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Lithium-Excess Cation-Disordered Rocksalt-Type Oxide with Nanoscale Phase Segregation: Li1.25Nb0.25V0.5O2 Mizuki Nakajima and Naoaki Yabuuchi* Department of Applied Chemistry, Tokyo Denki University, 5 Senju Asahi-Cho, Adachi, Tokyo 120-8551, Japan S Supporting Information *

ABSTRACT: A Li-excess cation-disordered rocksalt oxide, Li1.25Nb0.25V0.5O2, is proposed as a new high-capacity positive electrode material for rechargeable lithium batteries. Li1.25Nb0.25V0.5O2 delivers a reversible capacity of over 240 mAh g−1 with excellent capacity retention on the basis of the two-electron redox of V3+/V5+ as evidenced by X-ray absorption spectroscopy. Moreover, a small volume change is noted on electrochemical cycles, presumably associated with reversible vanadium migration from octahedral to tetrahedral sites. Additionally, a heat-treated sample shows unexpectedly good rate capability as electrode materials. Observation by scanning transmission electron microscopy reveals that the sample consists of vanadium-rich and niobium-rich nanoparticles formed by phase segregation, which hinders the kinetics as electrode materials. The heat-treatment process effectively changes the cation distribution in the particles and relieves strain induced by ball milling, leading to good rate capability. These findings open a path to design high-capacity electrode materials with the multielectron redox reaction.

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sible capacities are possibly increased to 300 mAh g−1 (1000 mWh g−1 vs Li metal) based on the redox reaction of anionic species, i.e., oxide ions. However, voltage decay on electrochemical cycles, which is because of an unfavorable phase transition into a spinel-like phase coupled with gradual oxygen loss, restricts its use for commercial applications. Recently, our group also reported Li3NbO4-and Li2TiO3-based electrode materials, and Mn-substituted samples deliver 300 mAh g−1 of reversible capacities with the contribution of the anionic redox reaction.7 Reversibility of an anionic redox reaction is effectively improved by the presence of elements possessing less covalent character with oxide ions, such as Nb5+ and Ti4+ when compared with the conventional Li2MnO3 system. Nevertheless, the common problems remain for the anionic redox process, i.e., large hysteresis and slow electrode kinetics on charge/discharge processes.8,9 Although high-energy density is a promising feature for battery applications, further studies are encouraged to solve these problems. Additionally, a Li2O-based system10,11 and Li4Mn2O512 are also proposed, but similar

INTRODUCTION To enable an energy-efficient society, the technology of electric vehicles equipped with rechargeable lithium batteries and electric motors as power sources has rapidly progressed in the past few years. Nevertheless, there is an ever-increasing demand on the energy density of rechargeable lithium batteries to extend a cruise distance of electric vehicles. However, the energy density of lithium batteries is currently restricted by the lack of advanced high-capacity positive electrode materials. As positive electrode materials, many oxide- and phosphate-based materials have been extensively studied in the past three decades.1 Ni-rich layered oxides, such as LiNi3/5Mn1/5Co1/5O2, are potentially used for commercial lithium batteries of electric vehicles. However, the available reversible capacity is limited to approximately 200 mAh g−1, and therefore the development of advanced positive electrode materials with high energy density is essentially required to increase the energy density of lithium batteries in the future. Reversible capacities of electrode materials are theoretically limited by the cationic redox reaction (in general, Ni2+/Ni4+, Ni3+/Ni4+, and Co3+/Co4+) of transition metal ions. Recently, Li-excess Mn layered oxides, Li2MnO3based materials, have been extensively studied for potential high-capacity positive electrode materials.2−6 Available rever© XXXX American Chemical Society

Received: June 6, 2017 Revised: July 21, 2017 Published: July 22, 2017 A

DOI: 10.1021/acs.chemmater.7b02343 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

kinetics is evidenced for Li insertion materials with the cationdisordered rocksalt structure. Moreover, the electrode performance of the sample is limited by the nanosize phase segregation in the single particles. The heat-treatment process influences the phase segregation and relieves strain induced by ball milling, leading to the reduction of electrode resistance. From these results, the future feasibility of high capacity electrode materials with two-electron redox of V ions for positive electrode materials of advanced rechargeable lithium batteries is discussed in more detail.

