Lithium Dendrite Formation inside Li3PS4 Solid Electrolyte Observed Via Multimodal/Multiscale Operando X-Ray Computed Tomography

Abstract

Sulfide-based all-solid-state batteries using a lithium metal anode are expected to be next-generation batteries due to their extremely high energy density. In order to use the lithium metal as the anode, suppressing dendrite of lithium metal during charge/discharge processes is essentially important. It has been reported that lithium dendrite formation occurs not from the lithium/sulfide solid electrolyte interface, but in the sulfide solid electrolyte, isolated from the interface1, 2. The formation of lithium dendrite within the sulfide solid electrolyte is caused by electron conduction in the sulfide solid electrolyte and at the sulfide solid electrolyte/void interface3. However, fundamental information on the mechanism of lithium dendrite formation in a sulfide solid electrolyte caused by its electron conduction is lacking. In this study, the three-dimensional morphological changes of the lithium dendrite in Li3PS4, which is a typical sulfide solid electrolyte, were observed directly using multimodal/multiscale operando computed tomography (CT) under an applied pressure.Li/Li3PS4/Li cells were constructed in a diameter of 10 mm and 1 mm for the critical current density measurements and X-ray CT measurements by using SPring-8 BL20XU respectively. X-ray CT images for behavior change with the electrochemical operation were collected in micro and nano scales at 25 °C every 30 mins. After a series of data processing steps, these images were converted to cross-sectional slices that were then stacked together to render a 3D reconstruction of the cell. The 3D imaging data coupled with precise species segmentation show that the lithium metal deposition start point is spatially separated from the lithium metal anode. The gradient in thickness of a lithium filament with repeated charging, widening the plating-susceptible region horizontally in the process and eventually led to cell failure. The lithium nucleation initiates along the pre-existing voids where local electronic conductivities are high during the plating. The deposition then widens from the nucleation across the electrolyte horizontally. Accompanied with streak fracture widening through the Li3PS4, does a Li/Li3PS4/Li cell finally short circuit. By combining the multimodal/multiscale operando X-ray computed tomography with X-ray absorption spectroscopy and electrochemical impedance spectroscopy measurements, we revealed that the electronic conduction of reductive decomposition products and the solid electrolyte/void interface cause the lithium deposition within the Li3PS4. These results suggests that the suppression of reductive decomposition and sulfide solid-state electrolytes with low electronic conductivity plays significant roles in suppressing the growth of lithium dendrites in the solid-state electrolyte layer.