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 Omega cm(2) 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/beta ''-alumina counterpart by means of the electrochemical impedance technique, which elucidates a stark difference between the activation energies for interfacial charge transfer: similar to 0.6 eV for Na/NASICON and similar to 0.3 eV for Na/beta ''-alumina. This result suggests the formation of a Na+-conductive interphase layer in pressing Na metal on the NASICON surface at room temperature.