Directly revealing the structure-property correlation in Na⁺-doped cathode materials

The introduction of Na⁺ is considered as an effective way to improve the performance of Ni-rich cathode materials. However, the direct structure–property correlation for Na⁺ doped NCM-based cathode materials remain unclear, due to the difficulty of local and accurate structural characterization for light elements such as Li and Na. Moreover, there is the complexity of the modeling for the whole Li ion battery (LIB) system. To tackle the above-mentioned issues, we prepared Na⁺-doped LiNi_0.6Co_0.2Mn_0.2O₂ (Na-NCM622) material. The crystal structure change and the lattice distortion with picometers precision of the Na⁺-doped material is revealed by Cs-corrected scanning transmission electron microscopy (STEM). Density functional theory (DFT) and the recently proposed electrochemical model, i.e., modified Planck-Nernst-Poisson coupled Frumkin-Butler-Volmer (MPNP-FBV), has been applied to reveal correlations between the activation energy and the charge transfer resistance at multiscale. It is shown that Na⁺ doping can reduce the activation energy barrier from ΔG = 1.10 eV to 1.05 eV, resulting in a reduction of the interfacial resistance from 297 Ω to 134 Ω. Consequently, the Na-NCM622 cathode delivers a superior capacity retention of 90.8 % (159 mAh.g⁻¹) after 100 cycles compared to the pristine NCM622 (67.5 %, 108 mAh. g⁻¹). Our results demonstrate that the kinetics of Li⁺ diffusion and the electrochemical reaction can be enhanced by Na⁺ doping the cathode material.