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Synthesis and Electrochemical Properties of Li4MoO5-NiO Binary System as Positive Electrode Materials for Rechargeable Lithium Batteries Naoaki Yabuuchi, Yoshiyuki Tahara, Shinichi Komaba, Satsuki Kitada, and Yoshio Kajiya Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04092 • Publication Date (Web): 31 Dec 2015 Downloaded from http://pubs.acs.org on January 1, 2016
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Chemistry of Materials
Synthesis and Electrochemical Properties of Li4MoO5-NiO Binary System as Positive Electrode Materials for Rechargeable Lithium Batteries Naoaki Yabuuchi,1* Yoshiyuki Tahara,2 Shinichi Komaba,2 Satsuki Kitada,1 and Yoshio Kajiya3 1
Department of Green and Sustainable Chemistry, Tokyo Denki University, 5 Senju Asahi-Cho, Adachi, Tokyo 120-8551, Japan 2 Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan 3 Technology Department, Electronic Materials Group, JX Nippon Mining & Metals Corporation, 6-3, Otemachi 2-Chome, Chiyoda-ku, Tokyo 100-8164, Japan KEYWORDS, Lithium Battery, Molybdenum, Nickel, Positive Electrode A binary system, x Li4MoO5 – (1 – x) NiO, is first studied as electrode materials for rechargeable lithium batteries. In this system, Li4NiMoO6 (x = 0.5) is found to be as a new material having the Li5ReO6-type structure. Although an initial charge capacity of Li4NiMoO6 reaches over 300 mAh g-1, irreversible phase transition associated with Mo migration results in large polarization on discharge. When Li4NiMoO6 is cycled between 1.5 and 4.1 V in a Li cell, Li4NiMoO6 delivers a reversible capacity of 130 mAh g-1 with excellent reversibility. Rechargeable lithium batteries are widely used as power sources originally developed for portable electronic devices. Application of rechargeable lithium batteries is extended to different fields in the past two decades, and now it is used for electric vehicles as a zero emission transportation system. However, the demand for increase in energy density of rechargeable lithium batteries is still growing. In general, lithium-containing transition metal oxides, such as LiMn2O4, LiCoO2 etc., are used as positive electrode materials for rechargeable lithium batteries. In the past decade, so-called lithium-excess manganese oxides, Li2MnO3 and its derivatives, have been extensively studied as high-capacity positive electrode materials for rechargeable lithium batteries.1-3 Li2MnO3 with tetravalent manganese ions had been thought to be electrochemically inactive as an electrode material because the oxidation of tetravalent manganese ions into higher oxidation states is difficult. However, the fact is that Li2MnO3 is electrochemically active, and its derivatives deliver large reversible capacity with good capacity retention. Charge compensation is partly achieved by negatively charged oxide ions, instead of transition metal ions, coupled with the conventional redox reaction of transition metal ions.4,5 Our group has recently reported that Li3NbO4 with pentavalent niobium ions is also used as a new host structure for high capacity electrode materials.6,7 Similar to Li2MnO3, Li3NbO4 is also classified as one of the cation-ordered rocksalt structures, and consists of four edge-shared NbO6 octahedra (Nb4O16 tetramer). Lithium ions are accommodated in a bodycentered cubic lattice consisting of Nb4O16 tetramers.6 Although Li3NbO4 crystallizes as the lithium-enriched rocksalttype phase (lithium to metal ratio reaches three), Li3NbO4 is electrochemically inactive because of the absence of electrons in a conduction band (4d0 configuration for Nb5+). Transition metals (Ni2+, Co2+, V3+, Fe3+ and Mn3+) substituted for Li+ and Nb5+ donate electrons in the conduction band. However, the metal substitution also results in the formation of the cation-
