Structures, electrochemical properties, and phase evolution behaviors of LiNi_1-xMn_xO_2-δ (0 ≤ x ≤ 1/2) as cathode materials for secondary lithium batteries

In an attempt to get Co-free layered materials for cathode of secondary lithium batteries, LiNi_1-xMn_xO_2-δ (0 ≤ x ≤ 1/2) samples were synthesized by co-precipitation reactions followed by heating in an O₂ atmosphere. The optimum heating temperature to get a single-phase hexagonal layered structure increased with Mn contents in the compound. Oxygen vacancies could be gradually reduced with Mn-substitutions. While the lattice constants, M-O distance, and MO₂ slab thickness increased with Mn contents, Li-O and inter-slab distances decreased. Electrical conductivities decreased with Mn contents because of the decreased carrier mobility that resulted from smaller primary particle sizes as well as lager amounts of cation mixing. Samples of x = 1/12 and 1/9 in LiNi_1-xMn_xO_2-δ exhibited 0.1 C-rate discharge capacities of 216.2 and 208.9 mAh g⁻¹ and first cycle efficiencies of 93.3 and 94.0%, respectively. The sample of x = 1/9 in LiNi_1-xMn_xO_2-δ exhibited the highest rate capability. From the systematically investigated phase evolution behaviors, a higher activation energy as well as a slower rate of phase transition reaction for higher Mn contents phase were observed.