Hierarchical Nanostructured MWCNT MnF2 Composites With Stable Electrochemical Properties as Cathode Material for Lithium Ion Batteries

Abstract

Multi-walled carbon nanotubes?manganese(II) fluoride (MWCNT?MnF2) composite was prepared by a simple two-step method. In the first step, a wet chemistry reaction between manganese (II) nitrate, Mn(NO3)2, and hexafluoroslicic acid, H2SiF6, produces manganese(II) hexafluorosilicate (MnSiF6). The thermal decomposition of MnSiF6 in presence of MWCNTs at 400 ?C under argon atmosphere releases SiF4 gas and forms MWCNT?MnF2 composite. Powder X-ray diffraction, Raman spectroscopy, and scanning electron microscopy confirmed the formation of pure phases of MnF2 nanoparticles with rutile crystalline structure with no impurity phases. MnF2 nanoparticles were dispersed among MWCNTs networks and partially decorating MWCNTs forming a hierarchically nanostructured MWCNT?MnF2 composite. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy tests showed that the MWCNT?MnF2 composite electrode underwent electrochemical lithiation/delithiation reactions with enhanced reversibility and stabilized solid?electrolyte interface (SEI) during cycling. Charge?discharge tests demonstrated that the MWCNT?MnF2 electrode displayed an irreversible specific capacity in the first cycle mainly linked to the decomposition of the electrolyte and the formation of the SEI film. After 100 cycles, a charge specific capacity of 480 mAh g?1 MnF2 was measured (80% of the initial capacity) with high coulombic efficiency (CE) (?100%), indicating the high reversibility of electrochemical conversion reactions. X-ray photoelectron spectroscopy, XRD, and SEM-EDX analyses for non-cycled and fully discharged MWCNT?MnF2 electrodes confirmed the irreversible lithiation of MnF2 into Mn and LiF during the ten first charge?discharge cycles. The higher electrochemical performance of the composite electrode compared to pure MnF2 can be attributed to the hierarchical structure of MWCNT?MnF2, which is capable to assimilate the volume changes, stabilize the solid?electrolyte interface (SEI), and facilitate Li+ ions and electrons transfer. ? 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim