Poly(vinyl benzoate)-b-Poly(diallyldimethyl ammonium TFSI)-b-Poly(vinyl benzoate) Triblock Copolymer Electrolytes for Sodium Batteries
Authors: Stigliano Pierre Luca , Gallastegui Antonela , Villacis-Segovia Carlos , Amores Marco , Kumar Ajit , O'Dell Luke , Fang Jian , Mecerreyes David , Pozo-Gonzalo Cristina , Forsyth Maria 
Published: December 2024

Due to the great source of raw materials, sodium-based batteries are emerging as a competitive technology in the field of energy storage to supplement the rising need for higher energy densities and storage capacities, currently sustained primarily by the well-established lithium-ion technology. Current sodium-based batteries share with lithium-ion technology the safety concerns linked to flammable and toxic liquid electrolytes. Because of their simplicity of processing and low flammability, polymer electrolytes have found a growing research interest. Furthermore, polymers have several structural choices that span from linear homopolymers to more complex architectures like block copolymers, which allow for tailoring of properties and application to a variety of applications.

Among the diverse options within polymer electrolytes, block copolymer electrolytes (BCEs) have emerged as particularly promising. BCEs exhibit phase-separated nanostructured morphologies, offering independent tuning of thermal, mechanical, and electrochemical properties—a key advantage for adapting to varied applications.

Block copolymers (BCPs) as solid electrolytes for batteries are usually designed to have an ion-solvating block for ion conduction and an ionophobic block for providing mechanical strength.

Various polymer chemistries have been explored for ion solvation, with polyethylene oxides, polycarbonates, and poly(ionic liquid)s among the most investigated. In particular, the poly(ionic liquid) poly(diallyldimethyl ammonium bis trifluoromethanesulfonimide), known as PDADMATFSI, has been demonstrated to have great versatility for lithium and sodium batteries.

Here, we explore a novel solid polymer electrolyte for sodium batteries based on an ABA triblock copolymer electrolyte, consisting of poly(vinyl benzoate)-b-poly(diallyldimethylammonium bis(trifluoromethanesulfonyl)imide)-b-poly(vinyl benzoate)PVBx-b-PDADMATFSIy-b-PVBx triblock copolymer. The SPE triblock copolymer comprises a polymerized ionic liquid (PIL) ion-solvating block as an internal block and an ionophilic PVB as an external block, combined with NaFSI salt. The choice of poly(vinyl benzoate) for the mechanical block stems from two key factors. The presence of a benzoate ring in the backbone confers rigidity and mechanical strength to the polymer system. In addition, we anticipate the possibility of coordination of Na ions with the oxygen in the vinyl benzoate block. To study the influence of PVB as a “conductive” mechanical block on the physiochemical and electrochemical properties of polymer electrolytes, we synthesized four different samples with different chain lengths of PVB and PDADMATFSI, and NaFSI was added for evaluation of the polymer compositions in solid polymer electrolytes. The choice of mixed anions (FSI/TFSI) in the system is motivated by previous research showing the superior physiochemical and electrochemical properties of mixed anions systems in ionic liquid-based and polymer electrolytes Four distinct compositions with varying chain lengths of the blocks were synthesized by reversible addition−fragmentation chain-transfer (RAFT) polymerization. The neat copolymers were subsequently mixed with NaFSI in a 2:1 mol ratio of Na to ionic monomer units. Through comprehensive analysis using differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR), it was revealed that the ion coordination within the polymer–salt mixtures undergoes changes based on the composition of the starting neat polymer. It was observed that the relative chain length of each block had a significant impact on the Tg and conductivity of the polymer electrolytes. From FTIR and NMR analyses, it was shown that the PVB blocks also coordinated to some extent with the Na+ ions, in particular when MW was higher than that of the PDADMATFSI. Electrochemical evaluations identified the optimal composition for practical application as PVB11.5K-b-PDADMATFSI33K-b-PVB11.5K, showing an ionic conductivity at 70 °C of 4.2 × 10−5 S cm−1. This polymer electrolyte formulation was investigated for sodium in Na|Na symmetrical cells, showing an overpotential of 200 mV at 70 °C at 0.1 mA cm−2. When applied in a sodium–air battery, the polymer electrolyte membrane achieved a discharge capacity of 1.59 mAh cm−2 at 50 °C, indicating the considerable promise and viability of the investigated polymer electrolyte for sodium batteries. This research further opens new avenues for designing novel polymer electrolytes based on block copolymers and reiterates the importance of careful choice of the Mw of each block.