Pressure‐Driven Structural Distortion in Cold Sintered LiFePO$_4$|Li$_{1.3}$Al$_{0.3}$Ti$_{1.7}$(PO$_{4}$)$_{3}$ Composite Solid Electrolyte Bilayers

The pressure applied during the fabrication of LiFePO4-composite solid electrolyte bilayers via Cold Sintering Process (CSP) is known to influence their electrochemical performance and structural integrity. In a previous work, the authors have reported that bilayers sintered at high pressure (720 MPa) achieve high densification, but exhibit rapid capacity fading and poorer cycling stability compared to those processed at lower pressures (300 MPa). This study employs the galvanostatic intermittent titration technique (GITT), operando X-ray diffraction (XRD), and ex situ X-ray photoelectron spectroscopy (XPS) to elucidate the underlying mechanisms hindering the performance in high-pressure bilayers. GITT reveals larger and progressively increasing overpotentials and internal resistances, as well as irregular ion diffusion in high-pressure bilayers, suggesting altered lithiation kinetics. Operando XRD identifies irreversible phase transformations alongside accumulation of inactive FePO$_4$ species, with XPS confirming the presence of oxidized iron (Fe$^{3+}$) in the discharged cathodes sintered at high pressure. Notably, lattice distortions in the b and c axes—key lithium diffusion pathways in the olivine structure—are more pronounced at 720 MPa, indicating pressure-induced crystalline deformation modifies the energy barrier for lithium ion migration. Conversely, bilayers sintered at 300 MPa maintain stable structural and kinetic behavior, enabling reversible cycling and superior performance. These findings underscore the delicate balance required in CSP pressure to optimize densification of multi-component structures without compromising electrode functionality, guiding the design of robust solid-state bilayers and multilayers for next-generation battery technologies.