First complete picture of lithium-ion battery performance

Addressing a lot of misconceptions in the field.

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The problem with today’s commercial batteries is they release about half of the lithium ions they contain. One solution on this is to cram cathodes with extra lithium ions, allowing them to store more energy in the same amount of space.

Be that as it may, for reasons, each new charge and discharge cycle gradually strips these lithium-rich cathodes of their voltage and capacity.

In a new study, scientists have provided a comprehensive model of this process, identifying what gives rise to it and how it ultimately leads to the battery’s downfall.

Study lead William Gent, a Stanford University Siebel Scholar said, “This research addressed a lot of misconceptions in the field. 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.”

Most lithium particle battery cathodes contain alternating layers of lithium and transition metal oxides – elements like nickel or cobalt combined with oxygen. In commercial batteries, each time a lithium atom leaves the cathode for the anode, an electron is caught from a change metal atom. These electrons make the electrical flow and voltage important to charge the material.

Be that as it may, something other than what’s expected occurs in lithium-rich batteries.

Michael Toney, a distinguished staff scientist at SLAC and a co-author of the paper said, “An unusual feature of lithium-rich cathodes is that the electron comes from the oxygen rather than the transition metal. 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.”

Oxygen oxidation does something similar. 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, sapping its voltage and capacity.

In this study, scientists showed that this is because yanking the electron from oxygen makes it need to frame another bond and transition metal particles need to move around to suit that bond, changing the nuclear structure.

Michael Toney, a distinguished staff scientist at SLAC and a co-author of the paper said, “It took the combination of theory and many experimental methods, done at Berkeley Lab’s Advanced Light Source (ALS) and Molecular Foundry as well as SLAC’s Stanford Synchrotron Light Source (SSRL), to disentangle this complicated problem.”

“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 said, is to find ways to produce those changes without totally disrupting the cathode’s crystal structure.”

The study was published this week in Nature Materials.