In Situ Investigations on LiNiO2 during Post-Synthesis Heating in Air

Abstract

Enhancing cathode materials with high energy density and low cost has become the main direction of development in battery industry [1]. In order to achieve this goal, LiNi x Co y Mn1-x-y O2 (NCM) and LiNi x Co y Al1-x-y O2 (NCA) cathode materials with increased Ni contents (0.6 ≤ x ≤ 1) have been gradually put into mass production and applied in electric vehicles [2]. If the Ni content is being increased continuously, finally we will probably go back to LiNiO2, a well-known yet traditional material. Numerous efforts have been made to improve its stability and usability up to now [3, 4]. Nevertheless, an easy and effective method is still waiting to be discovered. For this purpose, we study the structural changes of LiNiO2 during the heat treatment process in air using nuclear magnetic resonance, synchrotron radiation diffraction and absorption spectroscopy. Various reactions such as oxygen loss, lithium transport and phase transitions take place in different temperature regions. During the thermal treatment, a thin layer is formed in-situ on the surface of LiNiO2 as a consequence of oxygen loss. The thickness of this layer could be precisely adjusted through changing the heating temperature. A thicker layer has positive effects on protection of LiNiO2 but it blocks the migration of Li ions, resulting in larger over-potential and lower capacities. After heating to 400 °C an excellent stability has been observed with 93% capacity retention after 180 cycles. In summary, the properties of LiNiO2 could be adjusted through a post-synthesis treatment without additional chemicals. This method could also be applied to a larger class of NCM or NCA materials as a new modification strategy.