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Lithium-sulfur (Li−S) batteries are recognized as one of the most promising technologies with the potential to become the next-generation batteries. However, to ensure Li−S batteries reach commercialization, complex challenges remain, among which the tailoring of an appropriate electrolyte is the most important. This review discusses the role of electrolytes in Li−S batteries, focusing on the main issues and solutions for the shuttle mechanism of polysulfides and the instability of the interface with lithium metal. Herein, we present a background on Li−S chemistry followed by the state-of-the-art electrolytes highlighting the different strategies undertaken with liquid and solid electrolytes.

For many years, composite electrolytes (CEs) consisting of a mixture of inorganic solid electrolytes (ISEs) and polymer electrolytes (PEs) have been investigated as promising materials for the scalable production of solid-state batteries (SSBs). It is believed that CEs can overcome limitations of the single components, namely the low room-temperature conductivity and lithium ion transference number of PEs and the poor mechanical properties and high temperature processing necessary for ISE ceramics. To facilitate ion transport in the CE between the electrodes a low and stable charge transfer resistance between PEs and ISEs is required. In this study, we investigate by means of electrochemical impedance spectroscopy (EIS) how polymer crystallinity influences the charge-transfer resistance of hetero-ionic interfaces between polyethylene oxide (PEO)-based electrolytes and Li1.5Al0.5Ti1.5(PO4)3 (LATP) as well as Li6.25Al0.25La3Zr2O12 (LLZO) as ISEs. Crystallization of PEO based electrolytes below their melting temperature leads to an increased charge-transfer resistance. On the other hand, electrolytes based on the amorphous poly[2-(2-(2-methoxyethoxy)ethoxy)ethyl glycidyl ether (PTG) do not show an increased charge transfer resistance. Finally, the conductivity of ISE-rich CEs is measured as a function of their temperature and composition for elucidating how the interface resistance influences charge transport in ISE-rich composite electrolytes.

All-solid-state batteries (ASSBs) are promising candidates to significantly exceed the energy densities of today's lithium-ion batteries. However, for their successful commercialization, an easily scalable production procedure is needed. The tape casting procedure herein described allows to process a composite cathode by using an inert (polyoleophine) binder, conductive carbon, β-Li3PS4 and Li1+x[Ni0.6Mn0.2Co0.2]1-xO2. The inorganic electrolyte β-Li3PS4 layer is either coated onto the composite cathode or applied as a self-standing tape. In both cases a pressing step at room temperature is used to remove the porosity. Finally, a polymer electrolyte layer is added prior to the lithium metal anode to avoid the reaction between the latter and the inorganic solid electrolyte. Such dual-electrolyte, solid-state cells demonstrate good cycle performance and specific capacity. The type of conductive carbon in the composite cathode is seen to play a crucial role.

Electric vehicles allow emission-free transportation but offer only limited driving range. A technological jump to all-solid-state batteries (ASSBs) that safely implement a lithium metal anode, enabling high energy densities, would dramatically boost the battery performance. Recent research efforts have yield to a variety of inorganic solid electrolytes with extremely high ionic conductivity at room temperature. In particular, low density sulfidic electrolytes are attractive candidates for high-energy ASSBs. However, many issues still need to be addressed[1]. Herein, we focus on the limited electrochemical stability of sulfidic electrolytes that results in decomposition reactions accompanied by increasing resistances at the electrode/electrolyte-interface. In academic research, mostly pellet-type cells are used to investigate the electrochemical performance. However, this type of cell set-up is far from being practical for large scale battery production and application. A key advantage of sulfidic electrolytes is their ductility permitting this class of materials to be simply cold pressed. Thus, herein, slurry-based procedure is presented that easily allows to process a composite cathode. The cells demonstrate good electrochemical performance, although the type and amount of conductive carbon as well as the active material particles size, are crucial parameters. Reference [1] J. Janek, W. G. Zeier, Nat. Energy 2016, 1, 16141. Acknowledgment The research leading to these results has received funding from the German Federal Ministry of Education and Research under grant agreement no. 03XP0026F.