disordered rocksalt-type phase.6,7 Although the formation of cation-disordered rocksalt phase is the unavoidable penalty for facile lithium insertion/extraction, lithium ions are able to migrate through a percolation network for lithium-excess materials.8 In this study, Li4MoO5 is targeted as a potential candidate for a host structure of high-capacity positive electrode materials. Similar to Li3NbO4, Li4MoO5 crystallizes into the cationordered rocksalt-type structure,9 and is expected to be used as the potential host structure for new high-capacity electrode materials. Furthermore, lithium contents in the structure are further enriched with hexavalent molybdenum ions, in comparison to Li2Mn4+O3 and Li3Nb5+O4. Nevertheless, hexavalent molybdenum ions have no d-electrons in a conduction band, and therefore the electronic conductivity of Li4MoO5 is intrinsically low as the electrode material. Since NiO containing divalent nickel ions crystallizes into a rocksalt structure, NiO has structural compatibility with Li4MoO5. To effectively extract lithium ions from Li4MoO5, divalent nickel ions are, therefore, partly substituted for lithium and molybdenum ions based on x Li4MoO5 – (1 – x) NiO binary system consisting of two different rocksalt-type related structures. Synthesis of single-phase samples is achieved in the entire range in this binary system. Additionally, nickel substituted samples deliver large reversible capacity as electrode materials even though irreversible phase transition associated with molybdenum migration is also observed. Crystal structures and X-ray diffraction patterns of the samples on the binary system, x Li4MoO5 – (1 – x) NiO, are shown in Figure 1. Synthesis methods are described in the Supporting Information. For the sample of x = 1.0 without nickel ions, the sample crystallizes into Li4MoO5, which is classified as one of cation-ordered rocksalt structures with a space group P19 as seen in Figure 1a. Mo6+ ions occupy octahedral sites in a cubic close-packed (ccp) lattice of oxide ions, and lithium ions occupy remaining octahedral sites. Two MoO6 octahedra
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share the edge, forming a Mo2O10 unit in the ccp lattice. After the substitution of Ni2+ ions for Mo6+ and Li+ ions, a new phase that is different from both Li4MoO5 and NiO as a rocksalt-type structure with a space group Fm-3m appears in 0.17 ≤ x ≤ 0.5 in x Li4MoO5 – (1 – x) NiO in Figure 1b. From the Bragg diffraction lines, it is found that this new phase has structural similarity with Li5ReO6, which is another lithi umenriched material possessing the cation-ordered rocksalt
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Figure 1. (a) Schematic illustrations of crystal structures of Li4MoO5, Li5ReO6, and NiO as rocksalt-related structures. (b) Xray diffraction (XRD) patterns of x Li4MoO5 – (1 – x) NiO (x = 0, 0.17, 0.25, 0.5, and 1.0). SEM images of particle morphology of as-prepared Li4MoO5 (x = 1.0), Li4NiMoO6 (x = 0.5), and NiO (x = 0) are shown in (c). Schematic illustrations of the crystal structures (Fig. 1a) were drawn using the program VESTA.10
structure with a space group C2/m.11 In Li5ReO6, Re6+ ions are located at octahedral sites in the ccp lattice of oxide ions,and completely isolated without clustering of ReO6 octahedra. Re6+ ions are found in alternate layers perpendicular to [111] for the rocksalt-type structure. For the new phase, x = 0.5, its chemical formula is simply described as the 1 : 1 mixture of Li4MoO5 and NiO, i.e.,Li4NiMoO6. Li4NiMoO6 is further reformulated as Li4/3Ni1/3Mo1/3O2 when the chemical formula of oxygen is normalized to “two”, which is the same content with conventional layered materials, LiMeO2. The detailed crystal structure of Li4NiMoO6 was further analyzed by the Rietveld analysis using RIETAN-FP.12 Structural parameters of Li5ReO611 are used as a starting model for refinement. Re ions at 2a sites
