Lithium Metal Penetration Induced by Electrodeposition through Solid Electrolytes: Example in Single-Crystal Li6La3ZrTaO12 Garnet

Abstract: Solid electrolytes are considered a potentially enabling component in rechargeable batteries that use lithium metal as the negative electrode, and thereby can safely access higher energy density than available with today's lithium ion batteries. To do so, the solid electrolyte must be able to suppress morphological instabilities that lead to poor coulombic efficiency and, in the worst case, internal short circuits. In this work, lithium electrodeposition experiments were performed using single-crystal Li6La3ZrTaO12 garnet as solid electrolyte layers to investigate the factors that determine whether lithium penetration occurs through brittle inorganic solid electrolytes. In these single crystals, grain boundaries are excluded as possible paths for lithium metal propagation. However, Vickers microindentation was used to introduce sharp surface flaws of known size. Using operando optical microscopy, it was found that lithium metal penetration sometimes initiates at these controlled surface defects, and when multiple indents of varying size were present, propagates preferentially from the largest defect. However, a second class of flaws was found to be equally or more important. At the perimeter of surface current collectors, an enhanced electrodeposition current density causes lithium metal filled cracks to initiate and grow to penetration, even when the large Vickers defects are in close proximity. Modeling the electric field concentration for the experimental configurations, it was shown that a factor of 5 enhancement in field can readily occur within 10 micrometers of current collector discontinuities, which we interpret as the origin of electrochemomechanical stresses leading to failure. Such field amplification may determine the sites where supercritical surface defects dominate lithium metal propagation during electrodeposition, overriding the presence of larger defects elsewhere.

Evaluation of Lithium Metal Penetration through Solid Electrolytes in Rechargeable Batteries

The research paper “Lithium Metal Penetration Induced by Electrodeposition through Solid Electrolytes” presents a detailed experimental investigation into the factors affecting lithium metal penetration through solid electrolytes. The study addresses challenges in achieving high-energy density batteries safely, focusing on the use of Li6La3TaO12 (LLZTO) garnet, a solid electrolyte, in combination with lithium metal anodes. The experiments are aimed at understanding the mechanisms of lithium metal propagation which lead to internal short circuits, thus compromising the efficacy and safety of solid-state batteries.

Key Findings

This paper provides extensive insights into the behavior of lithium metal during electrodeposition at high current densities (5 to 10 mA/cm²). The researchers employed Vickers microindentation to introduce controlled surface flaws on single-crystal LLZTO garnets to study lithium penetration behavior under galvanostatic conditions. The study revealed that lithium metal penetration initiates preferentially at current collector discontinuities, rather than at deliberately introduced large surface defects. Indeed, the electric field near these perimeter sites amplifies by a factor of up to 5 times within 10 micrometers, leading to electrochemomechanical stresses that act as the primary drivers of lithium propagation.

Numerical simulations further support these findings, illustrating the current density being significantly concentrated at the electrode edges. Such concentrations are pivotal to the lithium filament growth, which penetrates through solid electrolytes, causing short-circuits in approximately one to five minutes only.

Implications and Developments

The findings highlight a previously unrecognized failure mode in all-solid-state batteries. The preferential metal penetration at the electrode perimeters suggests that modifications to electric field distributions can mitigate this short-circuit risk. Designing cells with larger positive electrodes relative to negative electrodes can diminish the field amplification and, consequently, may reduce the incidence of edge-induced failures during the charging process.

These observations have profound implications for battery design, particularly the aspect ratio and electrode area, calling for innovative adjustments to ensure reliability and longevity in solid-state configurations for high-energy density applications.

Future Directions

This research paves the way for further investigations into customizing the geometry of battery cells to enhance their safety and performance. Exploring different electrode configurations and material compositions may also offer novel solutions to minimize failure risks due to lithium dendrite formation. Future research could explore understanding the microstructural impacts on lithium dendrite behavior, offering targeted approaches to control battery imperfections at the molecular level. Furthermore, advanced modeling techniques could provide deeper insights into electric field distributions, thereby refining battery design methodologies.

In conclusion, the study underscores a critical need for strategically designing solid-state batteries that leverage high current densities safely. By addressing both geometric and material factors, upcoming developments in lithium metal batteries could revolutionize energy storage with enhanced stability and efficiency.