problems based on the anionic redox cannot be ignored with current battery technology. Another impactful high capacity system would be found in the two-electron redox of V ions. Historically, V oxides were extensively studied as positive electrodes for battery applications. After the historical finding of LiCoO2, its counterpart of a V system, LiVO2, was also studied in the 1980s.13 However, reversibility of LiVO2 as electrode materials is low, and this problem originates from V migration and the formation of a cation disordered rocksalt phase. In contrast, V2O5, which is known as one of the oldest host structures for Li ion insertion, delivers a large reversible capacity with high reversibility in Li cells. 1.5-electron redox of V ions is achieved and such large reversible capacity is realized by the structural changes into a cation disordered rocksalt phase (ω-Li3V2O5 phase).14 LiVO3 also delivers a large reversible capacity with the one-electron redox of V ions. Similar to ω-Li3V2O5, LiVO3 changes into the cation disordered rocksalt phase (Li2VO3).15 Historically, cation-disordered phases are believed to be electrochemically inactive because of the absence of a lithium conduction path. However, a recent theoretical prediction has revealed that a percolative lithium migration path is formed in the lithiumexcess systems,16,17 such as Li3V2O5 and Li2VO3. Nevertheless, both V2O5 and LiVO3 are lithium-deficient phases and cannot be directly used for positive electrode materials with a graphite negative electrode as a lithium-free system. New V chemistry is lately published in the literature, i.e., metal oxyfluorides with a cation-/anion-disordered rocksalttype stucture.18,19 For instance, a lithium vanadium oxyfluoride, Li2VO2F (Li1.33V0.67O1.33F0.67), delivers a large reversible capacity of 350 mAh g−1 in Li cells on the basis of a reversible process of V3+/V5+ two-electron redox. However, in general, the solubility of fluorides into carbonate solvents is higher than that of oxides,20 and therefore cyclability is insufficient for battery applications. Moreover, the lithium vanadium oxyfluoride is a thermodynamically metastable phase, and therefore a route for material synthesis is limited to specific methods, e.g., mechanical milling, which potentially hinders mass production. We have also recently demonstrated the highly reversible process for V3+/V5+ two-electron redox and proposed it as potential positive electrode material for battery applications.21 In this system, to prove our hypothesis, trivalent V ions were diluted into a Li-excess host material, and Li3NbO4 was targeted as a model material. The materials have been newly synthesized according to the chemical formula of the xLi3NbO4−(1 − x) LiVO2 binary system. The original composition selected was x = 0.43 on this binary system, which can be reformulated as Li1.3Nb0.3V0.4O2. There is in excess of 30% for Li ions in the crystal structure based on the stoichiometric layered materials, LiMeO2 (Me = trivalent transition metals). Li1.3Nb0.3V3+0.4O2 delivers a reversible capacity of nearly 180 mAh g−1 in Li cells with high reversibility. However, 0.5 mol of Li ions remains in the structure for the fully charged state (□0.8Li0.5Nb0.3V5+0.4O2; □ denotes vacant octahedral sites created by Li extraction).7 This fact suggests that there is room for further increase in the reversible capacity and energy density through optimization in chemical compositions on this binary system. In this study, the crystal structures and electrode performance of the xLi3NbO4−(1 − x) LiVO2 (0 ≤ x ≤ 1) binary system have been systematically examined. It is found that large reversible capacities of 250−300 mAh g−1 with high reversibility are obtained in this system, and surprisingly superior electrode

