Lithium-Based Materials are Promising Candidates for Solid-State Electrolyte

Today’s world depends on portable electronic devices such as camcorders, cameras, laptops, tablets or smartphones. Several of these devices are powered by lithium-ion batteries, which could be safer, lighter, smaller and more efficient if the liquid electrolytes they comprise of were replaced by solids.

A new class of materials based on lithium compounds, presented by Physicists from Switzerland and Poland, is considered to be a promising candidate for a solid-state electrolyte.

Lithium amide-borohydride is a promising candidate for a solid electrolyte. The crystalline structure of this material consists of two sub-lattices, shown in different colors. Under appropriate conditions, lithium ions (red), normally found in the elementary cells of only one sub-lattice (yellow), move to the empty cells of the second sub-lattice (blue) where they can freely propagate. (Source: IFJ PAN)

Lithium-ion batteries that are commercially available comprise of two electrodes connected by a liquid electrolyte. This electrolyte makes it tough for Engineers to reduce the weight and size of the battery, and additionally, it is subjected to leakage; the lithium present in the exposed electrodes then gets in touch with oxygen in the air and experiences self-ignition. Boeing's troubles, which for several months caused a full grounding of Dreamliner flights, are considered to be a remarkable example of the problems brought about by the use of modern lithium-ion batteries.

For years, laboratories have been searching for solid materials that have the potential to replace liquid electrolytes. The most common candidates comprise of compounds in which lithium ions are surrounded by sulfur or oxygen ions. However, a Swiss-Polish team of Scientists, in the journal Advanced Energy Materials, have presented a new class of ionic compounds where the charge carriers are lithium ions traveling in an environment of amine (NH2) and tetrahydroborate (BH4) ions.

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

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. 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!

Prof. Zbigniew Lodziana, the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN), Cracow

A new, unexplored direction in the search for a candidate for a solid-state electrolyte is opened up by the several dozen-fold boost in ionic conductivity of the new material – the effect of a change in the proportion of its constituents. Earlier, all over the world, the focus was almost wholly on variations in the composition of the chemical substance. Now it has become obvious that, at the production stage of the compound, a vital role can be played by the proportions themselves of the ingredients used for manufacturing them.

Our lithium amide-borohydride is a representative of a promising new class of solid-state electrolyte materials. 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.

Prof. Zbigniew Lodziana, the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN), Cracow

The prospects of the research prove to be promising. The Scientists from Empa, UG and IFJ PAN did not restrict themselves to only characterizing the physico-chemical properties of the new material. The compound was utilized as an electrolyte in a typical Li4Ti5O12 half-cell. The half-cell functioned well, in tests of running down and recharging 400 times it established its potential to be stable. Promising steps have been executed in order to resolve another significant issue. Only at about 40 °C did the lithium amide-borohydride described in the publication exhibit exceptional ionic conductivity. This has already been lowered to below room temperature in the latest experiments.

However, the new material theoretically remains a challenge. Until now, models have been developed for substances in which the lithium ions travel in an atomic environment. Ions travel among light molecules that alter their orientation to ease the lithium movement in the new material.

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 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.

Prof. Zbigniew Lodziana, the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN), Cracow

Only some of the observed features of the new material are explained by the theoretical description presented here. The mechanisms responsible for its elevated conductivity are indeed more complex. Their enhanced understanding should majorly speed up the search for optimal compounds for a solid-state electrolyte and subsequently shorten the process of commercialization of new power sources that are mostly expected to transform portable electronics.

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