10 technical indicators of graphite anode for lithium battery
Graphite anode materials are divided into artificial graphite and natural graphite; natural graphite and artificial graphite anode materials are characterized by high electrical conductivity, large lithium ion diffusion coefficient, high embedded lithium capacity and low embedded lithium potential.
Compared with other types of lithium ion battery anode materials, natural graphite and artificial graphite have comprehensive advantages in terms of battery specific capacity, first time efficiency, cycle life, safety, etc., and the raw materials are widely available and cheap.
The technical indexes of graphite anode materials mainly include specific surface area, particle size distribution, vibration density, compaction density, true density, first charge/discharge specific capacity, first efficiency and so on. In addition, there are electrochemical indicators such as cycling performance, multiplicity performance, swelling and so on. This article will specifically describe the knowledge of the ten technical indicators of graphite anode.
Table of Contents
Specific surface area of graphite anode
Refers to the surface area per unit mass object has, the smaller the particles, the larger the specific surface area will be.
The anode with small particles and high specific surface area has more channels and shorter paths for lithium ions to migrate, so the multiplicity performance is better, but due to the large contact area with the electrolyte, the area to form the SEI film is also large, and the first time efficiency will be lower.
Large particles, on the contrary, have the advantage of greater compaction density. The specific surface area of graphite anode material is less than 5m2/g is appropriate.
Particle size distribution of graphite anode
The effect of particle size on its electrochemical properties is manifested in the fact that the particle size of the anode material will directly affect the vibrational density of the material as well as the specific surface area of the material. The size of the vibration density will directly affect the bulk energy density of the material.
In the same volume of filling parts, the larger the particle size of the material, the wider the particle distribution, the smaller the viscosity of the slurry, the more conducive to improving the solid content and reducing the difficulty of coating.
In addition, when the particle size distribution of the material of the graphite anode is wide, the small particles in the system can be filled in the gaps of the large particles, which helps to increase the compaction density of the pole piece and improve the volumetric energy density of the battery.
The characteristic parameters of the particle size distribution of the material of the graphite anode are D50, D10, D90 and Dmax, of which D50 represents the particle size value corresponding to the accumulated amount of 50% in the cumulative distribution curve of the particle size, which can be regarded as the average particle size of the material.
In addition, the width of the particle size distribution of the material can be expressed by K90, K90=(D90-D10)/D50, the larger K90 is, the wider the distribution is. The particle size of graphite anode material is mainly determined by its preparation method, and the requirements for its particle size parameters in the graphite standard are D50 (about 20 μm), Dmax (≤70 μm) and D10 (about 10 μm).
Vibrational density of graphite anode
Relying on the vibration makes the powder present a more compact pile form, the mass per unit volume measured. It is an important indicator to measure the active material, lithium-ion battery volume is limited, if the vibration density is high, the active material mass per unit volume is high, the volume capacity is high.
Compacted density of graphite anode
Mainly for the pole piece, refers to the density of the anode active material and binder, etc. made into the pole piece, after rolling, compacted density = surface density / (the thickness of the pole piece after milling minus the thickness of the copper foil).
Compacted density is closely related to the specific capacity of the pole piece, efficiency, internal resistance and battery cycle performance. The higher the compaction density, the more active material per unit volume, the greater the capacity.
However, at the same time, the pores will be reduced, the performance of absorbing electrolyte becomes worse, the wettability is reduced, the internal resistance increases, and the lithium ion is difficult to be embedded and dislodged, instead of being unfavorable to the increase of capacity. Factors affecting the compaction density: the size, distribution and morphology of the particles have an effect.
True density of graphite anode
The weight of solid material per unit volume of the material of a graphite anode in an absolutely dense state (excluding internal voids). Since the true density is measured in the dense state, it will be higher than the vibrated density. Generally, true density > compacted density > vibrated density.
First charge/discharge specific capacity
During the first charging process of lithium-ion battery, with the embedding of lithium ions on the surface of the graphite anode material, the solvent molecules in the electrolyte are co-embedded, and the SEI passivation film is formed by decomposition on the surface of the graphite anode material. Only after the graphite anode surface is completely covered by the SEI film, the solvent molecules cannot be embedded, and the reaction is stopped.
The generation of SEI film consumes a part of lithium ions, which can not be dislodged from the surface of the anode during the discharge process, thus reducing the specific capacity of the first discharge.
First coulomb efficiency
An important indicator of the performance of graphite anode materials is their first charge/discharge efficiency, also known as the first coulomb efficiency. During the charging and discharging process, some lithium ions are detached from the positive electrode and embedded in the anode, but cannot return to the positive electrode to participate in the charging and discharging cycle, resulting in the first coulombic efficiency <100%.
