Dielectric barrier discharge plasma sulfonated carbon nanotube modified PVDF-TrFE-CFE copolymer electrolyte for high-performance flexible solid-state lithium metal batteries

Abstract

Herein, a method for in-situ sulfonation of carbon nanotubes (CNTs) utilizing dielectric-barrier discharge (DBD) plasma treatment has been meticulously designed that introduces sulfonic acid functional groups while preserving the structural integrity of the CNT walls. These sulfonated CNTs are subsequently incorporated into a poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE) copolymer that is dispersed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to fabricate a polymer solid-state electrolyte (PSE) exhibiting excellent ionic conductivity, thermal conductivity, and mechanical properties. In this PSE system, the PVDF-TrFE-CFE copolymer enhances the electrolyte's mechanical properties and lithium salt solubility, while the high content of LiTFSI with low dissociation energy provides additional lithium-ion transport pathways. The DBD plasma in-situ sulfonated CNT fillers enhance the electrochemical and thermodynamic stability of the PSE. This dual-doped PSE composed of LiTFSI and sulfonated CNTs exhibits an ionic conductivity of 1.67 × 10⁻⁴ S cm⁻¹ at 25 °C and a high critical current density (CCD) of up to 2.0 mA cm⁻². Furthermore, the Li|PVDF-T-C/p-CNTs-60|Li symmetric cell constructed using the PSE doped with sulfonated CNTs optimized by DBD plasma sulfonation time (p-CNTs-60) exhibits low polarization at 0.1 mA cm⁻² and 0.2 mA cm⁻², thereby achieving stable charge-discharge cycles for over 3600 h and 3000 h, respectively. The full cell Li|PVDF-T-C/p-CNTs-60|LFP shows impressive capacity retention at 90.2 % along with a coulombic efficiency of 99.75 % following 300 cycles. Similarly, the full cell Li|PVDF-T-C/p-CNTs-60|NCM811 exhibits a capacity retention of 90.96 % and maintains a coulombic efficiency of 99 % after 250 cycles.