analysis of the separator thickness and porosity on the performance of lithium-ion batteries.

1.
Introduction rechargeable LIBs are widely used in many types of electronic devices due to their high energy density and good electrical performance.
Development of Energy-
Storage devices with high energy and power density have developed at an unprecedented speed in the past decade [1]. Li-
Ion batteries, thanks to their excellent electrical performance and high capacity, are one of the most promising solutions in these storage systems.
Lithium iron phosphate (LiFeP[O. sub. 4]or LFP)
It is a very popular commercial cathode material for lithium batteries. It has the advantages of low cost, good environmental compatibility, relatively large capacity and good inherent stability. 2]. Lithium-
The ion battery consists of three important functional components: cathode, anode and electrolyte.
During charging and discharging, lithium ion moves from electrode to electrode through electrolyte.
Similar to the battery material, the separator plays a vital role in the operation of the battery [3].
The basic function of the separator is to separate the two electrodes and prevent the internal short circuit and the stability of the thermal runaway [4].
The separator does not directly participate in the battery reaction, but the physical properties play an important role in determining the performance of the battery (including energy density, power density and safety [])5].
Compared with other factors, studies on the effects of separator thickness and air holes on performance appear less in publications.
With a carbon-modified separator, the effectiveness of the fiberglass separator is regularly reported during battery operation.
The separator used in rechargeable batteries is usually 20-30 [micro]m [6].
Celgard 2400 is one of the most widely used separators.
The study of the manufacturing process focuses on the reduction of weight and the stability of the battery.
Most batteries use commercial separators based on micro-pore film single-layer and three-layer olefin [3, 7, 8].
The separator used in the analysis is Celgard-
2400, PP2075, H2013 and h2512.
The separator must be chemically stable in contact with the used electrolyte and electrode material.
They should not be degraded during charging and discharging [6].
During normal battery operation, it should also be heat stable at high temperature.
The humidity of the electrolyte is also an important feature of the battery separator because ion transport [requires electrolyte adsorption and stability of the separator]4].
The adsorption of electrolyte is very important for achieving low internal resistance and high ion conductivity [9].
Air holes are also an important feature of the separator [10, 11].
The aperture should be small enough to prevent penetration of electrode materials and active components of conductive additives.
The air holes of the separator must be evenly distributed to inhibit the branching lithium and to prevent the active particles from penetrating through the separator [6].
Usually, sub-micron aperture less than 1 [micro]
Separator [m required]10].
In order to maintain the electrolyte, the pore rate of the separator must be appropriate in order to provide sufficient ionic conductivity.
If the air hole rate is too high, due to low mechanical strength and large internal resistance, it will adversely affect the performance of the battery [12].
The separator must be hot and stable;
It should not shrink or curl when the temperature rises [6].
The typical multi-layer design of the separator provides the off function, one of which
Layers close to the heat out-of-control temperature melt to close the air holes and other layers provide oxidation resistance and mechanical strength.
Most separators facilitate ion transfer only when they are filled with electrolytic mass.
Electrolyte retention is also a key factor in long-term retention
[Semester performance]4, 6, 11]. 2.
Experimental LifeO. sub. 4](LFP)
Mix materials with wet method.
The influence of separator thickness and air hole on battery performance is analyzed from the charge angle
Analysis of discharge process, surface morphology and AC impedance.
LFP powder, carbon (
Conductive carbon (3%)+ super P (3%))
, And mixed polydiammonium by mass at an age ratio of 84: 6:10 percent.
First, three hours of polyammonium and NMP were mixed in a ball mill at 150 RPM.
Subsequently, conductive carbon, super P and NMP were added and mixed for 3 hours at 180 RPM.
Finally, add LFP powder to the machine at 200 RPM and mix for 5 hours.
Use the doctor blade method of the same thickness to coat the slurry on the carbon coated aluminum plate, and then dry in the standard convection oven for the night.
After that, they were cut in half and compressed with a calender.
The electrode density is calculated by the mass of the active material and the thickness of the active material on the sheet.
Finally, the LFP electrode is divided into the calender sample and the uncalender sample.
The electrode sheet is punched and kept overnight in a vacuum oven.
Later, they were used to assemble half a cell with LiFeP [O. sub. 4]
As a cathode, electrolyte from BASF, 1m lipf6.
EC: EMC 1:2, lithium metal as the negative electrode. Initial charge-
Due to a short circuit problem, the discharge test using a single-layer separator is unstable for the cycle.
