Since early 1990s lithium-ion batteries (LIBs) have become the most important secondary/rechargeable battery technology for portable electronic devices; and more recently for battery powered vehicles. Extensive research has been conducted on high-capacity electrode materials for LIBs due to the emerging demands for high energy density batteries for electric vehicles. Compared with the conventional cationic redox-based cathode materials, such as NMC, LMO and LFP, Li-excess, Manganese rich layered cathode (LLC) materials containing cationic and anionic redox process, show great potential as the next generation cathode materials because of their higher theoretical specific capacity and lower raw material cost.

Our work has shown that Ni content of the Co-free, LLC materials, Li_1.2Ni_xMn_0.8-xO₂ (x = 0.12, 0.24, 0.30 and 0.36) prepared by a citric acid method, has a significant impact on the material structure, electrochemical performance, and thermal stabilities. Generally, the primary particle size increased as the Ni content increased with a primary particle size distribution in the range of 200–600 nm for the samples investigated. Monoclinic phase (C2/m) and spinel (Fd3m) phase depending on the content of Ni have been identified by XRD. With increasing Ni content, the fraction of the spinel phase decreased and eventually the pure layered structure was obtained on the samples at x = 0.30 and 0.36. Our work demonstrates that the charging and discharging capacities are determined by the Ni or Mn content. The highest charging and discharging capacities were achieved when x = 0.24. As Ni content increased, the first cycle coulombic efficiency decreased, and the thermal stability deteriorated. But, an increase in Ni content results in an increase in average discharge voltage, higher electrochemical stability of the layered structure, fast Li ions diffusivity. Li_1.2Ni_0.12Mn_0.68O₂ sample exhibited the best capacity retention and thermal stability among the tested samples. However, its specific energy density was limited due to the lowest average discharge voltage. The Li_1.2Ni_0.36Mn_0.44O₂ sample which contained the highest Ni content exhibited the worst thermal stability, but relatively higher specific energy density due to the highest average discharge voltage.