Misfit strains inducing voltage decay in LiMnyFe1−yPO4/C

Abstract LiMnyFe1−yPO4 is considered a promising cathode material for next-generation lithium-ion batteries (LIBs) due to its high energy density and low cost. Its energy density degradation is often ascribed to the capacity loss during cycling. However, in this study, we find that the energy density degradation mainly roots in voltage decay. We have synthesized a series of LiMnyFe1−yPO4/C (0.5≤y≤0.8) and find this voltage decay is correlated with the Mn content. A high amount Mn leads to a heavier voltage decay. In-situ X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) reveal the nature of this effect, which show a mismatch along the b-axis of −2.68% (charge) and +3.4% (discharge), a volume misfit of −4.41% (charge) and +4.54% (discharge) between LixMnyFe1−yPO4 and MnyFe1−yPO4 during phase transitions. The resultant misfit strains during Li+ insertion compared to extraction result in structural degradations, such as amorphization and impurity (MnF3) accumulation after cycling. The voltage decay can be alleviated by kinetic relaxations and recovered by a wild reannealing. This work demonstrates effective strategies to improve the energy density and cycling performance of LiMnyFe1−yPO4/C, providing good references for other LIB cathodes, such as the Li-rich cathodes.