As a result, the separator for all samples has doubled to provide stability and obtain a consistent comparison.
The doubling of the separator has minimal impact on the performance of the battery and is therefore ignored.
A separator of different thickness, such as 40 [micro]
M has gaps of 45% and 48% and 50 [micro]
M with 41% and 50% gaps was used. JEOL JSM-
SEM analysis was carried out with 74 10f machine.
The surface morphology of the uncalving and calving electrode separator was analyzed using SEM.
Neware BTS is used to use constant current constant voltage (CC-CV)
Routine between 2. 0 and 4. 0 V.
Dynamic potential impedance spectroscopy was performed using Gamry Instruments.
The EIS is performed using an AC voltage of 10 mV and a frequency of 1 MHz to 0. 01 Hz. 3.
Results and discussion 3. 1.
Surface Morphology.
Characterize the morphology of the separator using a scanning electron microscope (SEM)at 5 KV.
Figure 1 shows the surface of the separator under the same magnification.
The air hole rate and thickness of the separator provided by the company are discussed in Table 1.
The separator does not conduct electricity;
Therefore, before observing the image, it was coated with gold particles for 1 minute in the spatula.
As shown in Figure 1, the holes of the separator are evenly distributed.
According to the manufacturer, the shrinkage of all separators is 0%;
This indicates that the separator does not shrink at 85 [degrees]
Bake in the oven for an hour.
The simplest micropore film is a single layer film.
It can be made from different polymer materials.
It is widely used in the manufacture of separators for its excellent mechanical strength and chemical stability.
Membrane morphology has no significant effect on battery performance at low C- TIMErate.
Usually, a separator with high air holes and small thickness is required.
However, under the same operating conditions, the weight of the active material, the performance results of each battery are observed, and the electrode density provides insight into the optimal air holes and thickness required for better battery performance.
The fiberglass separator is one of the commonly used separators.
However, they are thicker and heavier than olefin-based separators [11].
The size and thickness of the separator is very important to reduce the overall weight of the battery without affecting performance. Separators (a)and (b)
It is a three-layer separator with micro holes (c)and (d)
Is single-layer microporous membrane separator.
These separators have a smooth surface compared to fiberglass separators.
Figure 2 shows the surface of the LFP electrode without calving and calving under the same magnification.
For the purpose of this experiment, electrode samples from the same piece were used.
It can be seen from the SEM image that the density of the calender electrode is high.
The average density of the undelayed sample is calculated to be 0. 8 g/[cm. sup. 3]
The average density of the calender sample is 1. 2 g/[cm. sup. 3]. 3. 2. C-Rate Test.
We tested the effect of separator thickness and air hole rate on the performance of the LFP battery.
With lithium metal as anode and LFP as cathode, assemble the battery in the case of CR2032 coin battery.
For the analysis of the results, the electrodes with the same thickness (
There is a difference between the left and right [+ or -]2 [micro]m)
Weight is selected.
In addition, the battery was made with the calender and uncalender electrodes to effectively compare the thickness of the separator and the effect of the air holes.
All cells were tested at 1 °c.
The separator is classified according to the type of material used by the manufacturer.
The calender is an important process to increase the electrode density and reduce the internal resistance of the battery.
Figure 3 shows the charging and discharging performance of the battery with a single-layer separator 2400 and PP2075 for the calender and non-calender samples.
It should be noted that the legend in the chart is written in a sample type format-
Type of separator used.
Figure 4 shows the charging and discharging performance of the three-layer PP/PE/PP separator on the calender and non-calender electrode samples under similar operating conditions.
As shown in Figure 3 and Figure 4, the discharge performance of the calender sample is better than that of the non-calender sample.
In addition, it can be seen by looking at the figure in figure 3 that the performance of the Celgard 2400 separator is better than that of the Celgard PP2075 separator.
It can be explained according to the hole rate and thickness of the separator used.
The best hole rate and thickness when comparing single layer separator are 41% and 50 [micro]M respectively.
The change of separator thickness affects the battery at high C-
The application speed is increased due to high internal impedance.
For our experiments, it can be assumed that thickness plays a minimal role in performance due to C- low
The rate used for testing.
Similarly, as shown in figure 4, compared to the Celgard H2512 separator, the discharge performance of the calender sample using the Celgard h2015 separator produces better performance.
We can attribute the higher charging and discharge capacity of the battery manufactured using Celgard 2400 (monolayer)
And Celgard H2013 (trilayer)
Compared with cells made using PP2075 (monolayer)and H2512 (trilayer)
Porous structure of separator [13].
