Evaluation of Compressibility and Compaction Density of Lithium-ion Battery Powder Materials
With the rapid development of the lithium-ion battery industry, there are more and more safety problems in the use of batteries. Among them, the material problem is a major problem that cannot be ignored. The selection of materials and the composition of the system determine the safety performance of the battery. When selecting positive, negative active materials and separator materials, the manufacturer did not monitor the characteristics and matching of raw materials, and the battery will have many potential safety hazards. At current,during battery cell development process, the overall quality control of powder materials has also received a lot of attention, among which the compaction density index is also a key index affecting battery performance, the level of compaction density is closely related to the particle size and distribution of the key main material positive and negative electrode powders, and is closely related to capacity, battery internal resistance, battery life, etc. The measurement of compaction density is currently well known and recognized by material companies, battery companies, and research institutes in universities, there are relatively many influencing factors that may exist in the determination process, and further systematic analysis is required to determine the appropriate parameter conditions to complete the systematic determination.
According to the description in the book "Lithium-ion Battery Manufacturing Process Principles and Applications" by Mr. Yang Shaobin, the powder density is generally considered to be the total weight of the powder sample per unit volume, powder density has three forms, namely filling density, particle density and true density. Among them, the particle density is also called the apparent density, including the particle itself and the internal micropores, not including the gaps between particles. The true density mainly refers to the total volume of the powder and does not include the sum of the true volume of the micropores in the particles and the voids outside the particles. The total volume of the powder corresponding to the filling density includes the overall voids between the particles and the micropores inside the particles, also known as bulk density. The size order among different densities is: true density > particle density > filling density [1].
Filling density includes bulk density , tap density and compaction density. Bulk density is the density of free accumulation of particles under no pressure conditions. Tap density is mainly the filling density of the powder after vibration testing. Compaction density is the overall filling density of the particles after external pressure is applied. The order of filling density comparison is: Compaction density > Tap density > Bulk density. The electrode compaction density is one of the key indicators in the design process of lithium-ion batteries. The electrode compaction density = surface density / (thickness of the electrode after rolling - the thickness of the current collector), the compaction density of the powder material = powder weight after pressing / powder volume after pressing; the measurement of powder compaction density can effectively evaluate the difference of powder compaction density under different process modification conditions in powder research, and it is of great significance in the stability of powder production process and the monitoring of incoming materials【1】.
Powder has fluidity similar to liquid, compressibility similar to gas, and has the ability to resist deformation of solid. Powder research is mainly based on the science of the properties of aggregates of various shapes of particles. The particle size of the powder research is mostly between 0.1~100μm, and a small part of the particles can be as small as 1nm or as large as 1mm. The process of powder compression will be affected by the powder particle size and its distribution, shape, density, specific surface area, void distribution, surface properties, mechanical properties and flow properties, and finally show the difference in filling performance and compression performance. The electrode rolling process in the manufacturing process of lithium-ion batteries is actually a process of compacting positive and negative electrode materials, and a process of rearrangement and densification of powders (Figure 1 is a schematic diagram of the microstructure evolution during the electrode coating rolling process), so the powder performance research is also the focus of the current lithium-ion battery process modification research and development. In this paper, based on the actual measurement process of the compacted density of lithium battery powder, a systematic analysis is carried out to clarify the relevant indicators that affect the compacted density of the powder, the measurement of the compression performance and the selection of parameters, so as to ensure the effectiveness and rationality of the evaluation of the compacted density【1, 2】.
Figure 1 (a) Schematic diagram of the microstructure evolution of coating materials during the rolling process of the positive electrode (b) negative electrode sheet【2】
Powder Filling & Compression Properties
After the powder is pressed by external force, under the condition of small pressure, the filling between powder particles is not tight, and the porosity between powder is large; with the increase of external force, the powder particles flow and rearrange to form a compact packing state, and the void ratio between the particles also decreases; as the pressure continues to increase, the powder particles undergo elastic deformation, and the porosity between the particles does not change much, but the particle pore size will decrease; with the further increase of pressure, part of the powder particles will undergo irreversible plastic deformation, and the particle pore size will further decrease; at the same time, the brittle particle system will be broken, and the particle pore size will be significantly reduced. The actual compression process of powder is a complex composite process, elastic deformation will coexist with plastic deformation, elastic deformation is recoverable, and plastic deformation part is irreversible [1].
The compressibility of powder is the focus of the study of powder mechanical properties, and it has been relatively comprehensively studied in the field of pharmacy, in the field of lithium-ion batteries, people often pay more attention to the compression performance of finished batteries. With the development of the lithium-ion battery industry and the importance of material compaction density indicators, the compression performance of powder materials is gradually being paid attention to by researchers, and more and more researchers hope to determine the relevance of each stage of the process development process from the evaluation of multi-level compression properties of powder, electrode, and battery. The PRCD series powder resistance & compaction density meter produced by IEST currently has nearly 200+ customer groups in the lithium battery industry. Currently, it is mostly used as an effective means for evaluating the difference in process modification indicators of powder materials and batch stability evaluation, in addition to the basic resistance and compaction density index determination, the equipment can also realize the evaluation of the compressibility of powder materials.
