Microwave-assisted synthesis of graphene nanocomposites: recent developments on lithium-ion batteries

Introduction

With the development of electrical energy-storage materials to meet the increasing demand for the ever-growing energy consumption, lithium-ion battery (LIB), with excellence in terms of high energy density, no memory effect, long cycle life, and environmental friendliness, has been an attractive power source for portable mobile devices and stationary energy storage.^1–4 As the commercial anode for LIBs, graphite gradually cannot satisfy the requirements of the ever-growing advanced high-power LIBs and new anode materials with higher energy density and power density must be explored.^5–8 Since the discovery in 2004, graphene has attracted significant research concerns for various energy-relative applications including LIBs,^9–11 solar cells,^12–14 supercapacitors,^15–17 and fuel cells.^18–21 This is largely ascribed to its intriguing properties associated with the unique single-atom layered structure such as a large theoretical specific surface area of 2,600 m² g⁻¹,²² highly flexible but robust mechanical structure, and fast electronic conduction. As an anode for LIBs, graphene was reported with a reversible capacity of ~400–1,100 mAh g⁻¹,^23–26 and its unsatisfactory cyclability has been mainly ascribed to its heavy agglomeration during cycling, which leads to the loss of promising properties relative to the atomic-thickness structure. An effective strategy is to introduce the secondary phase to the graphene, which acts as a spacer to separate few-layer graphene nanosheets (GNS) and prevent their restacking to graphite platelets.^27–29 The introduced component is usually also highly active for lithium-ion storage, and therefore, there is no capacity loss for the resultant composites. Moreover, the presence of graphene is also very beneficial for the improvement of the electrical conductivity and mechanical stability to the introduced secondary phase. As a result, graphene-supported composites can exhibit synergetic effect with respect to lithium-ion storage properties and achieve better electrochemical performance, especially long cycle life and impressive high-rate capability.³⁰

GNS used for fabrication of lithium ion anode materials were mostly synthesized through the chemical oxidation and reduction approach. The exfoliation and reduction of graphene oxide (GO) is the most important step to obtain GNS with suitable functionalities and reduction extents, which strongly affect the electrochemical properties of graphene or graphene-based composite electrodes. However, the exfoliation and reduction of GO rely heavily on the usage of strong reducing agents or a very high temperature. Most of these approaches are complicated, energy, and cost intensive. Recently, microwave exfoliation has been proved to be an attractive method for graphene preparation^31–33 because it is a facile, time-efficient, and cost-effective process. Besides, the obtained graphene via microwave-assisted methods can exhibit larger average size, higher quality with residual functional groups, and better electrochemical properties for LIBs compared with those prepared from conventional methods.^34–36

Microwaves have been widely used in industrial applications such as food processing^37,38 and industrial materials.^39,40 Motivated by its advantages of facile, fast, secure, controllable, and energy-saving characteristics, microwave-assisted technique has achieved rapid development in the field of materials science.^41,42 Microwave-assisted techniques such as solid-state microwave irradiation, microwave-assisted solvothermal/hydrothermal process can provide simple and fast routes to synthesize nanomaterials without high temperature or high pressure. Furthermore, the microwave technique is particularly useful for a large-scale synthesis without complicated preparation conditions.^43–46 The rapid transfer of energy and fast decomposition of the precursors provided by microwave source would result in highly effective local reaction temperatures and significant enhancement in reaction rates. Besides, the microwave technique can provide an effective way to control particle size distribution and macroscopic morphology during the synthesis process because it can heat a substance uniformly and therefore a more homogeneous nucleation environment and a shorter crystallization time can be achieved compared to conventional heating.

In this review, we aim to investigate the mechanism of microwave-assisted syntheses of graphene and graphene-based nanocomposites, and summarize the recent development of graphene-supported nanocomposites for applications as electrodes for LIBs. Various types of graphene-based nanomaterials: mainly graphene-based transitional metal oxide anodes, tin/germanium/silicon based anodes, metal sulfides anodes, lithium-titanium-oxide-based anodes and graphene-decorated lithium iron phosphate cathodes are presented and discussed with respect to their morphological and size control in the microwave-assisted preparation process and their relation to the resultant lithium storage properties.

Mechanism of microwave-assisted syntheses

Microwave irradiation is an electromagnetic irradiation in the range of wavelengths from 0.01 m to 1 m with corresponding frequency range from 300 MHz to 300 GHz.⁴⁷ The domestic microwave generally owns a frequency of 2.45 GHz (a wavelength of 12.25 cm), while the industrial microwave usually owns two frequencies of 915 MHz and 2.45 GHz.³⁷ Microwave has been widely used for heating those materials, which can absorb microwave energy and convert it into heat. In the presence of moisture or water, dielectric heating happens due to the dipolar nature of water. These permanently polarized dipolar molecules could rearrange in the direction of the electric field at a high speed, which would cause internal friction of molecules and further result in the volumetric heating of the whole material. Besides the dipolar mechanism, microwave heating may also occur due to the ionic mechanism, and oscillatory migration of ions in the material would generate heat under a high-frequency oscillating electric field.⁴⁸ Consequently, microwave-assisted technology can provide a fast and effective approach to heat the material/system homogeneously from the interior. In contrast, traditional heating system, in which heat is transferred from the surface toward the center of the material under the help of heating mantle, water/oil bath or other external heat source, is relatively slow and inefficient.

