Recently, the team of Ma Cheng, a professor at the University of Science and Technology of China, designed a new type of fluoride-ion solid electrolyte, the perovskite fluoride-ion conductor, which for the first time achieved a stable long-term cycle of an all-solid-state fluoride-ion battery at room temperature. After continuous charge and discharge at 25°C for 4581 hours, the capacity did not significantly decay. Related research results were recently published in Small.

This achievement has created a world record for the longest cycle time and highest capacity retention rate in the field of all-solid-state fluorine-ion batteries, which gives people hope for the diversified development of batteries in the future.

Turn "impossible" into "possible"

"The most important significance of this achievement is that it is a 'from 0 to 1' breakthrough." Ma Cheng introduced that due to the lack of suitable electrolytes, fluoride-ion batteries have not been favored by the industry for a long time, and related research is extremely rare. The discovery of new solid-state electrolytes has turned "impossible" into "possible".

Three years ago, "Science" reported an organic liquid electrolyte that can transport fluoride ions, which was hailed as a "milestone" work for fluoride ion batteries. However, the fluoride-ion battery composed of it can only achieve stable charge and discharge for less than 10 cycles at room temperature, so there is still a long way to go before practical application.

According to Ma Cheng, it is extremely difficult to construct a liquid electrolyte that can transport fluoride ions, and even if it succeeds, there will be safety hazards. If all-solid-state batteries can be constructed using non-flammable inorganic solid-state electrolytes, it will undoubtedly be more practical. However, this technical route is quite challenging. Most of the fluoride-ion solid-state electrolytes have low ionic conductivity and can only work at high temperatures. Although a few all-solid-state fluoride-ion batteries can be charged and discharged at room temperature, their electrochemical window is extremely narrow. The capacity decays to almost zero after less than 10 charges and discharges, which has no practical application value.

In the case of a "dead knot" in the liquid electrolyte of the fluoride-ion battery, can it avoid the liquid electrolyte and directly explore the solid electrolyte?

"The smaller the ion and the less charge it is, the more likely it is to migrate rapidly in the material and become a suitable battery carrier." Ma Cheng said that as a carrier of a solid electrolyte, lithium ions are the cations with the smallest radius and the least charge except hydrogen cations, and fluoride ions are the anions with the smallest radius and the least charge except hydrogen anions. In the absence of a better cation than lithium ion, fluoride ion, as the closest anion to lithium ion, is a direction worth trying.

Since there are not many cases to learn from, Ma Cheng's team almost started from "zero". They developed a new type of fluoride-ion solid electrolyte, the perovskite fluoride-ion conductor, which took two years to develop. It adopts a perovskite structure that is particularly conducive to anion transport. It has a high ionic conductivity and a wide electrochemical window. It has broken through the major technical bottleneck of "high ionic conductivity" and "wide electrochemical window" in the past, and its stability against moisture far exceeds that of sulfide and chloride solid-state electrolytes commonly used in all-solid-state lithium batteries.

The performance of the fluoride-ion battery based on this solid electrolyte far exceeds that of the fluoride-ion battery based on the liquid electrolyte reported in Science. Industry insiders believe that this important breakthrough allows people to see the possibility of the practical application of all-solid-state fluorine-ion batteries.

This field "has a bright future"

"This is a field with many challenges, but the prospect is extremely attractive." Ma Cheng said that what brings him the most pleasure in research is to overcome these "seemingly impossible" challenges.

Compared with lithium-ion batteries using liquid electrolytes, the energy density and safety performance of all-solid-state lithium batteries have been greatly improved. However, compared with all-solid-state fluorine-ion batteries, the room for improvement in energy density, safety performance, and raw material supply is still quite limited.

Ma Cheng revealed that the theoretical energy density of all-solid-state fluorine-ion batteries can reach up to 5,000 watt-hours per liter, which is about eight times the energy density of current commercial lithium-ion batteries, and also exceeds that of lithium-air batteries under development.

In terms of safety performance, the short circuit caused by the growth of lithium dendrites in all-solid-state lithium batteries has always been an insurmountable bottleneck. Fluorine is the most electronegative element, it is extremely difficult to transform into the corresponding simple substance, and it is not easy to form lithium dendrites. Therefore, the safety performance of fluorine-ion batteries based on non-flammable inorganic solid-state electrolytes is undoubtedly better.

In addition, the crustal abundance of fluorine is about 50 times that of lithium, and the pressure on the supply of raw materials for fluorine-ion batteries is much lower than that of lithium-ion batteries. Ma Cheng told the "Chinese Journal of Science" that my country's fluorite (main component calcium fluoride) resources have obvious advantages in the world, and fluoride-ion batteries can make full use of this advantage.

"Xiaohe just showed her sharp corners"

"The all-solid-state fluoride-ion battery is like a newborn baby." Ma Cheng said frankly.

According to Ma Cheng's analysis, it is extremely difficult to construct a liquid electrolyte for fluoride-ion batteries, and it is easy to catch fire and has a high safety risk. In addition, the overall performance of solid-state electrolytes is worrying, and the research force is quite weak. "Only by paying more attention to this field can we break through the relevant bottlenecks."

All-solid-state fluorine-ion batteries are composed of solid-state electrolytes, positive electrode materials, and negative electrode materials. Only when the three have excellent performance at the same time can this kind of battery be put into practical application. Ma Cheng said that the new material they reported this time overcomes the bottleneck of solid electrolytes, but there is still no positive and negative electrode material with satisfactory performance. This will also become the key research direction of the research group in the future.

"For all-solid-state fluoride-ion batteries to truly enter the 'home of ordinary people', not only a breakthrough in basic science is required, but also a comprehensive consideration of cost and sustainable development is required, so it is still a long process." Ma Cheng said, "The discovery of perovskite fluoride-ion conductors brings hope to solve these problems. Once successful, all-solid-state fluoride-ion batteries will bring disruptive changes to fields that rely heavily on battery technology, such as new energy vehicles and energy storage, with excellent safety and high energy density." (Gui Yun'an)

[Source: Chinese Journal of Science]

Disclaimer: This article is reproduced for the purpose of conveying more information. If there is an error in the source labeling or violation of your legal rights, please contact the author with the proof of ownership, and we will correct and delete it in time, thank you.