High-performance lithium storage in an ultrafine manganese fluoride nanorod anode with enhanced electrochemical activation based on conversion reaction

A facile, one-step solvothermal reaction route for the preparation of manganese fluoride nanorods is successfully developed using manganese(ii) chloride tetrahydrate (MnCl₂·4H₂O) as the manganese source and the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF₄) as the fluorine source. X-ray diffraction, field-emission scanning electron microscopy and high-resolution transmission electron microscopy (HRTEM) are conducted to characterize the structural and microstructural properties of the synthesized MnF₂. The pure-phase tetragonal MnF₂ displays nanorod-like morphology with a diameter of about 20 nm and a length of several hundreds of nanometers. The electrochemical performance of the MnF₂ nanorod anode for rechargeable lithium batteries is investigated. A reversible discharge capacity as high as 443 mA h g^-1 at 0.1 C is obtained for the lithium uptake reaction with an initial discharge plateau around 0.7 V. The striking enhancement in electrochemical Li storage performance in ultrafine MnF₂ nanorods can be attributed to the small diameters of the nanorods and efficient one-dimensional electron transport pathways. Long cycle performance for 2000 cycles at 10 C with a stabilized capacity of about 430 mA h g^-1 after activation is also achieved. Furthermore, lithiated and delithiated MnF₂ anodes are analyzed with HRTEM to elucidate the conversion mechanism for the electrochemical reaction of MnF₂ nanorods with Li at a microscopic level.