Interface investigation between sulfidic and oxidic electrolytes


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Considering Li-ion solid-state batteries, the area of solid electrolytes can be divided into polymers, sulfidic and oxidic electrolytes. Each of the material classes and their representatives deal with individual advantages as well as drawbacks. Thiophosphates, such as Li7P3S11 (LPS), have high ionic conductivities of up to 10 mS/cm. LPS can be processed easily at room temperature, due to its ductility. On the other hand, LPS is sensitive to moisture and oxygen, and it is not electrochemically stable against typical potentials at the electrodes [1,2]. Oxidic electrolytes, such as the garnet LLZO (Li7Zr3La2O12), exhibit ionic conductivities of up to 1 mS/cm and a wide electrochemical stability window. Most importantly, LLZO type materials are reported to be stable against metallic lithium [3,4]. In contrast to the processability of sulfides, oxidic electrolytes require a sintering step to manufacture mechanically stable components [5].
One strategy to overcome the specific drawbacks of each material class is the concept of a layered hybrid battery, where multiple electrolyte layers are used. Very promising seems the combination of a sulfidic electrolyte in the composite cathode, due to it`s high ionic conductivity and facile processability, and an oxidic electrolyte separator, which is stable contact with metallic lithium. Such a concept introduces additional interfaces between the different electrolyte materials. Thus, the interface between the electrolyte materials needs to be well understood.
In our work we investigated the interface resistance between the sulfide material LPS and the oxidic solid electrolyte LLZO. We characterized a layered LPS/LLZO/LPS stack by impedance spectroscopy and determined the interface resistance between the materials by comparing the stack impedance with the pure materials properties. The interface resistance could be identified by comparing the relaxation time constants of the pure materials to the stack’s conduction mechanisms. The interface resistance was found to be depending on the applied pressure to the stack while measurement and on the interface area. Thus, the interface resistance is determined by the constriction resistance. The ionic transport of Li-ions across the interface could be proved by DC cycling experiments and even by charging and discharging of an NCM-based battery cell.

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[2] Lian P-J, Zhao B-S, Zhang L-Q, Xu N, Wu M-T, Gao X-P. Inorganic sulfide solid electrolytes for all-solid-state lithium secondary batteries. J. Mater. Chem. A 2019;7(36):20540–57.
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[4] Zhu Y, Connell JG, Tepavcevic S, Zapol P, Garcia‐Mendez R, Taylor NJ, Sakamoto J, Ingram BJ, Curtiss LA, Freeland JW, Fong DD, Markovic NM. Dopant‐Dependent Stability of Garnet Solid Electrolyte Interfaces with Lithium Metal. Adv. Energy Mater. 2019;9(12):1803440.
[5] Zhao N, Khokhar W, Bi Z, Shi C, Guo X, Fan L-Z, Nan C-W. Solid Garnet Batteries. Joule 2019;3(5):1190–9.

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