Probing the Dynamic Na Metal/Nasicon Interface in Sodium-Ion Batteries
Authors: Sukumaran Sivakkumaran , Skinner Stephen J , Lima Decio , Dawson Richard 
Published: February 2024

Interest in solid-state sodium-ion batteries (SSIBs) have resurged remarkably as a means of diversification in energy storage devices to address climate issues and further propelled by the movement of ‘Beyond-Li’. State of the art SSIBs not only offer energy security and sustainability but also comparable power and energy densities to their counter-parts. Yet to fully capitalise on the technology, the fundamental processes taking place between the anode and interface or ‘point of contact’ needs to be understood. Previous iterations of SSIBs have employed various solid-electrolyte candidates – most notable being beta-alumina – however with the recent advancements in synthesis and dopants more prominent solid-electrolytes have become interesting, one in particular being NaSICON (Sodium Super Ionic Conductor). NaSICON has several advantages, one of which is the reduction in operating temperatures of SSIBs due to its inherent fast sodium-ion transport1. This would broaden the potential applications of SSIBs, making the technology more accessible to a wider range of markets.
Current perspectives on NaSICON solid-electrolytes have focused on optimising conductivity through means of additive dopants2 and synthesis procedures3. Yet at present, the understanding of the fundamentals of interfacial inhomogeneities that affect the performance of these materials in SSIBs is limited. With consideration of the direct implications on performance that surface chemistry has, understanding of this is paramount.
Evaluation of the chemical evolution of the Na metal/NaSICON interface has been achieved through assessing the changes of the chemical landscape that take place at the surface and near-surface, through surface characterisation techniques that probe the area of interest at varying depths – X-Ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). This has been conducted on samples that have been electrochemically cycled and comparison made to samples in pristine condition. The compositions used to conduct this investigation are Na3Zr2Si2PO12 and Na3.4Zr2Si2.4P0.6O12 – the latter of which has been shown to have one of the highest room temperature sodium-ion conductivity4. Through this, the electronic structure, and hence chemical state, can be deduced alongside a 3D chemical map that will correlate surface chemical changes with electrochemical performance. Thus, a preliminary means of understanding the fundamentals of chemical evolution can be established.

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