Proportional advance for solid-state electrolytes

The modern world relies on portable electronic devices such as smartphones, tablets, laptops, cameras and camcorders. Many of these devices are powered by lithium-ion batteries, which could be smaller, lighter, safer and more efficient if the liquid electrolytes they contain were replaced by solid versions. A promising candidate for such a solid-state electrolyte is a new class of materials based on lithium compounds that has just been developed by physicists from Switzerland and Poland.

Commercially available lithium-ion batteries consist of two electrodes separated by a liquid electrolyte. This electrolyte makes it difficult for engineers to reduce the size and weight of the battery. It is also at risk of leaking, in which case the lithium in the exposed electrodes can come into contact with oxygen in the air and catch fire.

Laboratories have been searching for solid materials capable of replacing liquid electrolytes for years. The most popular candidates include compounds in which lithium ions are surrounded by sulphur or oxygen ions. However, in a paper in Advanced Energy Materials, the Swiss-Polish team of physicists report a new class of ionic compounds where the charge carriers are lithium ions moving in an environment of amine (NH₂) and tetrahydroborate (BH₄) ions.

The experimental part of the research project was carried out at Empa, the Swiss Federal Laboratories for Materials Science and Technology, in Dübendorf and the University of Geneva. The theoretical description of the mechanisms leading to the exceptionally high ionic conductivity of the new material was conducted by Zbigniew Lodziana from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow.

"We were dealing with lithium amide-borohydride, a substance previously regarded as not being too good an ionic conductor," explains Lodziana. "This compound is made by milling two constituents in a ratio of one to three. To date, nobody has ever tested what happens to ionic conductivity when the proportions between these constituents are changed. We were the first to do so and suddenly it turned out that by reducing the number of NH₂ groups to a certain limit we could significantly improve the conductivity. It increases so much that it becomes comparable to the conductivity of liquid electrolytes!"

The several dozen-fold increase in ionic conductivity of the new material, producing by changing the proportion of its constituents, opens up a new, unexplored direction in the search for a candidate for a solid-state electrolyte. Previously, scientists had focused on varying the composition of the electrolyte. It has now become apparent that a key role is played by the proportions of the ingredients used to manufacture the electrolyte.

"Our lithium amide-borohydride is a representative of a promising new class of solid-state electrolyte materials," says Lodziana. "However, it will be some time before batteries built on such compounds come into use. For example, there should be no chemical reactions between the electrolyte and the electrodes leading to their degradation. This problem is still waiting for an optimal solution."

Nevertheless, the research prospects are promising. The scientists from Empa, the University of Geneva and IFJ PAN did not just confine themselves to characterizing the physico-chemical properties of the new material. They also tested it as an electrolyte in a typical Li₄Ti₅O₁₂ half-cell and found that it performed well, proving stable over the course of 400 charge/discharge cycles.

Promising steps have also been taken towards resolving another important issue. The lithium amide-borohydride described in the paper exhibited excellent ionic conductivity only at temperatures of about 40°C; in the most recent experiments, the physicists have already lowered this to below room temperature.

Theoretically, however, the new material remains a challenge. Hitherto, models have been constructed for substances in which the lithium ions move in an atomic environment. In the new material, ions move among light molecules that adjust their orientation to ease the movement of the lithium ions.

"In the proposed model, the excellent ionic conductivity is a consequence of the specific construction of the crystalline lattice of the tested material," says Lodziana. "This network in fact consists of two sub-lattices. It turns out that the lithium ions are present here in the elementary cells of only one sub-lattice. However, the diffusion barrier between the sub-lattices is low. Under appropriate conditions, the ions thus travel to the second, empty sub-lattice, where they can move quite freely."

This theoretical description explains only some of the observed features of the new material; the mechanisms responsible for its high conductivity are certainly more complex. Nevertheless, the physicists’ enhanced understanding should significantly accelerate the search for optimal compounds for a solid-state electrolyte and consequently shorten the process of commercialization of new power sources.

This story is adapted from material from the Institute of Nuclear Physics of the Polish Academy of Sciences, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.