Enhancing Li ion transfer efficacy in PEO-based solid polymer electrolytes to promote cycling stability of Li-metal batteries

Abstract

Solid polymer electrolytes (SPEs) could effectively enhance energy density and safety of Li-metal batteries (LMBs). Poly(ethylene oxide) (PEO)-SPEs exhibit excellent properties at elevated temperature (>70 °C), while their insufficient ionic conduction at room temperature yields nonnegligible mass transfer polarization. Such a polarization of SPEs in LMBs will lead to huge capacity loss, affect rate performance, cause Li dendrite growth that initiates safety problems, and significantly limit the battery lifespan. Herein, we have developed a method for improving the transfer efficacy of Li⁺ in PEO electrolyte through the construction of composite PEO with Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO)/fluorinated carbonate (FEC) and succinonitrile (SN) (denoted as PLFS). By means of the decomposition reaction between LLZTO and FEC, interfacial compatibility between LLZTO and PEO has been improved, and fluorinated oligomers generated in the decomposition of FEC can simultaneously fix anions by the ‘fluorous effect’. Furthermore, SN with a lower Gutmann donor number is used to composite with PEO/FEC/LLZTO to weaken the strong complexation between Li⁺ and PEO. The construction of a fast Li⁺ transfer channel as well as the weakened interaction of Li⁺ to PEO is achieved, which facilitates the migration of Li⁺ of PEO-SPEs. Benefiting from the above design, composite PLFS-SPEs present an excellent Li⁺ transfer efficacy that is 41 times higher than that of pure PEO-SPEs, and significantly enhances rate performance and cycling life of LMBs. The Li/PLFS-SPEs/Li symmetric battery demonstrates a lifespan of more than 1100 h at room temperature. When PLFS-SPEs are applied in LMBs, the Li/PLFS-SPEs/LiFePO₄ battery can maintain nearly 80% of its initial capacity after 700 cycles at 1C.