Enhanced electrolyte and electrode for li-o2 battery discharge

• Main Author: Wijaya, Olivia
• Other Authors: Jason Xu Zhichuan
• Format: Theses and Dissertations
• Language: English
• Published: 2016
• Subjects: DRNTU::Engineering::Materials::Energy materials; DRNTU::Science::Chemistry::Physical chemistry::Electrochemistry; DRNTU::Science::Chemistry::Organic chemistry::Organic electrochemistry; DRNTU::Science::Chemistry::Physical chemistry::Catalysis; DRNTU::Engineering::Materials::Material testing and characterization
• Online Access: http://hdl.handle.net/10356/68862
• Institution: Nanyang Technological University

High energy storage demand from electrical vehicle (target set by U.S Advanced Battery Consortium: 350 Whkg-1), has sparked the interest to investigate battery system beyond the Li-ion batteries, which is projected to reach their highest energy density of only 250 Whkg-1. Li-O2 battery is one of such system that promises high theoretical energy density of 3460 – 3860 Whkg-1, ten times higher compared to the theoretical energy density of currently available Li-ion battery, LiCoO2 387 Whkg-1. Despite the potential of high theoretical energy density, Li-O2 battery has been reported to suffer from several limitations such as poor cycle efficiency, poor rate capability, and low cycle life. This thesis aims to investigate the factors affecting the discharge of Li-O2 battery and suggests ways to improve the battery performance using cathode and electrolyte modification. Based on the investigation of Li-O2 battery discharged at various discharge rates, the Li-O2 battery performance was found to be strongly correlated with the discharge history. Initial discharge at high rates will result in severely lowered subsequent discharge. This is caused by the growth of resistive Li2O2 layers at high discharge rates, most likely due to O2 deficiency. To address the issue of O2 deficiency in the electrolyte, fluorocarbon additives, known for their high O2 solubility are incorporated in the electrolyte of Li-O2 battery. Certain type of fluorocarbon additive, a gamma fluorinated ether (TE4), was identified to have a reasonable miscibility with organic solvent and Li salts, relatively stable during discharge, improve the O2 solubility up to four times compared to tetraglyme, and enhanced the discharge capacity up to ten times at a high discharge rate of 400 mAhgc-1. Electrolytes with improved O2 mass transport, such as an electrolyte using fluorocarbon additive, make it possible to use a cathode material with high activity towards ORR to construct a Li-O2 battery with a high discharge rate. For plain carbon electrodes, defect sites including edges and functional groups on the surface have been reported to catalytically promote ORR in various electrolytes and redox systems. In this thesis, carbon black and carbon nanofibers with varying degree of defects were investigated. The electrochemical experiments consistently showed that graphitic carbon had a lower ORR kinetic, more negative onset, and lower specific current than carbons with higher defect densities. This thesis also shows that at the low discharge rate, the effect of defective carbon (Acetylene Black) electrode was more significant than the effect of the fluorocarbon additive (TE4) in increasing the energy density. At a high discharge rate, the TE4 additive was more effective than defective carbon electrodes at enhancing the power density and energy density, in agreement with the high O2 solubility capability of the fluorocarbon additive. Meanwhile, the highest power density obtained by the cell, about 908 Whkgc-1, was obtained using the AB cathode and 20 vol % TE4 additive discharged at 400 mAgc-1.