The Chinese government proposed to accelerate the development of the next generation of lithium-ion power batteries in "Made in China 2025", and proposed that the energy density of power battery cells reach 300Wh/kg in the medium term and 400Wh/kg in the long term. In response to this requirement, for the anode material, the actual capacity of graphite is close to its theoretical limit, and it is necessary to develop a new material having a higher energy density and taking into account other indexes. Among them, the silicon carbon negative electrode can combine the conductivity of the carbon material and the high capacity of the silicon material, and is considered as the negative material of the next generation lithium ion battery, so the corresponding standard is also being drafted.
2 lithium battery anode material product standard technical specifications
2.1 Lithium-ion battery requirements for anode materials
As the core component of lithium-ion batteries, the anode material usually needs to meet the following conditions when applied:
1 The embedded lithium potential is low and stable to ensure a high output voltage;
2 allows more lithium ions to be reversibly deintercalated, with higher specific capacity;
3 The structure is relatively stable during charging and discharging, and has a long cycle life;
4 high electronic conductivity, ionic conductivity and low charge transfer resistance to ensure less voltage polarization and good rate performance;
5 can form a stable solid electrolyte membrane with the electrolyte to ensure high coulombic efficiency;
6 The preparation process is simple, easy to industrialize, and the price is cheap;
7 Environmentally friendly, it will not cause serious pollution to the environment during the production and actual use of materials;
8 resources are abundant.
For more than 30 years, although new anode materials for lithium-ion batteries have been reported, there are very few commercially available applications, mainly because few materials can meet the above conditions. For example, although materials such as metal oxides, sulfides, and nitrides having a conversion reaction mechanism have a high specific capacity, they have high plate potential, severe polarization, large volume change, and difficulty in forming a stable lithium intercalation process. SEI and high cost make it impossible to actually get practical applications.
Graphite is widely used because it satisfies the above conditions well. In addition, although Li4Ti5O12 has a low capacity and a high lithium insertion potential, it is structurally stable during charge and discharge, allowing high rate charge and discharge, and thus has certain applications in power batteries and large-scale energy storage.
The production of negative electrode materials is only a part of the entire battery manufacturing process. The development of standards will help battery companies judge the quality of materials. In addition, materials are inevitably affected by factors such as human, machine, material, environment and test conditions during production and transportation. Only by standardizing their physical and chemical properties can they be truly reliable.
In general, the key technical indicators of the anode material are: crystal structure, particle size distribution, tap density, specific surface area, pH, water content, main element content, impurity element content, initial discharge specific capacity, and first charge and discharge efficiency. The explanation will be expanded one by one below.
2.2 Crystal structure of the anode material
Graphite mainly has two crystal structures, one is hexagonal phase (a=b=0.2461nm, c=0.6708nm, α=β=90°, γ=120°, P63/mmc space group); the other is Ling The square phase (a = b = c, α = β = γ ≠ 90 °, R3m space group) (Table 3). In graphite crystals, the two structures coexist, except that the ratio of the two of the different graphite materials is different, which can be determined by X-ray diffraction test.
The degree of order of the crystal structure of the carbon material and the degree of difficulty in graphitization can be described by the degree of graphitization (G). The larger the G, the easier the carbon material is to be graphitized, and the more ordered the crystal structure. Where d002 is the interplanar spacing of the (002) peak in the XRD pattern of the carbon material, 0.3440 represents the layer spacing of the completely ungraphitized carbon, and 0.3354 represents the interlayer spacing of the ideal graphite, in units of nm. The above formula shows that the smaller the d002 of the carbon material, the higher the degree of graphitization, the less the corresponding lattice defects, the smaller the electron migration resistance, the higher the dynamic performance of the battery, and thus the GB/T24533—2009 “Lithium In the ion-based graphite anode material, the d002 values of various types of graphite are clearly defined.
Li4Ti5O12 is a cubic spinel structure belonging to the Fd-3m space group and has a three-dimensional lithium ion migration channel. Compared with the structure of the lithium intercalation product (Li7Ti5O12), the unit cell parameters are not much different (0.836nm→0.837nm). It is a "zero strain material" and thus has excellent cycle stability.
Li4Ti5O12 is usually prepared by sintering at a high temperature using TiO2 and Li2CO3 as raw materials. Therefore, a small amount of TiO2 may remain in the product, which affects the electrochemical properties of the material. To this end, GB/T30836—2014 “Lithium titanate battery and its carbon composite anode material” gives the upper limit value and detection method of TiO2 residue in Li4Ti5O12 product. The specific process is as follows: First, the diffraction pattern of the sample measured by XRD should conform to the provisions of JCPDS (49-0207); secondly, the (111) crystal plane diffraction peak and anatase TiO2 of Li4Ti5O12 are read from the spectrum ( 101) crystal plane diffraction peak, rutile TiO2 (110) crystal plane diffraction peak intensity; finally calculate anatase TiO2 peak intensity ratio I101/I111 and rutile TiO2 peak intensity ratio I110/I111, the requirements in the control standard Can make a judgment.
