Interfacial lithiation of lithium aluminum titanium phosphate explored by 7Li NMR.

Lithium aluminum titanium phosphate (LATP) is well-established as a crystalline electrolyte offering fast Li⁺ diffusion pathways. However, when in contact with lithium metal, LATP forms a mixed-conducting interphase, potentially impacting the performance of LATP-based batteries. During lithiation, Ti⁴⁺ is partially reduced to form Ti³⁺, and Li⁺ occupies vacant sites within the NaSICON-type structure. Here, we employed ⁷Li nuclear magnetic resonance (NMR) to investigate changes in Li⁺ diffusivity induced by chemical lithiation using n-butyllithium. Chemical lithiation allowed us to mimic the structural and dynamic changes occurring within a lithium metal battery. Our findings reveal that lithiation does not hinder Li⁺ diffusivity; rather, ⁷Li NMR relaxation measurements indicate enhanced Li⁺ ion hopping processes. Despite the formation of a lithiated interfacial layer that propagates inward, the dynamic properties of LATP-characterized by Li-rich and Li-poor domains-remain resilient. These results highlight that electrochemical degradation does not compromise the intrinsic ion dynamics of LATP.