SURPRISING CHEMISTRY IN LI-ION CELLS

Abstract

High voltage LiNixMnyCo(1-x-y)O2 (NMC) cells with conventional ethylene carbonate-based electrolytes do not have good charge-discharge capacity retention above 4.3 V. It is therefore important to investigate the failure of the NMC cells at high voltage. In this thesis, lithiated graphite and delithiated NMC electrode materials taken from high voltage NMC/graphite full cells were studied individually using the pouch bag method and thermogravimetric analysis coupled with mass spectrometry (TGA-MS). The results obtained from the pouch bag method show that charged NMC positive electrodes stored at elevated temperature in pouch bags create more gas than the same electrodes stored in pouch full cells at the same temperature. Surprisingly, the charged NMC electrodes removed from pouch bags after storage at elevated temperature had larger impedance than those removed from pouch full cells. These results suggest that electrolyte oxidation at the positive electrode mainly contributes to gas generation and impedance growth in Li-ion cells. The results also show that the use of an Al2O3 surface coating and fluorinated electrolyte can hinder impedance growth at the positive electrode. The results obtained from the TGA-MS experiments show that oxygen can be released from charged NMC electrodes at high voltage at mild temperature. The results show that the amount of oxygen released can be greatly limited by utilizing larger NMC particles. These results provide new support that the released oxygen from charged NMC electrodes causes electrolyte oxidation resulting in cell failure. In this thesis, the effects of different salt concentrations in ethylene carbonate (EC)-free electrolytes on cell performance and on their physical properties were also studied. The results show that 1.5 M salt concentration should be used to improve overall cell performance. Unexpectedly, at low salt concentrations below 0.4 M, the impedance of cells increased greatly. It was found that this result was due to the formation of contact ion pairs in the electrolyte. The results obtained from studies of the physical properties of EC-free electrolytes suggest that limited amount of dimethyl carbonate (a widely used linear carbonate) should be used in conventional electrolytes to improve low temperature cell performance.