Electrolyte Studies for High Voltage Lithium Ion Batteries

Abstract

Extensive research on next generation lithium ion batteries, specifically batteries operating at high voltage (>4.4V), is currently underway. There are several issues that need to be addressed in order to produce a viable product. For example, graphite anodes are used in current state of the art batteries but silicon-based anodes offer higher energy density. Also, cathode materials that are electrochemically active at high voltages are available, e.g. LiNi0.5Mn1.5O4 (LNMO) @ 4.7V and LiCoPO4 (LCP) @ 4.8V. Unfortunately, batteries composed of these materials with the state of the art electrolyte suffer from poor cycle life. New electrolytes are essential for the operation of next generation high voltage lithium ion batteries. This research focused on the electrolyte for LNMO-graphite based batteries. The importance of not only the high voltage stability of the electrolyte but also the low voltage stability will be discussed. Cyclic voltammetry (CV) on glassy carbon electrodes allow for the investigation of both the oxidative and reductive stability of the electrolytes. Cycle life data, Coulombic efficiency, and fade rate data was collected from LNMO-graphite coin cells. Differential capacity (dQ/dV) vs. V plots were also constructed from the coin cell data to show how the electrochemical behavior changed over time. The dQ/dV vs V data for the state of the art electrolyte show a buildup of impedance over time which was also evidenced via electrochemical impedance spectroscopy (EIS). Several different lithium salts and additives were investigated in this study. The results suggest that while the oxidative stability of the electrolyte is important, the reductive stability must not be ignored.