Unlocking Failure Mechanisms and Improvement of Practical Li-S Pouch Cells through in Operando Pressure Study
Authors: Li Bin 
Published: July 2022

Lithium-sulfur (Li-S) batteries have been considered a promising candidate for next-generation high-energy density storage technology due to their low cost and high theoretical capacity. However, since 2017, more and more attentions have been paid to the gap of lab cell characterization (coin cell) and prototype cell (pouch cell) development since misinterpretations and false expectations are frequently reported: material property impacts are often over-interpreted, while parameters with indirect impact (e.g., electrode and separator porosity, tortuosity, and pressure on cell stack) are neglected. In order to accelerate Li-S battery commercialization, the rapid transfer of material-related concept discovered in coin cells to a pouch cell level is essential, as some problems ignored or deemed minimal at the smaller level could have a greater effect on the performance of the larger pouch cell. The issues existing in practical pouch cell should be discovered, which would shed light on further battery materials development, or inspire the novel approaches to identify cell failure and improve cell performances at the pouch cell level.
Considering the gap between practical pouch cells and coin cells, in addition to the noticeable difference in electrode size (e.g., the electrode size of practical pouch cell is usually >100 times of that of coin cell), a much higher stack pressure (> 1Mpa) is usually applied inside the coin cell. It was taken for granted that stack pressure was playing a critical role, leading to inconsistent performance between pouch cells and coin cells. Furthermore, with increasing size of the cells (especially for multi-layer pouch cells), the electrolyte wettability needs to be taken seriously. Otherwise, the sulfur utilization would be largely reduced as ionic conduction pathways was significantly affected.
Herein, we rationally designed two kinds of cathode: Non-calendared sulfur electrode (NCSE) and Calendared sulfur electrode (CSE). The former’s porosity (ε) and tortuosity (τ) were proven to change with stack pressures while the latter’s do not change by simulations based on micro-XCT results with in-situ pressure applied. These two sulfur cathodes provide preconditions to distinguish the effects of stack pressure and porosity/tortuosity on Li-S pouch cell performances. For the first time, through in-situ monitoring of pressure applied onto Li-S pouch cells, the failure mechanisms of Li-S pouch cells were deeply understood, and the approaches to improve Li-S pouch cell performances were identified. It is found that highly porous structures of cathodes/separators and slow electrolyte diffusion through cathodes/separators can both lead to poor initial wetting. Additionally, Li-metal anode dominates the thickness variation of the whole pouch cell, which is verified by in situ measured pressure variation. Consequently, a real-time approach that combined normalized pressure with dP/dV analysis is proposed and validated to diagnose the morphology evolution of Li-metal anode. Moreover, applied pressure and porosity/tortuosity ratio of the cathode are both identified as independent factors that influence anode performance. In addition to stabilizing anodes, high pressure is proven to improve the cathode connectivity and avoid cathode cracking over cycling, which improves the possibility of developing cathodes with high sulfur mass loading. This work provides insights into Li-S pouch cell design (e.g., cathode and separator) and highlights pathways to improve cell capacity and cycling performance with applied and monitored pressure.

Figure 1