Classification of Carbonaceous Materials
Carbonaceous materials that are capable of reversible lithium reaction can be roughly classified into two categories according to their structures: graphitic and non-graphitic (disordered) carbon. The non-graphitic carbon can be further categorized into soft carbon/hard carbon upon annealing and high specific charge carbon/low specific charge carbon according to the capability of reversible lithium storage. Charge/discharge behaviors of each type of carbon are presented and analyzed in detail in the following context.
Graphitic carbon is a well-defined layered structure. Normally, a number of structural defects could appear in graphitic carbon. The term of “graphite” was derived from crystallographic point of view which should be only applied to carbons whose layered lattice structure follows a perfect stacking order of graphene layers. That is to say it contains the layer stacking order of either the common AB (hexagonal graphite, Figure 2.4 and Figure 2.5a) or the rather rare ABC (rhombohedral graphite). However, since the transformation energy from AB stacking to ABC stacking (and vice versa) is rather small, perfectly stacked graphite crystals are not readily available. Therefore the term of “graphite” is often used regardless of well-defined stacking order . The terms of natural graphite, artificial graphite, and pyrolytic graphite are commonly used, although the materials are polycrystalline . The actual structure of carbonaceous materials typically deviates more or less from the ideal graphite structure. Materials consisting of aggregates of graphite crystallites are called graphites as well.
Figure 2.4. Left: Schematics of the crystal structure of hexagonal graphite with an AB stacking order. Right: view perpendicular to the basal plane of graphite. Edges can be subdivided into arm-chair and zigzag faces. Cited from Ref. .
Non-graphitic (disordered) carbonaceous materials consist of carbon atoms that are mainly arranged in a planar hexagonal network but no crystallographic order in the c-direction compared to graphite, as shown in Figure 2.5c [7,9]. The structure of those carbons is characterized by amorphous areas embedded and cross-linked in the network. Non-graphitic carbons are mostly prepared by pyrolysis of organic polymer or hydrocarbon precursors at temperature below ~1500°C [10-12].
(a) graphite (b) graphitizable carbon (c) non-graphitizable carbon
Figure 2.5. Schematic indications of (a) graphite and (b) non-graphitic (disordered) carbonaceous material.
Heat treatment of most non-graphitic carbons (from ~1500 to ~3000°C) allows us to further classify non-graphitic carbon into two sub-categories: soft carbon and hard carbon. In the case of soft carbons, crosslinking between the carbon layers is weak and therefore the layers are mobile enough to form graphite-like crystallites and develop the graphite structure continuously during the heating process, as shown in Figure 2.5b . In the case of hard carbons, since the carbon layers are immobilized by crosslinking, they show no real development of the graphite structure even at temperatures of 2500 ~ 3000 °C . The representative figure is shown in Figure 2.5c.
Lithium Intercalation into Carbonaceous Materials
Lithium Intercalation into Graphitic Carbon Materials
Lithium-intercalated graphitic carbon compounds (GICs) are known with the configuration LixCn. It is well known that Li intercalation reaction occurs only at the edge plane of graphite. Through the basal plane, intercalation is possible only at defect sites [13-16]. The maximum lithium content for highly crystalline graphitic carbons is one Li guest atom per six carbon host atoms (i.e. n 6 in LiCn or x 1 in LixC6) at ambient pressure . That is to say it follo ws the equation as below: 6 C + x Li+ +x e- LixC6, where, x = 1 in LixC6 (the maximum Li conte nt).
In LiC6, lithium avoids to occupy the nearest neighbor sites due to the Columbic repulsive force of Li, as shown in Figure 2.6. Two major changes in graphite structure point of view occur when Li intercalats into graphite layers: (1) the stacking order of the carbon layers (i.e. graphene layers) shifts to AA stacking, see Figure 2.6a and Figure 2.6c. (2) The interlayer distance between the graphene layers increases moderately (10.3% has been calculated for LiC6) due to the lithium intercalation, as indicated in the right panel in Figure 2.6a [18-21].
