A team of scientists from two chinese universities has found the solution to improve the key chemical reaction that occurs in lithium-sulfur batteries, responsible for their rapid degradation. The use of mediators that work 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 lithium batteries, to which they improve significantly in energy density.
Until now, lithium-ion batteries have been able to meet the energy storage needs of mobile devices and electric vehicles for about three decades. Nevertheless, are approaching their theoretical limit in terms of energy density. Meanwhile, the energy storage devices of electric vehicles require ever greater capacities, greater autonomies and contained prices, which requires increasing energy density 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 capable of withstanding without an excessive degradation in its energy capacity being appreciated. In the case of lithium-sulfur batteries, there is some research that claims to have achieved several hundred cycles, but all of it at the expense of reduce other fundamental parameters such as recharging capacity and power, resiliency and even safety. The challenge for researchers is to meet the needs of an electric vehicle while meeting each of these parameters and including cost.
With a theoretical potential to reach energy densities of up to 2,600Wh/kg, lithium-sulfur batteries are one of the alternatives that scientists are studying with greater intensity. They have an attractive chemistry for the 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 translates into a short useful life. 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 useful life.
The cause responsible for this to occur 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 prevented the technology from reaching its potential in terms of theoretical energy density, much higher than that of lithium-ion batteries, and therefore from achieving similar or superior performance to the chemistries of the batteries that are used. currently.
The slow redox reaction is not exactly a small obstacle to overcome. This is because the high theoretical energy density in lithium-sulfur batteries originates 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, thus converting chemical energy into electrical energy.
A team of researchers from two Chinese universities has worked on this reaction. The results have been published in an article in the journal nanoresearch.
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 interlayer materials in lithium-sulfur batteries, to speed up the battery’s kinetics and thus improve its reactions.
Still, it’s not enough to prevent the gradual decline in battery performance due to solid lithium sulfide deposits that build up at active electrocatalytic sites, according to the researchers.
Digging deeper into their investigations, the team found that soluble redox mediators They are effective in promoting kinetics by chemically reducing or oxidizing lithium polysulfides and then regenerating them on the electrode surface. Initially, they showed that the use of these redox mediators is an effective mechanism to accelerate the slow specific kinetics in the button or button cell batteries, small and flat batteries used in devices such as hearing aids, car keys, or medical implants.
The next step was find a redox mediator that would work in lithium bag cell batteries, which can be used in higher power applications, as is the case in the automotive industry. To accomplish 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 energy-dense 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, a researcher at the Institute of Technology of Beijing, 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 that can reach their theoretical potential in applications that require a high power demand.
The next step of research is to develop more advanced redox mediators with the ultimate goal of achieving “high-energy-density, long-cycle lithium-sulfur batteries with low costs and high safety” for applications such as aerial vehicles, including spacecraft, Li explains.
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