Berkeley Lab researchers shed light on how lithium-rich cathodes work, opening the door to higher capacity batteries
Researchers at the Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) report a major advance in understanding how oxygen oxidation creates extra capacity in lithium-rich cathodes, opening the door to batteries with far higher energy density.
The nature of the findings shows “a clear and exciting path forward” to create the next-generation cathode materials with substantially higher energy density then current cathode materials, the researchers wrote in a paper on their work published in the journal Nature Chemistry.
The traditional design paradigm for Li-ion battery cathodes has been to create compounds in which the amount of extractable Li+ is well balanced with an oxidizable transition metal (TM) species (such as Mn, Fe, Co or Ni) to provide the charge-compensating electrons, all contained in an oxide or sulfide host. Transition metals have been considered the sole sources of electrochemical activity in an intercalation cathode, and as a consequence the theoretical specific capacity is limited by the number of electrons that the transition-metal ions can exchange per unit mass.
Recent observations have brought this simple picture into question and argued that oxygen ions in oxide cathodes may also participate in the redox reaction. This oxygen redox is often ascribed to covalency, following theoretical and experimental work in the last two decades that has demonstrated large electron density changes on oxygen when the transition metal is oxidized. However, covalency cannot lead to a higher capacity than would be expected from the transition metal alone, as the number of transition-metal orbitals remains unchanged when they hybridize with the oxygen ligands. The more important argument for the future of the Li-ion battery field is whether oxygen oxidation can create extra capacity beyond what is predicted from the transition-metal content alone, as has been argued for several Li-excess materials, such as Li1.2Ni0.2Mn0.6O2, Li2Ru0.5Sn0.5O2 and Li1.3Mn0.4Nb0.3O2.—Seo et al.
For the study, the research team used ab initio calculations to demonstrate which specific chemical and structural features lead to electrochemically active oxygen states in cathode materials. They uncovered a specific atomistic origin of oxygen redox and explained why this oxygen capacity is so often observed in Li-excess materials and why it is observed with some metals and not with others.
The research was led by Gerbrand Ceder of Berkeley Lab’s Materials Sciences Division. The lead authors were Dong-Hwa Seo and Jinhyuk Lee, and other co-authors were Alexander Urban, Rahul Malik, and ShinYoung Kang. Ceder also has an appointment at UC Berkeley’s Department of Materials Science and Engineering, and all the co-authors are also affiliated with the Massachusetts Institute of Technology (MIT), where some of the work was done.
In a conventional lithium-ion battery, the cathode material is a lithium transition metal oxide, with the content of the lithium and the transition metal, such as nickel or cobalt, balanced, as described above. In a lithium-rich (also called lithium-excess) cathode, there is a higher proportion of lithium than the transition metal.
Because transition metals are heavy and also expensive, reducing its content is a big benefit. The battery can be significantly cheaper and lighter, which are especially important factors for vehicle applications, where the battery is often one of the heaviest components of the vehicle.
This is a very exciting direction being pursued by battery scientists. It has been experimentally demonstrated many times that a lithium-excess cathode material can deliver higher energy density, about 50% higher than the current cathode materials in commercial lithium batteries.—Jinhyuk Lee
A major stumbling block has been that scientists had lacked a clear understanding of the chemistry in a lithium-rich cathode—specifically the role of oxygen. Normally when a battery is charged and discharged, the transition metal in the cathode oxidizes and releases electrons; those electrons then travel between the cathode and anode and create electricity.
What we and others have been claiming recently is that you can take an electron off the oxygen and put it back, which is fairly radical. That’s the big idea for this cathode design. This paper specifically shows that it’s true and more importantly, shows under which conditions that it becomes true.—Gerbrand Ceder
Currently there are only three transition metals—cobalt, nickel, and manganese—used in most commercial cathodes. That limited choice constrains battery design; further, their availability is limited. Demand for cobalt has been booming, and more than 45% of the world’s cobalt production now goes to lithium-ion batteries, Ceder noted.
It’s not scalable. If we’re ever to all drive electric vehicles, there’s no way a cobalt-only technology can make it.—Gerbrand Ceder
The research started two years ago after Ceder’s group discovered that a disordered cathode structure, previously dismissed by battery designers, could indeed be workable. This prompted the group to look into how and when oxygen is active in lithium-excess cathodes, which are similar in structure to disordered cathodes.
Ceder’s group developed a novel methodology of utilizing quantum mechanical simulations to study electron charge transfer in cathode materials with high accuracy. They used supercomputer facilities at the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility hosted at Berkeley Lab, and the Extreme Science and Engineering Discovery Environment (XSEDE), led by the University of Illinois.
… we have identified the chemical and structural features that lead to oxygen oxidation and therefore to extra capacity in lithium intercalation compounds. Oxygen redox activity in Li-excess layered and disordered materials originates from very specific Li–O–Li configurations that create orphaned oxygen states that are lifted out of the bonding oxygen manifold and become positioned in the TM-dominated complex of eg* and t2g states, making oxygen oxidation and TM oxidation compete with each other.
This effect is distinct from the prevailing argument that the holes are introduced in O 2p states that are hybridized with TM d states, which occurs due to the covalent nature of the TM–O bonding.
In stark contrast with this current belief, we demonstrated that oxygen oxidation in Li-excess materials mainly occurs by extracting labile electrons from unhybridized O 2p states sitting in Li–O–Li configurations and is therefore unrelated to any hybridized TM–O states. This distinction is important, because the number of electrons, and thus the capacity, from the hybridized TM d states (for example, the eg* state) stays the same, regardless of the oxygen contribution to these states. Only the energy (voltage) of these TM states is modified by their hybridization. Hence, unlike oxygen redox states created by Li–O–Li configurations, oxygen redox participation through (re)hybridization with TM states is not a useful mechanism to extend capacity beyond the conventional, TM-determined theoretical capacity limit.
Creating unhybridized (orphaned) oxygen states in Li-intercalation cathodes is a promising mechanism to achieve higher-energy-density cathode materials as it lifts the fundamental restriction on transition metal content that has existed for decades in the Li-ion battery field.—Seo et al.
The research was supported by DOE’s Office of Vehicle Technologies, the Robert Bosch Corporation, and Umicore Specialty Oxides and Chemicals.
Dong-Hwa Seo, Jinhyuk Lee, Alexander Urban, Rahul Malik, ShinYoung Kang & Gerbrand Ceder (2016) “The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials” Nature Chemistry doi: 10.1038/nchem.2524