Lithium ion batteries - the impact of electrode surface density/compaction/thickness on performance!

This article uses LiFePO4 as the positive electrode material and lithium sheet as the negative electrode material to prepare a button type lithium-ion battery. 

Three parameters, namely surface density, compaction density, and thickness consistency, 

are used as indicators to systematically study the influence of these parameters on battery performance, 

providing basic data and basis for the production process of lithium-ion battery electrode sheets.

1 Experiment

1.1 Polar film production

The positive electrode dosage is LiFePO4: acetylneblack: PVDF=8:1:1. 

The planetary ball mill will run the electrode material prepared in proportion at a speed of 300r/min for 1 hour in forward 

and reverse rotation alternately for 10 hours to prepare the positive electrode sheet slurry. 

Add an appropriate amount of NMP solvent to the slurry in the mortar and continue to grind it evenly to obtain the slurry. 

Apply the slurry onto aluminum foil (thickness 20um) at 2 ℃ in a dry environment, with three coating thicknesses of 100, 150, and 200um.

 After drying, roll press. Among them, the pole pieces with a coating thickness of 100um were rolled at different reduction rates, passes, and temperatures. 

The coating reduction rates were set to 30%, 40%, and 50%, and the passes were set to 1, 2, and 3 (when the pass was 1, the reduction amount was 50um; 

when the pass was 2, the reduction amount was 30/20um; when the pass was 3, the reduction amount was 20/20/10um). 

The temperatures were set to 20, 90, and 160 ℃, resulting in a total of 9 pole pieces made by different processes. 

Slicer slices 16mm to produce battery positive electrode sheets.

1.2 Battery Preparation

The positive electrode uses experimentally made positive electrode sheets, and the negative electrode uses lithium sheets. The positive electrode sheet, lithium sheet, and separator (Celgard2500) are placed in a glove box filled with high-purity argon gas and prepared into a CR2025 button type battery through processes such as liquid injection and sealing. Number the prepared batteries according to different electrode manufacturing processes. Table 1 shows the electrode manufacturing process parameters for different batteries. The surface density ρ 1 and compaction density ρ 2 involved in the table can be solved by equations (1) and (2):

In the formula: m1 is the mass of the polarizer;

 M2 is the quality of aluminum foil; 

W is the mass fraction of the active substance; 

S is the polar area; H1 is the thickness of the pole piece after rolling, and h2 is the thickness of the aluminum foil.

1.3 Testing Methods

Using an electron scanning microscope to observe the microstructure of the positive electrode sheet; 

Perform constant current charge and discharge tests on the battery using a blue battery tester; 

Perform cyclic voltammetry and alternating current impedance tests using an electrochemical workstation. 

Record the experimental data of the above tests by battery number.

1.4 Experimental Instruments

Planetary ball mill, coating machine, roller press, glove box, scanning electron microscope, blue electron tester, electrochemical workstation.

2 Results and Discussion

2.1 The influence of coating compression rate on battery performance

As shown in Figure 1, as the compression rate increases from 30% to 40% and then to 50%, 

the discharge specific capacity of the sample increases from 142mAh/g to 153mAh/g and then to 158mAh/g. 

This is because when the compaction density is too low, the distance between particles is large, there are many ion channels, and the electrolyte absorption capacity is large, 

which is conducive to ion movement. However, due to the small contact area between particles, 

it is not conducive to electronic conductivity, and the discharge polarization increases; 

To a certain extent, as the compaction density increases, the distance between raw material particles decreases, the contact area increases, 

the conductive channels and bridges increase, and the active area that can participate in the reaction increases, 

thereby significantly improving the specific capacity of the battery.

As shown in Figure 2, as the compression ratio increases from 30% to 40% and then to 50%, 

the charge transfer impedance of the positive electrode material decreases from 70 Ω to 35 Ω and then to 28 Ω. 

This is due to the high porosity caused by low compaction density, which results in some particles forming an insulating state and unable to participate in charging and discharging. 

High voltage density electrodes have higher fracture strength, thus avoiding electrode particle detachment and forming insulating particles during cycling. 

