A new cathode material exceeds the capacity limit of traditional lithium-ion batteries

As the demand for smartphones, electric vehicles and renewable energy continues to grow, scientists are looking for ways to improve lithium-ion batteries-lithium-ion batteries are the most common type of battery in household electronics and grid-scale energy storage Promising solutions. Increasing the energy density of lithium-ion batteries can promote the development of advanced technologies with long-lasting batteries, as well as the widespread use of wind and solar energy. Now, researchers have made significant progress in achieving this goal. A collaboration led by scientists from the University of Maryland (UMD), the Brookhaven National Laboratory of the U.S. Department of Energy (DOE) and the U.S. Army Research Laboratory has developed and studied a new type of cathode material that can triple the energy density of lithium ions. Battery electrode. Their research results were published in Nature Communications on June 13. 'Lithium-ion batteries consist of an anode and a cathode,' said Xiu Lin Xiu, a UMD scientist and one of the paper's lead authors. 'Compared with the large capacity of commercial graphite anodes used in lithium-ion batteries, the capacity of cathodes is more limited. Cathode materials have always been the bottleneck to further increase the energy density of lithium-ion batteries.' Scientists at UMD have synthesized a new cathode material , This is an improved engineering form of iron trifluoride (FeF3), composed of cost-effective and environmentally friendly elements-iron and fluorine. Researchers have been interested in using compounds such as FeF3 in lithium-ion batteries because they have a higher capacity than traditional cathode materials. 'The materials commonly used in lithium-ion batteries are based on intercalation chemistry,' said Enyuan Hu, a Brookhaven chemist and one of the paper's lead authors. 'This type of chemical reaction is very effective; however, it only transfers one electron, so the cathode capacity is limited. Some compounds such as FeF3 can transfer multiple electrons through a more complex reaction mechanism, called a conversion reaction.' FeF3 has the potential to increase the cathode capacity, but the performance of this compound in lithium-ion batteries is not ideal because its conversion reaction has three complications: poor energy efficiency (lag), slow reaction speed, and side reactions that may lead to poor cycle life . To overcome these challenges, scientists added cobalt and oxygen atoms to FeF3 nanorods through a process called chemical substitution. This allows scientists to manipulate the reaction pathway and make it more 'reversible.' 'When lithium ions are inserted into FeF3, this substance is converted into iron and lithium fluoride,' said Sooyeon Hwang, a co-author of the paper and a scientist at the Brookhaven Center for Functional Nanomaterials (CFN). 'However, the reaction is not completely reversible. After being replaced with cobalt and oxygen, the main skeleton of the cathode material is better maintained, and the reaction becomes more reversible.' In order to study the reaction pathway, scientists conducted research on CFN and National Synchrotron Light Source II (NSLS). -II)-Two DOE science user facility offices in Brookhaven conducted multiple experiments. First at CFN, researchers used powerful electron beams to observe FeF3 nanorods at a resolution of 0.1 nanometers-a technique called transmission electron microscopy (TEM). TEM experiments allow researchers to determine the exact size of the nanoparticles in the cathode structure and analyze how the structure changes between different stages of the charge-discharge process. They saw faster reactions to replace nanorods. 'TEM is a powerful tool for characterizing very small-scale materials, and it can also study reaction processes in real time,' said DongSu, a CFN scientist and co-author of the study. 'However, we can only use TEM to see a very limited sample area. We need to rely on the synchrotron technology of NSLS-II to understand the function of the entire battery.' On the X-ray powder diffraction (XPD) beamline of NSLS-II, Scientists guided ultra-bright X-rays through the cathode material. By analyzing light scattering, scientists can 'see' other information about the structure of the material. 'In XPD, we have performed a paired distribution function (PDF) measurement, which can detect a large number of subway ranks,' said Bai Jianming, a co-author of the paper and NSLS-II scientist. 'PDF analysis of the discharge cathode clearly shows that chemical substitution promotes electrochemical reversibility.' Combining highly advanced imaging and microscopy techniques on CFN and NSLS-II is a key step in evaluating the function of cathode materials. 'We have also performed advanced calculation methods based on density functional theory to crack the atomic-scale reaction mechanism,' said Xiao Ji, a scientist at UMD and co-author of the paper. 'This method shows that chemical substitution transforms the reaction into a highly reversible state by reducing the particle size of iron and stabilizing the rock salt phase.' Scientists at UMD said that this research strategy can be applied to other high-energy conversion materials, and future studies may use this Ways to improve other battery systems.