What Limits the Rate Capability of Li-S Batteries during Discharge: Charge Transfer or Mass Transfer?

Abstract

Li-S batteries exhibit poor rate capability under lean electrolyte conditions required for achieving high practical energy densities. In this contribution, we argue that the rate capability of commercially-viable Li-S batteries is mainly limited by mass transfer rather than charge transfer during discharge. We first present experimental evidence showing that the charge-transfer resistance of Li-S batteries and hence the cathode surface covered by Li 2 S are proportional to the state-of-charge (SoC) and not to the current, directly contradicting previous theories. We further demonstrate that the observed Li-S behaviors for different discharge rates are qualitatively captured by a zero-dimensional Li-S model with transport-limited reaction currents. This is the first Li-S model to also reproduce the characteristic overshoot in voltage at the beginning of charge, suggesting its cause is the increase in charge transfer resistance brought by Li 2 S precipitation. Achieving a high practical energy density in Li-S batteries requires increased sulfur loading and reduced electrolyte loading at cell level. However, increasing the sulfur-to-electrolyte mass ratio lowers the sulfur utilization as well as the rate capability.1-3 The charge rate of Li-S batteries is mainly limited by the large initial overpotential associated with the activation of precipitated Li 2 S, 4 as well as the slow subsequent dissolution that could lead to incomplete Li 2 S conversion at the end of charge. 5 The mechanisms behind the low discharge rate capability have been explained in terms of mass transport limitation and surface passivation caused by the accumulation of precipitated Li 2 S.The transport limitation could arise from high electrolyte viscosity and pore-blocking due to localized Li 2 S precipitation. We recently demonstrated that a Li-S cell discharged at high current can provide extra capacity after an hour of relaxation.6 With a one-dimensional Li-S model, it was found that slow ionic transport could force polysulfides to accumulate temporarily in the separator, leading to the known reduced capacity at high discharge rates. The capacity recovery phenomenon is the result of redistribution of polysulfides across the cell during relaxation. Danner et al. 7 further demonstrated with a multiscale Li-S model that localized precipitation tends to occur at the carbon-sulfur particle surface, leading to pore-blocking and transport-limited discharge capacity for high sulfur loading.The insulating and insoluble discharge product, Li 2 S, has been shown experimentally to cover active cathode surfaces during discharge and increase the charge-transfer resistance. The relation between discharge rate and morphology of the precipitate, however, remains under debate. Based on ex-situ SEM imaging of a carbon fiber ultramicroelectrode of a Li-S cell, Fan et al. 8 revealed that Li 2 S formed as a thin, continuous coating after a fast discharge but appeared as a small number of large particles after a slow discharge. It was therefore hypothesized that ...

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

“…The charge transfer and diffusion resistances increase throughout the discharge and become dominating toward the end of discharge, leading to large overpotentials and thus causing the end of discharge as a result of the voltage cut-off being reached. 20 As expected, a peak in the resistance appears at all temperatures at the transition between upper and lower plateaus, associated with a maximum in electrolyte resistance, as a result of high concentrations of dissolved polysulfides in this state. 21,22 The size and location of this peak is strongly temperature dependent.…”

Polysulfide Shuttle Suppression by Electrolytes with Low‐Density for High‐Energy Lithium–Sulfur Batteries

“…S 8 and Li 2 S precipitation and accumulation on the carbon surface impedes the charge transfer from the carbon scaffold to the active material. Furthermore, pore blocking effects appear upon accumulation, therefore, reinforcing active material loss . As a result, the internal resistance is believed to gradually increase resulting in higher overpotential during charge and discharge.…”

Quantitative Galvanostatic Intermittent Titration Technique for the Analysis of a Model System with Applications in Lithium−Sulfur Batteries

“…For example, a detailed analysis of impedance measurements of polysulfide solutions with well-behaved glassy carbon electrodes, complemented with UV-vis measurements, demonstrated the importance of disproportionation reactions, and the use of this technique to estimate values of the relevant rate constants and diffusion coefficients. [9] Impedance spectroscopy has also been used to evaluate the individual contributions from electrolyte resistance, charge transfer, ion diffusion and electrode surface layers, [10][11][12][13][14][15] which in turn has been used to refine mechanistic models of LiÀS cells, [16,17] as well as to investigate capacity fading. [18][19][20] Other experimental techniques used to elucidate redox and diffusion properties in LiÀS cells include XAS, [21][22][23][24][25][26][27][28][29][30][31][32] in-operando optical imaging of the temporal and spatial distribution of polysulfides, [33] UV-Vis spectroscopy, [34][35][36][37][38][39][40][41][42] NMR, [43][44][45][46] EPR/ESR, [40,47] XRD, [48][49]…”