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 (BmimBF(4)) as fluorine source and manganese (II) acetate tetrahydrate (Mn(CH3COO)(2)center dot 4H(2)O) 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.