Analysis of the Impact of Compaction Density on Lithium Battery Performance
Research has found that, in addition to the intrinsic properties of the active material in lithium-ion battery electrodes, the microstructure of the electrode also significantly influences the energy density and electrochemical performance of the battery.
In electrodes without compaction, only 50% of the space is occupied by the active material. Increasing the compaction density can effectively enhance both the volumetric and gravimetric energy density of the electrode.
Currently, there are four main factors affecting the compaction density of the positive electrode sheet:
- Material true density
- Material morphology
- Material particle size distribution
- Electrode sheet processing
By optimizing these influencing factors, it is possible to improve energy density by increasing the compaction density.
Impact of Compaction Density on Lithium Battery Performance
1. Material True Density
Currently, the true density of commercial positive electrode materials follows the order: lithium cobalt oxide > ternary materials > lithium manganese oxide > lithium iron phosphate. This aligns with the compaction density pattern, indicating that the impact of material true density on compaction density is immutable.
True Density and Compaction Density Ranges of Several Commercial Positive Electrode Materials
Positive Electrode Materials | Lithium Cobalt Oxide | Ternary materials | Lithium manganese oxide | Lithium iron phosphate |
True Density/ (g·cm-3) | 5.1 | 4.8 | 3.6 | 4.2 |
Compaction Density/ (g·cm-3) | 4.1-4.3 | 3.4-3.7 | 2.2-2.3 | 2.9-3.2 |
Note: Different components of ternary materials have different true densities. NCM111 is selected in this table.
Currently, the difference between the compacted density and the true density of lithium cobalt oxide is already less than 1.0 g·cm-3.
If ternary materials also reach this value, the compacted density can reach 3.8 g·cm-3.
The current methods to increase compacted density mainly focus on material morphology, material particle size distribution, and electrode manufacturing processes.
2. Material Morphology
Currently, commercial lithium cobalt oxide consists of primary particles with large single crystals.
Ternary materials, on the other hand, form secondary aggregates of small single crystals, as shown in the figure.
Aggregates of primary particles, several hundred nanometers in size, form secondary spheres of ternary materials, which inherently have many voids.
After being processed into electrode sheets, there are also numerous voids between these spheres, further reducing the compacted density of ternary materials.
(a) lithium cobalt oxide; (b) ternary materials
If the morphology of ternary materials is prepared to be similar to large single crystals like lithium cobalt oxide, it can effectively increase its compacted density (above 3.8 g·cm-3).
However, the current technology is not mature, and the product’s capacity and initial discharge efficiency are lower than conventional products.
3. Particle Size Distribution
The particle size distribution of ternary materials affects their compacted density, and this is related to the spherical morphology of ternary materials. When equal-sized spheres are stacked, there will be a large number of voids between the spheres. If there are no suitable small particle sizes to fill these voids, the stacking density will be low. Therefore, an appropriate particle size distribution can increase the compacted density of the material.
(a)SEM Image of Positive Electrode Material Prepared as an Electrode with Common Particle Size Distribution.
(b) SEM Image of Positive Electrode Material Electrode After Mixing Products with Two Particle Size Distributions.
Optimizing the particle size distribution of ternary materials can improve their compaction density. Materials with similar D50, but differences in D10, D90, Dmin, and Dmax, can also result in varying compaction densities.
Both too narrow and too wide particle size distributions can reduce the compaction density of the material.
Some battery manufacturers may impose requirements on positive electrode material producers regarding particle size distribution.
Other battery manufacturers achieve increased compaction density by blending products with different particle size distributions.
4. Electrode Sheet Processing
The areal density of the electrode sheet, as well as the amounts of binder and conductive agent used, can impact the compaction density. The true density of conductive agents and binders is very low, and the more added, the lower the compaction density of the electrode sheet.
Therefore, it is advisable to use highly conductive conductive agents during the electrode sheet manufacturing process to reduce the amount of conductive agent required.
Additionally, high-speed dispersion during the slurry mixing process, ensuring uniform dispersion of conductive agents and binders, can also enhance the compaction density.
While increasing compaction density is beneficial, moderation is key, as excessive compaction can have counterproductive effects.
Therefore, compaction density should not be excessively raised. Let’s explore the effects of overcompaction in the following section.
What are the Effects of Overcompaction on Ternary Material Electrode Sheets?
There are two main reasons for causing overcompaction of ternary material electrode sheets:
- One is that battery manufacturers pursue high energy density, leading to overcompaction of the electrode sheets.
- The other is that material manufacturers have lax process control, resulting in inconsistent compaction density among different batches of ternary materials.
Battery manufacturers, without analyzing the specific characteristics of the materials, may subject the electrode sheets to overcompaction by using conventional process parameters.
SEM of Overcompacted Electrode Sheet
Overcompaction of the electrode sheet can lead to issues such as reduced battery capacity, cycling degradation, and increased internal resistance.
Initially, the overcompaction causes widespread fracturing of the spherical ternary materials. The newly created surfaces consist of numerous primary small particles detached from the secondary spheres, either falling off the electrode sheet due to a lack of contact with PVDF or locally deteriorating the conductivity of the electrode sheet due to insufficient contact with the conductive agent.
The generation of new surfaces also increases the specific surface area, enhancing contact with the electrolyte, which in turn leads to increased side reactions, resulting in decreased battery performance, such as cell expansion and cycling decay.
Overcompaction can also cause deformation of the aluminum foil, making the electrode sheet brittle and prone to breakage, thereby increasing the internal resistance of the battery.
Furthermore, in overcompacted electrode sheets, the excessive compression between material particles results in a low porosity of the electrode sheet. This leads to a reduced absorption of electrolyte by the electrode sheet, making it challenging for the electrolyte to penetrate the interior of the electrode sheet.
The direct consequence of this is a deterioration in the specific capacity of the material. Batteries with poor liquid retention capacity exhibit significant polarization and rapid decay during cycling, accompanied by a noticeable increase in internal resistance.
Whether the electrode sheet is overcompacted can be determined by observing whether it is brittle, examining the material under an electron microscope for signs of breakage, and estimating the porosity of the electrode sheet. The porosity of the electrode sheet is a crucial indicator for assessing the liquid absorption capacity and absorption rate, directly influencing battery performance.
The porosity of the electrode sheet refers to the percentage of the volume of internal pores in the electrode sheet after rolling to the total volume of the electrode sheet after rolling：
– An excessively low porosity reduces the rate at which the electrolyte infiltrates the electrode sheet, affecting battery performance.
– While an excessively high porosity reduces battery energy density, wasting effective space. Therefore, overemphasis on achieving energy density should not lead to an excessive increase in compaction density!