Photo: An artistic illustration of the integration between two distinct battery cathode structures, rock salt (blue polyhedra) and polyanion olivine (red/yellow polyhedra). A novel hybrid structure is obtained by integrating polyanions (yellow polyhedra) into a rock salt (blue polyhedra) structure. Credits: Yimeng Huang
For the past decade, disordered rock salt has been a focal point for researchers aiming to revolutionize lithium-ion battery cathodes. This material holds the key to unlocking low-cost, high-energy storage solutions for a wide range of applications, from portable electronics to electric vehicles and renewable energy systems. A recent study from MIT is bringing this vision closer to reality.
A team led by Ju Li, the Tokyo Electric Power Company Professor in Nuclear Engineering and professor of materials science and engineering, has developed a new class of partially disordered rock salt cathodes. This innovative material, dubbed disordered rock salt-polyanionic spinel (DRXPS), integrates polyanions to achieve high energy density at high voltages while significantly enhancing cycling stability.
Yimeng Huang, a postdoc in the Department of Nuclear Science and Engineering and lead author of the paper published in Nature Energy, explains, “There’s usually a trade-off between energy density and cycling stability in cathode materials. Our goal is to push those boundaries by designing new cathode chemistries. This material family combines the advantages of both rock salt and polyanionic olivine cathode materials, resulting in high energy density and excellent cycling stability.”
A key advantage of this new material is its reliance on manganese, an earth-abundant element significantly more affordable than the nickel and cobalt commonly used in today’s cathodes. Li emphasizes the importance of this, stating, “Manganese is at least five times cheaper than nickel and about 30 times cheaper than cobalt. It’s also crucial for achieving higher energy densities. Having a more abundant and affordable material is a huge advantage.”
This affordability will be crucial as the world strives to build a sustainable energy future. Batteries are essential for decarbonizing transportation and addressing the intermittency of renewable energy sources like wind and solar. The reliance on expensive and less abundant materials like cobalt and nickel poses a significant obstacle to scaling up renewable energy infrastructure.
Li highlights the importance of this research, stating, “For true electrification of energy generation, transportation, and more, we need earth-abundant batteries to store intermittent photovoltaic and wind power. This is a step towards that dream.”
Gerbrand Ceder, the Samsung Distinguished Chair in Nanoscience and Nanotechnology Research and a professor of materials science and engineering at the University of California at Berkeley, echoes this sentiment, saying, “Lithium-ion batteries are crucial for the clean energy transition. Their continued growth and price reduction depend on developing inexpensive, high-performance cathode materials from earth-abundant materials, as demonstrated in this work.”
One of the main challenges with disordered rock salt cathodes has been oxygen mobility. While these materials offer high capacity (up to 350 milliampere-hours per gram compared to 190-200 milliampere-hours per gram in traditional cathodes), they suffer from instability. The high capacity is partially due to oxygen redox, which gets activated at high voltages. However, this activation makes oxygen mobile, leading to reactions with the electrolyte and material degradation.
To address this, Huang introduced phosphorus into the mix. This element acts as a stabilizing agent, binding the oxygen and preventing degradation. Li explains, “The key innovation is adding just the right amount of phosphorus to form polyanions with neighboring oxygen atoms within a cation-deficient rock salt structure. This effectively halts oxygen transport due to the strong covalent bonds between phosphorus and oxygen. We can utilize the oxygen-contributed capacity while maintaining good stability.”
This ability to charge batteries to higher voltages simplifies energy management systems. Li notes, “Higher voltage per cell means fewer cells are needed in series in a battery pack, simplifying the battery management system.”
While this research represents a significant leap forward, there are still areas for further exploration. Huang outlines some of these, including exploring new fabrication methods for scalability and morphology control, reducing the carbon content in the electrode to increase active material content, and experimenting with different chemical compositions within the DRXPS family.
With further optimization, this technology holds immense potential for electric vehicles, grid storage, and even consumer electronics. The future of battery technology seems brighter with this breakthrough in disordered rock salt cathodes.
This research was supported by the Honda Research Institute USA Inc., the Molecular Foundry at Lawrence Berkeley National Laboratory, the National Synchrotron Light Source II at Brookhaven National Laboratory, and the Advanced Photon Source at Argonne National Laboratory. MIT.nano’s facilities were also instrumental in this work.
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