A lithium-ion battery is a recyclable energy storage device, also known as a lithium-ion secondary battery, composed of a positive electrode, a negative electrode, a separator, and an electrolyte system. This type of battery is characterized by its high energy density compared to other primary batteries, the absence of a memory effect, and low self-discharge. The aggregate for the negative electrode material of lithium-ion batteries is mainly divided into artificial graphite and natural graphite. The raw materials for artificial graphite are primarily oil-based and coal-based needle coke.
Sony's commercially used lithium-ion battery anode material is petroleum coke. High-quality petroleum coke, represented by needle coke, has a series of advantages such as low thermal expansion coefficient, low porosity, low sulfur, low ash content, low metal content, high conductivity, and easy graphitization, and is therefore regarded as a high-quality raw material for lithium-ion battery anode materials.
High-quality petroleum coke is used in lithium-ion battery anode materials, generally requiring processes such as purification, crushing and particle size sieving, graphitization, and surface modification. The entire process is relatively long, and many factors influence the final result. Some of the most concerning issues are:
(1) The mechanism of carbon material structure change with temperature;
(2) The relationship between the performance of negative electrode materials and the structure of carbon materials;
(3) Are there any suitable carbon materials that meet the requirements for negative electrode materials in power lithium-ion batteries?
This paper will review the research in these areas, and finally discuss the structural characteristics of petroleum coke materials suitable for anode materials and the future development trend of petroleum coke anode materials.
1. The effect of post-processing temperature on the performance of high-quality petroleum coke
The post-heat treatment of high-quality petroleum coke consists of two stages: calcination and high-temperature graphitization. Calcination refers to the calcination process below 1500℃, while high-temperature graphitization refers to the high-temperature treatment process approaching 3000℃.
High-quality petroleum coke produced by delayed coking process undergoes calcination in a rotary kiln, significantly reducing moisture and volatile matter content, making transportation and storage more convenient. During graphitization, graphitization temperature is a crucial factor, influencing the degree of graphitization in high-quality petroleum coke.
Liu Chunfa et al. investigated the effect of calcination temperature on the electrochemical performance of needle-shaped petroleum coke-based lithium-ion battery anode materials using methods such as cycle performance, charge-discharge characteristics, and cyclic voltammetry curves. Within the temperature range of 700–1000℃, higher temperatures resulted in smaller interlayer spacing of the graphite in the carbonized samples and increased structural order; the coke at this stage can be termed soft carbon. Samples treated at this temperature exhibited an initial capacity exceeding the theoretical capacity of graphite by 372 mAh/g. However, needle-shaped petroleum coke-based lithium-ion battery anode materials struggled to achieve stable charge-discharge potentials and exhibited poor cycle performance.
The research group further extended the highest carbonization temperature to 2800℃, investigating the changes in the graphite microcrystalline structure and its electrochemical performance during heat treatment. The paper indicates that at 2800℃, the treated needle-shaped petroleum coke sample was nearly pure graphite. Battery charge-discharge experiments showed that the sample achieved a stable lithium intercalation capacity of 300 mAh/g and exhibited a stable charge-discharge plateau. The degree of change in graphite microcrystalline structure with temperature varied depending on the soft carbon structure.
Niu Pengxing et al. graphitized needle coke and pitch coke at 2800℃ and found that after 40 charge-discharge cycles, the lithium intercalation capacity of graphitized needle coke remained stable at 301 mAh/g, while that of graphitized pitch coke was only 240 mAh/g. This is because the raw material for needle coke is purified, and a broad-area mesophase can be formed during the coking process, making it easier and more graphitized.
Therefore, the effect of graphitization temperature on material properties is also related to the material's structure. Figure 1 shows the relationship between the capacitance properties of coke materials and processing temperature and internal structure; these two figures can also explain the above phenomenon.
2. Microstructure and Lithium Storage Mechanism of High-Quality Petroleum Coke
Isao Mochida's research group proposed a different structural model for carbon materials than Franklin's, offering new perspectives on easily graphitized and difficult-to-graphitize coke, as illustrated in Figure 2. Through direct observation of coke using scanning tunneling electron microscopy (STM), they found that regardless of whether the coke is easily or difficult to graphitize, the basic size of a single micro-region is approximately 2–5 nm. The difference lies in the fact that easily graphitized coke has a relatively uniform wide area, consisting of multiple tightly connected micro-regions, with an overall size increase of 20–70 nm after graphitization. In contrast, difficult-to-graphitize coke has a non-uniform wide area, consisting mostly of independent micro-regions with a few connected ones, and its size increase after graphitization is relatively small, ranging from 5–18 nm.
Difficult-to-graphitize coke is considered to have torsional stress between micro-regions, making it difficult for these micro-regions to connect and thus limiting crystal size. Therefore, low-quality coke will not achieve a high degree of crystallinity even at high temperatures, thus affecting its performance as a negative electrode material.
There are two lithium storage mechanisms using petroleum coke, as shown in Figure 3:
(1) Represented by soft carbon, there are multiple lithium storage mechanisms, such as interlayer lithium storage of graphite microcrystals, lithium storage of internal nanopores or cracks of soft carbon, and the reaction of surface defects or residual functional groups of carbon materials with Li+ to form a solid electrolyte membrane (SEI), etc.
