A team of scientists led by the U.S. Department of Energy’s (DOE) 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.
Consider that it only takes a few minutes to fill up the gas tank of a car but a few hours to charge the battery of an electric vehicle, said co-corresponding author Feng Wang, a materials scientist in Brookhaven Lab’s Interdisciplinary Sciences Department.
Figuring out how to make lithium ions move faster in electrode materials is a big deal, as it may help us build better batteries with greatly reduced charging time.
Lithium-ion batteries work by shuffling lithium ions between a positive and negative electrode (cathode and anode) through a chemical medium called an electrolyte.
Graphite is commonly employed as the anode in state-of-the-art lithium-ion batteries, but for fast-charging applications, LTO is an appealing alternative. LTO can accommodate lithium ions rapidly, without suffering from lithium plating (the deposition of lithium on the electrode surface instead of internally).
As LTO accommodates lithium, it transforms from its original phase (Li4Ti5O12) to an end phase (Li7Ti5O12), both of which have poor lithium conductivity.
Thus, scientists have been puzzled as to 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.
This electrochemical cell enabled 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.
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.
We look forward to examining transport behaviors in fast-charging electrodes more closely by fitting our newly developed electrochemical cell to the powerful electron and x-ray microscopes at Brookhaven’s CFN and National Synchrotron Light Source II (NSLS-II),” said Wang.
By leveraging these state-of-the-art tools, we will be able to gain a complete view of lithium transport in the local and bulk structures of the samples during cycling in real time and under real-world reaction conditions.