Salt and water could make a really cheap battery

Salt and water could make a really cheap battery

Technology News |
By Christoph Hammerschmidt

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.

The researchers found out that this salt solution has an electrochemical stability of up to 2.6 volts – almost twice as much as other aqueous electrolytes. The discovery could be the key to affordable and safe battery cells. Inexpensive also because the sodium FSI cells were safer and easier to build than the well-known Li-Ion batteries.
The system has already withstood a series of loading and unloading cycles successfully in the laboratory. Until now, however, the researchers have tested the anodes and cathodes of their test battery separately – against a standard electrode as a partner. In a next step, the two half cells are to be combined into a single battery. Then further charging and discharging cycles are planned. Empa’s research activities on new types of batteries for stationary power storage systems are embedded in the Swiss Competence Center for Heat and Electricity Storage (SCCER HaE), which coordinates research for new heat and power storage concepts at national level and is managed by the Paul Scherrer Institute. If the experiment succeeds, the inexpensive water battery is within reach.

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