Structural and Electrochemical Impacts of Mg/Mn Dual Dopants on the LiNiO₂ Cathode in Li-Metal Batteries

Doping chemistry has been regarded as an efficient strategy to overcome some fundamental challenges facing the "no-cobalt" LiNiO₂ cathode materials. By utilizing the doping chemistry, we evaluate the battery performance and structural/chemical reversibility of a new no-cobalt cathode material (Mg/Mn-LiNiO₂). The unique dual dopants drive Mg and Mn to occupy the Li site and Ni site, respectively. The Mg/Mn-LiNiO₂ cathode delivers smooth voltage profiles, enhanced structural stability, elevated self-discharge resistance, and inhibited nickel dissolution. As a result, the Mg/Mn-LiNiO₂ cathode enables improved cycling stability in lithium metal batteries with the conventional carbonate electrolyte: 80% capacity retention after 350 cycles at C/3, and 67% capacity retention after 500 cycles at 2C (22 °C). We then take the Mg/Mn-LiNiO₂ as the platform to investigate the local structural and chemical reversibility, where we identify that the irreversibility takes place starting from the very first cycle. The highly reactive surface induces the surface oxygen loss, metal reduction reaching the subsurface, and metal dissolution. Our data demonstrate that the dual dopants can, to some degree, mitigate the irreversibility and improve the cycling stability of LiNiO₂, but more efforts are needed to eliminate the key challenges of these materials for battery operation in the conventional carbonate electrolyte.