Yuriy Mikhaylik

This work reports a quantitative analysis of the shuttle phenomenon in Li/S rechargeable batteries. The work encompasses theoretical models of the charge process, charge and discharge capacity, overcharge protection, thermal effects, self-discharge, and a comparison of simulated and experimental data. The work focused on the features of polysulfide chemistry and polysulfide interaction with the Li anode, a quantitative description of these phenomena, and their application to the development of a high-energy rechargeable battery. The objective is to present experimental evidence that self-discharge, charge-discharge efficiency, charge profile, and overcharge protection are all facets of the same phenomenon. © 2004 The Electrochemical Society. All rights reserved.

A mathematical model is presented for a complete lithium–sulfur cell. The model includes various electrochemical and chemical (precipitation) reactions, multicomponent transport phenomena in the electrolyte, and the charge transfer within and between solid and liquid phases. A change in the porosity of the porous cathode and separator due to precipitation reactions is also included in the model. The model is used to explain the physical reasons for the two-stage discharge profiles that are typically obtained for lithium–sulfur cells.

The primary mechanisms limiting lithium sulfur (Li-S) cell cycle life and thermal stability are discussed. Two major cycle life limiting mechanisms are identified: development of rough surface morphology on the metallic lithium anode with cycling; and depletion of lithium and electrolyte components through chemical reaction. The approach taken here to mitigate these problems, by employing physical protection, including multi-functional membrane assemblies and non-isotropic pressure is presented. Sulfur utilization of 92%, at C/5 discharge rates, increased cycle life and elimination of thermal runaway in 300 mAh Li-S cells was achieved.

This paper relates to development of a new generation of Li/S rechargeable batteries with energy density exceeding 180 Wh/kg and rate capability comparable with aqueous systems. The cells demonstrate 5 C discharge rate at −40°C. Retained energy and capacity at −40°C exceeded 60 and 80%, respectively. At room temperature cells generate specific power up to 750 W/kg. Cells are inherently protected from overcharge at low and room temperatures by internal chemistry and can sustain 30 times overcharge. It was shown that at −40°C sulfur cathode discharge includes at least five steps, in contrast to the known two steps at room temperature. Thermal effects related to different charge-discharge steps were analyzed. Cell thermal model including Ohm and Arrhenius parts of polarization allowed simulation Ragone plots and capacity over a wide range of currents and temperatures. © 2003 The Electrochemical Society. All rights reserved.

The influence of the electrolyte solvents on the cell voltage in lithium-sulfur (Li-S) batteries is investigated. It is found that changing the solvent does not only alter the reaction mechanisms taking place during charge and discharge, but also exerts a pronounced influence on the cell voltage. The changes monitored upon switching from standard ether-based electrolytes to more polar solvents are quite substantial. An increase in the open circuit voltage of up to ∼400 mV could be observed. Both experimental evidence and theoretical calculations are presented in order to elucidate and quantify these effects. It is demonstrated that both the observed trends and the order of magnitude of the measured values can be explained by the free solvation energies of the respective ionic species in the electrolyte systems. Among them, the lithium cation contributes most to the phenomena described. Given that the final reaction products are solid and precipitate from the solution, these effects cannot be exploited to increase the overall energy densities of standard Li-S batteries. However, they are still important both with respect to the fundamental understanding of the electrochemical processes involved as well as practical applications such as liquid, polysulfide-based redox flow batteries.