With many advantages such as high energy density, long cycle life, low self-discharge, no memory effect and environmental friendliness, lithium-ion batteries (LIBs) have been widely used in consumer electronics, such as smartphones, smart bracelets, digital cameras and laptops, with the strongest consumer demand. At the same time, it is promoted in the markets of pure electric, hybrid electric and extended-range electric vehicles, with the fastest market share growth. In addition, LIBs are gaining momentum in large-scale energy storage applications, such as power grid peak regulation, household power distribution and communication base stations (Figure 1).
LIBs are mainly composed of positive electrodes, negative electrodes, electrolytes and separators, and the choice of anode materials will directly affect the battery energy density. Metal lithium has the lowest Standard Electrode Potential (SEP) (−3.04V, vs. SHE) and a very high theoretical specific capacity (3860mA·h/g), making it the first choice for anode materials of lithium secondary batteries. However, it is not used in practice as it is prone to generate dendritic crystal during charging and discharging, resulting in isolated lithium, which reduces battery efficiency and poses serious safety hazards.
It was not until 1989 that Sony Corporation discovered that petroleum coke could use as a substitute for lithium metal, which pushed LIBs to commercialization. In the subsequent development, graphite has occupied the main market for LIBs due to its advantages of low, stable lithium intercalation potential (0.01~0.2V), high theoretical specific capacity (372 mA·h/g), cheapness and environmental friendliness. In addition, although Li4Ti5O12 has a low capacity (175 mA·h/g) and high lithium intercalation potential (1.55V), its structure is stable during charging and discharging as a “zero-strain material”. Therefore, it has been applied in power batteries and large-scale energy storage, occupying a small market share. With the pursuit of higher energy density in LIBs, silicon and lithium metal will be the future trend for anode materials (Figure 2).
China has certain advantages in the industrialisation of anode materials for LIBs. Its battery industry chain is complete in terms of its raw material mining, electrode material production, battery manufacturing, recycling, etc. In addition, China is rich in graphite reserves, second only to Turkey and Brazil. After nearly 20-year development, China’s anode materials have gone abroad. Manufacturers like Shenzhen BTR New Energy Material Co., Ltd., Shanghai Shanshan Tech Co., Ltd. and Jiangxi Zichen Technology Co., Ltd. have reached the world advanced level in the R&D and production of anode materials.
To promote the healthy development of the lithium industry, China has successively promulgated relevant standards since 2009, involving raw materials, products and testing methods. Specifically, it proposes specific indicators for each parameter and the corresponding testing methods, which guided the production and application of anode materials. The types of anode materials in practical application are relatively concentrated (graphite and Li4Ti5O12), mainly related to four standards (Table 1). But, there are six standards under development or revision (Table 2), indicating that the variety of anode materials has increased and new standards are needed to regulate their development. This article will focus on the main points of the four promulgated standards.
Correlative Standards for Anode Materials for LIBs in China
Table 1 lists the relevant standards for anode materials for LIBs released in China in decades past, including three national standards and one industry standard. In terms of categories, there are three anode products and one test method involved. Graphite was the first anode material to be commercialized, so GB/T24533-2009 Graphite Anode Materials for Lithium-ion Battery was the first anode standard. Subsequently, a small amount of lithium titanate also entered the market, with the corresponding industry standard YS/T825-2012 Lithium Titanium and national standard GB/T30836-2014 Lithium Titanium Oxide and Its Carbon Composite Anode Materials for Lithium-ion Battery launched successively.
Graphite Anode Materials for Lithium-ion Battery divides graphite into natural graphite, mesocarbon microbead artificial graphite, needle coke artificial graphite, petroleum coke artificial graphite and composite graphite. Each category is divided into different grades according to its electrochemical performance (charge-discharge efficiency of the first cycle and the initial Coulombic efficiency), and each grade is divided into different varieties according to the average particle size (D50) of its material. The standard requires various physicochemical properties of different varieties of graphite. Due to space limitations, the following part only divides graphite into natural graphite, mesocarbon microbead, artificial graphite, needle coke artificial graphite, petroleum coke artificial graphite and composite graphite. Each category of indicators combines all the parameters of different grades and varieties of the graphite in that category.
