New discoveries may better Li-ion battery design

Materials World magazine
24 Sep 2018

Scientists have discovered how the lithium ions move in the battery, paving the way for improved design. Idha Valeur reports.

Using X-rays, an international research team has discovered the travel route that lithium ions take – the back and forth movement to charge and subsequently discharge – in a normal battery, which is more complex than originally believed. This discovery, led by the Department of Energy’s SLAC National Accelerator Laboratory at Stanford University and Lawrence Berkeley National Laboratory at the University of California, USA, may inform a new generation of lithium-ion (Li-ion) batteries.

Martin Bazant, Professor of Chemical Engineering at the Massachusetts Institute of Technology and one of the lead authors of the study, Fluid-enhanced surface diffusion controls intraparticle phase transformations published in Nature Materials, compared Li-ion batteries to a black box, making it hard to know what works and why.

‘You could see that the material worked pretty well and certain additives seemed to help, but you couldn’t tell exactly where the lithium ions go in every step of the process. You could only try to develop a theory and work backwards from measurements. With new instruments and measurement techniques, we’re starting to have a more rigorous scientific understanding of how these things actually work,’ Bazant said.

The pop-pop-popcorn effect

By employing two different X-ray techniques, the researchers, led by William Chueh, Assistant Professor of Materials Science at SLAC’s Stanford Institute for Materials & Energy Science, were able to examine how a Li-ion battery works on the inside. Researchers bounced X-rays off a sample of lithium iron phosphate, and were able to expose the atomic and electronic structure of the battery, revealing how the lithium ions move.

During the lithium ions’ movement inside the battery material, when the batteries charge and discharge, the ions flow from a liquid solution to a solid reservoir, and once in the solid side the lithium can rearrange itself. This can lead the material to split into two phases. This is what Chueh calls ‘the popcorn effect’ because the ions clump together into hot spots causing a shorter lifetime for the battery.

Chueh told Materials World, ‘We discovered that lithium ions move along the interface between the solid electrode and the liquid electrolyte in the battery. Shutting this transport pathway, we believe, will lessen the popcorn effect, thus allowing current to be more evenly distributed.’

While they used the bouncing technique at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), they used a different technique at Berkeley lab’s advanced light source (ALS). Here the researchers used X-ray microscopy to magnify the process, enabling the researchers to map how the concentration of lithium changes over time.

Changing direction

During the research period, the team noticed that lithium ions move in a different direction on the surface of the material, which goes against thoughts based on older models. It was formerly thought that lithium iron phosphate was only able to move in one direction.

Working with Saiful Islam, Chemistry Professor at the University of Bath, UK, using computer models and simulations, the team was able to uncover that lithium ions move in two additional directions, making lithium iron phosphate a three-dimensional conductor and not one-dimensional, as previously believed.

‘As it turns out, these extra pathways are problematic for the material, promoting the popcorn-like behavior that leads to its failure. If lithium can be made to move more slowly on the surface, it will make the battery much more uniform. This is the key to developing higher performance and longer lasting batteries,’ said Chueh.

Islam said, ‘Regardless of the application, the discovery and optimisation of high-performance materials are critical to future breakthroughs for next-generation rechargeable batteries. These advances depend on exploring new classes of compounds and gaining a better understanding of the underpinning materials science.’

Sodium vs. lithium

With the buzz around sodium as a potential competitor for lithium in rechargeable batteries, because of its abundance and therefore low cost, why hasn’t it replaced lithium yet?

Islam said, ‘Sodium ions have similar intercalation chemistry to lithium ions and so, not surprisingly, a lot of cathode materials tested for sodium batteries are similar to those used for lithium. But there are exciting opportunities for discovering new types of sodium-based materials with many structures yet to be investigated.’

‘It’s true that sodium is much more abundant than lithium. However, by weight, lithium is a small fraction of a battery. What will becoming limiting first, in terms of natural resources, is cobalt followed by nickel,’ Chueh said.