Normally, when liquids solidify, their molecules become locked in place, making it much harder for ions to move and leading to a steep decrease in ionic conductivity. Now, scientists have synthesised a new class of materials, called state-independent electrolytes (SIEs), that break that rule.

The team have achieved this result by designing a new class of organic molecular ions with special physical and electronic properties. Each molecule has a flat, disc-shaped centre surrounded by long flexible sidechains—like a wheel with soft bristles. Positive charge is spread out evenly across the molecule by the movement of electrons, which prevents it from tightly binding with its negatively charged partner. This allows the negative ions to move freely, flowing through the side-chains (the ‘soft bristles’).

Then, in the solid state, these organic ions naturally stack on top of each other, forming long rigid columns surrounded by many flexible arms: much like static rollers in a car-wash (see diagram in multimedia gallery). Despite forming an ordered structure, the flexible side chains still create enough space for the negative ions to continue moving as freely as they would in a liquid.

The result: a dynamic ordered structure that allows the negatively charged ions to move through just as easily in the solid state as in the liquid form, with no sharp decrease in ionic conductivity.

Lead author Professor Paul McGonigal (University of Oxford) says: “We designed our materials hoping that ions would move through the flexible, self-assembled network in an interesting way. When we tested them, we were amazed to find that the behaviour is unchanged across liquid, liquid-crystal, and solid phases. It was a really spectacular result – and we were happy to find it can be repeated with a few different types of ions.”

PhD student Juliet Barclay, first author on the study, says: “As a PhD student, it’s incredibly rewarding to discover something that changes how we think materials can work. We’ve shown that organic materials can be engineered so that the movement of ions doesn’t ‘freeze out’ when the material solidifies. This opens new possibilities for safer, lightweight solid-state devices that work efficiently over wide temperature ranges.”