Generally, compaction density is closely related to electrode specific capacity, efficiency, internal resistance, and battery cycle performance. Finding the optimal compaction density is crucial for battery design. Generally, within the material's allowable compaction range, a higher electrode compaction density allows for a higher battery capacity; therefore, compaction density is considered one of the reference indicators for material energy density. However, blindly pursuing high compaction not only fails to improve battery specific capacity but also severely reduces battery specific capacity and cycle performance.
Figure 1. Schematic diagram of electrode rolling production line
Higher compaction density leads to greater compression between material particles, resulting in smaller electrode porosity and poorer electrolyte absorption. This makes it harder for the electrolyte to wet the electrode, directly resulting in lower specific capacity, poorer electrolyte retention, greater polarization during battery cycling, faster battery degradation, and a significant increase in internal resistance. Therefore, a suitable positive electrode compaction density can increase battery discharge capacity, reduce internal resistance, minimize polarization loss, extend cycle life, and improve the utilization rate of lithium-ion batteries. Conversely, excessively high or low compaction density hinders lithium-ion intercalation and extraction. So, what factors influence the compaction density of the positive electrode?
Factors affecting compaction density
The main factors affecting the compaction density of positive electrode sheets are as follows: ① True density of the material ② Material morphology ③ Material particle size distribution ④ Electrode manufacturing process. True density of materials: The true density and achievable compaction density of several commercially available positive electrode materials are shown in the table (the ternary material selected in the table is NCM111). It can be seen that the true density of the materials is: lithium cobalt oxide > ternary materials > lithium manganese oxide > lithium iron phosphate, which is consistent with the trend of compaction density. It should be noted that the true density of ternary materials with different compositions varies with the composition.
True density and compacted density range of several commercial cathode materials
Material Morphology: The true density difference between ternary materials and lithium cobalt oxide is not significant. As shown in the table above, the true density difference between NCM111 and lithium cobalt oxide is only 0.3 g·cm⁻³. However, the compaction density of ternary materials is 0.5 g·cm⁻³ lower than that of lithium cobalt oxide, or even higher. There are many reasons for this result, but the most important reason is the difference in morphology between lithium cobalt oxide and ternary materials. Currently, commercially available lithium cobalt oxide consists of primary particles with large single crystals, while ternary materials are secondary aggregates of fine single crystals, as shown in the figure. As can be seen from the figure, the secondary spheres of ternary materials formed by the aggregation of primary particles of several hundred nm inherently have many voids; and after being fabricated into electrodes, there are also a large number of voids between the spheres. These factors further reduce the compaction density of ternary materials.
SEM images of lithium cobalt oxide and ternary materials; particle size distribution.
When spheres of equal diameter are stacked, there are numerous gaps between them. If suitable small-diameter spheres are not used to fill these gaps, the packing density will be very low. Therefore, a suitable particle size distribution can improve the compaction density of a material, while an unreasonable particle size distribution will significantly reduce the compaction density. Electrode manufacturing process: The areal density of the electrode, the amount of binder and conductive agent used all affect the compaction density. The true densities of common conductive agents and binders are shown in the table. As can be seen from the table,
True density of common conductive agents and adhesives
The influence of a material's true density on its compaction density is unchangeable, but a comparison between compaction density and true density shows that there is still considerable room for improvement in the compaction density of ternary materials.
How to increase compaction density
Currently, methods to improve compaction density mainly focus on three aspects: material morphology, particle size distribution, and electrode manufacturing process. For example, the morphology of ternary materials can be prepared into large single crystals similar to lithium cobalt oxide; the particle size distribution of ternary materials can be optimized; conductive agents with good conductivity can be used during electrode manufacturing to reduce the amount of conductive agent required; and high-speed dispersion during slurry preparation can ensure uniform dispersion of the conductive agent and binder. Below is an example of improving the compaction density of ternary materials by optimizing their morphology and particle size.
