Lithium-sulphur batteries with lean electrolyte: problem areas clarified
Snapshot of a cell layer during the charging cycle of the Li-S pouch cell using operando neutron tomography: regions that are well wetted appear green, while poorly wetted regions are shown in red.
© L Lu et al., Advanced Energy Materials 2025
Using a non-destructive method, a team at HZB investigated practical lithium-sulphur pouch cells with lean electrolyte for the first time. With operando neutron tomography, they could visualise in real-time how the liquid electrolyte distributes and wets the electrodes across multilayers during charging and discharging. These findings offer valuable insights into the cell failure mechanisms and are helpful to design compact Li-S batteries with a high energy density in formats relevant to industrial applications.
As one of the most promising next-generation battery technologies, lithium-sulphur (Li-S) batteries can achieve ultrahigh gravimetric energy densities (e.g. above 700 Wh/kg vs ~250 Wh/kg for the state-of-the-art Li-ion batteries), which makes them particularly attractive for aerospace, robots and long-range electric vehicle applications. Abundant and low-cost sulphur provides compelling alternative to the critical and geopolitically sensitive metals (e.g., Co, Ni) used in lithium-ion batteries.
Reducing weight is tricky
However, the practical energy density is often constrained by the high weight fraction of inactive materials such as the electrolyte. To increase the energy density of lithium-sulphur batteries at cell level, reducing the amount of electrolyte is necessary. However, the less electrolyte there is in the battery cell, the more challenging it becomes to fully wet the electrodes. Incomplete wetting disrupts the electrochemical processes and causes the battery to age faster or even fail. ‘Critical questions are how electrolyte wets electrodes, infiltrates electrode pores, and distributes in the Li-S pouch cells, and then how these properties affect cell performance. Because of the closed configuration of batteries, it is extremely challenging to observe the quality of electrolyte wetting non-destructively,’ says HZB chemist Prof. Dr. Yan Lu who led the study.
Neutron tomography: a deep look in real time
In order to observe the dynamic wetting of the pouch cells during charging and discharging in Li-S battery systems, the team of Yan Lu designed the multilayer pouch cells and operando experiments. The multilayer Li-S pouch cell batteries are manufactured at HZB's Pouch Cell Assembly Lab using lean electrolyte and in accordance with industrially relevant parameters. Dr. Ingo Manke and Dr. Nikolay Kardjilov from the HZB imaging group examined these samples using neutrons at the Institut Laue-Langevin in Grenoble, locating light elements such as lithium and hydrogen with the highest possible accuracy.
Behaviour of the electrolyte analysed
‘This enabled us to observe for the first time, how the liquid electrolyte behaves in real time and how the wetting changes locally in the different layers of a pouch cell over time. We gained some interesting insights from this,’ says Yan Lu.
During the rest phase of the battery at open circuit voltage, unwet areas accumulated in local regions, especially at the beginning of the resting phase. Cell rest improves the electrolyte wetting, however, a long rest phase has only a minimal effect on overall electrolyte wetting.
The discharge/charge processes significantly improve the homogeneity of the electrolyte and can therefore promote the electrochemical activation of sulphur, leading to enhanced capacity of batteries. The team, for the first time, observed unique “breath in ” and “breath out” wetting behaviours: these are periodic processes in the electrolyte wetting that correlate with the dissolution and precipitation of sulphur compounds. ‘The dynamic electrolyte wetting behaviour differs significantly from that of conventional Li-ion batteries due to the distinct chemistry of Li-S systems,’ says Dr. Liqiang Lu, a postdoctoral researcher in Yan Lu’s team and the first author of the publication.
‘This is an important contribution to our understanding of the mechanisms that lead to the rapid ageing and failure of such systems. These insights will help increase the energy density of Li-S batteries while maintaining their service life,’ says Yan Lu.
Note: This project is supported by the German Ministry of Education and Research (BMBF) within the research program Batterie 2020 (SkaLiS) and EU Horizon project (HealingBat).
- BESSY II: How intrinsic oxygen shortens the lifespan of solid-state batteriesAlthough solid-state batteries (SSBs) demonstrate high performance and are intrinsically safe, their capacity currently declines rapidly. A team from the TU Wien, Humboldt-University Berlin and HZB has now analysed a TiS₂|Li₃YCl₆ solid-state half-cell in operando at BESSY II using a special sample environment that allows for non-destructive investigation under real operating conditions. Data obtained by combination of soft and hard X-ray photoelectron spectroscopy (XPS and HAXPES) revealed a new degradation mechanism that had not previously been identified in solid-state batteries. They have gained some surprising insights, particularly regarding the harmful role played by intrinsic oxygen. This study provides valuable information for improving design and handling of such batteries.
- Spintronics at BESSY II: Real-time analysis of magnetic bilayer systemsSpintronic devices enable data processing with significantly lower energy consumption. They are based on the interaction between ferromagnetic and antiferromagnetic layers. Now, a team from Freie Universität Berlin, HZB and Uppsala University has succeeded in tracking, for each layer separately, how the magnetic order changes after a short laser pulse has excited the system. They were also able to identify the main cause of the loss of antiferromagnetic order in the oxide layer: the excitation is transported from the hot electrons in the ferromagnetic metal to the spins in the antiferromagnet.
- Electrocatalysts: New model for charge separation at the solid-liquid interfaceHydrogen is at the heart of the transition to carbon neutrality, as both an energy carrier and a reagent for green chemistry. However, large-scale production of hydrogen via electrolysis, as well as the production of many other chemical products, requires significantly cheaper and more efficient catalysts. A precise understanding of the electrochemical processes that take place at the interface between the solid catalyst and the liquid medium is highly useful for developing better electrocatalysts. In the journal Nature Communications, an European team has now presented a powerful model that determines charge separation at the interface, the formation of the electric double layer and local electric potential variations, and the resulting influence on the catalytic activity.