(389g) Considering Novel Transport Modes in Non-Aqueous Electrolytes

Jing Liu and Charles W. Monroe

Department of Chemical Engineering, University of Michigan

The characterization of Li-ion-battery electrolytes is often performed using symmetric electrochemical cells with lithium-metal electrodes. Properties are determined by experiments in which currents applied to these cells are controlled, while response voltages are recorded over time. Theoretical expressions are then used to fit the voltage responses, yielding transport properties such as diffusion coefficients and cation transference numbers, and thermodynamic properties such as Darken factors. It has been observed for a variety of concentrated electrolytic solutions that transport properties measured by standard methods do not always accurately predict the responses of cells subjected to high currents, or experiencing large concentration gradients. Voltage-relaxation data may be indicative of an elevated diffusion coefficient, the limiting current may be underpredicted, or cells may appear to have negative transference numbers [1,2]. These issues appear to be significant in non-aqueous electrolytic solutions.

Non-aqueous electrolytes behave differently than aqueous ones because organic salts tend to have very large partial molar volumes, which causes them to occupy large volume-fractions of the solutions containing them. Many discrepancies observed in Li-ion electrolyte characterization literature can be resolved by considering two novel transport modes that arise from solute-volume effects. One effect arises when concentration polarization induces density gradients in the solution, which induces volume redistribution through the solution’s equation of state. Another effect arises when interfacial electrochemical reactions induce volume flow throughout the electrochemical cell – a driving force for convection.

This discussion will focus on analytical approaches to the concentrated-solution multicomponent transport equations, which apply when solute-volume effects are significant. Dimensionless quantities that determine the importance of the ‘excluded-volume effect’ and ‘Faradaic convection’ will be addressed. The ramifications of the solute-volume effects on standard transport measurements will be described. In non-aqueous electrolytes solute-volume effects are particularly significant; their neglect can lead to models that under- or over-predict key characteristics of the cell (limiting current, Cottrell relaxation time) by ten percent or more.

References

[1] H. Hafezi and J. Newman, J. Electrochem. Soc. 147 (2000), 3036.

[2] M. Doeff, L. Edman, S. Sloop, J. Kerr, and L. De Jonghe, J. Power Sources 89 (2000) 227.