An overview about all-solid-state batteries research activities and characterization capabilities at PSI

All-solid-state batteries (ASSBs) are a rising alternative for boosting the volumetric energy density towards 500 Wh kg^-1 and are considered safer than conventional Li-ion batteries. However, Li-ion transport across the solid electrolyte (SE)/active materials (AMs) interfaces is limited by the parasitic (electro-)chemical side reactions and mechanical instability responsible for the impedance rise and lower electrochemical performance. The current research efforts in the Battery Materials and Diagnostics group (BMD) aim to shed light on the fundamental understanding of the interface chemo-mechanical processes of sulfide-based SEs and to mitigate interface resistivity alongside improved battery life cycle performance. BMD group is committed to address those challenges by the development of advanced in-situ and operando surface and bulk characterization methods using laboratory and synchrotron facility techniques at the Swiss Light Source (SLS). Therefore, we designed custom-made (i) electrochemical cells for accurate and reliable electrochemistry cycling of pellet-type ASSB materials, (ii) operando electrochemical cells for the various analytical methods and adapt/develop the synchrotron beamline end-stations to the requirements for ASSB studies. Furthermore, the BMD group dedicates large efforts to the interface engineering, with the purpose of mitigating the interface reactivities and improving cycling stability. 

At first, we highlight, the development of different generations of custom-made ASSB cells (Figure a) which is the first crucial step to accurately evaluate and determine the electrochemical properties and cycling performances of ASSBs. Currently, we have the generation 2 (gen.2) cell, capable of functioning at very low stabilized pressures (80 MPa – 2 MPa). Additionally, with the improved O-ring compression system exceptional tightness of 10^-8 mbar·l s^-1 can be achieved. The gen.2 cell has been used successfully in metallic lithium applications to enable excellent electrochemical cycling outside the glovebox. 

At second we highlight, the creation and development of a unique complementary in-situ/operando non-destructive surface and bulk characterization platform (Figure b) combining in-house X-ray photoelectron spectroscopy (XPS) and X-ray synchrotron techniques located at the SLS, employing X-ray absorption spectroscopy (XAS), X-ray photoemission electron microscopy (XPEEM) and X-ray tomographic microscopy (XTM). This state-of-the-art platform allows to study the surface, near surface and interface to understand the (electro-) chemical reactivity, charge carrier mobility as well as chemo-mechanical limitations between the SSE and different active materials (AMs). BMD has addressed the necessity of developing such advanced characterization at multiple scales of depth probing for both post-mortem and in-situ/operando studies. As an example, conventional laboratory XPS with high surface-sensitivity (~10 nm) is a powerful technique to study the chemical and electronic properties of materials. It is known to resolve the individual redox byproducts of the SEs and operando studies could correlate accurately the redox processes in ASSBs with their voltage dependency. In addition, operando XAS with its higher sensitivity towards transition metals (TMs) allows for a depth-profiling by following simultaneously signals evolving in real-time on the surface (~10 nm) and near-surface (~100s nm) of a composite working electrode, using total electron yield (TEY) and total fluorescence yield (TFY) detection modes, respectively. Furthermore, operando XPEEM is employed as a highly surface sensitive analytical technique capable of providing a unique combination of the nanoscale lateral resolution (~50 nm) and the spectroscopic capability of XAS, confined within a depth analysis range of 3 – 4 nm. Besides the utilization of synchrotron light for chemical surface and interface-oriented studies, the internal mechanical evolution like volume change and crack formation of the ASSBs is monitored by operando XTM allowing for a time-resolved 3D imaging of the ASSB interior during operation with sub-µm resolution. 

At third, we highlight, the surface and interface chemical engineering towards higher (electro-) chemical stability. BMD group is equipped with an ultra-high vacuum (UHV) cluster with a base pressure ~ 10^-9 mbar connected to the XPS spectrometer, employing Ar sputtering for surface treatment and depth profile analysis, RF sputtering for thin film deposition and electron gun evaporator for various metals or oxides thin film depositions. Such a unique setup offers a great variety of surface modifications (e.g., Li metal, current collector foils or pelletized SEs), with a direct in-situ XPS chemical analysis under vacuum. For thin coatings of powders with micron- to nanosized particles, wet-chemical methods (e.g., sol-gel) or gas-solid reactions with reactive gasses are more eligible.