(Invited) Solid State Sodium Satteries: From Solid Electrolytes to Functional Device
Authors: Fattakhova-Rohlfing Dina 
Published: December 2020

Li-ion batteries (LIBs) dominate the electrochemical energy storage technology because of their extraordinary performance, high energy density, and long cycle life. Still, the enormously growing demand of Li due to the expanding market raises concerns about the costs and the availability of lithium in the future, necessitating the development of beyond-lithium electrochemical energy storage concepts. Sodium is among the most investigated alternatives for lithium due to its abundance, widespread availability and much lower costs. The simple replacement of lithium with sodium in liquid electrolyte cells while keeping otherwise similar cell design results however in lower energy densities as compared to LIBs due to a higher molar mass, more positive redox potential and higher reactivity of sodium with liquid electrolytes as compared to lithium. In combination with solid electrolytes, however, much higher energy densities can be expected due to a possible use of metallic sodium as an anode instead of currently used hard carbons, opening new perspectives for the development of Na-based electrochemical energy storage systems.

Sodium is among the most investigated alternatives for lithium due to its abundance, widespread availability and much lower costs. The simple replacement of lithium with sodium in liquid electrolyte cells while keeping otherwise similar cell design results however in lower energy densities as compared to LIBs due to a higher molar mass, more positive redox potential and higher reactivity of sodium with liquid electrolytes as compared to lithium. In combination with solid electrolytes, however, much higher energy densities can be expected due to a possible use of metallic sodium as an anode instead of currently used hard carbons, opening new perspectives for the development of Na-based electrochemical energy storage systems.

Fundamental prerequisite for the development of high-performance solid state sodium batteries is a sufficiently high total conductivity of all its components at room temperature. The latter includes the ionic conductivity of the electrolyte (simultaneously acting as a separator in solid state batteries), the combined electronic/ionic conductivity of the cathode and the anode half-cells, as well as a high conductivity of key interfaces between the electrolyte and the electrode materials. The total conductivity critically relies on the bulk properties of individual materials, but also on their nanomorphology in the component assemblies as well as on the processing- and operation-dependent interface between the materials. The materials challenge therefore goes far beyond the bulk structure level, including the other fundamental aspects of materials chemistry (such as integration of materials into components, optimization of component microstructure, interface properties), electrochemistry (individual contributions to the total impedance, degradation mechanisms upon cell operation), and engineering (development of processing technology enabling fabrication of optimized components).

I will give an overview of the sodium solid state battery development that spans from the development of solid state sodium conductors to their processing to functional battery components and finally to the fabrication and testing of the complete cells. I will discuss bottlenecks in the battery performance and will show the concepts for the mitigation of critical steps.