and dCo−O clarified that the monoclinic distortion from the hexagonal symmetry starts to decrease below Ts2, although the crystal structure is still in the C2/m phase down to 100 K. This structural phase transition is not reported in the previous firstprinciples calculations for LixCoO2 with x ∼ 0.5.10 Although the crystal structure at x > 0.56. Furthermore, our low-T XRD measurements on the stoichiometric LixCoO2 compound with x = 0.6 indicate that the crystal structure maintains the R3̅m symmetry down to 90 K (see Figure 13).14 Recently, Ou-Yang et al.26 proposed a radical (mosaic) model for LixCoO2 by the X-ray Laue diffraction and χ measurements on the single crystal of LixCoO2; a minor Li0.5CoO2 phase exists preferentially on the surface or grain boundaries in the wide x range between 0.75 and 0.5 to stabilize the surface of the particle. Because the conventional XRD measurements reflect only a bulk structure of a particle, X-ray absorption fine structure analyses of the surface and Raman spectroscopy would be essential for investigating the correspondence between Ts1 and Ts2 on LixCoO2 [or Lix(LiδCo1−δ)O2−δ].

monoclinic phase (C2/m) at ambient temperature (T), while that, which is prepared with lithium overstoichiometric Li1+δCo1−δO2−δ with δ ≥ ∼0.02, maintains the R3̅m phase. The present low-T XRD measurements demonstrated that the crystal structure of the lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 samples with x = 0.56 and 0.51 transforms from the R3̅m to C2/m phases below 250 K (=Ts1), as in the case for stoichiometric LixCoO2. On the basis of the T dependence of the parameters that characterize a monoclinic distortion from a hexagonal symmetry (am/bm and Δβ), such monoclinic distortion was mainly caused by a gliding along the intraplane. The T dependence of Δβ and magnetic susceptibility (χ) also clarified another structural phase transition at 170 K (=Ts2); surprisingly, the degree of monoclinic distortion starts to decrease below Ts2, although its symmetry remains in the C2/m space group down to 100 K. The two structural phase transitions, Ts1 (330 K) and Ts2 (150 K), were also observed for the stoichiometric LixCoO2 compound with x ∼ 0.5. Considering the results for the isotropic atomic displacement parameter of the Li+ ions [Biso(Li)] and muon-spin rotation and relaxation (μSR) measurements, the Ts2 for both stoichiometric LixCoO2 and lithium overstoichiometric Lix(Li1.02Co0.98)O1.98 was attributed to the freezing/diffusion of the Li+ ions and/or an order− disorder transition of the Li+ ions. Thus, not a simple model of ordering between the Li+ ions and vacancies but at least two contributions such as diffusion of the Li+ ions and the distance between the LiO2/CoO2 layers were found to involve the monoclinic distortion in LixCoO2 at ambient temperature. We restricted our XRD measurements down to 100 K because of the limitation of the cooling system of liquid nitrogen. However, these results would provide crucial information for understanding the thermodynamics of LixCoO2, because the monoclinic distortion from the hexagonal symmetry becomes smaller below Ts2. Further neutron scattering measurements, Xray photoelectron spectroscopy, and Raman spectroscopy are underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

Temperature dependence of the XRD patterns for the lithium overstoichiometric Lix(Li0.02Co0.98)O1.98 samples with x = 1 and 0.56, simulated XRD patterns for the stoichiometric Li0.5CoO2 compound in the C2/m and P2/m space groups, and Biso(Li) values at the 3b site at 300 K as a function of x in stoichiometric LixCoO2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-561-71-7698. Fax: +81-561-63-6156. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS XRD measurements were taken at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, Proposal 2011A1854). We thank the staff of JASRI for help with the XRD measurements. We also appreciate Mr. S. Kosaka of TCRDL for ICP-AES analysis and Dr. T. Noritake of TCRDL for fruitful discussion on the Rietveld analyses. This

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SUMMARY A delithiated LixCoO2 compound with x ∼ 0.5, which is prepared with stoichiometric LiCoO2, indicates a structural phase transition between a rhombohedral phase (R3̅m) and a I

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work was partially supported by a Grant-in-Aid for Scientific Research (C) (25410207) from MEXT.

