Electrochemical Properties of LiNiO2 Integrated with Nanosize Li3PO4

Abstract

The demand of EVs is growing in the world and further increase in energy density of lithium-ion batteries (LIBs) is required. Among positive electrode materials for LIBs, LiNiO2 and its derivatives have been considered as an alternative material to LiCO2 because of its high reversible capacity, good rate capability, and relatively low material cost.[1] Nevertheless, LiNiO2 without cobalt ions shows insufficient irreversibility especially upon charge to high voltage. In this study, nanosized Li3PO4 is integrated into LiNO2, which is synthesized by mechanical milling and post heat treatment, and is tested as a positive electrode material in Li cells. We also discuss the possibility of high-performance and Co-free electrode materials for LIBs.A binary system of x Li3PO4 – (1 – x) LiNiO2 (x = 0, 0.03, and 0.05) with a rocksalt structure was synthesized by mechanical milling and used as a precursor. The precursor with a rocksalt structure was heated at 650 oC in oxygen, resulting in the formation of layered LiNiO2 integrated with nanosize Li3PO4 domains. Characterization of the samples was conducted by X-ray diffraction (XRD) and scanning electron microscopy (SEM). For the evaluation of electrochemical properties, composite electrodes consisting of 80 wt% active material, 10 wt% AB, and 10 wt% PVdF, were casted on aluminum foil. 1.0 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (3/7 by volume) solution was used as electrolyte.XRD patterns of x Li3PO4 – (1 – x) LiNiO2 (x = 0.03 and 0.05) binary system after milling are assigned into the rocksalt structure that is a metastable phase. After heating, the crystal structure of the samples is changed into the layered phase, and a uniform distribution of Ni and P was confirmed from energy dispersive X-ray elemental mapping. This finding suggests that nanosized Li3PO4 is successfully integrated into LiNiO2. Figure 1 shows the galvanostatic charge/discharge curves of the samples cycled in the voltage range of 2.5 and 4.5 V at a rate of 10 mA g-1 in Li cells. Layered LiNiO2 without Li3PO4 (x = 0) delivers a discharge capacity of 210 mA h g-1 at initial cycle. However, capacity degradation is non-negligible upon further cycles. In contrast, the sample of x = 0.03 exhibits a slightly reduced capacity of 190 mA h g−1, but the significant improvement of cyclability is evidenced even electrochemical cycle with charge to 4.5 V. From these results, we will further discuss the impact of integration of nanosize Li3PO4 into layered LiNiO2 and the possibility of Co-free electrode materials in the future. References (1) Y.-K. Sun et al., ACS Energy Lett. 2, 1150, (2017). Figure 1