Charge Carrier Dynamics of the Mixed Conducting Interphase in All‐Solid‐State Batteries: Lithiated Li1.3Al0.3Ti1.7(PO4)3 as a Case Study

All‐solid‐state batteries relying on Li metal as negative electrode material and a ceramic electrolyte may severely suffer from unwanted interfacial processes. Here, Li1.3Al0.3Ti1.7(PO4)3 (LATP) serve as a model electrolyte which is known to form an ionic‐electronic, that is, mixed conducting interphase (MCI) when in contact with metallic Li or any other Li source. Li1.3+xAl0.3Ti1.7(PO4)3 with x = 0.2, 0.6 and 1.3 is prepared via ex situ chemical lithiation to mimic the formation of MCIs taking place otherwise operando. The preparation of large amounts of lithiated LATP with controlled Li contents allowed us to use nuclear and electric techniques to study local structures and ionic/electronic dynamics in detail. The results point to the formation of a core‐shell two‐phase morphology with the Li‐rich Li3Al0.3Ti1.7(PO4)3 phase covering the nonlithiated Li‐poor regions. The originally poor electronic conductivity σeon of 6.5 × 10−12 S cm−1 (293 K) increases by ≈3 orders of magnitude, hence reaching the order of 6.6 × 10−9 S cm−1 for x = 0.6. At even higher loadings (x = 1.3), a decrease in conductivity is seen, i.e., not exceeding alarming values for σeon. Quantifying electronic and ionic transport processes will help assessing the extent of damage through MCI formation and discussing whether any strategies to mitigate such formation is necessary at all. Chemical lithiation is used to mimic the operando formation of a mixed conducting interphase in lithium aluminium titanium phosphate (LATP). While spectroscopic methods give evidence for the formation of a core‐shell structure with a Ti3+‐enriched layer covering the LATP crystallites, chronoamperometric polarization revealed that electronic conductivities do not reach alarming values for its application as solid electrolyte.