(2) The second type is represented by artificial graphite, which mainly stores lithium between graphite sheets, so its initial capacity is smaller than that of soft carbon.
In summary, the graphitization temperature ultimately affects the internal structure of high-quality petroleum coke and other carbon materials. A more ordered internal structure, making graphitization easier, results in higher capacity and better cycle efficiency for the anode. However, while highly graphitized carbon materials exhibit high capacity and a stable charge-discharge platform, their cycle performance and low-temperature performance are actually poor. This is because when Li+ ions intercalate into the graphite layer, they form intercalation compounds with the lamellar graphite, causing the graphite layer to expand. When Li+ ions are extracted, the graphite returns to its original state. This repeated expansion and contraction can easily damage the graphite layer structure and may also lead to solvent co-intercalation, thus reducing the cycle performance of the anode. Therefore, during the graphitization process of high-quality petroleum coke and other carbon materials, the degree of graphitization should be controlled. Some amorphous structure is needed between the microcrystals to maintain a certain structural strength.
3. Soft carbon as a negative electrode material for lithium-ion batteries
Power lithium-ion batteries have different requirements for negative electrode materials compared to ordinary lithium-ion batteries. They need higher rate performance to shorten charging time, good low-temperature performance to meet different working environments, large capacity to reduce battery size, and better stability to prevent safety issues during use.
Soft carbon, as an anode material, exhibits low initial efficiency and lacks a stable voltage plateau. Regarding the low initial cycle efficiency, Alcántara et al. offered two explanations:
(1) Due to the irreversible reaction between Li+ and aliphatic hydrocarbons in coke at low temperatures;
(2) The combination of Li+ with graphite fragments present at the exposed edges of the coke causes irreversible damage. In addition to the low initial cycle efficiency, the gaps between the layers cause lag in the charge and discharge voltage, resulting in electrode instability. However, the advantages of soft carbon anode materials are that they have a relatively high operating voltage, which can prevent lithium metal deposition from causing short circuits and other safety issues. Secondly, they are low in cost and do not require high-temperature graphitization.
Furthermore, Li Yang et al. compared the performance of soft carbon and mesophase carbon microspheres (MCMB) as anode materials for lithium-ion power batteries. They found that soft carbon materials were inferior to mesophase carbon microspheres in terms of initial charge-discharge capacity and coulombic efficiency, but had significant advantages in high-rate charging performance at room temperature and low-temperature charging performance. Therefore, if methods can be found to improve the shortcomings of soft carbon and leverage its strengths, it will promote the application of soft carbon as an anode material for power lithium-ion batteries.
Considering the advantages of soft carbon, Liu Ping et al. incorporated it into conventional graphite-based anode materials to improve the low-temperature charging performance of high-capacity lithium-ion batteries. They found that doping with 20% soft carbon achieved the expected low-temperature charging effect and exhibited good cycle life.
Pan Guanghong et al. combined soft carbon and hard carbon to obtain a soft/hard composite carbon lithium-ion battery anode material with significantly improved rate performance while maintaining high capacity and coulombic efficiency. Other researchers have also modified soft carbon using nano-coatings and conductive carbon layers to achieve good cycle performance and coulombic efficiency.
Alcántara significantly improved the capacitance and cycle stability of petroleum coke by modifying it with Fe2O3. He explained this phenomenon by stating that the oxide stabilizes the soft carbon structure, reduces surface active sites, and forms a stable protective layer on the surface.
In addition, Alcántara et al. pointed out that soft carbon, when used as an anode material in sodium batteries, exhibits higher capacity and cycle efficiency than high-temperature graphitized coke. Literature indicates that soft carbon is also suitable for lithium-ion capacitors, demonstrating safety and excellent cycle performance. After pre-lithiation treatment, soft carbon exhibits even better capacity and cycle stability, showing promise for applications in long-cycle power batteries.
4. Conclusion and Outlook
Petroleum-based coke, suitable for lithium-ion battery anode materials, has low content of heteroatoms such as sulfur (S) and oxygen (O), making it easy to graphitize. It also requires suitable particle size distribution and a small surface area. High-quality calcined petroleum coke and other soft carbon materials exhibit excellent performance at low temperatures and rate, making them increasingly popular in the field of power lithium-ion battery anode materials. However, issues related to cycle efficiency and stability still need to be addressed.
Calcination and graphitization can alter the internal structure of high-quality petroleum coke materials, thereby changing their electrochemical performance as a negative electrode material. However, graphitized materials still require further upgrading using materials engineering methods to achieve good cycle life, rate capability, and high capacity performance.
There are three future development trends for petroleum coke-based anode materials:
(1) To gain a deeper understanding of the structure of coke and its influencing factors, so as to achieve the purpose of customized preparation and to develop lithium-ion batteries with higher capacity and higher rate performance;
(2) Development and commercial application of novel composite coke-based anode materials;
(3) Development of new petroleum coke-based anode materials, including the mass production of petroleum coke-based carbon nanotube anode materials and new coke positive and negative electrode materials that match new battery systems.
