Tuning crystal water of -MnO₂ with enhanced diffusion kinetics for zinc-ion batteries

The tunnel-type MnO2 structure is regarded as one of the most promising cathode materials for aqueous zinc-ion batteries due to its high theoretical specific capacity and excellent rate capability. In this study, alpha-MnO2 nanorods (MnO2-r) and nanotubes (MnO2-t) were synthesized by reacting KMnO4 with different reducing agents, namely Cr(NO3)(3) and hydrochloric acid. Their molecular formulas were determined using various techniques as K0.8Mn8O16 center dot 1.44H(2)O and K0.8Mn8O16 center dot 0.80H(2)O, respectively. By taking advantage of the charge screening effect induced by the higher crystal water content including enhanced Zn2+ diffusion kinetics, the zinc-ion storage capacity of MnO2-r is significantly improved. This results in a consistently increasing specific capacity of 372 mAh.g(-1) on the 50th cycle at 0.1 A.g(-1), excellent cycle stability over 1600 cycles at 1.0 A.g(-1) and superior rate performance. Moreover, a two-stage process involving the insertion/extraction of H+ and Zn2+ accompanied by precipitation/dissolution of ZnSO4 [Zn (OH)(2)](3)center dot 3H(2)O (ZHS) on the electrode surface was tentatively proposed as the mechanism for Zn2+ storage in two electrode materials.