More power to you

Worldwide, two-thirds of the population – more than 5 billion people – now have a cell phone, tablet or some other mobile device. 

Most of these devices rely on a lithium-ion (Li-ion) rechargeable battery, a technology that is also increasingly used to power electric vehicles and as part of renewable energy installations.  

The UK is set to ban petrol and diesel vehicle sales from 2040, grid-scale renewable energy projects are increasing, and there's no end in sight to demand for consumer electronics. Our increasing need for energy storage is driving research into higher-performance batteries, and one area of focus is safe and rapid recharging. 

New anode materials are needed, and an international team think they may have just the thing. And it looks like they are on to something…

The current generation of Li-ion batteries uses a metal oxide positive electrode (cathode) and a graphite negative electrode (anode). Using graphite limits how fast the battery can recharge and how much power it can store. To allow rapid charging, lithium ions must move quickly through the electrode materials. The crystal structure of most battery materials doesn't give lithium ions enough mobility, and the usual way around this is to reduce the particle size down to the nanoscale so that the ions have shorter distances to travel.  

However, nanoparticles are expensive and difficult to produce in commercial quantities. Around 100,000,000 kilograms of graphite are used in batteries each year, and that quantity is expected to triple by 2025. 

New anode materials are needed, and an international team from the University of Cambridge, Diamond Light Source, the UK’s synchrotron, and at the Advanced Photon Source in the US, think they may have just the thing. And it looks like they are on to something having just been awarded the prestigious 2019 Charles Hatchett Award for their research.  

It’s elemental…  

Niobium has wide-ranging uses in everyday life, and in high-tech applications. It is used as an alloying element in, for example, steel bridges and buildings, jet engines and car body parts, medical prosthetics and magnets, and energy storage and electronic components. CBMM, the world's leading niobium producer, sponsors the Charles Hatchett Award to recognise and celebrate the importance of niobium and its contributions to civilisation.  

Their winning paper, published in Nature,  presents initial results on two complex niobium-tungsten oxides that show greater energy and power densities than current battery materials. The selection process for the award focuses on both technical excellence and originality and the social, economic and environmental advantages of the research. The 2018 Award also went to a paper on energy storage, recognising a trend in new applications for niobium in emerging technologies. 

Replacing graphite 

The University of Cambridge team studied niobium tungsten oxides, which can store an unexpectedly large quantity of energy. At Diamond, they used the Core X-ray Absorption Spectroscopy (XAS) beamline to investigate the chemical changes that occur in battery materials containing niobium and tungsten during use. Using XAS allowed them to distinguish each type of ion and the changes to its chemical state while the battery was in operation. 

Principal Beamline Scientist at Diamiond, Giannantonio Cibin explains:?"We need to develop new electrode materials for batteries that will improve both charge/discharge rates and increase storage capacities. This is really important for growing markets such as electric vehicles, portable appliances and large-scale energy storage. This research highlights why two complex niobium-tungsten oxides show higher energy and power densities than those in battery materials currently available."? 

Research into other battery materials that contain more than one type of ion has generally shown that the different ions are not active at the same time and that some are not active at all. In this work, the team have demonstrated that niobium and tungsten ions store 'extra' charge in the battery – that is to say, beyond what is expected – and that the ions work together. This research has shown that niobium tungsten oxides can offer high-rate battery performance with large-scale particles that can be synthesised more easily.

The research team performed electrochemical tests on electrodes made from niobium tungsten oxides at different applied currents for up to 1000 discharge/charge cycles. The results showed that these electrodes can store more than one lithium ion for each metal cation, allowing the battery to store more charge. This unusual effect is called multielectron redox. In order to understand why it occurs, the team needed to understand the separate roles played by the niobium and tungsten ions. 

X-ray absorption spectroscopy (XAS) is an advanced characterisation technique able to distinguish between different elements and to track the changes that occur during chemical reactions. In this work, the research team used XAS to see what happens when a niobium tungsten oxide reacts with lithium. They used beamline B18 to collect high-resolution data on niobium tungsten oxide compounds with the addition of lithium, up to as much as two lithium ions per metal ion. 

The samples tested were sensitive to air, and so Diamond's beamline scientists created a unique transfer device using 3D printed components. This device was used to move the samples from an inert argon glovebox to the beamline, and to maintain an inert helium atmosphere around the samples during data collection. 

By tracking X-ray absorption at different stages of the charging cycle, the team discovered that niobium and tungsten ions store charge simultaneously. Both elements undergo multielectron redox, contributing to the high storage capacity of these new materials. 

Seeing the complete picture 

The XAS measurements were crucial to gaining chemical insights about the activity of the niobium tungsten oxide materials, but the team combined them with other techniques to gather the complete picture. They used pulsed-field gradient nuclear magnetic resonance spectroscopy to measure lithium diffusion, and X-ray diffraction to analyse the crystal structure.

This multifaceted approach allowed them to develop an understanding of how the metal ions store excess charge, and to discover that lithium ions can readily move through these materials. Because the atomic structure undergoes minimal volume changes, there is minimal strain as the lithium is incorporated. 

Using niobium tungsten oxide as the negative electrode, with any of the positive electrode materials in commercial use, could deliver a high-performance lithium-ion battery with ultra-fast charging times, facilitate new technologies and accelerate the transition away from fossil fuels.  

And that is something the international panel for the Charles Hatchett Award seem to agree with. "This paper deals with a very important research topic with potential commercial applications involving the use of niobium: how to significantly increase the capacity and reduce the charging time of Li-ion batteries. The research used a very wide range of experimental methods, producing extensive experimental data. The potential applications of the technology could have clear sustainability-related impacts by promoting the use of non-carbon energy storage and usage." 

This award-winning research offers new electrode candidates for advanced battery technologies. It shows that fast charging and high-power capacity are achievable with large, easily-produced particles. By identifying the critical features of these high-performance compounds, it outlines a pathway for the discovery and design of future materials. 

CEO of Diamond, Professor Andrew Harrison adds: "As a leading-edge facility for scientific research supporting a wide range of users from both academia and industry, we are extremely proud of this potentially world-changing science that has been undertaken on our B18 beamline into new fast charging and high-power battery materials. We hope their award-winning research will unlock new benefits to our society and economy and further studies are eagerly expected."