Improved electrochemical performance of Na0.67MnO2 through Ni and Mg substitution

Abstract

Incorporating nickel and magnesium into P2–Na_0.67MnO₂ is expected to suppress Jahn–Teller active Mn³⁺ ions, increase the average potential of the electrode through Ni⁴⁺/Ni²⁺ redox couples and stabilize M–O layers resulting in a smooth cycling profile with better capacity retention. In this communication, P2–Na_0.67Ni_0.25Mg_0.1Mn_0.65O₂ is reported as the cathode material for rechargeable Na-ion batteries. It reversibly intercalates ∼0.52 moles of Na per formula unit in the voltage region of 1.5–4.2 V at C/10 rate which corresponds to a specific capacity of 140 mA h g⁻¹. X-ray photoelectron spectroscopy has attributed the origin of the capacity in this phase to both Ni and Mn redox species active at different potential ranges. Substituting part of Mn with Ni and Mg increases the average potential of Na_0.67MnO₂ and improves the energy density of Na_0.67Ni_0.25Mg_0.1Mn_0.65O₂ to 335 W h kg⁻¹.