Using neutrons for solid-state batteries

Solid-state batteries are already being used in smartphones because they charge faster and are safer than batteries with liquid electrolytes. © FRM II/ TUM

Solid-state batteries are already being used in smartphones because they charge faster and are safer than batteries with liquid electrolytes. © FRM II/ TUM

Electrolytes in batteries are responsible for storing and releasing electricity by ensuring the exchange of ions between the anode and cathode. Solid-state electrolytes have been the subject of intensive research for some time now, as they could be a more stable and safer alternative to liquid electrolytes. With the help of neutrons, a group from Shanghai has gained important insights into the relationship between the lattice structure and ionic conductivity of such solid-state electrolytes.

An essential requirement of an electrolyte is to allow the exchange of ions between the anode and the cathode. That is not-critical for liquid electrolytes, as the ions can flow freely within the solution. However, safety precautions are necessary due to the highly flammable and difficult-to-extinguish components (especially lithium). These precautions come with high costs, especially for larger energy storage systems. Solid-state electrolytes could be a solution to this problem. The property that one exploits in solid-state electrolytes is the superionic conductivity. One class of materials with this property are argyrodite-like crystal compounds, i.e. compounds with a similar lattice structure as the mineral argyrodit. To understand the original driving force of superionic conductivity in more detail, the group led by Professor Jie Ma from Shanghai Jiao Tong University in Shanghai took a closer look at Ag~8~SnSe~6~.

Complex dynamics makes research difficult
Superionic compounds, such as Ag~8~SnSe~6~, have complex structures that give rise to properties that change depending on the temperature. For example, the superionic conductivity of Ag8SnSe6 only occurs at temperatures above 355 K (approx. 82 °C). In addition, the material has a very low thermal conductivity, which could be attractive for thermoelectric applications. It has long been assumed that there is a connection between these two properties. All these interconnected effects are challenging to disentangle experimentally, making it difficult for researchers to unveil the underlying structural mechanisms.

Tracking lattice vibrations with neutrons
Professor Jie Ma’s group examined Ag~8~SnSe~6~ with X-rays and neutrons. Using X-rays, they found structural changes in the crystal lattice at approx. 82 °C, which are related to a phase transition to superionic conductivity. On the other hand, the crystal’s dynamic processes concern the atoms’ vibrations in the crystal lattice around their resting position. Jie Ma’s group investigated this in more detail using inelastic neutron scattering at the TOFTOF at MLZ and the AMATERAS spectrometer at the Japanese spallation neutron source J-PARC.

The measurements show inharmonic lattice vibrations between the silver atoms and the rest of the crystal structure. At temperatures above 82 °C, the silver atoms can hop from lattice site to lattice site and thus flow through the crystal. “The results confirm that the ultra low thermal conductivity and superionic conduction can be attributed to these inharmonic lattice vibrations. However, both phenomena occur at different temperatures and are not, as previously assumed, dependent on each other,” explains Jie Ma. “The work shows nicely how measurements with neutrons and X-rays complement each other and provide important impulses for optimizing these materials,” says Marcell Wolf, who was involved in parts of the measurements as an instrument scientist at TOFTOF.