Using Thin Films to Study Capacity Fade Mechanisms Observed in Conventional LiV₃O₈ cathodes

Capacity fade mechanisms can be difficult to pinpoint in conventional composite cathodes for Li-ion batteries. Capacity fade associated with the cathode can be caused by: (1) irreversible reactions involving the active material, possibly with the electrolyte, (2) degraded integrity of the composite structure, or (3) anode contamination by cathode dissolution products. Post-cycling characterization can be difficult, because these potential capacity fade mechanisms would occur in a complicated cathode structure with: porosity, possible microcracks, irregular surfaces, and multiple particulate phases (active material, binder, and conducting additive). Thin film cathodes were used to study how LiV3O8 (LVO) microstructure affects cycling performance without the added complications associated with a conventional composite cathode structure. Thin film cathodes were produced by magnetron sputtering a Pt current collector layer and an LVO layer onto alumina disks. The thin film geometry with its uniform layered structure was well suited for analysis of pristine and cycled LVO cathodes by several characterization techniques, including transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The cycling performance of heat-treated thin film cathodes was directly compared to that of conventional composite cathodes with heat-treated sol-gel derived LVO powder. Thin film and sol-gel derived LVO cathodes showed excellent microstructural and electrochemical agreement as a function of heat-treatment temperature. Depending on heat-treatment temperature, the microstructure ranged from amorphous (as-deposited or low temperature heat-treatment) to coarse-grained crystalline material (higher temperature). The maximum specific capacity and the maximum capacity fade were both observed for LVO nanocrystalline material fired at intermediate temperature, where reduced capacity and reduced capacity fade were observed for higher and lower firing temperatures. Capacity fade was linked to vanadium dissolution, where amorphous LVO cathodes showed much lower vanadium dissolution than nanocrystalline LVO material. This work was supported as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012673.