Researchers from the University of Cambridge, with colleagues from Argonne National Laboratory in the US and Diamond Light Source, Harwell Science and Innovation Campus, UK, have identified a group of materials—niobium tungsten oxides—that could be used to make even higher power batteries.

The researchers, led by Professor Clare Grey, found that lithium ions move through the materials at rates that far exceed those of typical electrode materials, which equates to a much faster-charging battery. Their paper is published in the journal Nature.

Charge, energy, and power densities of micrometer-sized Nb₁₆W₅O₅₅ and Nb₁₈W₁₆O₉₃ compared to state-of-the-art high-rate nanoscale materials and formulations. Griffith et al.

The maximum power output and minimum charging time of a lithium-ion battery depend on both ionic and electronic transport. Ionic diffusion within the electrochemically active particles generally represents a fundamental limitation to the rate at which a battery can be charged and discharged. To compensate for the relatively slow solid-state ionic diffusion and to enable high power and rapid charging, the active particles are frequently reduced to nanometre dimensions, to the detriment of volumetric packing density, cost, stability and sustainability.

As an alternative to nanoscaling, here we show that two complex niobium tungsten oxides—Nb₁₆W₅O₅₅ and Nb₁₈W₁₆O₉₃, which adopt crystallographic shear and bronze-like structures, respectively—can intercalate large quantities of lithium at high rates, even when the sizes of the niobium tungsten oxide particles are of the order of micrometers.

Measurements of lithium-ion diffusion coefficients in both structures reveal room-temperature values that are several orders of magnitude higher than those in typical electrode materials such as Li₄Ti₅O₁₂ and LiMn₂O₄. Multielectron redox, buffered volume expansion, topologically frustrated niobium/tungsten polyhedral arrangements and rapid solid-state lithium transport lead to extremely high volumetric capacities and rate performance.

Unconventional materials and mechanisms that enable lithiation of micrometre-sized particles in minutes have implications for high-power applications, fast-charging devices, all-solid-state energy storage systems, electrode design and material discovery.

—Griffith et al.

Although these materials do not result in higher energy densities when used under typical cycling rates, they come into their own for fast charging applications. Additionally, their physical structure and chemical behavior give researchers a valuable insight into how a safe, super-fast charging battery could be constructed, and suggest that the solution to next-generation batteries may come from unconventional materials.

When a lithium-ion battery is charging, lithium ions are extracted from the positive electrode and move through the crystal structure and electrolyte to the negative electrode, where they are stored. The faster this process occurs, the faster the battery can be charged.

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. 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.

—Kent J. Griffith, lead author

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. 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.

—Professor Grey

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. 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. 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.

—Kent Griffith

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. 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.

—Professor Grey

In addition to their high lithium transport rates, the niobium tungsten oxides are also simple to make.

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.

The research was funded in part by the European Union, the Science and Technology Facilities Council, and the Engineering and Physical Sciences Research Council.

Resources

• Kent J. Griffith, Kamila M. Wiaderek, Giannantonio Cibin, Lauren E. Marbella & Clare P. Grey (2018) “Niobium tungsten oxides for high-rate lithium-ion energy storage” Nature volume 559, pages 556–563 doi: 10.1038/s41586-018-0347-0