(518f) Study of Lithium Salt Dissociation and Diffusion in Block Copolymer Via Spectroscopy

Solid state batteries have increasingly attracted research interest due to their enhanced safety and high energy density. Block copolymer electrolytes are promising materials which can provide ionic conductivity and stable chemical structure. Suppression of dendrite growth from the lithium electrode surface and improvement of ionic conductivities are the major challenges to successfully replace the lithium-ion batteries. To develop high-performance batteries, it is required to understand the transport properties: ionic conductivity, diffusion coefficient, and cation transference number. There have been only a limited number of studies on diffusion coefficient and transference number compared to ionic conductivity because of difficulties of measurement. In our previous study, we have studied the diffusion of lithium salt through a polystyrene-poly(ethylene oxide) block copolymer (SEO) electrolytes with various salt concentrations using magnetic resonance imaging (MRI).¹ The diffusion coefficient showed an exponential dependence on salt concentration. This behavior is thought to be closely related to the segmental motion of the ethylene oxide (EO) chain which is affected by interaction between the EO repeating units and the ions.² Ion-pairing effects become more pronounced with increasing salt concentration.³ The relationship between the dissociation and the diffusion of salt in a block copolymer was investigated by spectroscopic techniques, FTIR-ATR and Raman spectroscopy, which can provide direct and simultaneous measurement of diffusion and ionic states. With increasing salt concentration, the diffusion coefficient had a non-monotonic trend, decreasing initially then passing through a minimum at 1.3 M and increasing with further increase in concentration. The diffusion coefficient results have much weaker concentration dependence than that observed in our MRI study but agree with other literature, indicating a shortcoming of the MRI study. We will attempt to connect ionic states with measured diffusion coefficients.

References

1. Chandrashekar, S.; Oparaji, O.; Yang, G.; Hallinan, D. Journal of the Electrochemical Society 2016, 163, (14), A2988-A2990.
2. Borodin, O.; Smith, G. D. Macromolecules 2006, 39, (4), 1620-1629.
3. Rey, I.; Lassegues, J.; Grondin, J.; Servant, L. Electrochimica Acta 1998, 43, (10-11), 1505-1510.