On the origin of phase transition suppression of P2–Na0.67MnO2 by substitution of Mn with Li

Abstract

Mn-based layered oxides are promising cathode materials for Na-ion batteries, but their low cyclability due to phase transition during charge/discharge remains a challenge. P2–Na_0.67MnO₂ compound undergoes a severe phase transition of P2 → O2 during charging. It has been proposed that this behavior results from the desodiation-induced change in the Jahn–Teller (J–T) activity of Mn after its oxidation from 3+ to 4+. In this work, we show that the driving force of the phase transition is indeed the oxidation of Mn³⁺ to Mn⁴⁺ but not the suppression of J–T activity with desodiation. Combining density functional theory calculations and electrostatic analyses indicates that the main factor stabilizing the P2 phase is the Na–Mn interaction, which strongly favors this phase over the O2 phase. Desodiation induced-weakening of this interaction leads to the formation of O2–Na_0.11MnO₂, which is driven by O–O interaction. Substituting Mn with Li stabilizes P2–Na_xLi_0.22Mn_0.78O₂ even at low Na content (x = 0.11). This is because the Na–Mn interaction is more favorable for the P2 phase, and this energy preference remains almost unchanged after desodiation. The absorption energy of Na at Na sites close to Li_TM is much stronger than at sites near Mn_Mn, and favors P2 phase. As the overall Na absorption energy (dictated by Na–Mn repulsion) is mainly determined by the nearest Na–Mn neighbor interaction, which does not change much with desodiation, no phase transition to O2 occurs for Na_xLi_0.22Mn_0.78O₂ at x = 0.11. Overall, the phase stability of Na-based layered oxide materials is driven by electrostatic forces, which can be tuned by substitution of Mn by a metal ion of appropriate charge and concentration.