GNS are usually obtained from graphite or GO, which is prepared by a modified Hummer’s method.^49–51 The reduction of GO is usually carried out by chemical methods in the presence of various hazardous reduction agents such as hydrazine and NaBH₄. In comparison, thermal treatment is a green method because no hazardous reduction agents are used. Instead of the conventional preparation of graphene in traditional heating system (furnace or oil bath), the eco-friendly microwave-assisted method has attracted increasing attentions in which the microwave-assisted solvothermal/hydrothermal methods can be adopted to treat GO^52,53 or natural graphite^54–57 in a microwave oven or microwave plasma-enhanced chemical vapor deposition (MPCVD) system.^58–60 It is worth noting that the microwave exfoliation is an attractive and effective method for graphene synthesis from GO, in which GO is exfoliated with nontoxic solvents within a short reaction time of 1–15 minutes at a relatively low temperature range of 180°C–300°C.^52,53 It was reported that the stable graphene suspension could be obtained from the GO suspension in an alkaline medium (pH ≈ 10) or polar solvents (N,N-dimethylformamide, ethanol, 1-butanol, and water) in a facile microwave-assisted solvothermal process.^61,62 Besides, the water-soluble polymer-grafted graphene sheets were prepared from GO in a household microwave oven at a power of 450 W for 4 minutes.⁶³ The synthesis of three-dimensional (3D) nanostructure of “graphene nano-cup” anchored on the few layered graphene substrate⁶⁴ under the microwave irradiation in a domestic microwave oven was reported by two steps: one-pot synthesis of graphene-coated metal nanoparticles anchored on the graphene sheets and the subsequent etching of metals. Furthermore, giant graphene sheets could be obtained by double microwave-assisted exfoliation of expandable graphite⁶⁵ and highly hydrogenated graphene could be produced from GO by a one-step microwave irradiation process in hydrogen plasma, in which the deoxidation and concurrent hydrogenation were both achieved.⁶⁶ A possible mechanism of graphene preparation by a microwave-assisted technology is illuminated in Figure 1. The microwave irradiation provides high local temperature and pressure atmosphere, and energy is transferred directly into the GO interior. Heat is produced from the interaction of irradiation with the polar bond of oxygen-containing functional groups on the surface and edge of GO sheets. Besides, the interaction between polar solvent and surface oxides on GO sheets is an important factor to determine the uniformity of deposits. Furthermore, the functional groups on the surface of GO are effectively reduced, and the reduction degree of graphene sheets is further improved.

Figure 1 Schematic illustration of the synthesis of graphene and graphene-based composites with the assistance of microwave irradiation.

The microwave-assisted technology for the synthesis of graphene has several obvious advantages. First, the microwave-assisted process is time-efficient without complicated synthesis procedure. Second, such process is cost-effective as the quantity of the used chemicals is greatly reduced compared to conventional approaches. Third, the average size of the obtained graphene from microwave-assisted technology can be ten times larger than those prepared by the conventional heating method. Finally, the graphene products from microwave-assisted technology are of high quality with controlled structure and residual functional groups.

Until now, microwave irradiation has been suggested as an effective tool to obtain carbon-relative composites with uniform dispersion and size and morphology control,^67–69 because the microwave energy allows rapid heating and extremely rapid rate of crystallization to produce the desired nanocrystalline products. Meanwhile, during the microwave-assisted synthesis process, it is possible to control the growth of the favorable crystallographic plane by varying reaction time and the relative concentrations of different organic surfactants. Furthermore, the obtained nanostructures would extend from small spherical nuclei to short nanorod or nanosheet.^70–72 As a result, various graphene-based nanocomposites with controlled size and shape, such as particle/crystal-on-sheet, nanorod/nanofiber-on-sheet, and nanosheet-on-sheet, can be obtained with the help of ecofriendly microwave-assisted technology, as illuminated in Figure 1. It is worthy noting that the hydrothermal/solvothermal processes operated in a single-mode microwave reaction or a multimode household microwave oven are the most used technology for the microwave-assisted synthesis of graphene-based nanocomposites. Nonuniform microwave is offered from multimode domestic microwave oven, in which there is considerable variation in the microwave intensity throughout the reactor chamber. Moreover, the multimode domestic microwave oven can only roughly provide the time and several stages of power control (such as high, medium, low). In comparison, very uniform microwave can be generated in the specialized single-mode microwave reactor with stable microwave intensity in the chamber in which power, temperature, and time can be fine-tuned with continuous magnetic stirring. Therefore, the reaction environment is more uniform in the single-mode microwave reactor and better size and shape control should be achieved.

GNS-supported transitional metal oxide anodes

The microwave irradiation method has been applied for the synthesis of GNS-supported transitional metal oxide electrodes such as Co₃O₄-GNS,^73–77 CuO-GNS,^78–83 and Fe_xO_y-GNS.^84–89 All these transitional metal oxides have approximately two to three times larger theoretical capacities than commercial graphite anode based on a well-known conversion mechanism of lithium storage. Lithium can reduce metal oxides to metal and form lithium oxide, and this reaction is reversible. The morphologies and lithium-storage properties of various graphene-supported transitional metal oxide anodes are summarized in Table 1. Among these GNS-supported transitional metal oxide anodes, Co₃O₄-GNS tends to form a particle-on-nanosheet morphology under microwave irradiation.^73–76 As reported by Wang et al,⁷⁷ two-dimensional (2D) porous Co₃O₄ nanosheets were obtained by a microwave solvothermal process at 180°C for 5 minutes (pressure: ~7.5 bar) in a single-mode microwave reactor (Nova, EU Microwave Chemistry, Shanghai, People’s Republic of China). As shown in Figure 2, these porous Co₃O₄ nanosheets have pore sizes of 60–100 nm and a thickness around 100 nm. After stacking with graphene, the Co₃O₄-GNS composite can form a sheet-on-sheet composite structure.⁷⁷ The sheet-on-sheet composite shows superior Li-ion storage performances. Initial reversible charge capacity of 1,235 mAh g⁻¹ is delivered, which decreases to 1,065 mAh g⁻¹ after 30 cycles. This capacity is larger than those of GO or GNS, and even the theoretical value of pristine Co₃O₄ (890 mAh g⁻¹). The composite also exhibits an impressive good high-rate capability (a reversible capacity of 931 mAh g⁻¹ at a large current rate of 5 C [4,450 mA g^–1, 5 C represents the current at which the cell capacity is charged/discharged in 1/5 h]).

