Hierarchical Nanostructured MWCNT–MnF 2 Composites With Stable Electrochemical Properties as Cathode Material for Lithium Ion Batteries

Multi‐walled carbon nanotubes–manganese(II) fluoride (MWCNT–MnF 2 ) composite was prepared by a simple two‐step method. In the first step, a wet chemistry reaction between manganese (II) nitrate, Mn(NO 3 ) 2 , and hexafluoroslicic acid, H 2 SiF 6 , produces manganese(II) hexafluorosilicate (MnSiF 6 ). The thermal decomposition of MnSiF 6 in presence of MWCNTs at 400 °C under argon atmosphere releases SiF 4 gas and forms MWCNT–MnF 2 composite. Powder X‐ray diffraction, Raman spectroscopy, and scanning electron microscopy confirmed the formation of pure phases of MnF 2 nanoparticles with rutile crystalline structure with no impurity phases. MnF 2 nanoparticles were dispersed among MWCNTs networks and partially decorating MWCNTs forming a hierarchically nanostructured MWCNT–MnF 2 composite. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy tests showed that the MWCNT–MnF 2 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–MnF 2 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 MnF 2 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–MnF 2 electrodes confirmed the irreversible lithiation of MnF 2 into Mn and LiF during the ten first charge–discharge cycles. The higher electrochemical performance of the composite electrode compared to pure MnF 2 can be attributed to the hierarchical structure of MWCNT–MnF 2 , which is capable to assimilate the volume changes, stabilize the solid–electrolyte interface (SEI), and facilitate Li + ions and electrons transfer.