There are many technical indicators of graphite anode materials, and it is difficult to take into account, mainly including specific surface area, particle size distribution, tap density, compaction density, true density, first charge and discharge specific capacity, first efficiency, etc. In addition, there are electrochemical indicators such as cycle performance, rate performance, swelling, and so on. So, what are the performance indicators of graphite anode materials? The following content is introduced to you by HCMilling(Guilin Hongcheng), the manufacturer of the anode materials grinding mill.

01 specific surface area

Refers to the surface area of an object per unit mass. The smaller the particle, the larger the specific surface area.

The negative electrode with small particles and high specific surface area has more channels and shorter paths for lithium ion migration, and the rate performance is better. However, due to the large contact area with the electrolyte, the area for forming the SEI film is also large, and the initial efficiency will also become lower. . Larger particles, on the other hand, have the advantage of greater compaction density.

The specific surface area of the graphite anode materials is preferably less than 5m2/g.

02 Particle size distribution

The influence of the particle size of graphite anode material on its electrochemical performance is that the particle size of the anode material will directly affect the tap density of the material and the specific surface area of the material.

The size of the tap density will directly affect the volume energy density of the material, and only the appropriate particle size distribution of the material can maximize the performance of the material.

03 Tap Density

The tap density is the mass per unit volume measured by the vibration that makes the powder appear in a relatively tight packing form. It is an important indicator to measure the active material. The volume of the lithium-ion battery is limited. If the tap density is high, the active material per unit volume has a large mass, and the volume capacity is high.

04 Compaction Density

The compaction density is mainly for the pole piece, which refers to the density after rolling after the negative electrode active material and the binder are made into the pole piece, compaction density = area density / (the thickness of the pole piece after rolling minus the thickness of the copper foil ).

The compaction density is closely related to the sheet specific capacity, efficiency, internal resistance and battery cycle performance.

Influencing factors of compaction density: particle size, distribution and morphology all have an effect.

05 True Density

The weight of solid matter per unit volume of a material in an absolutely dense state (excluding internal voids).

Since the true density is measured in a compacted state, it will be higher than the tapped density. Generally, true density > compacted density > tapped density.

06 The first charge and discharge specific capacity

The graphite anode material has irreversible capacity in the initial charge-discharge cycle. During the first charging process of the lithium-ion battery, the surface of the anode material is intercalated with lithium ions and the solvent molecules in the electrolyte are co-inserted, and the surface of the anode material decomposes to form SEI. Passivation film. Only after the negative electrode surface was completely covered by the SEI film, the solvent molecules could not intercalate, and the reaction was stopped. The generation of SEI film consumes a part of lithium ions, and this part of lithium ions cannot be extracted from the surface of the negative electrode during the discharge process, thus causing irreversible capacity loss, thereby reducing the specific capacity of the first discharge.

07 First Coulomb Efficiency

An important indicator for evaluating the performance of a anode materials is its first charge-discharge efficiency, also known as the first Coulomb efficiency. For the first time, the Coulombic efficiency directly determines the performance of the electrode material.

Since the SEI film is mostly formed on the surface of the electrode material, the specific surface area of the electrode material directly affects the formation area of the SEI film. The larger the specific surface area, the larger the contact area with the electrolyte and the larger the area for forming the SEI film.

It is generally believed that the formation of a stable SEI film is beneficial to the charging and discharging of the battery, and the unstable SEI film is unfavorable for the reaction, which will continuously consume the electrolyte, thicken the thickness of the SEI film, and increase the internal resistance.

08 Cycle performance

The cycle performance of a battery refers to the number of charges and discharges that the battery experiences under a certain charge and discharge regime when the battery capacity drops to a specified value. In terms of cycle performance, the SEI film will hinder the diffusion of lithium ions to a certain extent. As the number of cycles increases, the SEI film will continue to fall off, peel off, and deposit on the surface of the negative electrode, resulting in a gradual increase in the internal resistance of the negative electrode, which brings heat Accumulation and capacity loss.

09 Expansion

There is a positive correlation between expansion and cycle life. After the negative electrode expands, first, the winding core will be deformed, the negative electrode particles will form micro-cracks, the SEI film will be broken and reorganized, the electrolyte will be consumed, and the cycle performance will be deteriorated; second, the diaphragm will be squeezed. Pressure, especially the extrusion of the diaphragm at the right-angle edge of the pole ear, is very serious, and it is easy to cause micro-short circuit or micro-metal lithium precipitation with the progress of the charge-discharge cycle.

As far as the expansion itself is concerned, lithium ions will be embedded in the graphite interlayer spacing during the graphite intercalation process, resulting in an expansion of the interlayer spacing and an increase in volume. This expansion part is irreversible. The amount of expansion is related to the degree of orientation of the negative electrode, the degree of orientation = I004/I110, which can be calculated from the XRD data. The anisotropic graphite material tends to undergo lattice expansion in the same direction (the C-axis direction of the graphite crystal) during the lithium intercalation process, which will result in a larger volume expansion of the battery.

10 Rate performance

The diffusion of lithium ions in the graphite anode material has a strong directionality, that is, it can only be inserted perpendicular to the end face of the C-axis of the graphite crystal. The anode materialss with small particles and high specific surface area have better rate performance. In addition, the electrode surface resistance (due to the SEI film) and the electrode conductivity also affect the rate performance.

Same as the cycle life and expansion, the isotropic negative electrode has many lithium ion transport channels, which solves the problems of less entrances and low diffusion rates in the anisotropic structure. Most of the materials use technologies such as granulation and coating to improve their rate performance.

HCMilling(Guilin Hongcheng) is a manufacturer of anode materials grinding mill. HLMX series anode materials super-fine vertical mill, HCH anode materials ultra-fine mill and other graphite grinding mill produced by us have been widely used in the production of graphite anode materials. If you have related needs, please contact us for details of the equipment and provide follow information to us:

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