Table 1 Summary on the morphologies and electrochemical performances of graphene-supported transitional metal oxide anodes Abbreviations: GNS, graphene nanosheets; RGO, reduced graphene oxide.

Figure 2 The Co3O4-GNS composite and its electrochemical properties. Notes: (A) Schematic illustration of the growth process, (B and C) TEM images, (D) cycling performances at 0.1 C (89 mA g−1) and (E) rate capability performances of Co3O4-GNS sheet-on-sheet composite. Reproduced from Chen SQ, Wang Y. Microwave-assisted synthesis of a Co3O4-graphene sheet-on-sheet nanocomposite as a superior anode material for Li-ion batteries. J Mater Chem. 2010;20:9735–9739, with permission from The Royal Society of Chemistry.77 Abbreviations: GNS, graphene nanosheet; GO, graphene oxide; TEM, transmission electron microscopy.

Synthesized by the microwave-assisted technology, GNS-supported copper oxides can exhibit a variety of morphologies including zero-dimensional (0D) nanoparticle⁷⁸/nanosphere,^79,80 one-dimensional (1D) nanowire⁸¹/fusiform,⁸² and 2D nanoleaf⁸³/nanosheet⁸² morphologies. Among them, GNS-supported copper oxides with higher dimensional (1D and 2D)^81–83 or core-shell morphology⁷⁹ exhibit better electrochemical performance. By a fast single-mode microwave hydrothermal method, the GNS-supported sheet-like or fusiform-like CuO morphologies were obtained by varying the reaction temperature of microwave heating. As shown in Figure 3A–C,⁸² CuO-GNS sheet-on-sheet product was prepared at 170°C with CuO nanosheet of 0.3–0.5 μm in size, while the CuO-GNS fusiform-on-sheet material was obtained at a lower temperature of 110°C with fusiform CuO product owning the length around 0.4–0.8 μm and narrow tips. Figure 3D shows the electrochemical performance of the above two CuO-GNS composites in comparison with the physical mixture of CuO and GNS. Reversible charge capacities of 801 and 666 mAh g⁻¹ can be retained after 40 cycles for graphene-supported CuO nanosheet and fusiform composites, respectively. These reversible capacities are substantially larger than that of CuO-GNS (431 mAh g⁻¹) by a physical mixture after the same cycle numbers. As shown in Figure 3E, the graphene-supported CuO nanosheet composite exhibits an excellent rate capability with initial charge capacities of 981, 925, and 846 mAh g⁻¹ at 1, 2, and 5 C, respectively (1 C =700 mA g⁻¹).

Figure 3 The CuO-GNS composites and their electrochemical properties. Notes: (A) Schematic illustration of the growth process of CuO-GNS sheet-on-sheet and fusiform-on-sheet structures, (B) TEM image of the CuO-GNS sheet-on-sheet composite, (C) TEM image of the CuO-GNS fusiform-on-sheet composite, (D) cycling performances at 0.1 C (70 mA g−1) for various CuO-GNS composites, (E) rate capability performances of the CuO-GNS sheet-on-sheet composite. Reproduced from Lu LQ, Wang Y. Sheet-like and fusiform CuO nanostructures grown on graphene by rapid microwave heating for high Li-ion storage capacities. J Mater Chem. 2011;21:17916–17921, with permission from The Royal Society of Chemistry.82 Abbreviations: GNS, graphene nanosheets; TEM, transmission electron microscopy.

The Fe₂O₃-GNS composite with porous nanorod-on-sheet morphology was synthesized by a solvothermal process in a microwave oven.⁸⁴ The composite delivers a high reversible initial capacity of 1,016 mAh g⁻¹ at 0.1 A g⁻¹ (508 mAh g⁻¹ at 2 A g⁻¹ after 200 cycles). By comparison, the GNS-supported Fe₂O₃ nanoparticle composite⁸⁵ obtained from the similar hydrothermal process in microwave oven exhibits better electrochemical properties with discharge capacities of 1,693, 1,142, 1,120, 1,098, and 1,027 mAh g⁻¹ in the first, tenth, 20th, 30th, and 50th cycles, respectively. A high capacity of ~800 mAh g⁻¹ is also observed at a current density of 800 mA g⁻¹. Among the various GNS-supported Fe₃O₄ composites, Fe₃O₄ has a trend to form a particle or a porous particle morphology by the microwave-assisted technology.^86–89 As reported by Yu et al, a series of Fe₃O₄-GNS nanostructures were synthesized by a simple nonaqueous sol-gel approach in a single-mode microwave apparatus.⁸⁸ With different compositional ratios of Fe₃O₄ and GNS, reaction temperature, and times, as well as the synthesis method (Hummer’s method or modified Hummer’s procedure) of GO reactant, the obtained Fe₃O₄-GNS composites exhibit different particle sizes on the surface of GNS. The cycling performances of GNS-supported Fe₃O₄ composites are very stable even at high current density, which delivers high capacity of over 500 mAh g⁻¹ at 1,600 mA g⁻¹.⁸⁸ Moreover, the Fe₂O₃-GNS rice-on-sheet and particle-on-sheet nanocomposites were synthesized by single-mode microwave hydrothermal technique with and without NH₄H₂PO₄, respectively, as shown in Figure 4A.⁸⁹ The Fe₂O₃ nanorice is observed with a length of 200 nm and diameters in the range of ~40 nm in the middle part to only 3–5 nm in the tip (Figure 4C), while the nanoparticle is nearly a nanocube-like morphology of ~50–80 nm in size (Figure 4B). The Fe₂O₃-GNS rice-on-sheet composite exhibits large reversible charge capacities of 825, 762, and 633 mAh g⁻¹ at large currents of 1, 2, and 5 C (1 C = 1,000 mA g⁻¹) respectively. A high capacity of 582 mAh g⁻¹ can be observed at 1 C after 100 cycles. The rice-on-sheet composite also shows more stable cycle life and better high-rate performance than the particle-on-sheet composite (Figure 4D and E).

