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Title: Direct observation of the redistribution of sulfur and polysufides in Li-S batteries during first cycle by in situ X-Ray fluorescence microscopy

The demands on low cost and high energy density rechargeable batteries for both transportation and large-scale stationary energy storage are stimulating more and more research toward new battery systems. Since sulfur is an earth-abundant material with low cost, research on the high energy density Li–S batteries (2600 W h kg⁻¹) are getting more and more attention. The reactions between sulfur and lithium during charge–discharge cycling are quite complicated, going through multiple electron transfer process associated with chemical and electrochemical equilibrium between long- and short-chain polysulfide Li₂Sx intermediates (1 < x ≤ 8). It is reported that the long-chain polysulfides can be dissolved into electrolyte with aprotic organic solvents and migrated to the Li anode side. This so-called “shuttle effect” is believed to be the main reason for capacity loss and low columbic efficiency of the Li–S batteries. In the past few years, a great deal of efforts have been made on how to overcome the problem of polysulfide dissolution through new sulfur electrode construction and cell designs, as well as the modification of the electrolyte. Although it has been reported by several publications that some Li–S cells can sustain more than a thousand cycles based on the thin film electrodemore » configurations, the long-term cycling stability is still one of the major barriers for the real application of Li–S batteries. More in-depth studies on the fundamental understanding of the sulfur reaction mechanism and interactions among the different polysulfide species, the electrolyte and the electrodes are still greatly needed. Various in situ techniques have been developed and applied to study the mechanism of the sulfur chemistry in Li–S batteries during electrochemical cycling, such as transmission X-ray microscopy (TXM), X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), UV–visible spectroscopy, and electron paramagnetic resonance (EPR). The applications of these characterization techniques have demonstrated their power in probing the structure changes, morphology evolutions, and coordination of sulfur and polysulfides with the electrolyte in Li–S cells, providing complementary information to each other thus enhancing the understanding in Li–S battery systems. In this communication, in situ X-ray fluorescence (XRF) microscopy was combined with XAS to directly probe the morphology changes of Li–S batteries during first cycle. The morphology changes of the sulfur electrode and the redistribution of sulfur and polysulfides were monitored in real time through the XRF images, while the changes of the sulfur containing compounds were characterized through the XAS spectra simultaneously. In contrast to other studies using ex situ or single characterization technique as reported in the literatures, the in situ technique used in this work has the unique feature of probing the Li–S cell under operating conditions, as well as the combination of XRF imaging with spectroscopy data. By doing this, the morphology evolution and redistribution of specific sulfur particles during cycling can be tracked and identified at certain locations in a real time. In addition, this technique allows us to select the field-of-view (FOV) area from micrometer to centimeter size, providing the capability to study the Li–S reactions not just at the material level, but also at the electrode level. This is very important for both understanding Li–S chemistry and designing effective strategies for Li–S batteries.« less
Authors:
 [1] ;  [2] ;  [1] ;  [1] ;  [2] ;  [1] ;  [1] ;  [3] ;  [4] ;  [2] ;  [5] ;  [1]
  1. Brookhaven National Laboratory (BNL), Upton, NY (United States)
  2. Pacific Northwest National Laboratory, Joint Center for Energy Storage Research, Richland, WA (United States)
  3. Dongguk University-Seoul, Department of Energy and Materials Engineering, (Republic of Korea)
  4. Univ. of Massachusetts at Boston, Dept. of Chemistry, MA (United States)
  5. Argonne National Laboratory, X-ray Science Division, Lemont, IL (United States)
Publication Date:
OSTI Identifier:
1177848
Report Number(s):
BNL--107699-2015-JA
Journal ID: ISSN 1614-6832; R&D Project: MA453MAEA; VT1201000
Grant/Contract Number:
SC00112704
Type:
Accepted Manuscript
Journal Name:
Advanced Energy Materials
Additional Journal Information:
Journal Volume: 5; Journal Issue: 16; Journal ID: ISSN 1614-6832
Publisher:
Wiley
Research Org:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
Country of Publication:
United States
Language:
English
Subject:
25 ENERGY STORAGE energy storage; in situ techniques; lithium–sulfur batteries; microspectroscopy; reaction mechanism