All-solid-state batteries (ASSBs) based on noncombustible solid electrolytes are promising candidates for safe and high energy storage systems, but it remains a challenge to prepare systems with stable interfaces between the various solid components that survive both the synthesis conditions and electrochemical cycling. We have investigated cathode mixtures based on a carbon-coated LiFePO₄ active material and Li_3+xP_1–xSi_xO₄ solid electrolyte for potential use in all-solid-state batteries. Half-cells were constructed by combining both compounds into pellets by spark plasma sintering (SPS). We report the fast and quantitative formation of two solid solutions (LiFePO₄–Fe₂SiO₄ and Li₃PO₄–Li₂FeSiO₄) for different compositions and ratios of the pristine compounds, as tracked by powder X-ray diffraction and solid-state nuclear magnetic resonance; X-ray absorption near-edge spectroscopy confirms the formation of iron silicates similar to Fe₂SiO₄. Scanning electron microscopy and energy dispersive X-ray spectroscopy reveal diffusion of iron cations up to 40 μm into the solid electrolyte even in the short processing times accessible by SPS. Electrochemical cycling of the SPS-treated cathode mixtures demonstrates a substantial decrease in capacity following the formation of the solid solutions during sintering. Consequently, all-solid-state batteries based on LiFePO₄ and Li_3+xP_1–xSi_xO₄ would necessitate iron ion blocking layers. More generally, this study highlights the importance of systematic studies on the fundamental reactions at the active material–solid electrolyte interfaces to enable the introduction of protective layers for commercially successful ASSBs.