Effects of Oxygen Pressurization on Li⁺/Ni²⁺ Cation Mixing and the Oxygen Vacancies of LiNi_0.8Co_0.15Al_0.05O₂ Cathode Materials

Abstract

Ni-rich cathode materials are a low-cost and high-energy density solution for high-power lithium-ion batteries. However, Li⁺/Ni²⁺ cation mixing and oxygen vacancies are inevitably formed during the high-temperature calcination process, resulting in a poor crystal structure that adversely affects the electrochemical performance. In this work, the LiNi_0.8Co_0.15Al_0.05O₂ cathode material with a regular crystal structure was prepared through oxygen pressurization during lithiation-calcination, which effectively solved the problems caused by the high calcination temperature, such as oxygen loss and a reduction of Ni³⁺. The co-effect of oxygen pressure and calcination temperature on the properties of Ni-rich materials was systematically explored. Oxygen pressurization increased the redox conversion temperature, thus promoting the oxidation of Ni²⁺ and reducing Li⁺/Ni²⁺ cation mixing. Moreover, due to the strong oxidizing environment provided by the elevated calcination temperature and oxygen pressurization, the LiNi_0.8Co_0.15Al_0.05O₂ material synthesized under 0.4 MPa oxygen pressure and a calcination temperature of 775 °C exhibited few oxygen vacancies, which in turn suppressed the formation of microcracks during the electrochemical cycling. An additional feature of the LiNi_0.8Co_0.15Al_0.05O₂ material was the small specific surface area of the particles, which reduced both the contact area with the electrolyte and side reactions. As a result, the LiNi_0.8Co_0.15Al_0.05O₂ material exhibited remarkable electrochemical performance, with an initial discharge capacity of 191.6 mA h·g^-1 at 0.1 C and a capacity retention of 94.5% at 0.2 C after 100 cycles.