Understanding Lithium-Ion Conductivity in NASICON-Type Polymer-in-Ceramic Composite Electrolytes

Composite electrolytes comprising distinctive polyether (PEO) or polyester (PCL, P(CL-co-TMC)) polymers in combination with a high loading of Li1.4Al0.4Ti1.6(PO4)3 NASICON-type ceramic powders (LATP, 70 wt %) are investigated to gain insights into the limitations of their ion conductivity in resulting polymer-in-ceramic solid-state electrolyte systems. Here, LATP constitutes an advantageous ceramic Li-ion conductor with fair ionic conductivity that does not immediately suffer from limitations arising from interface issues due to the detrimental formation of surface species (e.g., Li2CO3) in contact with air and/or surrounding polymers. The Li-ion transport in all these composite electrolytes is found to follow a slow-motion regime in the polymer matrix, regardless of the nature of the polymer used. Interestingly, the weakly Li-coordinating polyester-based polymers PCL and P(CL-co-TMC) exhibit an exchange of Li+ ions between the polymer and ceramic phases and high Li-ion transference numbers compared to the polyether PEO matrix, which has strong Li–polymer coordination. LATP particle agglomeration is nevertheless observed in all the composite electrolytes, and this most likely represents a dominating cause for the lower Li-ion conductivity values of these composites when compared to those of their solid polymer electrolyte (SPE) counterparts. These findings add another step toward the development of functional composite electrolytes for all-solid-state batteries.