Study Of Disordered Rock Salts Leads To A Battery Breakthrough
For the past decade, disordered rock salt has been investigated as a potential breakthrough cathode material for lithium-ion batteries, as well as a key to developing low-cost, high-energy storage for anything from mobile phones to electric vehicles to renewable energy storage
A new MIT study is ensuring that the material achieves its promise
A team of researchers, led by Ju Li, the Tokyo Electric Power Company Professor in Nuclear Engineering and professor of materials science and engineering, describes a new class of partially disordered rock salt cathode integrated with polyanions — dubbed disordered rock salt-polyanionic spinel, or DRXPS — that delivers high energy density at high voltages while significantly improving cycling stability.
Cathode materials often face a trade-off between energy density and cycle stability. “And with this work, we hope to push the envelope by designing new cathode chemistries,” says Yimeng Huang, a postdoc in the Department of Nuclear Science and Engineering and first author of a paper reporting the work published today in Nature Energy. “This material family has high energy density and good cycling stability because it integrates two major types of cathode materials, rock salt and polyanionic olivine, so it has the benefits of both.”
Importantly, Li says, the new material family is mostly made up of manganese, a plentiful element that is substantially less expensive than metals like nickel and cobalt, which are commonly used in cathodes today.
“Manganese is at least five times less expensive than nickel and about 30 times less expensive than cobalt,” declares Li. “Manganese is also one of the keys to achieving higher energy densities, so having that material be much more earth-abundant is a tremendous advantage.”
A potential approach to renewable energy infrastructure
That advantage will be especially important, Li and his co-authors argued, as the world works to create the renewable energy infrastructure required for a low- or zero-carbon future.
Batteries are an especially important part of that picture, not only because they have the potential to decarbonize transportation with electric cars, buses, and trucks, but also because they will be critical in addressing the intermittent nature of wind and solar power by storing excess energy and feeding it back into the grid at night or on calm days, when renewable generation drops.
Given the high cost and relative scarcity of elements such as cobalt and nickel, they predicted that efforts to rapidly scale up energy storage capacity will result in dramatic pricing spikes and perhaps significant material shortages.
“If we want to have true electrification of energy generation, transportation, and more, we need earth-abundant batteries to store intermittent photovoltaic and wind power,” according to Li. “I think this is one of the steps toward that dream.”
Gerbrand Ceder, the Samsung Distinguished Chair in Nanoscience and Nanotechnology Research and a materials science and engineering professor at the University of California, Berkeley, expressed this sentiment.
“Lithium-ion batteries are a critical part of the clean energy transition,” Ceder tells me. “Their continued growth and price decrease depends on the development of inexpensive, high-performance cathode materials made from earth-abundant materials, as presented in this work.”
Overcoming problems with existing materials
The new work addresses one of the most significant issues with disordered rock salt cathodes: oxygen mobility.
While the materials have long been recognized for their extremely high capacity — up to 350 milliampere-hour per gram — as compared to standard cathode materials, which typically have capacities ranging from 190 to 200 milliampere-hour per gram, they are not very stable.
The high capacity is partially due to oxygen redox, which occurs when the cathode is charged to high voltages. However, when this occurs, oxygen gets mobile, causing interactions with the electrolyte and material degradation, ultimately rendering it effectively unusable after extensive cycling.
To address these issues, Huang added another element — phosphorous — which serves as a glue, binding the oxygen in place to prevent deterioration.
“The main innovation here, and the theory behind the design, is that Yimeng added just the right amount of phosphorus, formed so-called polyanions with its neighboring oxygen atoms, into a cation-deficient rock salt structure that can pin them down,” according to Li. “That allows us to basically stop the percolating oxygen transport due to strong covalent bonding between phosphorus and oxygen, meaning we can both utilize the oxygen-contributed capacity but also have good stability as well.”
According to Li, the ability to charge batteries to greater voltages is critical because it enables simpler systems to manage the energy they store.
“You can say the quality of the energy is higher,” according to him. “The higher the voltage per cell, then the less you need to connect them in series in the battery pack, and the simpler the battery management system.”
Pointing the path to future investigations.
While the cathode material presented in the study has the potential to improve lithium-ion battery technology, there are various avenues for future research.
Huang notes that future research should look into novel techniques to produce the material, particularly for shape and scalability factors.
“We are now using high-energy ball milling for mechanochemical synthesis, which results in non-uniform morphology and tiny average particle size (about 150 nanometers). “This method is also not very scalable,” he explains. Our goal is to improve battery performance by achieving a more uniform morphology with larger particle sizes through alternate synthesis methods. This will increase the volumetric energy density of the material and allow us to explore coating methods. Future approaches, of course, should be industrially scaleable.”
Furthermore, he claims that the disordered rock salt material itself is not a particularly good conductor, so substantial amounts of carbon — up to 20% of the cathode paste — were added to increase its conductivity. If the team can reduce the carbon percentage of the electrode without sacrificing performance, the battery’s active material content will grow, resulting in a higher practical energy density.
“In this paper, we just used Super P, a typical conductive carbon consisting of nanospheres, but they’re not very efficient,” Huang tells me. “We are now exploring using carbon nanotubes, which could reduce the carbon content to just 1 or 2 weight percent, which could allow us to dramatically increase the amount of the active cathode material.”
Aside from lowering carbon content, he adds that using thick electrodes is another technique to boost the battery’s practical energy density. This is another area of research that the team is pursuing.
“This is only the beginning of DRXPS research since we only explored a few chemistries within its vast compositional space,” he says. “We can play around with different ratios of lithium, manganese, phosphorus, and oxygen, and with various combinations of other polyanion-forming elements such as boron, silicon, and sulfur.”
According to him, the DRXPS cathode family is very promising in applications such as electric vehicles and grid storage, as well as consumer electronics, where volumetric energy density is critical. This is due to optimized compositions, more scalable synthesis methods, improved morphology that allows for uniform coatings, lower carbon content, and thicker electrodes.
The Honda Research Institute USA Inc. and the Molecular Foundry at Lawrence Berkeley National Laboratory provided funding for this work, which also utilized the National Synchrotron Light Source II at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory.