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in Li5ReO6 are replaced by Mo ions. Additionally, one Ni ion is substituted for one Li ion, resulting in the chemical formula of Li4NiMoO6. In the starting model used for the refinement, Ni2+ and Li+ ions in Li4NiMoO6 are randomly distributed in Li sites (2d, 4g, and 4h sites). A refined fitting result is shown in Figure 2, and refined structural parameters are summarized in Table 1. From the result of the Rietveld analysis, nickel ions slightly tend to occupy the 4g sites, which has the same z position with Mo ions at the 2a sites. However, the degree of ordering between Li and Ni ions, which have similar ionic radii (Li+; 0.76 Å and Ni2+; 0.69 Å) and electric charges (monovalent and divalent cations) for each other, is small. In contrast, perfect ordering is evidenced for Mo ions as a relatively small cation (Mo6+; 0.59 Å) with a high oxidation state. Recently, detailed crystal structure of Li4NiTeO6 has been reported in the literature.13 Li4NiTeO6 has similar chemical composition with Li4NiMoO6, and Te6+ ions are replaced for Mo6+ ions. Nevertheless, clear difference in the cation ordering is found in both samples. Ni ions cannot be found in the 4g sites for Li4NiTeO6, forming the layered structure. Particle morphology of the samples observed by SEM is shown in Figure 1c. The particle size of Li4MoO5 is found to be large (5 – 30 µm) even after grinding using a mortar and pestle. In contrast, NiO has clearly different particle morphology compared with that of Li4MoO5. Spherical secondary particles, which consist of sub-micrometer-sized NiO particles, are observed by SEM. Li4NiMoO6 crystallizes into large particles (5 – 15 µm) with smooth surfaces, and small primary particles, like NiO, are not observed. The electrode performance of Li4NiMoO6 was examined in Li cells. The theoretical capacity of Li4NiMoO6 reaches 385 mAh g-1 when all lithium ions are reversibly extracted/inserted from/into the crystal lattice. Although the as-prepared Li4NiMoO6 sample shows a large charge capacity of approximately 300 mAh g-1 on charge to 4.8 V (Supporting Figure S1), a reversible capacity observed is limited to 140 mAh g-1 with huge polarization and/or hysteresis. To reduce particle size and to improve electrical conductivity as a composite electrode, as-prepared Li4NiMoO6 and acetylene black (20 wt%) were thoroughly mixed by mechanical ball-milling. The particle size of Li4NiMoO6 is effectively reduced to a submicrometer scale and uniformly mixed with nano-sized carbon (Supporting Figure S1). Discharge capacity is, therefore, obs. Mo6+ calc. difference 2+ + Bragg positions Ni and Li
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Figure 2. A fitting result by the Rietveld analysis on Li4NiMoO6 (x = 0.5). Refined structural parameters are summarized in Table 1.
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Table 1. Refined structural parameters of Li4NiMoO6 by the Rietveld method: Monoclinic, space group C2/m, a = 5.101(1), b = 8.789(1), c = 5.085(1) Å, and β = 110.06(1)o, V = 214.19 Å3, Rwp = 4.28 %, and RI = 3.99 %. †Not refined.
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Figure 3. (a) Galvanostatic cycle data of x Li4MoO5 – (1 – x) NiO (x = 0.17, 0.25, and 0.5) samples in Li cells, and capacity retention of the cells are also shown in (b). (c) Galvanostatic oxidation/reduction curves of Li4NiMoO6 (x = 0.5) with different cut-off voltages at 10 mA g-1. Charge capacity retention with 4.11, 4.5, and 4.8 V cycles is also compared in (d). Differential curves of (c) are shown in Supporting Figure S3.
