Role of fluorinated-component positioning in Li metal battery performance

Abstract

To boost electrochemical performance, fluorine-containing constituents are often introduced into lithium metal batteries (LMBs) to achieve superior properties. However, the underlying mechanism governing positional effects of fluorinated species remains unclear. Herein, three typical gel polymer electrolytes: fluorinated solvent-dominated (FSD), fluorinated polymer-dominated (FPD), and dual-site fluorinated-dominated (DFD) are systematically fabricated to investigate the related influence. Ionic conductivity, XPS characterization and molecular dynamics confirm the contribution of fluorinated solvent to the formation of stable and anion-derived inorganic-rich solid electrolyte interphase (SEI) layer. Density functional theory (DFT) results demonstrate that fluorinated polymer exhibits enhanced interaction energies, leading to increased free ion concentration. While Consistent conclusions can be drawn from diffusion coefficient analysis and distribution of relaxation times (DRT) results. By synergistically combining the merits of fluorinated solvents and fluorinated polymer, an ionic conductivity of 1.19 mS cm⁻¹ is obtained for DFD and a minor polarization of 12 mV is achieved over 8000 h at 1 mA cm⁻¹ in Li//Li battery. After assembling with LiFePO₄, capacity retentions of 91.3 % and 91.6 % are maintained after 200 cycles at 1C under 25 °C and 60 °C, showing exceptional cycling stability, respectively, particularly under elevated-temperature operating conditions.