Tohoku University can offer a solution to the current strong demand to replace organic liquid electrolytes used in conventional rechargeable batteries, with solid-state ionic conductors which will enable the batteries to be safer and have higher energy density.

To get there a lot of effort has been devoted to finding materials with superior ionic conductivities. Among the most promising, are solid-state ionic conductors that contain polyanions such as B12H122-. They constitute a particular class of materials due to their unique transport behavior, which has the polyanions rotating at an elevated temperature, thereby greatly promoting cation conductivities.

Typical polyanions found in solids. (a) B12H122-, (b) MoH93-, and (c) OsH82-. â’¸Shigeyuki Takagi. Image Credit: Tohoku University. Click image for the largest view.

However, a major drawback is the high temperature (equaling energy) required to activate the rotation, which conversely on turn means low conductivities at room temperature.

To address that problem, a research group at Tohoku University, led by Associate Professor Shigeyuki Takagi and Professor Shin-ichi Orimo, has established a new principle for room-temperature superionic conduction. The group’s findings were recently published in Applied Physics Letters.

The research group was able to reduce the activation temperature by using transition metal hydride complexes as a new class of rotatable polyanions, wherein hydrogen is the sole ligand species, covalently binding to single transition metals. Unlike in B12H122- polyanions, the rotation of transition metal hydride complexes only requires displacements of highly mobile hydrogen and can therefore be expected to occur with low activation energy.

The group then studied the dynamics of transition metal hydride complexes in several existing hydrides, and found them reoriented – as if rotating by repeating small deformations  – even at room temperature.

This kind of motion is known as “pseudorotation,” and is rarely observed in solid matter. Due to the small displacements of hydrogen atoms, the activation energy of the pseudorotation is relatively low – more than 40 times lower than what’s reportedly needed for the rotation of B12H122-.

As a result of a cation conduction being promoted from a low temperature region by pseudorotation, the lithium ion conductivity in Li5MoH11 containing MoH93-, for example, can reach 79 mS cm-1 at room temperature. This is more than three times the world record of room-temperature lithium ion conductivity reported so far. This suggests that an all-solid-state lithium ion battery with shorter charging time at room temperature can be realized.

The discovered mechanism is quite general and would be useful in lowering the temperature required to activate the rotation of polyanions. This may positively contribute towards finding compositions that are amenable to room-temperature superionic conductors.

Your humble writer is aware this is a really technically written post. The issue with ionic conductors is the energy needed to get them warmed up and keep them warmed up – something that is simply a killer in practical applications. But the improvements to capacity *if* they can run at or below, say, room temperature would be a massive improvement in consumers’ ability to transport power with them. Most large markets for batteries would be improved dramatically.

There is now a path to, as the press release said, “This is more than three times the world record of room-temperature lithium ion conductivity reported so far.” Now that would be a really worthwhile improvement.


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