Electrochemical characterization of P2-type layered Na2/3Ni1/4Mn3/4O2 cathode in aqueous hybrid sodium/lithium ion electrolyte

P2-type layered Na2/3Ni1/4Mn3/4O2 has been synthesized by a solid-state method and its electrochemical behavior has been investigated as a potential cathode material in aqueous hybrid sodium/lithium ion electrolyte by adopting activated carbon as the counter electrode. The results indicate that the Na+/Li+ ratio in aqueous electrolyte has a strong influence on the capacity and cyclic stability of the Na2/3Ni1/4Mn3/4O2 electrode. Increase on the Li+ content leads to a shift of the redox potential towards a high value, which is favorable for the improvement of the working voltage of the layered material as cathode. It is found that the coexistence of Na+ and Li+ in aqueous electrolyte can improve the cyclic stability for the Na2/3Ni1/4Mn3/4O2 electrode. A reversible capacity of 54 mAh g−1 was obtained with a high cyclability as the Na+/Li+ ratio was 2:2. Furthermore, an aqueous hybrid ion cell was assembled with the as-proposed Na2/3Ni1/4Mn3/4O2 as cathode and NaTi2(PO4)3/graphite synthesized in this work as anode in 1 M Na2SO4/Li2SO4 (mole ratio as 2:2) mixed electrolyte. The cell shows an average discharge voltage at 1.2 V, delivering an energy density of 36 Wh kg−1 at a power density of 16 W kg−1 based on the total mass of the active materials.