Cu Doping Increases Capacity Retention in LiNi0.6Mn0.2Co0.2O2 (NMC622) by Altering the Potential of the Ni-Based Redox Couple and Inhibiting Particle Pulverization

Abstract

To discern the influence of Cu²⁺ as a dopant on both the structural and electrochemical characteristics of LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622), Cu²⁺(aq) was added to the coprecipitation synthesis from the constituent ions. At 5 mol % Cu²⁺, a single-phase Cu-NMC product results, evidenced by an increase in d-spacing along the [003] and [104] directions and a slight increase in the crystal volume of the R–3m hexagonal (rock-salt superstructure) lattice. XRD data and high-resolution TEM imaging support Cu²⁺ doping primarily on the transition metal 3b Wyckoff sites. Galvanostatic cycling of Cu-NMC shows a reversible gravimetric capacity of 102 mAh/g compared to 136 mAh/g for undoped NMC. Despite the lower capacity, the discharge capacity retention of Cu-NMC is 89% after 100 cycles compared to only 70% for NMC. XPS analysis reveals that this lower capacity is due to an increase in the concentration of Ni³⁺ ions at the surface, while XRD data collected at the top and bottom of charge show a smaller decrease in crystalline domain size for Cu-NMC (40.5% decrease) compared to NMC (74.7% decrease), translating to pulverization of the secondary particles.