Abstract
The interface stability versus Li represents a major challenge in the development of next-generation all-solid-state batteries (ASSB), which take advantage of the inherently safe ceramic electrolytes. Cubic Li7La3Zr2O12 garnets represent the most promising electrolytes for this technology. The high interfacial impedance versus Li is, however, still a bottleneck toward future devices. Herein, we studied the electrochemical performance of Fe3+-stabilized Li7La3Zr2O12 (LLZO:Fe) versus Li metal and found a very high total conductivity of 1.1 mS cm–1 at room temperature but a very high area specific resistance of ∼1 kΩ cm2. After removing the Li metal electrode we observe a black surface coloration at the interface, which clearly indicates interfacial degradation. Raman- and nanosecond laser-induced breakdown spectroscopy reveals, thereafter, the formation of a 130 μm thick tetragonal LLZO interlayer and a significant Li deficiency of about 1–2 formula units toward the interface. This shows that cubic LLZO:Fe is not stable versus Li metal by forming a thick tetragonal LLZO interlayer causing high interfacial impedance.
Highlights
Lithium-oxide garnets based on Li7La3Zr2O12 (LLZO) show very high ionic conductivities coupled with Li+ transfer numbers approaching 1
By combining Raman spectroscopy and nanosecond laserinduced breakdown spectroscopy we discovered that a Li-deficient tetragonal LLZO interlayer formed at the LLZO|
The RT impedance spectrum of LLZO is shown in Figure 2; it consists primarily of one semicircle
Summary
Lithium-oxide garnets based on Li7La3Zr2O12 (LLZO) show very high ionic conductivities coupled with Li+ transfer numbers approaching 1. The present study is aimed at understanding the interfacial phenomenon taking places at the LLZO|Li interface For this purpose we studied key electrochemical properties including the Li-ion conductivity. X-ray powder diffraction (XRPD) measurements were performed using a Bruker AXS D8 diffractometer with Cu Kα radiation This was done to characterize the synthetic products and to identify all phases present, as well as to determine the symmetry and unit-cell dimension of the garnet. Cyclic voltammetry (CV) was measured using the Li| LLZO:Fe|Au configuration to assess the electrochemical stability window of the garnet For this purpose, a Au blocking electrode was sputtered on one side of the garnet pellet, and the reversible Li electrode was applied on the other side. Isomer shift values are reported relative to the α-iron at RT
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