Electrochemical Conversion at the Nanoscale in Post-Li-Ion Batteries
Post-Li-ion batteries such as Li-S rely on different processes than today's intercalation cells. They use conversion reactions and insertion into complex nanostructures, where new solid phases form and dissolve through multi-step pathways, coupled to mass and charge transport. Capacity, rate capability, and cycle life emerge from the interplay of these processes across many length scales. Our understanding is weakest at the nanoscale, where conversion products nucleate and grow, charge is transferred at interfaces, and confined geometries decide how much active material is actually accessible.
The reason is partly methodological. Atomic-scale diffraction and spectroscopy, and micrometer-scale tomography, are well established. The range from nanometers to hundreds of nanometers is hard to reach: operando TEM struggles with beam-, air-, and sulfur-sensitive materials, and cryo-EM gives only local snapshots. At the same time, the parameter space (electrolyte, electrode architecture, operating conditions) is too large for trial and error. Our approach rests on four directions:
Scattering data of multiphase electrodes can only be interpreted with structural models. We use stochastic models (Gaussian, Plurigaussian, and Boolean random fields) that turn scattering curves into real-space structures. Neural networks accelerate the fitting by orders of magnitude, so large parameter spaces become tractable.
Quantitative data analysis
We use fractional factorial designs and Bayesian optimization with Gaussian processes to find the most informative next experiment. This reaches optimal conditions with far fewer experiments than grid search and gives an interpretable map of the parameter space.
Data-driven experimental design
We combine these tools with electrochemical methods (GITT, EIS), XPS, gas adsorption, and Raman spectroscopy to build quantitative models of electrode processes. In Li-O₂ batteries, our work showed Li₂O₂ formation through solution-mediated disproportionation, challenging the surface-passivation model. In Li-S batteries, we found a biphasic Li₂S/Li₂S₂ discharge product and a disproportionation-driven pathway, which explains why the theoretical capacity is rarely reached. The goal is rational design principles for better batteries.
Fundamentals and design principles
We develop operando X-ray and neutron scattering (SAXS/WAXS, SANS) and combine it with cryo-electron microscopy. Scattering gives time-resolved, bulk-averaged structure from the atomic to the nanometer scale. Cryo-EM adds real-space structural and chemical information. Together they make nanoscale structural evolution measurable during battery operation.
Operando metrologies and advanced characterization
Previous research
Physical charge storage in nanoporous confinement. The essential contribution of Christian Prehal’s Ph.D. in the group of Oskar Paris at the Institute of Physics (Montanuniversität Leoben, Austria) was to establish in situ small- and wide-angle x-ray scattering (SAXS/WAXS) and x-ray transmission to elucidate processes central to capacitive energy storage in supercapacitors: ion exchange, structural rearrangement, and interactions in nanoporous electrodes. The complex multiphase system consisting of ions, water molecules, and carbon required a new way of data analysis. A real-space picture was obtained from stochastic modeling and atomistic Monte Carlo simulations and correlated to the experimental scattering data via numerical Fourier Transformation (see Figure on the right). The synergy of in situ experiments and modelling unlock information that neither would provide alone. This allowed quantifying ion confinement and ion desolvation during device operation for the first time. The work provides conclusive experimental evidence for the long-debated “anomalous capacitance” increase in sub-nanometer pores.
Beyond ion electrosorption, the developed method can uniquely quantify any sub-nanometer structural rearrangements in disordered nanoporous hosts; for example, during water, hydrogen, or CO2 adsorption. The work related to supercapacitors and nanoporous carbons resulted in several publications: e.g., Energy & Environmental Science 2015, Nature Energy 2017, Phys. Chem. Chem. Phys. 2017, Nature Communications 2018, Carbon 2019, ACS Applied Materials and Interfaces 2019
Electrochemical phase transformation on the nanoscale. As a postdoc in the ERC StG project of Stefan Freunberger (former TU Graz, now IST Austria), Christian Prehal expanded the developed scattering methods to post-lithium-ion batteries. In situ SAXS/WAXS was used to study the reversible structural evolution of Li2O2, the active material in lithium-air (Li-O2) batteries, and its relation to physical-chemical mechanisms (PNAS 2021, Energy & Environmental Science 2019, Nature Chemistry 2021). For data analysis, the approach developed for supercapacitors was taken further to explicitly model Li2O2 nucleation and growth in a realistic 3D carbon nanopore model. The outcomes of this work explained capacity limitations and generalized the physicochemical reaction model (ACS Energy Letters 2022). In a fundamental collaborative study on iodide-based aqueous energy storage (Nature Communications 2020), the foundations for non-flammable aqueous batteries were set; with a capacity per cathode mass not far from lithium-ion batteries and a rate capability on par with supercapacitors. The work shows that the electrochemical oxidation of iodide in nanoporous carbons forms persistent solid iodine deposits. Confinement slows dissolution into soluble polyiodides, responsible for otherwise significant self-discharge.
In a recent collaborative project (Nature Communications 2022), Christian Prehal and colleagues investigated the nanoscale structural evolution and physicochemical mechanisms in lithium-sulfur (Li-S) batteries using operando small angle scattering, stochastic modeling, and electron microscopy. Stochastic modelling allowed fitting the SANS data and quantifying the chemical phase evolution in real space. The results show that the deposit nanostructure in Li-S batteries consists of more than one solid phase: the known nanocrystalline Li2S and a second solid Li2S2 phase. The found mechanism suggests that, even in liquid Li-S batteries, half of the capacity is passed as a solid-state process. Hence, solid-state conversion can be fast if active materials are correctly structured at nanometer length scales. Christian Prehal aims to realize such high-rate solid-state S conversion in his recently funded ERC Starting grant project.
Research projects
2025 – 2028; M-ERA.net, transnational consortium
Operando Techniques for Conversion Reaction Engineering in High-Energy Lithium Sulfur Solid state batteries (OCULUS)
Project partners: Fraunhofer IWS (Dresden, Germany), TU Dresden (Germany), Sixonia Tech (Germany), National Institute of Chemistry (Slovenia), and University of Salzburg (Austria);
From 09/2023; ERC Starting Grant
Systems Materials Engineering for High-Rate Bulk Solid-State Conversion in Metal-Sulfur Batteries (SOLIDCON);
Host Institution: University of Salzburg (Austria); Principal Investigator (PI)
2022 – 2025; M-ERA.net, transnational consortium
Advanced Lithium-sulfur batteries with ultramicroporous carbons (ALISA)
Project partners: INM Saarbrücken (Germany), Heraeus (Germany), National Institute of Chemistry (Slovenia) and ETH Zürich (Switzerland);
2022 – 2023; ETH Zürich Career Seed Grant
Bulk solid-state sulfur to lithium sulfide conversion in lithium-sulfur batteries
Principal investigator (PI), ETHZ funding to support independence of young researchers
2020 – 2022; Horizon 2020, Marie Skłodowska-Curie actions, Individual Fellowship,
Nanoscale phase evolution in lithium sulfur batteries (NanoEvolution),
Host Institution: ETH Zürich (Switzerland); Principal investigator (PI)
