Insights into Sodium Ion Transfer at the Na/NASICON Interface Improved by Uniaxial Compression

A robust ceramic solid electrolyte with high ionic conductivity is a key component for all-solid-state batteries (ASSBs). In terms of the demand for high-energy-density storage, researchers have been tackling various challenges to use metal anodes, where a fundamental understanding on the metal/solid electrolyte interface is of particular importance. The Na⁺ superionic conductor, so-called NASICON, has high potential for application to ASSBs with a Na anode due to its high Na⁺ ion conductivity at room temperature, which has, however, faced a daunting issue of the significantly large interfacial resistance between Na and NASICON. In this work, we have successfully reduced the interfacial resistance as low as 14 ω cm² at room temperature by a simple mechanical compression of a Na/NASICON assembly. We also demonstrate a fundamental study of the Na/NASICON interface in comparison with the Na/β′′-alumina counterpart by means of the electrochemical impedance technique, which elucidates a stark difference between the activation energies for interfacial charge transfer: ∼0.6 eV for Na/NASICON and ∼0.3 eV for Na/β′′-alumina. This result suggests the formation of a Na⁺-conductive interphase layer in pressing Na metal on the NASICON surface at room temperature.