A manganese hexacyanoferrate framework with enlarged ion tunnels and two-species redox reaction for aqueous Al-ion batteries

The recently emerging aqueous Al-ion batteries (AIBs) face the problems of poor rate capability (typically, less than 30% retention with a current density raised to 0.4 A g^–1) and limited cycle life with rapid capacity degradation (typically, 50% capacity retention after 60 cycles.) The battery failure was typically attributed to the poor stability of the host structure associated with the large polar interaction between the Al ions and host. In this study, a defective manganese hexacyanoferrate (MnFe-PBA) cathode is designed with enlarged ion transportation tunnels and weakened Coulombic interactions with Al ions. Moreover, two reactive metal-sites of MnFe-PBA are both successfully activated, resulting in a large cavity for 1.2 Al ions storage per unit cell. Such structural and active site features contribute to a remarkable rate capability (50.7% capacity retention at 1.0 A g^–1) and a high specific capacity of 106.3 mAh g⁻¹. The intrinsic 3-dimensional (3D) framework of MnFe-PBA also delivered superior cycling stability: an ignorable capacity loss after 50 cycles at 0.2 A g⁻¹ and 69.5% retention after 100 cycles at 0.5 A g⁻¹. Furthermore, nanofibrillated cellulose/polyacrylamide hydrogel electrolytes were applied and successfully prolonged the cycling life to 200 cycles, and the constructed flexible quasi-state Al//MnFe-PBA batteries displayed stable output under various deformations.