Polymer Electrolytes Based on Interactions between [Solvent-Li+] Complex and Solvent-Modified Polymer

Abstract

Poly(vinylidene fluoride) (PVDF) is the most popular electrode binder in the current lithium ion batteries (LIBs). Depending on solvent content, polymer electrolytes are classified into solid polymer electrolyte (SPE; solvent-free) and gel polymer electrolytes (GPE; solvent-rich). PVDF-based electrolytes with high contents of solvent undisputedly belong to GPE. However, the affiliation (SPE or GPE) of the PVDF-based electrolyte with low content of “solvent” is in argument as its “solvent” cannot be eliminated till 80°C in vacuum and its structure and ion transport mechanism remain unclear. By a series of studies, we demonstrate that this is actually a polymer electrolyte based on the interactions between the [solvent-Li+] complex and the polymer in structure and in Li-ion transport mechanism. The “residual solvent” (N,N-dimethylformamide, DMF, for example) plays critical roles in constructing the polymer electrolyte and in determining its ion transport and therefore such electrolyte cannot be assigned to any of the known polymer electrolytes. The Li+-associated DMF becomes difficult to be eliminated while the DMF-dehydrofluorinated PVDF has a larger dielectric constant, enhances the dissociation of the lithium salt and interacts with the [DMF-Li+] complex, the charge carrier of the electrolyte. These [solvent-Li+] complex-based polymer electrolytes have high ionic conductivity (10-4 S cm-1 at room temperature), Li-ion transference number (0.44) and Young’s modulus (45.87 MPa) and will find important applications in the solid lithium batteries and as an ion-conducting binder for the electrodes. These findings will enrich the gallery and deepen the fundamental understandings of the polymer electrolytes, spurring designing of novel electrolytes.

Keywords: poly(vinylidene fluoride)-based polymer electrolyte, ion transport mechanism, electrode binder, high voltage