A separator modified by high efficiency oxygen plasma for lithium ion batteries with superior performance

In order to improve the electrochemical performance of Li-ion batteries, especially at elevated temperatures, we, for the first time, modified the separator with the oxygen plasma method. Physical characterization including the contact angle measurement and Fourier transform infrared spectrometer analysis indicate that the O-2 plasma treatment can generate oxygen-containing groups on the surface of separator. The Li-ion battery using the O-2 plasma modified separator shows superior performance with a higher initial capacity (135.7 mA h g(-1)) and significantly improved cycling stability (maintaining 96.98% of its initial discharge capacity after 100 cycles), because of the presence of oxygen containing groups such as carbonyl and hydroxyl on the surface of the separator after the plasma modification. Moreover, when cycled at 1C, the discharge capacity of the Li-ion battery using a normal separator is only 86.7 mA h g(-1) whereas the Li-ion battery with the modified separator shows a much higher discharge capacity of 135.7 mA h g(-1), indicating an excellent rate performance due to the electrostatic repulsion between electronegative oxygen functional groups on the surface of modified separator. When tested at an elevated temperature (55 degrees C), the capacity retention of the cell using the modified separator is much higher (85.39%) than that of the normal separator (53.69%). All the tests show that the oxygen functional groups on the surface of the separator by the oxygen plasma modification play a critical role in improving the electrochemical performance of the Li-ion battery.