Lithium ion battery is a recyclable energy storage device, also known as lithium ion secondary battery, which is composed of a positive electrode, a negative electrode, a diaphragm and an electrolytic liquid system.This type of battery is characterized by high energy density compared to other primary batteries, no memory effect and low self-discharge.Lithium ion battery anode material aggregate is mainly divided into artificial graphite and natural graphite.The raw material of artificial graphite is mainly oil and coal needle coke.

High quality petroleum coke, represented by acicular petroleum coke, has a series of advantages such as low thermal expansion coefficient, low void, low sulfur, low ash, low metal content, high conductivity and easy graphitization, so it is regarded as high quality anode materials for lithium ion batteries.

High quality petroleum coke is used as anode material for lithium ion batteries, which generally requires purification, crushing, particle size screening, graphitization, surface modification and other processes.The whole process is relatively long, and the final effect is more influential factors.Some of the biggest concerns are:
(1) The mechanism of carbon structure changing with temperature;
(2) The relationship between the properties of anode materials and the structure of carbon materials;
(3) Are there any suitable carbon materials to meet the needs of the anode materials for lithium ion batteries?

1. The influence of post-treatment temperature of high quality petroleum coke on its performance

The post-heat treatment of high quality petroleum coke is divided into two stages: calcination and high temperature graphitization.Calcination refers to the calcination process below 1500, and high temperature graphitization refers to the high temperature treatment process close to 3000.
The high quality petroleum coke produced by the delayed coking process is calcined in the rotary furnace, which significantly reduces moisture and volatile matter, and is more convenient for transportation and storage.During the graphitization process, the graphitization temperature is a key factor affecting the graphitization degree of high-quality petroleum coke.

In the range of 700 ~ 1000, the higher the temperature, the smaller the graphite layer spacing of the carbonized sample, the increase of the structure order of the sample, this period of coke can be called soft carbon.The initial capacitance of the sample treated at this temperature is higher than the theoretical capacitance of graphite of 340 mAh/g.However, it is difficult to obtain stable charge and discharge potential for lithium ion battery anode materials made of needle-like petroleum coke.

After the graphitization of acicular petroleum coke and pitch coke at 2800, it was found that the graphitized acicular petroleum coke after repeated charging and discharging 40 times, its lithium capacity can be stable at 301mAh/g, while the graphitized pitch coke is only 240mAh/g.This is because the raw material of acicular petroleum coke is purified, and the wide area mesophase can be formed in the coking process. Finally, the acicular petroleum coke is easier to graphitization and the graphitization degree is higher.

2.Microstructure and lithium storage mechanism of high quality petroleum coke

(1) Represented by soft carbon, there are various lithium storage mechanisms, such as interlaminar lithium storage of graphite microcrystals, lithium storage by nano-pores or cracks in soft carbon, and solid electrolyte film (SEI) generated by reaction of surface defects or residual functional groups of carbon materials with Li+, etc.

(2) The second kind, represented by artificial graphite, is mainly the interlayer storage of lithium graphite, so the first capacity will be smaller than soft carbon.

To sum up, the final effect of graphitization temperature is the internal structure of high-quality petroleum coke and other carbon materials. If the internal structure of the material is more orderly and easier to graphitization, the final negative electrode capacity is higher and the cycle efficiency is better.However, although highly graphitized carbon materials have high capacity and stable charge-discharge platform, their cycle performance and low temperature performance are poor.This is because when Li+ is inserted into the graphite layer, it forms a graphite interlayer compound with lamellar graphite, and the graphite layer expands.When Li+ is extruded, graphite is restored to its original state.In the process of repeated expansion and contraction, the structure of graphite layer is easy to be destroyed, and it may cause co-embedding of solvent, so that the cycling performance of the negative electrode decreases.Therefore, the degree of graphitization should be controlled in the graphitization process of carbon materials such as high-quality petroleum coke, and some amorphous structures between microcrystals are needed to maintain a certain structural strength.

3.Soft carbon as anode material of lithium ion battery

Different from ordinary lithium-ion batteries, power lithium-ion batteries need higher rate performance to shorten charging time, good low-temperature performance to meet different working environments, large capacity to reduce the volume of the battery, and better stability to prevent safety problems.
Soft carbon as anode material for the first time has low efficiency and no stable voltage platform.Alcantara et al. offer two explanations for the low efficiency of the first cycle:
(1) Due to Li+ and low temperature aliphatic hydrocarbon reaction in coke caused irreversible;
(2) Li+ binds irreversibly with graphite fragments in the exposed edge of coke.In addition to the low efficiency of the first cycle, due to the gap between the layers, the charge and discharge voltage will lag and the electrode will be unstable.However, the advantage of soft carbon anode material is that the working voltage is relatively high, which can prevent the safe use of lithium metal precipitation caused by short circuit and other problems. Secondly, the cost is low, and no high temperature graphitization is required.

4. Conclusion and prospect

Petroleum coke suitable for lithium ion battery anode material S, O and other heteroatomic content is small, easy to graphitization, and need to have appropriate particle size distribution and small surface area, etc..Calcined high quality petroleum coke and other soft carbon materials have excellent performance in low temperature and rate performance, which makes them more attention in the field of lithium ion battery anode materials, but the problems of cycle efficiency and stability still need to be solved.
Calcination and graphitization can change the internal structure of high quality petroleum coke, and then change its electrochemical performance as a anode material.However, the graphitized material still needs to be upgraded using materials engineering methods to show good cycling, magnification and high volume properties.
There are three development trends of petroleum coke anode materials in the future:
(1) To have a deeper understanding of coke structure and its influencing factors, so as to achieve the purpose of customized preparation, oriented to higher capacity, higher rate performance lithium ion battery;
(2) Development and commercial application of new composite coke anode materials;
(3) Development of new petroleum coke anode materials, including batch preparation of petroleum coke based carbon nanoanode materials, and new coke anode and cathode materials matching with new battery systems.