Lithium-sulphur batteries with lean electrolyte: problem areas clarified

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).