Electrochemical Failure Mechanism of δ-MnO 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 batteries during the phase transitions from δ-MnO to λ-ZnMnO is presented via systematic first-principle investigation. The initial insertion of Zn 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-ZnMnO. The Mn relocation barrier for the λ-ZnMnO formation path with 1.09 eV is significantly lower than the δ-MnO 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 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-ZnMnO products accumulate on the electrode, causing continuous capacity decay of the Zn-MnO battery.