| AVS 61st International Symposium & Exhibition | |
| Energy Frontiers Focus Topic | Tuesday Sessions |
| Session EN+EM+NS-TuA |
| Session: | Charge Storage Materials and Devices |
| Presenter: | Ryan Thorpe, Rutgers, the State University of New Jersey |
| Authors: | R. Thorpe, Rutgers, the State University of New Jersey S. Rangan, Rutgers, the State University of New Jersey A. Howansky, Rutgers, the State University of New Jersey R.A. Bartynski, Rutgers, the State University of New Jersey |
| Correspondent: | Click to Email |
Li-ion conversion batteries can store 2-3 times more charge than intercalation batteries by utilizing the full range of oxidation states of their constituent divalent or trivalent transition metal compounds during discharge. A prototypical conversion compound is CoO, which follows the reaction
2Li+ + 2e- + Co(2+)O --> Li2O + Co(0).
Cobalt oxide and other transition metal oxides are attractive for use as Li-ion anodes in portable electronics due to their high charge storage capacity and moderate voltage versus Li+/Li0. However, the cycling stability of conversion electrodes is poor, and capacity losses have thus far prevented their implementation.
In order to understand phase progression during the conversion reaction of CoO, high-purity CoO thin films grown in UHV were sequentially exposed to atomic lithium. The electronic structure of the pristine films and of the products of lithiation was studied using x-ray photoemission spectroscopy (XPS), UV photoemission spectroscopy, and inverse photoemission spectroscopy. The crystal structure and film reorganization were probed in parallel with transmission electron microscopy (TEM) and scanning tunneling microscopy.
The amount of CoO reduction for a given Li dose was observed to be highly dependent upon the temperature at which lithiation was performed. At 150oC, Li mobility in the active material was sufficient to allow full reduction of the CoO film as confirmed by XPS. Consistent with electrochemically lithiated CoO electrodes, precipitation of Co nanoparticles in a Li2O matrix was observed in TEM images. However, at room temperature, the Li-rich overlayers that formed on the CoO film after initial lithiations inhibited further Li diffusion. This could be due to the intrinsically poor kinetic properties of Li2O or to the formation of Li2O2 and/or LiOH passivating films.
The reactivity of CoO films was also found to depend on the orientation of the film. CoO(100) films exhibited a higher degree of conversion for a given Li exposure than polycrystalline films. STM and angle-resolved XPS of these films have been used to investigate the differences between these two film morphologies upon exposure to Li.