(Invited) High Performance Single Conducting Polymer Electrolytes for Lithium Metal Batteries
Authors: Iojoiu Cristina , Bresser Dominic , Paillard Elie , Nguyen Huu-Dat , Lyonnard Sandrine 
Published: July 2021

Lithium-based batteries are a key technology for successfully transitioning from depleting fossil fuels to renewable energy sources, particularly in meeting the demands of a highly mobile society. Nevertheless, the commonly employed liquid organic electrolytes are highly flammable and release toxic degradation products[1]. Solid electrolytes are a promising alternative, since they offer an improved safety and potentially suppress dendritic lithium deposition[2]. Among them, due to their flexibility and modularity, solid polymer electrolytes (SPEs) are excellent candidates. For their commercial implementation, however, one must simultaneously ensure acceptable ionic conductivity (>10-4 S cm-1 at 25 °C), high mechanical strength, and an electrochemical stability window of at least 4-5 V. Conventional polymer electrolytes based on poly(ethylene oxide) (PEO) consist of a lithium salt dissolved in the polymer. Frequently, also a plasticizer is added to enhance the segmental relaxation-dependent charge transport by lowering the glass transition temperature (Tg)[3]. The need for a conducting salt, however, results in transference numbers (t+) far below 0.5. This leads to charge concentration gradient and reversed cell polarization issues, which worsen the long-term cycling stability[4]. SPEs with t+ = 1, also known as ‘single-ion polymer electrolytes’ (SIPEs), might overcome these issues. The simplest approach to achieve single-ion conductivity is to covalently tether the anion to the polymer backbone (so-called ‘ionomers’), thus ensuring that the only mobile species is the cation. Nevertheless, the conductivity of these systems is typically very low (10−
8 to 10−
7 S cm−
1 at 25 °C)[5
] and despite the ten-fold increase achieved through optimizing the ionomer structure by introducing weakly coordinating anionic species (∼10−
6 S cm−
1 at 25 °C)[
6] or designing block or grafted copolymers, their employment remains unfeasible. Moreover, for most block copolymer-based SIPEs, the anion is tethered to the rigid block, while the Li+ conduction occurs in the microphase-separated PEO block. Consequently, adequate conductivities are achievable only in the disordered state above the melting point of the PEO phase, so that the phase separation and, accordingly, the mechanical properties provided by the rigid block vanish. In this presentation we give SIPE that we recently synthetized[7,8] with very efficient charge transport pathways, high electrochemical stability and an ionic conductivity approaching that of liquid organic electrolytes. Moreover, these SIPEs allowed for lithium stripping/plating for more than 1000 h with a remarkably low overpotential and cycling of NMC811 with high reversible capacity at various temperatures, including 20 °C and even 0 °C, for more than 500 cycles without substantial capacity fading when applying an optimized charging mode. These are very remarkable and original results, since the materials’ properties are nicely tailorable with regard to the ionomer structure, charge carrier density, phase separation, and morphological appearance. The polymer nanostructure and cell performances are correlated.

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