Carbon materials have been widely used in electrochemical technology. In fact, this is partly due to the carbon materials possessing the advantages of good thermal/electrical conductivity, low density, corrosion resistance, low coefficient of thermal expansion, low elasticity, low cost and high purity; however, most of this is due to The flexibility and intricacy of carbon materials in functional structure. Carbon materials have numerous structures, each of which has a profound effect on the electrochemical properties of carbon.
The smallest spatial structures are formed by chemical bonds between carbon atoms. The carbon atoms are connected in the form of sp3, sp2 and sp hybrid orbitals. Carbon materials are usually formed in the form of repeating sp2 bonds connecting C-C atoms, and the planar hexagonal network structure (similar to a honeycomb) composed of carbon atoms is called a graphene layer. Graphene layers are sometimes doped with other elements, such as phosphorus, boron, nitrogen, and silicon, which disrupt the order of C-C sp2 bonds and alter the lithium intercalation properties of carbon. For example, Dahn’s team has tried to incorporate B[63] or N[64] elements into the C-C network, and found that N would intercalate into the carbon lattice leading to lower operating voltages, while substitution of B would make the operating voltage lower. voltage rises. As a result, N reduces the reversible capacity while B increases the reversible capacity of storage. In addition, there are many sp3 hybrid bonds such as C-C (rocking chair carbon structure), C-H, C-OH and C-COOH in the carbon atoms at the edges or defects of the graphite layer; the chemical activity of sp3 hybrid carbon atoms seems to be stronger than sp2-hybridized C C bonds. Many research groups have discovered graphitic carbon that stitches the edges of adjacent graphene layers to obtain a capped surface structure, which is similar to carbon nanotubes [65]. The edge-coupling process of graphene layers is accomplished through the connection of single-bonded carbon atoms. Capped graphitic carbon releases a tiny initial irreversible capacity as the chemically stable surface hinders the formation of SEI films.
In fact, each graphene layer can be thought of as a large co-rolled polymer. The layers are connected by van der Waals forces to form an ordered crystallite structure. Orderly stacking of graphite layers forms ideal crystallites in two forms, one in the …ABAB…order and the other in the …ABCABCABC…order. The former is a symmetrical hexagonal structure with higher thermal stability, and the latter is a symmetrical rhombic structure. Graphite usually contains these two crystal structures, but the content of rhombic structure is always less than 30% [66~68]. Several studies have shown that carbon with a high content of rhombic structure can suppress the exfoliation effect when solvated Li+ intercalates into the graphite layer, especially in PC-based electrolytes. [69] recently concluded that in most graphite crystals, the presence of rhombohedral stacking modes has no direct effect on the initial irreversible capacity, after an exhaustive study of high-temperature annealing effects and combustion treatments during the preparation of artificial graphite. The irreversible capacity is related to the decomposition of the PC-based electrolyte, and the exfoliation of graphite seems to depend on the surface morphology of the graphite.
The van der Waals force between the graphite layers is quite weak, and the layers are easy to slide. Therefore, random rotations and translations of graphene layers in the carbon matrix lead to stacking dislocations of varying degrees. Most of the carbon atoms deviate from their normal positions, and the periodic stacking is no longer continuous. This form of structure is called turbulent disorder. Its X-ray diffraction (XRD) pattern shows only broad (001) and asymmetric (hκ) diffraction peaks [70], which is due to the weak but roughly parallel three-dimensional spatial regularity, and the random stacking of graphite sheets can be avoided. been detected. For turbulent disordered carbon, the interlayer spacing is larger, usually larger than that of graphite. Turbulent disordered carbon forms in two forms: soft carbon, whose disordered structure is easily eliminated when heated to nearly 3000°C; and hard carbon, whose disordered structure is difficult to eliminate at any temperature. Franklin proposed structural models of soft carbon and hard carbon, as shown in Figure 1.