Figure 4 The Fe2O3-GNS composites and their electrochemical properties. Notes: (A) Schematic illustration of the growth process of Fe2O3-GNS particle-on-sheet and rice-on-sheet composites. TEM images of (B) Fe2O3-GNS particle-on-sheet and (C) Fe2O3-GNS rice-on-sheet composite. Electrochemical performances of Fe2O3-GNS composites: (D) cycling performances at 0.1 C (100 mA g−1) and (E) rate capability. Reprinted with permission from Zou YQ, Kan J, Wang Y. Fe2O3-graphene rice-on-sheet nanocomposite for high and fast lithium ion storage. J Phys Chem C. 2011;115:20747–20753. Copyright © 2011. American Chemical Society.89 Abbreviations: GNS, graphene nanosheets; TEM, transmission electron microscopy.

There are also several reports on GNS-supported other transitional metal oxides. Their electrochemical performances are summarized in Table 1. GNS-supported Mn₃O₄ particle composite was prepared by a microwave hydrothermal method.⁹⁰ It exhibits a high specific capacity of more than 900 mAh g⁻¹ at 40 mA g⁻¹ and no capacity fading up to 50 cycles. By a similar microwave hydrothermal process, the MoO₃ nanobelt/graphene film was also reported by Noerochim et al⁹¹ and exhibits initial discharge capacity of 291 mAh g⁻¹ at 100 mA g⁻¹ and 172 mAh g⁻¹ after 100 cycles. Moreover, 3D porous MoO₂/graphene microspheres were prepared in a microwave-assisted hydrothermal process. The obtained MoO₂/graphene composite exhibits excellent cycling stability of 1,300 mAh g⁻¹ after 80 cycles at 0.1 C and good rate capability of 913 and 390 mAh g⁻¹ at 2 and 5 C, respectively.⁹² The ZnO@graphene composite⁹³ was synthesized from ZnO nanoparticles via a microwave-assisted deposition on GO in a microwave oven. It exhibits improved electrochemical performance with a high capacity of 850 mAh g⁻¹ at 0.1 C. There is a small capacity decay of ~8% during 50 cycles of discharge and charge.

GNS-supported tin/germanium/silicon based anodes

Tin, germanium and silicon are high-capacity elements for lithium storage, whose theoretical capacities values are 990, 1,600, and 4,200 mAh g⁻¹, respectively. These theoretical values are calculated based on the lithium-ion reaction to form lithium alloys (Li_4.4Sn, Li_4.4Ge, and Li_4.4Si). It is worth noting that these elements are often used as oxides such as SnO, SnO₂, GeO₂, and SiO₂. The lithium ion can reduce these oxides to Sn, Ge, and Si at an early stage, followed by a similar lithium alloy and de-alloy storage mechanism. The oxygen element in these oxides is usually believed to be inactive for lithium ion storage. As summarized in Table 2, among the reported GNS-supported lithium alloy anodes by microwave-assisted technology, Sn^94–98 or SnO₂^99–106 usually tends to form a nanoparticle/nanocrystal morphology in the presence of GNS. GNS-supported Ge nanoparticle¹⁰⁷ or thin film¹⁰⁸ and GNS-supported Si nanoparticle¹⁰⁹ or thin film¹¹⁰ was also synthesized by microwave irradiation or microwave plasma methods.

Table 2 Summary on the morphologies and electrochemical performances of graphene-supported Sn/Ge/Si-based anodes Abbreviations: CNTs, carbon nanotubes; GNS, graphene nanosheets; RGO, reduced graphene oxide.

