A relaxor ferroelectric polymer with an ultrahigh dielectric constant largely promotes the dissociation of lithium salts to achieve high ionic conductivity

Abstract

The extremely low room-temperature ionic conductivity of solid-state polymer electrolytes (SPEs) ranging from 10⁻⁷ to 10⁻⁵ S cm⁻¹ seriously restricts their practical application in solid-state lithium metal batteries (LMBs). Herein, a unique relaxor ferroelectric (RFE) polymer of poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)] is first investigated as a matrix of SPEs. We find that the P(VDF-TrFE-CTFE) with an ultrahigh dielectric constant (ε_r) of 44 presents a stronger solvation ability towards lithium ions, which promotes the dissociation of LiN(SO₂CF₃)₂ to form more free charge carriers and enhances their mobility compared to the conventional PVDF with a low ε_r of 9. The P(VDF-TrFE-CTFE) based SPEs show a much higher ionic conductivity of 3.10 × 10⁻⁴ S cm⁻¹ at 25 °C and lower activation energy (0.26 eV) than PVDF based SPEs (1.77 × 10⁻⁵ S cm⁻¹ and 0.49 eV). The PVDF blended with the P(VDF-TrFE-CTFE) or dielectric fillers such as BaTiO₃ further confirm that the hybrid electrolytes with a larger ε_r show a higher ionic conductivity. In addition, very tight interfaces of P(VDF-TrFE-CTFE) based SPEs with both the cathode and Li metal anode are constructed to ensure a stable interfacial resistance during cycling. The LiFePO₄/Li and LiNi_0.8Co_0.1Mo_0.1O₂/Li batteries using P(VDF-TrFE-CTFE) based SPEs present a stable cycling performance at 25 °C. This work proposes a new strategy and opens a new research area to construct SPEs with high ionic conductivity by greatly increasing the ε_r of polymers.