Table 2 lists the standards for anode materials for LIBs, which are being formulated or revised in China. Except for Graphite Anode Materials for Lithium-ion Battery, which are revised standards, the other five are newly formulated. The newly developed Mesocarbon Microbeads initially belonged to a small category of graphite, but now it is listed separately, indicating that this type of graphite is increasingly significant. In addition, a new graphite species standard, Spherical Graphite, has been added. There are also two standards for soft carbon (Soft Carbon and Oil-Based Needle Coke). Soft carbon refers to a carbon material that can be graphitized at high temperature (<2500°C), and its carbon layer is less orderly than that of graphite, but higher than that of hard carbon. Soft carbon materials have the advantages of strong adaptability to electrolytes, good resistance to overcharge and over-discharge, high capacity and good cycling performance. They have been applied in energy storage batteries and electric vehicles, so the corresponding standards are being laid out (Table 2).
In Made in China 2025, the Chinese government proposes to speed up the development of next-generation lithium-ion power batteries, and to achieve the goal of reaching 300W·h/kg in the medium term and 400W·h/kg in the long term. In response, for anode materials, the actual capacity of graphite is close to its theoretical limit, so new materials with higher energy density and other indicators need to be developed. Silicon-carbon anode, which can combine the electrical conductivity of carbon with the high capacity of silicon, is considered to be the next generation of anode materials for LIBs, so the corresponding standards are being drafted (Table 2).
Standards and Specifications for Anode Materials for LIBs
Requirements for Anode Materials for LIBs
Anode materials, the core component of LIBs, are needed when the following conditions are met:
① The potential of lithium insertion is low and stable to ensure high output voltage;
② Allow more Li-ions to be reversibly deintercalated, with high specific capacity;
③ The structure is relatively stable during charging and discharging, with a long cycle life;
④ High electronic conductivity, ionic conductivity and low charge transfer resistance to ensure small voltage polarization and good rate capability;
⑤ It can form a solid electrolyte membrane (SEI) with the electrolyte to ensure high coulombic efficiency;
⑥ The preparation process is simple, easy to industrialize and inexpensive;
⑦ Environmentally friendly, it will not cause serious pollution to the environment during the production and actual use of materials;
⑧ Abundant resources, etc.
For more than 30 years, although new anode materials for LIBs have been reported, few have been commercially available, mainly because few materials can combine the above conditions. For example, although metal oxides, sulphides and nitrides have high specific capacity, their high potential, severe polarisation, considerable volume changes, difficulty in forming stable SEI and high cost during lithium intercalation prevent them from being applied in reality.
Graphite is widely used because it balances the above conditions. In addition, although Li4Ti5O12 has low capacity and high lithium intercalation potential, its structure is stable during charge and discharge, allowing high-rate charge and discharge, so it also has been applied in power batteries and large-scale energy storage.
The production of anode materials is only one part of the entire battery manufacturing process. The formulation of standards will help battery companies to judge the material quality. In addition, materials are inevitably affected by people, machines, materials, environment, test conditions, etc. in their production and transportation. Only by standardizing their physical and chemical properties can their reliability be ensured.
Generally, the key technical indicators of anode materials include crystal structure, particle size distribution, tapped density, specific surface area, pH, water content, major element content, impurity element content, first discharge specific capacity and first charge-discharge efficiency, etc. They will be explained below.
Crystal Structure of Anode Materials
Graphite mainly has two crystal structures, a hexagonal phase (a=b=0.2461nm, c=0.6708 nm, α=β=90°, γ=120°, P63/mmc space group) and a rhombohedral phase (a=b=c, α=β=γ≠90°, R3m space group) (Table 3). In graphite crystals, these two structures co-exist, but their ratio varies in different graphite materials, which can be determined by the X-ray diffraction test.
