Electrochemical Redox Behavior of β-Li3PS4 lithium-Ion Conducting Solid Electrolyte
Authors: Swamy Tushar , Chiang Yet-Ming 
Published: February 2020

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

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