High compaction density can significantly make the pore size and pore distribution of the electrode more uniform, the distribution of conductive agents and binders more uniform,

 and reduce the contact resistance and charge transfer impedance of the electrode.

2.2 Effect of Coating Thickness on Battery Performance

As shown in Figure 3, as the coating thickness increases from 100um to 150um and then to 200um, the capacity retention rate of the sample gradually decreases. 

The capacity retention rate after 50 cycles decreases from 95% to 93% and then to 91%. 

This is because the lower the surface density of the electrode, the thinner the electrode plate and the lower the internal resistance of the battery. 

The changes caused to the electrode plate structure by the continuous insertion and extraction of lithium ions during charge and discharge cycles are also smaller,

 resulting in better cycling performance and higher capacity retention rate. 

The electrode with a higher surface density makes the migration path of lithium ions longer, and the internal resistance of the battery is also higher. 

This makes it easy for congestion to occur on the migration path of lithium ions, 

which can lead to incomplete deintercalation of lithium ions in a short period of time and ultimately result in a loss of specific capacity.

Figure 4 shows the discharge specific capacity of each sample at different rates. 

The samples were charged and discharged 5 times at different rates of 0.1C, 0.5C, 1C, 2C, and 5C. 

As the charge and discharge rate increased, although the discharge specific capacity of each sample decreased, for the same reason as above, 

the discharge specific capacity of the sample with a coating thickness of 100um was higher than that of the sample with a coating thickness of 150um at the same rate, 

and the discharge specific capacity of the sample with a coating thickness of 150um was higher than that of the sample with a coating thickness of 200um. 

At a 5C rate, the sample with a coating thickness of 100um still has a discharge specific capacity of 100mAh/g,

while the samples with coating thicknesses of 150 and 200um only have discharge specific capacities of 60 and 40mAh/g, respectively.

2.3 The influence of rolling passes on the performance of electrodes and batteries

2.3.1 The influence of rolling passes on the consistency of electrode thickness

Figure 5 shows the thickness curve of a 100um coated polarizer under different rolling passes. 

When the pass is 1, press down 50um at once; When the number of passes is 2, the pressing amount is gradually reduced in batches of 30/20um; 

When the pass is 3, the pressing amount is sequentially divided into batches of 20/20/10um.

 As shown in Figure 5, as the number of rolling passes increases from 1 to 2 and then to 3, 

the thickness deviation of the polarizer decreases from ± 3.0um to ± 1.9um and then to ± 1.4um, and the consistency of polarizer thickness gradually improves. 

This is because as the number of rolling passes increases, the compaction density of the polarizer coating also gradually increases, 

making the surface thickness of the polarizer more uniform.

2.3.2 The influence of rolling passes on the surface morphology of polarizer

Figure 6 shows SEM images of the surface of the polarizer coating under different rolling passes. 

As shown in Figure 6, when the pass is 1, the particles on the surface of the polarizer coating are tightly bound in some areas, 

while some areas are not tightly bound enough; When the number of passes is 2, the area of tightly bound particles on the surface of the polarizer coating increases; 

When the number of passes is 3, the particles on the surface of the polarizer coating are tightly combined and almost integrated, further increasing the density of the coating. 

The different rolling passes change the distance and gap between particles in the polar coating, resulting in different degrees of density on the surface of the polar coating.

2.3.3 The influence of rolling passes on battery performance

As shown in Figure 7, when the number of rolling passes increased from 1 to 2 and then to 3, the cycling performance of each sample improved. 

The discharge specific capacities of the 50th cycle were 141, 151, and 157 mAh/g, respectively, due to the increased consistency of electrode thickness with the increase of rolling passes. 

If the thickness of the positive electrode is significantly uneven,

 the charging and discharging process will occur locally on the electrode, 

causing an increase in resistance at the interface between the electrode material and the electrolyte and a decrease in specific capacity.

Figure 8 shows the AC impedance spectra of each sample. 

The AC impedance spectrum consists of a semicircle in the high-frequency region and a straight line in the low-frequency region. 

The intercept of the semicircle in the high-frequency region on the Z 'axis represents the charge transfer impedance at the interface between the positive electrode and the electrolyte. 

