Elucidating Interfacial Phenomena between Solid-State Electrolytes and the Sulfur-Cathode of Lithium-Sulfur Batteries

Abstract

With the increasing demand for electric/hybrid vehicles, smart electronic devices, and rapid technological advancements, the need for batteries with improved properties to state-of-the-art Li-ion batteries has hastily grown over the last decade. In the screening of new technologies, Lithium-Sulfur (Li–S) batteries are one of the most promising candidates among rechargeable battery systems due to its reduced cost, low toxicity, and high theoretical specific energy and specific capacity. However, there are still several limitations that need to be overcome before Li-S batteries are feasible for commercialization. One of the main drawbacks of this technology is the “shuttle” effect – migration of soluble polysulfide species (PS) from the cathode to the anode, where are reduced to Li2S2 and Li2S, resulting in the formation of an SEI layer that can passivate the lithium anode. In addition, the use of liquid organic electrolytes rises safety concerns due to their flammability. For this reason, inorganic solid-state electrolytes (SSEs) are being explored as an alternative because unlike liquid organic-based electrolytes, SSEs are usually non-flammable and have the potential to suppress PS shuttle and Li dendrites. However, when working with SSEs, the compatibility between the electrodes and the SSEs is critical. Such compatibility is determined by the properties of the electrode/electrolyte interface. A good interface between an SSE and electrode requires fast ion transport, maximum contact area, and chemical stability during cycling. Therefore, it is imperative to gain insights regarding the SSE/Sulfur-cathode interface. In this work, we use density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations in order to study the SSE/S-cathode interface. Li3PS4, Li7P2S8I, and Li6PS5Cl were selected as representative materials of the sulfide-type family of SSEs, which has proven to exhibit high ionic conductivity and good mechanical properties. Here, two states of charge of the cathode (non- and fully-lithiated) in contact with the SSEs models are studied. Reaction mechanisms, the structure of the interface, and charge transfer evolution are carefully examined and characterized in detail. Finally, interfacial and surface energies are provided as descriptors of stability and compatibility of the electrode/cathode interfaces.