The degree of ordering of the crystal structure of carbon materials and the difficulty of graphitization can be described by the degree of graphitization (G). The larger the G, the easier the graphitization of the carbon material, and the higher the degree of order of the crystal structure. Specifically, d002 is the interplanar spacing of the (002) peak in the XRD pattern of carbon materials, 0.3440 is the interplanar spacing of completely ungraphitized carbon, and 0.3354 is the interplanar spacing of ideal graphite (All units are nm). The above formula shows that the smaller the d002 of the carbon material, the higher the degree of graphitization, the fewer the corresponding lattice defects, the smaller the electron migration resistance, and the dynamic performance of the battery will be improved. Therefore, the d002 values for each type of graphite are clearly defined in GB/T24533-2009 Graphite Anode Materials for Lithium-ion Battery (Table 3).
Li4Ti5O12 is a cubic spinel, belonging to the Fd-3m space group, with three-dimensional Li-ion migration channels (Figure 4). Compared with the structure of its lithium intercalation product (Li7Ti5O12), the unit cell parameters of Li4Ti5O12 have little difference (0.836nm→0.837nm), known as zero-strain materials, which results in excellent cycling stability. Li4Ti5O12 is usually prepared by high-temperature sintering with TiO2 and Li2CO3 as raw materials, so there may be a small amount of residual TiO2 in the product, which affects the electrochemical performance of the material. For this reason, GB/T30836-2014 Lithium Titanium Oxide and Its Carbon Composite Anode Materials for Lithium-ion Battery gives the upper limit of TiO2 residue in Li4Ti5O12 products and the detection method. The specific process is as follows. Firstly, the diffraction pattern of the sample measured by XRD should be under JCPDS (49-0207). Secondly, the intensity of the (111) crystalline diffraction peak, the anatase TiO2 (101) crystalline diffraction peak and the rutile TiO2 (110) crystalline diffraction peak of Li4Ti5O12 are read out from the spectrum. Finally, after the intensity of the anatase TiO2 peak I101/I111 and the rutile TiO2 peak I110/I111 TiO2 are calculated, judgement can be made against the requirements of the standard (Table 3).
Particle Size Distribution of Anode Materials
The particle size distribution of the anode material directly affects the slurry process and volumetric energy density of the battery. 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 (Figure 5), which helps to increase the solid content and reduce the difficulty of coating. In addition, with a wider particle size distribution, the small particles in the system can fill the voids of the large particles, which helps to increase the compaction density of the pole piece and to improve the volumetric energy density of the battery.
The particle size and size distribution of materials can be measured by laser diffraction particle size analysers and nanoparticle analysers. The laser diffraction particle size analyser works mainly based on static light scattering theory, i.e., particles of different sizes scatter light at different angles with different intensities. It is mainly used to measure the particle system at the micron level. The nanoparticle analyser works mainly based on dynamic light scattering theory, i.e., the more severe Brownian motion of nanoparticles affects not only the intensity of scattered light, but also its frequency, and thus the particle size distribution of the nanoparticles is determined.
The characteristic parameters of the material size distribution are D50, D10, D90 and Dmax. D50 is the particle size corresponding to 50% of the cumulative amount in the particle size cumulative distribution curve, which can be regarded as the average particle size of the material. In addition, the width of the material size distribution can be expressed by K90 = (D90 – D10)/D50. The larger the K90, the wider the distribution.
The particle size of the anode material is mainly determined by its preparation method. For example, the synthesis method of mesocarbon microspheres (CMB) is the thermal decomposition and thermal polycondensation of liquid-phase hydrocarbons under high temperature and high pressure. The particle size of CMB can be regulated by controlling the type of raw materials, reaction time, temperature, pressure, etc. The requirements for particle size parameters in the graphite standard are D50 (~20μm), Dmax (≤70μm) and D10 (~10μm), while the D50 required in the lithium titanate standard is considerably smaller than that of graphite (≤10μm, Table 4).
Density of Anode Materials
Generally, powder materials are porous, some with the outer surface of the particle, known as open pores or semi-open pores (connected at one end), and some not connected to the outer surface at all, known as closed pores. When calculating the density of a material, it can be divided into true density, effective density and apparent density according to whether these pore volumes are included, while apparent density is divided into compacted density and tapped density.
