## 2 Experimental Methods  
### Fabrication of AC-EPD LFP Cathode  
To prepare the AC-EPD suspension, 108 mg of LFP, 6 mg of carbon black, and 6 mg of PVDF were combined in 40 mL of acetone, yielding a total solid concentration of 3 mg\(\cdot\)mL\({}^{-1}\). The mixture was ultrasonicated and stirred to ensure a uniform dispersion. The resulting suspension was then used for AC-EPD, where an AC voltage of 50 V at a frequency of 4 Hz was applied for 10 min to deposit the LFP composite onto a stainless steel (SS) foil substrate, as shown in Figure 1a. The AC-EPD deposition parameters (50 V, 4 Hz) were optimized through preliminary experiments. Frequencies above 4 Hz resulted in negligible deposition due to rapid polarity switching, while lower frequencies (e.g., 2 Hz) caused particle agglomeration and non-uniform films. DC-EPD (0 Hz) led to uncontrolled thickness and poor film integrity. Therefore, 4 Hz was selected to balance deposition efficiency and particle stability, and 50 V was chosen to ensure sufficient electric field strength without inducing breakdown or solvent decomposition. This process produced a thin electrode film with a mass loading of approximately 0.5-0.7 mg\(\cdot\)cm\({}^{-2}\). Prior to AC-EPD, the SS substrates were cleaned by sequential ultrasonication in acetone, methanol, and deionized water (1 min each), then dried at 80 \({}^{\circ}\)C to remove residual solvents and ensure a clean deposition surface. After deposition, the coated substrate underwent five cycles of roll pressing to improve particle connectivity and electrode cohesion. The electrode was subsequently annealed under an argon atmosphere at 400 \({}^{\circ}\)C for one hour, using a temperature ramp rate of 5 \({}^{\circ}\)C per minute. This thermal treatment enhanced structural integrity and ensured good adhesion before cell assembly. Figure 1: \(Cont.\)  
Preparation of LATP Solid Electrolyte and Assembly of Li-Metal/AC-EPD LFP Batteries (Sample 1 and Sample 2) and Li-Metal/LiPF\({}_{6}\)/AC-EPD LFP (Sample 3)  
To prepare the LATP solid electrolyte, 850 mg of the LATP powder was first compacted into a green pellet using uniaxial pressing at 7 MPa for 10 min. The resulting pellet was then vacuum-sealed and subjected to cold isostatic pressing (CIP) at 40 MPa for 30 min to enhance its mechanical strength and bulk density. The densified pellet was subsequently sintered in an argon atmosphere with a two-step thermal process: an initial stage at 650 \({}^{\circ}\)C for 30 min, followed by high-temperature sintering at 900 \({}^{\circ}\)C (Sample 1) or 1000 \({}^{\circ}\)C (Sample 2) for 5 h with a controlled heating rate of 10 \({}^{\circ}\)C per minute. After sintering, the LATP pellet was mechanically polished to achieve a uniform thickness of ~350 \(\upmu\)m, making it suitable for cell integration. Ionic conductivity measurements using electrochemical impedance spectroscopy showed a value of 1.9 \(\times\) 10\({}^{-4}\) S-cm\({}^{-1}\), indicating sufficient ionic transport for solid-state applications. The ionic conductivity of the synthesized LATP was measured to be ~1.9 \(\times\) 10\({}^{-4}\) S-cm\({}^{-1}\), which lies within the well-reported optimized range for undoped LATP ceramics (typically 2-7 \(\times\) 10\({}^{-4}\) S-cm\({}^{-1}\)). While values as high as 1.3 \(\times\) 10\({}^{-3}\) S-cm\({}^{-1}\) have been achieved in certain highly doped or glass-ceramic LATP variants, our result represents stable, reproducible conductivity obtained under conventional sintering conditions without additives. This conductivity is suitable for evaluating the electrochemical interface in all-solid-state batteries, and further enhancement through compositional tuning is planned in future studies. For liquid-based Li-metal/LiPF\({}_{6}\)/AC-EPD LFP batteries (Sample 3), a standard 1 M LiPF\({}_{6}\) electrolyte dissolved in a 1:1 volume mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used. CR2032 coin cells were assembled in an argon-filled glove box to prevent contamination from air or moisture.