Using a technique called pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy, the researchers measured the movement of lithium ions through the oxides, and found that they moved at rates several orders of magnitude higher than typical electrode materials.
Although these oxides do not result in higher energy densities when used under typical cycling rates, they come into their own for fast charging applications, the researchers add. Additionally, their physical structure and chemical behaviour give insight into how a safe, super-fast charging battery could be constructed, and suggest that the solution to next-generation batteries.
In the search for new electrode materials, researchers normally try to make the particles smaller. "The idea is that if you make the distance the lithium ions have to travel shorter, it should give you higher rate performance," explains Dr Kent Griffith of Cambridge. "But it's difficult to make a practical battery with nanoparticles: you get a lot more unwanted chemical reactions with the electrolyte, so the battery doesn't last as long, plus it's expensive to make."
"Nanoparticles can be tricky to make, which is why we're searching for materials that inherently have the properties we're looking for, even when they are used as comparatively large micron-sized particles. This means that you don't have to go through a complicated process to make them, which keeps costs low," adds Professor Clare Grey of Cambridge. "Nanoparticles are also challenging to work with on a practical level, as they tend to be quite 'fluffy', so it's difficult to pack them tightly together, which is key for a battery's volumetric energy density."
The niobium tungsten oxides used in the current work have a rigid, open structure that does not trap the inserted lithium, and have larger particle sizes than many other electrode materials. Dr Griffith speculates that the reason these materials have not received attention previously is related to their complex atomic arrangements. However, he suggests that the structural complexity and mixed-metal composition are the very reasons the materials exhibit unique transport properties.
"Many battery materials are based on the same two or three crystal structures, but these niobium tungsten oxides are fundamentally different," continues Dr Griffith. The oxides are held open by 'pillars' of oxygen, which enables lithium ions to move through them in three dimensions. "The oxygen pillars, or shear planes, make these materials more rigid than other battery compounds, so that, plus their open structures means that more lithium ions can move through them, and far more quickly."
"In high-rate applications, safety is a bigger concern than under any other operating circumstances," says Prof Grey. "These materials, and potentially others like them, would definitely be worth looking at for fast-charging applications where you need a safer alternative to graphite."
Although the oxides have “excellent” lithium transport rates, they do lead to a lower cell voltage than some electrode materials. However, the operating voltage is beneficial for safety and the high lithium transport rates mean that when cycling fast, the practical (usable) energy density of these materials remains high, says the Cambridge team.
While the oxides may only be suited for certain applications, Prof Grey says that the important thing is to keep looking for new chemistries and new materials. "Fields stagnate if you don't keep looking for new compounds. These interesting materials give us a good insight into how we might design higher rate electrode materials.”