between Mo6+ and Te6+.13 By tailoring the inductive effect for n i c k e l i o n s , e l e c t ro c h e mi c a l re d o x p o t e n t i a l w i t h Ni2+/Ni4+would be effectively controlled and designed as electrode materials for different applications. Additionally, when the cut- off voltage is further lowered, ~1.5 V, an additional capacity below 2 V is observed (Figure 4b) with good capacity retention (Figure 4d). Such additional capacity further increases at elevated temperatures. Charge/discharge curves of ball-milled Li4-yNiMoO6 in the range of 1.5 – 4.1 V at 50 oC is shown in Figure 4c. Reversible capacity in the range of 2.0 – 1.5 V increases to approximately 50 mAh g-1, leading to 175 4.5
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increased by the ball-milling process as shown in Supporting Figure S1. Electrochemical properties of x Li4MoO5 – (1 – x) NiO (x = 0.17, 0.25, and 0.5) samples (after ball-milling with carbon) were further examined in Li cells in the voltage range of 1.5 – 4.8 V. Charge/discharge capacities increase as increase in the fraction of NiO (Figure 3a). Capacity retention is, however, insufficient as electrode materials as shown in Figure 3b. Although a large initial charge capacity (> 300 mAh g-1) is observed for Li4NiMoO6 (x = 0.5), approximately 50 % of reversible capacity is lost after 5 cycles. To improve the electrochemical reversibility, ball-milled Li4NiMoO6 was cycled with different cut-off voltages for the charge (oxidation) process (Figure 3c). As shown in Figure 3d, reversibility is improved by lowering the cut-off voltage even though reversible capacity is inevitably lowered. No capacity loss was observed for continuous 15 cycles in the range of 1.5 – 4.11 V. When the cut-off voltage is increased from 4.11 to 4.5 V, a discharge (reduction) voltage profile changes. A clear voltage plateau at 2 V appears after charge above 4.5 V, which is clearly different from the voltage profile observed for 4.11 V cycle. To examine the difference in voltage profiles for 4.11 and 4.5 V cycles, ex-situ XRD patterns for Li4-yNiMoO6 were collected after electrochemical cycles. Diffraction lines related to the Mo ordering, observed in a two theta range of 20 – 35o, have disappeared after the 4.5 V cycle as shown in Supporting Figure S2, and major diffraction lines on the XRD pattern can be assigned as a cation-disordered rocksalt structure. This observation suggests that an irreversible phase transition, at least including Mo migration in the bulk structure, occurs on the 4.5 V cycle. Clear difference is observed on the XRD pattern for the 4.11 V cycle. Diffraction lines related to the Mo ordering are still observed after charge to 4.11 V. It is hypothesized that partial oxygen loss occurs on oxidation to above 4.11 V, resulting in the structural reconstruction process as observed for Li2MnO3based electrode materials. 5 To further examine the electrode performance of ball-milled Li4-yNiMoO6 (with 10 wt% carbon) the sample was continuously cycled with the limited voltage ranges (Figure 4). As shown in Figure 4a, reversible capacity reaches 105 mAh g-1 with quite small polarization in the range of 2.5 – 4.1 V. Although available reversible capacity is similar to that of Li4yNiTeO6, the operating voltage observed (3.3 – 4.1 V) is much lower than that of Li4-yNiTeO6 (4 – 4.5 V). The fact clearly indicates that difference in the inductive effect for nickel ions
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Figure 4. Charge/discharge curves of ball-milled Li4NiMoO6 (x = 0.5) with different cut-off voltages at 5 mA g-1; (a) 2.5 – 4.1 V, (b) 1.5 – 4.1 V at room temperature, and (c) 1.5 – 4.1 V at 50 oC. Discharge capacity retention of (a) – (c) was also compared in (d). As-prepared Li4NiMoO6 was ball-milled with 10 wt% carbon and used as the carbon-composite sample.
