Reactive Sintering of Garnet Type LLZTO from Pyrochlore Synthesized Using a Nonaqueous Sol-Gel Method

Garnet-type solid-state electrolytes such as Li 7 La 3 Zr 2 O 12 (LLZO) are promising ceramic electrolytes for all-solid-state batteries because of their high (electro)chemical stability and ionic conductivity. LLZO is typically synthesized from oxide precursors via solid-state reaction, but use of other precursors can lead to improved phase purity and homogenous dopant distribution. We recently demonstrated that pyrochlores synthesized using molten salt synthesis can serve as starting precursor for LLZO, a process we call “pyrochlore-to-garnet” (P2G) [1]. Here, a new, scalable synthesis method for multi-doped pyrochlore is demonstrated using non-aqueous sol-gel methods [2]. The P2G method provides an alternative way to prepare dense LLZO material with greatly reduced sintering time and different microstructure properties compared to other methods. For Ta-doped LLZO (LLZTO), the dense LLZTO ceramic is directly obtained via reactive sintering of the multi-doped pyrochlore with a lithium salt and displays high relative density and ionic conductivity 0.4-0.7 mS/cm, which is comparable to LLZTO prepared by solid-state reaction (SSR), using only 2 hours sintering at 1100 degrees Celsius. Investigation of the synthesis parameters shows that liquid phase sintering is important for achieving high pellet densities and phase-purity in the P2G process. Microstructure analysis also shows that the P2G method results in LLZTO with average grain size of around 3 µm, which is much smaller than the grain sizes (as large as 20 µm) seen in SSR LLZTO. The P2G method also allows for preparation of cubic phase LLZO with other dopants using similar conditions. The P2G method is also applied to prepare other kinds of garnets that display mixed ionic and electronic conductivity, such as Ga and Cr doped Li 7 Pr 3 Zr 2 O 12 . To better understand the effects of microstructure and grain size on the lithium dendrite propagation behavior, a detailed investigation was carried out on LLZTO prepared with the P2G process and conventional SSR followed by conventional pressureless sintering [3]. Both preparation methods were controlled to keep the phase and elemental composition, ionic and electronic conductivity, relative density, and area specific resistance of the pellets constant. Reflection electron energy loss spectroscopy (REELS) and X-ray photoelectron spectroscopy (XPS) confirmed that both types of LLZTO have similar bandgaps and chemical states. Galvanostatic Li stripping/plating and linear sweep voltammetry measurements show that P2G LLZTO can withstand higher critical current densities (up to 0.4 mA/cm 2 in bidirectional cycling and > 1 mA/cm 2 for unidirectional) than those seen in SSR LLZTO. Postmortem examination reveals much less Li deposition along the grain boundaries of P2G LLZTO, particularly in the bulk of the pellet, compared to SSR LLZTO after cycling. The improved cycling behavior in P2G LLZTO despite the higher grain boundary area could be from more homogenous current density at the interfaces and different grain boundary properties arising from the liquid-phase, reactive sintering method. This work shows the P2G is a promising and versatile alternate method for the synthesis and processing of many kinds of LLZO materials. References: [1] Weller, J. M.; Chan, C. K. Pyrochlore Nanocrystals as Versatile Quasi-Single-Source Precursors to Lithium Conducting Garnets. Journal of Materials Chemistry A 2020 , 8 (34), 17405-17410. DOI: 10.1039/D0TA05842D. [2] Guo, J.; Weller, J. M.; Yang, S.; Bhat, M. H.; Chan, C. K. Reactive Sintering of Garnet-Type Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) from Pyrochlore Precursors Prepared Using a Non-Aqueous Sol–Gel Method. Ionics 2023 , 29, 581-590. DOI: 10.1007/s11581-022-04837-4. [3] Guo, J.; Chan, C.K. Lithium Dendrite Propagation in Ta-Doped Li7La3Zr2O12 (LLZTO): Comparison of Reactively Sintered Pyrochlore-to-Garnet vs LLZTO by Solid-State Reaction and Conventional Sintering. ACS Applied Materials & Interfaces 2024 , 16 (4) 4519-4529. DOI: 10.1021/acsami.3c11421.