A Study on the Formation of a Lithium-Metal Protective Layer to Stabilize the Anodic Interface in All-Solid-State Batteires

Abstract

Rechargeable batteries with high energy density and long cycle life are urgently required for electric vehicles and other energy storage systems. However, existing lithium-ion batteries that use flammable organic solvents encounter stability-related issues such as explosion and combustion. In this regard, all-solid-state batteries (ASSBs) exhibit significantly reduced risks because the battery components are made of a fireproof solid. Additionally, ASSBs can physically inhibit lithium dendrite growth, which can facilitate the development of lithium secondary batteries with high capacity and energy density using lithium metal as an anode. Sulfide-type solid electrolytes (SSEs) have attracted considerable interest owing to their relatively high lithium-ion conductivity; moreover, they can easily decrease the grain-boundary resistance through room-temperature compression without any high-temperature heat treatment. However, Li3PS4, a representative SSE, reacts with Li metal and yields byproducts such as Li2S and Li3P, which reduce the interfacial stability of the battery. Therefore, direct contact between Li metal and the solid electrolyte must be prevented for stable battery operation. The voids generated by poor solid–solid contact cause unstable interfacial contact. Lithium dendrite growth is induced in these voids, which also leads to deterioration of the Li-metal anode and stability-related problems such as short circuits. Therefore, to fabricate long-life and high-stability Li-ion secondary batteries, the interfacial contact should be improved using an effective manufacturing process for the solid electrolyte layer or inserting an intermediate layer.

Each chapter describes an investigation on constructing a Li-metal coating layer to enhance the stability of the lithium-metal/solid-electrolyte interface. The approach presented here is anticipated to provide new insight into increasing the interfacial stability of lithium metal in ASSBs. The contents of each chapter are summarized below.

Chapter 2 describes the construction of a Li-P-S-based Li-metal coating layer using chemical and electrochemical methods. A precursor solution was prepared using LiFSI as a salt in the DOL/TEG organic solvent electrolyte and by adding Li2S-P2S5 in various ratios. The chemical method involved the formation of a coating layer by immersing Li metal in a precursor solution or adding it dropwise. The electrochemical method involved the formation of a coating layer through cell charging and discharging using a precursor solution as an electrolyte. This investigation was a basic experiment conducted to form a Li-metal coating layer, which helped in establishing the conditions required for forming a Li-P-S-based lithium-metal coating layer.

In Chapter 3, the solution deposition method was discussed as a highly effective strategy to increase the interfacial stability between the Li metal and solid electrolyte; moreover, the essential conditions for using solution processing were determined. A thin and dense halide-doped Li-P-S-based Li-metal coating layer was formed by solution processing. The obtained coating layer completely covered the surface of lithium metal and enabled uniform lithium-ion deposition. The cycling performance was considerably improved owing to the enhanced contact and interfacial stability. This investigation aided in validating the importance of interfacial stability in ASSBs.

Chapter 4 introduces an Indium-Iodine-Li-P-S (IILPS) Li-metal coating layer prepared using the solution method. The IILPS layer assisted in developing electro-chemo-mechanically stable anodic interfaces in an ASSB. The IILPS coating layer was composed of Li-In and LiI embedded in an Li2S-P2S5 glass–ceramic matrix. The void-free interface prepared using the solution processing method and the components of the IILPS layer enhanced the interfacial stability between the Li metal and solid electrolyte and improved the electrochemical performance. The coated-Li-based symmetric cell exhibited both stable Li deposition/stripping over 550 h and notably superior stability at high currents. This research was published in Journal of Materials Chemistry A

This study investigated the development of a Li-metal protective layer in an ASSB to improve the interfacial stability of the anode. Future studies conducted using an anode interface stabilized by a protective layer are expected to accelerate the commercialization of Li-metal-containing ASSBs.