Air holes affect the ion charge compensation rate between positive and negative electrodes soaked in electrolyte solutions, thus affecting the overall impedance of the battery.
This is observed from the impedance results provided in Figures 6 and 7.
Figure 5 shows the dQ/dV curve for charging and discharging of single-layer and three-layer separators.
The peak value of charge and discharge using various separators is observed in the voltage range of 3. 3 V-3. 6 V.
In addition, the charge peak using the separator calender electrode has a higher differential capacity compared to the uncalender electrode.
This is consistent with the results observed for all samples using a single and three-layer separator. Tables 1(a)and 1(b)
Provide a comparison of the separator used for testing and a specific capacity observed at 1C discharge. 3. 3.
Electrical impedance spectrum (EIS).
Figures 6 and 7 show a comparison of the impedance spectrum results of the single-layer and three-layer groups of uncalving and calving electrode separators.
It can be observed from the EIS results that compared with PP2075 and H2512 separators, the ohm resistance of the batteries using 2400 and H2013 separators is lower.
Compared to the same type of material, the diffusion impedance of the battery using the 2400 and H2013 separators is also lower
Separator based on PP2075 and H2512 respectively.
Therefore, the result of the impedance supports the charge-
The discharge performance of the cells observed in Figure 3 and Figure 4.
In the single-layer and three-layer groups, cells with better performance were observed with a low-hole separator.
We attribute this behavior to the loss of circulating lithium in the initial stage of charge transfer and decomposition of the electrolyte that forms the solid electrolyte interface (SEI).
In addition, the decomposition product of the electrolyte has the possibility to block the hole of the separator [4].
This loss of lithium and the formation of the interface layer affect the charge transfer dynamics and diffusion process [14]. 4.
Conclusion The coin battery used in this trial was made using lithium iron phosphate material.
The calendering and uncalendering electrodes were characterized to investigate the difference between the effect of the pore rate and thickness of the separator on the performance of the battery. C-
The effect of separator hole rate and thickness was studied by rate test, scanning electron microscope analysis and impedance spectrum experiment.
It is observed from the results that the air holes of the separator obviously play an important role in the operation of the battery.
The battery performance with a higher hole rate separator is low.
This can be attributed to the initial loss of lithium and the decomposition of the electrolyte that forms the interface layer.
This affects the charge transfer dynamics and diffusion processes in the battery.
Since the separator is made of different materials, it cannot be compared under the same battery conditions.
The conclusion is that compared with other separators with higher hole rate, the three-layer separator with 45% hole rate and the single-layer separator with 41% hole rate have better performance.
Data Availability experimental data supporting the results of this study can be obtained from the corresponding authors as required.
Conflict of Interest authors declare they have no conflict of interest.
The work was supported by the National Science Foundation\'s ERC projectEEC-08212121.
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Rajan Dhevathi Rajagopalan Kannan, Krishna Pranaya Terala (iD), Pedro L. Moss (iD), and Mark H.
Department of Electrical and Computer Engineering, Florida State University and Florida State University, FL 32310 ahseere, USA, should write to Pedro L. Moss; plm1735@my. fsu.
Received by Edu on April 6, 2018;
Revised in May 29, 2018;
Accepted on June 14, 2018;
Academic Editor Published in July 8, 2018: Liu Haodong Title: Figure 1: three layers of Celgard commercial separator (a)H2013 (porosity 45%)and (b)H2512 (porosity 50%); monolayers (c)2400 (porosity 41%)and (d)PP2075 (porosity 48%).
Description: Figure 2: SEM image of LFP electrode :(a)
Electrode without delay; (b)
The calender electrode.
Description: Figure 3: charging and discharging performance of LFP batteries assembled with single-layer partitions PP2075 and 2400.
Description: Figure 4: charging and discharging performance of LFP batteries assembled with three-layer partitions H2512 and h2013.
Title: figure, details: Parameters of dQ/dV and voltage curve charging and discharging (a)
Single-layer separator and (b)
Three-layer separator.
Description: Figure 6: the measured EIS data of the three-layer separator calender and non-calender electrodes :(a)
Use the uncalved electrode of the H2512 and h2015 separator; (b)
Use the calender electrode of the H2512 and h2015 separator.
Description: Figure 7: measured EIS data for single-layer separator calender and non-calender electrodes :(a)
Use the uncalved electrode of the 2400 and PP2075 separator; (b)
Use the calender electrode of the 2400 and PP2075 separator.