Figure 2 is a schematic diagram of the PRCD series powder resistance & compaction density test equipment and compression performance test functions, among them, (a) & (b) are the pressure relief testing methods to evaluate the compression performance. The powder particles are compressed with elastic deformation and plastic deformation. When the pressure on the powder particles is released, the elastic deformation part will recover, combined with the pressure setting mode in Figure 2(a), the thickness of the powder after depressurization is deducted from the thickness of the powder after pressurization to define the rebound thickness of the powder, Figure 2(b) shows the variation curve of the rebound thickness difference between different materials with pressure, and the rebound thickness of the material gradually increases and tends to be stable with the increase of the applied pressure. Combined with the mechanism of the powder compression process, when the powder itself is broken, the irreversible plastic deformation accounts for a large proportion, and the rebound thickness of the material will not recover after pressure relief, this is also the original intention of the development of the pressure relief test method. It is hoped that the characterization of powder particle breakage can be achieved through the pressure relief test mode. Figure 2(c) and (d) show the steady-state test pressure mode and steady-state test results. This method mainly characterizes the powder compressive stress-compressive thickness deformation percentage curve. Among them, ① in (d) is the maximum deformation point of the material after compression, and with the unloading of the pressure, ② is the irreversible compression deformation part of the material, ①-② is the reversible deformation part of the material after pressurization and depressurization, and the results of the experiment will be significantly different for powder materials with different particle sizes or particle size ratios, in the actual material development, this method can be used to evaluate the stress-strain performance of materials.
Figure 2. IEST PRCD series powder resistance & compaction density test equipment and compression performance test function
Powder Compaction Density
During the compression process of powder materials, the voids between the powders and the particles themselves will change. The Heckel equation can be used to express the relationship between the void ratio and the compression pressure. It is also a semi-empirical formula that summarizes the changes in compression force and density. The porosity (1) and Heckel equation (2) are as follows [4]:
Among them, ρbulk is the filling density of the powder, ρbulk is the true density of the powder, p is the pressure; D is the relative filling density of the powder when the pressure is p, and the porosity ε=1-D, k and A are constants, can be obtained from the slope and intercept of the straight line portion of the empirical formula. The meaning of A can be clarified by combining the formula A= Ln [1/(1-DA)], where the relative density DA is the maximum density before deformation after particle rearrangement at low pressure, and this value may be closely related to the compaction density of the lithium-ion battery electrode layer; k is a parameter to measure the plasticity of the powder. The larger the value of k is, the greater the density change will be under the same pressure, and the plasticity of the powder material will also be greater. Powder compression is a very complex process, and the Heckel equation is usually applicable to powder materials with medium and high pressure and low void ratio.
The evaluation of powder compaction density in the current design and manufacturing process of lithium-ion batteries has become a key indicator for many material factories and OEMs, the stability measurement of the powder compaction density is particularly important. The measurement of the powder compaction density is actually the ratio of the total weight of the plate to the total volume after compression, which is the filling density of the powder after compression at different pressures, in the actual measurement process, people, machines, materials, methods, environment, etc. are all key indicators that affect the measurement. Appendix L of the national standard GB/T 24533-2019 stipulates the scheme for the measurement of powder compaction density, among them, the method of manually measuring the thickness of the powder after pressing the powder sample is mainly combined with the manual tablet press to obtain the thickness of the powder after compression, and then calculate the compaction density of the powder, the thickness measurement part of this standard method is measured after the pressure on the powder end is released after the pressure is completed, which is actually similar to the pressure relief test method in Figure 2(a). With the increasing attention to compaction density, there are more and more professional testing equipment for compaction density determination, compared with the method of tablet press-assisted testing, most of the current integrated equipment for pressurization and thickness measurement is equipped with a stable lower computer control system, and parameter instructions are sent through the upper computer software system, which can effectively improve the overall detection efficiency.
With reference to the current testing capabilities of different laboratories, the compaction density test mainly includes single-point pressure relief test, variable pressure multi-point test, variable pressure and pressure relief continuous test, figure 3 shows the compaction density test results of different materials under variable pressure conditions. This process is accompanied by the continuous pressure of powder materials, which is closely related to the powder compressibility. The application of the compacted density index in the research and development is usually measured under variable pressure conditions, and combined with the powder particle size, particle size distribution, specific surface area and void ratio for further analysis; at the same time, it can also be combined with the performance of the subsequent process to conduct correlation evaluation. In addition, in the application of compaction density in batch stability monitoring, it inevitably involves benchmarking results of different types of equipment from different manufacturers. the compaction density measurement itself is closely related to the pressurization method of the equipment, the thickness measurement method, the size selection of the test mold, the sampling volume and other indicators. If the benchmarking needs to be further clarified the correlation of each indicator, and finally determine the effective benchmarking parameters; if there is a large difference in the function of the equipment involved, the absolute difference of the test results of different equipment can be used to clarify the difference in test capability and then benchmark; in short, it is very important to clarify the parameter differences. Firstly, the parameters are specified and then tested and compared to prevent waste of time and cost.
Figure 3. Determination of compaction density of different powder materials under pressure swing conditions
Summary
The compressibility and compaction density of powder materials are closely related, and the compaction density index of powder is also a key index affecting battery performance, the level of compacted density is closely related to the particle size and distribution of the key main material positive and negative electrode powders, and is closely related to capacity, battery internal resistance, battery life, etc., which is of great significance to the evaluation of compacted density.
Reference Literature
【1】Yang Shaobin, Liang Zheng. The principle and application of lithium-ion battery manufacturing process.
【2】mikoWoo@ideal life. Lithium-ion battery electrodetheory and technology basis.
【3】B K K A , A S A , A H N , et al. Internal resistance mapping preparation to optimize electrode thickness and density using symmetric cell for high-performance lithium-ion batteries and capacitors[J]. Journal of Power Sources, 2018, 396:207-212.
【4】Lu Guoning, Huang Wanting, Li Gensheng, et al. Research on the application of different compression models in the compression of four powder excipients [J]. Chinese Journal of Pharmaceutical Sciences, 2018, 53(23): 8. DOI: CNKI: SUN: ZGYX.0.2018-23-008.