Figure 5A shows the schematic illustration of the preparation of graphene-supported Sn nanoparticles, which were synthesized via a single-mode microwave hydrothermal process in a microwave reactor, followed by hydrogen gas reduction.⁹⁵ Figure 5B and C show the obtained GNS-supported Sn nanoparticles. Interestingly, the size of Sn nanoparticles in the Sn-GNS composite is changed from 60–120 nm (Sn-GNS-1) to 10–20 nm (Sn-GNS-2) when the ratio of Sn and GNS is reduced from 1:1 to 1:4. When used as the anode for LIBs, the Sn-GNS-1 and Sn-GNS-2 composites deliver reversible charge capacities of 1,206 and 1,407 mAh g⁻¹, respectively, with the corresponding Coulombic efficiencies of 67.9% and 65.9%. As shown in the cycling performance curves of Figure 5D, the Sn-GNS-1 and Sn-GNS-2 electrodes deliver specific capacities of 772 and 1,100 mAh g⁻¹, respectively, after 30 cycles at 0.1 C, which are both higher than that for bare GNS (582 mAh g⁻¹) after the same cycle number. The enhanced high-rate properties are also observed for Sn-GNS-2, which exhibits specific discharge capacities of 1,247, 1,106, 946, and 876 mAh g⁻¹ at current densities of 0.5, 1, 2, and 5 C. Moreover, a new strategy for the growth of self-assembled Sn@CNT on vertically aligned graphene (VAGN) was suggested by the microwave plasma irradiation method in MPCVD system.⁹⁴ SnO₂ is reduced to Sn on VAGN and subsequently encapsulated in the catalyzed carbon nanotubes (CNTs). In the Sn@CNT product, pear-like Sn core with a diameter of about 30 nm and a length of 40–50 nm is encapsulated inside a cylindrical CNT with a length less than 100 nm. The whole Sn@CNT structure is anchored on the surface of GNS. Such a composite exhibits a high reversible capacity of 1,026 mAh g⁻¹ at 0.25 C, and a capacity of 140 mAh g⁻¹ is retained in a short discharge time of 12 seconds. A Sn@graphene on the VAGN structure⁹⁷ was also reported by the same authors as above, which exhibits a reversible capacity of 1,005 mAh g⁻¹ at 0.25 C even after 120 cycles. Meanwhile, there are several other reports on GNS-supported SnO₂ particles^99–106 prepared by microwave-assisted method. Excellent electrochemical properties (a stable capacity of ~890 mAh g⁻¹ without noticeable fading up to 80 cycles at 500 mA g⁻¹) were observed for the GNS-SnO₂ nanocomposite, which was synthesized by a microwave-assisted hydrothermal process.¹⁰¹

Figure 5 The Sn-GNS composites and their electrochemical properties. Notes: (A) Schematic illustration for the preparation process of Sn-GNS nanocomposites, (B) TEM images of Sn-GNS-1 (the weight ratios of Sn and GNS being 1:1), (C) TEM images of Sn-GNS-2 (the weight ratios of Sn and GNS being 1:4), (D) cycling performances of Sn-GNS composites at 80 mA g−1. Reprinted from J Power Sources, 216, Chen SQ, Wang Y, Ahn H, Wang GX, Microwave hydrothermal synthesis of high performance tin-graphene nanocomposites for lithium ion batteries, 22–27, Copyright © 2012 with permission from Elsevier.95 Abbreviations: GNS, graphene nanosheets; TEM, transmission electron microscopy.

A unique sandwich-structured C/Ge/graphene composite¹⁰⁷ was synthesized by a microwave-solvothermal process as shown in Figure 6A. A carbon coating layer is introduced on the surface of the graphene/germanium oxide composite precursor, followed by a reduction treatment. The composite consists of metallic germanium nanoparticles (20–30 nm in size) between graphene sheets and carbon layers (Figure 6B and C). The C/Ge/graphene composite shows better cycling performances (a capacity of 993 mAh g⁻¹ after 160 cycles, corresponding to 86.4% of the capacity at the second cycle) and rate capability (discharge capacity of 1,008 mAh g⁻¹ after 30 cycles at 5 C) than the Ge/C and Ge/graphene composites.

Figure 6 The C/Ge/graphene composite. Notes: (A) Schematic illustration of the C/Ge/graphene composite, (B and C) TEM images of C/Ge/graphene composite at different magnifications. Reproduced from Li D, Seng KH, Shi DQ, Chen ZX, Liu HK, Guo ZP. A unique sandwich-structured C/Ge/graphene nanocomposite as an anode material for high power lithium ion batteries. J Mater Chem A. 2013;1:14115–14121 with permission of The Royal Society of Chemistry.107 Abbreviation: TEM, transmission electron microscopy.

3D graphene scaffold-supported Si thin film composite was prepared by Wang et al.¹¹⁰ As shown in Figure 7A, a 3D graphene scaffold is first synthesized using a MPCVD approach, and Si is then deposited on the graphene scaffold using radio frequency sputtering. The obtained Si grains (0.3–0.5 μm) are deposited on the surface of 3D graphene (Figure 7B). Figure 7C shows that these Si grains are composed of numerous Si nanoparticles (several nanometers in size). When used for LIBs, such 3D graphene scaffold-supported Si electrode exhibits an outstanding cycling stability. A capacity of 1,314 mAh g⁻¹ can be observed after 500 cycles with capacity retention of 84% relative to the maximum capacity of 1,560 mAh g⁻¹ in the 50th cycle. The composite also exhibits good high-rate cycling performances and a large capacity of 1,083 mAh g⁻¹ is still retained after 1,200 cycles at a large current of 2.39 A g⁻¹.

Figure 7 The graphene supported Si composite. Notes: (A) Schematic illumination of the growth process of the 3D graphene scaffold supported Si electrode (GSSSE). The as-synthesized GSSSE: (B) SEM image and (C) TEM image. Reproduced from Wang CD, Chui YS, Ma RG, et al. A three-dimensional graphene scaffold supported thin film silicon anode for lithium-ion batteries. J Mater Chem A. 2013;1:10092–10098 with permission of The Royal Society of Chemistry.110 Abbreviations: SEM, scanning electron microscopy; TEM, transmission electron microscopy.

