## 3 Results and Discussion  
The LFP cathode was deposited on the SS foil using the AC-EPD method using the setup in Figure 1a. The crystallinity of the LFP film on the SS was confirmed using XRD in Figure 1b. (Sample 1: Li/LATP (900 \({}^{\circ}\)C)/LFP, Sample 2: Li/LATP (1000 \({}^{\circ}\)C)/LFP, and Sample 3: Li/LiPF\({}_{6}\) (liquid electrolyte/LFP). The XRD pattern corresponds to the LFP electrode fabricated by AC-EPD onto an SS foil substrate under the conditions of 50 V and 4 Hz for 10 min. The diffraction peaks match well with the orthorhombic olivine structure of LFP, confirming the successful deposition of phase-pure crystalline LFP on the substrate. The sharp and intense peaks suggest a high degree of crystallinity in the deposited LFP film, with no evidence of secondary phases such as Fe\({}_{2}\)O\({}_{3}\) or Li\({}_{3}\)PO\({}_{4}\), which could form during improper processing. The absence of significant peak broadening also indicates that the crystallite size is relatively large, and that the EPD process did not degrade the structural integrity of the LFP particles. The inset image in Figure 1a shows the surface of the EPD-deposited LFP film, which appears dense and uniform, further supporting the effectiveness of the AC-EPD method in forming a high-quality electrode layer. Overall, the XRD analysis in Figure 1b confirms that the AC-EPD process preserved the crystal structure of LFP and enabled its direct deposition onto conductive substrates for solid-state or conventional LIB applications. The rate performance of the three samples was compared across a range of current densities from 1 C to 5 C in Figure 2, demonstrating its excellent high-rate capability in Sample 1. The rate performance of the three LFP-based cells--each interfaced with a different electrolyte environment--is presented in Figure 2a, with the corresponding charge-discharge voltage profiles shown in Figure 2b-d. These results clearly highlight the effect of the electrolyte interface and LATP sintering temperature on the electrochemical behavior of the LFP electrodes under various current rates. Sample 1, using LATP sintered at 900 \({}^{\circ}\)C, demonstrates the best rate performance and stability among the three configurations. As shown in Figure 2a, it maintains a specific capacity of approximately 80-120 mAh\(\cdot\)g\({}^{-1}\) across a wide range of current rates (1 C to 5 C), and nearly full capacity is recovered upon returning to 1 C. The corresponding voltage profiles in Figure 2b display well-defined charge-discharge plateaus, even at high rates, suggesting low interfacial resistance and effective Li+ transport across the LFP/LATP interface. This performance indicates favorable interfacial contact and mechanical compliance between the LFP and LATP processed at this temperature. In contrast, Sample 2, which used LATP sintered at 1000 \({}^{\circ}\)C, shows significantly lower capacity and poor rate capability. The capacity drops below 50 mAh\(\cdot\)g\({}^{-1}\) at high rates and exhibits abnormal decay in the charging capacity when cycled back to 1 C. This is further supported by the voltage curves in Figure 2c, which reveal distorted and less-defined plateaus, indicating sluggish kinetics and possible interfacial degradation. The higher sintering temperature likely led to a denser LATP microstructure with reduced surface compliance, impairing interfacial contact and increasing resistance during cycling, which will be discussed later. Sample 3, which employed a liquid electrolyte (1 M LiPF\({}_{6}\)), exhibits moderate performance. The initial capacity is around 100 mAh\(\cdot\)g\({}^{-1}\) at 1 C and gradually declines with the increasing rate, but the capacity largely recovers when the current is reduced back to 1 C. Figure 2d shows smoother voltage profiles than Sample 2. While the liquid electrolyte allows better wetting and ion transport than poorly sintered LATP, it still falls short of the well-optimized solid interface seen in Sample 1.