Lithium-sulfur (Li-S) batteries display significantly higher practical energy density (~500Wh/kg) as compared to commercially available lithium ion based battery systems (~150Wh/kg)¹. This improved energy density arises from the fact that each mole of sulfur (S₈) reacts with sixteen moles of electron. In the long term, the lithium-sulfur based cells have the potential to be cheaper due to the usage of lower cost active materials. Hence, these light weight lithium-sulfur cells can significantly increase the range experienced by electric vehicles.

However, at present the lithium-sulfur cells display modest cycle life only under low power and low rates of operation¹. The staged, multi-step reaction during the discharge process causes performance limitation in Li-S batteries². In addition, shuttling of the long-chain polysulfides from the cathode to the anode and reaction with the lithium film not only reduces the effective capacity of the cell, but also forms a passivating layer on top of the lithium anode, which can significantly increase the anode overpotential¹. Deposition of insoluble lithium-sulfides on top of pristine solid sulfur can passivate the active material and result in capacity fade at higher rates of operation¹.

During the discharge of a lithium-sulfur battery, a major product is lithium-sulfide (Li₂S) which is insoluble in the electrolyte solvent and precipitates at cathode-electrolyte interface^{2, 3}. When a mole of S_8(s) completely coverts to Li₂S_(s), it occupies 80% more volume². This volume expansion is accommodated by the cathode system in two ways: a) By decreasing the pore space, and b) By increasing the total volume of the electrode which results in a higher value of porosity⁴. Computational models developed to capture the performance of lithium-sulfur cells do not take into account the effect of swelling of the entire electrode^{2, 3}. Also depending on the mechanical stability of the cathode microstructure, this precipitation induced volume expansion can give rise to significant stress and fracture of the cathode substrate. Subsequent detachment of the ruptured cathode microstructure form the conducting network can severely deteriorate the overall capacity of the lithium-sulfur cell¹. A poromechanics based computational study has been conducted to elucidate the effect of cathode material (in terms of elastic and fracture properties) and microstructure (in terms of pore shape and size) on the overall volume expansion and performance of lithium sulfur cells.

Preliminary studies indicate that both pore shape and size can significantly impact the overall volume expansion and mechanical integrity experienced by the cathode microstructure. From the effective stiffness point of view, small sized pores are preferred, because large pores experience higher volume expansion. However, extremely small pores contain very thin pore-walls, which can be prone to microcracking under a precipitation induced stress. A thorough analysis of the influence of cathode microstructure, including the pore morphology, size, and volume fraction, will be elucidated in this presentation.

References

1.         Wild, M.; O'Neill, L.; Zhang, T.; Purkayastha, R.; Minton, G.; Marinescu, M.; Offer, G. J. Lithium sulfur batteries, a mechanistic review. Energy and Environmental Science 2015.

2.         Kumaresan, K.; Mikhaylik, Y.; White, R. E. A mathematical model for a lithium-sulfur cell. J Electrochem Soc 2008, 155, A576-A582.

3.         Zhang, T.; Marinescu, M.; O'Neill, L.; Wild, M.; Offer, G. Modeling the voltage loss mechanisms in lithium-sulfur cells: the importance of electrolyte resistance and precipitation kinetics. Phys Chem Chem Phys 2015, 17, 22581-22586.

4.         Gomadam, P. M.; Weidner, J. W. Modeling volume changes in porous electrodes. J Electrochem Soc 2006, 153, A179-A186.