How making plastic salts is like playing tetris on hard-mode

My mum thought I was wasting time playing Tetris on the old Nintendo Gameboy. But all that rotating and positioning of blocky shapes has just helped me understand the latest breakthrough in the chemistry of ionic plastic crystals— a new class of material that could revolutionise battery and fuel cell technologies.

All batteries need an electrolyte—a material that lets ions pass through it but not electrons. In traditional batteries the electrolyte has always been liquid, like battery acid. The problem with liquid electrolytes is they can leak, and most give off toxic fumes.

But what if you could make the electrolyte from a solid instead?

Organic ionic plastic crystals may be the answer. They are a solid that conduct ions, they don’t give off fumes and are stable at temperatures well above 100 C.

Though ionic plastic crystals hold much promise, one problem is the rate that ions can flow through them is low compared to liquid electrolytes, and this limits battery output. The ionic conductivity can also be highly variable, and scientists didn’t know why.

Now, reporting in the prestigious Journal of the American Chemical Society, scientists have found out the origin of the variability, plus a way to boost the ionic conductivity tenfold.

Konstantin Romanenko, at the Institute for Frontier Materials at Deakin University, along with colleagues at the ARC Centre of Excellence for Electromaterals Science, has discovered that the key parameter was the speed at which the crystals were cooled from the melt-state during their synthesis.

When cooled rapidly, the crystals formed disordered arrangements because the crystal grains don’t have time to grow before bumping into another grain. Just like when I tried playing Tetris on ‘hard mode’, the result was a stack of misaligned blocks: game over.

But cooling the crystals slowly is a much more sedate affair. Like playing Tetris on ‘easy mode’, the molecules have time to grow and fit together to build up an ordered structure.

Romanenko found that the slow cooled crystals, with their more ordered structure, have much higher ionic conductivities. This is because the aligned grains provide long pathways for the ions to flow through, especially in the direction of alignment. This higher conductivity makes them much better electrolytes for batteries.

The team figured this out by using MRI imaging to visualize the crystal grains in three dimensions—a technique which has been used to image polymers, prostheses and bones, but which has never been used to image plastic salts before.

Besides batteries, the improved solid electrolytes could also be perfect for dye-sensitised solar cells, which spend their lives baking in the sun and have to be very cleverly designed so that their liquid electrolyte does not escape in the heat.

“The next stage of this research will be to develop ways to control the crystal alignment and to incorporate lithium ions to achieve even higher conductivities,” said Dr Romanenko.