The true density is the theoretical density of the powder material, and the volume used in the calculation is the particle volume excluding open and closed pores. The effective density refers to the density that the powder material can be effectively used, and the volume used is the particle volume including the closed pores. The effective volume is measured by the following steps. First, place the powder material in a measuring vessel. Then, add a liquid medium and allow the liquid to fully infiltrate the open pores of the particles. Last, subtract the volume of the liquid medium from the measured volume to obtain the effective volume.
In practice, manufacturers are more concerned about the apparent density of the material, including tapped density and compacted density. The principle of the compaction density test is as follows. First, a certain amount of powder is filled in the compaction density tester, which is continuously vibrated and rotated by the vibration device until the volume of the sample no longer decreases. Then, the compaction density is obtained by dividing the mass of the sample by the compacted volume.
The test principle of compaction density is that during the extrusion process of external force, as the powder moves and deforms, larger voids are filled, and the contact area between particles increases, thereby forming a compacted embryo with a certain density and strength, and the volume of the compacted embryo is the compacted volume. Generally, true density>effective density>compacted density>tapped density.
The density of the anode material directly affects the volumetric energy density of the battery. For the same material, the higher the compaction density, the higher the volumetric energy density, so the lower limits for each density are specified in the standard (Table 5). The true densities of different graphite materials are in the range of 2.20~2.26g/cm3, because they are essentially carbon materials, with different microstructures. In addition, due to the low initial conductivity of Li4Ti5O12, carbon coating is required to improve the rate capability of the battery, but at the same time, the corresponding tapped density decreases (Table 5).
Specific Surface Area of Anode Materials
Surface area is divided into the external and internal surface area, and the specific surface area of a material is the total area per unit mass. The ideal non-porous material has only an external surface area, with a small specific surface area, while the porous and multi-porous materials have a large internal surface area and a high specific surface area. The pore size of powder materials is divided into three categories: micropores (<2nm), mesopores (2-50nm) and macropores (>50nm). In addition, the specific surface area of a material is closely related to its particle size. The smaller the particle size, the larger the specific surface area.
Generally, the pore size and specific surface area of the material are determined by nitrogen adsorption and desorption experiments. The basic principle is that when the gas molecules collide with the powder material, they will stay on the surface of the material for a period, which is called adsorption. The amount of adsorption at constant temperature depends on the properties of the powder and gas and the pressure when the adsorption occurs. The specific surface area, pore size distribution and pore volume of the material can be calculated from the adsorption amount. In addition, the adsorption capacity of the powder to the gas will increase 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 for gases.
The specific surface area of the anode material has a great impact on the kinetic performance of the battery and the formation of the solid electrolyte membrane (SEI). For example, nanomaterials have been extensively investigated for their high specific surface area, which can shorten the transport path of Li-ions, reduce surface current density and improve the kinetic performance of the cell. Often, however, these materials are not used in practice, because the large specific surface area exacerbates the breakdown of the electrolyte during the first cycle, resulting in a lower first coulombic efficiency. Therefore, the anode material standard sets an upper limit for the specific surface area of graphite and lithium titanate. For example, graphite needs to be controlled to less than 6.5m2/g and Li4Ti5O12@C to less than 18m2 /g (Table 6).
pH and Moisture Requirements for Anode Materials
The trace moisture contained in powder materials can be measured by Karl Fischer coulometric titrator. The basic principle is that the water in the specimen can react with iodine and sulphur dioxide in the presence of an organic base and methanol (H2O+I2+SO2+CH3OH+3RN→[RHN]SO4CH3+2[RHN]I,) in which iodine is produced by electrochemical oxidation of the electrolytic cell (2I−—→I2+2e−). 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 power consumed by the electrolytic cell.
