2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
3, Stony Brook University, Stony Brook, New York, United States
4, Rensselaer Polytechnic Institute, Troy, New York, United States
5, Rensselaer Polytechnic Institute, Troy, New York, United States
6, Stony Brook University, Stony Brook, New York, United States
7, Stony Brook University, Stony Brook, New York, United States
8, Brookhaven National Laboratory, Upton, New York, United States
9, Rensselaer Polytechnic Institute, Troy, New York, United States
Cathode microstructure plays a vital role in the electrochemical performance of Li ion batteries. Understanding the crystallization process during annealing of electrode thin films can provide potential new avenues to control electrode microstructure in thin film batteries. A novel in-situ transmission electron microscopy (TEM) experiment is being designed to correlate microstructural evolutions to degradation mechanisms in LiV3O8 and V2O5 thin film battery systems. This design uses thin films of amorphous solid electrolyte (LiPON), amorphous carbon current collectors and a lithiated amorphous silicon anode.
Our first focus is understanding in detail the crystallization of the cathode films, which are amorphous as deposited. For these studies, c.50-100nm Li-V-O and V-O films are RF sputter deposited from LiV3O8 (Ar:O2=3:1) and V targets (Ar:O2=1:1) onto Si TEM discs which contain 50 nm thick silicon nitride membrane windows. Thermal annealing experiments are carried out both in vacuum (i.e. in-situ to the TEM) and Ar atmospheres for the Li-V-O thin films, and vacuum and ambient atmospheres for V-O films. Crystallization of Li-V-O thin films, depending on the annealing atmosphere (vacuum vs Ar) and temperature, resulted in either completely delithiated phases (V2O3, VO2 and V2O5) or uncatalogued Li-V-O phases but not the expected LiV3O8 phase highlighting the challenge in maintaining Li concentration. We overcome this challenge by using thicker films (~1 μm), where the Li concentration at the middle of these films is close to LiV3O8 phase and formed LiV3O8 electron transparent films (~100nm) by controlled FIB nanofabrication.
In-situ TEM annealing results of sputter deposited V-O films show the onset of V2O5 phase crystallization around 250oC in the areas not exposed to the e-beam. However, annealing to higher temperatures- 400oC and 500oC(1hr) in TEM atmosphere (pressure:10-6 Pa) results in partial phase transformation to oxygen deficient V-O phases (VO2, V4O7 and V6O13) depending on temperature and electron beam irradiation. Annealing of these as-deposited thin films in ambient atmosphere up to 500oC (1hr) results in desired V2O5 phase. Thus, annealing in ambient atmosphere is observed to form V2O5 thin films with different microstructures for in-situ TEM thin film battery testing.
These studies establish the ability to maintain suitable cathode stoichiometry and microstructure in ultra-thin film form, which is a crucial step for the in-situ TEM battery studies we are developing, and potentially for other applications.
Acknowledgements: - This work is supported as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Dept. of Energy, Office of Science, Basic Energy Sciences (award #DE-SC0012673). Work at RPI made extensive use of the cleanroom and characterization facilities in the Center for Materials, Devices and Integrated Systems (cMDIS).