Lab-based observations offer insights into battery performance

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Researchers at Cambridge University have developed a low-cost lab-based optical microscopy technique to study lithium-ion batteries, an advance that could help enable grid-scale storage and EV battery development.

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Currently, visualising active battery materials requires synchrotron X-ray or electron microscopy techniques, which can be difficult and expensive, and often cannot image quickly enough to capture the changes occurring in fast-charging electrode materials. Consequently, the ion dynamics on the length scale of individual active particles and at commercially relevant fast-charging rates remains largely unexplored.

In their work, which is detailed in Nature Materials, the researchers sent visible light into the battery through a small glass window, which allowed them to watch the dynamic process within the active particles of a niobium tungsten oxide in real time, under realistic non-equilibrium conditions. Nb14W3O­44 is regarded as among the fastest charging anode materials available.

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This revealed front-like lithium-concentration gradients moving through the individual active particles, resulting in internal strain which caused some particles to fracture. Particle fracture is a problem for batteries, since it can lead to electrical disconnection of the fragments, reducing the storage capacity of the battery.

a Optical images showing crack formation in a single rod-shaped particle of Nb14W3O¬44. Black dashed lines highlight Li-ion fronts propagating from the crack. b Optical image of a fractured particle, following 20 charge-discharge cycles. The more brightly scattering fragment has a higher Li-content, suggesting it has become inactive. Scale bars are 5 μm. c Extent of particle fracture in a fixed population of active particles, over 15 charge-discharge cycles - Research Team, Cavendish Laboratory, Department of Physics, Cambridge University

“Such spontaneous events have severe implications for the battery, but could never be observed in real time before now”, said co-author Dr Christoph Schnedermann, from Cambridge’s Cavendish Laboratory.

The high-throughput capabilities of the optical microscopy technique enabled the researchers to analyse a large population of particles, revealing that particle cracking is more common with higher rates of delithiation and in longer particles.

“These findings provide directly-applicable design principles to reduce particle fracture and capacity fade in this class of materials” said first author Alice Merryweather, a PhD candidate at Cambridge’s Cavendish Laboratory and Chemistry Department.

According to the researchers, the key advantages of the methodology – including the rapid data acquisition, single-particle resolution, and high throughput capabilities – will enable further exploration of what happens when batteries fail and how to prevent it. The technique can be applied to study almost any type of battery material, which will assist the development of next-generation batteries.

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