Based on the similar lithium alloy and de-alloy storage mechanism to Sn, Ge, and Si, Ag and Ag-based composites were also explored as the anodes for LIBs.^111,112 It is found in the literature that only GNS-supported Ag nanoparticle¹¹¹/nanorod¹¹² composites were synthesized by microwave-assisted methods and their electrochemical properties are presented in Table 2. The Ag nanorod on GNS¹¹² is obtained by reducing Ag-GO hybrid. The Ag-GO precursor is prepared by two steps: Ag nanorods are first synthesized in a microwave solvothermal process in a household microwave oven and then mixed with GO prepared from Hummer’s method. The Ag-GNS nanorod-on-sheet composite shows a high reversible capacity of 1,015 mAh g⁻¹ at 0.1 C with a high capacity retention rate of 64.1% at 5 C. By a similar microwave solvothermal process, GNS-Ag nanoparticle composite¹¹¹ was synthesized from natural graphite and silver salt, and it exhibits an initial charge capacity of 780 mAh g⁻¹, which decreases to 714 mAh g⁻¹ after 50 cycles.

GNS-supported metal sulfide anodes

Metal sulfides are also promising high-capacity anode materials for LIBs. The active element for lithium ions can be either metal (eg, Sn of SnS₂) or sulfur (eg, S of CoS and NiS). Metal and sulfur are also suggested to be both active for In₂S₃ materials, although most studies still believe the active element is only sulfur. The lithium ion storage mechanism of tin sulfide is similar to tin oxides, in which sulfide and oxygen are both inactive and lithium can reversibly react with tin to form Li_xSn alloys (the maximum x value is 4.4). The storage mechanism of active sulfur element is based on the reversible formation and decomposition of lithium sulfide.

There are few reports on the GNS-supported metal sulfide composites with microwave-assisted syntheses: GNS-supported SnS₂ nanosheets,¹¹³ SnS₂ nanoparticles,⁶⁹ In₂S₃ particles and In₂S₃ flowers¹¹⁴ composites were reported by Chen et al, Zou and Wang, and Gu and Wang, respectively. Their morphologies and lithium-storage properties are summarized in Table 3. A porous 3D SnS₂-reduced graphene oxide (RGO) sheet-on-sheet nanostructure was synthesized by a single-mode microwave solvothermal method at 180°C for 20 minutes in which SnS₂ nanosheets are distributed uniformly on the RGO surface.¹¹³ If the amount of the starting GO is increased, the obtained SnS₂ products are only nanoparticles.⁶⁹ This is ascribed to the presence of a large amount of surface functionalities (mainly oxygen-containing groups) of GO, which can significantly affect the nucleation process of SnS₂. As shown in Figure 8, large reversible capacities are observed from 1,077 to 896 mAh g⁻¹ at 0.1 C and 934 to 657 mAh g⁻¹ at 1 C in 40 cycles for the SnS₂-RGO sheet-on-sheet composite. Compared with bare graphene and pristine SnS₂ nanoflowers, such a sheet-on-sheet composite achieves better electrochemical performances, which is attributed to the synergetic effect for highly reversible lithium-ion storage that resulted from the closely contacted sheet-on-sheet morphology. These electrochemical properties of the sheet-on-sheet composite are also superior to previous SnS₂-GNS particle-on-sheet composite⁶⁹ in which an initial charge capacity of 858 mAh g⁻¹ is decreased to 652 mAh g⁻¹ after 40 cycles.

Table 3 Summary on the morphologies and electrochemical performances of graphene-supported metal sulfides and lithium titanium-based anodes Abbreviations: GNS, graphene nanosheets; RGO, reduced graphene oxide.

Figure 8 The SnS2-GNS composite and its electrochemical properties. Notes: (A) Schematic illustration of the growth process and applications of SnS2-RGO sheet-on-sheet nanostructure, (B and C) TEM images of SnS2-RGO sheet-on-sheet composite. The inset of (C) showing the selected area electronic diffraction (SAED) pattern of polycrystalline SnS2. Cycling performances of the products: (D) 0.1 C (66 mA g−1) and (E) 1 C. Reprinted with permission from Chen P, Su Y, Liu H, Wang Y. Interconnected tin disulfide nanosheets grown on graphene for Li-ion storage and photocatalytic applications. ACS Appl Mater Interfaces. 2013;5:12073–12082. Copyright © 2013. American Chemical Society.113 Abbreviations: LIB, lithium ion battery; RGO, reduced graphene oxide; TEM, transmission electron microscopy.

GNS-supported In₂S₃ nanoparticle and interconnected nanoflower composites were synthesized by the similar single-mode microwave hydrothermal method at 140°C under a pressure of 5.5 bars for 20 minutes.¹¹⁴ Black powders of In₂S₃-graphene particle-on-sheet composite are obtained instead of the tawny powders of In₂S₃-graphene flower-on-sheet when the dosage of graphene is increased. As indicated by Figure 9, In₂S₃ nanoparticles and nanoflowers are uniformly dispersed on GNS, forming sandwiched particle-on-sheet and unprecedented flower-on-sheet nanostructures. Compared with GNS and pristine In₂S₃, the GNS-supported In₂S₃ composites show extraordinary large reversible capacities and good cycling performances. Reversible initial capacities of 1,249, 913, 782, and 690 mAh g⁻¹ are observed at currents of 70, 700, 1,400, 3,500 mA g⁻¹, respectively, for the flower-on-sheet composite, while the particle-on-sheet composite shows slightly lower reversible capacities but more stable cycling performances at both small and high currents.

Figure 9 The In2S3-GNS composites and their electrochemical properties. Notes: (A) SEM image of the In2S3-GNS flower-on-sheet composite, (B) SEM image of the In2S3-GNS particle-on-sheet composite and the inset showing the energy-dispersive X-ray spectrum (EDS), (C) cycling performances at 0.1 C (70 mA g−1) and (D) In2S3 contribution in the composite. High-rate cycling performances: (E) In2S3-GNS flower-on-sheet and (F) particle-on-sheet composites at 1, 2, and 5 C. Reproduced from Gu Y, Wang Y. Microwave hydrothermal growth of In2S3 interconnected nanoflowers and nanoparticles on graphene for high-performance Li-ion batteries. RSC Adv. 2014;4:8582–8589, with permission from The Royal Society of Chemistry.114 Abbreviations: au, arbitrary unit; GNS, graphene nanosheets; SEM, scanning electron microscopy.

