Modeling the voltage loss mechanisms in lithium–sulfur cells: the importance of electrolyte resistance and precipitation kinetics

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...

Section: Resultsmentioning

confidence: 99%

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

Hunt

Zhang

Patel

et al. 2017

“…This decrease in capacity is mainly caused by the shortening of the lower discharge plateau, associated with the reduction to solid Li 2 S, and the increase of its overpotential (Figure 3c and Figure 3d). In recent modelling study this type of overpotential increase was attributed to differences in Li 2 S precipitation process 43 leading to: (i) a change in the electrolyte resistance due to the variation in concentration of polysulfides and Li-ions, which are formed in large quantities at high rates, (ii) the need of a larger activation overpotential with high currents to overcome the deposition of insulating Li 2 S, and subsequent decrease of available surface area, and (iii) a shift of the reduction potentials due to the consumption of the active species. In our work at discharge rates higher than 1C, the overpotential is so high that the lower-plateau reactions do not occur within the voltage window studied (Figure 3c and Figure 3d).…”

Section: Resultsmentioning

confidence: 99%

Pitfalls in Li–S Rate-Capability Evaluation

Schott

Novák

Trabesinger

2016

Rate-capability tests are widely used to highlight advances in Li-S-system development. Here we show, by studying the individual effects of a number of cycling-and electrolyte-related parameters on a simple Li-S cell, that the rate-capability results are sensitive not only to the applied current but also to the cycling prehistory of the cell. On the one hand, the performance is affected by the order in which cycling rate is changed. Slow initial rates aggravate material loss and reduce the achievable capacity. On the other hand, the results of rate-capability tests are significantly different when the rates are varied only on the charge or only on the discharge. The charge rate does not directly affect the measured capacity, whereas the discharge rate does, especially when discharge cutoff voltages are high, due to slow discharge reactions. High charge rates, however, do affect the long-term stability, which is difficult to predict from usual rate-capability tests. Our findings also provide insights into the relative reaction rates and the influence of the general cycling procedures, underlining the importance of understanding the kinetics of Li-S systems, while designing them for high performance batteries. The lithium-sulfur (Li-S) battery is one of the most promising electrochemical systems for next-generation energy-storage applications, given the abundance of sulfur, its low toxicity, and its high theoretical specific charge of 1672 mAh g sulfur -1 (often referred as well as capacity). However, the commercialization of Li-S cells is hindered by substantial challenges, including the insulating nature of sulfur and of its discharge product Li 2 S, preventing efficient activematerial utilization, and the so-called polysulfide shuttle, leading to a loss of active material and low coulombic efficiency. Considerable improvements on the electrode and cell levels were made over the past few years, alleviating these limitations and, in turn, leading to an exponentially growing interest in Li-S batteries.1-5 From the perspective of every-day battery use, having a high rate capability is very important, and therefore rate-capability tests, in which the capacity is measured while changing the cycling rate (i.e., while changing the applied current), are often employed to complete the characterization of a battery. These tests are also widely used in the characterization of Li-S systems, to quantify the improvements brought about by various enhancements to the Li-S electrodes, electrolytes, their additives, and cells. Although it is known that Li-S cells, due to their complex chemistry, are sensitive to a wide range of parameters, 6-8 the protocols used in rate-capability tests described in the literature vary significantly. A comparison of rate performances reported in publications for Li-S cells using commercially available materials 9-12 is plotted in Figure 1. Despite their similar electrode composition (∼60 wt% sulfur, ∼30 wt% carbon black, ∼10 wt% binder (PEO or PVDF)), they display significantly different rate per...

“…48 However, EIS studies carried out by Deng et al, 49 showed that the charge transfer resistance, corresponding to the activation overpotential, and the surface film resistance, are much larger than electrolyte resistance throughout the discharge event. This suggests that the cell potential can be limited by charge transfer and surface resistance even when the electrolyte conductivity is adequate.…”

Section: Methodology: Overall Construction and Governing Equationsmentioning

confidence: 99%

A Microstructurally Resolved Model for Li-S Batteries Assessing the Impact of the Cathode Design on the Discharge Performance

Thangavel

Xue

Mammeri

et al. 2016

This paper reports a discharge model for lithium sulfur (Li-S) battery cells, supported by a multi-scale description of the composite C/S cathode microstructure. The cathode is assumed to be composed of mesoporous carbon particles with inter-particular pores in-between and the sulfur impregnated into both types of pores. The electrolyte solutes such as sulfur, polysulfides and lithium ions, produced during the discharge, are allowed to exchange between the pores. Furthermore, the model describes the Li 2 S (solid) precipitation and its effects on transport and reduction reaction kinetics. Hereby it provides fundamental insights on the impact on the Li-S discharge curve of practically modifiable manufacturing parameters and operation designs, such as current density, carbon porosity, C/S ratio and sizes of carbon particles and pores. Lithium-ion battery technologies based on dual intercalation electrodes have come to totally dominate the consumer electronics market for mobile devices.1,2 However their capacities still limit the driving ranges and user modes for both electric and hybrid electric vehicles, 3 despite approaching the intrinsic maximum for intercalation reactions. 4,5 With the demand for batteries with even higher capacities, lithium sulfur (Li-S) batteries, 6,7 with a theoretical capacity of 1672 mAh.g −1 based on cathode solid sulfur mass 8 and a potential gravimetric energy density of about 600 Wh.kg −1 , 9 has (re-)gained attention in recent years.10-23 Li-S batteries usually have a lithium metal anode, a porous separator, and a porous C/S composite cathode where the carbon acts as a host and provides electronic wiring for the insulating sulfur, existing as S 8(solid) and Li 2 S (solid) . The pores in both the cathode and the separator are filled with an aprotic electrolyte, e.g. 1 M LiTFSI dissolved in dimethoxyethane (DME): 1,3-dioxolane (DOL) (1:1 volume ratio). 24 There is a general consensus that the reasons behind the theoretical capacity of Li-S batteries not being achieved are short-comings: low sulfur utilization due to poor wiring and soluble polysulfide intermediates giving rise to a parasitic shuttle effect. 25,26 One breakthrough in the last decade was the finding that a host of mesoporous carbon greatly enhanced the active material utilization as compared to a ground C/S mixture. 5 The mesoporous architecture of the cathode primarily assisted in retaining polysulfide intermediates in the cathode. 27Theory and mathematical modeling are powerful tools to assist the optimization of electrochemical devices, by providing insight in operating principles and identifying limitations. [28][29][30] Continuum models applied to Li-S batteries have been successful in simulating various battery operation phenomena. [31][32][33][34][35][36][37] However, most of the continuum models reported so far model the cathode as a homogenous porous medium, described by an effective porosity, not accounting for the * Electrochemical Society Member.h Present address: Institut Charles Gerhardt, CNRS and Univers...