Thermodynamics of Antisite Defects in Layered NMC Cathodes: Systematic Insights from High-Precision Powder Diffraction Analyses

While it is accepted that paired NiLi and LiNi antisite defects are present in the important family of NMC cathode materials with the general formula Li(Ni_xMn_yCo_z)O₂, their formation mechanism and influence on properties are not well understood due to the difficulty of accurately quantifying defects. In this work, novel high-precision powder diffraction methods have been used to elucidate the dependence of defect concentration on NMC composition. Formation energies for paired antisite defects (calculated under the assumption of equal state degeneracy) are observed to vary from about 320 to 160 meV, contradicting the constant defect formation energy that would be expected based on the previously proposed atomistic defect formation mechanism (size similarity of Ni²⁺ and Li⁺ cations). The present data support an alternative mechanism in which the equilibrium defect concentration is determined by the average size of transition-metal sites and thus suggest a new route by which chemical substitutions can be used to tune defect concentrations to optimal levels.