Optimized Processing Methods to Produce LLZO based Solid-State Li Metal Batteries

Li-ion batteries are the dominant electrical storage technology for both consumer electronics and electrified transportation. They have become predominant in these roles due to their low weight and high current density compared to other rechargeable battery designs. However, their liquid electrolytes are flammable, therefore the use of a solid electrolyte would provide greater safety. The solid electrolyte Li₇La₃Zr₂O₁₂ (LLZO) has high conductivity of approximately 3x10^-4 S/cm and is chemical stabile against lithium metal which makes it an attractive choice to replace liquid electrolytes. Improvements in LLZO processing and properties are needed for it to be a practical alternative for use in lithium batteries. This thesis addresses several incremental advances to improve sintering and boost the performance of LLZO.

Additions to LLZO to promote grain size refinement and glassy grain boundary additions are investigated to both improve resistance to Li dendrite propagation and increase the critical current density (CCD) of LLZO in Li-stripping experiments. MgO additions to LLZO refine grain size and yield samples with 94.5% of theoretical density with a conductivity of 3.5x10^-4 S/cm at 25 ºC. Glassy borates additions (Li₃BO₃ and Li_2.2C_0.8B_0.2O₃ (LCBO)) promote liquid phase sintering and increase the critical current density by 44.5% and 20% compared to 0 wt% borate LLZO.

The challenges of processing LLZO which are addressed in this material are wetting in ambient air, removal of surface contamination and optimization of cell processing by focusing on pressure, temperature, and sintering. Li-LLZO interfaces demonstrate high resistances due to poor wetting of Li to LLZO exposed to ambient air. Multiple approaches to modifying the anode-electrolyte interface are investigated to improve wetting and CCD. Polishing LLZO surfaces improves wetting and CCD but is time consuming. Alloying Li with Sn, Al, and Mg demonstrates low interfacial resistances and improves CCD. Intercalation coatings MoS₂ and graphite applied to LLZO yield low interfacial resistances, promote wetting of Li on LLZO, and improves CCD with graphite being the superior of the two coatings. LCBO interfacial coatings are also considered but decrease conductivity by an order of magnitude and cause almost immediate shorting during cycling.

Acid etching (HCl) LLZO successfully removes surface contaminants, promotes Li-LLZO wetting, lowers interfacial resistance, and increases LLZO atmospheric stability. However, extended etching damages the electrolyte and accelerates dendrite penetration.

Cell pressure during stripping experiments was found to play an important role. By promoting Li creep, deleterious pore formation is decreased and CCD is enhanced. Increasing cell temperature further aids Li creep and increases CCD. A cell pressure of 4.12 MPa at 70 ºC increased CCD by 1612% to 1284 µA/cm², the highest critical current density achieved in these studies.

Perhaps most importantly, a method for rapidly sintering LLZO to high density in short periods was developed. LLZO powder preparation methods needed modification to be compatible with rapid sintering rates (300 ºC/s). It was found necessary to finely mill the LLZO and remove adsorbed gasses prior to sintering to avoid cracking. The rapid heating schedule developed enabled densification of LLZO to 97% theoretical density with a conductivity of 3.6x10^-4 S/cm in only 93 seconds.

The advances that result from this work demonstrate that LLZO is a realistic alternative for use in lithium batteries. However, similar studies are needed on the cathode side to yield commercially viable batteries.