Lithium Battery Core Design Elements - Electrolyte Volume
Brief introduction of the lithium battery inner design
Lithium batteries include four main materials:
① positive active material ② negative active material ③ separator ④ electrolyte
Among them, the positive active material is a lithium-containing compound that can provide active Li+. While the negative active material accepts active Li+ and forms a potential difference with the positive active material. The two determine the high voltage and high capacity characteristics of the lithium battery. As the only inert material among the four main materials that does not participate in the electrochemical reaction, the separator usually has a pore size of no more than 1um, allowing Li+ to pass through freely, but not electrons. And it has the function of avoiding the contact short circuit of the positive and negative electrodes.
However, the process of removing Li+ from the positive electrode to intercalating into the negative electrode requires carrier transport. This carrier is the electrolyte. The released Li+ combines with electrolyte solvent molecules to form solvated Li+, which migrates to the negative electrode for reduction driven by the electric field and concentration difference.
Figure 1: Schematic diagram of the solvation and desolvation process of Li+ during charging
About the electrolyte of lithium battery
The electrolyte components include lithium salts, solvents and additives, among which lithium salts are used to provide active Li+ and solvents are used to dissolve lithium salts. There are many kinds of additives, including film-forming additives, overcharge additives, flame retardant additives, etc., which usually have certain functions chemical effect.
In order to ensure the capacity of the active material, the electrolyte must completely infiltrate the separator and electrodes to form a Li+ conductive path.
Influence of improper electrolyte volume
(1) If the amount of electrolyte is too small, it will inevitably lead to insufficient infiltration of some active particles, increased interface impedance, and deteriorated capacity and cycle life.
(2) But the electrolyte should not be too much, otherwise it will cause excessive gas production, interface problems, and the increase in side reactions will lead to low initial efficiency of the full battery. In addition, the BOM cost of lithium batteries will also increase.
The figure below is a schematic diagram of the electrolyte infiltration process of a cylindrical battery. And the yellow arrow represents the flow direction of the electrolyte. After the electrolyte is injected from the top of the battery, most of the electrolyte accumulates at the bottom of the battery case. And then it slowly penetrates into the pores of the separator and electrodes through capillary pressure. Therefore, the final destination of the electrolyte is the pores of the separator and electrodes.
Figure 2: Electrolyte infiltration process of cylindrical battery
Taking the production of soft-pack laminated batteries as an example, it contains three processes of “liquid injection-formation-pumping and sealing”.
- The liquid injection process is to inject the electrolyte into the inside of the cell, and then infiltrate it for an appropriate time.
- The formation process is to charge the lithium battery with a small current for the first time, forming an SEI film at the negative electrode interface and generating gas.
- The gas extraction and edge sealing process is to extract the gas and excess electrolyte generated during the formation process, and seal the battery to maintain a stable internal environment.
Figure 3: Flowchart of making a typical pouch laminated battery
It can be seen that how to quickly infiltrate the diaphragm and electrodes with the electrolyte after liquid injection is the key.
On the one hand, the liquid injection volume should be designed to fill the internal pores. And on the other hand, it must ensure that the electrolyte has a good penetration effect, such as penetration time, electrolysis Liquid intrinsic viscosity, aging temperature, electrode and diaphragm pore structure, etc. But in any case, the design of the electrolyte injection volume is more important than the penetration effect. Because sufficient electrolyte volume is the basic premise to ensure the wetting of the separator and electrodes.
In order to explore the effect of the amount of liquid injection on the internal resistance of lithium batteries, two sets of experiments were designed.
- The first group of liquid injection was 28g (rich liquid state)
- The second group of liquid injection was 20g (lean liquid state).
It is found that when the electrolyte is sufficient, the internal resistance of the battery will not change significantly. But when the electrolyte is not infiltrated enough, there is a significant correlation between the internal resistance and the amount of liquid injected.
Figure 4: Effect of electrolyte injection volume on battery internal resistance
Therefore, at the beginning of the design and development of new lithium battery products or new systems, the electrolyte consumption of single cells should be fully considered. Besides, the red line problem of “insufficient liquid injection” should be resolutely prevented.
In the process of designing the theoretical liquid injection volume, we have two ideas:
- One is to calculate the maximum theoretical value based on the volume of the internal space minus the volume of solid matter.
- The other is to calculate the minimum theoretical value based on the diaphragm, pole piece, and Overhang pores.
Considering the gas production and cost problems caused by excess electrolyte, we recommend choosing the second electrolyte volume design idea.
Figure 5: Design Ideas for Theoretical Dosage of Electrolyte
As previously mentioned in “Basic Issues of Lithium-ion Batteries-Design”, the porosity of the pole piece is related to the true density of the material and the compaction density during liquid injection, and the true density of the material is related to the material formula. But so far we have some newer understanding:
During the lithium intercalation process of the negative electrode material, the interlayer spacing will increase, and the macroscopic performance of the pole piece will thicken. And those will lead to a large increase in the electrode porosity, so more electrolyte is needed to ensure a smooth Li+ transmission path. According to the principle of constant volume, the formulas for calculating the average true density and porosity of the electrode coating are:
In the formula:
– “s” represents the expansion rate of the charged state of the pole piece.
– The larger the “s”, the greater the porosity of the coating.
It can be seen that the expansion of the pole piece is very important to the design of the theoretical dosage of the electrolyte.
Table 1: True Density Table of Commonly Used Materials
Let’s use an LCO+Gr design case to further illustrate. The detailed design parameters are shown in Attached Table 2.
According to the formula ratio:
- the average true density of the positive and negative electrodes is calculated to be 4.85g/cm3 and 2.14g/cm3 respectively
- the porosity of the electrodes is 16.1 % and 36.7%
- the final positive electrode sheet pore volume, negative electrode sheet pore volume, diaphragm pore volume and Overhang pore volume are 0.178mL, 0.805mL, 0.209mL, 0.255mL
- the total volume is 1.447mL
- the theoretical absorbable electrolyte coefficient is 1.824mL/Ah
Table 2: LCO+Gr Case Design Information Form
In order to further study the effect of electrode areal density and compaction density on the theoretical electrolyte dosage, the electrode pore volume under different areal density and compaction density was calculated respectively.
The results show:
When the areal density changes, the porosity of the electrode will not change. But under the premise of the same capacity, the pore volume of the positive electrode and Overhang don’t change much. However, the increase in the pore volume of the negative electrode and the decrease in the length of the separator lead to a significant decrease in the pore volume, and the theoretical absorbable electrolyte coefficient also decreases.
In terms of compaction density, it is obvious that as the compaction density of the positive and negative electrodes decreases, the porosity of the pole piece increases, while the diaphragm and Overhang pore volumes do not change much.
Figure 6: Effect of areal density and compaction density on electrode pore volume and theoretical absorbable electrolyte coefficient
It can be seen that the theoretical dosage of electrolyte changes with the process design such as surface density and compaction density. There is a certain relationship between the size and even the thickness of the foil. These factors should be fully considered when designing the amount of electrolyte to avoid “one size fits all”.
Summarize
1. Insufficient amount of electrolyte will significantly affect the internal resistance, rate and cycle performance of lithium batteries. In the early stage of new product or new system design and development, the minimum theoretical amount of electrolyte must be calculated.
2. The amount of electrolyte is related to many factors such as material system, process design, and internal structure size of the battery cell. The basic principle is that the electrolyte must fill all electrode pores to ensure ionic conduction paths.