A team of scientists from two Chinese universities found the solution to improve the key chemical reaction that occurs in lithium sulfur batteries, responsible for its rapid degradation. The use of mediators that function as an accelerator of the redox reaction in these batteries could solve the key problem of this technology and turn them into a possible replacement of the lithium batteriesat which they significantly improve the energy density.
So far, lithium-ion batteries have been able to meet the energy storage needs of mobile devices and electric vehicles for about three decades. However, are approaching their theoretical limit in terms of energy density. At the same time, the energy storage devices of electric vehicles require ever greater capacities, greater autonomy and contained prices, which requires increasing the energy density by using materials that are abundant in nature.
The useful life of a battery is measured by the number of charge and discharge cycles that it is able to support without an excessive degradation of its energy capacity being appreciated. In the case of lithium-sulfur batteries, some research claims to have achieved several hundred cycles, but all this at the expense of reduce other fundamental parameters such as recharging capacity and power, resilience and even security. The challenge for researchers is to meet the needs of an electric vehicle while respecting each of these parameters and including the cost.
With a theoretical potential to achieve energy densities of up to 2,600 Wh/kg, lithium-sulfur batteries are among the alternatives that scientists are studying with more intensity. They have an attractive chemistry for industry since the active material of the cathode, sulfur, is very abundant in nature, which makes it possible to control the cost of production and increases the ecological sensitivity of this component.
The great handicap of lithium-sulfur batteries is their rapid degradation during charge and discharge cycles, which results in a short lifespan. Unlike lithium-ion batteries, the chemical reaction that occurs inside sulfur batteries leads to the buildup of solid lithium sulfide and lithium polysulfide, which directly results in further battery degradation and therefore in a very limited lifespan.
The cause responsible for this is the slow oxidation-reduction (redox) reaction that occurs inside. This reaction is part of the process of converting chemical energy into electrical energy. This characteristic has impedido que la tecnología alcance su potencial en cuanto a densidad de energía teórica, mucho mayor que las baterías de las iones de litio, y por lo tanto que alcancen a rendimiento similar or superior a las químicas de las baterías que se utilizan currently.
The slow redox reaction is not exactly a small hurdle to overcome. Indeed, the high theoretical energy density in lithium-sulfur batteries comes from reactions between the sulfur cathode and the lithium anode. The slowness of the reaction occurs specifically during the download processwhen sulfur is reduced to dissolved lithium polysulfides and then to solid lithium sulfide, thereby converting chemical energy into electrical energy.
A team of researchers from two Chinese universities worked on this reaction. The results were published in a magazine article Nano-research.
The researchers believe that this problem would be eliminated if the device were subjected to the same level of stress that high energy density applications require. To solve the problem, they designed a variety of “promoter” additives, either sulfur hosts or spacer materials in lithium-sulfur batteries, to speed up the kinetics of the battery and thus improve its reactions.
Still, this is not enough to prevent the gradual decline in battery performance due to solid deposits of lithium sulfide accumulating at active electrocatalytic sites, according to the researchers.
Upon further investigation, the team discovered that soluble redox mediators They are effective in promoting kinetics by chemically reducing or oxidizing lithium polysulfides and then regenerating them at the electrode surface. First, they showed that the use of these redox mediators is an efficient mechanism to accelerate the slow specific kinetics in the button cells or coin cell, small and flat batteries used in devices such as hearing aids, car keys or medical implants.
The next step was finding a redox mediator that would work in lithium bag batteries, which can be used in higher power applications, such as in the automotive industry. To do this, the researchers designed a redox mediator using an organic molecule called 5,7,12,14-pentacenetetrone, or PT, to promote sulfur redox kinetics in high-energy-density lithium-sulfur pocket cells.
“Specifically, the PT redox mediator provides a chemical bypass for the reduction of lithium polysulfide to lithium sulfide, which reduces reaction resistance and improves deposition ability,” explained Bo-Quan Li, researcher at the ‘Beijing Institute of Technology, in a press. Release.
Showing that they could find a way to enhance the redox reaction in a high-energy-density battery leads scientists a step closer to the development of lithium-sulfur batteries capable of reaching their theoretical potential in applications that require high power demand.
The next step of research is to develop more advanced redox mediators with the ultimate goal of achieving “low-cost, high-safety, high-energy-density, long-cycle lithium-sulfur batteries” for applications such as air vehicles, including spacecraft, Li says.