One-Step solvothermal synthesis of nanostructured manganese fluoride as an anode for rechargeable lithium-ion batteries and insights into the conversion mechanism

A nanostructured manganese fluoride is successfully synthesized for the first time through a facile one-step solvothermal method. Ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF₄) as fluorine source and manganese (II) acetate tetrahydrate (Mn(CH₃COO)₂·4H₂O) as manganese source are used. By controlling the amount of manganese source and both the reaction time and temperature, pure phase tetragonal MnF₂ with a uniformly distributed nanocrystalline of 100-300 nm can be obtained. A possible formation mechanism related to the role of the IL is proposed. Electrochemical performance of MnF₂ nanocrystals as anodes for rechargeable lithium batteries is investigated. A low discharge plateau around 0.6 V at 0.1 C of the first cycle is obtained for lithium uptake reactions with a reversible discharge capacity as high as 300 mAh g^-1. The new MnF₂ anode is found to deliver significantly improved cycling performance than conventional conversion reaction electrodes with a capacity retention of 237 mAh g^-1 at 10 C even after 5000 cycles, indicating its promising utilization as anode material for future lithium-ion batteries with long cycle life. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy analyses for lithiated and delithiated MnF₂ electrodes are used to reveal the conversion mechanism for the reversible electrochemical reaction of MnF₂ with Li. A favorable MnF₂ anode is successfully synthesized based on ionic liquid and it delivers surprisingly inspiring electrochemical performance, with a capacity retention of 237 mAh g^-1 at 10 C after 5000 cycles. Redistribution of metallic Mn nanoparticles within the LiF matrix, resulting in the formation of Mn network facilitating electron transport, is considered as the dominant cause for improvement of reversibility and capability.