High-performance Ni-free sustainable cathode Na 0.67 Mg 0.05 Fe 0.1 Mn 0.85 O 2 for sodium-ion batteries.

Having in mind the remarkable economic and environmental issues involved in the presence of nickel and cobalt metals in electrode compositions, new Na 0.67 Mg 0.05 Fe 0.1 Ni x Mn 0.85-x O 2 (x=0.0, 0.05, 0.1, 0.15) with a P2 type layered structure, are synthesized to be essayed as positive electrodes in sodium-ion batteries. The sol-gel route here proposed favors the obtention of highly pure and crystalline samples with a homogeneous distribution of the constituting elements. Both galvanostatic and voltammetric tests reveal a superior electrochemical behavior for the Ni-free sample, which delivers 94 mA h g -1 at 5C. This excellent performance is associated with a good kinetic response in terms of low charge and discharge hysteresis, high Na + diffusivity, and low cell resistance. Ex-situ measurements evidenced the combined contribution of both the reversible electrolyte insertion and the formation of peroxo species. These advantageous properties allow this electrode to reach a remarkable behavior when is cycled either to low temperatures or high rates.