Looking for safe, affordable batteries for the future, the question is: Why don't we just use water as electrolyte? Water is inexpensive, available everywhere, does not burn and can conduct ions. However, water has a decisive disadvantage: it is chemically stable only up to a voltage difference of 1.23 V. A water cell therefore supplies three times less voltage than a standard lithium-ion cell with 3.7 volts, which is why it would hardly be suitable for applications in an electric car. For stationary power storage applications, however, a cost-effective water-based battery could become interesting. Ruben-Simon Kühnel and David Reber, researchers in Empa's Materials for Energy Conversion department, have now discovered a way to solve the problem: The salt containing electrolyte has to be liquid, but at the same time it has to be so highly concentrated that it does not contain any "excess" water. For their experiments, the two researchers used sodium FSI (exact name: sodium bis (fluorosulfonyl)imide). This salt is extremely soluble in water: seven grams of sodium FSI and one gram of water form a clear salt solution (see video clip). In this liquid, all water molecules are grouped around the positively charged sodium cations in a hydrate shell, hardly any unbound water molecules are present.