Combustion-synthesized sodium manganese (cobalt) oxides as cathodes for sodium ion batteries

We report on the electrochemical properties of layered manganese oxides, with and without cobalt substituents, as cathodes in sodium ion batteries. We fabricated sub-micrometre-sized particles of Na_0.7MnO_{2 + z} and Na_0.7Co_0.11Mn_0.89O_{2 + z} via combustion synthesis. X-ray diffraction revealed the same layered hexagonal P2-type bronze structure with high crystallinity for both materials. Potentiostatic and galvanostatic charge/discharge cycles in the range 1.5-3.8 V vs. Na | Na⁺ were performed to identify potential-dependent phase transitions, capacity, and capacity retention. After charging to 3.8 V, both materials had an initial discharge capacity of 138 mA h g^-1 at a rate of 0.3 C. For the 20th cycle, those values reduced to 75 and 92 mA h g ^-1 for Co-free and Co-doped samples, respectively. Our findings indicate that earlier works probably underestimated the potential of (doped) P2-type Na_0.7MnO_{2 + z} as cathode material for sodium ion batteries in terms of capacity and cycle stability. Apart from doping, a simple optimization parameter seems to be the particle size of the active material.