Balancing capacity and cycling stability of Ni-rich cathode : trace amount of cobalt dopant is unnecessary in single-crystal

Abstract

Cobalt is a crucial component in Ni-rich layered oxide cathodes. However, the role of trace amounts of cobalt has been debated and remains unclear. In this study, a Co-free single-crystal cathode, LiNi₀.₈₈Mn₀.₁₂O₂ (NM88) is proposed, exhibiting superior cycling performance compared with the Co-containing cathode, LiNi₀.₈₃Co₀.₀₅Mn₀.₁₂O₂ (NCM83). The results reveal that a small amount of Co in NCM83 facilitates an early onset of the phase transition, delivering higher capacity at the same cut-off voltage. However, 5% Co has no significant effect on Li-ion diffusion. The NCM83 suffers from more severe, irreversible surface reconstruction and rapid capacity decay. What's more, Ni dominates the change in oxidation state in NCM83, in contrast to Co, revealing the essence of capacity improvement in Co-containing cathode. Consequently, the highly oxidized Ni-ion in NCM83 leads to severe Ni⁴⁺ reduction, dissolution, and deposition. In contrast, although the NM88 exhibits slightly lower initial capacity, it demonstrates better stability with a higher capacity retention. This work unravels the role of trace amounts of Co and the origin of enhanced capacity in Co-containing single-crystal cathode, paving the way for the Co-free single-crystal cathode toward highly stable and low-cost Li-ion batteries.