Electrochemical Failure Mechanism of δ-MnO₂ 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-MnO₂ batteries during the phase transitions from δ-MnO₂ to λ-ZnMn₂O₄ is presented via systematic first-principle investigation. The initial insertion of Zn²⁺ 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-ZnMn₂O₄. The Mn relocation barrier for the λ-ZnMn₂O₄ formation path with 1.09 eV is significantly lower than the δ-MnO₂ 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 Zn²⁺ 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-ZnMn₂O₄ products accumulate on the electrode, causing continuous capacity decay of the Zn-MnO₂ battery.