Toward a Fundamental Understanding of the Lithium Metal Anode in Solid-State Batteries—An Electrochemo-Mechanical Study on the Garnet-Type Solid Electrolyte Li_6.25Al_0.25La₃Zr₂O₁₂

Abstract

For the development of next-generation lithium batteries, major research effort is made to enable a reversible lithium metal anode by the use of solid electrolytes. However, the fundamentals of the solid− solid interface and especially the processes that take place under current load are still not well characterized. By measuring pressure-dependent electrode kinetics, we explore the electrochemo-mechanical behavior of the lithium metal anode on the garnet electrolyte Li 6.25 Al 0.25 La 3 Zr 2 O 12 . Because of the stability against reduction in contact with the lithium metal, this serves as an optimal model system for kinetic studies without electrolyte degradation. We show that the interfacial resistance becomes negligibly small and converges to practically 0 Ω•cm 2 at high external pressures of several 100 MPa. To the best of our knowledge, this is the smallest reported interfacial resistance in the literature without the need for any interlayer. We interpret this observation by the concept of constriction resistance and show that the contact geometry in combination with the ionic transport in the solid electrolyte dominates the interfacial contributions for a clean interface in equilibrium. Furthermore, we show thatunder anodic operating conditionsthe vacancy diffusion limitation in the lithium metal restricts the rate capability of the lithium metal anode because of contact loss caused by vacancy accumulation and the resulting pore formation near the interface. Results of a kinetic model show that the interface remains morphologically stable only when the anodic load does not exceed a critical value of approximately 100 μA•cm −2 , which is not high enough for practical cell setups employing a planar geometry. We highlight that future research on lithium metal anodes on solid electrolytes needs to focus on the transport within and the morphological instability of the metal electrode. Overall, the results help to develop a deeper understanding of the lithium metal anode on solid electrolytes, and the major conclusions are not limited to the Li|Li 6.25 Al 0.25 La 3 Zr 2 O 12 interface.

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

“…A strong increase of the potential takes place only in the end. [26] The bulk transport (ω max ≈ 40 MHz) and the grain boundary charge transfer (ω max ≈ 600 kHz) contributions do not change with time, indicating the absence of lithium metal penetration through the solid electrolyte. The reason for this is that the global current density and thus the lithium transport in the bulk lithium and lithium alloy electrode seem to be not highly affected by the initial interface resistance.…”

“…In detail, we show current density, pressure, and temperature-dependent measurements of both Li-Mg alloy and lithium metal electrodes. [26] Figure S7 in the Supporting Information shows the trend of the interface capacitances. For all current densities, the depletion time t 0 of the Li 0.9 Mg 0.1 alloy is around a factor of two higher than for the bare lithium electrode.…”

“…[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. They are often performed with only limited cumulative charges passed, at which the kinetic limitations do not necessarily become critical.…”

“…[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.…”

“…[24] All previous results underline the need for sufficient and homogeneous contact between metal and SE during battery operation. [26,28] The morphological instability stems from the vacancy injection into lithium metal during anodic dissolution, which is a general phenomenon of parent metal electrodes. [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.…”