The reason why this part of lithium ions can not return to the positive electrode: (1) the existence of a part of irreversible embedded lithium, (2) the formation of SEI film on the surface of the anode, SEI film is an important factor affecting the Coulomb efficiency.
As SEI film is mostly formed on the surface of electrode materials, the specific surface area of electrode materials directly affects the formation area of SEI film, the larger the specific surface area, the larger the contact area with electrolyte, and the larger the formation area of 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 that kind of unstable SEI film is detrimental to the reaction, which will continuously consume the electrolyte, thicken the thickness of the SEI film, and increase the internal resistance.
Cycling performance
In terms of cycling performance, the SEI film will have a certain impediment to the diffusion of lithium ions, and with the increase in the number of cycles, the SEI film will continue to fall off, peel off, and deposit on the surface of the anode, resulting in a gradual increase in the internal resistance of the graphite anode, which will bring about heat accumulation and capacity loss.
Magnification performance of graphite anode
The diffusion of lithium ions in graphite anode materials is highly directional, i.e., it can only be inserted perpendicular to the end face of the C-axis aspect of the graphite crystal. The graphite anode materials with small particles and high specific surface area have better multiplicity performance. In addition, the electrode surface resistance (brought about by the SEI film) and electrode conductivity also affect the multiplicity performance.
Same as cycle life and expansion, isotropic anode with many lithium ion transport channels solves the problem of fewer entrances for embedding and disengagement and low diffusion rate in anisotropic structure, which is also useful for high-current charging and discharging.
Expansion properties
Expansion and cycle life are positively correlated. After the graphite anode expands, (1) it will cause core deformation, microcracks in anode particles, rupture and reorganization of SEI film, consumption of electrolyte, and deterioration of cycle performance;
(2) The lithium battery separator will be squeezed, especially at the right-angle edge of the lugs, which is more serious, and it is very easy to cause micro-short-circuit or micro-lithium metal precipitation with the charging and discharging cycle.
The amount of expansion is related to the orientation of the graphite anode, orientation = I004/I110, which can be calculated by XRD data, the anisotropic graphite anode material tends to expand the lattice to the same direction (the C-axis direction of the graphite crystals) during the lithium embedding process, which will lead to a larger volume expansion of the battery.
Hi, I am Lucky, graduated from a well-known university in China, now mainly engaged in article editing on lithium motorcycle batteries, and the battery swapping station, I am committed to offering services and solutions about battery swap station for various industries.
10 technical indicators of graphite anode for lithium battery
Graphite anode materials are divided into artificial graphite and natural graphite; natural graphite and artificial graphite anode materials are characterized by high electrical conductivity, large lithium ion diffusion coefficient, high embedded lithium capacity and low embedded lithium potential.
Compared with other types of lithium ion battery anode materials, natural graphite and artificial graphite have comprehensive advantages in terms of battery specific capacity, first time efficiency, cycle life, safety, etc., and the raw materials are widely available and cheap.
The technical indexes of graphite anode materials mainly include specific surface area, particle size distribution, vibration density, compaction density, true density, first charge/discharge specific capacity, first efficiency and so on. In addition, there are electrochemical indicators such as cycling performance, multiplicity performance, swelling and so on. This article will specifically describe the knowledge of the ten technical indicators of graphite anode.
Specific surface area of graphite anode
Refers to the surface area per unit mass object has, the smaller the particles, the larger the specific surface area will be.
The anode with small particles and high specific surface area has more channels and shorter paths for lithium ions to migrate, so the multiplicity performance is better, but due to the large contact area with the electrolyte, the area to form the SEI film is also large, and the first time efficiency will be lower.
Large particles, on the contrary, have the advantage of greater compaction density. The specific surface area of graphite anode material is less than 5m2/g is appropriate.
Particle size distribution of graphite anode
The effect of particle size on its electrochemical properties is manifested in the fact that the particle size of the anode material will directly affect the vibrational density of the material as well as the specific surface area of the material. The size of the vibration density will directly affect the bulk energy density of the material.
In the same volume of filling parts, the larger the particle size of the material, the wider the particle distribution, the smaller the viscosity of the slurry, the more conducive to improving the solid content and reducing the difficulty of coating.
In addition, when the particle size distribution of the material of the graphite anode is wide, the small particles in the system can be filled in the gaps of the large particles, which helps to increase the compaction density of the pole piece and improve the volumetric energy density of the battery.
The characteristic parameters of the particle size distribution of the material of the graphite anode are D50, D10, D90 and Dmax, of which D50 represents the particle size value corresponding to the accumulated amount of 50% in the cumulative distribution curve of the particle size, which can be regarded as the average particle size of the material.
