Effect of Li⁺ Affinity on Ionic Conductivities in Melt-Blended Nitrile Rubber/Polyether

Abstract

Substituting flammable liquid electrolytes with solid polymer electrolytes (SPEs) presents a serious challenge in improving the safety of lithium-ion batteries. Even though SPEs are a safer choice, their ionic transport properties are still lower than those of their liquid counterparts (<10 −4 S•cm −1 at room temperature). Here, we report the preparation of a blend of polymers used as SPEs in lithium-ion batteries. Composed of an elastomer, hydrogenated nitrile butadiene rubber (HNBR), and poly(ethylene oxide) (PEO), this blend combines the high conductivity of PEO and the stable properties of HNBR and shows better flexibility than a pristine PEO SPE. It is worth noting that the addition of HNBR, coupled with the intrinsic LiTFSI salt concentration, also reduces the crystallinity and melting temperature of typical PEO-LiTFSI SPEs; this also explains the higher ionic conductivity at low temperature (1.18 × 10 −4 S•cm −1 at 40 °C). Given these initial results, we may conclude that this polymer blend is a promising candidate as an SPE for all solid-state lithium-ion batteries.

Challenges in Solvent-Free Methods for Manufacturing Electrodes and Electrolytes for Lithium-Based Batteries

“…This process allowed several SPEs with different HNBR:PEO ratios to be produced, demonstrating the versatility of the melt-process. The ionic conductivity of the resulting films was tested by impedance spectroscopy and the blend comprising 30 wt.% HNBR and 70 wt.% PEO, with 24.5 wt.% of LiTFSI, had a conductivity of 3.03 × 10 −5 S•cm −1 at 30 • C, which is higher than that of 100 wt.% PEO with the same quantity of LiTFSI [30].…”

Extrusion of Polymer Blend Electrolytes for Solid-State Lithium Batteries: A Study of Polar Functional Groups

“…The DSC analysis was used to determine the melting enthalpy (Δ H m ) and calculate the degree of crystallinity ( X c ) via the following equation: where Δ H m 0 is the melting enthalpy of the pure polymer with 100% crystallinity.…”

“…In blended systems, both polymers can dissolve lithium salts as a result of possessing polar functional groups. According to previous studies, blending two polymers affects ionic transport properties, as polymer composition and architecture are known to affect ion transport mechanisms in SPE. , For example, in a PEO|HNBR blend SPE, Li + ions have been reported to predominately coordinate to the ether groups on PEO rather than to the nitrile groups on HNBR. This phenomenon explains why the ionic conductivity of this blend is close to that of PEO at temperatures above the melting temperature of PEO .…”

Thermal and Electrochemical Properties of Solid Polymer Electrolytes Prepared via Lithium Salt-Catalyzed Epoxide Ring Opening Polymerization

“…Therefore, materials that are amorphous at lower temperatures tend to have better ionic conductivities over a larger temperature range [46,47]. The T g of PEO-LiTFSI has been reported to be around −35 • C depending on the salt concentration [48]. Decreased T g has been linked to improved polymer mobility and therefore improved ionic conductivity.…”