Development and Characterization of Hybrid Inorganic-Organic Solid Electrolytes for Their Application in Solid State Sodium Batteries
Authors: Röttgen Niklas , Holzapfel Michael , Klein Franziska , Tübke Jens 
Published: December 2024

In recent years sodium-ion batteries (SIB) have received significant attention from academia and industry. Switching the charge carrier inside the battery from lithium to sodium enables the use of non-critical, widely available, affordable resources and in turn offers the potential for low-cost batteries. One drawback of SIB is their lower energy-density compared to state-of-the-art lithium-ion batteries. [1] To increase the energy density of sodium-based batteries, researchers are exploring strategies to implement sodium metal anodes because of their high specific capacity (1165 mAh g-1) and low redox potential (- 2.71 V vs standard hydrogen electrode). One of these strategies is the development and application of solid states electrolytes (SSE) that are compatible with sodium metal. In addition to an increased energy density, SSE might also offer safety improvements due to the absence of flammable liquid solvents and an increased thermal stability window. [1]
Among these SSE there are organic solid polymer electrolytes (SPE) and inorganic ceramic electrolytes (ISE). Both types of SSE offer certain advantages and disadvantages. While SPE enable a good contact to the electrodes due to their flexibility and are easily produced, they lack in ionic conductivity at ambient temperatures (typically < 10-4 S cm-1). ISE in turn achieve room temperature ionic conductivities in the range of 10-3 S cm-1. Furthermore, they possess a very high tensile strength which might help with preventing internal short circuits due to dendritic sodium metal puncturing the separator. However, owing to their rigidity and brittleness it is challenging to produce mechanically robust, thin separator membranes and achieve a good contact with the electrodes without high stack pressures. [2] To compensate for these disadvantages research has been conducted into the hybridization of SPE and ISE. [3]
In this work we produced hybrid inorganic-organic solid-state electrolytes via introducing fine NaSICON (Na superionic conductor, Na1+xZr2SixP3-xO12) particles and sodium bis(trifluoromethylsulfonyl)imide salt (NaTFSI) into a crosslinked polyether-based polymeric host structure. The hybrid electrolytes were analyzed for their temperature dependent ionic conductivity, electrochemical stability window and compatibility with sodium metal anodes. We could show that the addition of NaSICON to the polymer electrolyte can have a beneficial effect on the ionic conductivity in the measured temperature range (e. g. increase from 0.8 mS cm-1 to 2 mS cm-1 at 80 °C). Nonetheless, significantly lower ionic conductivity at lower temperatures is observed. Preliminary impedance analysis revealed an interface resistance between the NaSICON and polymer electrolyte, that might interfere with unlocking the high ionic conductivity of the NaSICON at room temperature. Additionally, the plating-stripping capability was improved for the hybrid electrolytes, while the electrochemical stability towards oxidation was maintained. Overall, we show that combining NaSICON with polyether-based SPE has beneficial effects. We also highlight remaining challenges for further development of these hybrid electrolytes for their successful application in solid state sodium batteries.
[1] Huang, J.; Wu, K.; Xu, G.; Wu, M.; Dou, S.; Wu, C.; Recent progress and strategic perspectives of inorganic solid electrolytes: fundamentals, modifications, and applications in sodium metal batteries. Chemical Society reviews 2023. DOI: 10.1039/d2cs01029a.
[2] Vasudevan, S.; Dwivedi, S.; Balaya, P.; Overview and perspectives of solid electrolytes for sodium batteries. Int J Applied Ceramic Tech 2023, 20 (2), 563–584. DOI: 10.1111/ijac.14267.
[3] Su, Y.; Xu, F.; Zhang, X.; Qiu, Y.; Wang, H.; Rational Design of High-Performance PEO/Ceramic Composite Solid Electrolytes for Lithium Metal Batteries. Nano-micro letters 2023, 15 (1), 82. DOI: 10.1007/s40820-023-01055-z.