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REFERENCES

(1) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783. (2) Nazri, G.-A.; Pistoia, G. Lithium Batteries: Science and Technology; Kluwer Academic: Norwell, MA, 2003. (3) Ohzuku, T.; Ueda, A. J. Electrochem. Soc. 1994, 141, 2972. (4) Ménétrier, M.; Saadoune, I.; Levasseur, S.; Delmas, C. J. Mater. Chem. 1999, 9, 1135. (5) Antolini, E. Solid State Ionics 2004, 170, 159. (6) Reimers, J. N.; Dahn, J. R. J. Electrochem. Soc. 1992, 139, 2091. (7) Reimers, J. N.; Dahn, J. R.; von Sacken, U. J. Electrochem. Soc. 1993, 140, 2752. (8) Kikkawa, S.; Miyazaki, S.; Koizumi, M. J. Solid State Chem. 1986, 62, 35. (9) Shao-Horn, Y.; Levasseur, S.; Weill, F.; Delmas, C. J. Electrochem. Soc. 2003, 150, A366. (10) Van der Ven, A.; Aydinol, M. K.; Ceder, G.; Kresse, G.; Hafner, J. Phys. Rev. B 1998, 58, 2975. (11) Mukai, K.; Sugiyama, J.; Ikedo, Y.; Nozaki, H.; Shimomura, K.; Nishiyama, K.; Ariyoshi, K.; Ohzuku, T. J. Power Sources 2007, 174, 711. (12) Mukai, K.; Sugiyama, J.; Ikedo, Y.; Andreica, D.; Amato, A.; Brewer, J. H.; Ansaldo, E. J.; Russo, P. L.; Chow, K. H.; Ariyoshi, K.; Ohzuku, T. J. Phys. Chem. Solids 2008, 69, 1479. (13) Mukai, K.; Ikedo, Y.; Nozaki, H.; Sugiyama, J.; Nishiyama, K.; Andreica, D.; Amato, A.; Russo, P. L.; Ansaldo, E. J.; Brewer, J. H.; Chow, K. H.; Ariyoshi, K.; Ohzuku, T. Phys. Rev. Lett. 2007, 99, 087601. (14) Mukai, K.; Nozaki, H.; Ikedo, Y.; Sugiyama, J.; Ariyoshi, K.; Ohzuku, T. J. Power Sources 2009, 192, 684. (15) Sugiyama, J.; Mukai, K.; Ikedo, Y.; Nozaki, H.; Månsson, M.; Watanabe, I. Phys. Rev. Lett. 2009, 103, 147601. (16) Levasseur, S.; Ménétrier, M.; Suard, E.; Delmas, C. Solid State Ionics 2000, 128, 11. (17) Levasseur, S.; Ménétrier, M.; Shao-Horn, Y.; Gautier, L.; Audemer, A.; Demazeau, G.; Largeteau, A.; Delmas, C. Chem. Mater. 2003, 15, 348. (18) Izumi, F.; Ikeda, T. Mater. Sci. Forum 2000, 198, 321. (19) Noritake, T.; Aoki, M.; Matsumoto, M.; Miwa, K.; Towata, S.; Li, H.-W.; Orimo, S. J. Alloys Compd. 2010, 491, 57. (20) Yashima, M.; Takizawa, T. J. Phys. Chem. C 2010, 114, 2385. (21) Mukai, K.; Kishida, Y.; Nozaki, H.; Dohmae, K. J. Power Sources 2013, 224, 230. (22) Ohzuku, K.; Ueda, A.; Nagayama, M. J. Electrochem. Soc. 1993, 140, 1862. (23) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (24) Nakamura, K.; Ohno, H.; Okamura, K.; Michihiro, Y.; Moriga, T.; Nakabayashi, I.; Kanashiro, T. Solid State Ionics 2006, 177, 821. (25) Chiang, Y.-M.; Birnie, D. P., III; Kingery, W. D. Physical Ceramics: Principles for Ceramics and Engineering; John Wiley & Sons: New York, 1996. (26) OuYnag, T. Y.; Huang, F.-H.; Shu, G. J.; Lee, W. L.; Chu, M.W.; Liu, H. L.; Chou, F. C. Phys. Rev. B 2012, 85, 035120.

J

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