1 / 33

Cluster-continuum model study to guide the choice of anions for Li + -conducting ionomers

Cluster-continuum model study to guide the choice of anions for Li + -conducting ionomers. Huai-Suen Shiau, 1 Wenjuan Liu, 2 Michael J. Janik, 1 Ralph H. Colby 2 1 - Department of Chemical Engineering 2 – Department of Materials Science and Engineering Pennsylvania State University.

yannis
Download Presentation

Cluster-continuum model study to guide the choice of anions for Li + -conducting ionomers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Cluster-continuum model study to guide the choice of anions for Li+-conducting ionomers Huai-Suen Shiau,1 Wenjuan Liu,2 Michael J. Janik,1 Ralph H. Colby2 1 - Department of Chemical Engineering 2 – Department of Materials Science and Engineering Pennsylvania State University

  2. Ion-containing polymers (ionomers) can offer “single-ion conduction” Ionomers Polymer-salt mixtures + • Advantage of ionomers : avoid anion concentration polarization1 • Disadvantage of ionomers: the conductivity of ionomers is lower than that of polymer/electrolyte mixtures2 1) D. R. Sadoway et al. J. Power Sources 97-98 (2001) 621-623 2) Klein et al. Macromolecules 40 (2007) 3990

  3. Computational method as a tool to aid in ionomer design Overall objective: design ionomers for enhanced Li-ion conduction Approach: Experimental synthesis and characterization (Dr. Colby and collaborators), quantum chemical calculations (Dr. Janik) The choices for Li+ ionomer compositions are extensive! • Side chains – Interact with Li to free? Solvate anion? • Anion – weaken Li+ interaction? Avoid clustering? Question: find the anions for Li+ ionomers with high conducting ion fraction side chain=? A-=? s ≈emLipLi

  4. Model of ion states: Four major ion states in ionomers Free Ion Ion Pair Triple Ion Quadrupole

  5. Charge carried by “interstitial pair”3 Four state model used to represent Li+ ion states in ionomers Solvent separated pairs observed in PEO-salt systems4 MD simulation1 sees ion clusters SAXS data consistent with quadrupoles2 Four state model: A- Li+ A- Li+ A- Li+ + A- Li+ A- Li+ A- Li+ A- Li+ or Separated ions (free) Ion clusters (quadrupoles) Ion pairs Charged ion clusters (triple ions) 1) K. –J. Lin, J. Maranas. Unpublished 2) W. Wang et al. Macromolecules 43 (2010) 4223 3) M. Duclot et al. Solid State Ionics 136 –137 (2000) 1153 4) G. Mao et al. Phys. Rev. Lett 84 (2000) 5536; G. MacGlashan et al. Nature 398 (1999) 792

  6. Cluster-Continuum Model (CCM) can accurately capture local and long-range solute-solvent interactions Parameter: dielectric constant (εof diethyl ether used here). dielectric continuum solvent (long-range interaction)  of diethyl ether used atomistic solvent, DME (local interaction) Solvation free energies in DMSO at 298 K (kcal/mol)1 Cluster continuum model produces solvation free energies in agreement with experiment1-2 • Tissandier et al, J. Phys. Chem. A 102 (1988) 7787 • Bryantsev et al., J. Phys. Chem. B 112 (2008) 9709; Pliego Jr. and Riveros, J. Phys. Chem. A 105 (2001) 7241

  7. Dimethyl Ether (DME) used as a model for PEO length scale of QM: 150-200 atoms Full representation of polymers is prohibitive Modeling choices: How many DME to include in cluster portion? reduce PEO into DMEn cut cut cut

  8. Strongly bound DME identified and included in the explicit cluster region LiA + N DME  LiA(DME)N N=0 PCM

  9. Strongly bound DME identified and included in the explicit cluster region LiA + N DME  LiA(DME)N N=1 PCM

  10. Strongly bound DME identified and included in the explicit cluster region LiA + N DME  LiA(DME)N N=2 PCM

  11. Strongly bound DME identified and included in the explicit cluster region LiA + N DME  LiA(DME)N N=3 PCM

  12. Strongly bound DME identified and included in the explicit cluster region: Example – Pair State LiA + N DME  LiA(DME)N N=4 N=3 sufficient PCM

  13. Strongly bound DME identified and included in the explicit cluster region LiALi+:N=6 Li+: N=4 LiALiA:N=4 LiA:N=3 ALiA-:N=2 LixAy + N DME  LixAy(DME)N Li+:N=4 N=4 LiALi+:N=6 LiALi+:N=6 LiALiA:N=4 PCM

  14. Solvated state energies can be used to determine the equilibrium distribution among states 4DME Separated ions Li+ + A- A- Li+ Li+ A- A- Li+ A- Li+ A- Ion pairs Negative triple ions quadrupoles 3DME 2DME 4DME Li+ A- Li+ Positive triple ions 6DME

  15. CCM model is necessary to get reasonable locally charged ion concentrations, but not necessary for ranking of anions TFSI- FSI- PF6- BF4- C6F5SO3- CH3SO3- CF3SO3- C6H5SO3- F- CF3SO3- TFSI- s ≈emLipLi FSI- C6F5SO3- Cluster continuum model Gas phase, no solvation Solvation necessary to predict charged ion fractions on order of magnitude of experiment Solvation does not change the ordering of anions for maximizing charged Li+ concentration

  16. s ≈emLipLi Question: what is the activation barrier to Li+ hopping for conduction?

