Off-stoichiometric design of a manganese-rich mixed olivine Li-ion cathode for improved specific energy

Lithium phospho-olivine cathodes operating with iron (Fe) and manganese (Mn) redox centers are considered technologically important materials that can make the development of Li-ion batteries sustainable. Although large Mn content is desirable to achieve high specific energy at a material level, the mixed olivine cathodes require particle nanostructuring and post-synthesis treatment to demonstrate reasonable energy storage properties at an electrode level. In this work, we have investigated the effect of off-stoichiometry on the electrochemical properties of a Mn-rich mixed olivine cathode material that does not require complex optimization processing. An off-stoichiometric form of LiFe_0.25Mn_0.75PO₄ is synthesized with nominal composition of LiFe_0.225Mn_0.675P_0.95O_3.8. X-ray diffraction and electron microscopy indicate that off-stoichiometry leads to phase separation into stoichiometric LiFe_0.25Mn_0.75PO₄ crystalline particles with non-crystalline surface phases. The off-stoichiometric cathode has an improved specific energy of 622 Wh/kg at C/5, outperforming the stoichiometric cathode. The off-stoichiometric cathode also exhibits improved rate capability, delivering 120 mAh/g at 20C and 78 mAh/g at 40C discharge, respectively, due to reduced interfacial and charge transfer resistances. This work highlights off-stoichiometry as an effective approach to engineer Mn-rich mixed olivine cathode materials with desirable electrochemical properties, providing a practically feasible route for materials optimization.