Hybrid solid electrolytes (HSEs), composed of mixtures of ion-conducting inorganic electrolytes and polymer electrolytes, are considered as one of the most promising classes of solid-state electrolytes for lithium battery applications. Supposedly, HSEs show enhanced properties with respect to both inorganic and polymer electrolytes, such as higher flexibility than inorganic electrolytes and improved transport properties compared to polymer electrolytes. Unfortunately, this is often not the case, as the ionic conductivity of HSEs usually decreases with increasing inorganic electrolyte content. Lithium-conducting inorganic particles dispersed in a polymer electrolyte matrix contribute negligibly to the long-range charge transport process, owing to the high resistance for the lithium-ion transfer across the inorganic and polymer phases. The origin of this high interface resistance has been attributed to a variety of factors, but there is still lack of understanding of the interfacial processes within HSEs.
In this study, we have deeply analyzed the ions coordination environment, transport properties and local ions dynamics of a series of HSEs, containing LATP-type Li+-conducting filler, by means of Electrochemical Impedance Spectroscopy (EIS) and solid-state NMR. After introduction of LATP, the total ionic conductivity decreases, owing to a high resistance for the inter-phase lithium transfer process. The latter is confirmed to be a thermally activated process, characterized by higher activation energy than the conductivity in the individual phases. At high temperatures (T > 60 °C), however, the interface resistance becomes negligible, and the inorganic particles contribute effectively to the long-range charge transfer process. This was also confirmed by 6Li NMR experiments, carried out after cycling an HSE between 6Li-enriched electrodes. Nonetheless, even in at high temperatures, the overall diffusivity is still limited by the transport properties in the polymer matrix. In addition, Pulsed Field Gradient (PFG) NMR shows that the anions diffusivity decreases with increasing concentration of LATP, which translates into a moderate increase of the lithium transference. This partially compensates the loss in total conductivity, that is, the lithium-ion conductivity remains constant up to a concentration of 10 vol% LATP.
In conclusion, this study confirms that inclusion of moderate amounts of LATP-type filler in HSEs can enhance the transport properties of HSEs, although only at temperatures high enough to overcome the interface resistance between LATP particles and polymer matrix. This has interesting implications for the use of HSEs in next-generation solid-state lithium batteries.