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EXPERIMENTAL SECTION

Synthesis of Material. The xLi3NbO4−(1 − x) LiVO2 (0 ≤ x ≤ 1) binary system was prepared by a solid-state reaction from stoichiometric amounts of Li2CO3 (98.5%, Kanto Kagaku), Nb2O5 (99.9%, Wako Pure Chemical Industries), and V2O3 (98%, SigmaAldrich Japan). The precursors were mixed thoroughly by wet ball milling and then dried in air. The obtained mixtures of the samples were pressed into pellets. The pellets were heated at 950 °C for 12 h in an inert atmosphere. Thus, prepared samples were stored in an Arfilled glovebox until use to avoid oxidation by moist air. Electrochemistry. The electrode performance of the samples was examined for the carbon composite sample prepared by ball milling. The as-prepared sample was mixed with acetylene black (sample: AB = 90:10 in wt %) using a planetary ball mill (PULVERISETTE 7, FRITSCH) at 300 rpm with a zirconia container and balls. The composite positive electrode consisted of 81 wt % active material, 14 wt % acetylene black, and 5 wt % polyacrylonitrile, pasted on an aluminum foil used as a current collector. The composite electrodes were dried at 120 °C in a vacuum. Metallic lithium (Honjo Metal) was used as the negative electrode. The electrolyte solution used was 1.0 mol dm−3 LiPF6 dissolved in ethylene carbonate:dimethyl carbonate (1:1 by volume) (battery grade, Kishida Chemical). A polyolefin microporous membrane was used as a separator. Two-electrode cells (TJ-AC, Tomcell Japan) were assembled in the Ar-filled glovebox. The cells were cycled in the voltage range 1.5−4.8 V at a rate of 10 mA g−1 at either room temperature or 50 °C. Charge/discharge tests were carried out by using a battery cycler (TOSCAT-3100, Toyo System). Electrochemical impedance measurements were carried out by using a potentiostat equipped with a frequency response analyzer (SP-200, Bio-Logic). Characterization of Samples. Crystal structures of the obtained samples were examined using an X-ray diffractometer (D2 PHASER, Bruker) equipped with a high-speed one-dimensional detector. Nonmonochromatized Cu Kα radiation was utilized as an X-ray source with a nickel filter. All measurements were carried out with an airtight sample holder to avoid air exposure. Structural analysis was carried out using RIETAN-FP.22 Schematic illustrations of crystal structures of samples were drawn using the program, VESTA.23 Morphological features of the samples were observed using a scanning electron microscope (JCM-6000, JEOL) and scanning transmission electron microscope (JEM-2100F, JEOL). Hard X-ray absorption spectroscopy at the V K-edge and Nb K-edge was performed at beamline BL-12C of the Photon Factory Synchrotron Source in Japan. The hard X-ray absorption spectra were obtained with a silicon monochromator in transmission mode. The intensity of the incident and transmitted X-ray was measured using an ionization chamber at room temperature. Composite electrode samples were prepared using coin cells at a rate of 10 mA g−1. The composite electrodes were rinsed with dimethyl carbonate and sealed in a water-resistant polymer film in an Ar-filled glovebox. Normalization of the XAS spectra was carried out using the program code, IFEFFIT.24 The postedge background was determined using a cubic spline procedure.

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RESULTS AND DISCUSSION Synthesis of Materials. X-ray diffraction (XRD) patterns of the binary system of xLi3NbO4−(1 − x) LiVO2 (0 ≤ x ≤ 1) are shown in Figure 1. Both end members (x = 1 and 0) B


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Figure 1. Crystal structure and electrode performance of x Li3NbO4− (1−x) LiVO2 binary system: (a) X-ray diffraction patterns of the binary system; (b) schematics of the crystal structures of LiVO2, Li1.3Nb0.3V0.4O2 (x = 0.43), and Li3NbO4.

crystallize into cation-ordered rocksalt structures, in which Li, Nb, and V ions are located at distinct octahedral sites. In contrast, the sample of x = 0.43 (Li1.3Nb0.3V0.4O2) crystallizes into a cation-disordered rocksalt structure. This observation is consistent with our previous report,7,21 and Li, Nb, and V ions are located at the same octahedral 4a sites. As fractions of V ions increase, the peak intensity of a 003 diffraction line at 18° in the 2θ range for the layered phase evolves. The highest theoretical capacity is expected for the sample of x = 0.25 (Li1.2Nb0.2V0.6O2) based on the two-electron redox for V ions. However, synthesis of a single phase sample was not successful, and two phases (presumably, the V-rich layered phase and Nbrich rocksalt phase) are found to coexist. For the sample of x = 0.33 (Li1.25Nb0.25V0.5O2), the crystal structure cannot be assigned to the cation-disordered rocksalt phase because a weak 003 peak is observed. This phase is most probably assigned as a partially cation-ordered layered phase as shown in Supporting Information Figure S1. However, the extent of cation ordering is limited (approximately 20% mixing between 3a and 3b sites with a symmetry of R3̅m), and a ratio of chex./ ahex. (lattice parameters in a hexagonal setting) is close to 4.9, which is identical to a cubic symmetry.25 Therefore, Miller indices for each diffraction in Figure 1a are assigned with a cubic symmetry. For example, the 003 line for the layered phase is reassigned as the 111 line with a large (2 × 2 × 2) cubic lattice. The local environment of cations is, however, very close to that of the cation-disordered rocksalt structure. The particle morphology of the samples are shown in Figure S2. The particle size is nonuniform (10−100 μm size), as observed in the SEM images. Nanostructures of the samples will be discussed in a later section. Electrode Performance. Electrochemical properties of the samples were examined in Li cells. Since the particle morphology of the as-prepared samples is nonuniform,

