Charge/Discharge Reaction Mechanisms of Nanosized Li-Excess Li2TiO3–LiVO2 System

Abstract

The demand of high-energy Li-ion batteries (LIBs) is rapidly growing for electric vehicles (EVs) to reduce dependence on fossil fuel. Although currently, Ni-rich layered oxides are mainly used as positive electrode materials for LIBs, the cost of Ni is raising because of increase demand of EVs. Therefore, the development of alternative materials without Co/Ni ions is necessary. In this study, vanadium ions with double electron redox reaction (V3+/V5+) are targeted as potential high capacity electrode materials. Stoichiometric LiVO2 is electrochemical inactive due to the phase transition from a layered to rocksalt-type structure.[1] Nevertheless, a Li-excess V-based oxide, Li1.25Nb0.25V0.5O2, with the rocksalt-type structure delivers a large reversible capacity of 250 mA h g-1 with percolative Li migration in the structure. [2] In this study, Li-excess Li2TiO3–LiVO2 binary system is synthesized and tested as potential high-capacity electrode materials. Among the tested samples, Li8/7Ti2/7V4/7O2 (x = 0.33 in x Li2TiO3–(1–x) LiVO2 binary system delivers the largest reversible capacity of approximately 300 mA h g-1 with no capacity degradation for over 100 cycles (Fig. 1). To clarify detailed reaction mechanisms, synchrotron X-ray total scattering was utilized and obtained pair distribution function (PDF) data are shown in Fig. 2. It is revealed that the crystallinity of the sample is lowered and V migration from original octahedral to tetrahedral sites on charging, which is consistent with the finding by X-ray absorption spectroscopy. Small V5+ ions are energetically stabilized at tetrahedral sites. Nevertheless, this process is highly reversible, and the high crystallinity phase with V ions at octahedral sites is obtained after discharging. From these data together with operando XRD data, the origin of the long cycle life for Li8/7Ti2/7V4/7O2 will be discussed in more details.[1] J. B. Goodenough, et al, Mater. Res. Bull., 15, 783-789 (1980)[2] N. Nakajima and N. Yabuuchi, Chem. Mater., 29, 6927 (2017). Figure 1