High-performance Ni-free sustainable cathode Na_0.67Mg_0.05Fe_0.1Mn_0.85O₂ 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_0.67Mg_0.05Fe_0.1Ni_xMn_0.85-xO₂ (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.