Graphene-supported lithium titanium oxide based anodes

Lithium-titanium-oxide-based materials are important high-rate anodes for lithium ion batteries, which are based on the lithium insertion mechanism of lithium storage. Although their theoretical capacities are even lower than graphite anodes, they exhibit very good cyclabilities because there is only a small volume change during the process of lithium insertion and extraction. A summary of morphologies and electrochemical performances of graphene-supported lithium-titanium-oxide-based anodes is given in Table 3. For example, Li₄Ti₅O₁₂ has been suggested as a zero-strain material with almost no structure change during cycling.^115,116 The microwave-hydrothermal method was reported to prepare Li₄Ti₅O₁₂ microspheres composed of nanoflakes wrapped in GNS.¹¹⁵ The obtained structure can avoid the restacking of GNS and offer rapid lithium diffusion; therefore, the composite exhibits highly desirable Li-ion storage properties in terms of a large capacity (168 mAh g⁻¹ at 0.2 C) approaching the theoretical value, stable cycling performance, and excellent rate capability. The Li₄Ti₅O₁₂/graphene composite was also prepared by the lithiation of the alkali titanate with the assistance of microwave.¹¹⁶ The composite exhibits a reversible capacity of 168 mAh g⁻¹ at a current rate of 1 C with a high capacity retention rate of 59% at a very large current rate of 50 C. Kim et al also reported Li₄Ti₅O₁₂ nanoplatelet/RGO hybrid,¹¹⁷ which was obtained from TiO₂/RGO nano-hybrid in LiOH aqueous solution via a microwave hydrothermal process. The composite can deliver a discharge capacity of 154, 128, and 101 mAh g⁻¹ (based on Li₄Ti₅O₁₂) at 1, 50, and 100 C, respectively. As reported by Yan et al, a facile microwave solvothermal process was developed to prepare an anatase TiO₂ anode material that maintains multiple properties including high surface area, high crystallinity, uniform mesoporous structure, perfect microspheres, and uniform particle size.¹¹⁸ Using this fine anatase TiO₂ product, a TiO₂/RGO hybrid material¹¹⁸ was prepared under UV-light irradiation. The incorporation of RGO improves the electrochemical kinetics of the TiO₂ microspheres (Figure 10), which results in superior electrochemical performance in terms of specific capacity, rate capability, and cycle stability. The lithium storage mechanism of the anatase TiO₂ is also a lithium insertion mechanism. A reversible lithium insertion and extraction reaction between TiO₂ and Li_0.5TiO₂ leads to a theoretical capacity of ~168 mAh g⁻¹. The composite also shows a large discharge capacity of 156 mAh g⁻¹ at a large current rate of 5 C. Even at 60 C, a very high discharge capacity of 84 mAh g⁻¹ is still obtained.

Figure 10 The TiO2/RGO composite. Notes: (A) TEM image and (B) HRTEM image. Reproduced from Yan X, Li YJ, Du F, et al. Synthesis and optimizable electrochemical performance of reduced graphene oxide wrapped mesoporous TiO2 microspheres. Nanoscale. 2014;6:4108–4116 with permission of The Royal Society of Chemistry.118 Abbreviations: HRTEM, high resolution transmission electron microscopy; RGO, reduced graphene oxide; TEM, transmission electron microscopy.

Other GNS composite anodes

In the literature, there are also some reports about other types of GNS-based anode composites. Kang et al reported a CuCo₂O₄/RGO composite by a solvothermal reaction in a microwave reactor, followed by calcination treatment. Porous CuCo₂O₄ nanocubes are well wrapped by RGO sheets.¹¹⁹ Based on the observation of the Brunauer–Emmett–Teller (BET) results, the porous CuCo₂O₄ nanocube/RGO composite exhibits a higher specific surface area (34.4 m² g⁻¹) than pristine CuCo₂O₄ nanocubes (10.9 m² g⁻¹). This would result in the increased contact areas between the electrode and the electrolyte solution when they are used as anodes for LIBs. Such a composite exhibits a high stable capacity of ~570 mAh g⁻¹ at 1,000 mA g⁻¹ after 350 cycles. Impressive high-rate performance is also observed (a high capacity of ~450 mAh g⁻¹ even at a high current density of 5,000 mA g⁻¹). The morphologies and electrochemical properties of other graphene-based anode materials^119–124 for LIBs synthesized via similar microwave hydrothermal/solvothermal process in microwave oven/reactor are summarized in Table 4. Multilayer GNS were prepared by a microwave hydrothermal technique and then mixed with single-walled carbon nanotube by vacuum filtering.¹²⁰ The obtained free-standing GNS-single-walled carbon nanotube film exhibits a large d-spacing of 0.41 nm and a reversible capacity of ~300 mAh g⁻¹ during 50 cycles. Among these reports,^119–124 3D GNS-CNT-Ni,¹²³ and GNS-CNT-Fe¹²⁴ composites show good lithium ion storage properties. They were both synthesized by similar microwave hydrothermal method in a microwave oven/reactor, in which vertically aligned CNTs are grown directly on graphene sheets under the catalysis of Ni and Fe nanoparticles, respectively.

