vast improvements in energy density may be achieved with lithium metal anodes owing to their high gravimetric capacity (3869 mA h g −1 ) and low density (0.534 g cm −3 ). However, adoption of rechargeable lithium metal batteries has been unsuccessful thus far due to safety concerns associated with short circuits that occur when Li dendrites grow through the liquid electrolyte during the charging process. [4][5][6] Although several approaches have reduced dendrite formation, [7][8][9][10] to date the phenomenon has not been avoided under all relevant conditions. Nonflammable inorganic solid electrolytes, paired with Li metal anodes, could result in high energy density yet safe rechargeable lithium batteries. [11][12][13][14][15] As reviewed by Takada, [12] inorganic solid electrolytes have now been widely studied, [16][17][18][19][20] but are not yet commercialized. Monroe and Newman have suggested that dendrite growth during the plating process may be suppressed if the liquid electrolyte is replaced with a Li-ion conducting solid electrolyte of a sufficiently high shear modulus. [21,22] According to this criterion, numerous inorganic solid electrolytes should be able to suppress dendrite formation. However, multiple research groups have recently reported cases where ceramic solid electrolytes paired with a Li metal anode experience a short circuit Li deposition is observed and measured on a solid electrolyte in the vicinity of a metallic current collector. Four types of ion-conducting, inorganic solid electrolytes are tested: Amorphous 70/30 mol% Li 2 S-P 2 S 5 , polycrystalline β-Li 3 PS 4 , and polycrystalline and single-crystalline Li 6 La 3 ZrTaO 12 garnet. The nature of lithium plating depends on the proximity of the current collector to defects such as surface cracks and on the current density. Lithium plating penetrates/infiltrates at defects, but only above a critical current density. Eventually, infiltration results in a short circuit between the current collector and the Li-source (anode). These results do not depend on the electrolytes shear modulus and are thus not consistent with the Monroe-Newman model for "dendrites." The observations suggest that Li-plating in pre-existing flaws produces crack-tip stresses which drive crack propagation, and an electrochemomechanical model of plating-induced Li infiltration is proposed. Lithium short-circuits through solid electrolytes occurs through a fundamentally different process than through liquid electrolytes. The onset of Li infiltration depends on solid-state electrolyte surface morphology, in particular the defect size and density.
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...
Young's modulus, hardness, and fracture toughness are measured by instrumented nanoindentation for amorphous Li 2 S -P 2 S 5 Li-ion solid electrolyte. Although low modulus suggests good ability to accommodate chemomechanical strain, highly brittle behavior can lead to disruptive crack formation.
Dislocations are mobile at low temperatures in surprisingly many ceramics but sintering minimizes their densities. Enabling local plasticity by engineering a high dislocation density is a way to combat short cracks and toughen ceramics.
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