Safe and efficient energy storage is important for American prosperity and security. With the adoption of both renewable energy sources and electric vehicles on the rise around the world, it is no surprise that research into a new generation of batteries is a major focus. Researchers have been developing batteries with higher energy storage density and, thus, longer driving range. Other goals include shorter charging times, greater tolerance to low temperatures and safer operation.
One of the more promising such batteries has a lithium-containing cathode supplemented with nickel, manganese and cobalt (NMC). At the U.S. Department of Energy's (DOE) Argonne National Laboratory, a team of scientists has recently developed a new coating method for NMC cathodes with high nickel content, which boosts the energy density substantially. The cathode is the positively charged battery component that supplies lithium ions that shuffle between it and the battery's negatively charged electrode, called the anode, during cycling.
"An NMC cathode was invented at Argonne in the early 2000s and has been used for lithium-ion batteries in many electric cars," said Guiliang Xu, a chemist at Argonne. "Consumers want such batteries to have an even higher energy storage capacity, and thus longer driving range, and to charge faster. The nickel-rich version of NMC offers that. But these performance demands have historically caused rapid cathode degradation with repeated charging and discharging. The key problem has been particle cracking."
The repeated charging of batteries under conditions of high voltage and rapid recharge leads to structural instability and breakdown over time. To overcome the problem, Argonne scientists developed a new coating that allows the cathode particles to withstand the fracturing in their crystalline structure that had previously occurred upon cycling. They call this material "epitaxial entropy-assisted coating," or EEC for short. According to Xu, "entropy assistance" ensures that the coating helps to prevent the breakdown of the material beneath it due to a thermodynamic effect, which leads materials to naturally become destabilized over time.
The resulting cathode allows fast charging with longer lifespan and greater durability. The coated cathode also can better tolerate operation in cold weather.
"The industry standard is to spray the coating onto the material, which does not always provide good coverage," Xu said. "The material we developed can be applied much more uniformly." He stressed, however, that balancing uniformity in the material and the quality of the material chosen is key. "If you do not coat the material uniformly, it will not protect the particles. At the same time, if it is coated too thickly or is of bad conductive quality, it will cause problems with repeated charging."
Cell tests showed that the EEC coating maintained much better cathode stability than more traditional coatings, even after hundreds of recharge cycles. Traditional coatings typically broke up into fragmented pieces after repeated recharging.
A key aspect of the team's research involved analyzing the structure of the coated cathode down to the nanoscale, including while it was being charged and discharged under different operating conditions. This involved use of two of Argonne's DOE Office of Science user facilities, the Advanced Photon Source (APS) and Center for Nanoscale Materials (CNM). The many analysis techniques possible at these facilities made it possible to examine the ways in which the EEC coating acted and interacted with the cathode material during cycling at the atomic scale, and then to determine why these reactions occurred.
At the APS, the 17-BM beamline was used to investigate structural changes and strain evolution in the cathodes during initial fabrication and under operational conditions. The researchers there also determined the thermal stability of cycled cathodes. With this information in hand, the team could optimize the EEC cathode fabrication and performance. In addition, APS's 34-ID-C beamline visualized the structural changes and strain evolution of single crystals from the cathode materials during initial charging. The results provided valuable insights into material behavior and degradation mechanisms under operational conditions.
At the CNM, scientists determined the composition and structure of the coating layers by using transmission electron microscopy. "From high-resolution electron microscopy imaging and composition analysis, we characterized the epitaxial entropy-assisted coating at the nanoscale," said Argonne nanoscientist Yuzi Liu.
"This is one of the advantages of Argonne," added Argonne physicist Tianyi Li. "We have powerful scientific tools to let us understand challenges in our projects, develop strategies to overcome them and then examine how these strategies will perform in real-world situations."
Xu noted that the new coating represented a dramatic improvement. "Overall, the new coating performs much more effectively than industry-standard coatings, and even than a previous Argonne-produced coating," he said. "It enhances mechanical stability of the particles and allows the materials to function more efficiently and safely for longer periods of time."
This paper appeared on February 29 in Nature Energy. The work was funded by the DOE Office of Vehicle Technologies, Office of Energy Efficiency and Renewable Energy. In addition to Xu, Argonne contributors include Chen Zhao, Xiang Liu, Inhui Hwang, Tianyi Li, Xinwei Zhou, Junjing Deng, Yan Qin, Zhenzhen Yang, Guanyi Wang, Wenqian Xu, Chengjun Sun, Wonsuk Cha, Ross Harder, Yi Jiang, Tekin Bicer, Wenquan Lu, Luxi Li, Yuzi Liu and Khalil Amine.