The pH and moisture of the anode material have effects on the stability of the material and the pulping process. For graphite, its pH is around neutral (4~9), while Li4Ti5O12 is alkaline (9.5~11.5) with a certain residual alkalinity (Table 7). It is mainly because in the preparation of Li4Ti5O12, to ensure that the reaction proceeds adequately, the lithium source is excessive, and they mainly exist in the form of Li2CO3 or LiOH, making the final product alkaline. When the amount of residual alkali is too high, the stability of the material becomes poor, and it is easy to react with water and carbon dioxide in the air, which will directly affect the electrochemical performance of the material. In addition, since the graphite-based anode slurry is an aqueous system, its requirement for moisture (≤0.2%) is not as harsh as that of the cathode material (the slurry is an oily system, ≤0.05%), which is of significance in reducing the production cost of the battery and simplifying the process.
Major Element Content of Anode Material
Although the graphite anode has high capacity and low, stable lithium intercalation potential, it is very sensitive to the composition of the electrolyte, easy to peel off, and has poor resistance to overcharging. As a result, the commercially used graphite is modified graphite. The modification methods include surface oxidation, surface coating, etc., which leaves some impurities in the graphite. Graphite is composed of fixed carbon, ash and volatile component. Fixed carbon is the electrochemically active component. The standard requires that the content of fixed carbon needs to be more than 99.5% (Table 8), which can be determined by indirect determination of carbon.
For Li4Ti5O12, the theoretical content of lithium is 6%, with an allowed deviation of 5% to 7% in the actual product (Table 8). The content of elements can be measured by electron-coupled plasma atomic emission spectrometry, and the basic principle is as follows. First, the working gas (Ar) generates plasma in the presence of high-frequency current. Then, the sample interacts with the high-temperature plasma to emit photons. The wavelength of photons is related to the elemental species, which can be determined from the excitation wavelength. In addition, as Li4Ti5O12 has low electrical conductivity, carbon coating strategy is usually adopted to enhance the reaction kinetics of the battery. However, the coated carbon layer should not be too thick, as it will not only affect the migration rate of lithium ions, but also reduce the tapped density of the material. Therefore, the carbon content is limited to less than 10% in the standard (Table 8).
Impurity Element Content of Anode Material
Impurity elements in anode materials are components other than the main elements and elements introduced by encapsulation and doping. Impurity elements, which are introduced through raw materials or during the production process, can affect the electrochemical performance of a battery, so they need to be controlled at the source. For example, some metal impurity components will not only reduce the proportion of active materials in the electrode, but also catalyse the side reaction between the electrode material and the electrolyte, and even pierce the separator, posing a safety hazard. In addition, as artificial graphite is mostly prepared by petroleum cracking, small amounts of organic products, such as sulphur, acetone, isopropanol, toluene, ethylbenzene, xylene, benzene, ethanol, PBBs and PBDEs often remain in them (Table 9).
The European Union’s RoHS standard, Restriction of Hazardous Substances in Electrical and Electronic Equipment, are referenced in the standards developed in China. For example, some anode materials contain restricted elements, such as cadmium, lead, mercury, hexavalent chromium and its compounds, which are harmful to animals, plants and the environment, so there are strict restrictions on such substances in the standard (graphite≤20ppm, lithium titanate≤100ppm, 1ppm=10-6) (Table 10). In addition, the production equipment of anode materials is mostly stainless steel and galvanized steel sheet. The products often contain magnetic impurities, such as iron, chromium, nickel and zinc, which can be collected by magnetic separation. Hence, the requirements for such impurities in the standards are stricter (graphite≤1.5 ppm, lithium titanate≤20 ppm).
First Reversible Specific Capacity and First Efficiency of Anode Materials
The first reversible specific capacity of the anode material is the first-cycle delithiation capacity, and the first-time efficiency is the ratio of the first-cycle delithiation capacity to the lithium intercalation capacity, which can reflect the electrochemical performance of the electrode material to a great extent. During the first-cycle of lithium intercalation, the graphite anode will decompose the electrolyte to form an SEI film, which allows the passage of Li-ions and hinders the passage of electrons, which prevents further depletion of the electrolyte, thus broadening the electrochemical window of the electrolyte.
