Lithium-Metal Growth Kinetics on LLZO Garnet-Type Solid Electrolytes

Abstract

Li 7 La 3 Zr 2 O 12 (LLZO)-based garnet materials are recently being investigated as suitable electrolytes for solid-state batteries with lithium-metal electrodes. Unfortunately, lithium-metal penetration through polycrystalline garnet-type electrolytes limits the electric current density during cell charging. In this study, we introduce an electrochemical operando approach that is well suited to get insights into the early stage of lithium-metal penetration that was yet only accessible with very elaborate neutron measurements. Combined with in situ as well as ex situ electron microscopic techniques, we investigate the morphological instability of the lithium-metal anode on garnet-type solid electrolytes under cathodic load and demonstrate the inter-relationship between microkinetic aspects and lithium-penetration susceptibility.

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

“…Nevertheless, the depletion times of the more strongly pressed electrodes  0 t area almost identical to the depletion times t 0 of the more gently pressed electrodes with higher initial interface resistance, for both lithium and the Li-Mg alloy. [24] This was observed for all measurements in this study and corroborates the absence of soft short circuits. In the following studies, measurements with a high initial interface resistance are preferred as changes in the impedance can be detected, which would be invisible for a negligibly small initial interface resistance.…”

“…[17,18] Lithium penetration through garnet-type SEs currently limits the possible charge rates. [24] All previous results underline the need for sufficient and homogeneous contact between metal and SE during battery operation. [24] All previous results underline the need for sufficient and homogeneous contact between metal and SE during battery operation.…”

“…[26,32,33] Quite a number of studies reported on lithium alloying interlayers, such as Ge, Al, Sn, Au, Si, Mg, and Ag, aiming to improve the interface kinetics. [24,39,41,42] Thus one must assume that most of the interlayers mainly act as contact mediator during cell assembly and new solutions need to be found to address and circumvent the vacancy diffusion limitation of lithium metal anodes. [41,42] A closer look shows that the operating principle of alloys and alloy interlayers is still not well understood.…”

“…[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. [24][25][26] Indeed, while the intrinsic charge transfer kinetics of the lithium|LLZO interface was found to be sufficiently fast for practical applications (R int < 2 Ωcm²), [26,27] recent work shows that the morphological instability of the (pure) lithium metal anode on solid electrolytes under anodic load is an inherent, fundamental problem that needs to be solved for battery designs that do not allow high operation pressures in the MPa range. Thus, it is of upmost importance for lithium metal solid-state battery development to prevent pore formation and growth at the anode interface during battery discharge.…”

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

“…Nevertheless, attempts to use LLZO‐based solid electrolytes in lithium cells have revealed potential technical challenges, such as unexpectedly premature short circuiting. 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.…”