A Single-Ion Polymer Superionic Conductor

Abstract: All-solid-state batteries (ASSBs) have been considered a promising candidate for the next-generation electrochemical energy storage because of their high theoretical energy density and inherent safety. Lithium superionic conductors with high lithium-ion transference number and good processability are imperative for the development of practical ASSBs. However, the lithium superionic conductors currently available are predominantly limited to hard ceramics. Practical lithium superionic conductors employing flexible polymers are yet to be realized. The rigid and brittle nature of inorganic ceramic electrolytes limits their application in high-performance ASSBs. In this study, we demonstrate a novel design of a ternary random copolymer single-ion superionic conductor (SISC) through the radical polymerization of three different organic monomers that uses an anion-trapping borate ester as a crosslinking agent to copolymerize with vinylene carbonate and methyl vinyl sulfone. The proposed SISC contains abundant solvation sites for lithium-ion transport and anion receptors to immobilize the corresponding anions. Furthermore, the copolymerization of the three different monomers results in a low crystallinity and low glass transition temperature, which facilitates superior chain segment motion and results in a small activation energy for lithium-ion transport. The ionic conductivity and lithium-ion transference number of the SISC are 1.29 mS·cm−1 and 0.94 at room temperature, respectively. The SISC exhibits versatile processability and favorable Young's modulus (3.4 ± 0.4 GPa). The proposed SISC can be integrated into ASSBs through in situ polymerization, which facilitates the formation of suitable electrode/electrolyte contacts. Solid-state symmetric Li||Li cells employing in situ polymerized SISCs show excellent lithium stripping/plating reversibility for more than 1000 h at a current density of 0.25 mA·cm−2. This indicates that the interface between the SISC and lithium metal anode is electrochemically stable. The ASSBs that employ in situ polymerized SISCs coupled with a lithium metal anode and various cathodes, including LiFePO4, LiCoO2, and sulfurized polyacrylonitrile (SPAN), exhibit acceptable electrochemical stability, including high rate performance and good cyclability. In particular, the Li||LiFePO4 ASSBs retained ~ 70% of the discharge capacity when the charge/discharge rate was increased from 1 to 8C. They also demonstrate long-term cycling stability (> 700 cycles at 0.5C rate) at room temperature. A capacity retention of 90% was achieved even at a high rate of 2C after 300 cycles at room temperature. Furthermore, the SISCs have been applied to Li||LiFePO4 pouch cells and exhibit exceptional flexibility and safety. This work provides a novel design principle for the fabrication of polymer-based superionic conductors and is valuable for the development of practical ambient-temperature ASSBs.