Improved electrochemical performance of P2-type concentration-gradient cathode material Na0.65Ni0.16Co0.14Mn0.7O2 with Mn-rich core for sodium-ion batteries

The full concentration-gradient Na _0.65Ni _0.16Co _0.14Mn _0.7O ₂ (CG-NCM), in which the composition varies gradiently from core Na _0.65Ni _0.01Co _0.01Mn _0.98O ₂ to shell Na _0.65Ni _0.31Co _0.27Mn _0.42O ₂ , was prepared by a co-precipitation method plus high-temperature calcination, and was investigated by XRD, SEM, TEM and electrochemical measurements. Benefiting from the special concentration-gradient core-shell structure, CG-NCM possesses higher diffusion coefficients of Na ⁺ and the consequently better rate capability and cyclability than concentration-constant Na _0.65Ni _0.16Co _0.14Mn _0.7O ₂ (CC-NCM). Galvanostatically cycled at 30 mA g ⁻¹ between 1.5 and 4.1 V (vs Na ⁺/Na), CC-NCM and CG-NCM have initial discharge capacities of 168.5 and 162.5 mA h g ⁻¹, and the capacity retentions of 11.5 % and 51.4 % after 100 cycles, respectively. CG-NCM presents high initial discharge capacities of 144.5, 138.2 and 121.8 mA h g ⁻¹ at 75, 300 and 750 mA g ⁻¹, respectively, and exhibits capacity retentions of 70.6 %, 73.2 % and 76.9 % after 100 cycles, respectively. However, CC-NCM shows initial discharge capacities of 149.5, 136.5 and 107.3 mA h g ⁻¹ at 75, 300 and 750 mA g ⁻¹ , respectively, and the matching capacity retentions are as low as 49.8 %, 52.3 % and 53.9 %, respectively. The results indicate that the construction of concentration-gradient oxide electrode materials is an effective strategy for improvement of the electrochemical performance of oxides cathodes.