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 (LIBs). Among SPEs, a new class of polyelectrolytes, namely poly(ionic liquid)s (PILs) has deserved considerable attention. Although significant progress has already been achieved with cationic PILs/Li salt composites, the power delivery of such electrolytes is hampered by the strong concentration gradients formed during battery operation. As an alternative the anionic polyelectrolytes or so called “polymeric single-ion conductors” have been recently suggested.

In this work, we present two approaches for the preparation of the innovative families of single-ion polymer electrolytes.

1. First approach consists in the synthesis of ionic block copolymers via RAFT polymerization technique.  Such copolymers comprise poly(lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethylsulfonyl)imide) and poly(ethylene glycol) methyl ether methacrylate blocks (Fig. 1A). This method allowed to vary the molecular weight of PILs and to gain the desired control over polymer’s T_g and ionic conductivity. The “best” obtained polyelectrolyte with M_n = 2.5x10⁴ g/mol shows the T_g as low as -61 ºC, ionic conductivity as high as 2.3×10^-6 and 1.2×10^-5 S cm^-1 at 25 and 60^o C, respectively, wide electrochemical stability (4.5 V vs Li⁺/Li) and a lithium-ion transference number close to unity (0.83).
2. Second utilized approach involves the free radical copolymerization of ionic liquid like monomers with poly(ethylene glycol) methyl ether methacrylate for the preparation of random ionic copolymers. Here, polymer’s ionic conductivity and T_g were controlled by the nature of ionic monomer and the molar composition of statistic copolymer (Fig. 1B). The “optimal” coPIL with M_w = 4.2x10⁵ g/mol demonstrates T_g = -56 ºC, ionic conductivity equal to 1.8×10^-6 and 1.5×10^-5 S cm^-1 at 25 and 60^o C, respectively, wide electrochemical stability (4.2 V vs Li⁺/Li) and high lithium-ion transference number (0.91).

Owing to the combination of all mentioned properties, the prepared polymer materials were used as solid polyelectrolytes as well as binders in the elaboration of truly solid Li/coPIL/LiFePO₄ battery prototypes working at 60-70^oC and delivering large capacities (up to 130 mAh/g), with an impressive charge/discharge efficiency and the capability to reversibly operate at relatively high rates up to C/5 (Fig. 1C).

This work was supported by the Russian Foundation for Basic Research (projects no. 14-29-04039_ofi_m and 16-03-00768_a) and by European Commission (project no. 318873 «IONRUN»). J.R.N. gratefully acknowledges financial support from MARS-EV project (FP7/2007-2013, under grant agreement n° 609201).