Figure 2. (a) Galvanostatic charge/discharge curves of the samples in of x Li3NbO4−(1 − x) LiVO2 binary system and (b) observed first discharge capacities of the samples and comparison with theoretical capacities.

including huge particles, average particle sizes of the samples were reduced by ball milling. Acetylene black (10 wt %) was also added for the ball milling process to enhance electrical conductivity (hereafter denoted as “carbon composite samples”). As shown in Figure S3, after the ball milling process, the size of the particles is drastically reduced. The electrode performance of the carbon composite samples is much better compared with as-prepared samples. The reversible capacities of Li1.25Nb0.25V0.5O2 are increased from 40 mAh g−1 for the as-prepared sample to 250 mAh g−1 for the carbon composite sample (Figure S3). The electrode performance of the carbon composite samples with different V ion contents are compared in Figure 2. The observed reversible capacities increase from the sample of x = 0.43 to 0.33, but reach almost the same value with the sample of x = 0.25 whereas theoretical capacities increase from 238 mAh g−1 for x = 0.43 to 360 mAh g−1 for x = 0.25. This trend would be C


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delivers a large reversible capacity of 250 mAh g−1, which corresponds to >80% of the theoretical capacity based on the two-electron redox of V ions (300 mAh g−1). Phase segregation for Li1.2Nb0.2V0.6O2 (x = 0.25) results in reduction of the reversible capacity as electrode materials apart from the theoretical capacity. Impact of Heat Treatment on Electrode Performance. In this study, the carbon composite samples were prepared by ball milling. However, in general, a ball milling process often induces strain and defects in samples.26 Therefore, heat treatment was conducted to relieve strain and defects in the oxide particles. The carbon composite Li1.25Nb0.25V0.5O2 samples were heat-treated in Ar at 500 or 800 °C for 1 h, and the samples are denoted as HT-500 and HT-800 hereafter. Changes in XRD patterns before and after heat treatment are shown in Figure 3. No significant change is found for lattice parameters on heating. Similarly, the heat-treatment process does not influence the particle morphology as shown in Figures S4 and S5. Agglomerated acetylene black is not observed for all the samples, indicating that carbon and oxide particles were uniformly dispersed in the composites. In contrast, clear changes are noted for profiles of the diffraction patterns. The full-width at half-maximum of the diffraction lines becomes narrow by heat treatment, indicating that the crystallinity of the samples is increased. Moreover, the bottom part of each diffraction line for the carbon composite sample before heat treatment is anomalously broad (see highlighted 104 lines and black arrows in Figure 3b), and such a profile was not found for

Figure 3. Impact of heat treatment for Li1.25Nb0.25V0.5O2: (a) changes in XRD patterns (highlighted patterns for the 104 lines are shown in b), high-resolution TEM images for (c) the carbon composite Li1.25Nb0.25V0.5O2 sample (before heat-treatment), and (d) after heat treatment at 800 °C (HT-800).

consistent with the experimental fact that a single phase sample was not obtained for x = 0.25. Li1.25Nb0.25V0.5O2 (x = 0.33)

Figure 4. Comparison of rate capability of Li1.25Nb0.25V0.5O2 Li cells before (a) and after (b) heat treatment. Rate capability tests were conducted after three cycles at 10 mA g−1. The cells were charged at 100 mA g−1 to 4.6 V, and then voltage was held at 4.6 V for 15 min and then discharged at different rates to 1.5 V. Impedance data of the carbon composite electrodes of Li1.25Nb0.25V0.5O2 before and after heat treatment at 800 °C; (c) Nyquist and (d) Bode plots. D


