Nanostructured alkali cation incorporated δ-MnO2 cathode materials for aqueous sodium-ion batteries

Nanostructured delta-MnO2 incorporated with alkali cations (A-delta-MnO2, A = K+, Na+) has been synthesized and evaluated as a cathode material for aqueous sodium-ion batteries. It is observed that Na+ ions are easier than K+ ions to intercalate into the layered delta-MnO2 due to their smaller ion radius. The incorporation of K+ and Na+ into delta-MnO2 shows a great influence on the electrochemical performance of the layered delta-MnO2. The full cell with (K, Na)-co-incorporated delta-MnO2 (K : Na : Mn = 0.15 : 0.26 : 1) hierarchical nanospheres as the cathode and NaTi2(PO4)(3) as the anode exhibits a specific capacity of 74.6 mA h g(-1) at 150 mA g(-1), and the capacity remains at similar to 62% at a high current density of 600 mA g(-1). The electrochemical cycling does not induce observable structural degradation even after 200 cycles. K-incorporated and (K, Na)-co-incorporated delta-MnO2 electrodes have superior capacity and rate capability, which can be ascribed to their hierarchical structure and adequate crystallinity.