Factors determining the Li+ conductivity in high-performance PVDF-based composite electrolytes revealed by solid-state NMR

Abstract

Composite polymer electrolytes (CPEs) are considered as promising electrolytes for next-generation lithium batteries due to their superior advantages in safety, mechanical stability/flexibility, cathode compatibility, etc. However, achieving high Li⁺ conductivity remains a major challenge, particularly at low temperatures. A key obstacle lies in the limited understanding of the complex interplay among amorphous components, including fillers, plasticizers, and residual solvents, which significantly hampers the rational design of high-performing CPEs. In this contribution, a polyvinylidene fluoride (PVDF)-based composite electrolyte has been developed, exhibiting high room-temperature ionic conductivity/mobility (>1 mS cm⁻¹/0.95 × 10⁻¹¹ m² s⁻¹), along with excellent electrochemical performances, including a wide stability window (4.8 V vs. Li/Li⁺), superior charge/discharge capacity, and reversibility. By performing advanced solid-state nuclear magnetic resonance (ssNMR) techniques, in combination with systematic investigations into solid polymer electrolytes (SPEs), gel polymer electrolytes (GPEs), and CPEs, we establish an efficient NMR-based strategy for deconvoluting the structural and dynamic features of complex electrolyte systems. Notably, the simple ¹H magic-angle spinning (MAS) NMR spectroscopy enables the identification and monitoring of nearly all components in the composite matrix. Motion-sensitive ¹H-¹³C and ¹H-⁷Li correlation experiments further reveal that the rigidity of PVDF polymer chain segments and the presence of residual solvents are two critical factors governing Li⁺ mobility. Moreover, we demonstrate that the order of the filler and plasticizer addition during the CPE fabrication significantly influences the performance of the electrolyte by regulating the retention of residual solvents. This work not only provides molecular-level insights into the structure-ion mobility relationships in the PVDF-based CPEs but also establishes a general NMR-based characterization approach for investigating other complex composite electrolyte materials.