Evaluation of Oxide|Sulfide Heteroionic Interface Stability with LLZO|Li7P3S11 and LLZO|Li6PS5 Cl As Fundamental Case Studies
Authors: Merola Leonardo , Palmer Max , Singh Vipin K. , Eckhardt Janis K. , Benz Sebastian L. , Fuchs Till , Richter Felix H. , Nazar Linda F. , Sakamoto Jeff , Janek Jürgen 
Published: December 2024

Despite the different classes of solid electrolytes (SEs) discovered and developed, no single SE has been found suitable to address all the requirements for developing lithium metal anode solid-state batteries (LMA-SSBs). Indeed, the designated single SE should be chemically and electrochemically stable against lithium metal and cathode active materials and achieve ionic conductivity comparable to the liquid electrolyte (≈ 10 mS cm−1). A possible solution would be using a multilayerLMA-SSB, where the drawbacks of one SE are solved using layers of different SEs.1 For example, garnet-based SEs show superior performance in contact with lithium metal, while the ionic conductivity and mechanical properties of sulfide-based SEs make them suitable candidates for the composite cathode.
However, the research on lithium-ion inorganic SE1|SE2 heteroionic interfaces is quite preliminary. Especially, the garnet|sulfide interface still poses several research questions: What is the resistance at the garnet|sulfide interface? Are such heteroionic interfaces chemically and electrochemically stable upon cycling at high capacity (1.0 mAh cm−
2)?2,3 In this work, we carried out an in-depth electrochemical investigation of the heteroionic interface between garnet- and sulfide-based SEs. As a fundamental case of study, Li6.25Al0.25La3Zr2O12 (Al-LLZO) is chosen as a garnet-based SE in contact with two representative materials for the sulfide-based SE: the glass-like Li7P3S11 (LPS711) and the argyrodite-based Li6PS5Cl (LPSCl).
Impedance analysis and pressure- and temperature-dependent measurement of symmetric cells (i.e. sulfide|Al-LLZO|sulfide) in equilibrium (i.e. no current load) allow us to identify the electric properties of the oxide|sulfide interface. We interpret the pressure-dependent kinetics of the Al-LLZO|LPSCl interface signal with the concept of current constriction and we show that its resistance is lower compared to the Al-LLZO|LPS711 interface. Furthermore, the effect of Al-LLZO surface treatment is evaluated as well. Such investigation reveals that heat treatment of the Al-LLZO pellet leads to a decrease in the activation energy of the interface impedance, showing an influence of the Li2CO3/LiOH passivation layer on the heteroionic interface. Additionally, cycling symmetric cells (In/InLi|sulfide|Al-LLZO|sulfide|In/InLi) for >900 hours at 1.0 mAh cm−2 reveals that the Al-LLZO|LPSCl interface has a smaller impedance increase than Al-LLZO|LPS711, especially if the garnet pellet is heat treated.
To conclude, with this work we emphasize that the garnet|argyrodite interface is a promising combination for developing LMA-SSBs. However, we encourage further investigation of LLZO surface treatment and chemical analysis of the garnet|sulfide interface.
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