Insight into Sulfur Reactions in Li–S Batteries

Abstract

Understanding and controlling the sulfur reduction species (Li2Sx, 1 ≤ x ≤ 8) under realistic battery conditions are essential for the development of advanced practical Li-S cells that can reach their full theoretical capacity. However, it has been a great challenge to probe the sulfur reduction intermediates and products because of the lack of methods. This work employed various ex situ and in situ methods to study the mechanism of the Li-S redox reactions and the properties of Li2Sx and Li2S. Synchrotron high-energy X-ray diffraction analysis used to characterize dry powder deposits from lithium polysulfide solution suggests that the new crystallite phase may be lithium polysulfides. The formation of Li2S crystallites with a polyhedral structure was observed in cells with both the conventional (LiTFSI) electrolyte and polysulfide-based electrolyte. In addition, an in situ transmission electron microscopy experiment observed that the lithium diffusion to sulfur during discharge preferentially occurred at the sulfur surface and formed a solid Li2S crust. This may be the reason for the capacity fade in Li-S cells (as also suggested by EIS experiment in Supporting Information ). The results can be a guide for future studies and control of the sulfur species and meanwhile a baseline for approaching the theoretical capacity of the Li-S battery.

Progress in Mechanistic Understanding and Characterization Techniques of Li‐S Batteries

“…[ 113 ] The result ( Figure 13 ) demonstrates that during sulfur reduction, lithium diffusion preferentially occurred at the surface of the sulfur particle and formed a solid Li 2 S crust on the bulk material. The Li-S reaction in the cell is through surface diffusion, and the insulating character of the formed Li 2 S layer makes the radial diffusion of Li + ions through the interface into the bulk more diffi cult compared to that along the contour of the particle.…”

“…The characterization techniques for dry lithium polysulfi de powders include XAS, [ 22,111 ] NMR, [ 22,112 ] and synchrotron XRD. [ 113 ] XAS provides an element-specifi c probe of local electronic structure and provides details of the oxidation state and/or atomic coordination. In sulfur K-edge spectra from X-ray absorption near edge structure (XANES) spectroscopy, polysulfi de di-anions have a characteristic pre-peak at 2470.5 eV and sulfur peak at 2472 eV, [ 22,111,112 ] which are signifi cantly different from those of crystalline Li 2 S (2474 eV).…”

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

“…Measuring the properties of individual polysulfide species is challenging due to the complex chemical equilibria among polysulfide species 23 . 15 we have assumed that the electrolyte conductivity is only a function of the total anion concentration (represented by C Li + ) instead of the concentrations of each individual ionic species.…”

A simple, experiment-based model of the initial self-discharge of lithium-sulphur batteries

“…However, in the pathway of sulphur reduction, lithium polysulphides of variable chain length, Li 2 S n , are formed [13][14][15][16][17][18][19][20][21]:…”