In recent years, polymer gel electrolyte has been widely investigated to improve the poor ionic conductivity of polymer electrolyte. Organic carbonate is the most common type liquid component in polymer gel which enhances ionic conductivity effectively. However, the use of organic liquid cause electrochemical and safety concerns. To overcome this obstacle, organic liquid was substituted by ionic liquid to form an ionic liquid polymer gel. It is becoming one of the promising electrolyte systems to enhance safety and reliability of lithium batteries. Although there are some limitations in using ionic liquid such as high viscosity, it shows several advantages over the conventional organic liquid polymer gel due to the specific property of ionic liquid such as low flammability, low volatili...[ Read more ]

In recent years, polymer gel electrolyte has been widely investigated to improve the poor ionic conductivity of polymer electrolyte. Organic carbonate is the most common type liquid component in polymer gel which enhances ionic conductivity effectively. However, the use of organic liquid cause electrochemical and safety concerns. To overcome this obstacle, organic liquid was substituted by ionic liquid to form an ionic liquid polymer gel. It is becoming one of the promising electrolyte systems to enhance safety and reliability of lithium batteries. Although there are some limitations in using ionic liquid such as high viscosity, it shows several advantages over the conventional organic liquid polymer gel due to the specific property of ionic liquid such as low flammability, low volatility and electrochemical stability.

In this thesis, composition of electrolyte system was optimized in terms of electrochemical property. Ionic polymer gel membrane is prepared by solution casting technique. N-N-butyl-N-methylpyrrolidinium, N,N bis(trifluoromethane) sulfonamide-lithium, N,N-bis(trifluoromethane) sulfonamide (Py₁₄TFSI-LiTFSI), ionic liquid solution with lithium salt was immobilized in poly(vinylidenefluoride)-hexafluoropropylene copolymer (PVdF-HFP) matrix with different compositions. In order to improve the ion transfer property, nano-scale SiO₂ particles were dispersed in the membrane to form composite ionic liquid polymer gel electrolyte. Effects of polymer/ionic liquid ratio, concentration of lithium salt and nano SiO₂ were investigated.

The optimal composition was found to be 70wt% Py₁₄TFSI-0.6MLiTFSL with 5wt% SiO₂. Ionic conductivity in the order of 10^-3 S/cm has been observed at room temperature with lithium ion transference number being 0.33. Besides, It is electrochemically stable up to 5.1V (vs Li/Li⁺) on both inert and active working electrodes which is high enough to work with high working voltage cathode materials. When applying the electrolyte membrane to LFP/Li cell, discharge capacity was 145mAh/g under 0.1C with efficiency close to 100%. Battery performance of electrolyte membrane on LMNO/Li battery was also evaluated under different temperature. Charging efficiency was improved up to 12% when comparing with conventional organic liquid electrolyte. It implies that the extent of irreversible electrochemical reaction under high temperature and high voltage is decreased by using composite ionic liquid polymer gel electrolyte. Discharge capacity under 1 C rate is also satisfactory at elevated temperature. These findings show that the composite ionic liquid polymer gel electrolyte is one of the promising candidates for high voltage lithium ion batteries.