New polymeric single-ion conductors for rechargeable lithium batteries

In recent years, wide research efforts have been devoted to the development of solid polymer electrolytes (SPEs) with the goal to enhance the intrinsic safety and replace the traditional flammable liquid electrolytes employed in the lithium-ion battery technology. Very commonly, SPEs are composed of a lithium salt dissolved either in a neutral polymer (e.g., PEO) or in an ion-conducting polymer matrix. The latter usually is represented by a new class of polyelectrolytes, namely poly(ionic liquid)s (PILs). Although significant progresses have already been achieved with cationic PILs, the motion of lithium ions carriers in such PIL/Li salt composite separators constitutes only a small fraction (1/5th) of the overall ionic current. This leads to the formation of a strong concentration gradient during battery operation, with deleterious effects such as favored dendritic growth and limited power delivery. Anionic PILs or polymeric single-ion conductors have been recently suggested as an alternative. Differently from other SPEs, a single-ion conductor is composed of a polymeric backbone bearing a covalently bonded anionic moiety and a Li counter-ion free to move and responsible for the ionic conductivity. Given the single-ion nature of the above-mentioned systems, the lithium-ion transport number is noticeably close to the unity. In this work, we present an innovative family of single-ion polymer electrolytes based on specifically developed lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethanesulfonyl) imide ionic monomer. Varying the macromolecular architecture of the polyelectrolytes (i.e., random or block copolymers with poly(ethylene glycol) methyl ether methacrylate or crosslinked networks with poly(ethylene glycol)dimethacrylate) it was possible to develop the SPE with the tailored high ionic conductivity (up to 2.7×10-6 at 25 °C). A full overview of the electrochemical and thermal properties for the synthesized SPEs will be presented. Finally, the performance of prototype Li-ion batteries using the best PILs will be shown, which demonstrates their highly promising prospects as next-gen all-solid safe electrolytes

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

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