## 3 Results and Discussion
These factors result in slower ion transport and more distributed electrochemical reactions, leading to peak broadening and increased polarization  
The diffusivity analysis presented in Figure 4 compares the Li-ion diffusion behavior of three LFP-based electrodes interfaced with different electrolytes and processing conditions. The current response at anodic and cathodic peaks plotted against the square root of the scan rate (Figure 4a-c) was used to extract diffusion coefficients based on the Randles-Sevcik equation. The resulting diffusivity values are summarized in the bar graph shown in Figure 4d, providing insights into how interface characteristics affect Li-ion transport kinetics. Sample 1, which uses LATP sintered at 900 \({}^{\circ}\)C, exhibits the highest diffusivity values for both anodic and cathodic processes, in the order of \(\sim\)10\({}^{-9}\) cm\({}^{2}\cdot\)s\({}^{-1}\). This suggests that the interface between LFP and LATP is well formed, likely due to optimal sintering that balances densification and ionic conductivity. The high diffusion coefficient is consistent with the previously observed strong rate capability and minimal polarization in Sample 1, indicating efficient Li\({}^{+}\) transport across the electrode-electrolyte interface. In contrast,  
Figure 3: Cyclic voltammetry (CV) analysis and \(b\)-value determination for AC-EPD LFP electrodes in three different battery configurations. (**a**,**c**,**e**) CV curves recorded at scan rates ranging from 0.1 to 1.0 mV/s for (**a**) Sample 1 (Li/LATP 900 \({}^{\circ}\)C/LFP), (**c**) Sample 2 (Li/LATP 1000 \({}^{\circ}\)C/LFP), and (**e**) Sample 3 (Li/LIPF\({}_{6}\)/LFP). (**b**,**d**,**f**) Corresponding log(i) vs. log(scan rate) plots at anodic and cathodic peak currents for the three samples, used to extract \(b\)-values. Sample 2, based on LATP sintered at 1000 \({}^{\circ}\)C, shows the lowest diffusivity among the three. Both anodic and cathodic values drop significantly, approaching \(\sim\)10\({}^{-10}\) cm\({}^{2}\cdot\)s\({}^{-1}\). This reduction reflects poor Li\({}^{+}\) transport, which may be attributed to excessive grain growth or reduced surface compliance in the over-sintered LATP, leading to inferior contact with the LFP cathode. The compromised interface in Sample 2 results in sluggish ion kinetics, which aligns with its poor capacity retention and distorted voltage profiles seen in rate performance measurements. Sample 3, using a liquid electrolyte (1 M LiPF\({}_{6}\)), shows intermediate diffusivity values between Samples 1 and 2. Although not as high as Sample 1, the liquid electrolyte provides sufficient ion mobility and wetting, facilitating moderate diffusion (\(\sim\)10\({}^{-9}\) cm\({}^{2}\cdot\)s\({}^{-1}\)). This is reflected in the CV symmetry and decent capacity recovery in rate cycling tests. However, limitations such as SEI formation and side reactions may hinder performance at higher current rates. The diffusivity results reinforce that interfacial quality is a critical factor in determining Li-ion transport in LFP electrodes. Properly sintered LATP at 900 \({}^{\circ}\)C (Sample 1) yields the most favorable conditions for fast and reversible diffusion, whereas excessive sintering (Sample 2) degrades ion transport. The liquid system (Sample 3) performs reasonably well, but lacks the structural robustness and long-term stability of an optimized solid-state interface. Figure 4: The determination of Li-ion diffusivity in AC-EPD LFP electrodes for three battery configurations. (**a–c**) The plots of peak current versus the square root of the scan rate (\(\
u^{1/2}\)) for anodic and cathodic processes in (**a**) Sample 1 (Li/LATP 900 \({}^{\circ}\)C/LFP), (**b**) Sample 2 (Li/LATP 1000 \({}^{\circ}\)C/LFP), and (**c**) Sample 3 (Li/LiPF\({}_{6}\)/LFP), based on the Randles–Sévécík equation. Linear fits confirm diffusion-controlled behavior. (**d**) Extracted diffusivity values for anodic (red circles) and cathodic (blue squares) processes.