The Na|NaSICON interface in sodium solid-state batteries
Creators: name: Quérel, Edouard L. P.
Publisher: name: Imperial College London
Year: 2022

description: In the last decade, solid-state batteries (SSB) have attracted scientific interest by virtue of their predicted superiority in terms of energy density and safety in comparison to conventional Li-ion batteries. The defining characteristic of SSBs is that their electrolyte is not a liquid but a solid. Solid electrolytes could, in theory, unlock many of the limitations imposed by liquid electrolytes regarding the choice of electrode materials. In particular, substantial energy density gains could be obtained from employing high capacity alkali metal negative electrodes instead of carbonaceous ones. Among the broad family of solid electrolytes, inorganic oxide solid electrolytes (ISEs) often display a good balance between a high room temperature ionic conductivity and a wide electrochemical stability window. This thesis brings the challenges of combining alkali metal electrodes with ISEs in SSBs under special focus. For this, a model system where a Na+ conducting ISE of the NaSICON family (Na3.4Zr2Si2.4P0.6O12, NZSP) interfaces with Na metal electrodes will be studied. This thesis more specifically explores how the reactions occurring at metal|ISE interfaces can affect the power performance and longevity of SSBs. After briefly introducing the general context explaining why new generations of batteries with higher energy densities and safety are awaited, the benefits of Na-SSBs will be outlined in Chapter 1. Chapter 2 introduces the theoretical background on ionic mobility in crystalline ISEs. A review of the pre-existing literature regarding processes affecting metal|ISE interfaces is provided in Chapter 3, at the end of which the scope and originality of this study are clarified. These first three chapters, serving as a prologue, are followed by a description of the experimental methods employed in this study (Chapter 4). The first chapter of results of this thesis (Chapter 5) focuses on the synthesis of NZSP solid electrolytes and the characterization of their structure, microstructure, and electrochemical performances. In Chapter 6, relations between the surface chemical composition of NZSP pellets and the Na|NZSP interface resistance of symmetrical cells are investigated. An important discovery from this chapter, exposed thanks to a combination of surface characterization and first principle calculations, is that a thin sodium phosphate layer terminates the surface of thermally treated NZSP samples and improves their electrochemical performances. Chapters 7 and 8 investigate the stability of Na|NZSP interfaces: Chapter 7 reveals that impurities contained in Na metal electrodes can poison the Na|NZSP interface and could play a pivotal (and often neglected) role in the aging dynamics of cells; Chapter 8 probes the electrochemical stability (in that context, the ability to withstand reduction) of NZSP in contact with contaminant free Na metal via an operando XPS experiment. Finally, the last two chapters of this thesis look at challenges affecting the Na|NZSP interface under cycling conditions: Chapter 9 looks at the formation of interfacial pores during stripping and the impact this has on the critical current density that symmetrical cells can withstand; Chapter 10 introduces two interface design strategies to increase the power density of SSBs.