Optimizing Sintering Conditions of Garnet Electrolytes for Scalable All Solid State Li-Ion Batteries
Authors: McOwen Dennis W. , Hitz Gregory , Wen Yang , Gong Yunhui , Hamann Tanner , Wachsman Eric D. 
Published: February 2020

Li-ion batteries have become prevalent in several consumer markets due to the many inherent advantages of Li-ion chemistry. However, concerns continue to grow over the extreme toxicity and flammability of the organic electrolyte employed in commercial Li-ion batteries. Several approaches to mitigate these concerns mainly involve the use of different types of additives acting in different capacities to reduce the possibility and/or extremity of cell failure. However, these additives too often come at the cost of decreased cell lifetime, efficiency, and/or power. On the other hand, solid-state electrolytes made of lithium garnet-type ceramics (e.g. Li7La3Zr2O12 or LLZ) are inherently non-flammable and contain no potentially toxic fluorine. Another key advantage of garnet electrolytes are that they are electrochemically stable to Li metal, thermally stable, and mechanically strong. Thus, solid-state Li-ion batteries promise to drastically increase energy density by enabling the use of Li metal electrodes, and eliminate the hazards typically associated with the use of traditional Li-ion batteries (e.g., venting of toxic gases, explosion). Over the last several years, researchers have made great strides in developing solid-state electrolytes with high Li ion conductivity (> 10-4 S cm-1)1that are now comparable with organic electrolytes.
A main criticism of garnet electrolytes is the high temperature (~1200 °C) and long heating time (≥ 12 h) required for sintering to obtain a high density, high conductivity electrolyte (> 10-4 S cm-1). However, the lithium in the garnet structure is volatile above 800°C, which can cause loss of the garnet phase. To alleviate this, additional garnet powder is typically used as a powder bed to increase the lithium activity in the vapor phase, reducing lithium loss and protecting the electrolyte.
The sintering conditions of LLZ and other garnets can thus be energy-intensive and use excessive garnet material for a powder bed. This has negative implications for the cost and scalability of solid-state Li-ion batteries and must be addressed. Most research concerning garnet electrolyte for Li-ion batteries, however, is focused on improving the ionic conductivity. Reducing the sintering time/temperature is at least as critical as the conductivity for the large-scale feasibility of solid-state Li-ion batteries using garnet electrolyte, and reduction or substitution of the powder bed with another lithium containing material is also desirable. Recent studies using dopants in the garnet structure (Li7La2.95Ca0.05Zr1.75Nb0.25O12)2 have reduced the sintering temperature significantly to 1050 °C, while achieving a conductivity of 6 x 10-4 S cm-1, demonstrating there are ways to influence the sintering and process conditions. In addition to composition, particle geometry and sintering atmosphere also play important roles in the sintering process—their roles and impact on electrolyte structure, density, and conductivity will be presented. Recent progress in developing methods to further decrease the sintering temperature and time while maintaining a high density and high conductivity electrolyte product will be discussed.

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