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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

Proposed ternarydiagram describing the pathway of the Li–Sdischarge. The system can only be explained using a ternary phasediagram as the electrolyte is actively involved in the discharge pathway.The approximate discharge profile would exhibit a plateau when passingthrough the three-phase region, a voltage drop upon exiting the three-phaseregion, and another plateau when passing through the two-phase regionsthereafter. This would result in two plateaus, as seen in experiment.
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fig8: Proposed ternarydiagram describing the pathway of the Li–Sdischarge. The system can only be explained using a ternary phasediagram as the electrolyte is actively involved in the discharge pathway.The approximate discharge profile would exhibit a plateau when passingthrough the three-phase region, a voltage drop upon exiting the three-phaseregion, and another plateau when passing through the two-phase regionsthereafter. This would result in two plateaus, as seen in experiment.

Mentions: We propose a ternary diagram thatfacilitates the visualizationof the Li–S discharge pathway and explains the formation ofthe observed phases by in situ NMR (Figure 8). The electrolyte in general contains several species but in thecontext of a Li–S electrochemical cell can be treated as asingle component. The electrolyte forms one corner of this diagram.We note that “electrolyte” could be replaced with “electrolytesolvent” as the electrolyte salt is relatively inert with respectto the performance and mechanisms of the cell discharge.52 The diagram exhibits a prominent single-phaseregion corresponding to Li+ and polysulfide species insolution. This phase must exist because Li+ + polysulfidesolutions can be prepared easily. Point F represents the solubilityof solid S in the electrolyte solvent, which should be somewhat lowfor ethereal solvents.53 The single phaseregion then dips strongly into the ternary in order to represent thehigher solubility of long chain polysulfides (high S:Li ratio) andlower solubility of short chain polysulfides, which will be true forany low dielectric constant electrolyte solvent like ethereal solvents.2 The bottom line of the phase diagram is the Li–Sbinary axis and only one solid phase exists on this line, which, asdiscussed above, is Li2S according to DFT calculations(Figure 1b) and experiments. The two-phasecoexistence between S and Li2S along the binary Li–Saxis will expand into a ternary composition space as a three-phaseregion. We propose that this three-phase region consists of S (pointA), Li+, and polysulfides dissolved in the electrolytesolvent (point B) and Li2S (point C) as schematically illustratedby the blue triangle in Figure 8. The phasediagram also shows a large two-phase region between solid Li2S and the electrolyte solvent. The tie lines (thin red lines) inthe two-phase region represent compositions of constant chemical potentials.The particular electrolyte chemistry determines the solubility ofshort chain polysulfides and therefore also the size and shape ofthe electrolyte single-phase region in Figure 8. The lower solubility of sulfur in the electrolyte in comparisonto the polysulfides is represented by point F along the S–electrolytebinary.


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)

Proposed ternarydiagram describing the pathway of the Li–Sdischarge. The system can only be explained using a ternary phasediagram as the electrolyte is actively involved in the discharge pathway.The approximate discharge profile would exhibit a plateau when passingthrough the three-phase region, a voltage drop upon exiting the three-phaseregion, and another plateau when passing through the two-phase regionsthereafter. This would result in two plateaus, as seen in experiment.
© Copyright Policy
Related In: Results  -  Collection

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

fig8: Proposed ternarydiagram describing the pathway of the Li–Sdischarge. The system can only be explained using a ternary phasediagram as the electrolyte is actively involved in the discharge pathway.The approximate discharge profile would exhibit a plateau when passingthrough the three-phase region, a voltage drop upon exiting the three-phaseregion, and another plateau when passing through the two-phase regionsthereafter. This would result in two plateaus, as seen in experiment.
Mentions: We propose a ternary diagram thatfacilitates the visualizationof the Li–S discharge pathway and explains the formation ofthe observed phases by in situ NMR (Figure 8). The electrolyte in general contains several species but in thecontext of a Li–S electrochemical cell can be treated as asingle component. The electrolyte forms one corner of this diagram.We note that “electrolyte” could be replaced with “electrolytesolvent” as the electrolyte salt is relatively inert with respectto the performance and mechanisms of the cell discharge.52 The diagram exhibits a prominent single-phaseregion corresponding to Li+ and polysulfide species insolution. This phase must exist because Li+ + polysulfidesolutions can be prepared easily. Point F represents the solubilityof solid S in the electrolyte solvent, which should be somewhat lowfor ethereal solvents.53 The single phaseregion then dips strongly into the ternary in order to represent thehigher solubility of long chain polysulfides (high S:Li ratio) andlower solubility of short chain polysulfides, which will be true forany low dielectric constant electrolyte solvent like ethereal solvents.2 The bottom line of the phase diagram is the Li–Sbinary axis and only one solid phase exists on this line, which, asdiscussed above, is Li2S according to DFT calculations(Figure 1b) and experiments. The two-phasecoexistence between S and Li2S along the binary Li–Saxis will expand into a ternary composition space as a three-phaseregion. We propose that this three-phase region consists of S (pointA), Li+, and polysulfides dissolved in the electrolytesolvent (point B) and Li2S (point C) as schematically illustratedby the blue triangle in Figure 8. The phasediagram also shows a large two-phase region between solid Li2S and the electrolyte solvent. The tie lines (thin red lines) inthe two-phase region represent compositions of constant chemical potentials.The particular electrolyte chemistry determines the solubility ofshort chain polysulfides and therefore also the size and shape ofthe electrolyte single-phase region in Figure 8. The lower solubility of sulfur in the electrolyte in comparisonto the polysulfides is represented by point F along the S–electrolytebinary.

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