Electrolyte breakthrough improves sodium battery performance

Researchers from the University of Geneva (UNIGE) have created a suitable solid electrolyte for use with sodium batteries, a less dangerous and more durable alternative to lithium. They also defined the ideal pressure to be applied to the battery for it to operate efficiently.

Lithium-ion (Li-ion) batteries currently power most of our electronic devices and electric vehicles; however, they have two major shortcomings. The liquid electrolyte they contain, which allows positive ions to flow between the two electrodes of the battery, is highly flammable: if it leaks, it can react violently with oxygen, posing a hazard to users. The supply of lithium is also problematic: unevenly distributed around the globe, it is at the heart of major geopolitical issues in the same way as oil.

By contrast, sodium is abundant all over the Earth and in the sea, and is cheaper and easier to recycle than lithium. However, its use in batteries is underdeveloped as, according to UNIGE researcher Fabrizio Murgia, “The production of this type of batteries involves a different technology from that used for lithium-based ones.”

Because sodium is heavier than lithium, its ions move less easily in the liquid electrolyte. The solution is to design a solid electrolyte that is also non-flammable. However, the electrolytes of this type developed up to now, which are composed of hydridoborates (boron and hydrogen), have not been able to achieve the performance of lithium batteries. Two recent studies conducted by the UNIGE crystallography laboratory, headed by Professor Radovan Cerny, have succeeded in solving this problem.

The first study, published in the journal ACS Applied Materials & Interfaces, saw the team modify the crystal structure of a material composed of sodium, carbon, boron and hydrogen (sodium carbo-hydridoborate, or NaCB₁₁H₁₂). As explained by Prof Cerny, “Originally, this material which is used in nuclear medicine is not conductive. By modifying the structure of its crystals, and more precisely the spatial arrangement of the atoms, we have succeeded in making it conductive, which makes it the most efficient means of transporting sodium ions currently available.”

To achieve this result, the research team subjected the compound to high shocks, generating high temperatures, inside a ball mill. This is an energy-efficient method that is widely used in the cement industry.

The second project, published in Advanced Materials Interfaces, consisted of putting this material in situation. For a battery to work, the electrolyte, whether liquid or solid, must be in intimate contact with the positive and negative electrodes of the battery. It must therefore be contained firmly within the battery.

“To achieve that, pressure must be applied by means of screws or springs. We looked for the ideal ‘force’ to exert on our solid electrolyte,” explained Matteo Brighi, former postdoctoral scholar in the crystallography laboratory. It was shown that this force should be around 400 atmospheres, which can be achieved very easily with a few turns of a screw.

Those discoveries pave the way for easier production of sodium batteries, especially in the automotive industry. According to Murgia, “Because of the slightly heavier weight of these batteries, they could be used primarily to power cars. The cost-effectiveness of manufacturing them has also yet to be assessed, but it is now important that industry realise that the material we have discovered is really interesting.”

Image caption: The solid electrolyte developed by the UNIGE team lets the ions move efficiently within the sodium batteries. Image ©Xavier Ravinet/Olivier Gaumer — UNIGE.

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