Lithium-ion conducting solid electrolytes are a promising alternative to conventional liquid electrolytes for making fire resistant and thus safer Li-ion batteries. Recent improvements in the conductivity of sulfide-based glass/ceramic electrolytes (> 0.1 mS/cm) have prompted further investigation into their material properties and interfacial characteristics [1]. Particularly, β-lithium thiophosphate (β-Li₃PS₄), owing to its sufficiently high conductivity (~0.2 mS/cm), scalable synthesis route, and favorable bulk mechanical properties, is a potential electrolyte candidate for solid-state Li-ion batteries [2]. Existing issues, however, surround the interface between β-Li₃PS₄ and conventional 4 V cathodes due to the limited electrochemical stability window of the electrolyte (1.7-2.31 V, theoretical [3]). The resulting solid electrolyte interface poses high impedance resulting in poor rate capability. While oxide-based coatings of cathode active material particles have shown improvement, long term mechanical reliability of the coating is in question [4]. In order to address the interfacial impedance issue with the β-Li₃PS₄ solid electrolyte, it is important to characterize its redox behavior outside its stable voltage window.

We have developed a novel technique based on cyclic voltammetry, which treats the solid electrolyte as an active material electrode and uses internal redox-capable standards (in this case, elemental phosphorus and sulfur) to systematically determine the species makeup of β-Li₃PS₄ at the β-Li₃PS₄/carbon interface as a function of the applied potential. Through this technique, we show that even in the absence of active material, β-Li₃PS₄ undergoes decomposition at the solid electrolyte/carbon interface producing redox active species of phosphorus and sulfur. Further, as the cell voltage is cyclically swept between 0-5 V vs. Li/Li⁺ and the oxidation states of these species change as a consequence, the result is a constantly changing interfacial composition during typical cell operation. This is unlike in case of liquid electrolytes which decompose into electrochemically irreversible species under normal operating conditions. In addition, via ex-situ XPS surface analysis of the β-Li₃PS₄/C interface, we have observed the formation of elemental sulfur and P₂S₅ upon oxidation of β-Li₃PS₄ solid electrolyte to 5 V vs. Li/Li⁺, confirming DFT-based theoretical predictions [3]. Formation of elemental sulfur is shown via the developed cyclic voltammetry based technique to be the cause for the apparent reversible cyclability of the sulfide-based solid electrolytes reported in literature [5]. Through our analysis we highlight the dynamic nature of the interface between β-Li₃PS₄ solid electrolyte and a high voltage material. Our technique may be used to characterize other superconducting sulfide-based solid electrolytes such as Li₇P₃S₁₁ and Li₁₀GeP₂S₁₂. Determining the properties of this redox capable solid-electrolyte/cathode interface and developing methods to mitigate the formation of high impedance species is an essential next step for sulfide-based solid electrolyte research.

Acknowledgments

We gratefully acknowledge support from the US Department of Energy’s Office of Basic Energy Science for the Chemo-mechanics of Far-From-Equilibrium Interfaces (COFFEI) small group, through award number DE-SC0002633.

References

1. K. Takada, Acta Mater., 61, 759–770 (2013)

2. Z. Liu et al., J. Am. Chem. Soc., 135, 975–978 (2013)

3. Y. Zhu, et al., J. Mater. Chem. A, 4, 3253–3266 (2016)

4. N. Ohta et al., Electrochem. commun., 9, 1486–1490 (2007).

5. F. Han, et al., Adv. Mater., 27, 3473–3483 (2015)