  17. Increase in molar conductance with salt content was attributed to triple ion formation2, but also possibly to the increase in dielectric constant. A significantly higher ion mobility was found in the higher ion content ionomers.3 (with a lower exponent n) Positive triple ion is also a conducting ion?! Hypothesis: An interstitial cationic pair (called positive triple ion) is formed by dissociation of an ion pair, followed by interstitial pair migration with the polymer motion. 1 1. M. Duclot et al., Solid State Ionics 136 –137 (2000) 1153 –1160 2. P. G. Bruce et al., J. Chem. Soc. Faraday Trans., 1993, 89(17), 3187-3203 3. Daniel Fragiadakis et al., JCP 130, 064907 (2009)

  18. Li+ hopping between contact ion pairs fixed 11 Angstrom S – S distance Daniel Fragiadakis et al., JCP 130, 064907 (2009)

  19. Hopping potential energy surface Li+A- Li+ A-Li+ LiALi+:N=6 Li+: N=4 Li+A- Li+A-Li+ Li+A-Li+ A-Li+ LiALiA:N=4 LiA:N=3 0.6 eV = 58 kJ/mol ALiA-:N=2 1. Specific solvation by 3 DME in the case of CCM_3DME can stabilize the transition state by 39 kJ/mol, compared with the case of PCM without DME. 2. What if we add a fourth DME to the cluster? Lower transition state by another 0.6 eV = 58 kJ/mol and make separated triple ion more stable than contact triple ion!

  20. Hopping potential energy surface in the presence of 4DME Li+ between two benzene sulfonate - Li+ ion pairs effective activation barrier = 8.7 kJ/mol ~ 3.5 kT

  21. Summary Cluster-continuum model mimics solvation of ionic states in PEO-based ionomers. N=3 LiALi+:N=6 LiALiA:N=4 Solvent-separated pairs are more stable than contact ion pairs. The effective activation barrier for S-S=11 angstrom is about 3.5 kT.

  22. Acknowledgments Iman Dr. Colby and his group Dr. Janik and his group All participants in the DOE project Computational Support: High Performance Computing Group at PSU Penn State Materials Simulation Center Funding: National Science Foundation: CBET Energy for Sustainability (CBET-0933391)

  23. Hopping potential energy surface Li+A- Li+ A-Li+ LiALi+:N=6 Li+: N=4 Li+A- Li+A-Li+ Li+A-Li+ A-Li+ LiALiA:N=4 0.6 eV = 58 kJ/mol 0.4 eV = 39 kJ/mol LiA:N=3 ALiA-:N=2 What if we add a fourth DME to the cluster? Lower transition state by another 0.6 eV = 58 kJ/mole and make separated triple ion more stable than contact triple ion!

  24. Li-ion conductivity is affected by distribution of ions among various states Dielectric spectroscopy modeling: <0.004% of Li+ are mobile1 Hypothesis: Increasing the concentration of “locally charged” species will increase conduction. Assume concentration impact on conductivity is separable from mobility s ≈ emLipLi σ: conductivity μLi: Li ion mobiliy p Li : mobile Li ion concentration Charged Ion Clusters Li+-(O)norA- Neutral Ion Clusters Ion pairs Li+A-Li+ or A-Li+ A- Li+ Li+ A- “Free” ions A-Li+A- Relative energy Li+ states in ionomer 1) Klein et al., J. Chem. Phys. 124 (2006) 144903

  25. [ACAC] = [AC] = [ACA] = [CAC] = [A] = [C] = ACAC ----> AC + AC ACAC ----> CAC + A ACAC ----> ACA + C AC ----> A + C Charge balance Mass balance (cation conservation) Assumptions: Limited to the 4 states considered, ZPVE corrected 0K energy used to consider equilibrium – no entropic considerations Solvated state energies can be used to determine the equilibrium distribution among states

  26. Li+

  27. m+ s = em+p+ 100% : Tg=240K <49% : Tg=230K

  28. electrode polarization (ionic polarization) slowest mechanism occur at lowest frequency contribute to ion conduction along polymer chains polymer relaxation (dipolar relaxation) faster mechanism occur at higher frequency

  29. If scale factor goes to infinity, the molecule is placed as if in vacuum even though PCM is used. • 2. The larger scale factor is, the weaker the interaction between solute and continuum solvent is. space between red and blue circle is vacuum hypothetical atom radius scale factor = 1.3 real atom radius The scale factor is justified by considering that the first solvation layer does not have the same dielectric properties as the bulk of the solvent.* * Vincenzo Barone, J. Chem. Phys., Vol. 107, No. 8, 22 August 1997

  30. Quadrupole/pair equilibrium dominates smaller anion (d decreases) increases Quadrupole increases Pair decreases

  31. Locally positively charged states have small populations A- Li+ Li+ A- Li+ A- Li+ Li+ A- Li+ + A- Li+ + A- >95% of ions are in aggregates at physically relevant temperatures Fraction of charged species is 2x10-8 at room temperature Triple ions are more populous than “free ions”

  32. Li+ vs Na+ with benzene sulfonate: ab initio model compared to experiment Li+ vs Na+: ab initio model matches dielectric spectroscopy model for Na+ vs Li+ (EaNa+ < EaLi+) Discrepancy in slope of charged ion concentration versus 1000/T is 20 kJ/mol Expt data: The Journal of Chemical Physics 124, 144903 (2006)

More Related