Research into disordered rock salts leads to battery breakthrough
Over the past decade, disordered rock salt has been studied as a potential breakthrough cathode material for use in lithium-ion batteries and a key to creating low-cost, high-energy storage for everything from cell phones to electric vehicles to renewable energy storage.
A new MIT study ensures that the material lives up to this promise.
Led by Ju Li, the Tokyo Electric Power Company Professor of Nuclear Engineering and professor of materials science and engineering, a team of researchers describes a new class of partially disordered rock salt cathodes integrated with polyanions – also called disordered rock salt polyanionic spinel, or DRXPS – which delivers high energy density at high voltages with significantly improved cycling stability.
“Cathode materials typically involve a trade-off between energy density and cyclic stability… and with this work we aim to push the boundaries by designing new cathode chemistries,” said Yimeng Huang, a postdoc in the Department of Nuclear Science and Engineering. Engineering and first author of a paper describing the work published in Nature Energy. “(This) material family has high energy density and good cycle stability because it integrates two major types of cathode materials, rock salt and polyanionic olivine, so it has the advantages of both.”
Importantly, Li adds, the new family of materials consists mainly of manganese, an element that is abundant in the Earth and is significantly cheaper than elements such as nickel and cobalt, which are commonly used in cathodes today.
“Manganese is at least five times cheaper than nickel and about 30 times cheaper than cobalt,” Li says. “Manganese is also one of the keys to achieving higher energy densities, so it’s a huge advantage that that material is much more abundant on Earth.”
A possible path to renewable energy infrastructure
That advantage will be especially crucial, Li and his co-authors wrote, as the world looks to build the renewable energy infrastructure needed for a low- or no-carbon future.
Batteries are a particularly important part of that picture, not only because of their potential to decarbonize transportation with electric cars, buses and trucks, but also because they will be essential for addressing the intermittent problems of wind and solar energy by storing excess energy. Feed back into the electricity grid at night or on quiet days, when renewable energy generation decreases.
Given the high cost and relative rarity of materials like cobalt and nickel, they wrote, efforts to rapidly scale up electrical storage capacity would likely lead to extreme cost spikes and potentially significant material shortages.
“If we want to achieve true electrification of power generation, transportation and more, we need batteries that are abundant on Earth to store intermittent photovoltaic energy and wind energy,” Li says. “I think this is one of the steps toward that dream.”
That sentiment was echoed by Gerbrand Ceder, the Samsung Distinguished Chair in Nanoscience and Nanotechnology Research and professor of materials science and engineering at the University of California at Berkeley.
“Lithium-ion batteries are a crucial part of the clean energy transition,” says Ceder. “Their continued growth and price decline depends on the development of low-cost, high-quality cathode materials made from materials that are abundant on Earth, as presented in this work.”
Overcoming obstacles in existing materials
The new study focuses on one of the biggest challenges facing disordered rock salt cathodes: oxygen mobility.
Although the materials have long been recognized as offering very high capacitance (up to 350 milliamp-hours per gram) compared to traditional cathode materials, which typically have capacities between 190 and 200 milliamp-hours per gram, they are not very stable. .
The high capacity is partly caused by oxygen redox, which is activated when the cathode is charged to high voltages. But when that happens, oxygen becomes mobile, leading to reactions with the electrolyte and breakdown of the material, ultimately leaving it effectively unusable after prolonged cycling.
To overcome these challenges, Huang added another element: phosphorus, which essentially acts like a glue, holding the oxygen in place to reduce degradation.
“The key innovation here, and the theory behind the design, is that Yimeng added just the right amount of phosphorus, called polyanions formed with the adjacent oxygen atoms, into a cation-deficient rock salt structure that can pin them down,” Li explains. “That allows us to basically stop percolating oxygen transport due to the strong covalent bond between phosphorus and oxygen… meaning we can both use the capacity contributed by oxygen, but also have good stability.”
That ability to charge batteries to higher voltages is critical, Li says, because it allows simpler systems to manage the energy they store.
“You can say the quality of the energy is higher,” he says. “The higher the voltage per cell, the less you have to connect them in series in the battery pack and the simpler the battery management system.”
Point the way to future studies
While the cathode material described in the study could have a transformative impact on lithium-ion battery technology, there are still several avenues for research in the future.
Areas for future research, according to Huang, include efforts to explore new ways to fabricate the material, particularly for morphology and scalability considerations.
“Currently we use high-energy ball mills for mechanochemical synthesis, and… the resulting morphology is non-uniform and has a small average particle size (about 150 nanometers). This method is also not completely scalable,” he says. . “We are trying to achieve a more uniform morphology with larger particle sizes by using some alternative synthesis methods, which would allow us to increase the volumetric energy density of the material and could allow us to explore some coating methods… that could further improve quality.” battery performance. Future methods must of course be industrially scalable.”
Furthermore, he says, the disordered rock salt material is not a particularly good conductor on its own, so significant amounts of carbon – as much as 20 percent by weight of the cathode paste – were added to increase its conductivity. If the team can reduce the carbon content in the electrode without sacrificing performance, there will be a higher active material content in a battery, leading to increased practical energy density.
“In this paper, we just used Super P, a typical conductive carbon made up of nanospheres, but they are not very efficient,” says Huang. “We are now exploring the use of carbon nanotubes, which could reduce the carbon content to as little as 1 or 2 percent by weight, which could allow us to dramatically increase the amount of active cathode material.”
In addition to lowering carbon levels, he adds, making thick electrodes is yet another way to increase the battery’s practical energy density. This is another area of research the team is working on.
“This is just the beginning of DRXPS research, as we have only explored a few chemistries within its vast compositional space,” he continues. “We can play with different ratios of lithium, manganese, phosphorus and oxygen, and with different combinations of other polyanion-forming elements such as boron, silicon and sulfur.”
With optimized compositions, more scalable synthesis methods, better morphology that enables uniform coatings, lower carbon content and thicker electrodes, he says, the DRXPS cathode family holds promise in electric vehicle and network storage applications, and possibly even in consumer electronics. , where the volumetric energy density is very important.
Research report:“Integrated rock salt polyanion cathodes with excess lithium and stabilized cycles”