Enhanced rate performance and cycle stability of LiNi_0.8Co_0.15Al_0.05O₂ via Rb doping

Rubidium modified Ni-rich LiNi_0.8Co_0.15Al_0.05O₂ cathode materials were successfully synthesized through a nano-milling assisted solid-state method. Rietveld refinement revealed that the Rb ions were incorporated into the lattice by replacing the original Li ions. It resulted in the enlarging of Li layer spacing together with the reducing of the thickness of the transition metal slab. Such changes in crystal structure led to the increase in the diffusion coefficient of Li ions in the lattice (D_Li). Calculated diffusion coefficient presented the highest Li-ion diffusion coefficient of 1.54 × 10⁻¹⁰ cm² s⁻¹ for the sample Rb_0.01Li_0.99Ni_0.8Co_0.15Al_0.05O₂. It was also in good accordance with the changes in Li slab thickness. Improved electrochemical performance in specific capacity, capacity retention and rate performance were observed. Among the samples synthesized, the Rb_0.01Li_0.99Ni_0.8Co_0.15Al_0.05O₂ sample exhibited the highest initial discharge capacity and the best rate performance. It delivered specific discharge capacities of 190.5, 177, 169, 161 and 151 mAh g⁻¹ at 0.1C, 0.2C, 0.5C, 1C and 5C, respectively. It also showed the best cycle stability with capacity retention of 91.31% after 100 cycles at 2C.