Pmma Based Polymer Lithium-ion Battery Research

Name: Liu Lu (Materials Physics & Chemistry) Directed by Prof. Xie JingyingThis paper is focus on two kinds of PMMA-based gel type polymer Li-ion batteries: PMMA gel type through radical polymerization and PVDF-HFP/PMMA blended type.The coin type Liquid Lithium and Li-ion batteries are assembled, meanwhile their first charge-discharge property, cycle performance, rate discharge performance, overcharge property and ac impedance property are studied. Charge/discharge processes of the batteries are studied by differential capacity method. The liquid Li-ion batteries show discharge capacity (based on cathode active material) of 127mAh/g and efficiency of 81% during the first cycle, maintain 90% of the initial discharge capacity at 50 cycle upon the repeated charge/discharge at C/3 rate, and release discharge capacity of 118mAh/g at 2C rate (94% of discharge capacity at C/5 rate). During the overcharge process, Lithium dendrites begin to grow and cause some soft-short-circuits when the voltage is higher than 5.0V, at the same time the liquid electrolyte begins to decompose. The voltage will jump to 10V when the battery is charged to 7.8V.PMMA gel type Li-ion batteries are prepared by radical polymerization. The effects of the MMA monomer, initiator and methods of polymerization are studied. Initiator will reduce the cycle stability of the battery, but its effect can be eliminated by heat treatment. MMA monomer can reduce the discharge capacity (60mAh/g at first cycle) and impair the cycle performance of the battery (discharge capacity of 10mAh/g at 15th Cycle). Liquid electrolyte will reduce the efficiency of the initiator. Through mass polymerization and temperature elevating or the reaction time increase will improve the discharge capacity (90mAh/g at first cycle) and cycle performance (discharge capacity of 75mAh/g at 15th Cycle) of the battery.The PVDF-HFP/PMMA blended polymer electrolytes are fabricated and evaluated. The addition of PMMA will reduce the crystallinity of blends and improve of the uptake of liquid electrolyte (260%) and the ion conductivity (ImS/cm). At the same time the PVDF-HFP maintains the mechanical strength of blends film acting as polymer framework.The PVDF-HFP/PMMA blended gel type polymer Li-ion batteries are studied.The ratios of the PVDF-HFP and PMMA affect the performance of the batteries. The blended gel type batteries show properties as good as the liquid type: discharge capacity of 128mAh/g and efficiency of 83% during the first cycle, 91% of the initial discharge capacity at 50th cycle upon the repeated charge/discharge at C/3 rate, and releases discharge capacity of 105mAh/g at 2C rate (86% of discharge capacity at C/5 rate). The plateau of overcharge between 5.0V and 6.0V is prolonged, this indicates that the blended gel type polymer Li-ion batteries have better anti-overcharge property.