A schematic of the mini electrochemical cell that the scientists built to chase lithium ions (orange) moving in the lattice of LTO (blue). Image: Brookhaven National Laboratory.

A team led by scientists at the US Department of Energy (DOE)’s Brookhaven National Laboratory and Lawrence Berkeley National Laboratory has captured in real time how lithium ions move in lithium titanate (LTO), a fast-charging battery electrode material made of lithium, titanium and oxygen.

The scientists discovered that distorted arrangements of lithium and surrounding atoms in LTO ‘intermediates’ (structures of LTO with a lithium concentration in between that of its initial and end states) provide an ‘express lane’ for the transport of lithium ions. Their discovery, reported in a paper in Science, could provide insights into designing improved battery materials for the rapid charging of electric vehicles and portable consumer electronics such as cell phones and laptops.



As LTO accommodates lithium, it transforms from its original phase (Li4Ti5O12) to an end phase (Li7Ti5O12), both of which have poor lithium conductivity. This creates a puzzle for scientists over how LTO can be a fast-charging electrode. Reconciling this seeming paradox requires knowledge of how lithium ions diffuse in intermediate structures of LTO (those with a lithium concentration in between that of Li4Ti5O12 and Li7Ti5O12), rather than a static picture derived solely from the initial and end phases.

But performing such characterization is a nontrivial task. Lithium ions are light, making them elusive to traditional electron- or x-ray-based probing techniques – especially when the ions are shuffling rapidly within active materials, such as LTO nanoparticles in an operating battery electrode.

In this study, the scientists were able to track the migration of lithium ions in LTO nanoparticles in real time by designing an electrochemical cell to operate inside a transmission electron microscope (TEM). This electrochemical cell allowed the team to conduct electron energy-loss spectroscopy (EELS) during battery charge and discharge. In EELS, the change in energy of electrons after they have interacted with a sample is measured to reveal information about the sample’s local chemical states. In addition to being highly sensitive to lithium ions, EELS, when carried out inside a TEM, provides the high resolution in both space and time needed to capture ion transport in nanoparticles.



“Computational modeling was very important to understand how lithium can move so fast through this material,” said co-corresponding author and CEDER group leader Gerbrand Ceder, professor in the Department of Materials Science and Engineering at the University of California, Berkeley and a senior faculty scientist in the Materials Science Division at Berkeley Lab. “As the material takes up lithium, the atomic arrangement becomes very complex and difficult to conceptualize with simple transport ideas. Computations were able to confirm that the crowding of lithium ions together makes them highly mobile.”


The team’s analysis revealed that LTO has metastable intermediate configurations in which the atoms are locally not in their usual arrangement. These local ‘polyhedral’ distortions lower the energy barriers, providing a pathway through which lithium ions can quickly travel.

“Unlike gas freely flowing into your car’s gas tank, which is essentially an empty container, lithium needs to ‘fight’ its way into LTO, which is not a completely open structure,” explained Wang. “To get lithium in, LTO transforms from one structure to another. Typically, such a two-phase transformation takes time, limiting the fast-charging capability. However, in this case, lithium is accommodated more quickly than expected because local distortions in the atomic structure of LTO create more open space through which lithium can easily pass. These highly conductive pathways happen at the abundant boundaries existing between the two phases.”

Next, the scientists will explore the limitations of LTO – such as heat generation and capacity loss associated with cycling at high rates – for real applications. By examining how LTO behaves after repeatedly absorbing and releasing lithium at varying cycling rates, they hope to find remedies for these issues. This knowledge will inform the development of practically viable electrode materials for fast-charging batteries.

“The cross-institutional efforts combining in situ spectroscopy, electrochemistry, computation and theory in this work set a model for conducting future research,” said Zhu.


This story is adapted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.