A relaxor ferroelectric polymer with an ultrahigh dielectric constant largely promotes the dissociation of lithium salts to achieve high ionic conductivity†
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 (εr) 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 εr 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 εr 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 LiNi0.8Co0.1Mo0.1O2/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 εr of polymers.