The morphology of several common ternary materials and their SEM images of the electrodes (after roll forming) are shown in the figure. Among them, (a), (c), and (e) are SEM images of three ternary materials with different morphologies, with the same magnification. (b), (d), and (f) are low-magnification SEM images of the electrodes after roll forming of (a), (c), and (e), respectively. (a) shows the most common morphology of ternary materials, namely secondary agglomerates of small single crystals. The SEM image of the rolled electrode is shown in (b). There are large gaps between the secondary particles, and some secondary particles have been crushed. Some small single crystals that did not come into contact with the binder have detached. (c) shows a morphology of a primary single-crystal ternary material, but the single crystals are slightly larger than those in (a). From the corresponding electrode (d), it can be seen that there are a small number of gaps between the single crystal particles. Because there is no issue of secondary particle breakage, as long as the binder is evenly dispersed, there is no problem of single crystals detaching from the electrode. (e) is also a secondary agglomerate, but the single crystals are very large, and the contact between the single crystals is not very tight. From the corresponding electrode (f), it can be seen that there are very few gaps between the particles. If a high-speed mixer is used to prepare the slurry, the effect will be better. The compaction density results of the three morphologies (a), (c), and (e) in the figure correspond to a, c, and e in (g). As can be seen from the figure, the material with morphology (a) has the lowest compaction density, but it is not much different from that of (c). The compaction density of (e) is much higher than that of (a) and (c), reaching 3.9 g·cm-3.
SEM images and compaction density comparisons of ternary materials with different morphologies and their electrodes; optimization of particle size distribution.
Materials with similar D50 values will also have different compaction densities if their D10, D90, Dmin, and Dmax values differ. Both excessively narrow and excessively wide particle size distributions will reduce the material's compaction density. Regarding the impact of particle size distribution, some battery manufacturers impose requirements on cathode material manufacturers, while others achieve higher compaction densities by mixing products with different particle size distributions, as shown in the figure.
SEM images of cathode materials with different particle size distributions under overvoltage conditions.
There are two reasons for over-pressure on ternary material electrodes. One is that battery manufacturers over-pressure the electrodes in pursuit of high energy density, for example, compressing ternary materials with a compaction density of only about 3.6 g·cm⁻³ to 3.7 g·cm⁻³ or even higher. The other is that the material manufacturers do not strictly control the process, resulting in inconsistent compaction densities of different batches of ternary materials. The battery manufacturers did not analyze the specific conditions of the materials and over-pressured the electrodes when preparing them according to conventional process parameters.
SEM image of the electrode after overvoltage
Over-voltage on the electrode can cause problems such as reduced battery capacity, worsened cycle life, and increased internal resistance. Firstly, over-voltage causes large-scale breakage of the spherical ternary material. The newly formed surfaces contain many small primary particles that have detached from the secondary spheres. These particles either fall off the electrode because they haven't come into contact with the PVDF (polyvinyl chloride) or because they haven't come into contact with the conductive agent, leading to localized deterioration of the electrode's conductivity. The formation of new surfaces also increases the specific surface area, increasing the contact area with the electrolyte and increasing side reactions, thus reducing battery performance, such as battery swelling and cycle degradation. Over-voltage can also cause aluminum foil deformation, making the electrode brittle and prone to breakage, and increasing the battery's internal resistance. Furthermore, in over-voltage electrodes, the excessive compression between material particles results in low electrode porosity, reducing the amount of electrolyte absorbed by the electrode. The electrolyte struggles to penetrate the electrode's interior, directly resulting in poorer specific capacity. Batteries with poor electrolyte retention exhibit significant polarization during cycling, leading to rapid degradation and a marked increase in internal resistance. Whether an electrode is over-pressed can be determined by observing whether it is brittle, examining the material for breakage using electron microscopy, and estimating the electrode porosity. Electrode porosity is a crucial indicator for assessing the electrolyte absorption capacity and rate, directly impacting battery performance. Electrode porosity refers to the percentage of the volume of internal pores within the rolled electrode relative to its total volume. Too low a porosity reduces the electrolyte wetting rate, affecting battery performance; too high a porosity reduces energy density and wastes valuable space.
