Revealing the Conversion Mechanism of Transition Metal Oxide Electrodes during Lithiation from First-Principles

Transition metal oxides such as Co₃O₄ and NiO are of significant interest as conversion anode materials for lithium-ion batteries (LIBs), due to their remarkably high theoretical capacities and low cost. While many previous experiments have found that the charge/discharge reactions of Co₃O₄ and NiO can be highly reversible, detailed information about the mechanisms of these reactions, such as the origin of the voltage hysteresis (>1.0 V) between the charge/discharge cycles, is still poorly understood. In this work, we develop and utilize a new computational mechanistic approach that helps elucidate the hysteresis and nonequilibrium reaction pathways associated with these conversion materials. We apply this methodology to investigate a variety of lithiation reaction pathways of Co₃O₄ and NiO by systematically exploring the energetics of a large number of equilibrium and nonequilibrium Li_xCo₃O₄ (0 ≤ x ≤ 8) and Li_xNiO (0 ≤ x ≤ 2) structural configurations using first-principles calculations. The overall value of the voltages from our nonequilibrium pathway is in much better agreement with experimental lithiation than the calculated equilibrium voltage while the overall value of the latter reasonably agrees with experimental delithiation. Hence, we propose the charge and discharge processes proceed through equilibrium and nonequilibrium reaction paths, respectively, which contribute significantly to the experimentally observed voltage hysteresis in Co₃O₄ and NiO. Additionally, we find a low-energy, lithiated intermediate phase (Li₃Co₃O₄) with an oxygen framework equal to that of the initial Co₃O₄ spinel phase. This intermediate phase represents the capacity threshold below which limited volume expansion and better reversibility can be realized and above which reactions lead to structural degradation and huge expansion.