GoKawiilLi dendrite growth in solid electrolytes
LLZTO solid electrolytes were obtained from Toshima Manufacturing Co., Ltd. The pellets were mechanically ground to a thickness of approximately 150 μm, with final polishing performed using a 0.05-μm alcohol-based colloidal silica suspension. To reduce the interfacial resistance between lithium and the solid electrolyte, the thin LLZTO discs were immersed in 1 M HCl for 30 s to remove surface contaminants, following the procedure demonstrated in ref. 51. Immediately after the acid treatment, the solid electrolyte discs were transferred into an argon-filled glovebox (O 2 and H 2 O < 0.5 ppm). Lithium foil (MaTeck Material Technologie & Kristalle GmbH) was scraped using a plastic tweezer to expose a fresh, shiny surface. A 3-mm-diameter lithium pad was then punched out and stuck to the LLZTO disc. The assembled cell was placed on a hotplate and baked at 130 °C for 1 h.
To study the interaction between lithium dendrites and specific features of interest, a single lithium metal pad was used as the counter electrode. A tungsten probe was placed on the surface of the solid electrolyte to serve as the working electrode, where lithium dendrites nucleated and grew41. A constant current was applied between the lithium metal pad and the tungsten needle using a SP-200 potentiostat (Bio-Logic Science Instruments GmbH). Electrochemical impedance spectroscopy (EIS) data were recorded in the frequency range between 10 Hz and 7 MHz with an amplitude of 50 mV using a SP-200 impedance analyser (Bio-Logic). The growth of lithium dendrites was conducted entirely within the glovebox and monitored using a camera mounted on a stereo microscope (KERN & SOHN GmbH).
Lithium dendrite growth through the symmetric cell configuration was cycled using the same potentiostat equipped with a pressure stand (Imada Inc.). Before applying the bias, the symmetric cell was heated to 130 °C using a heating sleeve (RS Components Ltd.) to improve the interfacial contact between the lithium metal and the solid electrolyte. After short-circuiting, the lithium metal was removed using sandpaper with a grit size of 1,200. The short-circuited solid electrolyte was then taken out of the glovebox, soaked in epoxy overnight for curing and subsequently polished to the region in which features resembling lithium dendrites could be observed, as shown in Extended Data Fig. 1f,g. Extended Data Fig. 1e schematically illustrates the sample preparation procedure. Extended Data Fig. 1f shows the surface after rough polishing with 320-grit sandpaper and Extended Data Fig. 1g shows the result after fine polishing using a 0.1-μm SiO 2 polishing suspension. The lithium metal on the plating side, where dendrite growth occurred, could be easily peeled off by hand, as shown in Supplementary Figs. 10, 11 and 17. Therefore, no sandpaper was used to remove the lithium electrode, in contrast to the procedure used for the samples shown in Extended Data Fig. 1 and Supplementary Fig. 6.
Cryogenic FIB, SEM and EBSD
Using an inert high-vacuum (< 10−7 mbar) cryogenic transfer suitcase (Ferrovac AG), hereafter referred to as the ‘suitcase’, the LLZTO disc was transferred from the argon-filled glovebox to a Thermo Fisher Scientific Helios 5 CX Ga FIB/SEM system. The Helios 5 is equipped with an Aquilos cryo-stage featuring free rotation capability and a Thermo Fisher Scientific EZ-Lift tungsten cryogenic micromanipulator. Both the cryo-stage and the manipulator were maintained at −190 °C using active heating control and a nitrogen flow rate of 190 mg s−1. All operations inside the FIB/SEM system—including SEM imaging, FIB cutting, TEM lamella preparation and EBSD—were conducted at a stable temperature of −190 °C. The TEM lamella was welded onto both the micromanipulator needle and a copper grid by means of redeposition induced by line cuts, as shown in Supplementary Fig. 33. Detailed lamella preparation procedures have been described in previous works52,53. Once thinned to below 150 nm, the lamella and the bulk sample were transferred back into the argon glovebox using the suitcase. The interaction between the electron beam and the solid electrolyte is strongly suppressed at cryogenic temperatures. No electron-beam-induced lithium nucleation was observed under cryogenic conditions, in contrast to the artefacts frequently encountered at room temperature11,54.