Table 4 Summary on the morphologies and electrochemical performances of other graphene-based anode materials Abbreviations: CNT, carbon nanotube; GNS, graphene nanosheets; RGO, reduced graphene oxide; SWCNT, single-walled carbon nanotube.

GNS-decorated cathodes

Graphene is an active anode for LIBs within a comparatively low voltage window, however there is almost no lithium storage capacity for graphene at a higher voltage for cathode. Therefore graphene has been investigated with more research concerns as anodes for LIBs. In comparison, only a small amount of graphene is used for cathode composites with the purpose to improve the electrical conductivity of the cathode. And the GNS-decorated cathodes prepared by microwave-assisted methods are summarized with their morphologies and electrochemical properties in Table 5. Among various graphene-based cathodes by microwave irradiation, graphene-LiFePO₄ composite has attracted more interests.^125–127 As shown in Figure 11, a GNS-supported LiFePO₄ nanorod composite was synthesized by a novel one-pot microwave solvothermal process within 15 minutes at a temperature below 300°C in an microwave synthesis system.¹²⁵ The obtained graphene/LiFePO₄ nanohybrid exhibits a discharge capacity of ~164 mAh g⁻¹ (close to the theoretical value of 170 mAh g⁻¹) at 0.1 C and good cyclability up to 70 cycles. LiFePO₄/C/graphene composite was also reported by Shi et al.¹²⁶ After a rapid, one-pot, microwave-assisted hydrothermal method (15 minutes at 200°C), followed by sintering at 600°C for 2 hours under a H₂/Ar (5:95, v/v) atmosphere, the obtained LiFePO₄ particles have sizes around 150 nm. These particles are wrapped in crumpled micrometer-size graphene sheets. The LiFePO₄/C/graphene composite exhibits obviously improved electrochemical performance with highly stable reversible capacity of 88 mAh g⁻¹ at 10 C. Around 99% of the initial capacity can be retained after 40 cycles. Wang reported a similar LiFePO₄/(C + graphene) structure.¹²⁷ The composite was obtained via a direct solid-state heating reaction in a microwave oven, which delivers a large discharge capacity (157.8 mAh g⁻¹) at 0.1 C and more stable cycling performance than those of LiFePO₄/C. A LiTi₂(PO₄)₃/RGO particle-on-sheet composite was synthesized via microwave hydrothermal method in a microwave oven.¹²⁸ The particle-on-sheet composite can deliver a reversible capacity of 138 mAh g⁻¹ at 0.1 C, and over 93.2% of its initial capacity can be retained after 100 cycles at 1 C. With the similar microwave hydrothermal method, the LiMn₂O₄/RGO nanoparticle-on-sheet composite was obtained.¹²⁹ The composite can deliver a high specific capacity of 137 mAh g⁻¹ at 1 C and a remarkably high discharge capacity of 117 mAh g⁻¹ and 101 mAh g⁻¹ at 50 and 100 C, respectively. Moreover, a FeF₃/RGO composite exhibits a nanoparticle-on-sheet morphology by a microwave solvothermal process.¹³⁰ It delivers a stable capacity of 150 mAh g⁻¹ after 50 cycles when used as a cathode for LIBs.

Table 5 Summary on the morphologies and electrochemical performances of graphene-decorated cathodes Abbreviations: GNS, graphene nanosheet; RGO, reduced graphene oxide.

Figure 11 The LiFePO4/GNS composite. Notes: (A) TEM image, (B) HRTEM image, and (C) Elemental mapping images. Reproduced from Praneetha S, Murugan AV. A rapid, one-pot microwave-solvothermal synthesis of a hierarchical nanostructured graphene/LiFePO4 hybrid as a high performance cathode for lithium ion batteries. RSC Adv. 2013;3:25403–25409 with permission of The Royal Society of Chemistry.125 Abbreviations: HRTEM, high resolution transmission electron microscopy; GNS, graphene nanosheets; TEM, transmission electron microscopy.

Conclusion

Microwave synthesis has been demonstrated as a fast, uniform, energy-efficient, and scalable approach to prepare graphene-supported various electrodes. Representative examples such as graphene-supported transitional metal oxides, metal sulfide, tin/germanium/silicon/lithium titanium oxide based anodes, graphene-decorated lithium iron phosphate based cathodes, and some other graphene-based composite electrodes have been discussed. The fast microwave heating offers homogenous reaction environment and leads to good control of shape, size, size distribution, and agglomeration of the products. The surface functionalities on graphene can be controlled to different extents and the introduced second-phase component to graphene can be also tuned with 0D, 1D, 2D morphologies, and their stacked 3D network. These graphene-based composites usually exhibit strong synergetic effect when used for LIBs. They deliver larger capacity and better cyclability and high-rate performance compared to individual component of the composite. These improved electrochemical properties have been attributed to the preserved promising properties of graphene and the improved electrical conductivity and more stable mechanical structure of graphene-supported materials. These synthesized graphene composites with the assistance of microwave irradiation may find wide applications for other energy-storage applications such as supercapacitors and fuel cells. Furthermore, the microwave-assisted technology would be used more and more in the recent future to synthesize materials with controlled size and shape for the energy-storage application due to its simple, quick, inexpensive, uniform, and energy-efficient advantages.

Acknowledgments

The authors gratefully acknowledge the follow-up Program for Professor of Special Appointment in Shanghai (Eastern Scholar), the National Natural Science Foundation of China (51271105 and 51201095), Shanghai Municipal Education Commission (13YZ012) and Innovative Research Team (IRT13078) for financial support. The authors also thank Lab for Microstructure, Instrumental Analysis and Research Center, Shanghai University, for materials characterizations.

Disclosure

The authors report no conflicts of interest in this work.

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