Insights into the conversion behavior of SiO-C hybrid with pre-treated graphite as anodes for Li-ion batteries
Graphical abstract
Introduction
Portable electronics, electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV) generate rapid increasing market demands for high-energy-density and low-cost lithium-ion batteries (LIBs) [1]. Nevertheless, the commercially used graphite anode materials with low theoretical capacity (372 mA h g−1) limit the development of high-performance lithium-ion batteries. Silicon has been considered as a promising anode material for lithium-ion batteries owing to its high specific capacity (4200 mA h g−1) and ideal potential for lithium insertion/extraction (<0.5 V vs.Li+/Li) [2], [3]. However, two drawbacks of silicon are apparent: the low intrinsic electric conductivity and large volume expansion (>300%) during lithium alloying-dealloying. The large volume expansion can lead to the damage on structural integrity and fast decay of specific capacity [4], [5], [6]. Solid SiO is considered as a promising candidate for high-performance Si-based anode materials, because it combines the advantages of high specific capacity and better cycling stability comparing with silicon [7]. The reaction between SiO and lithium in the initial discharge process produces Li2O and Li4SiO4 matrix, which could relieve the huge volume changes of Si during lithiation and delithiation [8], [9].
Several attempts about the combination of SiO and carbon were reported to improve the cyclic performance of SiO, because carbonaceous materials can significantly relieve the huge volume expansion while functioning as conducting medium. Doh et al. have found that SiO-C composite, synthesized through directly ball milling of SiO and graphite powders, delivers a reversible charge capacity of 688 mA h g−1 at 30 cycles [10]. This demonstrates that the addition of graphite greatly improves the electrochemical performance of SiO. Other carbonaceous materials (such as carbon nanofiber [11], amorphous carbon [12], carbon nanotubes [13] and graphene [14]) were also employed to improve the cyclic performance of SiO anodes. In addition, Morita et al. and Lu et al. reported SiO/graphite/amorphous carbon composites with high reversible capacity (∼700 mA h g−1 at 100th cycle), where Si particles are generated by using the disproportionation of silicon mono-oxide during high temperature heat-treatment (>850 °C) [15], [16]. Although they deliver high reversible capacity, the disadvantage of their low initial coulombic efficiency is obvious (∼65%), which hinders the practical application. From an application perspective, several critical factors must be considered: high specific capacity, cyclic performance, initial coulombic efficiency, easy preparation and low cost.
In this work, we prepared SiO-C composite via high energy ball-milling (HEBM) and heat treatment process, using inexpensive SiO powder, graphite and glucose as starting material. The added graphite is to improve the electrical conductivity and the amorphous carbon is to achieve a better effect of coating or to prevent the direct contact between SiO particles and the electrolyte. Two types of carbon are used aiming to get a synergistic effect. It is found that the pretreatment of graphite by ball-milling can greatly enhance the electrode performance of the SiO-C hybrid anode. The highly enhanced electrochemical performance of SiO-C hybrid can be ascribed to the uniform mixing of the components and good effect of carbon coating. The preparation process is so simple that it can be adaptive for industrial applications. Insights into the conversion behavior of SiO in the first cycle are provided by HRTEM: scattered amorphous silicon particles with size about 5 nm and irreversible Li4SiO4 as the detected resultants.
Section snippets
Preparation of SiO-C composite
The used graphite was obtained by ball-milling of commercially available graphite at a rotational speed of 400 rpm for 10 h. The mixture of SiO powders and ball-milled graphite was treated by HEBM at a rotational speed of 400 rpm for 15 h. Then glucose was added for further blend by ball-milling at a rotational speed of 400 rpm for 5 h. The mass ratios of SiO/ball-milled graphite/glucose were set to 2/2/5(approximately 2/2/1 for SiO/graphite/amorphous carbon considering the ratio of carbonization of
Structure characterization
The X-ray diffraction (XRD) curve of the SiO-C composite is displayed in Fig. 1a. The strong diffraction peaks can be indexed to graphite (JPCDS No. 41-1487) and the broad bands from 20° to 30° can be assignable to amorphous SiO and carbon [17], [18]. It is worth noting that no peaks corresponding to Si are detected, suggesting that ball milling and heat treatment do not cause the disproportionation of SiO. Raman technique has been used to gain detailed information regarding the composition of
Conclusions
In summary, an effective and low-cost method for the synthesis of SiO-C composite is demonstrated via the techniques of ball-milling and calcination, which have been widely applied in industry. The SiO-C composite exhibits high capacity, relatively high initial coulombic efficiency, excellent cycling stability and stable power rate as anode materials for LIBs. The superior electrochemical performance is ascribed to evenly dispersion of the components, good effect of carbon coating, and Li4SiO4
Acknowledgements
This work was supported by the Program of China (2011AA11A255), Natural Science Foundation of Tianjin, China (13JCZDJC32000) and the MOE Innovation Team (IRT13022).
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