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(NixMnyCoz)­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.