Understanding Voltage Hysteresis and Voltage Fade in Li-Rich Mn-Based Layered Oxide Cathode Materials

Lithium-rich manganese-based layered oxide cathodes exhibit high reversible capacity from a combination of transition metal ion redox and oxygen redox. However, these Li-rich materials suffer a loss of energy density during cycling, due to hysteresis and fade in their voltage-capacity curve, which has been associate with their oxygen redox chemistry [1]. Two different forms of voltage hysteresis can be recognised: a large first-cycle hysteresis, resulting in an irreversible voltage loss, and a smaller, persistent hysteresis on later cycles, causing a round-trip cycling inefficiency [2]. A further phenomenon of voltage fade, a gradual drop in average discharge voltage at each cycle, also results in energy density loss over long-term cycling [3]. To improve the practical viability of Li-rich Mn-based cathodes, it is vital that these detrimental electrochemical features are understood and mitigated. Unfortunately, the mechanisms of oxygen redox, including host-framework structural rearrangements, void formation and oxygen loss [4], that are associated with voltage hysteresis and fade, are not well understood at the atomic or nanoscale.<br/><br/>To understand the origin of voltage fade and hysteresis in an exemplary Li-rich Mn-based cathode: Li_1.2Mn_0.8O₂, we employ atomistic modelling, using density functional theory, ab initio molecular dynamics and machine-learning potentials. By combining these modelling methods, we can map a large structural and chemical space in the material during charge and discharge. In doing so, we resolve distinct atomic- and nanoscale origins for the first-cycle hysteresis, persistent hysteresis, and voltage fade, and identify structural modifications that will mitigate each process. Insights from this work provide clear guidelines for the design of Li-rich cathodes with better short-term and long-term cycling stability.<br/><br/>[1] <u>K. McColl</u>, S.W. Coles, et al., <i>Nature Materials</i>, <b>23</b>, 826–833 (2024)<br/>[2] <u>K. McColl</u>, R.A. House, et al., <i>Nature Communications</i>, <b>13</b>, 5275 (2022)<br/>[3] R.A. House, G.J. Rees, <u>K. McColl</u>, et al., <i>Nature Energy</i>, <b>8</b>, 351–360 (2023)<br/>[4] P. Csernica, <u>K. McColl</u> et al., Submitted, <i>ChemRxiv</i> (2024)