The compaction density should not be excessively increased in pursuit of energy density. Porosity can be tested using methods such as mercury intrusion porosimetry, nitrogen adsorption, liquid absorption, and estimation. Mercury intrusion porosimetry is a commonly used method. The specific steps of the liquid absorption method are as follows: cut an appropriate amount of electrode sheet and measure its mass m; measure its volume V; place the electrode sheet in a container containing electrolyte or other solvent (solvent density ρ), completely immerse the electrode sheet, and soak it for a certain period of time; remove the electrode sheet, place it on filter paper, and absorb it until a constant weight is reached, measuring its mass m1; calculate the porosity ε of the electrode sheet according to the formula ε=(m1–m)/ρV×100%. The estimation method is relatively simple; the porosity of the electrode sheet can be estimated based on the difference between the true density of the material and the compaction density of the electrode sheet. The equation for calculating electrode porosity is as follows: Electrode porosity (%) = (True density of mixture – Electrode compaction density) / True density of mixture × 100% The table below shows the porosity of ternary materials and lithium cobalt oxide at different compaction densities, and the data are calculated from the above formula. The calculations in the table below are based on the following: A ternary electrode contains 95% ternary material, 3% conductive agent, and 2% binder (all by mass fraction). The true density of the ternary material is 4.8 g·cm⁻³. The density of the conductive agent is approximately 1.9 g·cm⁻³, and the density of the binder is 1.78 g·cm⁻³. Therefore, the true density of the mixture is approximately 4.65 g·cm⁻³. A lithium cobalt oxide electrode contains 95% lithium cobalt oxide, 3% conductive agent, and 2% binder. The true density of LiCoO₂ is 5.1 g·cm⁻³. The density of the conductive agent is approximately 1.9 g·cm⁻³, and the density of the binder is 1.78 g·cm⁻³. Therefore, the true density of the mixture is approximately 4.94 g·cm⁻³.
Typical porosity values of ternary materials and lithium cobalt oxide at different compaction densities
Method for calculating the length of cylindrical battery electrode
The battery winding core is an Archimedean spiral. According to relevant theories, the relationship between the radius r of the winding core and the total rotation angle ϕ can be calculated by the following formula:
When the radius of the core winding needle is r0, we have:
Where ϕ is the total winding rotation angle, r0 is the diameter of the core needle, and the helical parameter a is calculated as follows:
t is the thickness of the basic unit in the core. For a cylindrical battery, t is equivalent to the thickness of the positive and negative electrode sheets and the thickness of the two separators, as shown in Figure 1.
Figure 1. Thickness of the basic constituent unit in the core. Based on Archimedes' spiral theory, the core arc length and the overall arc length are calculated using the formulas below. The difference between the two is the positive pole length:
The common outer diameter, casing thickness, and internal space diameter of cylindrical batteries are shown in the table below.
The diameter of the core winding needle is mainly determined by two factors: (1) the space in the middle of the core is sufficient to weld the bottom tab to the inside of the battery casing; (2) the minimum bending radius at which the coated electrode will not crack.
For example, the parameters for the 21700 battery used by Tesla are: positive electrode thickness 174μm; negative electrode thickness 143μm; separator thickness 10μm; core basic unit thickness t = (174+143+10*2)μm = 337μm; helix parameter a = t / 2π = 53.66μm.
The core needle diameter is 2mm, and the rotation radius of the hollow core is ϕ = r/a = 1 mm / 53.66 μm = 18.63. The corresponding number of turns is ϕ/2π = 18.63/(2*3.14) = 2.97;
The internal diameter of the shell is 20.4 mm. Considering the expansion space of the core, the core diameter is 19.4 mm. Therefore, the rotational radius ϕ containing the hollow core is ϕ = r/a = (19.4/2) mm / 53.66 μm = 180.77. The corresponding number of turns is ϕ/2π = 180.77/(2*3.14) = 28.77.
Therefore, the actual number of turns of the positive electrode winding is 28.77 - 2.97 = 25.8. According to the following formula...
The length of the rotating arc in the hollow core is l = 9.4 mm. The length of the rotating arc containing the hollow core is l = 874.2 mm. Therefore, the actual length of the positive electrode is 874.2 – 9.4 = 864.8 mm. The theoretically calculated positive electrode length matches the actual measured value of 865 mm. The formula is quite complex and will be further simplified.
In the formula, ϕ is generally quite large, for example, for a 21700 battery, ϕ = 180.77. Simplifying, 1 + ϕ^2 ≈ ϕ^2. Furthermore, ln(ϕ + √(1 + ϕ^2)) ≈ ln(ϕ + ϕ) ≈ ln(2ϕ), whose value is 2~3. These are also negligible. Therefore, we have...
Calculate the core arc length and the overall arc length using the above formulas. The difference between the two is approximately the length. Therefore, as shown in Figure 2, the core diameter d, the outer diameter D, the thickness t of the basic components in the core, the thickness of the positive and negative electrode sheets, and the sum of the thicknesses of the two separators are known.
Figure 2. Schematic diagram of the core. The method for estimating the electrode length is as follows:
Taking Tesla's 21700 battery as an example, with d=2mm, D=19.4mm, and t=337μm, the electrode length L is calculated to be 867.8 mm using the formula. The calculated value is not much different from the 864.8 mm calculated by the first method and the actual measured value of 865 mm.
