New insights into the kinetics of metal|electrolyte interphase growth in solid-state-batteries via an operando XPS analysis - part II: modelling

Understanding the interfacial dynamics of batteries is crucial to control degradation and increase electrochemical performance and cycling life. If the chemical potential of a negative electrode material lies outside of the stability window of an electrolyte (either solid or liquid), a decomposition layer (interphase) will form at the interface. To better understand and control degradation at interfaces in batteries, theoretical models describing the rate of formation of these interphases are required. Yet, experimental data which could support these models are challenging to obtain considering that the decomposition reaction is dynamic in nature and occurs at a deeply buried interface making it inaccessible to quantitative non-destructive techniques such as X-ray photoelectron spectroscopy (XPS) which has a limited depth of analysis. In the first article of this two-parts study, an experiment was designed to study the formation of an interphase at the interface between a Na metal negative electrode and a NaSICON solid electrolyte (Na3.4Zr2Si2.4P0.6O12 or NZSP) using a recent XPS protocol. Data was collected operando as a Na metal layer was plated on top of the NZSP electrolyte inside the XPS chamber. It was demonstrated that an interphase forms at the Na0|NZSP interface but that a native Na3PO4 layer present on thermally activated NZSP samples can minimize the extent of decomposition. In this second article, it is demonstrated that the rate of plating and interphase formation at the Na0|NZSP interface can be accurately described by adapting the theory of coupled ion-electron transfer (CIET). Models are fitted using experimental data from the first part of this study (in particular, the peak positions and peak areas as a function of Na0 plating time). This second part of the study therefore highlights the depth of information which can be extracted from this single operando experiment.