Researchers at Cambridge University have identified a group of materials that could be used to make batteries with higher power.
The team from the university's Department of Chemistry looked at niobium tungsten oxide as part of the search for materials with high rate battery performance: that is, capable of charging faster and delivering higher power output.
Niobium tungsten oxide has a complex atomic arrangement, which according to postdoctoral researcher Kent Griffith, might explain why it has so far been neglected in battery studies. However, the team has found that this unusual structure allows lithium ions to pass through it much faster than conventional materials used in battery electrodes, such as graphite.
In typical batteries, electrons are extracted from the positive electrode during the charge cycle. They then move through its crystal structure and the battery electrolyte to be stored in the negative electrode. The faster this happens, the faster the battery charges. One way that researchers try to speed this process up is to make electrodes from nanoparticles, because this should reduce the distance the ions have to travel. But this has drawbacks: nanoparticle electrodes tend to encourage unwanted chemical reactions in the electrolyte, reducing the battery lifetime, Griffith said. Moreover, as team leader Prof Clare Grey explained, nanoparticles tend to be difficult to make and to pack together efficiently.
Niobium tungsten oxide is not a nanoparticle material. It has a fundamentally different crystal structure from many battery materials, Griffith explained. The structure is held open by “pillars” of oxygen. "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," he said.
The method Grey’s team used to analyse the material performance was also unusual: pulsed field gradient NMR uses short, timed pulses of magnetic field specific coherences, or waves with the same phase difference, frequency and waveform as they propagate through a material. This technique allowed the team to measure the movement of lithium ions through the oxides, revealing that they moved at several orders of magnitude faster than the typical electrode materials.
In a paper in Nature, Grey, Griffith and their colleagues explain that graphite has particular drawback. Although it has a high energy density, in fast charge cycles spindly fibres of metallic lithium known as dendrites form on its surface, and can create short circuits that sometimes lead to fires and explosions.
"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."
Another advantage is that niobium tungsten oxides are relatively easy to make, Griffith said. "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."
A potential drawback is that the cell voltage is lower than with some electrode materials, but the usable energy density remains high. "Fields stagnate if you don't keep looking for new compounds," Grey commented. "These interesting materials give us a good insight into how we might design higher rate electrode materials."
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