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mAh g-1 of total reversible capacity with relatively good capacity retention. This process shows large hysteresis with slower kinetics in Li cells. Studies on the origin of the reversible capacity below 2 V is currently in progress in our group. In this article, x Li4MoO5 – (1 – x) NiO binary system has been first studied as electrode materials for rechargeable lithium batteries. A series of single-phase samples is obtained in this system and Li4NiMoO6 (x = 0.5) is found to crystallize into the Li5ReO6-related structure. Although lithium ions can be extracted from Li4NiMoO6 with an initial charge capacity of over 300 mAh g-1, irreversible phase transition has been observed after charge to high voltage. By lowering cut-off voltage, capacity retention as electrode materials is effectively improved. These findings will contribute further development of lithium-enriched high-capacity positive electrode materials for rechargeable Li batteries in the future. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI:
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Rechargeable Lithium Batteries, in-press, DOI: 10.1039/C5CC08034G. (8) Lee, J.; Urban, A.; Li, X.; Su, D.; Hautier, G.; Ceder, G., Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries, Science 2014, 343, 519. (9) Hoffmann, R.; Hoppe, R., Zwei neue Ordnungs-Varianten des NaCl-Typs: Li4MoO5 und Li4WO5, Zeitschrift für anorganische und allgemeine Chemie 1989, 573, 157. (10) Momma, K.; Izumi, F., VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data, J. Appl. Crystallogr. 2011, 44, 1272. (11) Betz, T.; Hoppe, R., Über Perrhenate. 2. Zur Kenntnis von Li5ReO6 und Na5ReO6 – mit einer Bemerkung über Na5IO6, Zeitschrift für anorganische und allgemeine Chemie 1984, 512, 19. (12) Izumi, F.; Momma, K., Three-Dimensional Visualization in Powder Diffraction, Solid State Phenom. 2007, 130, 15. (13) Sathiya, M.; Ramesha, K.; Rousse, G.; Foix, D.; Gonbeau, D.; Guruprakash, K.; Prakash, A. S.; Doublet, M. L.; Tarascon, J. M., Li4NiTeO6 as a positive electrode for Li-ion batteries, Chemical Communications 2013, 49, 11376.
Experimental details, Electrode performance of asprepared sample, SEM image of ball-milled sample, XRD patterns of the samples after electrochemical cycles, differential capacity plots. AUTHOR INFORMATION Corresponding Author *N. Yabuuchi. E-mail:
[email protected] ACKNOWLEDGMENT This study was partly founded by Inoue Foundation for Science. REFERENCES (1) Johnson, C. S.; Kim, J. S.; Lefief, C.; Li, N.; Vaughey, J. T.; Thackeray, M. M., The significance of the Li2MnO3 component in 'composite' xLi(2)MnO(3) center dot (1-x)LiMn0.5Ni0.5O2 electrodes, Electrochem. Commun. 2004, 6, 1085. (2) Lu, Z. H.; MacNeil, D. D.; Dahn, J. R., Layered cathode materials Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 for lithium-ion batteries, Electrochem. Solid State Lett. 2001, 4, A191. (3) Robertson, A. D.; Bruce, P. G., Mechanism of electrochemical activity in Li2MnO3, Chem. Mat. 2003, 15, 1984. (4) Koga, H.; Croguennec, L.; Mannessiez, P.; Menetrier, M.; Weill, F.; Bourgeois, L.; Duttine, M.; Suard, E.; Delmas, C., Li1.20Mn0.54Co0.13Ni0.13O2 with Different Particle Sizes as Attractive Positive Electrode Materials for Lithium-Ion Batteries: Insights into Their Structure, Journal of Physical Chemistry C 2012, 116, 13497. (5) Sathiya, M.; Rousse, G.; Ramesha, K.; Laisa, C. P.; Vezin, H.; Sougrati, M. T.; Doublet, M. L.; Foix, D.; Gonbeau, D.; Walker, W.; Prakash, A. S.; Ben Hassine, M.; Dupont, L.; Tarascon, J. M., Reversible anionic redox chemistry in high-capacity layeredoxide electrodes, Nat Mater 2013, 12, 827. (6) Yabuuchi, N.; Takeuchi, M.; Nakayama, M.; Shiiba, H.; Ogawa, M.; Nakayama, K.; Ohta, T.; Endo, D.; Ozaki, T.; Inamasu, T.; Sato, K.; Komaba, S., High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure, Proceedings of the National Academy of Sciences 2015, 112, 7650. (7) Yabuuchi, N.; Takeuchi, M.; Komaba, S.; Ichikawa, S.; Ozaki, T.; Inamasu, T., Synthesis and Electrochemical Properties of Li1.3Nb0.3V0.4O2 as a Positive Electrode Material for
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