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.

尽管学界已公认，通式为Li(NixMnyCoz)O₂的重要NMC（Nickel Manganese Cobalt，镍锰钴）正极材料家族中普遍存在成对的NiLi与LiNi反位缺陷，但由于难以精准量化缺陷，其形成机制与对材料性能的影响尚未得到充分阐明。本研究采用新型高精度粉末衍射方法，阐明了反位缺陷浓度随NMC材料组分的变化规律。研究发现，成对反位缺陷的形成能（基于态简并度相等的假设计算得到）约在320至160毫电子伏特之间波动，这与此前基于Ni²+与Li+阳离子尺寸相似性提出的原子级缺陷形成机制所预期的恒定缺陷形成能相矛盾。本研究数据支持一种全新的缺陷形成机制，即平衡态缺陷浓度由过渡金属位点的平均尺寸决定，据此提出了一条通过化学取代调控缺陷浓度至最优水平的新途径。