Water is a practical electrolyte. It is found virtually everywhere, is cheap, does not burn – and conducts ions. Water nevertheless has one major disadvantage: it remains chemically stable only up to a voltage difference of 1.23V. A water cell thus supplies three times less voltage than a commercially available 3.7 V lithium-ion cell, which makes it largely unsuitable for applications in electric cars. However, a low-cost, water-based battery could be extremely interesting for stationary power storage applications.

For such applications, researchers at the Swiss Materials Testing and Research Institute (Empa) are using the special salt sodium FSI, which is extremely soluble in water. In this liquid, all the water molecules around the positively charged sodium cations are part of a hydration shell, and there are hardly any unbound water molecules. The researchers discovered that this salt solution has an electrochemical stability of up to 2.6 V – that is, nearly twice as high as other aqueous electrolytes.

The discovery could be the key to inexpensive and safe battery cells. One of the factors influencing the price is that sodium FSI cells are safer and therefore easier to build than customary Li-ion batteries. The system has already successfully withstood a series of charge and discharge cycles in the laboratory. The researchers have, however, so far tested their test battery’s anodes and cathodes separately – against a standard electrode as a partner. The next step will be to combine the two half-cells into a single battery. Should the experiment succeed, a reasonably-priced saltwater battery could soon be within reach.