Insights into conversion mechanism of restriction strategy and self-activation enabled high-performance manganese fluoride anodes

Abstract

Metal fluorides are expected to serve high-energy lithium-ion batteries (LIBs) due to their prominent lithium storage capacity led by multi-electron transfer reactions. However, slow kinetics and voltage hysteresis limits its practical utilization. Herein, the carbon-coated submicron binary fluoride MnF₂ (MFMC) is synthesized via a reverse-micellar microreactor and tannic acid coating to physically restrict the growth of the precursor and MnF₂, respectively. A novel conversion reaction in the preliminary discharge is proposed that the mesophase Li₂MnF₄ can serve as a transition from MnF₂ to LiF to reduce lattice strain and energy consumption, which is confirmed by first-principles density functional theory (DFT) calculation and HRTEM. More remarkably, the (110) crystal planes of the Mn nanodomains refined after cycling possess an ultra-low migration barrier of 0.044 eV. Mn cooperates with the newly generated amorphous phase to accelerate the migration of Li⁺ into the interior of the particles, causing continuous activation of the battery. The optimized anode shows superior stability and lithium storage capacity (629 mAh g^-1 after 1300 cycles at 1 A g^-1). This work effectively enhances the kinetics with a feasible restriction strategy and proposes novel insights into the conversion mechanism, inspiring the design of fluoride electrodes.