High Capacity Li-Excess Vanadium Oxides for Positive Electrode Materials

Abstract

Further demand for higher energy density of lithium-ion batteries (LIBs) is growing, especially for the development of electric vehicles to reduce dependence on fossil fuel. Although Co/Ni ions are used as positive electrode materials, its depletion of material resources is an emerging problem. Among electrode materials with 3d transition metal ions, LiVO2 with a layered rocksalt structure (s.g. R-3m) is known to be electrochemical inactive, associated with phase transition during charge. Nevertheless, our group has reported Li-excess Li3NbO4–LiVO2 binary oxides, and Li1.25Nb0.25V0.5O2 on this binary system with a cation disordered rocksalt structure delivers a large reversible capacity of 250 mA h g-1 with two-electron redox of V3+/V5+ at room temperature. (1) In this study. Instead of Li3NbO4, Li2TiO3–LiVO2 binary oxides are targeted as potential high capacity positive electrode materials. We also discuss the possibility of high capacity and long cycle life batteries without Co/Ni ions in the future. Li2TiO3–LiVO2 binary oxides were prepared by conventional calcination method from stoichiometric amounts of Li2CO3, anatase type TiO2, and V2O3. The precursors were mixed by wet ball milling and dried in air, and then calcined at 900 oC for 12 h in argon atmosphere. Thus obtained oxides were mechanically milled at 600 rpm for 36 h to prepare nanosized oxides. All of synthesized samples were stored in an argon filled glovebox to prevent the contact with oxygen and water. Electrode performance of the oxides was examined after reducing particle sizes by ball milling with 10 wt% acetylene black. Crystal structures and electrochemical properties of the oxides were studied by X-ray/neutron diffraction and galvanostatic charge/discharge measurement in two-electrode cells. XRD patterns and SEM images of LiVO2 (x = 0) and Li8/7Ti2/7V4/7O2 (x = 0.33) in the binary system x Li2TiO3–(1 – x) LiVO2 before and after mechanical milling are shown in Fig. 1. As-prepared LiVO2 and Li8/7Ti2/7V4/7O2 crystallized into the layered rocksalt structure (with partial cation disordering for Li8/7Ti2/7V4/7O2) change into nanosized and cation-disordered rocksalt structure (s.g. Fm-3m) after mechanical milling. The mechanical milled samples consist of nanosized, less than 10 nm, and low crystallinity oxides, which are agglomerated for each other, forming (sub-)micrometer-sized secondary particles. Discharge capacities of LiVO2 and Li8/7Ti2/7V4/7O2 obtained by galvanostatic charge/discharge are plotted in Fig. 2. LiVO2 after mixing with carbon by milling delivers a reversible capacity of 150 mA h g-1 in the Li cell, and a much higher reversible capacity of 270 mA h g-1 is observed for Li8/7Ti2/7V4/7O2. XAS study reveals reversible two-electron vanadium redox reaction (V3+/V5+) is activated for Li8/7Ti2/7V4/7O2. Moreover, the nanosized and rocksalt sample prepared by mechanical milling delivers over 300 mA h g-1 even though capacity deterioration is non-negligible in the Li cell with 1 M LiPF6 used as electrolyte. Nevertheless, electrode reversibility is significantly improved for the Li cell with the concentrated electrolyte, LiFSA:DMC = 1:1.1 in a molar ratio,(2) and excellent capacity retention is achieved for continuous 100 cycle test. From these results, we further discuss the origin of excellent capacity retention as electrode materials and possibility of high capacity LIBs with V3+/V5+ two-electron redox in the future.Reference(1) M. Nakajima and N. Yabuuchi, Chem. Mater., 29, 6927 (2017).(2) A. Yamada, et al, Nat. Commun, 7, 12032 (2016). Figure 1