Identification of lithium–sulfur battery discharge products through 6Li and 33S solid-state MAS and 7Li solution NMR spectroscopy

Abstract

Li and 33 S solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy were used to identify the discharge products in lithium-sulfur (Li-S) battery cathodes. Cathodes were stopped at different potentials throughout battery discharge and measured ex-situ to obtain chemical shifts and T 2 relaxation times of the products formed. The chemical shifts in the spectra of both 6 Li and 33 S NMR demonstrate that long-chain, soluble lithium polysulfide species formed at the beginning of discharge are indistinguishable from each other (similar chemical shifts), while short-chain, insoluble polysulfide species that form at the end of discharge (presumably Li 2 S 2 and Li 2 S) have a different chemical shift, thus distinguishing them from the soluble long-chain products. T 2 relaxation measurements of discharged cathodes were also performed which resulted in two groupings of T 2 times that follow a trend and support the previous conclusions that long-chain polysulfide species are converted to shorter chain species during discharge. Through the complementary techniques of 1-D 6 Li and 33 S solid-state MAS NMR spectroscopy, solution 7 Li and 1 H NMR spectroscopy, and T 2 measurements, structural information about the discharge products of Li-S batteries is obtained.

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

“…The identifi cation of the formation of these species is important to understand the Li-S redox reactions, as some researchers attribute the capacity fading in Li-S cell to the formation of an insulating Li 2 S layer on the cathode. [ 16,25,154 ] However, XRD results were controversial regarding the formation of crystalline Li 2 S during discharge [ 23,25,124,155 ] and the reappearance of sulfur. [ 23,58,123,156 ] Recently, several in-situ XRD studies on the sulfur electrode during charge and discharge agreed on the formation of Li 2 S during discharge and its conversion to crystalline S 8 during the following charge.…”

“…[ 20 ] The technique of 7 Li NMR has the capability to distinguish solid vs. dissolved products containing Li + and can detect small crystallites containing Li and S with low X-ray scattering signals. However, the work done by Huff et al using 6 Li and 33 S magic angle spinning (MAS) NMR demonstrated that Li 2 S is produced only at the end of the discharge in cathodes discharged to 1.5 V. [ 154 ] The contradictions in these studies can be attributed to the complexity of the Li-S system, Adv. Energy Mater.…”

In Situ Techniques for Developing Robust Li–S Batteries

“…Also 33 S NMR has been used in LSB study because of its large quadrupole moment, the low natural abundance (0.8%), and low receptivity. Therefore, in situ NMR spectroscopy can provide critical real‐time information about anode lithium‐dendrite growth, LPS formation, and sequential migration in the LSB research field …”

“…In situ NMR spectroscopy has also provided insights into the complex electrochemistry of LPS formation in the LSB system, especially in tracking transient reactions during cycling . Huff et al investigated the reduction products of an LSB cell with a cathode (4.29 wt% sulfur showing the initial capacity of 1550 mA g −1 at 0.4C) in an electrolyte (1 m LiTFSI in 1:1 DOL:DME) by in situ NMR spectroscopy. They found that Li 2 S is exclusively produced at the end of an LSB discharge event (1.5 V), along with the formation of small amounts of longer‐chain LPS species.…”

“…However, there are still some areas in which there is not a complete agreement. For example, Li 2 S 2 has sometimes been considered a final stable product at the end of discharge . However, by using a combination of the synchrotron‐radiation techniques, in situ laboratory XRD and XANES, two groups in 2013 separately confirmed that Li 2 S is the only stable crystalline phase at the end of discharge.…”