Effect of valence states of Ni and Mn on the structural and electrochemical properties of Li1.2NixMn0.8−xO2 cathode materials for lithium-ion batteries

Abstract

Severe capacity fading and voltage decay of Li-rich layered oxides for lithium-ion batteries remain the major bottlenecks to commercialization. Herein, we studied the effect of valence states of Ni and Mn on the structure and electrochemical performances of Li_1.2Ni_xMn_0.8−xO₂. The oxidation states of the transition-metal (TM) were evaluated by X-ray photoelectron spectra (XPS) and validated by cyclic voltammetry (CV). With increasing Mn³⁺ content, the Li₂MnO₃ component increases due to less Li loss. The phenomenon of capacity increase upon cycling has been observed owing to the gradual activation of Li₂MnO₃ caused by the existence of Mn³⁺. It is worth mentioning that Li_1.2Ni_0.1Mn_0.7O₂ reaches the highest discharge capacity value of 231.5 mA h g⁻¹ from the initial value of 76 mA h g⁻¹ after 75 cycles and still keeps 100% of the capacity at 0.1C after 100 cycles. With the increase of Ni³⁺, Li/Ni mixing decreases because of its smaller size than Ni²⁺, the content of Li₂MnO₃ decreases as a result of the decrease of Li and Mn in the TM layers, and the discharge voltage increases on account of the higher reduction potential of Ni⁴⁺/Ni³⁺ compared with Ni⁴⁺/Ni²⁺ and Mn⁴⁺/Mn³⁺. Furthermore, Li_1.2Ni_0.4Mn_0.4O₂ delivers a first capacity value of 192.5 mA h g⁻¹ and operating voltage of 3.89 V at 0.1C, and capacity retention of 92.13% and a voltage drop of 0.25 V after 100 cycles, which indicates that the stability of capacity and voltage is improved by a large margin with increasing Ni³⁺.