Consequences of Dynamic Electrolyte Behavior during Fast-Charging of Li-Ion Systems

Abstract

The choice of electrolyte is a chief consideration for fast-charge architectures. This is challenging due to parallel requirements of high performance and acceptable battery life. A core issue in fast charging of Li-ion batteries is the dynamic of concentration polarization (CP), where for charge conditions there develops ionic depletion at the anode and ionic enrichment at the cathode. The magnitude and persistence of CP are key factors in battery limitations at high charge rates, as they influence the extent of impedance increase within the electrolyte and double-layer regions in direct proximity to electrode surfaces. CP will have profound impact on local electrolyte properties, especially within the double-layers where viscosity, conductivity and diffusivity can vary appreciably with bulk values.This work employs a molecular-based chemical physics model called the Advanced Electrolyte Model (AEM) to determine over 85 property metrics for multi-component electrolytes. AEM has been used within the U. S. DOE XCEL program to screen and characterize electrolytes intended for 10-minute charging of Li-ion cells [1]. This presentation will focus on how local electrolyte properties change due to CP across a cell, and how ionic velocities can vary accordingly to produce transit times that decrease with charge time. Parallel to this is solvent migration across a cell due to faradaic co-transport of solvent that is involved with lithium ion solvation. Thus, relaxation of CP following charge or discharge must involve a dissipation toward equilibrium of ionic and solvent gradients. Electrolyte design should consider solvents that have self-diffusion rates that are equal to or higher than that of solvated ions so that solvent gradients do not become rate limiting to CP relaxation. Within these concepts, comparisons will be made between a baseline electrolyte (e.g., EC-EMC-LiPF6) and those designed for fast charge conditions. Ningshengjie Gao, Sangwook Kim, Parameswara Chinnam, Eric J. Dufek, Andrew M. Colclasure, Andrew Jansen, Seoung-Bum Son, Ira Bloom, Alison Dunlop, Stephen Trask, and Kevin L. Gering*, “Methodologies for Design, Characterization and Testing of Electrolytes that Enable Extreme Fast Charging of Lithium-ion Cells,” Submitted for publication.