Lithium Metal Penetration Induced by Electrodeposition through Solid Electrolytes: Example in Single-Crystal Li₆La₃ZrTaO₁₂Garnet

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. Broader ContextAll-solid-state batteries can potentially store electricity at higher energy density and with greater safety than existing lithium-ion technology but require the use of lithium metal electrodes.Towards these goals, it is critical to understand possible failure modes when lithium metal electrodes are used with solid electrolytes, and especially the processes of metal dendrite formation and propagation. Here, we test the stability limits of lithium metal electrodeposition using high quality single crystals of LLZTO garnet solid electrolyte, at high current densities (5 to 10 mA/cm 2 ) equivalent to charging a battery at 1C-2C rates (1h to 0.5h charge times). We surprisingly observe that lithium metal filled cracks initiate at the edges of surface metal current collectors, rather than on millimeter-scale deliberately introduced surface cracks. At these current densities, lithium metal penetrates to short-circuit through ~2mm electrolyte thickness on the minute time scale. The results highlight a previously unrecognized failure mode for all-solidstate batteries and suggest that control of electric field distributions will be...

The Role of Interlayer Chemistry in Li‐Metal Growth through a Garnet‐Type Solid Electrolyte

“…Moreover, metal‐island clusters continued to preferentially grow into filamentary lithium metal (indicated by arrows in Figure 1i,j). Considering the relatively low current density of 0.1 mA cm −2 , these observations imply that lithium tends to first fill flaws on the LLZO surface rather than be uniformly deposited, even at a low current density, which is consistent with a previous report in which asperity flaws were observed to induce concentrated electric fields as well as low barriers for lithium nucleation;22,23 they are also consistent with our previous modeling study that suggested that the filamentary lithium‐growth mechanism is promoted by local perturbations in the surface current density around uneven lithium depositions 38. In Figure 1k, we schematically propose how lithium‐deposition proceeds at the electrode/LLZO interface in the presence of possible processing defects.…”

“…In contrast to its projected role as a mechanical barrier, short circuiting through the LLZO has frequently been observed during cycling and was found to arise from lithium‐metal growth penetrating through the LLZO 18–21. Several recent studies have suggested that intrashort circuits are likely to be formed because of the localized electrodeposition of lithium in pre‐existing flaws due to the build‐up of crack‐tip stress and the corresponding crack propagation that finally causes the solid electrolyte to mechanically fail 22–24. Other researchers have also proposed that the interfacial resistance between an electrode and LLZO is not only the source of slow SSB kinetics, but it significantly contributes to short‐circuit failure 20,25–29.…”

Fracture behavior in battery materials

“…Besides, anisotropic reaction kinetics accelerates fracture. For example, Si has a more sever lattice parameter change along the [110] direction than germanium does [39], because Li ions diffuse fast in the [110] direction in c-Si nanopillars. Consequently, the critical diameter of <111> Si pillars for lithiation-induced fracture is about 300 nm, while the critical diameter related to lithiation-induced fracture of <111> Ge pillars is~1.2 µm.…”

“…The crack-opening stress is produced by Li metal deposition inside the flaw. as shown in Figure 24 [110], a crack propagates from the deposited Limetal tip on a single-crystal electrolyte. For an amorphous electrolyte, no grain boundary exists.…”

Diffusion Limitation of Lithium Metal and Li–Mg Alloy Anodes on LLZO Type Solid Electrolytes as a Function of Temperature and Pressure

“…[15,16] Nevertheless, certain issues at the lithium|solid electrolyte interface remain unsolved. [19][20][21][22][23] In this context, it was found that good contact to a small reservoir of lithium metal is highly beneficial to prevent inhomogeneous lithium nucleation, which then reduces the lithium penetration susceptibility. [19][20][21][22][23] In this context, it was found that good contact to a small reservoir of lithium metal is highly beneficial to prevent inhomogeneous lithium nucleation, which then reduces the lithium penetration susceptibility.…”