Date of Award
Master of Science (MS)
School of Materials Science and Engineering
Solid-state lithium ion batteries are currently at the forefront of investigations to replace conventional lithium ion batteries in order to improve overall safety and device performance. Researchers have investigated many substitutes to organic based conventional liquid electrolytes that result in high levels of Li ion conductivity. Cubic Li7La3Zr2O12 (LLZO) is a leader among solid-state electrolyte research. Unfortunately, pure LLZO at room temperature is generally a tetragonal structure that is significantly less conductive than cubic LLZO. This research uses a gallium dopant to reach the highly conductive cubic LLZO structure. However, a dopant is not enough to ensure a highly conductive LLZO sample. Lithium evaporates during the calcining and sintering stages of sample preparation. In order to reach an actual composition close to the targeted composition, additional lithium must be added, or lithium loss must be prevented.
The goal of this research is to investigate the best methods and amounts of excess lithium to add in order to obtain a composition as close as possible to the targeted compositions. The most common method in literature to combat lithium loss is the addition of an excess lithium precursor to the initial set of precursors. This research studied the effect of excess Li2CO3 precursors in LLZO at zero, ten, twenty, and thirty weight percent excess. Another method studied in this research to prevent lithium loss was using a boating technique with excess lithium carbonate. Half a gram of lithium carbonate was placed on the edges of the sintering crucible, while the sample pellets were in the middle of the crucible untouched by the excess powder. The lithium carbonate powder evaporated during sintering resulting in a build-up of lithium vapor pressure in the crucible which will aid in lithium retention as it is more difficult to evaporate in high vapor pressure conditions.
This research found that gallium doped LLZO (Ga0.5Li5.5La3Zr2O12) with ten weight percent excess Li2CO3 precursor along with the boating technique resulted in the highest density and highest conductivity of all samples tested. While more testing needs to be done on this research, the data shows how important lithium content is to produce a highly conductive solid electrolyte.
Bolek, Katherine, "The Effect of Excess Lithium on the Phase Formation, Structure and Electrical Properties of LLZO Garnet Structured Solid-State Electrolyte" (2020). All Theses. 3405.