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Optimizing Sintering Conditions of Garnet Electrolytes for Scalable All Solid State Li-Ion Batteries
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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