Investigation of Surface Coating on High Voltage Lithium Ion Cathode Materials Using Combined TEM and Atom Probe Tomography
EMSL Project ID
46397
Abstract
Lithium ion batteries are the top candidates for on-board energy storage in plug-in hybrid electric vehicles (PHEV) and fully electric vehicles due to a higher energy density and rate capability compared to other battery chemistries such as Ni-MH. One group of materials that shows promising characteristics is the layered Li-excess nickel manganese oxides which has a high operating voltage (4.6 V vs Li) and more than 250 mAh/g capacity. This results in a much higher energy density than current commercial Li-ion batteries. In contrast, LiCoO2, at an operating voltage of only 3.7 V, only has a reversible capacity of about 140 mAh/g. Currently, the properties of electrode materials still cause them to fall short of practical performance goals. Since these materials are complex oxides, they usually display less than desirable transport properties, leading to low performance at high rates of discharge. In addition, lithium excess nickel manganese oxides experience significant permanent capacity loss on the first cycle, limiting energy density for later cycles.Many surface coatings have been used on lithium ion battery electrode materials to improve transport properties and reduce irreversible capacity (IRC) loss in many cases. Generally, the mechanism for performance improvement is well understood, such as coating non-conductive LiFePO4 with electrically conductive carbon. However, there are cases, in particular with lithium excess layered oxides, where coating with a normally non-conductive material such as Al2O3 or a Li-Ni-PO4 compound actually improves electrode properties. Whether the coating protects the surface from lithium-consuming reaction with the electrolyte, or restricts the anomalous cation migration to the particle surface is unclear, as is the mechanism by which it does these things. These lithium excess nickel manganese oxides are known for large irreversible capacity after the first charge. The reason for this is still not fully understood. A combined study using electron microscopy with electron energy loss spectroscopy (EELS), synchrotron x-ray diffraction, and first-principles computation has been carried out by Meng et al. on the aforementioned Li-excess nickel manganese oxides to look at surface and bulk structural changes that occur due to lithiation/delithiation. This and other studies point to a migration of transition metal atoms to the surface of the particles and a subsequent loss of capacity after cycling. Atom probe tomography (APT) would be an ideal technique to precisely identify the nature of the particle surface, since it can map the type and 3-D location of each atom to within less than a nanometer. Using this to understand nature of the first cycle capacity loss, researchers can design more effective materials for this family of electrodes.
APT is a powerful technique for studying surface layers of any material, especially when combined with high-resolution transmission electron microscopy (TEM).
Though we are encouraged by advances in recent years on the analysis of wider varieties of materials, very little work has been done on the family of materials used in Li-ion batteries. We intend to establish standards for this technique in our work since interfaces and particle coatings are so important to electrode performance.
Project Details
Project type
Exploratory Research
Start Date
2011-11-07
End Date
2012-11-11
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Santhanagopalan D, DK Schreiber, DE Perea, R Martens, Y Janssen, P Kalifah, and YS Meng. 2015. "Effects of Laser Energy and Wavelength on the Analysis of LiFePO4 Using Laser Assisted Atom Probe Tomography." Ultramicroscopy 148:57-66. doi:10.1016/j.ultramic.2014.09.004