Advanced Design of Lifsi-Based Electrolyte for High Performance Li-Ion Battery

Abstract

In recent years, Li-ion batteries have come to be used for high power applications such as automotive, and thus their charge and discharge rate performance is of crucial importance. There are many factors that affect to charge and discharge rate of Li-ion batteries and the most important factor varies according to ambient and operating conditions. However, the rate-determining process of charge-discharge reaction has been considered de-solvation process of ethylene carbonate (EC) molecules from lithium ion. This occurs at an electrode/electrolyte interface accompanied by lithium ion intercalation into active material [1]. That rate-determining process is certainly true for most conditions because lithium ion diffusion in active material particles as well as lithium ion conduction in conventional electrolytes are sufficiently fast for the required charge and discharge rate. Recently, we achieved significant reduction of a reaction resistance at the de-solvation process of EC molecule from lithium ion by a novel electrolyte design focusing on the solvation state of lithium ion. A lithium ion is solvated by three EC and one linear carbonate molecules in conventional electrolytes containing large amount (30 – 50 vol.%) of EC [2]. Although the solvation number of EC to lithium ion should be reduced as much as possible to improve charge and discharge rate performance, a simple decrease in EC content causes decreasing dissociation degree of lithium salts. Therefore, we focused on the interaction between lithium ions and counter anions. In our previous study on an ionic liquid electrolyte composed of 1-ethyl-3-methylimidazorium bis(fluorosulfonyl)imide (EMImFSI) and LiFSI, we already understood a unique feature that the interaction between lithium ion and FSI anion is quite weak [3]. Based on this knowledge, we solved such a dissociation degree issue of lithium salts in low EC content-based binary solvents by applying LiFSI as a lithium salt. The LiFSI-based low EC content electrolyte developed in our previous study had low solvation number of EC to lithium ion while maintaining high ionic conductivity comparable to conventional electrolytes. Moreover, we successfully improved charge and discharge rate performance of Li-ion batteries drastically. In the present work, we optimize our novel electrolyte to respond to demand of further high rate performance by a different perspective to solvation state of lithium ion. In addition, we investigate the low temperature (< −20°C) and high temperature (> 60°C) operation performance of our novel electrolyte and contrive suitable electrolyte designs for respective operating conditions. As described above, we can customize a LiFSI-based low EC content electrolyte to specific needs. However, our electrolytes have a big problem that aluminum current collector is easily corroded by FSI anions at a high potential (> 4.0 V vs. Li/Li+) and we have not found an effective solution to suppress the aluminum corrosion yet. Since aluminum corrosion is a fatal problem for practical application of our novel electrolytes, we must solve this problem. In this presentation, we introduce further advanced designs of LiFSI-based electrolyte that can be applied to a graphite / LiNi1/3Mn1/3Co1/3O2 cell.