A lithium solid electrolyte of acrylonitrile copolymer with thiocarbonate moiety and its potential battery application

Highlights: • RAFT polymerization technique reduces molecular weight, incorporates thiocarbonate. • Ion conductivity of the LiFSI doped PAN copolymer reaches 6.1 × 10⁻⁴ s cm⁻¹. • Lithium transference number of the SPE is 0.318. • The capacity of a lithium metal battery with the SPE reaches 150 mAh g⁻¹. -- Abstract: Researchers study the solid polymer electrolyte (SPE) to raise the safety and capacity of current lithium ion battery technology to a higher level. We opt to work on the polymer host based on acrylonitrile (PAN), since the nitrile group provides an admirable electrochemical stability and a high polarity, which are critical to the polymeric electrolyte. Yet, the nitrile group is also the origin of high glass transition temperature T_g, which requires synthetic efforts to reduce T_g through tuning the SPE composition. The RAFT mediated polymerization technique is employed to decrease the molecular weight and simultaneously incorporate a substantial amount of thiocarbonate moiety in the backbone. The synthesized PAN copolymer, molecular weight ~1600 g mol⁻¹, contains 43.3% (by mole) carbon, 2.1% sulfur, and 8.7% nitrogen. Copolymerization with dodecyl acrylate increases the free volume of host. Substitution of LiFSI for LiTFSI reduces the glass transition temperature effectively since LiFSI is easier to dissociate and more effective in plasticization. The synthesis efforts result in the highest ion conductivity 6.1 × 10⁻⁴ S cm⁻¹ at room temperature and 1.1 × 10⁻³ S cm⁻¹ at 50 °C. This SPE also displays a lithium transference number 0.318, a high stability when interfacing the lithium metal, and tolerates a potential window of 6.0 V. When sandwiched between lithium anode and NMC622 cathode, the cell of SPE reaches 150 mAh g⁻¹ in charge and discharge.