1. Introduction
In order to meet its pledge to peak carbon dioxide emissions by 2030 and target carbon neutrality by 2060, China is taking great actions in developing green and low-carbon transportation. As a result, electric vehicles (EVs) have been robustly developed in the past three years. According to the China Association of Automobile Manufacturers, the number of EVs sold annually has grown from 1.3 million in 2020 to a whopping 6.8 million in 2022, and it is expected to reach 9 million in 2023
[1]. Along with the robust development of EVs, the demand and upgradation of lithium-ion batteries (LIBs) as the main power source are greatly promoted, leading to a surging decommission stream in the next 3–5 years due to the limitation of service life
[2][3][4]. Considering the limited resources and environmental sustainability, the sustainable recycling of spent LIBs is of great significance. However, the recycling of spent graphite is far behind the development of LIBs. Currently, the industrial-scale reclamation of spent LIBs is mainly based on pyrometallurgy and hydrometallurgy, with the focus on recovering valuable metal elements only
[5]. In contrast, the resource utilization of anode materials that account for 12–21 wt%
[6] of the total mass of spent LIBs (about 11 times that of lithium) is still immature, resulting in most anode materials being directly discarded or incinerated as fuel. At present, the commercial anode materials mainly include graphite, hard carbon, soft carbon, etc. Among them, graphite is the main one, accounting for about 98% of the LIB anode material market
[7]. As an important strategic resource, graphite is known as “black gold”. Directly discarding or incinerating spent graphite will not only cause significant waste in terms of resources but also lead to environmental problems, such as carbon emissions. Therefore, the resource recovery and utilization of spent graphite is a process of great urgency and significance. According to the working principle of LIB and structural composition of the anode, spent graphite usually contains electrolytes (composed of lithium salts (e.g., LiPF
6 or LiClO
4) dissolved in organic solvents (e.g., ethylene carbonate, dimethyl carbonate)), adhesives (e.g., polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene–butadiene rubber), a solid electrolyte interfacial (SEI) layer, copper foil, and other pollutants
[8][9]. Owing to these impurities, spent graphite presents great differences compared to fresh graphite in terms of structure and surface morphology, making it unable to be reused directly. Impurity removal is essential in order to reuse the spent graphite. Furthermore, due to electrochemical oxidation, there are oxygen-containing functional groups attached to the layer surface of spent graphite, resulting in the conductive sp2 hybrid structure being changed into the nonconductive sp3 hybrid structure. Accordingly, structure repair is required for the reuse of spent graphite, which usually adopts high-temperature annealing treatment
[10]. However, compared with natural flake graphite, the interlayer distance of spent graphite is greatly enlarged due to the repeated intercalation/deintercalation of lithium ions during battery charge and discharge
[11], leading to the interlayer van der Waals force being remarkably weakened, and thereby it is much easier to be intercalated to form graphite intercalation compounds (GICs) or exfoliated to obtain graphene and its derivatives. On basis of the abovementioned characteristics of spent graphite, its recycling can be classified into three main technical routes, including impurity removal and regeneration, high-temperature repair, preparation of GIC, expanded graphite (EG), graphene and its composite nanomaterials, etc.
Despite the availability of certain chemical treatment processes, thermal treatment is essential for the recycling of spent graphite. Microwave heating is different from traditional heating methods that rely on thermal conduction, convection, and radiation. It operates by causing frictional collisions between polar molecules and free ions/electrons. The specific mechanisms are dipolar polarization and ionic polarization/Joule heating, correspondingly, leading to an excellent heating effect that mainly manifests in rapidity, selectivity, uniformity, and easy control. In spite of Joule heating, a discharge phenomenon can be induced when a conducting material (e.g., metal strip/wire/fiber, graphite, carbonaceous material) is implanted in a microwave field, leading to instantaneous release of significant heat and generation of active species and plasmas
[12][13]. For example, microwave treatment facilitates the recovery of metals from spent LIBs as an alternative to traditional pyrometallurgy, where the anode graphite acts as a microwave absorber and reductant
[14]. Notably, due to the excellent microwave absorbing property and conductivity of graphite, microwave–graphite interaction can induce Joule heat–discharge–plasma coupled effect, leading to the rapid heating process. Especially when discharge occurs, there is a thermal shock effect (heating rate can amount to 10
5~10
6 K/s), with the generation of a large number of high-energy electrons and active materials, which can remarkably strengthen the gas–solid reactivity and trigger reactions that cannot be completed or are difficult to complete under conventional conditions (e.g., etching, controllable doping, stripping, and reduction)
[15]. Therefore, based on the unique microwave–graphite interaction, microwave heating can be tailored to assist the impurity removal, structure repair, and graphite-derived materials preparation.
