Perylene as Cathode for Magnesium Batteries

Abstract

Magnesium ion batteries (MIB) have attracted much attention from battery researchers around the globe. Magnesium is divalent in nature and offer a higher theoretical capacity than that of lithium. However, the magnesium research is still in the niche stage and the search continues for better electrolyte systems and for high voltage cathode materials. Currently, extensive research is being done in employing organic materials for battery cathode materials. Organic materials are made from naturally occurring compounds and are easy to dispose since they have no metals. Perylene diimide is an organic material gaining importance as cathode material in metal ion batteries.

The goal of the project is to determine the voltage window of perylene in lithium and magnesium battery systems. Cyclic Voltammetry (CV) is employed to measure the electrochemical activity of the cell. The output of the CV is a scan of the current versus the voltage. During the operation of the cell, duck shaped peaks are observed which correspond to the reduction/oxidation activity of the cathode and the anode respectively. The current corresponding to the peaks is used to determine the cathodic and anodic current of the cell. Once the voltage and the current are known, the area under the peaks is calculated to determine the
charge/discharge capacity of the cell.

Since no prior research was done on magnesium, the most common cathode material (inorganic), chevrel phase molybdenum sulphide is synthesized and tested. Research with the perylene as cathode material is started with lithium because lithium is being studied extensively in the research group. Tests with both the monomer and the polymer has been conducted against lithium and magnesium battery systems. The lithium cell employing perylene is optimized as much as possible and is shown to be electrochemically active. The lithium cell shows a redox voltage of 2.5V vs Li/Li+. In the magnesium system, perylene is active as small peaks are observed at 1.5V and 1.7V vs Mg/Mg2+. However, the cell fails to operate after the first charge. This is most likely due to the electrolyte forming a passive film on the surface of the anode. It is recommended to disassemble the magnesium cell after the first discharge cycle to observe the magnesiation on the cathode and the passive film formation on the anode.