Cr-induced enhancement of structural stability in δ-MnO2 for aqueous Zn-ion batteries

δ-MnO2 was proposed as a promising cathode material for aqueous Zn-ion batteries. Nonetheless, its potential application has been impeded by its unstable crystal structure and sluggish reaction kinetics with Zn ions. Herein, we utilized Cr to modify δ-MnO2 by a simple hydrothermal method to develop cathode materials for aqueous Zn-ion batteries with better stability and electron transport properties. The introduction of Cr enhanced the specific capacity and cycling stability of δ-MnO2, in particular, the 3% Cr doped δ-MnO2 electrode exhibited a large specific capacity of 229.2 mAh·g-1 at 0.1 A·g-1 and an excellent capacitance retention of 86.0% after performing 300 cycles. The density functional theory (DFT) calculations indicate that the introduction of Cr decreases the band gap of MnO2 and enhances electron migration in the electrode material. Meanwhile, the introduction of Cr substantially improved the Zn ion adsorption and enhanced the binding energy of δ-MnO2, indicating that the addition of Cr promoted the interaction between the Cr doping δ-MnO2 (Cr-MnO2) electrode materials with Zn ions, while making the structure of the electrode materials more stable. As a result, this suppresses the Jahn-Teller distortion and reduces the solvation and phase transition of Mn.