As the production of current-generation energy storage technologies is being scaled up for widescale implementation in electric vehicle and large-scale grid storage, cost and sustainability are becoming more limiting factors for battery utilization than the capacity, which is already sufficient for many applications. Here, the future supply of Li has been pointed out as potentially problematic, [1] with in particular Na, with its similar size, reduction potential and chemistry, but with far greater abundancy and wider global distribution, highlighted as a natural candidate for more sustainable “beyond-Li” energy storage [2].

The notable similarities between Li and Na translate into similarities in the battery chemistries such that the research on Li-ion batteries can be used as a starting point for the development of Na-ion batteries. Unfortunately, these similarities also mean that the combination of high energy densities, high voltages and flammable organic electrolyte solvents that is cause for major safety concerns in Li-ion batteries [3] is also deeply concerning for Na-ion batteries. This can be mitigated by using solid polymer electrolytes, with negligible vapor pressure and inherent mechanical stability, in place of the traditional liquid electrolytes to improve battery safety and stability. These electrolytes are traditionally based on polyethers, in particular poly(ethylene oxide) (PEO) as the ion-solvating and -transporting host material. Unfortunately, for both Li and Na, this material shows poor ionic conductivity at room temperature because of crystallization, in combination with low cation transference numbers [4], requiring elevated temperatures in order to sustain cell cycling.

There are, however, a large range of potential host materials that extends beyond the polyether paradigm and that has been surveyed for Li-based energy storage [5] but has yet to be implemented for Na-ion batteries. We have focused on materials that can coordinate to Na⁺ by means of carbonyl groups. Using the host material poly(trimethylene carbonate) in combination with NaTFSI, fully amorphous electrolytes were obtained that could sustain Na⁺ transport in Na metal cells at 60 °C [6]. Improved stability was seen with NaFSI salt, where also notably high ionic conductivities were attained at high salt concentrations, but at reduced cycling stability. Instead, at moderate salt concentrations, stable cycling over >80 cycles was seen. Finally, using a polyester–polycarbonate copolymer host material, ionic conductivities on the order of 10⁻⁵ S cm⁻¹ were attained in combination with a cation transference number close to 0.5, allowing for room-temperature cycling of solid-state Na-ion full cells.

1. Tarascon, J.-M. Chem. 2010, 2, 510-510
2. Deng, J. et al. Energy Mater. 2018, 8, 1701428
3. Robinson, A. L. et al. MRS Bull. 2016, 41, 188-189
4. Qi, X. et al. ChemElectroChem 2016, 3, 1741–1745
5. Mindemark, J. et al. Polym. Sci. 2018, 81, 114–143
6. Mindemark, J. et al. Electrochem. Commun. 2017, 77, 58-61