Investigating the Mechanisms of Li Dendrite Formation in Sulfide Solid Electrolytes for All-Solid-State Batteries

Abstract

As the primary use of Li-ion batteries shifts from small electronic devices to electric vehicles, there is a need to increase the stability and energy density of Li-ion batteries. In order to achieve high energy density, use of lithium metal as an anode has being considered due to its high theoretical capacity (3860 mAh g-1) and low reduction potential (-3.04 V vs. SHE). However, the liquid electrolyte used in Li-ion batteries is not only dangerous due to the risk of fire in case of leakage, but also has the disadvantage of shortening the lifetime of the battery due to the generation of lithium dendrites during cycling when lithium metal is used as anode. To overcome these disadvantages, all-solid-state batteries using solid electrolytes have emerged, which are expected to improve safety and cycling stability even when using lithium metal anodes. Among the various types of solid electrolytes, sulfide solid electrolytes are widely used due to their high ionic conductivity and ductility, which allows easy formation of good interfaces, resulting in high performance all-solid-state batteries.To date, many attempts have been made to use lithium metal anodes with solid electrolytes, but dendrite growth has not been completely suppressed.1 Dendrite growth is affected by many interface-related factors, such as void ratio, grain size, elastic modulus, reductive decomposition products, and electrical conductivity of the electrolyte, which must be clarified before the use of lithium metal anodes can be realized.2 It is essential to observe and analyse the changes during electrochemical tests in order to reveal the formation of Li dendrites under the conditions in which actual real cells operate. We have attempted to elucidate the dynamic structural changes at the interface using multimodal/multiscale operando X-ray CT under an applied pressure, as well as the reactions taking place at the interface by observing the interface products using X-ray absorption spectroscopy. In this study, X-ray diffraction, X-ray absorption spectroscopy, time-resolved impedance measurements and multi scale operando X-ray CT are used to determine the electrochemical and chemical mechanisms of Li dendrite formation in different sulfide solid electrolytes. By investigating and comparing the particle size, porosity, doping effect of halides and reduction resistance of solid electrolytes used in lithium metal solid-state batteries, the mechanism of Li dendrite growth and how each factor affects dendrite growth are investigated.Here, we discuss Li3PS4, halogen-doped Li3PS4, and argyrodite-based Li6PS5Cl as typical sulfide-based solid electrolytes; Li3PS4 is known to have a low elastic modulus, and a simple cold press could be used to reduce the inter-particle voids. However, Li3PS4 has a very narrow potential difference, which resulted in unwanted oxidation and reduction byproducts during battery cycling. The high electronic conductivity of the reduction products at the interface also led to the formation of electronic pathways, resulting in the concentration of current in several locations and ultimately triggering the formation of Li dendrites. On the other hand, in argyrodite Li6PS5Cl, the formation of an interfacial phase with low electronic conductivity, which is mainly composed of lithium chloride at the interface, suppresses the reduction reaction and thus the formation of Li dendrites. In the presentation, the relationship between the lithium dendrite formation and the operating conditions such as temperature and pressure will also be presented.