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Study provides insight into key process in lithium-rich cathodes that both helps and hurts battery performance; oxygen oxidation

A new study led by researchers from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory has provided insight into a phenomenon that both helps and hurts Li-ion battery performance.

Today’s commercial battery materials are only able to release about half of the lithium ions they contain. One solution is 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.

The new study, published in Nature Materials, 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 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.

—study lead William Gent, a Stanford University Siebel Scholar and winner of an Advanced Light Source and Molecular Foundry Doctoral Fellowship at Berkeley Lab

Lithium-rich battery cathodes are like super-absorbent sponges, able to 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 such as 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. 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.

—Michael Toney, a distinguished staff scientist at SLAC and a co-author

The authors’ previous study, published in Nature Communications, 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 new study, the researchers 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.

Reversible high-voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen-anion redox has garnered intense interest for such applications, particularly lithium-ion batteries, as it offers substantial redox capacity at more than 4 V versus Li/Li+ in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis and voltage fade, which currently preclude its widespread use.

By comprehensively studying the Li2−xIr1−ySnyO3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure–redox coupling arises from the local stabilization of short approximately 1.8 Å metal–oxygen π bonds and approximately 1.4 Å O–O dimers during oxygen redox, which occurs in Li2−xIr1−ySnyO3 through ligand-to-metal charge transfer.

Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighboring cation sites, driving cation disorder. These insights establish a point-defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling.

—Hong et al.

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, to disentangle this complicated problem.

This combination allowed the team to demonstrate conclusively 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 completely 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. So far, progress in this space has been largely incremental, with improvements of only a few percent per year. If we can find a way to make this work, it would be a huge step forward in making this technology practical.

—William Chueh, Assistant Professor of Materials Sciences at Stanford, who co-led the study

The research team also included researchers from the University of California, Berkeley and DOE’s Argonne National Laboratory. SSRL, ALS and the Molecular Foundry are DOE Office of Science user facilities. This research was supported by the DOE offices of Science and Energy Efficiency and Renewable Energy Office of Vehicle Technologies, Battery Materials Research Program.

Resources

  • Jihyun Hong, William E. Gent, Penghao Xiao, Kipil Lim, Dong-Hwa Seo, Jinpeng Wu, Peter M. Csernica, Christopher J. Takacs, Dennis Nordlund, Cheng-Jun Sun, Kevin H. Stone, Donata Passarello, Wanli Yang, David Prendergast, Gerbrand Ceder, Michael F. Toney & William C. Chueh (2019) “Metal–oxygen decoordination stabilizes anion redox in Li-rich oxides” Nature Materials doi: 10.1038/s41563-018-0276-1

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