Tuning crystal water of α -MnO 2 with enhanced diffusion kinetics for zinc-ion batteries

The tunnel-type MnO 2 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, α-MnO 2 nanorods (MnO 2 -r) and nanotubes (MnO 2 -t) were synthesized by reacting KMnO 4 with different reducing agents, namely Cr(NO 3 ) 3 and hydrochloric acid. Their molecular formulas were determined using various techniques as K 0.8 Mn 8 O 16 ·1.44H 2 O and K 0.8 Mn 8 O 16 ·0.80 H 2 O, respectively. By taking advantage of the charge screening effect induced by the higher crystal water content including enhanced Zn 2+ diffusion kinetics, the zinc-ion storage capacity of MnO 2 -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 Zn 2+ accompanied by precipitation/dissolution of ZnSO 4 [Zn (OH) 2 ] 3 ·3H 2 O (ZHS) on the electrode surface was tentatively proposed as the mechanism for Zn 2+ storage in two electrode materials.