After oxygen, silicon is the most abundant element in the Earth’s crust. Its reserves are practically inexhaustible and it is cheap. When researchers look at an alternative to the current lithium-ion battery, it makes sense that they turn to silicon. Silicon-air batteries are not only smaller and lighter than their lithium-ion counterparts, but also have a much higher energy density. In addition, they are insensitive to external influences and environmentally friendly. Thus far, they have however only achieved relatively short running times. Scientists at Jülich’s Institute of Energy and Climate Research (IEK) reckon they have finally discovered why.
Questions as to why the flow of current in silicon-air batteries stops after a relatively short period have included:
- Is there a problem with the air electrode?
- Does a protective layer form spontaneously on the silicon anode?
- Is the electrolyte at all suitable?
Sadly, attempts at improving the components to rectify this problem have failed. The only solution that worked to a certain degree was using a special, high-quality electrolyte based on an ionic liquid. Although this has increased the battery’s running time to several hundred hours, it was not a viable alternative due to the cost. The reason for the study is after all to provide a cost-effective alternative to lithium-ion batteries.
As part of the AlSiBat project funded by Germany’s Federal Ministry of Education and Research, scientists at IEK tackled the problem from another angle. Rather than questioning the suitability of the electrolyte, they suspected that the consumption of the electrolyte was the cause for the short running time. To test this theory, a pump system was developed to refill the electrolyte fluid – potassium hydroxide dissolved in water – from time to time.
Hermann Tempel from the IEK’s Fundamental Electrochemistry department claims that the battery will continue running as long as the silicon anode remains in contact with the electrolyte. With this new design, the battery has clocked up an impressive 46 days, or 1,100 hours of running time. It only stops when the silicon anode is fully used up. The battery has to be ‘recharged’ mechanically by replacing the anode.
Although the study has proven a principle, the battery is not yet perfect and Tempel’s next step is to look for a way to keep the battery running without having to refill the electrolyte. Adding an additive to the electrolyte might prevent the electrolyte fluid from being used up.