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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

Integrated area of the dissolved Li+, the Li+-containing solid, and the sum of all Li+ components takenfrom the fits of the 7Li NMR spectra of the Li–Scell as the discharge progresses. The slope of the Li+ formationis linear which would be expected if the Li+ formationwas due to electrochemical processes. Initially, the rates of formationof solid and dissolved products are almost the same. The linear trendlineswere fit using the highlighted linear regions. A fit to the plot ofthe sum of Li+ components gives a slope of 0.27 au hr–1 if the fit is constrained to the same region usedfor the individual components.
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fig7: Integrated area of the dissolved Li+, the Li+-containing solid, and the sum of all Li+ components takenfrom the fits of the 7Li NMR spectra of the Li–Scell as the discharge progresses. The slope of the Li+ formationis linear which would be expected if the Li+ formationwas due to electrochemical processes. Initially, the rates of formationof solid and dissolved products are almost the same. The linear trendlineswere fit using the highlighted linear regions. A fit to the plot ofthe sum of Li+ components gives a slope of 0.27 au hr–1 if the fit is constrained to the same region usedfor the individual components.

Mentions: Further evidence that the Li+-containing solidis aresult of electrochemical processes comes from the observed rate offormation. Because one electron is pulled from the anode to produceone Li+ ion, the current and the rate of formation of Li+ should be equal if the Li+ formation is due toelectrochemical processes. Indeed, the integrated area as the dischargeprogresses exhibits a linear trend, as would be expected for a galvanostaticdischarge, and thus we suggest that the formation of the Li+-containing solid is a result of electrochemical reactions (Figure 7). Near the beginning of discharge, the formationof the solid species is very likely due to electrochemical processesand not chemical reactions due to shorting, i.e., the deposition ofLi2S at the anode due to the reduction of polysulfide ions(i.e., the polysulfide shuttle),46 sincewe observe a linear rate of formation. A positive deviation from linearitywould be expected if Li+ formation was due to Li2S deposition on the anode due to an increase in Li+ contentthat is unaccounted for by the current. Absence of a positive deviationfrom the linear fit is consistent with our suggestion that the diffusionof polysulfides into the bulk electrolyte is minimal.


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)

Integrated area of the dissolved Li+, the Li+-containing solid, and the sum of all Li+ components takenfrom the fits of the 7Li NMR spectra of the Li–Scell as the discharge progresses. The slope of the Li+ formationis linear which would be expected if the Li+ formationwas due to electrochemical processes. Initially, the rates of formationof solid and dissolved products are almost the same. The linear trendlineswere fit using the highlighted linear regions. A fit to the plot ofthe sum of Li+ components gives a slope of 0.27 au hr–1 if the fit is constrained to the same region usedfor the individual components.
© Copyright Policy
Related In: Results  -  Collection

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

fig7: Integrated area of the dissolved Li+, the Li+-containing solid, and the sum of all Li+ components takenfrom the fits of the 7Li NMR spectra of the Li–Scell as the discharge progresses. The slope of the Li+ formationis linear which would be expected if the Li+ formationwas due to electrochemical processes. Initially, the rates of formationof solid and dissolved products are almost the same. The linear trendlineswere fit using the highlighted linear regions. A fit to the plot ofthe sum of Li+ components gives a slope of 0.27 au hr–1 if the fit is constrained to the same region usedfor the individual components.
Mentions: Further evidence that the Li+-containing solidis aresult of electrochemical processes comes from the observed rate offormation. Because one electron is pulled from the anode to produceone Li+ ion, the current and the rate of formation of Li+ should be equal if the Li+ formation is due toelectrochemical processes. Indeed, the integrated area as the dischargeprogresses exhibits a linear trend, as would be expected for a galvanostaticdischarge, and thus we suggest that the formation of the Li+-containing solid is a result of electrochemical reactions (Figure 7). Near the beginning of discharge, the formationof the solid species is very likely due to electrochemical processesand not chemical reactions due to shorting, i.e., the deposition ofLi2S at the anode due to the reduction of polysulfide ions(i.e., the polysulfide shuttle),46 sincewe observe a linear rate of formation. A positive deviation from linearitywould be expected if Li+ formation was due to Li2S deposition on the anode due to an increase in Li+ contentthat is unaccounted for by the current. Absence of a positive deviationfrom the linear fit is consistent with our suggestion that the diffusionof polysulfides into the bulk electrolyte is minimal.

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