Polymer Electrolytes for Stable and Safer Sodium-Based Batteries

Abstract

Lithium-ion batteries (LIBs) have powered portable electronics and electric vehicles owing to their high energy density. In addition, the LIBs demand has significantly increased in the last years, raising concerns about the long-term availability and cost of the critical raw materials used in LIB production, e.g., cobalt, lithium, natural graphite, and copper [1,2]. In this scenario, alternative batteries must be developed based on low-cost and environmentally friendly raw materials. Sodium-ion batteries (SIBs) have become real energy storage devices for large-scale stationary applications and light electromobility due to the almost infinite and widely distributed sodium resources, the possibility to be made with environmentally friendly and sustainable raw materials, and lower cost.Liquid electrolytes are the electrolyte of choice for SIBs due to their high ionic conductivity, low viscosity, and good contact with electrodes. Nevertheless, they pose serious safety hazards due to their high volatility, flammability, severe decomposition reactions with the electrode, and risk of leakage. Therefore, alternative electrolytes must be explored for manufacturing safer SIBs. Polymer electrolytes are the best candidate to replace liquid electrolytes due to their higher thermal stability, wider electrochemical stability window, and less reactivity with the electrode. Nevertheless, their practical application is limited because they usually exhibit low ionic conductivity at room temperature and high interfacial resistance, which impoverish the performance of the sodium cells [3]. Adding plasticizers or inorganic fillers into the polymer is one way to enhanced ionic conductivity at room temperature. To keep the safety property of polymer electrolytes, ionic liquids (ILs) are more attractive plasticizers than carbonate-based solvents due to their non-volatility, non-flammability, wide electrochemical stability window, and high ionic conductivity, as well as thermal and chemical stability [4].This work describes, on the one hand, the physicochemical and electrochemical properties of gel polymer electrolytes using ILs as plasticizers. Their applicability has been demonstrated in sodium-metal cells using the P2-Na2/3Ni1/3Mn2/3O2 layered oxide as cathode material. The gel polymer electrolyte based-sodium-metal cell delivers a good specific capacity, with improved cycling stability of 99% and excellent Coulombic efficiency of 99.9% at room temperature, overcoming the electrochemical performance of the counterpart liquid electrolyte based-cell. On the other hand, a single sodium-ion conducting polymer based on bis(trifluoromethanesulfonyl)imide anionic center sodium salt monomer will be introduced. These results open the way for extending t sodium-based batteries’ long-term stability and safety without impoverishing the electrochemical performance.