2.3 Particle size distribution of the anode material
The particle size distribution of the negative electrode material directly affects the pulping process of the battery as well as the volumetric energy density. In the case of the same volume filling fraction, the larger the particle size of the material, the wider the particle size distribution, and the smaller the viscosity of the slurry, which is advantageous for increasing the solid content and reducing the coating difficulty. In addition, when the particle size distribution of the material is wide, small particles in the system can be filled in the voids of the large particles, which helps to increase the compaction density of the pole piece and increase the volumetric energy density of the battery.
The particle size and particle size distribution of the material can generally be measured by a laser diffraction particle size analyzer and a nanoparticle analyzer. The laser diffraction particle size analyzer is mainly based on the static light scattering theory, that is, the scattering angle and intensity of the particles of different particle sizes to the incident light are mainly used to measure the micron-scale particle system. The nanoparticle analyzer is mainly based on the theory of dynamic light scattering, that is, the more severe Brownian motion of the nanoparticle not only affects the intensity of the scattered light, but also affects its frequency, thereby determining the particle size distribution of the nanoparticles.
The characteristic parameters of the material particle size distribution mainly include D50, D10, D90 and Dmax, wherein D50 represents the corresponding particle size value when the cumulative amount in the particle size cumulative distribution curve is 50%, which can be regarded as the average particle diameter of the material. In addition, the width and width of the material particle size distribution can be represented by K90, K90 = (D90-D10) / D50, the larger the K90, the wider the distribution.
The particle size of the negative electrode material is mainly determined by its preparation method. For example, the synthesis method of mesocarbon microbeads (CMB) is a thermal decomposition and thermal polycondensation reaction of liquid hydrocarbons under high temperature and high pressure, and the particle size of CMB can be controlled by controlling the kind of raw materials, reaction time, temperature and pressure. . The requirements for the particle size parameters of the graphite standard are: D50 (about 20 μm), Dmax (≤70 μm) and D10 (about 10 μm), while the D50 required in the lithium titanate standard is significantly smaller than graphite (≤10 μm, Table 4). .
2.4 Density of the anode material
Powder materials are generally porous, and some communicate with the outer surface of the particles, called open or semi-open holes (one end communicates), and some do not communicate with the outer surface at all, called closed cells. When calculating the material density, it can be divided into true density, effective density and apparent density according to whether or not these pore volumes are included, and the apparent density is further divided into compaction density and tap density.
The true density represents the theoretical density of the powder material, and the volume used in the calculation is the volume of the particles from which the open and closed cells are removed. The effective density refers to the density value at which the powder material can be effectively utilized, and the volume used is the volume of the particles including the closed cells. The effective volume is tested by placing the powder material in a measuring container, adding a liquid medium, and allowing the liquid to fully wet into the opening of the particle, and subtracting the volume of the liquid medium from the measured volume to obtain an effective volume.
In practical applications, manufacturers are more concerned about the apparent density of materials, which mainly include tap density and compaction density. The test principle of tap density is: a certain amount of powder is filled in the tap density tester, and the vibration device continuously vibrates and rotates until the volume of the sample is no longer reduced. Finally, the mass of the sample is divided by the tapping. The volume is the tap density.
The test principle of compaction density is: in the extrusion process of external force, as the powder moves and deforms, larger voids are filled, and the contact area between the particles increases, thereby forming a compact with a certain density and strength. The volume of the preform is the compacted volume. Generally, true density > effective density > compacted density > tap density.
The density of the negative electrode material directly affects the volumetric energy density of the battery. For the same material, the higher the compaction density and the higher the volumetric energy density, the lower limit of each density is required in the standard (Table 5). Among them, the true density range of different graphite materials is the same, both 2.20 ~ 2.26g / cm3, because they are essentially carbon materials, but the microstructure is different. In addition, due to the low initial conductivity of Li4Ti5O12, it is usually necessary to enhance the rate performance of the battery by carbon coating, but at the same time, the corresponding tap density decreases (Table 5).
2.5 specific surface area of the anode material
The surface area is divided into an external surface area and an internal surface area, and the specific surface area of the material refers to the total area per unit mass. The ideal non-porous material has only an external surface area, the specific surface area is usually small, and the porous and porous materials have a large internal surface area and a high specific surface area. Further, the pore diameter of the powder material is generally classified into three types, micropores of less than 2 nm, mesopores of between 2 and 50 nm, and macropores of more than 50 nm. In addition, the specific surface area of the material is closely related to its particle size, and the smaller the particle size, the larger the specific surface area.
