Control of number of graphene layers using ultrasound in supercritical CO2 and their application in lithium-ion batteries

• Preparation of graphene by ultrasonic in supercritical CO2 is developed.
• Single- and few-layer graphene sheets are exfoliated from graphite directly.
• Ultrasonic power affects the number of graphene layers and their lateral sizes.
• As-exfoliated graphenes heighten electrochemical performance of LiFePO4 cathode.
• The specific capacity of the LiFePO4/graphene composite cathode achieves 160 mAh/g.

This study reports a novel strategy using ultrasound in supercritical CO2 for exfoliating graphite directly into single and few-layer graphene sheets. The mutually complementary characterizations of the as-exfoliated samples via atomic force microscopy, transmission electron microscopy and Raman spectroscopy indicate that the ultrasonic power greatly affects the number of layers and the lateral size of the graphene. Single-layer graphene with a lateral size of 50–100 nm and two-layer graphene with a lateral size of 0.5–10 μm are obtained using an ultrasonic power of 300 and 120 W, respectively. As-exfoliated graphene sheets heighten the electrochemical performance of LiFePO4 cathode materials, demonstrating graphene's remarkable electrical conductivity. The specific capacity of the LiFePO4/graphene composite cathode achieves 160 mAh/g and displays stable cycling for more than 15 cycles. This technique will enable cost-effective mass production of graphene sheets with good quality, and the as-exfoliated graphene will find wide applications, including lithium-ion batteries.

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