With the widespread use of portable electronic devices and electric vehicles (EVs), the development of next-generation battery systems emphasizing both safety and high energy density has become increasingly crucial. In this context, all-solid-state batteries (ASSBs) featuring inorganic electrolytes have emerged as leading candidates due to their superior safety features, particularly their non-flammable nature. Additionally, the potential for high energy density in ASSBs is attributed to the adoption of a Li-metal anode with a specific capacity of 3860 mAh·g⁻¹ and the lowest redox potential (0 V versus Li/Li⁺) among lithium-ion batteries [1]. Among inorganic electrolytes, cubic garnet-type Li₇La₃Zr₂O₁₂ (LLZO) electrolytes have garnered significant interest for their high ionic conductivities at room temperature (~1 mS·cm⁻¹) and their thermodynamic stability against Li-metal [2-4]. Despite these advantages, practical application has been hindered by premature short-circuits caused by the penetration of Li dendrites through the rigid LLZO. Studies have shown dendrite penetration even at low current densities of 0.1 to 1.0 mA·cm⁻² at room temperature, falling short of meeting fast-charging targets for EVs [5].

Recent research has highlighted that one crucial factor in Li dendrite formation is the interfacial inhomogeneity arising from poor Li wettability between Li metal and the LLZO electrolyte [6]. This inhomogeneous contact can generate uneven electrical distribution sites, leading to extremely high localized current densities [7]. Efforts to improve the lithiophilic properties of LLZO electrolytes have been extensive, aiming to mitigate this inhomogeneity [8-10]. Studies have demonstrated that removing interface impurities effectively enhances lithium wettability [11]. Additionally, the insertion of lithiophilic interlayers, such as metal/metal oxide, at the Li-metal–LLZO interface has shown notable performance improvements [12-14]. However, Li dendrites still occur even at low critical current densities (CCD) below 3.0 mA·cm⁻² [15].

Our approach aims to address the inhomogeneity between Li metal and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) electrolyte by utilizing a molten salt technique involving metal chloride. The interlayer, facilitated by the molten salt technique, significantly reduces the interfacial area-specific resistance (ASR) due to the improved wettability of Li metal. Consequently, this results in more uniform Li plating/stripping, achieving a higher CCD at 25°C in a Li/LLZTO/Li symmetric cell. Galvanostatic cycling in the symmetric cell demonstrates exceptional long-term durability.