By Battery Power Staff
August 9, 2018 | Researchers from the University of Cambridge have been working on a class of materials—niobium tungsten oxides—to use as a battery electrode. Two particular oxides have a complex crystalline structure, and researchers report that lithium ions move through them at rates that far exceed those of typical electrode materials, which equates to a much faster-charging battery. They don’t result in higher energy densities when used under typical cycling rates. The researchers reported their findings last month in Nature (https://doi.org/10.1038/s41586-018-0347-0).
The two oxides adopt crystallographic shear and bronze-like structures, and lithium diffusion happens quickly, even when the sizes of the niobium tungsten oxide particles are of the order of micrometers. The larger size–as opposed to nanoparticles–may be easier to make. “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,” said Kent Griffith, a postdoctoral researcher in Cambridge’s Department of Chemistry and the paper’s first author, in a university press release.
“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,” said Clare Grey, the paper’s senior author, also from the Department of Chemistry. “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. The atomic arrangements are complex, but Griffith 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,” said 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.”
Using a technique called pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy, which is not readily applied to battery electrode materials, 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.
Most negative electrodes in current lithium-ion batteries are made of graphite, which has a high energy density, but when charged at high rates, tends to form dendrites, which can create a short-circuit and cause the batteries to catch fire and possibly explode.
“In high-rate applications, safety is a bigger concern than under any other operating circumstances,” said 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.”
In addition to their high lithium transport rates, the niobium tungsten oxides are also simple to make. “A lot of the nanoparticle structures take multiple steps to synthesise, and you only end up with a tiny amount of material, so scalability is a real issue,” said Griffith. “But these oxides are so easy to make, and don’t require additional chemicals or solvents.”
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.
While the oxides may only be suited for certain applications, 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,” she says. “These interesting materials give us a good insight into how we might design higher rate electrode materials.”