Synthesis of LiMnPO4/C with superior performance as Li-ion battery cathodes by a two-stage microwave solvothermal process

Abstract

Lithium manganese phosphate, LiMnPO₄ (LMP), has 20% higher redox potential than that of LiFePO₄, and is thus considered a potential cathode replacement material for LiFePO₄ in lithium ion batteries. However, its cyclic stability and rate performance are restricted by the sluggish kinetics of electron and lithium ion migration in it. A high-efficiency two-step microwave solvothermal method has been established to synthesize carbon-coated LMP nanoparticles and the reaction conditions have been investigated in detail. In particular, nanocrystals with more active planes for efficient Li⁺ extraction and insertion can be selectively synthesized. This is of great importance for the preparation of a high performance LMP electrode material for Li-ion batteries. The synthesis and carbon coating of LMP were both performed using microwave irradiation, which shortens the time required to only a couple of minutes. A sample synthesized at 160 °C for 10 min exhibits a high and constant reversible capacity of 155 mA h g⁻¹ at 0.5 C (theoretical capacity: 170 mA h g⁻¹) even after 100 cycles and shows outstanding rate performance. For example, a high capacity of 118 mA h g⁻¹ was obtained at 10 C, which to our knowledge has never been previously reported. Moreover, the sample has an excellent low-temperature performance with 140 mA h g⁻¹ at 0.5 C at 8 °C and 133 mA h g⁻¹ at 2 °C. Such an excellent electrochemical performance can be attributed to the (020) plane of nanosized LMP, regular micromorphology and high conductivity of a uniform carbon coating containing some elemental nitrogen. The reported process is potentially scalable with a much shorter synthesis time than existing methods.