Working Principle of an Ionic Liquid Interlayer During Pressureless Lithium Stripping on Li_6.25Al_0.25La₃Zr₂O₁₂ (LLZO) Garnet‐Type Solid Electrolyte

Abstract

Solid‐state‐batteries employing lithium metal anodes promise high theoretical energy and power densities. However, morphological instability occurring at the lithium/solid–electrolyte interface when stripping and plating lithium during cell cycling needs to be mitigated. Vacancy diffusion in lithium metal is not sufficiently fast to prevent pore formation at the interface above a certain current density during stripping. Applied pressure of several MPa can prevent pore formation, but this is not conducive to practical application. This work investigates the concept of ionic liquids as “self‐adjusting” interlayers to compensate morphological changes of the lithium anode while avoiding the use of external pressure. A clear improvement of the lithium dissolution process is observed as it is possible to continuously strip more than 70 μm lithium (i. e., 15 mAh cm−2 charge) without the need for external pressure during assembly and electrochemical testing of the system. The impedance of the investigated electrodes is analyzed in detail, and contributions of the different interfaces are evaluated. The conclusions are corroborated with morphology studies using cryo‐FIB‐SEM and chemical analysis using XPS. This improves the understanding of the impedance response and lithium stripping in electrodes employing liquid interlayers, acting as a stepping‐stone for future optimization.

Increasing the Pressure‐Free Stripping Capacity of the Lithium Metal Anode in Solid‐State‐Batteries by Carbon Nanotubes

“…As a counter electrode, a resistance-free, ideally reversible electrode Li id was prepared following previous reports utilizing high isostatic pressure (380 MPa for 30 min). [8,37] Composite electrodes were attached to the SE by a small hand-pressing tool under an applied torque of 10 N m. Cells used for testing the CCD were prepared by polishing the Ta:LLZO following the procedures of Sharafi et al, [39] and then heat treating and applying lithium electrodes following the procedures of Wang et al [15] Electrochemical characterization was carried out using a VMP300 potentiostat by BioLogic in combination with the software EC-Lab (V. 11). Temperature dependent measurements were carried out in a climate chamber WKL 64 by WEISS.…”

“…To analyze the dissolution and impedance of Li-CNT composite electrodes, Li id |LLZO|Li-CNT cells were prepared, where the ideally reversible electrode (Li id ) shows negligible interfacial impedance, and thus, acts as a combined reference and counter electrode. [8,37] Therefore, any observed interfacial impedance and changes thereof solely originate from the Li-CNT|LLZO-interface. Using this method, the maximum current density is limited to around 200 µA cm −2 , as the Li id anode is not resistant to dendrites at higher current densities.…”

“…Lithium lanthanum zirconium oxide (LLZO) bulk and grain boundary (GB) contributions are identified at high (4 MHz) and middle (50 kHz) frequencies, respectively. [8,37] At low frequencies (1 kHz), a small interfacial contribution is observed. Hence, our equivalent circuit consists of a series combination of three parallel R-Q elements, where R and Q represent a resistor and nonideal capacity (constant-phase element).…”

“…The activation energies for bulk (E A,Bulk = 0.32 eV), GB (E A,GB = 0.41 eV) and interfacial transport (E A,Int = 0.37 eV) fit well to previously reported results. [8,37,39,40] It is reasonable that the Li-CNT|LLZO interface has a similar activation barrier as the Li|LLZO interface, as charge transfer occurs at the direct contact areas between lithium and LLZO.…”

“…Electrodes were stripped until contact loss to the SE was observed, which is indicated by a steep increase in voltage and interfacial resistance (i.e., lithium depletion). [8,17,37] Figure 3a shows the voltage profiles when stripping with 100 µA cm −2 for Li and Li-CNT-composites. The displayed interfacial resistances are extracted via galvanostatic electrochemical impedance spectroscopy (GEIS) according to the fit procedure exemplified in Figure 2.…”