Mechanically Clamped Liquid-PEO Based Block Copolymer Electrolytes for Solid-State Lithium-Ion Batteries

During the past two decades, many efforts were devoted to increase the performance of Li-ion batteries[1]. Currently, Li-ion technology using carbonate based liquid electrolyte exhibits the best performances but it’s also famous for safety reason: leakages of corrosive products, explosion hazard and narrow stability window. Solid-state electrolytes are the perfect solution to develop safer lithium devices[2]. By combining lightweight, easy implementation, flexibility and mechanical strength, solid polymer electrolyte appears to be the most attractive alternative. Among various polymers developed, only polyethylene oxide (PEO) is known to solvate a substantial amount of lithium salt. However, relevant ionic conductivities are obtained mainly above the melting point (>65°C), which render this material unpractical for most of applications[3]. 

Herein, we disclose the design of a mechanically clamped liquid PEO based electrolyte integrating both, high ionic conductivity and mechanical strength at room temperature[4]. Through macromolecular engineering, a bi-functionnal block copolymer is designed combining an anchoring polystyrene (PS) block attached to a polymethacrylate oligoethylene glycol (POEGMA) ionic conductive block. By tuning the relative size of the two blocks, the mechanical properties were finely adjusted to offer excellent mechanical strength. Only minority PS block was necessary to generate glassy PS-nanodomains that provide a robust mechanical anchor to the electrolyte confirmed by rheological measurements (Fig. 1a). The ionic conductivities appear to be barely affected by the PS block, with conductivities approaching 10^-5 S/cm, similar to pure POEGMA system at room temperature. To meet the highest performances for mechanically clamped electrolyte, the size of the oligomeric side chain was finely tuned to offer the conductivities above 10^-5S/cm. 

Furthermore, the bi-functional electrolyte was found to exhibit a wide electrochemical stability window, above 5 V vs. Li/Li⁺, which renders this material applicable to high voltage cathode batteries. The bi-functional electrolyte also displays a more stable behavior at anodic potentials allowing the formation of a stable electrode interphase after only few cycles (Fig. 1b). Finally, a prototype LiFePO₄/PS-b-POEGMA/Li battery is realized demonstrating the applicability of the mechanically clamped liquid PEO based electrolyte for all-solid-state Li-ion batteries. 

References

[1] Armand, M., Tarascon, J. M. Building better batteries Nature 451, 652–657 (2008).

[2] Murata, K., Izuchi, S., Yoshihisa, Y. An overview of the research and development of solid polymer electrolyte  batteries. Electrochim. Acta 45, 1501-1508 (2000).

[3] Bouchet, R., et al. Single-ion BAB triblock copolymers as highly efficient electrolytes for lithium-metal batteries Nature Materials 12, 452-457 (2013).

[4] Rolland, J., et al. submitted.