Electrochemical and structural design of pouch-wound batteries!
I. Lithium-ion Battery Design Principles 1.1 Safety
In product design, it is essential to eliminate any potential hazards to the personal safety and property of end customers.
1.2 Customer Needs
Customer satisfaction is the top priority. Project managers must communicate with customers frequently to understand their experience using the product.
1.3 Costs Reducing costs without compromising the user experience demonstrates responsibility to both the company and its customers.
1.4 Regulations Products must comply with the relevant laws and regulations of both the country of origin and the country of consumption. II. Electrochemical Design Section 2.1 Positive Electrode Formulation
2.2 Negative Electrode Formulation
2.3 Areal density/specific mass/compacted density design
2.4 Negative Electrode Capacity Margin Design Negative Electrode Capacity Margin: Because the negative electrode needs to continuously consume lithium ions from the initial formation of the SEI film to the cycle repair of the SEI, a certain loss margin needs to be set for the negative electrode.
2.5 Battery Capacity Design Battery design capacity margin: Due to operational and equipment reasons, the capacity of batteries in the same batch will be normally distributed. In order to ensure that the overall capacity distribution is basically above the nominal capacity, a design margin needs to be set.
2.6 Electrolyte Type Selection
Batteries are classified in various ways according to different user environments and habits. The main characteristics of the electrolytes used in these battery classifications are also different: (1) High temperature type: 85 degrees Celsius for 12 hours storage test meets industry requirements and is used for GPS products with high temperature requirements; (2) Ordinary type: 70 degrees Celsius for 12 hours storage test meets industry requirements and has a relatively high discharge platform and is used for MID products; (3) Low cost type: meets the requirement of maintaining 80% capacity after 300 cycles at 0.5C and is used for low cost products such as power banks; (4) High voltage electrolyte: meets the requirement of 4.35V-4.40V use and has a relatively high discharge platform requirement and is used for high voltage battery products; (5) Low temperature type: has a discharge capacity ratio of more than 50% at -40 degrees Celsius and 0.2C and is used for products used in low temperature environments; (6) High rate type: meets the requirement of discharge rate above 20C and is used for products such as drones and starting power supplies.
2.7 Electrolyte Formulation (1) Lithium Salt: LiPF6 is mainly used as lithium salt in the current lithium-ion battery industry, and its concentration is generally 0.8～1.2mol/L; (2) Solvent: Generally a binary or ternary component, the composition is EC/DEC/PC/EMC/DMC, etc., and the content is 90%～95%; (3) Additives: divided into film-forming additives, overcharge prevention additives, low temperature additives, and conductivity enhancement additives, with a content of 5%～10%; (4) Parameter requirements: Density: 1.1～1.2g/cm3 Conductivity: 6.0～9.0mS/cm (ordinary type), 10.0～14.0mS/cm (rate type) Moisture content: ≤20ppm HF content: ≤20ppm 2.8 Separator Separator material: single layer or multi-layer PE, PP Thickness specifications: 12um～30um 2.9 Outer Packaging - Aluminum-Plastic Film The main thicknesses of the aluminum-plastic film are 88um/113um/122um/153um. The main structure of the aluminum-plastic film consists of three layers of Nylon/Al/CPP, and two layers of adhesive; the Nylon layer is approximately 20um, the Al layer is approximately 40um, and the remaining thickness is the CPP thickness. III. Structural Design Section 3.1 Model Determination The battery model is mainly limited by the space available for placing the battery within the device, and the expansion during battery use also needs to be considered. This section uses P455268/2500Ah as an example to illustrate the battery's structural design: Model: P455268 Nominal Capacity: 2500mAh Dimensions: Maximum thickness 4.5mm, maximum width 52.0mm, maximum length 68.0mm
3.2 Aluminum-Plastic Film Packaging Shell Perforation Length (Outer Perforation) = Battery Length - Top Sealing Width - Deviation Coefficient C Deviation Coefficient C: A coefficient caused by various errors in the length direction, which needs to be subtracted to ensure the battery does not exceed the allowed length; Single-perforation battery: C = 1.0mm; Double-perforation battery: C = 1.5mm; Perforation Width (Inner Perforation) = Battery Width - Folding Width C Folding Width C: The width space required for the battery folding edge. When battery thickness T ≤ 2.8mm, C = 2.5mm; When 2.8mm < battery thickness T ≤ 3.5mm, C = 2.0mm; When battery thickness T > 3.5mm, C = 1.5mm. This type of battery has a single folding edge; if the manufacturing process capability is sufficient, Cmin = 1.0mm. Perforation Depth (Single Perforation) = Core Thickness - 0.2mm Perforation Depth (Double Perforation) = Core Thickness (sum of both perforations) When the casing depth is greater than 5.0mm, a double-pit design is recommended, with the depth difference between the two pits (positive and negative) approximately 1.0mm. Aluminum-plastic film thickness selection: 88um aluminum-plastic film: Casing depth ≤ 3.0mm, suitable for batteries with a thickness ≤ 3.5mm. 113um aluminum-plastic film: Casing depth ≤ 4.0mm, suitable for single-pit batteries with a thickness ≤ 5.0mm or double-pit batteries with a thickness ≤ 8.0mm. 152um aluminum-plastic film: Casing depth ≤ 6.5mm, suitable for double-pit batteries with a thickness > 8.0mm.