Figure 2.6. Structure indications of LiC6. (a) Left: schematic drawing showing the AA layer stacking sequence with Li intercalation. Right: simplified representation. (b) Perpendicular view to the basal plane of LiC6. (c) Enlarged schematic of AA stacking order. Cited and modified from Ref. [20-21].
An important feature of Li intercalation into graphite is the “stage formation”. Stage formation means a stepwise formation of a periodic pattern of unoccupied graphitic layer gaps at low concentrations of Li [23-31]. This stepwise process can be described by the stage index, s (s = I, II, III, IV) which is equal to the number of graphene layers between two nearest guest layers as shown in Figure 2.7. Note that stage IV is not indicated in the figure because Li concentration is too low in graphene layers. It is also known as a dilute stage when s > IV . Two factors determine the formation of stages during Li intercalation into graphite i) the energy required to expand van der Waals gap between two graphene layers [31,33] and ii) the repulsive interactions between guest species. Therefore, compared to a random distribution of Li in the graphitic layers during charge process, Li ions prefer to occupy van der Waals gaps with high density first to reach an energetically stable state .
Figure 2.7. Schematic indication of stage formation during Li ion intercalation into graphite layers.
Charge/Discharge Profile of Graphitic Carbon Materials
Stage formation as mentioned above is one of the most important characteristic of charge profile for graphitic carbon. It can be easily observed in the form of plateaus by constant current measurement (i.e. in charge/discharge curve), as indicated in Figure 2.8. The associated stages are marked in bottom panel of the figure. The plateaus indicate the coexistence of two phases [24,34]. The formation of stages II, IIL (a transition stage of stage II and stage III), III, and IV have been identified from experimental electrochemical curves [18,35,37-40] and confirmed by X-ray diffraction and Raman spectroscopy [17,25,27-28,35-38]. A schematic potential / composition curve for galvanostatic reduction of graphite to LiC6 is shown in the bottom panel in Figure 2.8.
Figure 2.8. Constant current charge/discharge curves of the graphite (Timrex KS 44, Cirr is the irreversible specific charge, and Crev is the reversible specific charge). Modified and replotted from Ref. .
Ideally, Li+ intercalation into carbons should be fully reversible and the maximum Li storage capacity should not exceed 372 mAh/g according to LiC6 configuration. However, the charge accumulated in the first cycle usually larger than the maximum theoretical specific capacity, as shown in Figure 2.8. Compared to the first charge, the first discharge capacity is much smaller. The excess charge generated in the first cycle which cannot be recovered can be ascribed to a film formation of the solid electrolyte interface (SEI) which is caused by the decomposition of the Li+ containing electrolyte, such as propylene carbonate and ethylene carbonate [41-46]. The decomposition of electrolyte usually takes place at less than 1 V vs. Li/Li+ and appears as the first plateau in the charge curve, as indicated in Figure 2.8 . The advantage of the SEI formation is that it can prevent further electrolyte decomposition and create a rather stable state for the surface of GIC [48-53]. On the other hand, the formation of SEI is a charge-consuming side reaction in the first few Li+ intercalation/deintercalation cycles, especially in the 1st charge cycle. Considering that the positive electrode is responsible to provide the Li ion in LIB, the charge and lithium losses are detrimental to the specific energy of the whole cell and have to be minimized. Because of the irreversible consumption of lithium and electrolyte, a corresponding charge loss exists, so called “irreversible specific charge” as indicated in Figure 2.8. The reversible lithium intercalation is called “reversible specific charge”.
Lithium Intercalation into Non-Graphitic Carbon Materials
According to the capability of reversible lithium storage, non-graphitic carbons can be further classified into two categories: high specific charge carbon and low specific charge carbon.
Low Specific Charge Carbon
Low specific charge carbons are carbonaceous materials which incorporate only a considerably lower amount of lithium than graphite. That is to say it follo ws the equation as below:
6C + xLi+ +xe- LixC6, where x = 0.5~0.8 in LixC6 at the maximum stoichiometry.
