## 1 Introduction  
The development of all-solid-state Li-metal batteries (ASSLBs) has gained significant attention as a promising route toward next-generation energy storage systems with enhanced energy density, safety, and longevity [1, 2, 3, 4]. Traditional Li-ion batteries (LIBs) are limited by flammable liquid electrolytes and low-capacity graphite anodes, which pose risks of leakage, dendrite formation, and thermal runaway [5, 6]. In contrast, replacing the liquid electrolyte with a solid-state electrolyte (SSE) and utilizing Li metal as the anode offer substantial improvements in volumetric and gravimetric energy density while greatly reducing safety concerns [7, 8]. ASSLBs are particularly appealing for high-demand applications such as electric vehicles and grid-level storage, where both energy density and safety are paramount. Among various solid-state configurations, the LFP/Li\({}_{1.3}\)Al\({}_{0.3}\)Ti\({}_{1.7}\)(PO\({}_{4}\))\({}_{3}\) (LATP)/Li system presents a highly attractive platform [9, 10, 11]. LFP is a widely studied cathode material due to its exceptional thermal stability, flat voltage profile, long lifecycle, and compatibility with solid-state architectures [12; 13; 14]. LATP, a NASICON-type solid electrolyte, offers high Li-ion conductivity and relatively good stability against LFP [15; 16]. Furthermore, Li metal as an anode maximizes energy density. This tri-layer system combines safety, cost-effectiveness, and electrochemical performance, making it a suitable model to explore the fundamental and practical aspects of ASSLBs. A key challenge in solid-state battery development lies in the solid-solid interface between the electrolyte and electrodes [17; 18]. Unlike liquid electrolytes that get wet and conform to electrode surfaces, solid electrolytes require intimate physical and chemical contact to enable efficient Li-ion transfer. Interfacial resistance, mechanical mismatch, and chemical reactivity can all hinder ion transport and degrade performance over time. Therefore, understanding and controlling the electrode/electrolyte interface are critical to improving ionic conductivity, cycling stability, and overall device performance in ASSLBs [19; 20]. Compared to previous reports on Li/LATP/LFP solid-state batteries, the present study offers a distinct approach in both material processing and mechanistic understanding. In contrast to the work by He et al. [21], which employed a mesoporous LFP-LATP scaffold filled with the MEEP polymer to achieve high areal capacities, our study eliminates the use of polymer fillers entirely, instead relying on a dense LATP pellet with mechanically stable contact. This polymer-free design not only simplifies the cell architecture but also mitigates issues such as polymer degradation or dendrite formation within polymer-rich regions. While Yao et al. explores Li/LATP/LFP ASSLBs using high-temperature sintered LATP and slurry-cast cathodes with polyvinylidene fluoride (PVDF) binders [22], our study utilizes an alternative fabrication method--alternating current electrophoretic deposition (AC-EPD)--which enables the uniform and binder-integrated deposition of LFP electrodes directly onto current collectors. This approach simplifies processing while ensuring strong adhesion and consistent morphology. In contrast to the tape-casting or manual mixing methods used in prior studies, AC-EPD offers better control over electrode thickness and composition, contributing to improved interface formation and mechanical integrity--critical factors for solid-state battery performance. Moreover, our study extends beyond the electrochemical testing emphasized in previous studies [22] by providing a quantitative correlation between Li-ion diffusivity, \(b\)-value analysis (from cyclic voltammetry), and morphological evolution [22; 23; 24]. For instance, while Mazor et al.