Solid-state sodium batteries (Na-SSBs) are emerging as a promising technology due to the abundance and low cost of sodium, offering an attractive alternative to lithium batteries with similar electrochemical properties. This study marks a breakthrough in solid-state sodium batteries by focusing on “anode-free” or “reservoir-free” Na-SSBs. Unlike traditional batteries, this approach eliminates the to include the metallic negative electrode during assembly. Instead, the cell is assembled with an "empty" negative electrode, dynamically formed during the initial charge cycle. This will enhance the safety and simplicity of the assembly process by removing the most reactive component of the cell, providing a high purity in-situ formed metallic anode with higher practical energy density(no excess Na added). The Na_3.4Zr₂Si_2.4P_0.6O₁₂ (NZPS) NaSICON has numerous advantages such as an increased safety, greater stability, and higher energy density. In addition, NZSP has a remarkable ionic conductivity (up to 5*10^-3 Scm^-1)^1,2 and a high resistance to moisture and air. It has recently been recognised to facilitate rapid charge transfer at the Na metal/NaSICON interface by stabilising a conductive solid-electrolyte interface (SEI) layer through thermally activated surface decoration.

Understanding and optimising the interface between NZSP and sodium is crucial, as it is indispensable to prevent the formation of an SEI layer that could negatively affect the cell performance. Furthermore, investigating sodium nucleation in anode-free batteries, where the anode is formed in situ, is essential to understand its interaction with the electrolyte-current collector interface. Several factors come into play, such as the chemical and morphological nature of the interface and the interaction of Na the system during platting and stripping.  

The choice of current collector is paramount as it interacts with the electrolyte surface and can play a pivotal role on the nucleation overpotential and stability of the interface upon cycling. By altering the morphology, size and composition of the current collector, we can investigate the growth of sodium and its interaction with both the electrolyte surface and the current collector. For example, metals including alloying (Sn) and non-alloying (Au) are examined for their respective effects on battery performance. Important factors include variations in nucleation energy to facilitate reversible cycling and stability. Understanding these aspects is crucial to optimize battery performance during the charging process.

Electrochemical experiments have been conducted to study the interface, employing both symmetric cells and anode-free cells with various electrolyte surfaces and current collectors. Additionally, modified surfaces have been analyzed using characterization techniques such as XPS and Raman spectroscopy.