Garnet-based lithium ion conducting solid electrolyte, with nominal composition Li7La3Zr2O12, is a promising solid electrolyte for all solid-state batteries due to its high Li-ion conductivity and stability with lithium metal anode . However, lithium dendrite growth during lithium deposition on the negative electrode causes short-circuit with increasing current density [2, 3]. In this work, we prepared garnet-based Li6.5La3Zr1.5Ta0.5O12 (LLZT) pellets, with different synthesis conditions, and investigated correlation between the pellet structure and short-circuit prevention.
LLZT powder and pellets were prepared using a solid-state reaction and spark plasma sintering technique, respectively, as reported in our previous work . The prepared pellets are designated as LLZT-air (conventional method), LLZT-w/o-air (without exposing to air), and LLZT-2calc (calcined twice with addition of LiOH) . A combination of structural and chemical characterization techniques, such as scanning electron microscopy and fourier transform infrared spectroscopy, revealed presence of LiOH and Li2CO3 on the LLZT powder and effects of excessive lithium salt on the microstructure of the pellets. To investigate the short-circuit prevention, symmetric cells of Li | LLZT | Li were assembled. The cells were cycled at various current densities (1 hour per cycle) at 25°C, which were gradually increased until the cells showed voltage drop [2, 3]. Direct current (DC) polarization curves demonstrate that the voltage fluctuation during cycling increased, as the current density increased. This is due to reduced effective electrode area because voids were formed by the Li dissolution at the Li | LLZT interface. The critical current density (CCD), at which voltage drop occurred, depended on the specimens. LLZT-2calc exhibited the highest CCD of 0.6 mA cm−2, while LLZT prepared without exposing to air showed rather poor short-circuit prevention (CCD of 0.15 mA cm−2). This suggests that Li2CO3 and/or LiOH on surface of starting LLZT powder is effective to improve density of grain boundaries in the pellets to suppress the short circuit. Microstructural analysis and detailed electrochemical impedance analysis demonstrated that Li2CO3 and LiOH not only facilitate sintering on the grain boundary, but also physically suppress the lithium growth by filling voids among LLZT grains .
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