Swiss EMPA researchers have succeeded in doubling the electrochemical stability of water with a special saline solution bringing us one step closer to using the technology commercially. This research could very well lead to a basis for a future with particularly inexpensive rechargeable batteries.

The press release begins by asking during the hunt to find safe, low-cost batteries for the future, eventually we have to ask ourselves a question: Why not simply use water as an electrolyte? Water is inexpensive, available everywhere, non-flammable and can conduct ions. However, water has one major drawback: It is chemically stable only up to a voltage of 1.23 volts. In other words, a water cell supplies three times less voltage than a customary lithium ion cell with 3.7 volts, which makes it poorly suited for applications in an electric car. A cost-effective, water-based battery, however, could be extremely interesting for stationary electricity storage applications.

Ruben-Simon K├╝hnel 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.

Sodium bis(fluorosulfonyl)imide based aqueous electrolytes exhibit a wide electrochemical stability window of up to 2.6 V when the water-to-salt molar ratio falls below 2:1, enabling the fabrication of high-voltage rechargeable aqueous sodium-ion batteries. Image Credit EMPA. Click image for the largest view.

For their experiments, the two researchers used the special salt sodium FSI (precise name: sodium bis(fluorosulfonyl)imide). This salt is extremely soluble in water: seven grams of sodium FSI and one gram of water produce a clear saline solution (see video clip). In this liquid, all water molecules are grouped around the positively charged sodium cations in a hydrate shell, virtually no unbound water molecules remain. Thus making an aqueous sodium ion battery.

The research paper has been published in the American Chemical Society’s Energy Letters.

The researchers discovered that this saline solution displays an electrochemical stability of up to 2.6 volts – i.e. nearly twice as much as other aqueous electrolytes. The discovery could be the key to inexpensive, safe battery cells; inexpensive because, apart from anything else, the sodium FSI cells can be constructed more safely and thus more easily than the familiar lithium ion batteries.

The system has already withstood a series of charging and discharging cycles in the lab. Until now, however, the researchers have been testing the anodes and cathodes of their test battery separately – against a standard electrode as a partner. In the next step, the two half cells are to be combined into a single battery. Then additional charging and discharging cycles are scheduled.

Empa’s research activities on novel batteries for stationary electricity storage systems are embedded in the Swiss Competence Center for Heat and Electricity Storage (SCCER HaE), which coordinates research for new heat and electricity storage concepts on a national level and is led by the Paul Scherrer Institute (PSI). If the experiment succeeds, inexpensive water batteries will be within reaching distance.

This research may have immense implications for the intermittent energy producers. Storing the energy in the face of way too expensive production strongly needs full productive output and metered release to gain more credible traction in the market. For now these alternatives can only operate with government forcing, a prospect that can not last much longer with competitive pressure mounting.


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