EBSD patterns of the LLZTO pellet were collected at cryogenic temperature (−190 °C) using a direct electron detector (Clarity Plus, EDAX LLC). Kikuchi patterns were acquired under an accelerating voltage of 10 kV and a beam current of 2.8 nA. To analyse diffraction from a lithium dendrite within the solid electrolyte, a lamella was prepared following the same procedure described above, except the final lamella thickness was maintained at approximately 1 μm. Supplementary Fig. 34 shows the TKD lamella, which maintains its mechanical integrity without any observable bending or distortion induced by ion-milling preparation. Moreover, because the sample was prepared using Ga+ FIB at cryogenic temperature, strain rearrangement during ion milling is expected to be strongly suppressed and therefore experimentally negligible, as reported in several previous studies37,38,55. TKD patterns of the lithium dendrite were also acquired using the same direct electron EBSD detector. The diffraction patterns of the lithium dendrite were analysed using spherical indexing56—a new technique that enables improved pattern recognition and orientation determination for low-symmetry or low-quality patterns. In contrast to the classical analysis technique that uses a Hough transform for detection of the Kikuchi bands in Kikuchi patterns57, spherical indexing is an advanced image matching technique, in which the experimental pattern is compared with a theoretical master pattern. The comparison is done by developing both the experimental and the master pattern into a series of spherical harmonic functions and comparing them by a spherical cross-correlation function. Because spherical indexing matches the whole pattern, this technique can be applied very robustly with weak diffraction patterns, typically obtained from lithium. Furthermore, because the master pattern can be calculated for any diffraction voltage and because the matching is executed directly on the diffraction sphere, the technique is independent of the acceleration voltage of pattern generation and can also be applied to low-voltage patterns. The classical Hough transform, which detects straight lines, fails in this case because of the high curvature of low-energy Kikuchi lines. Spherical indexing, together with the necessary image preprocessing (static and dynamic background subtraction and contrast enhancement) were done using an early build of the software OIM Analysis 9.1 produced by Ametek EDAX. The master pattern was calculated for 10 kV and 20° of sample tilt in transmission. The bandwidth, a parameter that describes the amount of details that is matched in the pattern, was set to 127.
The incident angles between the dendrite and grain boundaries were measured from EBSD results for both intergranular and transgranular fractures. In both cases, the incident angle values follow a normal distribution. The mean values, along with the 95% confidence intervals extracted from Fig. 1f and Supplementary Fig. 4b, were fitted and plotted in Fig. 1g. The positions of the red and blue dots were placed such that their error bars just begin to intersect the boundary between intergranular and transgranular regions, as indicated by the dashed line.
Cryo-STEM
The STEM lamella was loaded in a Mel-Build holder inside an argon-filled glovebox and then kept under inert argon atmosphere during sample transfer. All analysis was performed at cryogenic conditions (−150 °C). STEM was performed on a Titan Themis microscope (Thermo Fisher Scientific) operated at 300 kV. The aberration-corrected probe has a convergence semiangle of 23.8 mrad. High-angle annular dark-field and annular bright-field STEM micrographs were collected using respective angular ranges of 73–200 and 8–16 mrad. STEM energy-dispersive X-ray spectroscopy spectrum imaging was acquired using a Super-X detector. STEM-EELS spectrum imaging was performed using a Quantum ERS spectrometer (Gatan) with a collection angle of 35 mrad. To facilitate comparison with EELS spectra reported in the literature, we opt to show raw EELS spectra from selected areas in Supplementary Figs. 12b and 14d,e. Multivariate statistical analysis was performed on the spectrum imaging datasets to separate backgrounds and signals from different lithium-containing phases29,58,59. For lithium count maps shown in Fig. 2f,g, power law background was modelled for components 1 and 2, with respective fitting windows of (45, 50) eV and (45, 57) eV. The integration window was kept to (57, 67) eV. As evidenced in Supplementary Fig. 12, the Li K-edge onsets of the LLZTO and the Li/LiOH phases are different. The quantification of lithium is hence facilitated by multivariate statistical analysis29, for which most of the spatial variance in EELS signal can be expressed in components 1 and 2. As shown in Supplementary Fig. 12c–f, component 1 is mainly located in the dendrite area and the spectral feature is LiOH-like; component 2 relates to the LLZTO area surrounding the dendrite, with LLZTO-like spectral feature. Component 3 no longer resembles a physical spectrum, as it represents small differential signals to modify the two leading components. This observation confirms the dominance of the Li/LiOH and LLZTO phase in this area. Four-dimensional STEM diffraction imaging was recorded using the pixelated detector Electron Microscope Pixel Array Detector (EMPAD, Thermo Fisher Scientific) and a probe convergence semiangle of 0.65 mrad.
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