Systems Materials Engineering for High-Rate Bulk Solid-State Conversion in Metal-Sulfur Batteries

Project Information

SOLIDCON

Grant agreement ID: 101078271

DOI

10.3030/101078271

EC signature date 10 February 2023

Start date 1 September 2023

End date 31 August 2028

Funded under

• European Research Council (ERC)

Total cost

€ 2 374 448,00

EU contribution

€ 2 374 448,00

Coordinated by

PARIS-LODRON-UNIVERSITAT SALZBURG
Austria

Periodic Reporting for period 1 - SOLIDCON (Systems Materials Engineering for High-Rate Bulk Solid-State Conversion in Metal-Sulfur Batteries)

Reporting period: 2023-09-01 to 2026-02-28

Summary of the context and overall objectives of the project

Batteries will be key in our efforts to reduce CO2 emissions but require major progress in sustainability, cost, and energy density. Liquid-electrolyte metal-sulfur batteries, in particular lithium-sulfur (Li–S) systems, would be game-changers in many respects: a theoretical capacity amongst the highest of all batteries paired with the low cost and sustainability of sulfur. However, intrinsic obstacles are imposed by the electronically and ionically insulating nature of sulfur. Converting sulfur during discharge/charge is fundamentally different from mixed-conducting storage materials. While Li-ion battery materials transform in the solid-state, sulfur converts to metal sulfides in a solid-liquid-solid process. This causes poor cycle life and insufficient energy densities.
In the SOLIDCON project, we aim to redirect sulfur conversion towards a high-rate, bulk solid-state mechanism, combining high capacity with improved cycle life, reduced parasitic dissolution, and a higher packing density of active material. To achieve this, we develop advanced characterization methods, such as cryo-transmission electron microscopy (cryo-TEM) and operando scattering techniques, supported by machine-learning-based stochastic modelling to monitor and understand conversion processes at nanometer length scales, ranging from approximately 1 to 1000 nm. We systematically study three mechanistic regimes of electrochemical sulfur/sulfide conversion: (i) conventional solid-liquid-solid conversion in solvating electrolytes; (ii) quasi solid state conversion in sparingly solvating systems; and (iii) true solid state conversion in carbon confinement and with carbonate based electrolytes.
Based on these insights, we design composite cathodes, incorporating structured conductive hosts (nanoporous carbons) and ion/electron transport networks, targeting high rate, high density solid state sulfur conversion. The expected impact is the development of battery materials and methods that push metal-sulfur systems closer to practical, high energy, sustainable storage solutions, contributing to climate mitigation and enabling next generation electrochemical energy storage.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During the first project period, we established the methodological framework for SOLIDCON and achieved several milestones in each of the three electrochemical S/Li2S conversion regimes.
(i) Solid-liquid-solid conversion (standard solvating electrolyte)
We built a multimodal characterization workflow combining cryo TEM, EELS/EFTEM and operando small angle neutron scattering (SANS). This allowed high-resolution imaging of electrode morphology and chemistry together with time-resolved quantification of nanostructure evolution from scattering. We proposed a two-step mechanism for the final discharge process: precipitation of solid Li2S2 from solution followed by solid‐state reduction to Li2S. The finding that Li2S and a second solid polysulfide phase coexist during discharge, provides a deeper mechanistic understanding of the conversion pathway and underscores the importance of metastable intermediates.
(ii) Quasi solid state conversion (sparingly solvating electrolyte)
In systems with restricted polysulfide solubility, our operando SAXS/WAXS and ex situ cryo TEM/EELS showed that sulfur and Li2S coexist over wide charge/discharge states. Long-range polysulfide transport was strongly suppressed, and conversion was found to occur via localized intermediates near the carbon-electrolyte interface, rather than by full dissolution and migration. This insight shifts the mechanistic paradigm for such electrolyte systems.
(iii) Solid-state conversion (carbonate-based electrolyte, confined microporous hosts)
Using microporous carbons with pore sizes < 2 nm, operando SANS confirmed the formation of a stable cathode-electrolyte-interphase (CEI) penetrating the pore network during first discharge; operando XRD showed sulfur and Li2S remained amorphous, indicating non-crystalline conversion. Electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration (GITT) revealed that the nanopore architecture governs rate performance: carbons with smaller pores achieved higher capacities and improved rate capability, with charging identified as the kinetic bottleneck.
Methodological innovations:
We developed / implemented three major methodological advances: (1) a machine learning enhanced stochastic modelling workflow (using a Plurigaussian Random Field model and a convolutional neural network) to fit operando scattering data quickly and quantitatively; (2) a cryo TEM sample preparation protocol for realistic Li-S cathodes without cryo FIB, including a quench cooling method to avoid sulfur sublimation; (3) a Bayesian optimization/active learning platform for process optimization in battery systems (initially applied to zinc ion electrodeposition, now being extended to Li-S and Na ion systems).
These achievements have established a transferable platform for analyzing conversion-type battery chemistries, and they lay the groundwork for future cathode engineering and materials design in SOLIDCON.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The results achieved so far have advanced the state-of-the art in different ways.
Our integrated methodology goes beyond conventional characterization: whereas many operando scattering studies rely on simplified models or qualitative interpretation, our approach combines high-resolution cryo TEM imaging, operando scattering, machine-learning supported stochastic modelling to yield quantitative, real-time reconstructions of nanostructure evolution. This enables rigorous insight into phase evolution, morphology changes, and transport phenomena under operating conditions, something that has largely remained inaccessible in conversion-type batteries until now.
From a materials and mechanism standpoint, we have uncovered new mechanistic paradigms: the recognition of a second solid polysulfide intermediate (Li2Sx) in the classical conversion regime; the confined quasi-solid-state conversion in sparingly solvating electrolytes; and the pore structure-governed solid-state conversion in confined microporous hosts. These findings challenge existing models and open new design rules for high-rate, high-density Li-S batteries.
In the longer term, the insights gained through SOLIDCON are expected to inform the design of sulfur-based electrodes that enable solid-state conversion with reduced shuttle effects, improved stability, and better sulfur utilization. This could support the development of more practical metal-sulfur batteries for both mobile and stationary applications.
Overall, SOLIDCON has provided a strong methodological and mechanistic foundation for understanding sulfur conversion processes. The approaches developed in the project are broadly transferable and may be applied to other battery chemistries or reactive porous systems where structural changes at the nanoscale are central to performance.

Combined use of operando SANS and cryo-TEM to quantify S/Li2S conversion