The impact of low-level cobalt doping on ultra-high nickel cathode materials: From the perspective of structure and electrochemical properties

Abstract

Ni-rich Co-free cathode materials have been emerged as a promising candidate material due to its theoretical high energy density and relatively low cost. However, the absence of cobalt poses significant challenges for these materials in achieving both high specific capacity and practical structural stability. Here, we conducted a systematic investigation into the impact of substituting 3 % of Mn with Co in Ni_0.925Mn_0.075(OH)₂ hydroxide precursors on both the structure and the lithiation reaction behavior. Low-level Cobalt doping promotes the fusion and growth of primary grain boundaries during calcination, as a result, it reduces the optimal calcination temperature and restrains the lattice Li/Ni disorder. A higher proportion of Ni²⁺ is present in LiNi_0.925Mn_0.075O₂ to maintain charge balance due to a relative higher concentration of Mn⁴⁺ ions in the bulk, resulting in a more severe cation mixing (3.94 %) when compared to that of LiNi_0.925Co_0.03Mn_0.045O₂. The electrochemical performance test results indicate that low Cobalt substitution obviously promotes the specific capacity, cycle stability, and rate capability of the Ni-rich cobalt-free cathode materials. LiNi_0.925Co_0.03Mn_0.045O₂ cathode materials synthesized at the optimized calcination temperature of 750 °C shows a high capacity retention of 80.07 % after 100 cycles at 1C rate. Furthermore, the specific discharge capacity of 3 % Co doped sample at a 10C rate exceeds that of the cobalt-free sample by >15 mAh g⁻¹.