Reducing Impedance at a Li-Metal Anode/Garnet-Type Electrolyte Interface Implementing Chemically Resolvable In Layers

Abstract

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is a potential electrolyte material for all-solid-state Li-ion batteries mainly because of its reported excellent chemical stability in contact with Li metal. But good wettability of LLZO and 100% surface coverage of lithium are still a challenge. This study elucidated the suitability of magnetron-sputtered indium in Li(In)/LLZO/Li(In) symmetrical model cells as one of the promising interfacial modifications reported in the literature. Importance was given to the impact of preparation parameters on the surface coverage of Li(In)/LLZO interfaces and the consequences of impedance, cycling stability, and critical current density. SEM and EDXS analyses of In layers of thickness 100 nm to 1 μm revealed complete dissolution of indium in the lithium anode after annealing; 300 nm In layers annealed at 220 °C/10 h provided a surface coverage of >80%, best reproducibility, and a supreme interface resistance <i>R</i>_int of 12.4 Ω·cm². Presuming a surface coverage of 100%, an ultimate interface resistance close to 1 Ω·cm² can be expected. The critical current density was determined as 200-500 μA/cm² at a charge of 100-250 μAh, whereas 500 μA/cm² and above affected cell stability. The increasing voltage plateau was assigned to the increase of the interface resistance <i>R</i>_int and the electrolyte resistance <i>R</i>_G+GB. SEM, EDXS, and X-ray microtomography analyses after voltage breakdown confirmed Li-dendrite growth along grain boundaries into LLZO, often curved parallel to the interface, indicating short-circuiting of the solid electrolyte. Grain boundary characteristics are supposed to be decisive for lithium deposition in and failure of garnet-type solid electrolytes after cycling.