However, the generation of an SEI film also results in a large irreversible capacity, reducing the first coulombic efficiency. Especially, for full cells, a low first coulombic efficiency means the loss of a limited source of lithium. In contrast, Li4Ti5O12 has a higher lithium intercalation potential (~1.55V) and will not generate SEI film in the first circle, so its first efficiency is higher than that of graphite (≥90%, Table 11). The first efficiency of high-quality Li4Ti5O12 can reach more than 98%. In addition, the first-cycle reversible specific capacity of the cell can partly reflect the stable capacity of the material in subsequent cycles, which is of practical importance.
Recommendations for Future Standard-setting Work
With the basic principle of practicality, the formulation of standards helps to serve enterprises and meet market demands. However, the current LIB electrode material products are changing rapidly, bringing challenges to the development of standards. Taking the currently implemented Graphite Anode Materials for Lithium-ion Battery as an example. The standard involves five categories: natural graphite, mesophase carbon microsphere artificial graphite, needle coke artificial graphite, petroleum coke artificial graphite and composite graphite. Each category is divided into different varieties according to its electrochemical properties and average particle size. However, these criteria are not well applied from the customer’s point of view.
In addition, the standard contains too much content and is less targeted. It is suggested that separate standards be established for natural graphite, intermediate phase carbon microsphere artificial graphite, needle coke artificial graphite, petroleum coke artificial graphite and composite graphite. In addition, neither the rate performance nor the cycle life of the anode materials is clearly defined in the standard. As these two indicators are also the key parameters to measure whether the electrode material can be practically applied, it is suggested that these two indicators be added to the subsequent standards.
Raw materials and testing methods are important factors for battery consistency. For LIB cathode materials, there are independent standards for raw materials (e.g. lithium carbonate, lithium hydroxide and cobaltosic oxide) and test methods (e.g. lithium cobaltate electrochemical performance tests – first time discharge specific capacity and first time charge-discharge efficiency test methods). However, few such standards have been addressed for the anode side of LIBs. As the performance of different anode materials varies considerably, it is necessary to be specific in the testing methods. Therefore, it is recommended to formulate independent standards for different raw materials of anode materials and different anode material testing methods in the future.
For silicon anodes, there are two main technical routes, nano-silicon carbon and silicon oxide, whose basic performance differs considerably. The first coulombic efficiency and specific capacity of the nano-silicon carbon anode are high, but its volume expansion is large and the cycle life is relatively low, while the volume expansion of silicon oxide is relatively small and the cycle life is better, but the first-time efficiency is low. As the exact route to be developed also depends on the market and customer demand for the product, it is suggested that the formulation of the standard for silicon anode can be divided into two different systems: nano-silicon carbon and silicon oxide, so that the parameters in the standard are more pertinent and practical.
Also, hard carbon is a conventional anode material for LIBs. It is used in a narrow range, mainly by incorporating graphite anode to improve the rate performance of anode materials. However, in the future, the market share of hard carbon may gradually increase as the applications of LIBs diversify. Therefore, it can be standardized at the right time. In addition, lithium-sulphur and lithium-air batteries are new battery systems with high energy density, so metal lithium is also the future direction for the development of anode materials. However, as the development of lithium metal batteries is still in its infancy and will not be widely used in the short term, it is still too early to formulate standards for lithium metal anodes.
To sum up, the standard of anode material is mainly based on five aspects: crystal structure, particle size distribution, tapped density and specific surface area, pH and water content, main and impurity element contents, first-time reversible specific capacity and first charge-discharge efficiency, to achieve high energy density, high power density, long cycle life, high energy efficiency, low cost of use, and environmental friendliness in batteries (Figure 6). These standards regulate the parameters of anode materials for LIBs, which can be used to guide their production and application.
In recent years, with strong national support, the LIB industry has gained momentum and anode materials have ushered in unprecedented opportunities. Due to the increasingly high energy density requirements of LIBs in the new energy industry, the properties of graphite and lithium titanate materials are constantly being optimized. At the same time, the next-generation LIB anode material, silicon, is also being commercialized. Therefore, it is necessary to upgrade the original anode standards, or even compile new ones, to promote the healthy and sustainable development of China’s LIB industry.