To aid the development of such faster-charging and longer-lasting batteries, scientists need to be able to understand the processes occurring inside an operating battery, to identify the limitations to battery performance. Currently, visualising the active battery materials as they work requires sophisticated synchrotron X-ray or electron microscopy techniques, which can be difficult and expensive, and often cannot image quickly enough to capture the rapid changes occurring in fast-charging electrode materials. As a result, the ion dynamics on the lengthscale of individual active particles and at commercially-relevant fast-charging rates remains largely unexplored.
Researchers at the University of Cambridge have overcome this problem by developing a low-cost lab-based optical microscopy technique to study lithium-ion batteries. They examined individual particles of Nb14W3O44, which is among the fastest charging anode materials to-date. Visible light is sent into the battery through a small glass window, allowing the researchers to watch the dynamic process within the active particles, in real time, under realistic non-equilibrium conditions. 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. "Such spontaneous events have severe implications for the battery, but could never be observed in real time before now", says 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" says first author Alice Merryweather, a PhD candidate at Cambridge's Cavendish Laboratory and Chemistry Department.
Moving forward, 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, making it an important piece of the puzzle in the development of next-generation batteries.
