Charge Carrier Dynamics of the Mixed Conducting Interphase in All‐Solid‐State Batteries: Lithiated Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 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, Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 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. Li 1.3+ x Al 0.3 Ti 1.7 (PO 4 ) 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 Li 3 Al 0.3 Ti 1.7 (PO 4 ) 3 phase covering the nonlithiated Li‐poor regions. The originally poor electronic conductivity σ eon 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 σ eon . 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.