Reviewed by Lexie CornerSep 27 2024
A recent article in Nature Communications employed optical and ex-situ dark-field X-ray microscopy (DFXM) to examine the interplay between dendrite proliferation and dislocation formation in solid-state electrolytes. Electrolyte strain patterns and lattice orientation changes related to dendrite growth were studied in LiSn∣Li6.5La3Zr1.5Ta0.5O12∣LiSn symmetric cells.
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Background
Lithium dendrites pose a significant challenge to the development of high-energy Li metal batteries. They can fracture solid-state electrolytes, compromising device safety and performance. Therefore, understanding the factors driving dendrite nucleation and growth is essential for preventing their formation.
Experimental investigations of dislocations on dendrites in solid electrolytes are limited mainly due to the issues in introducing dislocations into ceramics. Additionally, only a limited number of characterization techniques are capable of resolving and visualizing dislocations in ceramic materials. Transmission electron microscopy (TEM), commonly used for this purpose, can be problematic due to the sensitivity of electrolytes to the preparation process.
X-ray techniques with nano resolution, like DFXM, offer promising alternatives for the non-destructive study of bulk materials. DFXM can accurately characterize the intricate three-dimensional orientation states and strain in solid electrolytes. Therefore, this study utilized optical microscopy and DFXM to investigate the microenvironment around a dendrite formed in single-crystalline Li6.5La3Zr1.5Ta0.5O12 (LLZTO).
Methods
Czochralski-derived LLZTO single crystals were oriented using Laue Diffraction and then cut into 3×2×0.5 mm3 cuboids. The 3×2 mm2 sides were mechanically thinned to 200 μm, polished, and cleaned to remove Li2CO3 contamination layers. The two opposite 200 μm × 3 mm sides were coated with a molten LiSn (30 wt.% Sn) alloy. The coated sample was assembled in a homemade brass setup.
An optical microscope was focused on the region of interest, and a current density between 1 and 10 mA/cm2 was applied for 5-20 s to the symmetric LiSn∣LLZTO∣LiSn cell using a potentiostat to facilitate dendrite propagation. The current was stopped when the dendrite reached around halfway through the electrolyte. Subsequently, the samples were stored in an argon-filled glovebox until further characterization.
DFXM was performed on the LLZTO samples containing dendrites. Three scans were conducted: rocking, mosaicity, and axial strain scans. Rocking scans involved a tilt angle (ϕ) range of 0.15 ° to map displacement gradient tensor field components. Mosaicity and axial strain scans assessed distortions along orthogonal tilts (χ and ϕ) and 2θ axis, respectively.
Voxels were associated with (HKL) pole figure subsets to generate center of mass (COM) maps for voxel-level (HKL) orientation. Finally, axial strain scans were used to quantify residual strain by scanning the 2θ axis (Δ2θ = 0.01 °) and reconstructing into the COM maps.
Results and Discussion
The LLZTO sample’s overall angular spread was less than 0.15 ° in the COM maps, with the edges showing more inhomogeneity than the interior. Near the dendrite, the local orientation differed from the surrounding material, indicating that the lattice distortion and curvature around the dendrite were distinct from the matrix.
The rocking curve’s full width at half maximum (FWHM) map showed a significantly higher orientation spread near the dendrite compared to the matrix. This inhomogeneous orientation distribution extended over more than 350 μm. The near-field topography maps from the rocking curve projection provided an effective initial characterization of the sample and dendrite.
The section DFXM maps of mosaicity and strain samples exhibited a two-dimensional (2D) layer (600 nm thick) in the z direction within the bulk. A 2D section of the dendrite was also evident as a line, revealing the complex orientation distribution over ϕ rotation with a 100 nm spatial resolution.
Both the nearfield projection and DFXM results demonstrated a certain orientation inhomogeneity around the dendrite. Notably, dendrite acted as a boundary between negative and positive orientation distribution relative to the rocking curve peak. Additionally, isolated individual dislocations were identified within the field of view, one dislocation pinning the dendrite’s edge.
Similarly, in the χ COM map, the dendrite acted as a boundary separating zones with different local orientation distributions. The dislocation at the dendrite’s edge was more prominent in the COM map, highlighted by negative and positive orientation around it. Additionally, the strain map displayed tensile and compressive strain regions at each side of the dendrite.
Conclusion
The study successfully utilized microscopy techniques to directly observe dislocations at the tips of growing dendrites in bulk LLZTO samples. DFXM allowed a detailed examination of the micro-environment around dendrite tips, revealing a correlation between dendrite growth and dislocation formation.
The researchers found that low dislocation mobility and density suggest these dislocations likely arise from the stress induced by dendrite expansion. These dislocations could weaken the material, facilitating easier fractures and branching of the dendrite. The insights gained from this study could help control dislocation density in ceramics to mitigate dendrite growth in future solid-state batteries.
Journal Reference
Yildirim, C., et al. (2024). Understanding the origin of lithium dendrite branching in Li6.5La3Zr1.5Ta0.5O12 solid-state electrolyte via microscopy measurements. Nature Communications. DOI: 10.1038/s41467-024-52412-4, https://www.nature.com/articles/s41467-024-52412-4