Unraveling the dynamic phase transition behavior of LiMn0.7Fe0.3PO4 nanoparticle electrodes for lithium-ion batteries

Abstract

The Mn-rich olivine phosphate LiMn_0.7Fe_0.3PO₄ (LMFP) has attracted great research interest because of its favorable electrochemical properties and higher operating voltage than those of commercial LiFePO₄. Understanding the phase transformation mechanisms and kinetic properties is crucial for advancing next-generation lithium-ion battery cathode materials. This work investigates the rate-dependent structural transition mechanism of LMFP at various currents: at slow rates (0.2C), LMFP reveals that the Fe²⁺/Fe³⁺ reaction zone (plateau at ∼3.5 V, vs. Li⁺/Li) indicates an entirely single-phase solid solution, whereas the Mn²⁺/Mn³⁺ reaction zone (plateau at ∼4.1 V, vs. Li⁺/Li) is roughly divided into one-half of a single-phase and one-half of a two-phase zone. Upon increasing the current, the solid solution region becomes progressively larger, whereas the two-phase region is delayed, resulting in a smaller proportion. At fast charging/discharging rates (20C), LMFP reveals a short-memory characteristic and delivers a hysteresis phase transition, followed by a solid solution phase transition process without the formation of Mn_0.7Fe_0.3PO₄ (i.e., without a two-phase reaction). Moreover, some of the Fe²⁺ and Mn²⁺ ions in LMFP both participate in the redox reaction and form a lithium-containing Li_0.79Mn_0.7Fe_0.3PO₄ (mixed valence of Fe²⁺, Fe³⁺, Mn²⁺ and Mn³⁺) phase at 20C. This study offers new insights into the dynamic phase transition behavior of LMFP, providing valuable guidance for advancing this cathode material toward commercialization.