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the discharge capacity at 2000 mA g−1 whereas HT-800 surprisingly delivers 150 mAh g−1 at the same rate. Electrochemical impedance measurements further reveal that such significant improvement in rate capability originates from drastic reduction of electrode impedance (Figure 4c, d). Heat treatment does not change the absolute values of Z for a higher frequency range (>1 kHz). However, drastic changes in impedance are noted for a middle frequency range (1 Hz−1 kHz), in which through-plane resistance of the composite electrode (often denoted as SEI film resistance in the literature) coupled with a charge transfer process for V ions would be dominated. The reduction of impedance in the middle frequency range realizes the improvement of electrode kinetics. Raman spectroscopy also suggests that microstructures of carbon are also influenced by heat treatment (Figure S6). Nevertheless, the origin of reduction of impedance and unexpected electrode kinetics for HT-800 is still unclear. Nanoscale Phase Segregation in Individual Particles. To answer this question, nanostructures of Li1.25Nb0.25V0.5O2 were examined by scanning transmission electron microscopy (STEM). The outcomes of energy dispersive X-ray (EDX) mapping of the carbon composite samples before and after heat treatment (HT-800) conducted by high-resolution STEM imaging are shown in Figure 5. Low magnification EDX maps are also shown in Figure S7. In the low magnification maps, a uniform contrast for Nb/V ions is found, indicating that the same chemical compositions are expected for each oxide particle. In contrast to the low magnification maps, nonuniform distribution for Nb/V ions in individual particles is clearly evidenced from the high resolution images as shown in Figure 5. STEM imaging reveals that the secondary particles of Li1.25Nb0.25V0.5O2 consist of nanosize primary particles with a size of approximately 30−40 nm. Moreover, such nanosize particles have different chemical compositions as clearly evidenced by STEM/EDX mapping. Nb-rich and V-rich domains are uniformly dispersed in the single particles. The sample before heat treatment contains Nb enriched domains, and electric conductivity is expected to be locally reduced for these domains.28 This fact makes it difficult to analyze a detailed and real crystal structure of the sample. Note that the cubic crystal structure mentioned in Figure 1 and Figure S1 is, therefore, simply described as the “averaged” structure. The heat-treatment process clearly changes the distribution of Nb and V ions. A contrast between Nb and V ions for EDX maps becomes more uniform after heat treatment, and this fact increases electric conductivity throughout particles, leading to the lower resistance of electrodes, as shown in Figure 4c, d. However, it is noted that nanosize segregation is still clearly observed after heat treatment. Moreover, the crystallinity of nanosize V-rich and Nb-rich domains is also increased by heat treatment without strain and defects. It is hypothesized that lithium migration within particles is promoted by the presence of grain boundaries formed by the nanosize domains.29−31 Li migration at grain boundaries formed by nanoscale phase segregation possibly assists slow electrode kinetics for percolative Li migration in the crystal lattice. Further studies on the factors affecting electrode kinetics for Li-excess cationdisordered rocksalt oxides are required to prove this hypothesis. Note that nanoscale phase segregation for HT-800 is also consistent with the outcome of high-resolution TEM imaging shown in Figure 3d. The presence of nanosize domains in the individual particles makes lattice fringe observation more

Figure 5. Nanoscale phase segregation revealed by STEM/EDX mapping; (a) before heat treatment and (b) after heat treatment at 800 °C (HT-800).

