Date Published: June 23, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Jiantie Xu, Yuhai Dou, Zengxi Wei, Jianmin Ma, Yonghong Deng, Yutao Li, Huakun Liu, Shixue Dou.
Lithium‐ion batteries (LIBs) with higher energy density are very necessary to meet the increasing demand for devices with better performance. With the commercial success of lithiated graphite, other graphite intercalation compounds (GICs) have also been intensively reported, not only for LIBs, but also for other metal (Na, K, Al) ion batteries. In this Progress Report, we briefly review the application of GICs as anodes and cathodes in metal (Li, Na, K, Al) ion batteries. After a brief introduction on the development history of GICs, the electrochemistry of cationic GICs and anionic GICs is summarized. We further briefly summarize the use of cationic GICs and anionic GICs in alkali ion batteries and the use of anionic GICs in aluminium‐ion batteries. Finally, we reach some conclusions on the drawbacks, major progress, emerging challenges, and some perspectives on the development of GICs for metal (Li, Na, K, Al) ion batteries. Further development of GICs for metal (Li, Na, K, Al) ion batteries is not only a strong supplement to the commercialized success of lithiated‐graphite for LIBs, but also an effective strategy to develop diverse high‐energy batteries for stationary energy storage in the future.
Since its discovery in 2004,1 graphene, consisting of a layer of sp2 bonded carbon atoms arranged in a hexagonal or honeycomb lattice, has received strong attention. Owing to its high surface area (≈2630 m2 g−1), high thermal conductivity (≈5000 W mK−1), large charge carrier mobility (≈200 000 cm2 V−1 s−1), strong mechanical strength (≈130 GPa), and large Young’s modulus (≈1 TPa), graphene holds great promise for practical applications in energy storage/conversion systems, such as supercapacitors (SCs), lithium ion batteries (LIBs), fuel cells, and solar cells.2 Graphite is a mineral with a layered structure, which is composed of many layers of graphene. Graphite intercalation compounds (GICs), with intercalated species between graphene layers, exhibit excellent physical and chemical properties comparable to those of pristine graphite. The physical and chemical properties of GICs are mainly related to the intercalant species, including alkali metal, metal oxides, metal chlorides, bromides, fluorides, oxyhalides, acidic oxides, and Lewis acids, as well as the quality of the graphene (e.g., lateral size, degree of exfoliation, conductivity, and defects).3 Since the concept of the GIC was published in 1841, the development of GICs has experienced several historical periods, leading to GICs with high conductivity, superconductivity (e.g., high transition temperature), and superb storage of hydrogen/lithium ions (Table1).4 There have been a large number of approaches developed for the mass production of GICs, as well as the exfoliation of high quality graphene using GICs.5 Most of these GICs have already been intensively reported for various applications in electrical/thermal conductors, catalysis, and energy storage.6, 7
According to the character of their bonding (covalent and ionic), GICs can be generally classified into two categories: covalent GICs and ionic GICs.41 Covalent GICs include graphite oxide (GO), carbon monofluoride, and tetracarbon monofluoride. In contrast, ionic GICs include graphite salts (e.g., graphite nitrate, graphite bisulphate), graphite−alkali‐metal compounds, graphite‐halogen compounds, and graphite‐metal chloride compounds. Ionic GICs have received more attention than covalent GICs owing to the change in the electronic properties of graphite, which is ascribed to the π‐bonds in graphite that can accept/donate electrons from/to the intercalation, respectively. According to Rüdorff and Daumas–Hérold’s models (Figure1), ionic GICs are further classified in terms of “staging”.40, 42 The stage (n) of GICs is determined by the number of graphene layers between two intercalant layers. For instance, in a stage‐1 GIC, each graphene sheet is separated from the others by intercalant galleries, while the stage‐2 GIC is composed of layers of two adjacent graphene sheets between intercalant galleries. The detailed gallery expansion of ionic GIC along the direction perpendicular (c‐axis) to the hexagonal plane of graphite (e.g. (002), Δd) can be described as:42, 43(1)Δd=Ic−3.35 Å⋅n=di+3.35 Å (n−2)=l⋅dobs−3.35 Å⋅n where Ic, di,dobs, and l are the periodic repeat distance, the intercalant gallery height, the observed value of the spacing between two adjacent planes, and the index of (00l) planes oriented in the stacking direction, respectively.
Several electrochemical strategies have been widely used to prepare GICs as electrodes for energy storage systems (e.g., supercapacitor and metal‐ion batteries, as shown in Table 1) over past decades. In an electrochemical system, ionic GICs (CXm), are formed by insertion (intercalation) of chemical species between the layers of graphene. The reaction can be simply expressed as: (2)C+mX→CXmwhere the interaction between host (graphite) and the guest (X) is a reversible redox process. If the insertion of X into graphite is an anion (Xm−), the reversible reaction is expressed as: (3)Cx+X↔Cxm+⋅Xm−
The intercalation chemistry of GICs is extremely rich in terms of various kinds of intercalants (X). Among all types of X, the alkali elements, group 1 of the Periodic Table (group IA), are very reactive and find it easy to lose their outermost electrons to become cations with charge. This helps alkali atoms to form ionic bonds with other elements. In addition, the alkali elements in group 1 are similar to each other. Therefore, many efforts have been devoted to exploring alkali based GICs. There has been a long development history of cationic GICs with alkali ions (X = Li, Na, K) as species intercalated into graphite, including the chemical formation of Na(NH3)2C12, Li(HMPA)C32, and Na(HMPA)C27,44 where HMPA = hexamethylphosphoramide, and the electrochemical formation of LixC6, NaxC6, KC6, and C–X with various C patterns (e.g., one‐dimensional (1D) carbon nanotube (CNT), two‐dimensional (2D) graphene, three‐dimensional (3D) carbon, and amorphous carbon).45 Unlike cationic‐type GICs intercalated by alkali ions (X = Li, Na, K) that are used as anodes in electrochemical systems, the intercalation of GICs with anions from alkali‐based electrolyte taking place on the cathode side have also been considered recently.
In summary, GICs for alkali ion‐based batteries will continue to hold great promise in energy‐related storage systems in the near future because of their unique chemical and physical properties, and the rich and excellent electrochemical activity of GICs. Graphite with a lithiation mechanism resulting in LiCx suffers from limited electrochemical performance in particular, low capacity and poor rate capability). Owing to the comparatively high natural abundance, low cost, and many similar chemical/physical properties of Na and K to Li, many efforts have also been made to develop graphitic materials for SIBs and PIBs, similar to their LIB counterparts. Compared with the research on LIBs, the development history of SIBs and PIBs (especially PIBs) is quite short. It is very much expected that a great number of developed graphitic anode materials or promising ones undergoing development, as well as carbonaceous materials, for LIBs with high theoretical capacities, high surface area, high conductivity, and superb chemical stability would also be available for SIBs and PIBs. Apart from the development of carbonaceous materials for advanced metal ion batteries, the discovery of advanced electrolytes leading to the formation of ternary GICs (tGICs) with co‐intercalation of electrolyte solvents and alkali ions is also a promising and insightful topic for investigation in research on graphite, as well as other related carbonaceous materials.
The authors declare no conflict of interest.