Understanding and Engineering Interfacial Adhesion in Solid-State Batteries with Metallic Anodes

Funding Information: The authors acknowledge funding for this work from the Engineering and Physical Sciences Research Council (EP/R002010/1, EP/R024006/1 and EP/P003532/1), Shell Global Solutions International B.V., the Spanish government (TED2021‐129254B‐C22) and Horizon Europe HORIZON‐CL5‐2021‐D2‐01 “SEATBELT” 101069726.Peer reviewedPublisher PD

The Na|NaSICON interface in sodium solid-state batteries

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.Open Acces

Operando characterization and theoretical modelling of metal|electrolyte interphase growth kinetics in solid-state-batteries - 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. This study focuses on the growth kinetics of the interphase forming between solid electrolytes and metallic negative electrodes in solid-state batteries. More specifically, we demonstrate that the rate of interphase formation and metal plating during charge can be accurately described by adapting the theory of coupled ion-electron transfer (CIET). The model is validated by fitting experimental data presented in the first part of this study. The data was collected operando as a Na metal layer was plated on top of a NaSICON solid electrolyte (Na3.4Zr2Si2.4P0.6O12 or NZSP) inside a XPS chamber. This study highlights the depth of information which can be extracted from this single operando experiment, and is widely applicable to other solid-state electrolyte systems

Operando characterization and theoretical modelling of metal|electrolyte interphase growth kinetics in solid-state-batteries - Part I: experiments

To harness all the benefits of solid-state battery (SSB) architectures in terms of energy density, their negative electrode should be an alkali metal. However, the high chemical potential of alkali metals makes them prone to reduce most solid electrolytes (SE), resulting in a decomposition layer called an interphase at the metal|SE interface. Quantitative information about the interphase chemical composition and rate of formation are challenging to obtain because the reaction occurs at a buried interface. In this study, a thin layer of Na metal (Na0) is plated on the surface of a SE of the NaSICON family (Na3.4Zr2Si2.4P0.6O12 or NZSP) inside a commercial XPS system whilst continuously analysing the composition of the interphase operando. We identify the existence of an interphase at the Na0|NZSP interface, and more importantly, we demonstrate for the first time that this protocol can be used to study the kinetics of interphase formation. A second important outcome of this article is that the surface chemistry of NZSP samples can be tuned to improve their stability against Na0. It is demonstrated by XPS and time-resolved electrochemical impedance spectroscopy (EIS) that a native Na3PO4 layer present on the surface of as-sintered NZSP samples protects their surface against decomposition