Validating Concentrated Lithium-Ion Electrolyte Transport Models with in-Situ MRI

Abstract

Spatiotemporal understanding of chemical species concentrations and electrochemical potentials, on both micro- and macroscopic scales, is vital to the enhancement of lithium-ion and lithium-metal battery performance. To understand transport in a concentrated binary electrolyte system comprising lithium hexafluorophosphate in ethyl methyl carbonate (LiPF6:EMC), we compared the responses of the concentration distribution and cell voltage captured with in situ MRI to simulations based on concentrated-solution transport theory. The transport simulations were parameterized using a suite of indirect thermodynamic, voltammetric, and rheometric measurements, including measurements of liquid-junction potentials, restricted-diffusion and Hittorf experiments, conductometry, impedance spectroscopy, and densitometry [1]. Close agreement between the simulations and experiment (see fig 1) suggests that the local states within super-concentrated electrolytes under applied currents can be predicted using a multicomponent transport theory that accounts for pairwise diffusional interactions, thermodynamic activity, and solution-volume effects [2]. The exceptionally close agreement between the measured and simulated transient states during the relaxation step of pulse-relaxation experiments suggests that simulation results can be used to extract kinetic overpotentials from experimental cell voltages during the pulse step. Experimental data is used to elucidate charge-transfer kinetics at lithium-metal electrode interfaces. Moving beyond binary electrolytes, we will also discuss the validity of pseudo-binary assumptions used to model current state-of-the-art co-solvated electrolytes, wherein preferential ion solvation by one of the solvents may impact salt activity [3].[1] Wang, A et al, Shifting-Reference Concentration Cells to Refine Composition-Dependent Transport Characterization of Binary Lithium-Ion Electrolytes. Electrochim. Acta (2020)[2] Liu, J.; Monroe, C. W. Solute-Volume Effects in Electrolyte Transport. Electrochim. Acta (2014)[3] O. Borodin et al., Competitive lithium solvation of linear and cyclic carbonates from quantum chemistry. Phys Chem Chem Phys (2016) Figure 1