3.3 Diaphragm Width Diaphragm width = Aluminum-plastic film stamping shell length (outer rim) - (0.0～0.5mm)
3.4 Electrode Size Design Negative electrode width = Separator - D1 Positive electrode width = Negative electrode width - D2 D1: The allowance for deviation between the negative electrode width and the separator width, preventing the negative electrode from misaligning beyond the separator's range. Generally 2.0～3.0mm. D1=2.0mm, electrode length ≤500mm & electrode width ≤50.0mm D1=2.5mm, 500mm < electrode length ≤1500mm D1=3.0mm, electrode length >1500mm D2: Misalignment deviation between the negative and positive electrodes, generally 0.0～2.0mm D2=0.0mm, battery capacity ≤200mAh D2=1.0mm, electrode length ≤500mm & electrode width ≤50.0mm D2=1.5mm, 500mm < electrode length ≤1000mm D2=2.0mm Electrode length > 1000mm Positive electrode length = Theoretical width of the winding needle × Number of positive electrode layers + P1 + P2 Negative electrode length = Positive electrode coating length × 0.5 + Negative electrode length allowance Theoretical width of the winding needle = Actual width of the winding needle + Winding needle thickness + Parameter adjustment value (0.2～0.5mm) P1: Sum of arc lengths of the positive electrode at even-numbered folds P2: Sum of arc lengths of the positive electrode at odd-numbered folds -- Refer to process requirements for specific calculations Negative electrode length allowance: Generally 3.0～5.0mm
3.5 Calculation of Positive/Negative Electrode Aspect Density ρ<sub>negative</sub> = ρ<sub>positive</sub> × 2S<sub>positive</sub> × R<sub>positive</sub> × C<sub>positive</sub> × E / (2S<sub>positive</sub> × R<sub>negative</sub> × C<sub>negative</sub>) = ρ<sub>positive</sub> × R<sub>positive</sub> × C<sub>positive</sub> × E / (R<sub>negative</sub> × C<sub>negative</sub>) ρ<sub>positive</sub>: Aspect density of a single side of the positive electrode ρ<sub>negative</sub>: Aspect density of a single side of the negative electrode Aspect area of a single side of the positive electrode R<sub>positive</sub> R<sub>negative</sub> C<sub>positive</sub> C<sub>negative</sub> T<sub>0</sub> - 2 × t) / E / T<sub>layer</sub> = (T<sub>0</sub>: Average design thickness of the battery t: Thickness of the aluminum-plastic film T<sub>layer</sub>: Thickness of a single layer M: Thickness of the positive electrode N: Thickness of the negative electrode L: Thickness of the separator E: Coefficient of battery expansion
3.7 Separator Length Separator Length = Negative Electrode Length × 2 + Needle Width × 2 + 40mm Allowance 3.8 Battery Thickness Estimation Battery Thickness = TA × EA × LA + TC × EC × LC + TM × LM + TA1 × 2 TA: Positive Electrode Rolling Thickness EA: Positive Electrode Expansion Rate LA: Number of Positive Electrode Layers TC: Negative Electrode Rolling Thickness EC: Negative Electrode Expansion Rate LC: Number of Negative Electrode Layers TM: Separator Thickness LM: Number of Separator Layers TA1: Aluminum-Plastic Film Thickness
IV. Conclusion
This design specification is for reference only, and some coefficients need to be adjusted according to actual conditions. For lithium-ion battery technology, the above introduction is only a small aspect. In order to make good batteries, we need to comprehensively consider all factors in battery design and production, including people, machines, materials, methods, and environment, and incorporate these factors into every detail of the process.
