Tuning electrolyte solvation structures to enable stable aqueous Al/MnO2 battery

MnO2 is regarded as a promising cathode material owing to its low cost, environmental friendliness and high theoretical capacity. However, it is plagued by sluggish intercalation/extraction of multivalent cations into MnO2 lattice structure in diluted electrolytes, leading to short cycle life and fast capacity decay of aqueous MnO2 cathode. Here, we select Al/MnO2 aqueous battery as a demonstration and report a new approach of using concentrated aqueous electrolytes to tune the solvation structures of Al3+ cations to eliminate their intercalation into MnO2, ensuring high reversibility of MnO2 cathode in the MnO2/Mn2+ deposition/stripping chemistry. Molecular dynamics simulation and experimental analysis reveal that the radius of Al3+ solvation sheath in the concentrated aqueous electrolytes is larger than the lattice spacing of MnO2, thereby avoiding their intercalation into the MnO2 cathode. As a result, the newly developed Al/MnO2 full cell in the optimized electrolyte delivers a high discharge voltage of ∼1.9 V, large specific capacity of ∼493 mAh/g and long-term stability of over 1000 cycles. This work provides new opportunities to the development of highly reversible MnO2 cathode for aqueous multivalent metal/MnO2 batteries.