Valence Engineering via In Situ Carbon Reduction on Octahedron Sites Mn3O4 for Ultra‐Long Cycle Life Aqueous Zn‐Ion Battery

Abstract In recent years, rechargeable aqueous zinc‐ion batteries (ZIBs) have received much attention. However, the disproportionation effect of Mn 2+ seriously affects the capacity retention of ZIBs during cycling. Here, the capacity retention of the Mn 3 O 4 cathode is improved by effective valence engineering. The valence engineering of Mn 3 O 4 is caused by bulk oxygen defects, which are in situ derived from the Mn‐metal organic framework during carbonization. Bulk oxygen defects can change the (MnO 6 ) octahedral structure, which improves structural stability and inhibits the dissolution of Mn 2+ . The ZIB assembled from bulk oxygen defects Mn 3 O 4 @C nanorod arrays (O d ‐Mn 3 O 4 @C NA/CC) exhibits an ultra‐long cycle life, reaching 84.1 mAh g −1 after 12 000 cycles at 5 A g −1 (up to 95.7% of the initial capacity). Furthermore, the battery has a high specific capacity of 396.2 mAh g −1 at 0.2 A g −1 . Ex situ characterization results show that initial Mn 3 O 4 is converted to ramsdellite MnO 2 for insertion and extraction of H + and Zn 2+ . First‐principles calculations show that the charge density of Mn 3+ increases greatly, which improves the conductivity. In addition, the flexible quasi‐solid‐state ZIB is successfully assembled using O d ‐Mn 3 O 4 @ C NA/CC. Valence engineering induced by bulk oxygen defects can help develop advanced cathodes for aqueous ZIB.