HT-800. Inhomogeneous residual strain causes compression and extension of the crystal lattice, leading to peak broadening.27 This observation suggests that inhomogeneous residual strains are induced by ball milling, and such strains are effectively relieved by heat treatment. This trend is further supported by high-resolution TEM images shown in Figure 3c, d. As clearly shown in Figure 3c, a Moire pattern is a characteristic feature for the carbon composite sample before heat treatment and is indicative of strains in the particles. This feature is less pronounced for HT-800 (Figure 3d). Nevertheless, clear lattice fringes are not still observed for HT-800, and its origin is further discussed in a later section. The heat-treatment process drastically increases the electrode performance of the samples. The rate capability of the carbon composite Li1.25Nb0.25V0.5O2 sample before and after heattreatment at 800 °C is compared in Figure 4a, b. Discharge capacities observed at a low rate of 10 mA g−1 are comparable for both samples, but a clear difference is noted at higher rates. The sample before heat treatment delivers only 20 mAh g−1 of E


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Figure 6. Reaction mechanism of Li1.25−yNb0.25V0.5O2 on electrochemical cycles: (a) ex situ XRD patterns and (b) ex situ V K-edge XAS spectra. A scheme of V ion migration on charge is shown in (c).

difficult because the orientations of the nanosize domains are not coherently aligned in the particles. Reaction Mechanisms on Lithium Extraction/Insertion. To further examine the reaction mechanisms as electrode materials, ex situ X-ray diffractometry and X-ray absorption spectroscopy (XAS) were applied for Li1.25−xNb0.25V0.5O2 with different charge/discharge conditions (Figure 6). The original crystal structure (a partially cation-ordered rocksalt structure) is lost on charge. The 111 diffraction line at 18° disappears after the charge to 160 mAh g−1, indicating loss of the partial cation ordering with a cubic close-packed (ccp) array of oxide ions. It is also noted that the 222 diffraction line at 38° (111 line for rocksalt) is not visible. Such a profile change is indicative of cation migration from the octahedral to tetrahedral site with the ccp array for anions.7 After discharge to 1.5 V, the 111 diffraction line is again clearly visible. This phase is assigned as the cation-disordered rocksalt structure, in which all cations are located at octahedral sites in the ccp anion lattice. Moreover, a volume change on charge/discharge processes is anomalously small. Only a 1% difference in unit cell volumes is noted between full charge and full discharge states. This small change would be expected to originate from cation migration of V ions as revealed by XAS in the following section.

XAS spectra of Li1.25−xNb0.25V0.5O2 (HT-800) with different charge/discharge conditions are shown in Figure 6b. The energy of an XAS spectrum at the V K-edge for the as-prepared sample is similar to that of LiVO2, indicating that the oxidation state of V ions is the trivalent state. Upon charge, the energy of the XAS spectra shifts toward to the higher energy region. The energy position after full charge to 4.8 V resembles that of LiV3O8, implying that the oxidation state of V ions changes to nearly the pentavalent state. Moreover, a clear increase in the absorption peak at the pre-edge region at 5469 eV is noted. This fact suggests that small V5+ ions migrate to face-shared tetrahedral sites as shown in Figure 6c. Pre-edge peak intensity is drastically increased at a tetrahedral symmetry, especially for d0-configulation.32 Moreover, this result is also consistent with a theoretical prediction published in our literature.7 Additionally, these observations are a highly reversible process. After discharge to 1.5 V, the energy of spectra returns to its original position, and the pre-edge peak intensity is also reduced. V ions again migrate from small tetrahedral sites to face-shared octahedral sites on discharge. Such a mobile character of V ions results in the phase transition from partially cation-ordered rocksalt to a cation-disordered rocksalt system. Similar observation is also noted for the mobile Mo/Cr system.16 It is also proposed that the mobility of V ions correlates with a F


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conventional layered oxides, e.g., LiCoO2 and LiNiO2, extraction of 1.0 mol of Li ions from host structures results in the formation of the O1 phase33 (additionally, some stacking faults are noted for LixNiO234), and thus drastic shrinkage of interlayer distances cannot be avoided. This fact results in unacceptable capacity retention for battery applications when all Li ions are extracted from oxides. Such anisotropic phase transition originates from a specific character as layered materials. In contrast, in the case of the cation-disordered rocksalt system, volume changes occur in an isotropic manner, which potentially increases the fractions of Li ions used for reversible electrochemical cycles. Therefore, high-capacity electrode materials with a long cycle life is anticipated. Although the penalty of electrode kinetics cannot be avoided for the cation-disordered rocksalt structure, the unique nanostructure of nanoscale phase segregation possibly assists the facile Li diffusion at grain boundaries. In this study, the asprepared powder sample was simply mixed with carbon by ball milling and tested as the electrode material. Therefore, there is plenty of room for further increase in electrode performance through the optimization of particle sizes, carbon coating, nanoscale phase segregation, etc. The operating voltage as electrode materials is relatively low (less than 4 V vs Li), potentially leading to good thermal stability as electrode materials. The cost of Nb ions would be disadvantageous for practical applications. Nevertheless, Ti ions, which provide similar chemistry with Nb ions, are possibly substituted for Nb ions.8

Figure 7. Electrode performance of Li1.25Nb0.25V0.5O2 (HT-700): an accelerated cycle test at 50 mA g−1 at rt and (b) galvanostatic cycle test at 10 mA g−1 at 50 °C.