The pore size and specific surface area of the material are generally determined by nitrogen adsorption and desorption experiments. The basic principle is: when a gas molecule collides with a powder material, it will stay on the surface of the material for a period of time. This phenomenon is adsorption. The adsorption amount at constant temperature depends on the nature of the powder and gas and the pressure at the time of adsorption. The amount of adsorption can be used to derive the specific surface area, pore size distribution and pore volume of the material. In addition, the adsorption amount of the powder to the gas increases as the temperature decreases, so the adsorption experiment is generally carried out at a low temperature (using liquid nitrogen) to improve the adsorption capacity of the material to the gas.
The specific surface area of the negative electrode material has a great influence on the dynamic performance of the battery and the formation of a solid electrolyte membrane (SEI). For example, nanomaterials generally have a high specific surface area, can shorten the lithium ion transport path, reduce the surface current density, and improve the dynamic performance of the battery, and thus have been extensively studied. However, such materials are often not practically used, mainly because the large specific surface area exacerbates the decomposition of the electrolyte during the first cycle, resulting in a lower first coulombic efficiency. Therefore, the negative electrode material standard sets an upper limit value for the specific surface area of graphite and lithium titanate. For example, the specific surface area of graphite needs to be controlled to be 6.5 m 2 /g or less, and Li 4 Ti 5 O 12 @ C is also less than 18 m 2 /g.
2.6 pH and moisture requirements for anode materials
The trace moisture contained in the powder material can be measured by a Karl Fischer Coulometric titrator. The basic principle is that the water in the sample can react with iodine and sulfur dioxide under the conditions of organic alkali and methanol. H2O+I2+SO2+CH3OH+3RN→[RHN]SO4CH3+2[RHN]I, wherein the iodine is By (2I?-→I2+2e?) generated by electrochemically oxidizing the electrolytic cell, the amount of iodine produced is proportional to the amount of electricity passing through the electrolytic cell, so the water content can be obtained by recording the amount of electricity consumed by the electrolytic cell.
The pH and moisture of the negative electrode material have an important influence on the stability of the material and the pulping process. For graphite, the pH is usually about neutral (4 to 9), while Li4Ti5O12 is alkaline (9.5 to 11.5) with a certain residual alkalinity. This is mainly because in the preparation of Li4Ti5O12, in order to ensure the full progress of the reaction, the lithium source is generally excessive, and they are mainly present in the form of Li2CO3 or LiOH, so that the final product is alkaline. When the amount of residual alkali is too high, the stability of the material is deteriorated, and it is easy to react with water and carbon dioxide in the air, which directly affects the electrochemical properties of the material. In addition, since the graphite-based negative electrode slurry is currently mainly an aqueous system, its moisture requirement (≤0.2%) is not as harsh as that of the positive electrode material (the slurry is usually an oily system, ≤0.05%), which lowers the battery. Production costs and simplified processes have a certain meaning.
2.7 The main element content of the negative electrode material
Although the graphite anode has a high capacity and a low and stable lithium insertion potential, it is very sensitive to the composition of the electrolyte, is easy to peel off, and has poor overcharge resistance. Therefore, the graphite used in commercial use is modified graphite, and the modification method mainly includes surface oxidation and surface coating, and the surface treatment also causes some impurities to remain in the graphite. Graphite is mainly composed of three parts: fixed carbon, ash and volatile matter. Fixed carbon is a truly electrochemically active component. The standard requires fixed carbon content to be greater than 99.5% (Table 8). Indirect carbon determination can be used to determine Fixed carbon content.
For Li4Ti5O12, the theoretical content of lithium is 6%, and the allowable deviation in actual products is 5% to 7% (Table 8). The content of general elements can be measured by inductively coupled plasma atomic emission spectroscopy. The basic principle is that the working gas (Ar) generates a plasma under the action of high-frequency current, and the sample interacts with the high-temperature plasma to emit photons, and its wavelength is Related to the type of element, the type of element can be judged by the excitation wavelength. In addition, Li4Ti5O12 has a low conductivity and usually uses a carbon coating strategy to enhance the reaction kinetics of the battery. However, the coated carbon layer should not be too thick, otherwise it will not only affect the migration rate of lithium ions, but also reduce the tap density of the material, so the carbon content in the standard is limited to less than 10%.
2.8 Impurity element content of anode material
The impurity element in the negative electrode material means other components than the main element and the element introduced by the coating and doping. Impurity elements are generally introduced through raw materials or during production, which can seriously affect the electrochemical performance of the battery and therefore need to be controlled from the source. For example, certain metal impurity components not only reduce the proportion of active materials in the electrode, but also catalyze side reactions of the electrode material and the electrolyte, and even pierce the separator, posing a safety hazard. In addition, since artificial graphite is mostly prepared by petroleum cracking, a small amount of organic products such as sulfur, acetone, isopropanol, toluene, ethylbenzene, xylene, benzene, ethanol, polybrominated biphenyls and polybrominated biphenyls are often left in such products. Ether, etc.
The European Union's RoHS standard, the Directive on the Restriction of Certain Substances in Electrical and Electronic Equipment, defines various types of hazardous substances, and the standards set by China are also
