Effect of aluminum doping on cathodic behaviour of LiNi_0.7Co_0.3O₂

Solid solutions of the formula LiNi_0.7Co_0.3-zAl_zO₂ (0.0≤z≤0.20) have been synthesized and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and surface area and density measurements. Single-phase materials are obtained for z≤0.15. Their cathodic behaviour in coin cells with Li metal as anode and a liquid electrolyte is examined by charge-discharge cycling at the 0.13C rate between 2.7 and 4.3 V and by cyclic voltammetry. Irreversible capacity loss occurs for all the compositions, but the phases with z = 0.05 and z = 0.10 show much less capacity fading at the end of 50 cycles as compared to those with z = 0.0 and z>0.10. The 5 at.% Al-doped compound showed a discharge capacity of 137 mA h^-1 g after 100 cycles that corresponds to 70% capacity retention. The beneficial effect of Al-doping (z≤0.10) is corroborated by cyclic voltammograms and differential scanning calorimetry (DSC) data on charged cathodes.