Abstract:

Graphene-wrapped natural spherical graphite (G/SG) composites were prepared using the encapsulation–carbonization approach. The morphology and structure of the composites were characterized by scanning electron microscopy and X-ray diffraction analysis. The electrochemical performance of the composites with different graphene contents as anode materials for lithium-ion batteries was investigated by various electrochemical techniques. In the absence of acetylene black (AB), the G/SG composites were found to exhibit high specific capacity with high first-cycle coulombic efficiency, good cycling stability, and high rate performance. Compared with the natural spherical graphite (SG) electrode, the G/SG composite electrode with 1% graphene exhibited higher reversible capacity after 50 cycles; this capacity performance was equal to that of the SG + 10%AB electrode. Moreover, when the addition of 2.5% graphene, the composite electrode exhibited higher initial charge capacity and reversible capacity during 50 cycles than the SG+10%AB electrode. The significant improvement of the electrochemical performance of the G/SG composite electrodes could be attributed to graphene wrapping. The graphene shell enhances the structural integrity of the natural SG particles during the lithiation and delithiation processes, further improving the cycling stability of the composites. Moreover, the bridging of adjacent SG particles allows the formation of a highly conductive network for electron transfer among SG particles. Graphene in the composites serves as not only an active material but also a conductive agent and promotes the improvement of electrochemical performance. When 5%AB was added, the reversible capacity of the 5%G/SG electrodes significantly increased from 381.1 to 404.5 mAh·g^-1 after 50 cycles at a rate of 50 mA·g^-1 and from 82.5 to 101.9 mAh·g^-1 at 1 A·g^-1, suggesting that AB addition improves the performance of the G/SG composite electrodes. AB particles connect to G/SG particles through point contact type and fill the gaps between G/SG. A more effective conductive network is synergistically formed via graphene-AB connection. Although graphene wrapping and AB addition improve the performance of natural graphite electrodes, such as through increase in electrical conductivity and enhancement of Li-storage performance, including improvement of reversible capacity, rate performance, and cycling stability, electrode density typically decreases with graphene or AB addition, which should consider the balance between the gravimetric and volumetric capacities of graphite anode materials in practical applications. These results have great significance for expanding the commercial application scope of natural graphite. Our work provides new understanding and insight into the electrochemical behavior of natural SG electrodes in lithium-ion batteries and is helpful for the fabrication of high-performance anode materials.

Key words: Graphene, Graphene-wrapped, Natural spherical graphite, Lithium-ion battery, Anode material, Conductive agent, Acetylene black