Our modern society relies on portable electronic devices like smartphones, tablets, laptops, cameras or camcorders. These devices are powered by lithium-ion batteries that could be smaller, lighter, safer and more efficient of the liquid electrolytes contained in the batteries were replaced by solids. A promising candidate for solid-state electrolyte is a new class of materials that are based on lithium compounds.
The commercially available lithium-ion batteries are made of two electrodes connected by a liquid electrolyte. The electrolyte makes it difficult for engineers to reduce the size and weight of the battery and it is subjected to leakages. The lithium in the exposed electrodes then comes into contact with oxygen and undergoes self-ignition.
Laboratories have been searching for solid materials that are capable of replacing liquid electrolytes for many years. The popular candidates include compounds in which lithium ions are surrounded by Sulphur or oxygen ions. But a team of scientists has developed a new class of ionic compounds that the charge carriers are lithium ions moving in an environment of amine (NH2) and tetrahydroborate (BH4) ions.
"We were dealing with lithium amide-borohydride, a substance previously regarded as not being too good an ionic conductor. This compound is made by milling two constituents in a ratio of 1 to 3. To date, nobody has ever tested what happens to ionic conductivity when the proportions between these constituents are changed," said Prof. Lodziana, a professor at the Institute of Nuclear Physics of the Polish Academy of Sciences in Cracow and person responsible for the theoretical description of the mechanisms. "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 scientists from Empa, UG and IFJ PAN didn’t just focus on characterizing the physicochemical properties of the new material. The compound was used as an electrolyte in a typical Li4Ti5O12 half-cell. The half-cell performed well in tests of running down and recharging 400 times and it proved to remain stable. The lithium amide-borohydride showed excellent iconic conductivity at about 40˚C. In recent experiments, this has already been lowered to below room temperature.
This new material is still a challenge. Hitherto models have been constructed for substances where the lithium ions move in an atomic environment. In the new material, ions move among light molecules that adjust orientation to ease the lithium movement.
"In the proposed model, the excellent ionic conductivity is a consequence of the specific construction of the crystalline lattice of the tested material. This network 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," explained Prof. Lodziana.
The theoretical description explains only some of the observed features of the new material. The mechanisms that are responsible for high conductivity are more complex. Their better understanding should significantly accelerate the search for optimal compounds for a solid-state electrolyte and shorten the process of commercialization of a new power source that is most likely to change portable electronics.
A paper on this research was published in Advanced Energy Materials.