Electrochemical Characterization of Thiophosphate- Ionic Liquid Hybrid Lithium Electrolytes Against Li Metal
Authors: Woolley Henry Michael , Vargas-Barbosa Nella 
Published: September 2023

(Almost) solid-state batteries that utilize thiophosphate solid electrolytes (SE) are an exciting technology emerging as a potential alternative to lithium-ion batteries. When used alongside a lithium metal anode they can offer the high energy densities [1] required to meet the increasing demand for energy storage. However, they suffer from numerous issues which predominately occur at the cathode- or anode-SE interface. Issues include dendrite propagation through gaps, pores and grain boundaries of the solid electrolyte which can eventually puncture electrolyte crystallites and lead to cell failure. [2] Thiophosphate electrolytes are also unstable both chemically and electrochemically. As a result of the SEs having a low electrochemical stability window reduction or oxidation can occur at the anode or cathode interface, forming a resistive solid electrolyte interphase (SEI). [3] Finally, the ionic contact between Li metal anode and electrolyte is poor and thus high interfacial impedances can arise. These impedances can be dynamic which increase during stripping of the lithium and decrease during plating. [4] To solve some of the interfacial issues there is the option to add a small of amount of liquid electrolyte at the lithium metal-electrolyte interface. The liquid electrolyte can fill in the gaps and pores at the interface thus improving the ionic contact whilst allowing a more stable interface. Whilst the ionic contact can be improved the inherent instability of thiophosphate electrolytes against the liquid electrolyte means that a new interphase known as the solid-liquid electrolyte interphase (SLEI) can form. The presence of this interphase can therefore lower the energy density and round-trip efficiencies of cells which utilize hybrid electrolytes meaning that minimizing the SLEI resistance and maximizing total ionic conductivities is important in hybrid cells. [5]

In this work a hybrid of the thiophosphate argyrodite Li6PS5Cl and the ionic liquid x-LiTFSI-1-butyl 1-methylpiperidinium TFSI (BMPipTFSI) with LiTFSI concentrations of 0.25 M and 0.5 M was studied. The choice of SE and liquid electrolyte boils down to the high ionic conductivity of the SE and the electrochemical and thermal stability of the ionic liquid. Temperature-dependent ionic conductivity measurements showed that hybrid systems exhibit lower in room temperature ionic conductivities and higher total activation energies. This hints at the presence of a SLEI forming between the LE and SE. To study how the SLEI resistances changes over time, ion blocking potential impedance spectroscopy measurements were performed. These measurements were performed at 10 °C to allow for the resistance contributions to be better resolved and showed a SLEI resistance of around 45-50 Ω cm2 for both hybrids. Over the period of 130 hours this resistance changed minimally (around 5 Ω cm2 on average) indicating good stability of the SLEI. To further test the suitability of this hybrid alongside lithium metal anodes impedance measurements in symmetrical lithium cells (Li0|LE|LPCL|LE|Li0) were undertaken. In this case galvanostatic impedance spectroscopy (GEIS) with an applied current density of ±0.4 mA cm-2 was used to probe the changes in resistance contributions in the system over the period of stripping (positive current) and plating (negative current). For cells with just SE a large change in the resistance owing to the electrochemical reaction (ECR) (Li0 ↔ Li+ + e-) occurred during stripping and plating indicating the dynamic nature of the ionic contact at the interface. For the hybrid electrolyte cells, this ECR resistance is decreased and becomes more stable however a larger interphase resistance is present. This resistance is a combination of the resistances of both the SLEI (the interphase between the LE and SE) and the SEI (the interphase between LE and Li anode) and it changes over stripping and plating showing that the S(L)EIs which are present are dynamic. Finally, post-mortem SEM/EDX of the surface of samples show a change in morphology and the presence of decomposition products from both the liquid and solid electrolytes. These studies show that the LPCL-BMPipTFSI hybrid is stable and improve ionic contact at the lithium metal anode interface. Further testing in half Li-S cells will determine the suitability of the use of the ionic liquid at the cathode side of the cell.

References

[1] J. Janek and W. G. Zeier, Nat Energy, 2016, 1, 16141. [2] M. B. Dixit et al. Matter, 2020, 3, 2138-2159 [3] G. Dewald et al. Chem Mater, 2019, 31, 8328-8337. [4] T. Krauskopf et al. Chem. Rev, 2020, 7745-7794. [5] H. M. Woolley and N. M. Vargas-Barbosa, under review.