a. Schematic diagram of the structure and composition of double gradient particles (SCDG) with a gradient structure from order to disorder toward the outer layer and a gradient composition from the Co-free bulk to the Co-enriched surface. b. The capacity of SCDG compared with the conventional layered NMC cathode and the 1st generation total concentration gradient cathode. c. The superior cycling stability of SCDG cathode with negligible capacity loss within 500 cycles in full cells. Credit: Liu et al.
Recent efforts to develop more advanced battery technologies have focused largely on designing new cathode materials. This is because existing cathodes do not perform well at high voltages and can contribute to rapid loss of battery capacity.
Layered oxide cathodes, a class of cathode materials with a layered crystalline structure, have shown particular promise for the development of next-generation batteries. Early findings suggest that these materials could improve the performance of lithium-ion batteries, while reducing their manufacturing costs and limiting their environmental impact.
Researchers at Argonne National Laboratory have recently designed new, ultra-stable NMC cathodes, a type of layered oxide cathode composed of nickel (Ni), manganese (Mn), and cobalt (Co). These newly designed materials, presented in a paper published in Natural energyhave proven capable of delivering high performance to lithium-ion batteries without significant capacity losses.
“To further advance NMC cathodes, our team developed a series of concentration gradient NMC cathodes to optimally exploit the beneficial characteristics of Ni, Mn, and Co,” Dr. Khalil Amine, Argonne Distinguished Fellow and lead author of the paper, told Tech Xplore.
“In this concentration gradient cathode, the nickel concentration decreases linearly while the manganese concentration increases linearly from the center to the outer layer of each particle.”
This full gradient cathode design, patented by Dr. Amine in 2012, exploits the high energy density of Ni (present at the core of the cathodes), as well as the high thermal stability and long lifetime of Mn in the outer layers of the cathode. This design has already been licensed to various battery technology and materials manufacturers.
“In pursuit of higher energy density and lower cost for next-generation batteries, we have pushed NMC cathodes for higher voltage operation (≥4.5V) to achieve high capacity, which overcomes the voltage limitation of conventional layered structure and leads to rapid capacity loss,” said Dr. Amine.
“In addition, current bottlenecks in cobalt (Co) supply have negatively impacted commercial battery production and inspired the development of cathode materials less dependent on Co.”
To overcome the limitations of existing NMC cathode designs, Dr. Amine and colleagues set out to design a second, updated version of their gradient cathodes. This second generation of cathodes is characterized by concentration- and structure-related gradients, which collectively fill the gaps in existing cathodes with high-voltage layered structures.
The researchers also lowered the concentration of Co in the cathodes. This change in composition could significantly reduce the manufacturing cost of cathode materials as well as their environmental impact.
“Previous layered cathodes suffer from a trade-off between capacity, cyclability, and safety. For example, increasing the operating voltage could improve their capacity, but at the expense of cycle life,” explains Dr. Amine. “As a result, most batteries used in electric vehicles can only operate at a voltage below 4.3 V, because the inherent structure tends to degrade at high voltages, resulting in reduced cycle life and a high safety risk.”
The new cathodes introduced in this recent study feature a unique composition and dual-gradient design, which address the voltage ceiling observed in other existing cathodes. By combining the advantages of different material components and structures in a single cathode, the team was able to achieve exceptional performance.
“In detail, the high-Ni bulk layered structure is capable of providing high capacitance, and the surface disordered rock salt structure could withstand high voltage up to 4.7 V without serious structural changes,” said Dr. Tongchao Liu, co-author of the paper.
“Therefore, this dual-gradient cathode could simultaneously achieve high capacity and longer lifetime during high-voltage operation (>4.5 V). Moreover, this design could reduce Co consumption by up to 1% and maximize its functionality and reduce safety risks.”
The researchers’ newly introduced materials depart from conventional cathode designs, which typically use a single structure and high Co concentrations. In early experiments, the new cathodes have proven to perform remarkably well, enabling high-capacity, high-voltage operation of the batteries at 4.5 V without any capacity loss, as well as negligible capacity loss when operating up to 4.7 V.
“By integrating the high energy density of the layered phase with the structural stability of the disordered rock salt phase, our design addresses the long-standing tradeoff between capacity, lifetime, and safety,” said Dr. Amine. “This innovation not only improves the overall performance of the cathode, but also broadens the research directions for cathode material design, enabling the creation of new materials that far outperform existing ones.”
This recent research opens up new possibilities for the development of lithium-ion batteries with lower Co concentrations, which maintain high capacities for longer periods, even when operating at high voltages. Moreover, the cathodes introduced by Dr. Amine and his colleagues may soon inspire other research teams to design similar materials with dual-gradient structures.
“The next steps in our research will be to further optimize the dual-gradient design to further reduce the use of Co and Ni while improving its energy density and scalability,” said Dr. Amine. “We are interested in exploring other material compositions and structural modifications to push the boundaries of energy density and stability even further.”
As part of their future work, the researchers also plan to integrate their cathodes into complete battery systems, which will allow them to test their performance in real-world conditions and assess their compatibility with existing battery components. To conduct these tests, Dr. Amine has patented his updated design and is initiating collaborations with battery manufacturers.
“In the long term, we envision our dual-gradient design inspiring a new generation of high-performance, cost-effective, and sustainable battery materials,” added Dr. Liu. “By reducing reliance on cobalt and improving the structural integrity of high-voltage cathodes, our work could have a significant impact on the development of next-generation batteries for electric vehicles, portable electronics, and grid storage.”
Argonne’s Advanced Photon Source and Nanoscale Materials Center (both part of DOE’s Office of Science User Facilities) and Brookhaven National Laboratory performed a series of experiments using X-ray, electron, and imaging techniques to characterize the new cathode material at rest and in operation.
These tests collectively evaluated the material at the cathode, particle and atomic levels and provided a complete picture of its composition, structure and performance.
More information:
Tongchao Liu et al, Ultrastable cathodes enabled by a dual compositional and structural gradient design, Natural energy (2024). DOI: 10.1038/s41560-024-01605-8
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