Many studies have been carried out on the relationship between the turbulent disordered structure and the lithium intercalation behavior of carbon. Through extensive research, Dahn’s group [71,72] developed an automatic structure refinement procedure based on XRD to deal with turbulent disordered carbon. Many basic parameters are calculated by this program, and the storage capacity of Li+ in carbon can be quantitatively calculated. This procedure was applied to more than 40 soft carbon materials and proved valuable. In addition, the Osaka Gas research group proposed the following equation from a mathematical point of view to predict the carbon storage capacity [73]:
The R&D team of Toshiba Corporation of Japan discovered the relationship between the discharge capacity of several soft carbons and the experimentally obtained average interlayer spacing (d002) (as shown in Fig. 2) [74]. The figure actually reflects two trends in the selection of carbon materials in lithium-ion secondary batteries. The value of d002 is 0.344 nm, which is practically equivalent to turbostratic carbon. The minimum value of 0.344nm can be selected as the starting point, and focus on two directions: one is the direction in which the d002 value decreases, which means that more graphitic carbon is obtained; the other is the direction in which the d002 value increases, This means screening out more disordered carbons. High-capacity carbon can be obtained either way.
Figure 3: Relationship between P1 and 1—P1 and the reversible capacity of soft carbon in different voltage ranges
Tatsumi et al found soft carbon in the voltage range. The relationship between P1 and capacity at 0~0.25V (for Li/Li + ) [75]. P1 represents the volume ratio of regularly stacked graphite crystals in carbon. In contrast, Dahn et al. used P to denote the probability of random stacking of two adjacent graphene layers [22, 71, 72, 76], i.e. P1=1-P. Furthermore, Tstsumi et al. denoted the capacity change in the voltage range 0.25–1.3 V (for Li/Li+) with the fraction of turbulent disordered structure as 1-P1. The relationship between P, -P1 and the reversible capacity in different voltage ranges is shown in Fig. 3.
Fujimoto et al. calculated an ideal probability function to simulate the normal (hk) XRD peaks of soft carbon with adjacent parallel graphene layers [77~78]° twisted layers of turbulent disordered carbon. Structures with different stacking sequences all have “ripple” pattern structures. Li+ can be embedded in the AA stacking “islands”, but not in the AB layer part. In Tatsumi’s research inference, the maximum Li+ storage capacity of the turbulent disordered structure can be estimated by Li0.2C6.
The surface structure of carbon also has an important influence on the battery performance. Carbon has two surface structures: one at the basal plane of the graphitic layers and the other at the end faces at the edges of each layer of graphite. The study of Li+ intercalation in highly oriented pyrolytic graphite (HOPG) shows that the basal plane is inert, while the end-face intercalation of Li+ is active. Nine Li+ is mainly intercalated into the graphite host through the end-face, and only a small amount of Li+ can pass through the basal plane. of defects into the graphite body. In addition, different functional groups on the end face also have a significant impact on Li+ intercalation. Some literatures have also reported a direct relationship between specific surface area and irreversible capacity.
Another structure of carbon is the textured structure, which is the way the crystallites bond. Texture is usually characterized by the degree of orientation of the crystallites from random to ordered arrangements. If the crystallite size is small enough and there is no well-defined orientation, the carbon will be in an amorphous state. Controlling the texture structure does not alter the properties of the individual crystallites, but can alter the properties of the crystallite aggregates, such as electrical conductivity and active surface area. A comparative study of mesophase pitch-based carbon fibers with different textures [83] shows that the radial texture is more favorable for Li+ intercalation than the concentric spherical texture, but the radial texture is more likely to be broken into flakes by solvated Li+.
The aggregation of different forms of textured structures can be thought of as a special larger-sized carbon structure. Since the electrochemical properties depend in part on the macrostructure of the electrode material, the aggregation state also plays a crucial role in the electrochemical performance. For carbon black [84,85], the adsorption capacity of dibutyl phthalate (DBP) can be used to characterize the relationship between the aggregation state of carbon black primary particles and their electrochemical properties. The neck position (that is, the adsorption position of DBP) connects two primary carbon black particles in the agglomerate and is parallel to the graphite plane to intercalate Li+, however, the primary carbon black particles themselves have a concentric spherical layered structure, which is not conducive to the intercalation and desorption of Li+ embedded. Examples of other aggregated structures: graphite + carbon black (acetylene) composite electrode [39,86], carbon fiber combined with carbonized epoxy resin [87], graphite coated with carbon coating (core-shell structure) [8~90], Or in some carbon fibers, two different texture structures coexist in one fiber [83].
Read more: What are the latest developments in cathode materials for lithium-ion batteries?
