Bulk defect-induced phase transition assisting high-flux magnesium ions intercalation in manganese tetroxide

The industry consensus is that finding appropriate electrode materials is a major obstacle to the widespread use of magnesium ion (Mg2+) energy storage devices due to challenges like high charge density, intense polarization, and strong interaction with the surrounding matrix of Mg2+. Manganese tetroxide electrodes for magnesium ion energy storage suffer from the limited capacity resulting by pseudocapacitance reaction on the subsurface. Herein, manganese tetroxide with bulk cation-anion dual defects (CADDs-Mn3O4) is constructed for the first time by a facile hydrothermal reduction method. The as-prepared bulk CADDs-Mn3O4 electrode in the three-electrode system exhibits a high discharge capacity of 320.93 mAh g-1 contributing by the intercalation of high flux Mg2+, which achieves capacity breakthrough that ever reported. Coupled with the activated carbon anodes in a full-cell configuration, aqueous magnesium ion capacitors display an energy density of 128.39 Wh/kg with an ultra-long cycling capability, giving 85 % capacitance retention after 6000 cycles. This outstanding performance can be attributed to the metastable region created by unique bulk cation-anion dual defects. The transition of Mn3O4 to layered MnO2 is promoted, achieving the quick migration of magnesium ions in the bulk phase in a large flux, thus breaking the constraint of the spinel structure on the insertion of divalent magnesium ions. Moreover, the manganese cation defects reduce the band gap and increase the electrical conductivity of the electrodes, and the oxygen anion defects change the spin state of the electron and weaken the bonding energy of Mg-O. This work provides a detailed understanding of the structure-function relationship based on the introduction of cationanion dual defects, and thus opens up new pathways for modifying energy storage materials.