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CONCLUSIONS The binary system of Li3NbO4−LiVO2 has been systematically examined as high-capacity electrode materials for rechargeable lithium batteries. Among the samples found in this binary system, Li1.25Nb0.25V0.5O2 delivers a reversible capacity of over 240 mAh g−1 with excellent capacity retention. Ex situ XRD and XAS measurements reveal a highly reversible two-electron redox of V3+/V5+ with a small volume change on electrochemical cycles, presumably originating from reversible V migration from octahedral to face-shared tetrahedral sites. Heat treatment of the sample significantly improves the rate capability as electrode materials. The unique nanostructure of the sample clarified by high-resolution STEM imaging is proposed to be responsible for the improvement of rate capability. The sample consists of V-rich and Nb-rich nanoparticles formed by phase segregation, and Nb-rich domains before heat treatment suppresses the electronic conductions, resulting in the inferior electrode kinetics as evidenced by impedance measurements. The high reversibility of the two-electron redox of V ions would be beneficial for the development of high-capacity and potentially safe rechargeable lithium batteries in the future.

small volume change observed in this system. In general, oxidation of transition metal oxides and lithium extraction result in volume shrinkage because of reduction of ionic radii for transition metal ions at the same octahedral sites. However, migration of transition metals into small and narrow tetrahedral sites would increase electrostatic interaction, and would expand the crystal lattice on “oxidation”, leading to the anomalously small volume change on electrochemical cycles. In contrast, Nb ions are not responsible for redox processes as shown in Figure S8, and would provide a rigid framework structure as electrode materials. Practical Assessment for Battery Applications. In this article, Li1.25Nb0.25V0.5O2 is proposed for a new high-capacity positive electrode material. Cyclability is an important factor for practical applications, and therefore an accelerated cycle test was conducted. Figure 7a shows capacity retention of a Li/ Li1.25Nb0.25V0.5O2 cell. First three cycles are operated at 10 mA g−1 and then cycled at 50 mA g−1 for continuous 100 cycles. Good capacity retention as electrode materials is evidenced, and such high reversibility is advantageous in using the twoelectron redox reaction of V ions for battery applications. Moreover, reversible capacities are further enhanced at elevated temperatures. Li1.25Nb0.25V0.5O2 delivers a reversible capacity of approximately 300 mAh g−1 at 50 °C at least for 15 cycles as shown in Figure 7b. The energy density available reaches 770 mWh g−1 vs Li metal, and this value exceeds those of conventional layered oxides and phosphates (but lower than those of Li2MnO3-based oxides with an anion redox). Experimentally observed 300 mAh g−1 of the reversible capacity nearly corresponds with that of the theoretical capacity based on the two-electron redox of V ions, indicating that 1.0 mol of Li ions is reversibly extracted from the host structure. For the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02343. Comparison of simulated and experimental XRD patterns for Li1.25Nb0.25V0.5O2, SEM images of asprepared powders of x Li3NbO4−(1 − x) LiVO2 binary system, electrode performance and SEM images and Raman spectra of as-prepared Li1.25Nb0.25V0.5O2 and ball G


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milled Li1.25Nb0.25V0.5O2 with AB, SEM images of carbon composite sample of Li1.25Nb0.25V0.5O2 and heat-treated samples, low magnification STEM/EDX mapping of the carbon composite sample of Li1.25Nb0.25V0.5O2, ex situ Nb K-edge XAS spectra for Li1.25−yNb0.25V0.5O2 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Naoaki Yabuuchi: 0000-0002-9404-5693 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synchrotron X-ray absorption work was done under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2015G529).

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REFERENCES

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