(ii) Examples of Low Specific Charge Carbon
Cokes [68,77-83] and carbon blacks [81,84-85] are typical disordered carbons with low specific charges. During the charge process, Li intercalation-induced formation of AA stacking is hindered due to the existence of crosslinking of carbon sheets as mentioned in chapter 2.1.2. This will eventually affect the accommodation of a higher Li amount into graphitic sites and deliver a lower specific charge [86-88].
Turbostratic carbon [43,86-90] which can also be classified into the category of graphitizing/soft carbon is one type of low specific charge carbon. The lower amount of Li intercalation than graphite is due to not only the effect of crosslinking as mentioned in cokes and carbon blacks, but also larger amount of wrinkled and buckled structural segments existing in the structure, and thus available lithium intercalation sites is rather low therefore the specific charge is lower than graphite [91-92].
(iii) Charge/Discharge Profile of Low Specific Charge Carbon
Figure 2.9 shows the first Li+ intercalation/deintercalation cycle of a coke-containing electrode. The potential profile of low specific carbon differs considerably from that of graphite, as the reversible intercalation of Li+ begins at around 1.2 V vs. Li/Li+, and the curve slopes without distinguishable plateaus. This behavior is a consequence of the disordered structure providing electronically and geometrically nonequivalent sites, whereas for a particular intercalation stage in highly crystalline graphite, the sites are equivalent [93-94].
Table of contents :
Bibliography of Introduction
Chapter 1. Overview of Rechargeable Lithium Ion Battery
1.1 Electrochemical Energy Storage Systems
1.2 Rechargeable Lithium Based Battery
1.3 Rechargeable Lithium Ion Battery
Bibliography of Chapter 1
Chapter 2. Carbonaceous Materials as An Anode of Li-Ion Battery
2.1 Operation Mechanism of Li-Ion Battery
2.2 Classification of Carbonaceous Materials
2.2.1 Graphitic Carbon
2.2.2 Non-graphitic Carbon
2.3 Lithium Intercalation into Carbonaceous Materials
2.3.1 Lithium Intercalation into Graphitic Carbon Materials
2.3.1-2 Charge/Discharge Profile of Graphitic Carbon Materials
2.3.2 Lithium Intercalation into Non-graphitic Carbon Materials
2.3.2-1 Low Specific Charge Carbon
2.3.2-2 High Specific Charge Carbon
2.4 Summary of Chapter Two
Bibliography of Chapter 2
Chapter 3. Silicon-Coated Carbon Nanofiber Mat for Anode of Lithium Ion Battery
3.1 One Dimensional Carbon Materials as an Anode Material for LIB
3.1.1 General Introduction of CNFs and CNTs
3.1.2 CNFs and CNTs Using as an Anode Material for LIB
3.1.3 Fabrication Methods of CNFs and CNTs
3.1.3-1 Chemical Vapor Deposition for Both CNFs and CNTs
3.1.3-2 Electrospinning Method for CNFs Mat
3.2 Electrospinning Fabricated CNFs Mat as an Anode Material for LIB
3.2.1 SEM and Raman Characterization of CNFs Synthesized Through Electrospinning
3.2.2 Anode Performance of CNFs Synthesized Through Electrospinning
3.3.3 Anode Performance of CNF-Si Mat
3.4 Summary of Chapter Three
Bibliography of Chapter 3
Chapter 4. Diffusion Mechanism of Lithium Ions through Basal Plane of Layered Graphene
4.1 Brief Introduction of Two Dimensional Graphene
4.1.1 General Physical Properties of Graphene
4.1.2 Synthesis Methods of Graphene
4.2 Diffusion Mechanism of Lithium Ions through Basal Plane of Layered Graphene
4.2.1 Material Preparation
4.2.2 Transfer Process of Graphene
4.2.3 Characterization of Graphene
4.2.4 Anode Performance of Graphene
4.3 Summary of Chapter Four
Bibliography of Chapter 4