Electrochemical Failure Mechanism of δ-MnO 2 in Zinc Ion Batteries Induced by Irreversible Layered to Spinel Phase Transition.

Phase transitions of Mn-based cathode materials associated with the charge and discharge process play a crucial role on the rate capability and cycle life of zinc ion batteries. Herein, a microscopic electrochemical failure mechanism of Zn-MnO 2 batteries during the phase transitions from δ-MnO 2 to λ-ZnMn 2 O 4 is presented via systematic first-principle investigation. The initial insertion of Zn 2+ intensifies the rearrangement of Mn. This is completed by the electrostatic repulsion and co-migration between guest and host ions, leading to the formation of λ-ZnMn 2 O 4 . The Mn relocation barrier for the λ-ZnMn 2 O 4 formation path with 1.09 eV is significantly lower than the δ-MnO 2 re-formation path with 2.14 eV, indicating the irreversibility of the layered-to-spinel transition. Together with the phase transition, the rearrangement of Mn elevates the Zn 2+ migration barrier from 0.31 to 2.28 eV, resulting in poor rate performance. With the increase of charge-discharge cycles, irreversible and inactive λ-ZnMn 2 O 4 products accumulate on the electrode, causing continuous capacity decay of the Zn-MnO 2 battery.