What is the difference between hard carbon powder and graphite powder?

As two key anode materials,hard carbon powder and graphite powder are widely used in the field of lithium batteries, but there are many significant differences between them, which extend from the microstructure to the macro performance, and then profoundly affect the performance and applicability of lithium batteries in different application scenarios.

1、Differences in structural characteristics

(1) Crystal structure of graphite powder

Graphite has a typical layered crystal structure in which carbon atoms are tightly packed in a hexagonal pattern to form a layered plane, with layers stacked together by weaker van der Waals force interactions. This unique crystal structure gives graphite good electrical conductivity, as electrons are able to move freely within the layers, while also providing relatively structured embedding and exiting channels for lithium ions. In the process of charge and discharge, lithium ions can shuttle smoothly between graphite layers, so that graphite shows relatively stable electrochemical properties. However, this layered structure also has certain limitations, for example, under extreme conditions such as large external forces or high temperatures, the interlayer stripping of graphite may occur, thus affecting its structural integrity and electrochemical properties.

(2) Amorphous state and chaotic layer structure of hard carbon powder

The Irregular hard carbon powder shows the structure characteristics of amorphous state or chaotic layer. Compared with graphite, its atomic arrangement lacks long-range order, and although there may be graphite-like microstructures in local regions, the overall state of disorder appears. This structure allows the hard carbon powder to have more micropores and defects, which not only increase the specific surface area of the material, but also provide a rich variety of adsorption and storage sites for lithium ions. The storage mechanism of lithium ions in hard carbon powder is more complex, including surface adsorption, pore filling and other ways in addition to embedding reaction. This unique structure makes the hard carbon powder have better performance than graphite in some aspects.

2、the difference in lithium storage performance

(1) Lithium storage capacity and mechanism of graphite powder

The lithium storage of graphite is mainly based on the interlayer embedding and removal process of lithium ions, and its theoretical specific capacity is usually about 340-370 mAh/g. In practical application, the reversible capacity and charge-discharge performance of graphite can be improved to some extent by optimizing the preparation process and surface treatment method.

(2) The high capacity characteristics of hard carbon powder and the way of lithium storage

Because of its unique structure, hard carbon powders exhibit a high theoretical specific capacity, usually up to 500 mAh/g or more, and even close to 1000 mAh/g in some studies. This is mainly due to its abundant microporous structure and surface active sites, which allow lithium ions to be stored inside the material in a variety of ways.

3、the difference in charge and discharge performance

(1) Charge and discharge curve and rate performance of graphite powder

Graphite presents a relatively flat voltage platform during the charge and discharge process, which means that during the process of lithium ion embedding and removal, the voltage change of the battery is slower, and it can provide a more stable voltage output for the external circuit. This property makes graphite an advantage in some applications where voltage stability is required.

(2) Advantages of high rate charging and discharging of hard carbon powder and voltage lag

Due to its abundant lithium ion diffusion path and rapid surface adsorption reaction, hard carbon powder shows remarkable advantages in high rate charging and discharging conditions. Even at higher current densities, the hard carbon powder is still able to maintain a high capacity retention rate, which allows lithium batteries to provide more electrical energy in cases where fast charging and high power discharge are required, thereby reducing charging time and improving the power performance of the device.

4、the difference in cycle life

(1) The cyclic stability and attenuation mechanism of graphite powder

Graphite can show better cycle life under suitable charging and discharging conditions. Due to its relatively stable structure, the capacity attenuation of the graphite electrode is relatively slow after several charge and discharge cycles. However, with the increase of the number of cycles, the graphite electrode will still face some problems of performance attenuation.

(2) Cycle performance challenges and improvement strategies of hard carbon powder

Although the hard carbon powder has high initial capacity and good high rate performance, its cycle life has certain challenges compared with graphite. Due to the high first irreversible capacity of the hard carbon powder, the reconfiguration of the SEI film during the cycle consumes more lithium ions and electrolytes, resulting in relatively fast capacity decay. Moreover, the structure of the hard carbon powder will also change to a certain extent during the long-term cycle, such as the plugging of micropores and the collapse of the microstructure, which further affects its lithium storage performance and cycle stability.

5、Differences in security

(1) Safety hazards and preventive measures of graphite powder

Graphite in the normal charge and discharge process has a good safety, but in some extreme conditions, such as overcharge, overdischarge and high temperature environment, there may be safety problems. When the battery is overcharged, lithium evolution may occur on the surface of the graphite electrode, forming lithium dendrites. The growth of lithium dendrites will not only consume the active lithium in the battery, leading to capacity attenuation, but also may puncture the diaphragm and cause a short circuit of the positive and negative electrodes, which will lead to thermal control of the battery and cause serious safety accidents such as fire or explosion.

(2) Safety advantages and potential risks of hard carbon powder

Because of its unique structure, hard carbon powder has certain advantages in terms of safety. The complex amorphous state and chaotic layer structure make the lithium ions more uniform in the process of embedding and exiting, and it is difficult to form lithium dendrites. Even under a certain degree of overcharge or overdischarge conditions, the structural change of the hard carbon powder electrode is relatively small, which can better maintain the safety of the battery.