The Effect of Current Inhomogeneity on the Performance and Degradation of Li-S Batteries

Abstract

The effect of thermal gradients on the performance and cycle life of Li-S batteries is studied using bespoke single-layer Li-S cells, with isothermal boundary conditions maintained by Peltier elements. A temperature difference is shown to cause significant current imbalance between parallel connected single-layer cells, causing the hotter cell to provide more charge and discharge capacities during cycling. During charge, significant shuttle is induced in the hotter Li-S cell, causing accelerated degradation of it. A bespoke multi-tab cell in which the inner layers are electrically connected to different tabs versus the outer layers, is used to demonstrate that noticeable current inhomogeneity occurs during the operation of practical multilayer Li-S pouch cells, which is expected to affect their performance and degradation. The observed thermal and current inhomogeneity should have a direct consequence on battery pack and thermal management system design for real world Li-S battery packs. Lithium-sulfur (Li-S) batteries are a promising post lithium-ion battery technology due to their high theoretical energy density of 2600 Wh/kg along with other potential benefits such as low cost, improved safety and good low-temperature performance.1-4 The main challenges in the commercialization of Li-S batteries are to improve their cycle life, rate performance, charging efficiency, and high-temperature performance. 5,6The performance of Li-S batteries depends on a number of temperature-dependent mechanisms. During discharge, lithium and sulfur react electrochemically to form soluble polysulfide species of different chain lengths, such as Li 2 S 6 and Li 2 S 4 , which are then reduced further to produce Li 2 S, which is insoluble and precipitates. When the cell is subsequently charged, the precipitated Li 2 S must re-dissolve before being oxidized back to sulfur and lithium. The solubility of polysulfides as well as the rates of precipitation and dissolution are dependent on temperature:7,8 high temperatures promote dissolution of both sulfur and Li 2 S, whereas low temperatures limit dissolution and encourage precipitation. Model predictions indicate that a decrease in the solubility of polysulfides and their dissolution rates leads to reduced discharge and charge capacities for Li-S batteries. 9 The power capability of Li-S batteries is also critically dependent on temperature. At low temperatures, Li-S cells suffer from larger polarization from reduced ionic conductivity and diffusion rates, resulting in lower rate performance. 10The polysulfide shuttle, a characteristic phenomenon of the Li-S battery, is itself temperature-dependent. The shuttle is strongly correlated to many of the performance challenges that inhibit the mainstream uptake of Li-S batteries, 5 such as the self-discharge, low charge efficiency and irreversible degradation of Li-S batteries at high statesof-charge (SoC). Shuttle occurs when the highly soluble dissolved long chain polysulfides migrate to the lithium anode, where they react to form shorter ch...

Current Status and Future Prospects of Metal–Sulfur Batteries

“…In spite of this progress in lithium–sulfur technology, the research community has also witnessed bottlenecks in mass production and fierce competition with established commercial lithium‐ion batteries . Lithium–sulfur cells are challenged with several intrinsic material characteristics, including the insulating nature of sulfur, the irreversible relocation of polysulfides, the volume change of sulfur cathode, and the degradation of lithium‐metal anode .…”

The Effects of Lithium Sulfur Battery Ageing on Second-Life Possibilities and Environmental Life Cycle Assessment Studies

“…The trend that shows an initial linear capacity fade transitioning into a stabilization period and then continuing with a capacity loss was indicated in [47] for cells cycled at C/10 and C/5, where stabilization periods of 150 to 350 cycles were observed. Furthermore, an increase of the capacity during the initial cycles, as observed for cell J33, has been highlighted in [48] for Li-S cells cycled at C/10 and C/5 and temperatures of 20, 30 and 40 • C. Thus, it seems reasonable to state that Li-S battery ageing does not behave like the Lithium ion in response to changes in temperature and C-rate. In fact, Lithium ion battery ageing is strongly affected by several factors that accelerate the ageing in different ways [49,50].…”

A graphical model for evaluating the status of series-connected lithium-ion battery pack

“…The difference between the performance of single cell and that of battery pack originates from the constraints that are set during pack integration . For cells connected in parallel, they share a same voltage but with current reformulated within the parallel‐connected circuit . In this paper, we focus on the behavior of series‐connected battery pack; those parallel‐connected cells that share a same voltage are regarded as a single cell.…”

“…12,13 For cells connected in parallel, they share a same voltage but with current reformulated within the parallel-connected circuit. 14,15 In this paper, we focus on the behavior of series-connected battery pack; those parallel-connected cells that share a same voltage are regarded as a single cell. To avoid overcharge/overdischarge problem for the series-connected battery pack, the charge/discharge process should be terminated when the highest/lowest voltage among the cells reaches the limit.…”