Nanoscale phase evolution in lithium-sulfur batteries

Project Information

NanoEvolution

Grant agreement ID: 894042

DOI

10.3030/894042

Project closed

EC signature date 20 April 2020

Start date 1 July 2020

End date 30 June 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 191 149,44

EU contribution

€ 191 149,44

Coordinated by

EIDGENOESSISCHE TECHNISCHE HOCHSCHULE ZUERICH
Switzerland

Periodic Reporting for period 1 - NanoEvolution (Nanoscale phase evolution in lithium-sulfur batteries)

Reporting period: 2020-07-01 to 2022-06-30

Summary of the context and overall objectives of the project

Reducing global greenhouse gas emissions is amongst the most pressing societal and technological challenges of our times. Batteries will be key in our efforts to reduce CO2 emissions but require major progress in sustainability, cost, and energy density. Practical Li-sulfur (Li-S) batteries 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. Years of research left a central question in Li-S batteries open: what is the very fundamental mechanism to reversibly convert sulfur (S) into lithium sulfide (Li2S) and back? Particularly, how is insulating, insoluble Li2S electrodeposited and stripped?
As the battery discharges, S is reduced into soluble polysulfides (Li2Sn), which eventually need to be reduced to solid, insulating, and insoluble Li2S. Only converting large amounts of S into Li2S allows to fully exploit the outstanding capacity of Li-S electrochemistry. Notably, while reducing S8 via various polysulfides to Li2S2 affords half of the capacity, converting Li2S2 to Li2S affords the other half, highlighting the importance of this step alone.
The MSC project NanoEvolution aimed at understanding the fundamentals of the solid-liquid-solid S/Li2S conversion and the relevant structure-property relationships by developing structure-sensitive operando scattering methods, combined with stochastic modelling and electron microscopy.

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

The project established operando small- and wide-angle X-ray scattering (SAXS / WAXS) and operando small-angle neutron scattering (SANS) as suitable tools to track the growth and dissolution of solid deposits from atomic to sub-micron scales during charge and discharge of Li-S battery cathodes. Stochastic modelling based on the SANS data allowed quantification of the chemical phase evolution in real space. Transmission electron microscopy (TEM) provided complimentary local structural information to verify the model input for analyzing the operando scattering data via stochastic modeling. The concept of plurigaussian random fields allowed fitting the SAXS and SANS data and generating a statistically representative 3D structure of the solid multiphase discharge products.
Using these methods provided direct evidence for the existence and persistence of a second solid discharge product in Li-S batteries, next to the known nanocrystalline Li2S. By combining operando X-ray and neutron scattering and stochastic modelling (plurigaussian random fields), we showed that crystalline Li2S aggregates are embedded in a matrix of persisting smaller Li2Sx particles. Raman spectroscopy indicates that this second phase is likely Li2S2. Charging reverses the process. On charging, the solid Li2S2 particles are found to initially grow while Li2S already disappears.
These results were disseminated by the following publication and presentations at conferences (among others):

Scientific publications:
C. Prehal, J.M. von Mentlen, S. Drvarič Talian, A. Vizintin, R. Dominko, H. Amenitsch, L. Porcar, S.A. Freunberger, V. Wood, On the nanoscale structural evolution of solid discharge products in Lithium-Sulfur batteries using neutron, x-ray and electron techniques, currently under review, preprint: https://doi.org/10.21203/rs.3.rs-818607/v3
C. Prehal, S. Mondal, L. Lovicar, S. A. Freunberger, Exclusive solution discharge in Li-O2 batteries? ACS Energy Lett. 2022, 7, XXX, 3112–3119

Oral presentations at conferences or seminar talks:
C. Prehal, J.M. von Mentlen, S. Drvarič Talian, A. Vizintin, R. Dominko, H. Amenitsch, Lionel Porcar, S.A. Freunberger, V. Wood, Mechanism of Li2S formation and dissolution in Lithium-Sulfur batteries, Swiss Battery Days 2022, EMPA, Dübingen, Switzerland. 08/2022
C. Prehal, S. A. Freunberger, V. Wood, In situ exploration of in-pore redox processes for beyond intercalation-type energy storage, CIMTEC conference 2022, Perugia Italy, 06/2022
C. Prehal, H. Amenitsch, S. A. Freunberger, V. Wood, Operando X-ray scattering with stochastic modelling to quantify the nanoscale phase evolution in post-Li-ion batteries, MRS fall meeting 2021, online, Boston USA 12/2021
C. Prehal, H. Amenitsch, V. Wood, S. A. Freunberger, Nanoscale phase evolution in conversion-type lithium-sulfur and lithium-air battery cathodes, EUROMAT 2021, online, Graz Austria 09/2021
C. Prehal, H. Amenitsch, V. Wood, S. A. Freunberger, Mechanisms of reversible active material electrodeposition in Li-O2 batteries and beyond, ISE annual meeting 2021, online, South Korea 09/2021

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)

Knowing that the solid discharge products are a Li2S/Li2S2 composite, shifts paradigms of how to influence the reaction and discharge capacity.
First, some previous studies suggested electron transport through a passivating surface film to limit capacity and rate, demanding high surface carbons. Contrary to that, the project’s results show mass transport to limit discharge capacity and rate. Handles to influence them are, therefore, species solvation, diffusivities in the electrolyte and the Li2S2/Li2S solids, and applied current density. Design strategies to improve device performance must therefore change.
Second, the resulting mechanism explains why theoretical sulfur capacities have never been achieved. We cannot fully convert all S into Li2S; a certain amount of polysulfides remains as a second solid phase (Li2S2). Key to increase practical capacities in the future is to account for the found solid-state conversion.
More broadly, solid-state conversion requires Li (Li+ + e¬–) mobility in the solid Li2S2 deposit to convert it to the fullest into the Li2S. Surprisingly, despite their insulating nature, mobility appears sufficient for aggregates of a several 100 nm in size. This implies that solid-state S-to-Li2S conversion (SSC) is possible at practical rates if S/Li2S structures are properly engineered; a crucial message for all Li-S design strategies that avoid the polysulfide shuttling problem by utilizing SSC, but so far struggled to convert practical S amounts.
Next to electron microscopy and Raman spectroscopy, the essential tools for these insights were operando SAXS/WAXS, operando SANS with contrast matching and advanced data analysis using stochastic modelling. NanoEvolution showed that the combination of these techniques offers unique quantitative structural insights into the complex Li2S/Li2S2 composite structure. Their strength is to provide seamless operando insights on the sub-nm to sub-micron scale, which are hardly accessible to any other methods. The example of Li2S/Li2S2 conversion demonstrated the power of operando SAXS/WAXS and SANS to clarify mechanisms in complex energy materials more generally and that structural information on both molecular and nanoscopic length scales holds key to important mechanistic detail.

Results from operando small angle neutron scattering