Layered lithium transition metal oxide cathodes feature a relatively high capacity, making them of importance for Li-ion batteries. However, they also suffer from crystal and interfacial structural instability under aggressive electrochemical and thermal driving forces; this leads to rapid performance degradation and severe safety concerns.
Now, researchers at the US Department of Energy’s (DOE) Argonne National Laboratory, with colleagues in China and the US, have developed a new coating for layered lithium transition metal oxide cathodes that can help solve these and several other potential issues with lithium-ion batteries all in one stroke. A paper describing the development is published in Nature Energy.
… we report a transformative approach using an oxidative chemical vapour deposition technique to build a protective conductive polymer (poly(3,4-ethylenedioxythiophene)) skin on layered oxide cathode materials. The ultraconformal poly(3,4-ethylenedioxythiophene) skin facilitates the transport of lithium ions and electrons, significantly suppresses the undesired layered to spinel/rock-salt phase transformation and the associated oxygen loss, mitigates intergranular and intragranular mechanical cracking, and effectively stabilizes the cathode–electrolyte interface. This approach remarkably enhances the capacity and thermal stability under high-voltage operation. Building a protective skin at both secondary and primary particle levels of layered oxides offers a promising design strategy for Ni-rich cathodes towards high-energy, long-life and safe lithium-ion batteries.—Xu et al.
An illustration of the structural stability of both secondary/primary particle coating and secondary particle coating only after long-term cycling. The oCVD process led to conformal PEDOT coating on both secondary and primary particles, resulting in no particle cracking after a long cycle life, while secondary particle coating only by conventional processes resulted in particle cracking after a long cycle life. Xu et al.
The coating we’ve discovered really hits five or six birds with one stone.—Khalil Amine, Argonne distinguished fellow and battery scientist and co-corresponding author
In the research, Amine and his colleagues took particles of Argonne’s pioneering nickel-manganese-cobalt (NMC) cathode material and encapsulated them with a sulfur-containing polymer called PEDOT. This polymer provides the cathode a layer of protection from the battery’s electrolyte as the battery charges and discharges.
Unlike conventional coatings, which only protect the exterior surface of the micron-sized cathode particles and leave the interior vulnerable to cracking, the PEDOT coating had the ability to penetrate to the cathode particle’s interior, adding an additional layer of shielding.
In addition, although PEDOT prevents the chemical interaction between the battery and the electrolyte, it does allow for the necessary transport of lithium ions and electrons that the battery requires in order to function.
This coating is essentially friendly to all of the processes and chemistry that makes the battery work and unfriendly to all of the potential reactions that would cause the battery to degrade or malfunction.—Argonne chemist Guiliang Xu, first author
The coating also largely prevents another reaction that causes the battery’s cathode to deactivate. In this reaction, the cathode material converts to another form called spinel.
The combination of almost no spinel formation with its other properties makes this coating a very exciting material.—Khalil Amine
The PEDOT material also demonstrated the ability to prevent oxygen release, a major factor for the degradation of NMC cathode materials at high voltage. The PEDOT coating was also found to be able to suppress oxygen release during charging, which leads to better structural stability and also improves safety.
Amine indicated that battery scientists could likely scale up the coating for use in nickel-rich NMC-containing batteries. With the coating applied, the researchers believe that the NMC-containing batteries could either run at higher voltages—thus increasing their energy output—or have longer lifetimes, or both.
To perform the research, the scientists relied on two DOE Office of Science User Facilities located at Argonne: the Advanced Photon Source (APS) and the Center for Nanoscale Materials (CNM). In situ high-energy X-ray diffraction measurements were taken at beamline 11-ID-C of the APS, and focused ion beam lithography and transmission electron microscopy were performed at the CNM.
The research was funded by DOE’s Office of Science, Office of Basic Energy Sciences and the Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office.
Gui-Liang Xu, Qiang Liu, Kenneth K. S. Lau, Yuzi Liu, Xiang Liu, Han Gao, Xinwei Zhou, Minghao Zhuang, Yang Ren, Jiadong Li, Minhua Shao, Minggao Ouyang, Feng Pan, Zonghai Chen, Khalil Amine & Guohua Chen (2019) “Building ultraconformal protective layers on both secondary and primary particles of layered lithium transition metal oxide cathodes” Nature Energy doi: 10.1038/s41560-019-0387-1