Graphene/nanosized silicon composites for lithium battery anodes with improved cycling stability

Abstract

Graphene/nanosized silicon composites were prepared and used for lithium battery anodes. Two types of graphene samples were used and their composites with nanosized silicon were prepared in different ways. In the first method, graphene oxide (GO) and nanosized silicon particles were homogeneously mixed in aqueous solution and then the dry samples were annealed at 500 °C to give thermally reduced GO and nanosized silicon composites. In the second method, the graphene sample was prepared by fast heat treatment of expandable graphite at 1050 °C and the graphene/nanosized silicon composites were then prepared by mechanical blending. In both cases, homogeneous composites were formed and the presence of graphene in the composites has been proved to effectively enhance the cycling stability of silicon anode in the lithium-ion batteries. The significant enhancement on cycling stability could be ascribed to the high conductivity of the graphene materials and absorption of volume changes of silicon by graphene sheets during the lithiation/delithiation process. In particular, the composites using thermally expanded graphite exhibited not only more excellent cycling performance, but also higher specific capacity of 2753 mAh/g because the graphene sheets prepared by this method have fewer structural defects than thermally reduced GO.

Introduction

To meet requirement for electric vehicles and renewable sources of energy, the state-of-the-art energy storage and conversion devices should be endowed with higher energy density and power density, and longer lifespan [1]. Lithium-ion batteries are now widely used as energy storage devices for portable electronic devices, and they are also very attractive for power tools, electric vehicles and renewable sources of energy. Current dominant electrodes based on intercalation reactions have the limitations in the specific charge storage capacity, even though they have the advantages of small structure and volume changes and fast Li⁺ transfer rate [2]. Recently, Si has attracted much attention as a replacement of graphite (372 mAh/g), due to its high theoretical specific capacity of about 4200 mAh/g corresponding to the formation of Li₂₂Si₅ alloy [3], [4]. The alloying process is accompanied with a huge volume expansion (about 400%) and significant structure stress, which cause cracking and pulverization of the Si anode. As a result, electrical contact becomes worse and worse with cycling and eventual capacity fades dramatically. To fight against the volume change, many approaches have been considered, such as reducing the Si particle size to dozens of or even several nanometer [5], [6], [7], preparing Si/C composite by dispersing the Si particles into a carbon matrix [8], [9], and improving the structure of the electrode by using special binder or abundant conductive additives [10], [11]. All efforts were made to buffer the volume change by introducing plenty of free spaces or enhancing the linkage of Si particles.

Recently, graphene with superior electrical conductivities, high surface areas of over 2600 m²/g, excellent thermal property and mechanical property, has attracted much attention in the field of materials science [12], [13], [14], [15]. Perfect graphene is one-atom-thick two-dimensional layers of sp²-hybridized carbon. Graphene materials can be prepared in many ways. The most popular way is to prepare graphene oxide (GO) first and then obtain graphene sheets by chemical reduction or thermal reduction [16], [17], [18]. Another economic way to get large-scale graphene-based materials is to thermally expand the expandable graphite at a very high heating rate to about 1000 °C [19], [20]. Usually, the time for preparation must be short enough to avoid aggregation and graphitization under so high temperature. In addition, the graphene sheets also could be obtained by exfoliating graphite directly via mechanical or electrochemical routes [21], [22], or via bottom-up routes, e.g. epitaxial growth [23], [24], [25], chemical vapor deposition and solvothermal method [26], [27], [28]. The properties of graphene sheets usually depend on the route for their preparation. As a novel anode material for the lithium-ion batteries, graphene sheets mostly exhibit a higher reversible capacity than graphite [16], [29], [30]. But the cycling performance is not as good as graphite when the graphene sheets are prepared by chemical reduction of GO, which is mainly due to facile stacking of graphene sheets and severe side reactions between graphene and the nonaqueous electrolyte arising from its high surface area and the existence of many defects. However, the graphene sheets prepared by thermal reduction of GO, exhibited higher specific capacity and improved cycling performance [31], [32], [33]. Definitely, the superior electrical conductivity as well as high surface area makes it very promising to prepare some composite materials with excellent performance [34], [35], [36], [37]. There are at least two advantages for the graphene-based composites. One is the improved electronic conductivity of the electrode system where graphene plays the role of a highly efficient conductive additive. The other is that the rest in the composite can effectively prevent graphene from stacking and also depress the side reactions between graphene and the electrolyte, which is promising to improve the cycling performance of the anode. Therefore, the graphene-based composites are welcome and very promising for the advanced lithium-ion batteries.

Chou and his co-workers prepared graphene/nanosized Si composite by mixing nanosized Si particles with graphene synthesized via a solvothermal method in a mortar, and demonstrated that graphene/nanosized Si composite could accommodate large volume change of Si and maintain good electronic contact by graphene [38]. But it is difficult to form a homogeneous composite just by mortar, which is believed as a key issue for a composite to exhibit good performances. Recently, Lee and his co-workers reported the graphene/nanosized Si paper composite as anode for lithium-ion batteries [39]. But from the X-ray diffraction (XRD) results, the sharp peak at 26.4° indicated that there was a large amount of graphite in the composite paper, which could be a possible reason for the poor cycling performance. Since the properties of graphene materials prepared via different ways usually have distinct difference, the performance of the graphene-based composites should also strongly depend on the preparation route of graphene. Here in our investigation, graphene/nanosized Si composites were prepared via two facile methods with different graphene structures. In the first method, stable GO suspension was used to prepare homogeneous graphene/nanosized Si composites, and the existence of –OH on the surface of Si particles is helpful to form the stable composite by chemical interactions with GO. The GO/nanosized Si composites were then treated at 500 °C to give thermally reduced GO/nanosized Si composites. In the second method, the graphene sheets were obtained from thermal expansion of expandable graphite at 1050 °C. The expandable graphite is graphite intercalated compound along with slight oxidation, and the obtained graphene sheets are flat and smooth, not like that with many ripples derived from GO. Thus the graphene sheets from the expandable graphite are believed as more excellent conductive components for the graphene/nanosized Si composites which can be prepared by simple mechanical blending. The effect of the graphene sheets with different structures on the cell performance of graphene/nanosized Si in the lithium-ion batteries was investigated.