Self-Standing Single-Ion Borate Salt-Based Polymer Electrolyte for Lithium Metal Batteries

Abstract

Polymer electrolytes (PEs) with excellent flexibility and superior compatibility toward lithium (Li) metal anodes have been deemed as one of the most promising alternatives to liquid e l e c t r o l y t e s . H o w e v e r , c o n v e n t i o n a l l i t h i u m b i s -(trifluoromethanesulfonyl)imide (LiTFSI)-based dual-ion PEs suffer from a low Li ion transference number and notorious Li dendrite growth. Here, a single-ion conducting polyborate salt without any fluorinated groups, polymeric lithium dihydroxyterephthalic acid borate (PLDPB), is presented for addressing the issues of Li metal batteries. Owing to a nearly immovable bulky anion and the presence of a rigid benzene structure, the PLDPB@ poly(ethylene oxide) (PEO) PE exhibits an ultrahigh Li ion transference number (0.94) and excellent mechanical strength, which could significantly restrict the growth of Li dendrites. Postmortem analysis reveals that a fluorine-free solid electrolyte interphase (SEI) enriched with B−O and benzene-containing species is formed on the surface of the Li metal anode, thereby facilitating elimination of excessive parasitic reactions and simultaneously suppressing the formation of Li dendrites. Consequently, the LiFePO 4 /Li cells with PLDPB@PEO PEs show an improved long-term cycling performance and high capacity retention (90.0%) and Coulombic efficiency (99.9%) after 500 cycles. This work may inspire new ideas to boost the development of single-ion conducting salts for dendrite-free Li metal batteries.

Functionally Modified Polymer Electrolyte Based on Noncovalent Interaction for Stable Lithium Metal Batteries

“…Solid-state lithium metal batteries with both high energy density and high safety have competitive potential in the development of next-generation portable energy storage devices, electric vehicles, and renewable energy systems. − Among them, solid polymer electrolytes (SPEs) are one of the technologies considered most likely to realize the full electrochemical potential of Li/Li + redox pair for lithium metal batteries. , However, the classical SPEs that dissolve lithium salts through polar polymer chains are usually dual-ion conductors . The strong coupling coordination between Li + and Lewis basic sites on the polymer chains, as compared to anions that have much weaker or even no interactions with the polymer chains, makes the Li + transference number ( t Li + ) of SPEs typically much less than 0.5 at room temperature. , Furthermore, the excessive aggregation of anions at the electrolyte–electrode interface will lead to concentration polarization that is difficult to eliminate, thus inducing side reactions destabilizing the interface. − Currently, there are three main improvement strategies for the nonuniform migration of anions and cations in polymer electrolytes: (1) immobilizing anions by covalent binding between the functional groups of the polymer chain and the anions to achieve single-ion conductivity of electrolytes; − (2) increasing the molecular size of the anion by molecular structure design to increase its migration energy barrier; − (3) functionalizing the polymer molecules by introducing functional groups that can form noncovalent interactions with anions, thus effectively limiting anion migration. − …”