When the number of rolling passes increases from 1 to 2 and then to 3, the charge transfer impedance of each sample decreases from 45 Ω to 32 Ω and then to 28 Ω.

2.4 The Effect of Rolling Temperature on the Performance of Polar Plates and Batteries

2.4.1 The influence of rolling temperature on the consistency of electrode thickness

Figure 9 shows the thickness curve of a 100um coated polarizer at different rolling temperatures. 

As shown in the figure, as the rolling temperature increases from 20 ℃ to 90 ℃ and then to 160 ℃, 

the thickness deviation of the polarizer decreases from ± 1.9um to ± 1.3um and then to ± 0.8um, and the consistency of polarizer thickness gradually improves. 

This is because as the rolling temperature increases, the deformation resistance of the polarizer coating decreases, 

the plasticity improves, and the surface thickness of the polarizer becomes more uniform.

2.4.2 The influence of rolling temperature on the surface morphology of polarizer

Figure 10 shows SEM images of the surface of the polarizer coating at different rolling temperatures. As shown in the figure, when the rolling temperature is 20 ℃, some areas of the surface of the polarizer coating have relatively tight particle bonding, while some areas are not tight enough and have a small number of micropores; When the rolling temperature is 90 ℃, the degree of tight bonding between particles on the surface of the polarizer coating increases, the area of tight bonding increases, and the number of micropores decreases; When the rolling temperature is 160 ℃, the degree of tight bonding between particles on the surface of the polarizer coating further increases, the area of tight bonding further increases, and the number of micropores further decreases. The different rolling temperatures alter the deformation resistance of the coating, resulting in varying degrees of density on the surface of the polarizer coating.

2.4.3 Effect of Rolling Temperature on Battery Performance

As shown in Figure 11, when the rolling temperature is increased from 20 ℃ to 90 ℃ and then to 160 ℃, 

the Coulomb efficiency of the sample also increases. 

Coulomb efficiency is the ratio of discharge specific capacity to charge specific capacity in the same charge discharge cycle. 

As the thickness consistency of the electrode increases, the resistance decreases and Coulomb efficiency also increases.

Figure 12 shows the cyclic voltammetry performance curves of each sample. 

It can be seen that among the three samples, when the rolling temperature is 160 ℃, 

the symmetry between the upward oxidation peak and the downward reduction peak is good, the peak position difference is also the smallest, 

and the reversibility of charging and discharging is also the best, proving that the Coulomb efficiency is inevitably high.

3 Conclusion

The manufacturing process of lithium-ion battery positive electrode involves three important parameters: surface density, 

compaction density, and consistency of electrode thickness. 

These parameters all affect the performance of the battery by affecting its internal resistance. 

Reducing surface density, increasing compaction density appropriately, 

and improving the consistency of electrode thickness can all reduce the internal resistance of the battery, 

especially by reducing the charge transfer impedance at the interface between the electrolyte and the positive electrode. 

Any parameter setting that can reduce the internal resistance of the battery will improve its performance, otherwise it will reduce its performance:

(1) The coating thickness increased from 100um to 150um and then to 200um, 

and the surface density correspondingly increased from 2.48mg/cm2 to 3.72mg/cm2 and then to 4.96mg/cm2. 

The capacity retention rate of the 50th cycle decreased from 95% to 93% and then to 91%, 

and the discharge specific capacity of the 5th rate decreased from 100mAh/g to 60mAh/g and then to 40mAh/g.

(2) The compression rate increased from 30% to 40% and then to 50%,

 and the compaction density correspondingly increased from 0.35g/cm3 to 0.42g/cm3 and then to 0.49g/cm3. 

The discharge specific capacity of the sample increased from 142mAh/g to 153mAh/g and then to 158mAh/g.

(3) When the number of positive electrode rolling passes increased from 1 to 2 and then to 3, the consistency of electrode thickness gradually improved. 

The discharge specific capacity of the 50th cycle increased from 141mAh/g to 151mAh/g and then to 157mAh/g.

(4) When the rolling temperature of the positive electrode is increased from 20 ℃ to 90 ℃ and then to 160 ℃, 

the consistency of the electrode thickness gradually improves, 

the reversibility of charging and discharging improves, and the Coulomb efficiency also increases.