Dual strategies of mild C–F scissoring fluorination and local high-concentration electrolyte to enable reversible Li–Fe–F conversion batteries

Batteries taking conversion-type iron fluorides as energy-dense cathodes provide the possibility for the power electrification of the transportation and aviation industries. However, a safe and low-toxicity synthesis method for fluorides and the design of a compatible electrolyte formula are still challenging. Here, we propose a dual strategy of mild C-F scissoring fluorination and a local high-concentration electrolyte (LHCE) to enable highly reversible Li-Fe-F conversion batteries. A facile and safe scissoring strategy at a low temperature (95 °C) enables the preparation of a carbon-iron fluoride composite with a porous cubic cage-like structure. CF_x plays a double role as a solid fluorination agent and an in situ conductive network after defluorination. The as-prepared fluoride cathode delivers a reversible capacity as high as 300 mA h g^-1 over 100 cycles. The further LHCE strategy not only enhances the oxidation stable voltage of the electrolyte (>5 V) and the transference number of Li⁺ (0.74), but also realizes dual protection of the fluoride cathode and Li metal anode by facilitating the construction of robust cathode- and anode-electrolyte interfaces, respectively. The LHCE-assisted fluoride battery releases a higher reversible capacity of 335 mA h g^-1 after 130 cycles. This work provides a solution to high-performance carbon-fluoride conversion cathodes by a synergetic effect of tailored synthesis, electroactive particle texture and electrolyte formula.