Suppressing water splitting for "high" voltage aqueous ion batteries

F.R. Hughson, R. Borah, J.H. Johnston, T. Nann
Victoria University of Wellington,
New Zealand

Keywords: batteries, electrolytes, aqueous, supercapacitors, flow batteries, ion batteries, electrochemical window

Summary:

Commercial rechargeable ion batteries use flammable and often toxic organic electrolytes. Water is an attractive alternative for ion batteries but it has a relatively narrow electrochemical window. Above a potential difference of 1.23 V, water is undesirably electrolysed into hydrogen and oxygen. Where a standard lithium ion battery can achieve a cell potential of above 4 V, conventional aqueous ion batteries are practically limited to a potential of approximately 1 V. This results in a 4-fold reduction in the energy density of the cell. We have discovered a series of cheaply available additives that innovatively extend the electrochemical window of a water-based electrolyte to as large as 5 V. This new electrolyte can be used in a variety of devices from ion batteries to flow batteries and supercapacitors. As a result of the extended electrochemical window, this new electrolyte consistently outperforms similar aqueous systems. This electrolyte is non-toxic, non-flammable and non-moisture sensitive. This means that all devices can be prepared in air, on a bench-top and retain full performance. In our electrolyte the extended electrochemical window can be observed through cyclic voltammetry. Oxidation is observed only above 2.5 V (vs. Ag/AgCl) whereas the onset of the reduction is approximately -2 V (vs. Ag/AgCl). This means that any redox reaction whose potential lies within this window can be used in a battery. The larger the window, the greater number of redox couples that can be used which increases the versatility of the system as well as determining the maximum cell potential that can be achieved. For example, a lithium ion battery constructed using this new electrolyte achieved a discharge capacity of 140 mAh/g after 100 cycles which out-performs literature results for aqueous batteries by a large margin. This new electrolyte would significantly reduce production costs of lithium ion batteries and other similar devices. The electrolyte is also environmentally friendly, using only easily sourced, non-hazardous materials. Therefore, the battery would be completely safe to use along with all its components being recyclable.