## 3 Results and Discussion
The measured diffusivity (\(\sim\)10\({}^{-9}\) cm\({}^{2}\cdot\)s\({}^{-1}\)) is sufficient to support fast interfacial reactions, though it does not fully compensate for limitations in bulk ion transport. The liquid electrolyte provides good ionic conductivity and wetting, which enhances surface-driven processes but may still be constrained by interfacial stability or side reactions during extended cycling. Sample 1 achieves superior rate performance due to its high diffusivity and well-formed solid-state interface that supports diffusion-controlled kinetics. Sample 2 is limited by low diffusivity and poor interfacial contact, while Sample 3 benefits from surface capacitive effects and moderate diffusivity but lacks the structural stability of an optimized solid-state system. These findings collectively demonstrate that high-rate capability in LFP electrodes depends on a delicate balance between bulk diffusion, interfacial contact, and surface kinetics, as summarized in Table 1.  
The SEM images in Figure 5a-d illustrate the morphological evolution of AC-EPD LFP electrodes under various cycling and interfacial conditions. In Figure 5a, the as-deposited LFP electrode before cycling shows a loosely packed morphology with distinguishable primary particles. The structure appears porous, with noticeable interparticle spacing, indicating a typical surface formed by AC-EPD without compaction from cycling-induced stress or interface interactions. After cycling in contact with a liquid electrolyte (1 M LiPF\({}_{6}\)), as shown in Figure 5b, the morphology significantly changes. The surface becomes smoother and more compact, suggesting surface reconstruction and particle merging likely caused by electrolyte-induced side reactions or binder/electrolyte degradation. The reduction in contrast and particle definition may also point to the formation of a surface film or decomposition products, such as SEI-like layers, which can passivate the electrode surface. In contrast, LFP electrodes cycled in contact with solid-state LATP electrolytes sintered at different temperatures show distinct morphological behavior. In Figure 5c, the interface with LATP sintered at 900 \({}^{\circ}\)C preserves a relatively granular texture, with LFP particles still visible and partially interconnected. This suggests moderate interfacial contact and mechanical pressure during cycling, likely promoting Li transport while avoiding extensive surface degradation. However, in Figure 5d, where LATP was sintered at 1000 \({}^{\circ}\)C, the LFP morphology becomes more densely packed and featureless. The clear particle boundaries visible in Figure 5a,c are significantly diminished, implying enhanced densification or possible interfacial reactions at elevated sintering-induced interface conditions. The higher sintering temperature likely increased LATP grain boundary conductivity and mechanical stiffness, leading to stronger physical contact and potentially more intense interfacial stress or reactions during cycling. The dense and featureless morphology observed in Figure 5d for LFP cycled with LATP sintered at 1000 \({}^{\circ}\)C may be attributed to mechanical stress accumulation at the rigid LATP interface, leading to particle compaction. Additionally, potential interfacial reactions--such as elemental diffusion from LATP (e.g., Ti or P)--could chemically modify the LFP surface, suppressing the original granular texture. These mechanisms, though not directly confirmed here, are consistent with the reported degradation pathways in high-temperature-processed solid-state interfaces and warrant further investigation using post-mortem structural and compositional analyses.