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 Na0.65Ni0.16Co0.14Mn0.7O2 (CG-NCM), in which the composition varies gradiently from core Na0.65Ni0.01Co0.01Mn0.98O2 to shell Na0.65Ni0.31Co0.27Mn0.42O2, was prepared by a coprecipitation 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 Na0.65Ni0.16Co0.14Mn0.7O2 (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. (c) 2023 Elsevier B.V. All rights reserved.