Graphite Material: Structure and Properties
A comprehensive analysis of graphite's crystalline structure, properties, and applications in rechargeable lithium ion batteries, highlighting its significance as a key material in energy storage technology.
Composition and Structure
Graphite is a crystalline material with a layered structure composed of carbon atoms arranged in a hexagonal network, forming parallel stacked planes. It belongs to the hexagonal crystal system with a P63/nc space group, making it particularly suitable for applications in rechargeable lithium ion batteries due to its unique atomic arrangement.
Within each graphite plane, carbon atoms are arranged in a hexagonal pattern, where each carbon atom forms covalent bonds with three adjacent carbon atoms through sp² hybrid orbitals. The carbon-carbon bond length is 0.1421nm, while electrons in the p orbitals form delocalized π bonds, contributing to the excellent electrical conductivity within the graphite layers. This conductivity is one of the key reasons graphite is widely used in rechargeable lithium ion batteries.
Carbon atoms in adjacent layers are not stacked in an aligned manner but rather form two distinct structures: hexagonal and rhombohedral. The hexagonal structure follows an ABABAB... stacking model, while the rhombohedral structure adopts an ABCABCABC... stacking pattern, as illustrated in Figure 2-16.
The theoretical interlayer spacing is 0.3354nm, with unit cell parameters: a₀ = 0.246nm and c₀ = 0.670nm. These precise measurements are critical for understanding how lithium ions intercalate within the structure, a fundamental process in rechargeable lithium ion batteries.
Figure 2-16: Crystal Structures of Graphite
(a) Hexagonal Structure
ABABAB... stacking model
(b) Rhombohedral Structure
ABCABCABC... stacking model
Graphite Intercalation Compounds
Graphite intercalation compounds exhibit a staging phenomenon, where the stage number equals the number of graphite layers between two adjacent intercalated layers. Examples include stage 1 LiC₆, stage 2 LiC₁₂, stage 3 LiC₁₈, and stage 4 LiC₂₄ compounds, all of which are critical for the operation of rechargeable lithium ion batteries.
During the charging process of graphite in rechargeable lithium ion batteries, the charging voltage gradually decreases, forming voltage plateaus that correspond to the transformation of higher-stage compounds to lower-stage compounds. These voltage plateaus directly relate to the transitions between adjacent stage intercalation compounds.
Low concentrations of lithium are randomly distributed throughout the graphite lattice, existing in a dilute stage 1 form. The transition voltage from dilute stage 1 to stage 4 is 0.20V, while the transition from stage 4 to stage 3 occurs over a continuous voltage range. The transition from stage 3 to dilute stage 2 (2L stage) has an overvoltage of 0.14V, followed by the transition from 2L stage to stage 2 at 0.12V, and finally from stage 2 to stage 1 at 0.09V.
Figure 2-19: Relationship Between Graphite Intercalation Stages and Voltage Plateaus
When fully lithiated, the stage 1 compound LiC₆ is formed, which has a specific capacity of 372 mA·h/g — the theoretical maximum for graphite under normal temperature and pressure conditions in rechargeable lithium ion batteries.
Researchers have successfully prepared graphite intercalation compounds (LiₓC₆) with x > 1 under high-temperature and high-pressure conditions, demonstrating that rechargeable lithium ion batteries can potentially achieve higher capacities under high-pressure environments. This discovery opens new avenues for improving the energy density of rechargeable lithium ion batteries beyond current theoretical limits.
SEI Film Formation
Graphite anode materials exhibit a distinct charging plateau around 0.8V during the first charge cycle in rechargeable lithium ion batteries. This plateau disappears during the second discharge and is considered an irreversible process, as shown in Figure 2-20. This phenomenon is unrelated to the intercalation compound formation but is instead associated with the formation of the Solid Electrolyte Interphase (SEI) film.
The SEI film is an ion-conductive, electron-insulating solid electrolyte interface layer that forms on the surface of graphite anodes in rechargeable lithium ion batteries. During the first charge, the edge and basal planes of graphite carbon layers are in an exposed state, exhibiting a very low electrochemical potential and strong reducing properties.
It is now widely accepted that no electrolyte can resist the low electrochemical potential of lithium and highly lithiated carbon. Therefore, during the initial charging of graphite anode materials in rechargeable lithium ion batteries, the electrolyte and solvent undergo reduction reactions on the graphite surface.
The reaction products include solid compounds such as Li₂CO₃, LiF, LiOH, and organic lithium compounds. These solid products deposit on the surface of the carbon material, creating a layer that conducts ions while preventing electron conduction, thereby stopping further decomposition of the electrolyte. This significantly reduces irreversible reactions in rechargeable lithium ion batteries, resulting in stable cycling performance.
In essence, the formation of an ion-conductive, electron-insulating SEI film is what enables carbon materials to exhibit stable and reversible lithium intercalation/deintercalation capabilities in rechargeable lithium ion batteries.
Figure 2-20: First Charge-Discharge Curve of Graphite
Key Observations:
- Irreversible SEI formation plateau at ~0.8V
- Lithium intercalation plateaus during charging
- Lithium deintercalation during discharge
- Capacity difference indicating irreversible loss
Irreversible Reactions
Beyond electrolyte decomposition and SEI film formation, other factors contribute to irreversible capacity loss in rechargeable lithium ion batteries. These include irreversible reduction of water and oxygen adsorbed on the graphite surface, as well as decomposition of surface functional groups.
Factors Affecting Irreversible Capacity
- Specific surface area of the graphite anode material
- Number of surface functional groups
- Ratio of basal planes to edge planes
- Lithium ion co-intercalation at graphite edges
- Self-discharge processes within the electrode
Research has shown that these factors significantly impact battery performance metrics in rechargeable lithium ion batteries:
- Larger specific surface area typically increases irreversible capacity
- More surface functional groups reduce first-cycle Coulombic efficiency
- Edge planes contribute more to irreversible capacity than outer surfaces
Properties and Applications
Graphite anode materials have a theoretical specific capacity of 372 mA·h/g, making them ideal for rechargeable lithium ion batteries. Batteries constructed with graphite anodes exhibit several advantageous characteristics that have established them as industry standards.
High Operating Voltage
Maintains high and stable operating voltages throughout the discharge cycle, a critical feature for consistent performance in rechargeable lithium ion batteries.
Excellent Cycle Performance
Demonstrates exceptional stability over numerous charge-discharge cycles, extending the lifespan of rechargeable lithium ion batteries.
High Coulombic Efficiency
Exhibits high first-cycle charge-discharge efficiency, minimizing energy loss in rechargeable lithium ion batteries.
Due to these favorable properties, graphite is currently the most widely used anode material in industrial applications for rechargeable lithium ion batteries. However, graphite anode materials exhibit poor compatibility with PC-based electrolytes, which has led to the development of modification techniques.
Carbon coating modification has emerged as an effective solution to enhance the structural stability and electrochemical performance of graphite in rechargeable lithium ion batteries. This surface treatment improves electrolyte compatibility while maintaining the desirable properties that make graphite such a valuable material for energy storage applications.