Operando characterization of degradation phenomena in all-solid-state batteries with a sulfide-based solid electrolyte

Abstract

The pursuit for a low carbon emission society is enabled through the widespread utilization of alternative energies such as wind and solar energy. However, most of the so-called green energy forms are characterized by their intermittent nature which places energy storage devices into the center of any green energy utilization strategies. The lithium-ion batteries (LiB) have proven themselves over the last three decades as robust, high energy density storage devices for mobile electronics applications and become one of the key enablers for the digitalization of our society. During the past ten years, the emergence of electric vehicles and grid-scale energy storage require ever-increasing size of the LiB with ever increasing energy density. The massive deployment of these batteries soon also reveals their inherent limitation in power density, cycle life and, in some instances, safety. As a radical improvement of the current LiB technology, the all-solid-state battery (SSB) promises higher energy and power density along with a compact cell design and removal of complex battery thermal management system. All the mentioned beneficial characteristics of an SSB are enabled by the superionic conducting solid electrolyte (SE) with minimal volatility. Particularly thiophosphate based solid electrolyte attracted much of the attention in both academic and industrial research and development due to their outstandingly ionic conductivities, their unity transference number, their compliant mechanical properties, and their processability. Despite the favorable properties of the thiophosphate-based SE, several fundamental issues still limit their practical use: (i) thiophosphate-based SEs exhibits inherently narrow electrochemical stability window due to the electrochemical reactivity of sulfur; (ii) the elements sulfur and phosphorous are both prone to parasitic reactivities in presence of transition metal oxides as typically used for LiB electrode active material (AM); (iii) the mechanical degradation of SSB due to volume changes of the AM is poorly understood. The first two issues have been empirically tackled through the engineering of an amorphous oxide coating between the AM and SE. However, in long-term cycling and/or at high current rates, these coatings tend to fail. The incremental improvement of those coatings through the trial-and-error approach is tedious and promises only little success. A fundamental understanding of the parasitic (electro) chemical reactivity at the AM/SE interface could provide a more systematic bottom-up approach in order to propose functional coatings. Concerning the mechanical degradation as mentioned in point (iii), due to limited experimental data, no reliable prediction could be made to which extent the mechanical compliance of the thiophosphate SE could tolerate volume expansion of the AM and how the crack formation and propagation could eventually limit ion transport in SSB. In the course of this thesis, we consider the real-time and destruction-free characterization techniques as essential methods for the unambiguous conclusion of the above mentioned scientific questions. For the investigation of the parasitic (electro)chemical reactivities at the AM/SE interface, we consider X-ray photoelectron spectroscopy (XPS) as the method of choice due to its surface sensitivity and the ability to resolve various oxidation states of most elements. As a further development of the state-of-the-art ex-situ XPS characterization, operando XPS provides real-time chemical information of the reactive interfaces and enables unambiguously the identification of the experimental electrochemical stability window of the SE as well as the SE degradation reaction byproduct. The development of operando XPS for SSB is based on the formulation of a fundamental physical model which describes the observed physical shift of the binding energy as a function of the applied cell voltage. The careful analysis of this physical shift provides a methodology for a direct and contactless measurement of the local surface (over)potential of the elements and reveals the electronic properties of the interfacial species on top of their chemical nature. This fundamental measurement principle is applied in order to thoroughly describe the SE’s interfaces with LiCoO2 and Li4Ti5O12. In the former, by identifying the electronic properties of interfacial species, AM-SE cross reaction products could be identified and led to strong experimental evidence of a space charge layer between SE and LiCoO2 due to fixed negative charges. In the latter, the insulator-metal transition as a consequence of the lithiation of Li4Ti5O12 is revealed by operando XPS and the importance of preserving the reactive surface and interface degradation products in the ultra-high-vacuum conditions is highlighted. For a deeper understanding of the mechanical degradation phenomena in SSB, operando X-ray tomographic microscopy (XTM) has been developed at the synchrotron facility Swiss Light Source. The three-dimensional, high-resolution imaging of an electrode composite containing Sn as an AM with large volume expansion reveals a variety of mechanical degradation phenomena. For the first time, we were able to describe the reversible electrode expansion and contraction in SSB as a consequence of the volume changes of the AM. This result is rationalized by the elasticity of the SE and contrdicts the common perception of a rigid cell stack for SSB. Additionally, we investigated the evolution of cracks in SE at high volume expansion of the AM and ascribed the increase in tortuosity factor to the anisotropic crack formation and propagation in SE. Thus, we were able to draw important conclusions on the altered ion transport properties due to mechanical degradation. In conclusion, the fundamental (electro)chemical and mechanical degradation phenomena provide important guidelines for prospective development of AM coatings and electrode engineering. The developed methodologies go beyond the application for SSBs and can be applied to other solid-state electrochemical systems.