Stabilizing effect of Mg on the energetics of the Li(Ni,Co,Al)O₂ cathode material for lithium ion batteries

Mg-doped LiNi_0.80Co_0.15Al_0.05O₂ cathode material shows less degradation than the undoped material during charge/discharge cycles, resulting in significantly improved cell performance. The improved performance may arise from thermodynamic stabilization and/or from kinetic factors, both of which may act to hinder decomposition. To investigate the possible thermodynamic contribution, the change in enthalpy of formation from oxides with Mg doping was investigated using high temperature oxide melt solution calorimetry. The measured enthalpy data plus the estimated configurational entropies indicate that the Mg-doped sample is 3.3 ± 0.9 kJ/mol more stable in free energy than the undoped material. This stabilization is suggested to be mainly due to short range ordering of Ni cations around Mg²⁺, providing stronger bonding in the structure. This modest thermodynamic stabilization leading to greater short range order may inhibit the formation of a NiO - like phase during cell operation.