Scalable, low-cost synthesis of high volumetric capacity LiMn0.5Fe0.5PO4 cathode for lithium-ion batteries

Abstract

LiMn_xFe_1−xPO₄ (LMFP) cathodes in lithium-ion batteries (LIBs) can improve the energy density by up to 15% compared to LiFePO₄. However, the smaller primary particle size needed to overcome sluggish Mn^2+/3+ redox kinetics results in lower electrode density, limiting the practical energy density of LMFP cells. We present here a scalable, wastewater-free synthesis of high energy density LMFP cathodes by employing a spinel LiMnFeO₄ precursor obtained by a facile solid-state reaction. The LMFP synthesized with LiMnFeO₄, referred to as S-LMFP, has uniform secondary particles formed via spray drying and greater tap density than LMFP synthesized with a conventional co-precipitation route, referred to as P-LMFP. While S-LMFP offers a lower specific capacity (147 mA h g⁻¹) than P-LMFP (153 mA h g⁻¹) at C/5 rate due to inferior Li⁺ diffusion kinetics during Mn^2+/3+ redox, it exhibits a ∼15% higher electrode-level energy density due to its exceptional electrode packing density (2.6 g cm⁻³). Rate capability testing of LMFP cathodes with different electrode densities reveals that at low rates (<1C), electrode density is more important than specific capacity in determining the energy density of LMFP cathodes. Overall, this work highlights the complex tradeoff between Mn^2+/3+ redox kinetics and electrode density, which dictates the practical energy density of LMFP cathodes. With further optimization of composition, particle size, and synthesis conditions, LMFP synthesized from LiMnFeO₄ is promising to improve the energy density of the next generation of LIBs.