2. Microwave-Assisted Preparation of Graphene and Graphene-Derivative Functional Material
2.1. Graphene
The graphene preparation methods include mechanical stripping
[16], chemical vapor deposition
[17], liquid stripping
[18], epitaxial growth
[19], chemical oxidation–reduction
[20], electrochemical
[21], organic synthesis
[22], and so on. Among them, the oxidation–reduction method has been the most widely used method for production. The process can be approximately divided into three parts: graphite oxidation, exfoliation of oxidized graphite to obtain graphene oxides (GO), and reduction of GO to produce reduced graphene oxides (rGO). As mentioned in the preparation of GICs, the unique structure of spent graphite can effectively decrease the intercalation difficulty and reduce the consumption of oxidants. In addition, Chen et al.
[23] found that the exfoliation efficiency of spent graphite was 3–11 times higher than that of natural graphite in a study on the preparation of graphene by sonication-assisted liquid-phase exfoliation. Furthermore, due to the lithium-ion intercalation and deintercalation in the graphite during charge/discharge, the spent graphite exhibits an irregular expansion and the pre-expansion process enabled four times enhancement in graphene productivity compared with the pristine graphite
[24]. Therefore, spent graphite as a raw material for graphene preparation can reduce oxidant consumption and improve exfoliation efficiency, which has inherent advantages.
In addition, a large amount of oxygen functional groups can be removed from the few-layer graphene by microwave irradiation
[25]. Voriy et al.
[26] reported in
Science that the rapid high temperature generated by microwave-induced discharging can almost wholly remove oxygen functional groups and rearrange the carbon atoms in the graphene basal plane to obtain microwave-reduced graphene oxide (MWrGO), which provides a new technological route for the efficient preparation of high-quality graphene from spent graphite. The MwrGO can be prepared by 1 to 2 s pulses of microwave radiation, presenting a highly ordered structure compared to rGO obtained by general thermal reduction. Jiang et al.
[27] conducted research based on the effect of graphite as it pertains to inducing microwave discharge plasma to assist the reduction of GO to prepare MwrGO. By adding 5 wt% of prepared MwrGO into the cathode material of LIBs, the layered structure of MwrGO increases the contact area between lithium ions and electrolytes, which promotes the rapid transfer of lithium ions and electrons, thereby enhancing the electrochemical performance. Yan et al.
[28] utilized a microwave-assisted solvothermal treatment to reduce graphene oxide by glucose to prepare rGO, presenting shortened reaction time and improved reaction efficiency compared to the conventional oxidation–reduction method. The obtained rGO possesses good electrochemical properties (a specific capacitance of 179 F g
−1 at a scan rate of 2 mV s
−1).
Furthermore, ultrasonic–microwave synergistic assisted preparation of graphene by liquid phase exfoliation has also been extensively studied. Sreedhar et al.
[29] exposed graphite to domestic microwave radiation to obtain EG, which was sonicated in an ethanol environment to obtain graphene. Song et al.
[30] obtained an rGO suspension by heating up to 110 °C under microwave radiation at 300 W and discontinuous sonication for 30 min, which shortened the reduction time of GO and enhanced the firmness of the rGO anchored onto the modified material. Therefore, the ultrasonic–microwave synergistic method is remarkably effective in terms of improving exfoliation and reduction efficiency, which is a novel and efficient approach for preparing high-quality graphite in bulk.
Conclusively, compared with most conventional graphene preparation methods that feature complicated steps, high reagent input, high energy costs, and are time-consuming, the adoption of spent graphite as a raw material integrated with a microwave-assisted exfoliation/reduction process can achieve efficient preparation of high-quality graphene, promoting the batch production of graphene and value-added utilization of spent graphite.
2.2. Graphene-Derivative Functional Material
As mentioned above, GO is rich in oxygen-containing functional groups (e.g., epoxide, carbonyl, carboxyl, hydroxyl groups) compared to pristine graphene, and microwave radiation can efficiently remove oxygen-containing groups, which provides opportunities for doping modification of graphene and compounding with functional materials. Dai et al.
[31] utilized a microwave process to carry out deep reduction and nitrogen doping in GO, using ethylenediamine (EDA) as the nitrogen source. Functional graphene sheets (FGS) were synthesized by the ring-opening reaction of EDA with the epoxy group of GO. After microwave radiation treatment for 1 min, the polar oxygen-containing functional groups showed obvious decomposition, and nitrogen long-pair electrons were conjugated to the graphene π system to obtain nitrogen-doped graphene sheets (NGS). Fei et al.
