The electrochemical stability window of solid electrolyte is overestimated by the conventional experimental method using a Li/electrolyte/inert metal semi-blocking electrode because of the slow kinetics of the decomposition reactions due to the small contact area between LGPS and inert metal. Since the battery is cycled in the overestimated stability window, the decomposition of the solid electrolyte at the interfaces occurs but has been ignored as a cause for high interfacial resistances in previous studies, limiting the performance improvement of the bulk-type solid-state battery despite the decades of research efforts. Thus, there is an urgent need to identify the intrinsic stability window of the solid electrolyte. We propose a new experimental method to measure the intrinsic electrochemical stability window of solid electrolytes using a Li/electrolyte/electrolyte-carbon cell. The Li/electrolyte/electrolyte-C cell provides improved kinetics from large and continuous physical contacts between solid electrolyte and carbon to facilitate the thermodynamically favorable decomposition reactions of the solid electrolyte. Moreover, the use of the Li/electrolyte/electrolyte-C cell mimics the cell configuration in the bulk-type solid-state battery and represents the real microstructural architectures in the solid-state electrode composite, where carbon and solid electrolyte are mixed with the active material. The intrinsic stability window of the most promising solid electrolytes, Li₁₀GeP₂S₁₂ (LGPS) and cubic Li-garnet Li₇La₃Zr₂O₁₂(LLZO), were examined using the novel experimental method. The results indicate that both of these solid electrolyte materials have much narrower electrochemical window than what was previously claimed (0 – 5 V), as shown in Figure 1. The cathodic and anodic decomposition products for both electrolytes were also characterized by XPS test. The measured stability windows as well as the decomposition products agree well with our first-principles calculations, indicating that our proposed cell design could obtain the intrinsic electrochemical stability window of solid electrolyte.

Our work offers a new research orientation towards the minimization of the interfacial resistance of all-solid-state batteries. At the cycling voltages beyond the stability window of the solid electrolyte, the decomposition products of the solid electrolyte would form as an interphase at the electrolyte/electrode and electrolyte/carbon interfaces. Therefore, the performance of the bulk-type solid-state battery is greatly influenced depending on the properties of the decomposition interphases, such as ionic conductivity, electronic conductivity, and electrochemical reversibility. The influences of the decomposition of different solid electrolytes on the interfacial resistance will be discussed in detail. Specific recommendations for the engineering of sulfides and oxides solid electrolyte materials will be provided to obtain a higher performance all-solid-state batteries. Our most recent experimental results about the interface engineering of LGPS-based all-solid-state battery with consideration of the decomposition of LGPS will also be presented.

On the other hand, the reversible decomposition of the solid electrolyte at two different voltages allows us to design an all-solid-state lithium ion battery using a single material. Based on this concept, a single-LGPS battery (Figure 2) was prepared using LGPS-C as cathode and anode, and pure LGPS as solid electrolyte, while the carbon mixed in the electrodes can be considered as an extension of current collector. The single-LGPS battery exhibited a remarkably low interfacial resistance due to the improvement of interfacial contact, the modification of the interfacial interactions, and the suppression of the strain/stress at the interface, providing a promising direction to address the most challenging interfacial problem in all-solid-state batteries. Additional implications of the single-material battery concept will also be presented.

References:

1. F. Han, Y. Zhu, X. He, Y. Mo, C. Wang, Electrochemical Stability of Li₁₀GeP₂S₁₂ and Li₇La₃Zr₂O₁₂ Solid Electrolytes, Advanced Energy Materials, DOI:10.1002/aenm.201501590.

2. F. Han, T. Gao, Y. Zhu, K. J. Gaskell, C. Wang, A Battery Made from a Single Material,   Advanced Materials, 27 (2015) 3473.