New research offers a complete understanding of why a promising approach of stuffing more lithium into battery cathodes leads to their failure. This knowledge could be the key to smaller phone batteries and electric cars that drive farther between charges.
The Lithium-ion batteries within electric vehicles and phones charge and discharge by transferring lithium ions back and forth between two electrodes, an anode, and cathode. The more Lithium ions the electrodes can absorb and release, the more energy the battery can store.
One issue in today’s commercial battery materials is that they are only able to release about half of the Lithium ions they contain. One solution has been to cram cathodes with extra Lithium ions, allowing them to store more energy in the same amount of space. But for some reason, every new charge and discharge cycle slowly strips these Lithium-rich cathodes of their voltage and capacity.
A new study provides a comprehensive model of this process, identifying what gives rise to it and how it ultimately leads to the battery’s downfall.
“This research addressed a lot of misconceptions in the field,” says study lead William Gent, a Stanford University and winner of an Advanced Light Source and Molecular Foundry Doctoral Fellowship at Berkeley Lab. “There’s a long way to go, but now we have a foundational understanding of the properties that lead to this process that’s going to help us harness its power rather than just stab at it in the dark.”
The cycling of Lithium through a battery is like a sponge relay. The more absorbent the sponge, the more water can be squeezed into the second bucket.
Lithium-rich battery cathodes are like very absorbent sponges and can soak up nearly twice as many Lithium ions as commercial cathodes, packing as much as twice the energy into the same amount of space. This could allow for smaller phone batteries and electric vehicles that travel farther between charges.
Most Lithium-ion battery cathodes contain alternating layers of Lithium and transition metal oxides – elements like nickel or cobalt combined with oxygen. In commercial batteries, every time a lithium atom leaves the cathode for the anode, an electron is snagged from a transition metal atom. These electrons create the electrical current and voltage necessary to charge the material. But something different happens in Lithium-rich batteries.
An unusual feature of Lithium-rich cathodes is that the electron comes from the oxygen rather than the transition metal,” says Michael Toney, a distinguished staff scientist at SLAC and a co-author of the paper. “This process, called oxygen oxidation, enables cathodes to extract about 90 percent of the lithium at a high enough voltage that it boosts the energy stored in the battery.”
The authors’ previous study showed that every time Lithium ions cycle out of the cathode into the anode, some transition metal atoms sneak in to take their place and the atomic structure of the cathode becomes a little messier. The layered structure essential to the cathode’s performance slowly falls apart.
In this new study, the team showed that this is because yanking the electron from oxygen makes it want to form another bond and transition metal atoms have to move around to accommodate that bond, changing the atomic structure.
“This is the first paper that provides a complete model as to why these things are related and
In order to figure out this problem, Toney says it took the combination of theory and many experimental methods, done at SLAC’s Stanford Synchrotron Light Source (SSRL) as well as Berkeley Lab’s Advanced Light Source (ALS) and Molecular Foundry.
This combination allowed the team to conclusively demonstrate the strong driving force behind changes in the cathode’s bonding configuration during oxygen oxidation. The next step, Toney says, is to find ways to produce those changes without totally disrupting the cathode’s crystal structure.
“Because oxygen oxidation gives rise to extra energy density, being able to understand and control it is potentially a game changer in electric vehicles,” says William Chueh, Assistant Professor of Materials Sciences at Stanford, who co-led the study. “So far, progress in this space has been largely incremental, with improvements of only a few
The study was led by researchers from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory.
Source: SLAC National Accelerator Laboratory