[32] used amine-functionalized GO (AGO) as a nitrogen source and accomplished the reduction and N-doping of the AGO simultaneously through the high-energy environment created by short-time microwave radiation. The amino groups were easily released and formed chemical bonds, such as C–N bonds, under microwave thermal reduction, resulting in nitrogen-doped graphene with a high doping rate. Through microwave-assisted heteroatom (e.g., nitrogen, boron) doping, graphene exhibits superior structural characteristics and enhanced physicochemical properties, showing great potential in energy storage, catalysis, sensors, and so on.
The microwave thermal shock effect can facilitate the effective integration of diverse functional materials with graphene to prepare graphene-derivative nanocomposites. These nanocomposites can be divided into two categories: graphene/inorganic nanocomposites and graphene/polymer nanocomposites, which may be obtained by adding graphene to materials with special functional properties or vice versa. In the case of graphene/inorganic nanocomposites, the inorganic class can be divided into metals (e.g., Cu
[33][34], Mg
[35]), metal oxides (e.g., MnO
2 [36], SnO
2 [37]), and nonmetals (e.g., Si
[38], ceramic
[39]). The addition of graphene nanosheets (GNs) significantly improved the mechanical, tribological
[33], and energy storage
[36] properties of the nanocomposites, broadening their application areas.
Fei et al.
[32] further embedded atomic metals into graphene while achieving nitrogen doping and found the microwave-assisted thermal reduction of GO created defects or vacancies during the removal of oxygen-containing functional groups, which could serve as anchoring sites for metal atoms and effectively prevent the aggregation of mass atomic metals. The graphene-supported single atomic metals synthesized by the microwave heating process have special catalytic, magnetic, and electronic properties that are difficult to achieve via other methods. Kim et al.
[37] proposed a method for the synthesis of graphene/SnO
2 nanocomposites based on the ability of microwave-generated plasma to perform rapid surface chemical reactions. The composite material obtained by microwave treatment has extremely high and selective sensitivity to NO
2 gas, which makes it useful in gas sensor applications. Kumar et al.
[40] prepared ZnO/rGO nanocomposites by chemical oxidation and microwave radiation. During the preparation process, the high temperature generated by microwave radiation promoted the exfoliation and reduction of GO (500 W, 90 s) and the formation of ZnO nanoparticles (900 W, 45 s). The ZnO/rGO nanocomposites obtained can serve as excellent electrode materials for supercapacitors, with outstanding cyclic stability (82.5% for 3000 cycles at high scan rate 100 mV/s). Zhang et al.
[41] utilized a one-step ultra-fast microwave approach to prepare nickel–cobalt sulfide (NCS)/graphene composite in 1 min. The prepared NCS/graphene composite can be used as a promising electrode material for supercapacitors with high specific capacitance and prominent cycling stability.
In addition to the graphene/inorganic nanocomposites, the microwave aspect also plays an important role in assisting the fabrication of graphene/polymer nanocomposites. Hou et al.
[42] synthesized reduced graphene oxide/polystyrene (rGO/PS) nanocomposites by using microwave treatment to achieve the reduction of GO and the polymerization of styrene simultaneously. The accelerated monomer polymerization by microwave radiation facilitates the production of polymer-based dielectric nanocomposites with tunable dielectric constants, expanding their applications in energy storage. Aldosari et al.
[43] conducted microwave-assisted preparation of polymethylmethacrylate–graphene (PMMA/RGO) nanocomposites and found that the composites obtained by microwave radiation had better morphology, superior dispersion, and higher thermal stability than those obtained by natural polymerization or the direct polymerization of free radical initiators. Based on the excellent properties of graphene, PMMA/RGO composite materials have significantly enhanced electrical, thermal, and optical properties, exhibiting promising applications as it pertains to energy storage materials, catalyst carriers, lightweight fillers, etc.
As a new material in the 21st century, graphene has fascinating properties in many aspects. However, its batch production is still challenged by complex processes, harsh reaction conditions, an inefficient exfoliation rate, and secondary pollution caused by the major wastewater discharge. Therefore, developing an efficient, simple, and controllable technology for graphene preparation is of great significance. The microwave-featured technology provides a novel, efficient, and simplified route for the preparation of graphene and graphene-derivative functional nanomaterials. Firstly, microwave-induced impurity gasification significantly enlarges the graphite layer space, which provides a favorable basis for the exfoliation process. Secondly, microwave-induced Joule heat and thermal shock effect can accelerate the intercalation and expansion steps, resulting in effective high-quality exfoliation and reduction of graphene. Lastly, the featured high-temperature environment and plasma effect facilitate surface chemical reactions to fabricate graphene-derivative functional materials. As an efficient technical route, microwave-assisted fabrication of graphene and graphene-derivative material has significant meaning for the value-added recycling of spent graphite.