Nowadays, lithium-ion batteries (LIBs) are not only used to power mobile electronic devices but also for electromotive and stationary storage applications. The growing demand has raised concerns about the long-term availability and cost of the critical raw materials employed, as well as environmental aspects, in LIB production [1]. Among the "beyond" lithium electrochemical storage systems, sodium-ion batteries (SIBs) are a promising and appealing technology, especially for large-scale stationary applications and light electromobility [2]. As sodium is one of the most abundant elements in the Earth’s crust, sodium resources are virtually unlimited everywhere, easily accessible and of low cost [3]. Additionally, all cell components can be based on non-critical raw materials and renewable resources. Implementing SIBs as an alternative or complement to the lithium-ion technology therefore has the potential to reduce cost, improve sustainability and decrease the environmental impact of battery production [4].

Most state-of-the-art SIBs employ carbonate- or ether-based electrolytes due to their high ionic conductivity, optimal viscosity and satisfactory electrochemical performance [2]. As for LIBs, the use of conventional liquid electrolytes involves serious safety hazards owing to their high volatility, flammability and the risk of leakage. Side reactions between electrode and electrolyte, leaching of active material ions and dendrite formation are additional issues detrimental to battery performance [5]. Transitioning to solid state systems by using polymer electrolytes for example has the potential to overcome these shortcomings. Amongst others, ternary polymer electrolytes incorporating ionic liquids have attracted interest in the past, as they combine high safety, good mechanical properties and sufficient ionic conductivity at room temperature [6].

In this work, the liquid electrolyte in sodium-based batteries is replaced by a solvent-free ternary polymer based on polyethylene oxide (PEO), sodium salt and ionic liquid. Synthesized polymers have been thoroughly characterized by a set of thermal and electrochemical analysis techniques to study the thermal and electrochemical stability, as well as thermal transitions. Electrochemical impedance spectroscopy has been applied to determine the temperature dependent ionic conductivity and unravel the polymer/sodium metal interface. Finally, the polymer electrolytes have been tested in cells comprising the well-studied layered P2-Na_2/3Ni_1/3Mn_2/3O₂ as a reference cathode and sodium metal as anode. The results obtained in terms of columbic efficiency, practical capacity and capacity retention are outstanding examples that polymer electrolytes can improve the safety and long-term cycling stability of sodium-based batteries.

References:

[1] C. Vaalma, D. Buchholz, M. Weil, S. Passerini, Nat. Rev. Mater. 3 (2018) 18013.

[2] I. Hasa, S. Mariyappan, D. Saurel, P. Adelhelm, A. Y. Koposov, C. Masquelier, L. Croguennec, M. Casas-Cabanas, J. Power Sources 482 (2021) 228872.

[3] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636.

[4] G. G. Eshetu, G. A. Elia, M. Armand, M. Forsyth, S. Komaba, T. Rojo, S. Passerini, Adv. Energy Mater. 10 (2020) 2000093.

[5] J. Yang, H. Zhang, Q. Zhou, H. Qu, T. Dong, M. Zhang, B. Tang, J. Zhang, G. Cui, ACS Appl. Mater. Interfaces 11 (2019) 17109

[6] I. Osada, H. de Vries, B. Scrosaty, S. Passerini, Angew. Chem. Int. Ed. 55 (2016) 500.