Lithium-sulfur (Li-S) batteries with organic electrolyte have attracted much interest recently because of their relatively high energy density. Being environmentally friendly and avoiding some of the disadvantages of other competing types of batteries (such as metal-air batteries, which are open systems and subject to external contamination), Li-S batteries have the potential to replace conventional Li-ion batteries in many applications. Although Li-S batteries are currently being investigated experimentally extensively by the research community, there are a limited number of theoretical studies predicting the limitations of these systems and, unfortunately, the existing studies are often contradictory.

In this presentation we analyze the limitations of Li-S batteries using a comprehensive approach that involves modeling, simulation, as well as experimental measurements. First, in agreement with some recent computational and experimental results published in the literature [1], we show that the capacity of Li-S batteries is limited by surface passivation: Li₂S and possibly higher order lithium polysulfides (LiPS) deposit on the surface of the cathode and passivate the active carbon-electrolyte interface. Because of surface passivation, Li ions are prohibited from reaching the electron conductive material in the cathode (i.e. the electron conductive material), limiting the capacity of the battery. We show that tunneling effects across the solid Li₂S layer can be neglected and, because of the high resistivity of Li₂S, the cathode material becomes unreachable as soon as the solid Li₂S deposits on the surface of the electron conductive material. The solid reaction product does not deposit uniformly on the surface of the electron conductive material but it appears as nuclei that grow non-uniformly during discharge, in agreement with the classical nucleation theory. We also show that, because of the relatively high diffusivities, long range diffusion processes (from separator to cathode) can also be neglected for most organic electrolytes reported in the literature.

Finally, a differential model that describes the nucleation and precipitation of Li₂S and LiPS in the cathode is presented and the discharge curves are compared with experimental data. Unlike the existing works in the literature, which keep track of the statistical distribution and growth of the nuclei at each location in the cathode, our model is expressed entirely in the form of a differential system of equations that can be solved using finite-difference or finite-element methods. In this way, the numerical implementation of the model is much simplified compared with the existing models in the literature and can be easily implemented in standard finite element simulators.

The theoretical model presented in this work is based on a differential implementation of the Kolmogorov [2] and Avrami [3] precipitation models and takes into consideration discrete nuclei formation and size evolution on the surface of the electron conductive material. The number of nuclei and growth rate is expressed locally in the form of two ordinary differential equations that are solved self-consistently with the transport equations for sulfur and Li ions. Notice that in reference 1 the number and size of nuclei is described in integral form at each node of the finite-element (or finite-difference) discretization. A detailed analysis of the limitations of Li-S batteries will be presented at the conference. In addition, a few possible solutions to overcome these limitations will also be proposed.

References:

[1] Y. X. Ren, T. S. Zhao, M. Liu, P. Tan and Y. K. Zeng, Journal of Power Sources, 336, 115 (2016).

[2] A. N. Kolmogorov, Izv. Akad. Nauk SSSR Ser. Mat., 3, 355 (1937).

[3] M. Avrami, J Phys Chem A, 7, 1103 (1939).