In addition, the width of the particle size distribution of the material can be expressed by K90, K90=(D90-D10)/D50, the larger K90 is, the wider the distribution is. The particle size of graphite anode material is mainly determined by its preparation method, and the requirements for its particle size parameters in the graphite standard are D50 (about 20 μm), Dmax (≤70 μm) and D10 (about 10 μm).
Vibrational density of graphite anode
Relying on the vibration makes the powder present a more compact pile form, the mass per unit volume measured. It is an important indicator to measure the active material, lithium-ion battery volume is limited, if the vibration density is high, the active material mass per unit volume is high, the volume capacity is high.
Compacted density of graphite anode
Mainly for the pole piece, refers to the density of the anode active material and binder, etc. made into the pole piece, after rolling, compacted density = surface density / (the thickness of the pole piece after milling minus the thickness of the copper foil).
Compacted density is closely related to the specific capacity of the pole piece, efficiency, internal resistance and battery cycle performance. The higher the compaction density, the more active material per unit volume, the greater the capacity.
However, at the same time, the pores will be reduced, the performance of absorbing electrolyte becomes worse, the wettability is reduced, the internal resistance increases, and the lithium ion is difficult to be embedded and dislodged, instead of being unfavorable to the increase of capacity. Factors affecting the compaction density: the size, distribution and morphology of the particles have an effect.
True density of graphite anode
The weight of solid material per unit volume of the material of a graphite anode in an absolutely dense state (excluding internal voids). Since the true density is measured in the dense state, it will be higher than the vibrated density. Generally, true density > compacted density > vibrated density.
First charge/discharge specific capacity
During the first charging process of lithium-ion battery, with the embedding of lithium ions on the surface of the graphite anode material, the solvent molecules in the electrolyte are co-embedded, and the SEI passivation film is formed by decomposition on the surface of the graphite anode material. Only after the graphite anode surface is completely covered by the SEI film, the solvent molecules cannot be embedded, and the reaction is stopped.
The generation of SEI film consumes a part of lithium ions, which can not be dislodged from the surface of the anode during the discharge process, thus reducing the specific capacity of the first discharge.
First coulomb efficiency
An important indicator of the performance of graphite anode materials is their first charge/discharge efficiency, also known as the first coulomb efficiency. During the charging and discharging process, some lithium ions are detached from the positive electrode and embedded in the anode, but cannot return to the positive electrode to participate in the charging and discharging cycle, resulting in the first coulombic efficiency <100%.
The reason why this part of lithium ions can not return to the positive electrode: (1) the existence of a part of irreversible embedded lithium, (2) the formation of SEI film on the surface of the anode, SEI film is an important factor affecting the Coulomb efficiency.
As SEI film is mostly formed on the surface of electrode materials, the specific surface area of electrode materials directly affects the formation area of SEI film, the larger the specific surface area, the larger the contact area with electrolyte, and the larger the formation area of 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 that kind of unstable SEI film is detrimental to the reaction, which will continuously consume the electrolyte, thicken the thickness of the SEI film, and increase the internal resistance.
Cycling performance
In terms of cycling performance, the SEI film will have a certain impediment to the diffusion of lithium ions, and with the increase in the number of cycles, the SEI film will continue to fall off, peel off, and deposit on the surface of the anode, resulting in a gradual increase in the internal resistance of the graphite anode, which will bring about heat accumulation and capacity loss.
Magnification performance of graphite anode
The diffusion of lithium ions in graphite anode materials is highly directional, i.e., it can only be inserted perpendicular to the end face of the C-axis aspect of the graphite crystal. The graphite anode materials with small particles and high specific surface area have better multiplicity performance. In addition, the electrode surface resistance (brought about by the SEI film) and electrode conductivity also affect the multiplicity performance.
Same as cycle life and expansion, isotropic anode with many lithium ion transport channels solves the problem of fewer entrances for embedding and disengagement and low diffusion rate in anisotropic structure, which is also useful for high-current charging and discharging.
Expansion properties
Expansion and cycle life are positively correlated. After the graphite anode expands, (1) it will cause core deformation, microcracks in anode particles, rupture and reorganization of SEI film, consumption of electrolyte, and deterioration of cycle performance;
(2) The lithium battery separator will be squeezed, especially at the right-angle edge of the lugs, which is more serious, and it is very easy to cause micro-short-circuit or micro-lithium metal precipitation with the charging and discharging cycle.
The amount of expansion is related to the orientation of the graphite anode, orientation = I004/I110, which can be calculated by XRD data, the anisotropic graphite anode material tends to expand the lattice to the same direction (the C-axis direction of the graphite crystals) during the lithium embedding process, which will lead to a larger volume expansion of the battery.
Related articles on lithium ion anode can also see hard carbon anode, silicon based anode, top 10 silicon-based anode material companies.