Ionic Liquid Based Electrolyte Systems Enabling Operation of High Voltage Cathode Materials

Lithium batteries are becoming increasingly important in hybrid and electric vehicle applications. The common cathode materials for these batteries, when based on metal oxides, are reaching their limits in terms of gravimetric capacity (mAh/g). In an effort to push the boundaries in energy density (Wh/kg) for these devices, there is an increasing push to higher voltage cathode materials such as LiMn_0.33Ni_0.33Co_0.33O₂ and the LiNi_0.5Mn_1.5O₄ type materials, where the highest capacities of the materials are accessed at potentials greater than 4.6 V vs. Li|Li⁺. With this push for higher energy and higher voltage there is a limit to the types of electrolytes that are compatible with these materials and allow for the higher operating voltages. Standard battery electrolytes such as (1.2 M LiPF₆ in EC/DMC) are well known to be unstable at these high voltages as well allowing severe Aluminium current collector corrosion. A key enabler to increasing the energy density of a lithium battery is an improved electrolyte.

At CSIRO, we have been at the forefront of developing new ionic liquid based electrolytes to enable the next generation of high voltage electrodes. The key factors to consider besides essential properties such as solution conductivity, Lithium ion diffusivity, viscosity, etc are the electrochemical windows of these solvents (i.e. stability at high cathodic voltages), the stability of the Aluminium current collector (i.e. limited corrosion in the solvent) and the compatibility of the high voltage cathode materials in these electrolytes (i.e. limited solubility of charge/discharge products in the solvent).  At this time, there is a significant effort to identify solvents (blends), lithium salts and / or additives that can stabilize the cathode in contact with the electrolyte and preclude Aluminium current collector corrosion. 

Specifically described in this work is our progress in developing ionic liquids electrolytes as well as blends of these with other solvents and additives for high voltage cathode applications. SECM characterisation to detect cathode dissolution and current collector corrosion has been carried out, as well as long term cycling of lithium cells incorporating the high voltage cathodes.