Synthesis and optimized formulation for high-capacity manganese fluoride (MnF2) electrodes for lithium-ion batteries

Abstract Electrochemical activity of poorly conductive metal fluorides in Li-ion batteries is contingent on their nanostructuration to reduce diffusion lengths and increase reaction kinetics. In that regard, we optimize the synthesis and electrode formulation of MnF2 to enable sufficient electrochemical activity required to study its electrochemical conversion reaction mechanism. Solvothermal synthesis in a water–ethanol mixture (1:1 Vol.), using Mn acetate and a slight excess of hydrofluoric acid (HF), results in pure phase, nanosized (˜30 nm diameter) rutile-type MnF2 (P42/mnm). High energy ball-milling of MnF2‒carbon mixtures leads to an amorphization of MnF2 and its partial phase transformation to the α-PbO2-type structure, without significant improvement of the electrochemical performance. Changing the electrode binder, however, from typical polyvinylidene fluoride (PVDF) to water-soluble Na-alginate, leads to a significant improvement of the reversibility of the electrochemical reaction. We attribute this drastic improvement to the improved adherence and homogeneity of the electrode film prepared with Na-alginate.