Coupled Transport in Li-ion Battery Electrolytes

Abstract

Electrolytes are materials that contain mobile charge carriers – ions. The electrolyte in lithium-ion batteries enables the transport of positive Li+ ions in between the electrodes during battery operation. The transport properties of the electrolyte determine how quickly and efficiently the batteries can be charged and discharged. Currently, lithium-ion batteries are the best rechargeable batteries available on the market. However, to enable electric aircraft, and long-distance transport by electric trucks, batteries with higher energy density that can be quickly charged are required. We believe that a fundamental understanding of the electrolyte transport properties is beneficial when developing new and better battery technologies.

In this work, we have studied the transport of common battery electrolytes using Molecular Dynamics (MD) simulations. We run these simulations on supercomputers. Molecular Dynamics is a simulation method that allows us to study the interactions and motions of atoms and molecules, and we can calculate transport properties that are relevant for battery operation. One such property is the lithium-ion transport number, which is how much of the total current inside the electrolyte is carried by the lithium ions. We want the lithium-ion transport number to be as high as possible, ideally 1. Lithium-ion transport numbers below one lead to build-up of concentration differences that steal energy from the battery.

Electrolytes for lithium-ion batteries are highly complex mixtures containing several components. The motions of the components influence each other during battery operation, and so-called coupling effects arise. There are coupling effects in between mass transport of the various components. In addition, there can also be coupling between temperature differences and battery performance. Using MD simulations, we have studied and quantified all these coupling effects and their effect on the battery performance, which is not done before. We find that the coupling effects are significant and must be considered for better understanding of batteries.