Design of Sulfide-based Hybrid Electrolytes for Stable All-Solid-State Batteries

Abstract

The growing demand for high energy density and safety in energy storage systems (ESSs) and electric vehicles (EVs) has led to the exploration of next-generation batteries using Li metal anodes and solid electrolytes (SEs) for all-solid-state batteries (ASSBs). High capacity (3,860 mAh/g) and the lowest reduction potential (-3.040 V vs. Li/Li+) characterize lithium metal as a promising candidate, since SEs offer theoretical prevention of lithium dendrite growth. Especially, sulfide solid electrolytes (SSEs) exhibit high ionic conductivity (1~25 mS/cm) and soft mechanical properties at room temperature, which are advantageous for battery performance. Among SSEs, Li6PS5Cl (LPSCl) has shown promising results but suffers from unstable electrodeelectrolyte interface issues, limiting their practical application. This study investigates hybrid liquid electrolyte/sulfide solid electrolyte (LE/SSE) systems to enhance interfacial stability focusing on commonly employed LEs in lithium-ion batteries (LIBs). Carbonate-based (1M LiPF6, EC/DEC = 3/7) and ether-based (1M LiTFSI, DOL/DME = 5/5) LEs were applied with small amounts to Li6PS5Cl (LPSCl) solid electrolyte. Additionally, carbonate-based (1M LiTFSI, EC/DEC = 3/7) LE was combined with LPSCl to examine the effect of Li salts. First of all, ether-based LE caused serious side reaction with LPSCl with a high donor number. Accordingly, the ether-based hybrid electrolyte showed low ionic conductivity and poor symetric cell performance. The hybrid system using carbonate-based LE demonstrated improved interfacial contact on the anode side, leading to the formation of an in-situ fluorinerich solid electrolyte interphase (SEI) layer and extended cell lifespan (over 450 hours for symmetric cell and over 100 cycles with high CE of ~95 % for half cell) even under conditions without assembly pressure or additional processes. The reactivity of carbonate-based LE and LPSCl is negligible in actual cell performance, and by introducing a small amount of liquid electrolyte to the interface, lithium ion movement is promoted and crack formation is much suppressed than bare solid electrolyte and ether-based hybrid electrolyte. However, challenges persisted at high voltage ranges due to serious side reactions. Potential solutions to overcome these limitations include implementing a 3D structure on the anode to restrict the physical contact between the cathode and the LE, using cathodes that operate within a low voltage range to limit the overall voltage band to 2.5 V, and identifying an optimal electrolyte composition that minimizes side reactions between LE and LPSCl under high voltage ranges. This study highlights the benefits and drawbacks of hybrid LE/SSE systems, providing valuable insights and potential solutions for future development.