Doped cation radius optimization of Li0.99M0.01Ni0.8Co0.1Mn0.1O2 (M=Ca, Sr, Ba) based on DFT calculation and electrochemical performance of Li-ion battery

To explore the influence of cation doping with cation radius variation on LiNi_0.8Co_0.1Mn_0.1O₂, a series of layered Li_0.99M_0.01Ni_0.8Co_0.1Mn_0.1O₂ (M-NCM, M=Ca, Sr, Ba) cathode materials were synthesized. Through density functional theory (DFT) calculations, all doped models have low band gaps and more relative electrons near the Fermi energy level than the pristine model. With increasing cation radius, the improvement in electronic conductivity diminishes. X-ray diffraction data and structural refinement results show that lattice parameters and lithium layer increase with increasing doping cation radius. The Ba-NCM sample, which has the largest radius, has the highest initial capacity (246.6 mAh·g⁻¹) at 0.1 C. The Ca-NCM sample, with the least doping cation radius, has the highest capacity retention, which reaches to 90.06% after 100 cycles at 1 C. The Sr-NCM sample exhibits the highest remaining capacity, which could retain 146.5 mAh·g⁻¹ at 5 C and 158.7 mAh·g⁻¹ after 100 cycles at 1 C. This is attributed to the highest Li⁺ diffusion coefficient (8.49 × 10⁻¹¹ cm²·s⁻¹) and moderate electronic conductivity and radius of Sr²⁺. Doping Li sites in LiNi_0.8Co_0.1Mn_0.1O₂ with divalent cations of proper radius could be effectively improved the comprehensive electrochemical performance. This effect is based on not only the pillar effect and high electronic conductivity but also the trade-off between the expansion of lithium layer thickness and introduction of steric hindrance, as a consequence of the cations doping.