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 (epsilon(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 epsilon(r) of 9. The P(VDF-TrFE-CTFE) based SPEs show a much higher ionic conductivity of 3.10 x 10(-4) S cm(-1) at 25 degrees C and lower activation energy (0.26 eV) than PVDF based SPEs (1.77 x 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 epsilon(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 LiNi0.8Co0.1Mo0.1O2/Li batteries using P(VDF-TrFE-CTFE) based SPEs present a stable cycling performance at 25 degrees C. This work proposes a new strategy and opens a new research area to construct SPEs with high ionic conductivity by greatly increasing the epsilon(r) of polymers.

Automated summary

In this study, a novel approach using a relaxor ferroelectric (RFE) polymer of P(VDF-TrFE-CTFE) as the matrix of solid-state polymer electrolytes (SPEs) significantly increased the ionic conductivity in solid-state lithium metal batteries. The high dielectric constant of P(VDF-TrFE-CTFE) enabled enhanced solvation ability towards lithium ions and promoted dissociation, resulting in improved charge carrier mobility. The hybrid electrolytes with larger dielectric constant showed higher ionic conductivity, and tight interfaces of P(VDF-TrFE-CTFE) based SPEs ensured stable interfacial resistance during cycling, leading to stable cycling performance in LiFePO4/Li and LiNi0.8Co0.1Mo0.1O2/Li batteries at 25 degrees C. This work suggests a new research direction to construct SPEs with high ionic conductivity by increasing the dielectric constant of polymers.