Self-assembly of Li single-ion-conducting block copolymers for improved conductivity and viscoelastic properties

Abstract

Single-ion conducting polyelectrolytes (SICPs) with mobile Li cation have recently gathered significant attention as an “ideal” electrolyte for safe solid-state rechargeable lithium batteries, because they eliminate salt concentration gradients and concentration overpotentials, allowing transference number (tLi+) values close to unity. In this work, a series of single ion conducting block copolymers, namely [(LiM)n-r-(PEGM)m]-b-(PhEtM)k (A-b-B), is synthesized via reversible addition-fragmentation chain transfer (RAFT) copolymerization of 1-[3-(methacryloyloxy)propylsulfonyl]-(trifluoromethanesulfonyl)imide (LiM), poly(ethylene glycol)methyl ether methacrylate (PEGM) and 2-phenylethyl methacrylate (PhEtM) with controlled PEGM:LiM ratio, molecular weights (Mn = 25.8 ÷ 85.9 kDa) and narrow polydispersity (Mw/Mn = 1.12 ÷ 1.21). The bulk ionic conductivity, solid-state morphology and thermal properties of block copolymers are studied as a function of their composition. Block copolymers having molecular weights in the range of 46 ÷ 63 kDa and any ratio of PEGM:LiM (from 3:1 to 7:1) tend to evolve in quasi-hexagonally-packed cylinders, while copolymers with higher molecular weights (Mn > 74 kDa) and the ratio of PEGM:LiM = 5:1 and MA/MB ≤ 2.0 show lamellar phase separation. The lamellar long-range ordering in poly[(LiM17-r-PEGM86)-b-PhEtM131] and poly[(LiM17-r-PEGM86)-b-PhEtM194] results not only in the improved viscoelastic (mechanical) performance compared to parent copolymer poly[LiM17-r-PEGM86] (complex viscosity = 2.5 × 108 mPa s and 8.7 × 104 mPa s at 25 °C, respectively), but also in the demonstration of sufficiently high ionic conductivity despite the decrease in Li+ amount (σ = 3.8 × 10−7 and 4.1 × 10−7 S/cm at 25 °C, correspondingly). The selected poly[(LiM17-r-PEGM86)-b-PhEtM131] further shows high tLi+ (0.96 at 70 °C) and wide electrochemical stability (4.4 V vs. Li+/Li at 70 °C), which results in reversible and stable cycling at high specific capacities (up to 150 and 118 mAh g−1 at C/20 and C/5 rates, respectively) when assembled in lab-scale truly-solid-state Li metal cells with Li/copolymer/LiFePO4 configuration.