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Ab initio structure search and in situ 7Li NMR studies of discharge products in the Li-S battery system.

See KA, Leskes M, Griffin JM, Britto S, Matthews PD, Emly A, Van der Ven A, Wright DS, Morris AJ, Grey CP, Seshadri R - J. Am. Chem. Soc. (2014)

Bottom Line: We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li2S) which forms near the beginning of the first plateau, in the cell configuration studied here.The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li2S).A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Biochemistry and Materials Research Laboratory and ∥Materials Department, University of California, Santa Barbara (UCSB) , Santa Barbara, California 93106, United States.

ABSTRACT
The high theoretical gravimetric capacity of the Li-S battery system makes it an attractive candidate for numerous energy storage applications. In practice, cell performance is plagued by low practical capacity and poor cycling. In an effort to explore the mechanism of the discharge with the goal of better understanding performance, we examine the Li-S phase diagram using computational techniques and complement this with an in situ (7)Li NMR study of the cell during discharge. Both the computational and experimental studies are consistent with the suggestion that the only solid product formed in the cell is Li2S, formed soon after cell discharge is initiated. In situ NMR spectroscopy also allows the direct observation of soluble Li(+)-species during cell discharge; species that are known to be highly detrimental to capacity retention. We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li2S) which forms near the beginning of the first plateau, in the cell configuration studied here. The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li2S). A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition.

No MeSH data available.


Related in: MedlinePlus

(a) The discharge profile of the Li–S bag celldischargedat C/20 and studied by NMR spectroscopy. The spectrawere fit using the three components described in Figure 5 to extract the changes in (b) chemical shift and (c) integratedintensities as a function of discharge. The errors bars in (b) and(c) indicate the error of the fit as reported by the DMFit program.
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fig6: (a) The discharge profile of the Li–S bag celldischargedat C/20 and studied by NMR spectroscopy. The spectrawere fit using the three components described in Figure 5 to extract the changes in (b) chemical shift and (c) integratedintensities as a function of discharge. The errors bars in (b) and(c) indicate the error of the fit as reported by the DMFit program.

Mentions: Noticeable changes inintensity and shifts of the resonances occuras the cell discharges (Figure 6). The spectrumat the end of the discharge (Figure 5b) canbe deconvoluted with three different contributions, the two sharpresonances from the dissolved Li+ contributions and a broaderresonance. The entire in situ NMR data set was therefore fit fixingthe widths of the sharp resonances but allowing the line width ofthe broad resonance to float and good fits are obtained throughout(Figure 6b,c). The integrated area of the higherfrequency, sharp Li+ resonance increases as the cell discharges,which we ascribe to the dissolution of polysulfides (Figure 6c). We therefore assign this resonance to the Liions in the electrolyte close to the carbon electrode, which willcontain a higher concentration of polysulfides. The shift of the higherfrequency resonance to even higher frequencies agrees well with theconclusion from the ex situ data that high ion concentrations causea positive shift in the Li+ resonance (Figure 6b). The quantity of Li+ exhibiting alower frequency resonance stays relatively constant as the cell dischargeswhich suggests that the dissolution of polysulfides is limited mostlyto electrolyte near the cathode structure and the migration of thepolysulfides through the separator to different regions of the batteryis not significant at least during the first discharge. This was alsosuggested previously by in situ transmission X-ray microscopy.8


Ab initio structure search and in situ 7Li NMR studies of discharge products in the Li-S battery system.

See KA, Leskes M, Griffin JM, Britto S, Matthews PD, Emly A, Van der Ven A, Wright DS, Morris AJ, Grey CP, Seshadri R - J. Am. Chem. Soc. (2014)

(a) The discharge profile of the Li–S bag celldischargedat C/20 and studied by NMR spectroscopy. The spectrawere fit using the three components described in Figure 5 to extract the changes in (b) chemical shift and (c) integratedintensities as a function of discharge. The errors bars in (b) and(c) indicate the error of the fit as reported by the DMFit program.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4353022&req=5

fig6: (a) The discharge profile of the Li–S bag celldischargedat C/20 and studied by NMR spectroscopy. The spectrawere fit using the three components described in Figure 5 to extract the changes in (b) chemical shift and (c) integratedintensities as a function of discharge. The errors bars in (b) and(c) indicate the error of the fit as reported by the DMFit program.
Mentions: Noticeable changes inintensity and shifts of the resonances occuras the cell discharges (Figure 6). The spectrumat the end of the discharge (Figure 5b) canbe deconvoluted with three different contributions, the two sharpresonances from the dissolved Li+ contributions and a broaderresonance. The entire in situ NMR data set was therefore fit fixingthe widths of the sharp resonances but allowing the line width ofthe broad resonance to float and good fits are obtained throughout(Figure 6b,c). The integrated area of the higherfrequency, sharp Li+ resonance increases as the cell discharges,which we ascribe to the dissolution of polysulfides (Figure 6c). We therefore assign this resonance to the Liions in the electrolyte close to the carbon electrode, which willcontain a higher concentration of polysulfides. The shift of the higherfrequency resonance to even higher frequencies agrees well with theconclusion from the ex situ data that high ion concentrations causea positive shift in the Li+ resonance (Figure 6b). The quantity of Li+ exhibiting alower frequency resonance stays relatively constant as the cell dischargeswhich suggests that the dissolution of polysulfides is limited mostlyto electrolyte near the cathode structure and the migration of thepolysulfides through the separator to different regions of the batteryis not significant at least during the first discharge. This was alsosuggested previously by in situ transmission X-ray microscopy.8

Bottom Line: We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li2S) which forms near the beginning of the first plateau, in the cell configuration studied here.The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li2S).A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Biochemistry and Materials Research Laboratory and ∥Materials Department, University of California, Santa Barbara (UCSB) , Santa Barbara, California 93106, United States.

ABSTRACT
The high theoretical gravimetric capacity of the Li-S battery system makes it an attractive candidate for numerous energy storage applications. In practice, cell performance is plagued by low practical capacity and poor cycling. In an effort to explore the mechanism of the discharge with the goal of better understanding performance, we examine the Li-S phase diagram using computational techniques and complement this with an in situ (7)Li NMR study of the cell during discharge. Both the computational and experimental studies are consistent with the suggestion that the only solid product formed in the cell is Li2S, formed soon after cell discharge is initiated. In situ NMR spectroscopy also allows the direct observation of soluble Li(+)-species during cell discharge; species that are known to be highly detrimental to capacity retention. We suggest that during the first discharge plateau, S is reduced to soluble polysulfide species concurrently with the formation of a solid component (Li2S) which forms near the beginning of the first plateau, in the cell configuration studied here. The NMR data suggest that the second plateau is defined by the reduction of the residual soluble species to solid product (Li2S). A ternary diagram is presented to rationalize the phases observed with NMR during the discharge pathway and provide thermodynamic underpinnings for the shape of the discharge profile as a function of cell composition.

No MeSH data available.


Related in: MedlinePlus