Demonstrating a new approach to lithium-ion batteries

A team of researchers from the University of Cambridge, Diamond Light Source and Argonne National Laboratory in the US have demonstrated a new approach that could fast-track the development of lithium-ion batteries that are both high-powered and fast-charging. 

     

In a bid to tackle rising air pollution, the UK government has banned the sale of new diesel and petrol vehicles from 2040, and the race is on to develop high performance batteries for electric vehicles that can be charged in minutes, not hours. The rechargeable battery technology of choice is currently lithium-ion (Li-ion), and the power output and recharging time of Li-ion batteries are dependent on how ions and electrons move between the battery electrodes and electrolyte. In particular, the Li-ion diffusion rate provides a fundamental limitation to the rate at which a battery can be charged and discharged.

Crystal structure and particle morphology of Nb16W5O55 and Nb18W16O93.

a–c, Nb16W5O55 is built from blocks (red rectangles) of 4 × 5 (Nb,W)O6 octahedra, with adjoining blocks forming crystallographic shear planes. (Nb,W)O4 tetrahedra connect the corners of the blocks. a, A view down the b direction of the structure. b, c Electron images of the micrometre-sized particles. d–f, Nb18W16O93 is a superstructure of the tetragonal tungsten bronze (blue) with pentagonal tunnels (grey) partially filled by –W–O– chains that form pentagonal bipyramids. d, A view down the c direction, depicting the various tunnels. e, f, Electron images. In a and d, the black boxes indicate the unit cells.
a–c, Nb16W5O55 is built from blocks (red rectangles) of 4 × 5 (Nb,W)O6 octahedra, with adjoining blocks forming crystallographic shear planes. (Nb,W)O4 tetrahedra connect the corners of the blocks. a, A view down the b direction of the structure. b, c Electron images of the micrometre-sized particles. d–f, Nb18W16O93 is a superstructure of the tetragonal tungsten bronze (blue) with pentagonal tunnels (grey) partially filled by –W–O– chains that form pentagonal bipyramids. d, A view down the c direction, depicting the various tunnels. e, f, Electron images. In a and d, the black boxes indicate the unit cells.

The conventional approach to improving the Li-ion diffusion rate is to create electrodes with nanoscale structures, which decrease the distance Li-ions must travel, and greatly increase the surface area of the electrode that is in contact with the electrolyte. However, there are many downsides to this approach: it reduces the volumetric energy density (the amount of energy the battery can store), and producing electrodes with nano-architectures is slow and expensive, and can produce large amounts of chemical waste products.

In a paper recently published in Nature, researchers have demonstrated a completely different approach to developing new materials for battery electrodes, which does not rely on nanoscaling, but instead goes back to the drawing board to find materials with better ionic diffusion properties.

The idea behind this breakthrough arose from previous research into complex niobium oxides, with the team identifying structures that were associated with favourable diffusion properties, and might therefore offer extremely high diffusion rates with larger, micrometre-sized particles. This led them to investigate two complex niobium tungsten oxides (Nb16W5O55 and Nb18W16O93), using X-ray Absorption Near-Edge Structure) XANES on Diamond’s B18 spectroscopy beamline to analyse the charge storage mechanism.
 

Their results show that these two materials are capable of multi-electron reduction on both niobium and tungsten, leading to high capacities. It was also discovered that the oxides have Li-ion diffusion rates several orders of magnitude above those of current electrode materials, even when the niobium tungsten oxide particles are micrometre sized. This means batteries made with these materials would combine both high power and fast charging properties, and would operate in a similar voltage region to the well-studied, and generally considered ‘safe’ anode materials currently in use.

This work proves that large, micrometre-sized particles can be used for high-rate electrodes with certain atomic structures and illustrates that nanoscaling is not always the most appropriate strategy to improve performance. It has implications for high-power applications, fast-charging devices, all-solid-state energy storage systems, electrode design and material discovery.
 

Related paper

Griffith KJ et al. Niobium tungsten oxides for high-rate lithium-ion energy storage. Nature (2018). DOI: 10.1038/s41586-018-0347-0