Fast Li⁺ Conduction Mechanism and Interfacial Chemistry of a NASICON/Polymer Composite Electrolyte

The unclear Li⁺ local environment and Li⁺ conduction mechanism in solid polymer electrolytes, especially in a ceramic/polymer composite electrolyte, hinder the design and development of a new composite electrolyte. Moreover, both the low room-temperature Li⁺ conductivity and large interfacial resistance with a metallic lithium anode of a polymer membrane limit its application below a relatively high temperature. Here we have identified the Li⁺ distribution and Li⁺ transport mechanism in a composite polymer electrolyte by investigating a new solid poly(ethylene oxide) (PEO)-based NASICON-LiZr₂(PO₄)₃ composite with ⁷Li relaxation time and ⁶Li → ⁷Li trace-exchange NMR measurements. The Li⁺ population of the two local environments in the composite electrolytes depends on the Li-salt concentration and the amount of ceramic filler. A composite electrolyte with a [EO]/[Li⁺] ratio n = 10 and 25 wt % LZP filler has a high Li⁺ conductivity of 1.2 × 10^-4 S cm^-1 at 30 °C and a low activation energy owing to the additional Li⁺ in the mobile A2 environment. Moreover, an in situ formed solid electrolyte interphase layer from the reaction between LiZr₂(PO₄)₃ and a metallic lithium anode stabilized the Li/composite-electrolyte interface and reduced the interfacial resistance, which provided a symmetric Li/Li cell and all-solid-state Li/LiFePO₄ and Li/LiNi_0.8Co_0.1Mn_0.1O₂ cells a good cycling performance at 40 °C.