As the “ion channel” and “safety barrier” of lithium batteries, the parameter design and process selection of separators directly affect core indicators such as battery internal resistance, self-discharge rate, and low-temperature performance. For separators of the same material, even small differences in thickness or porosity may lead to vastly different battery performance; the two processes of dry and wet methods will endow separators with completely different performance characteristics.
Through systematic experiments, the research team compared the impact of 6 types of separators with different parameters and processes on LiFePO₄/graphite system pouch batteries, and revealed the correlation law between separators and battery performance with accurate data. This article will detail the experimental results, providing actionable basis for researchers and producers in separator selection and parameter optimization.
1. Experimental Design: Comparative Verification Focusing on Core Variables
To clarify the impact of separator parameters and processes on battery performance, the experiment adopted the control variable method and built a standardized test system:
1. Experimental Samples and Variable Setting
A total of 6 types of separator samples were prepared, with core variables divided into two categories:
Wet-process PE separators (A-E): Covering different thicknesses (12μm, 16μm, 25μm) and porosities (37%, 42%, 52%). Among them, A, B, and C have the same thickness (12μm) but different porosities; B, D, and E have similar porosities (42%-43%) but different thicknesses;
Dry-process PP separator (F): Thickness 25μm, porosity 43%, forming a process comparison with wet-process separator E.
2. Test System Construction
Separator Physical Property Test: Observe the microstructure by Scanning Electron Microscopy (SEM), test porosity and tortuosity with a water pressure porosimeter, test air permeability with an air permeability meter, and evaluate electrolyte wettability through contact angle experiments;
Battery Performance Test: Assemble different separators into pouch batteries, and test key performance indicators such as Alternating Current Resistance (ACR), Direct Current Resistance (DCR), room/high-temperature self-discharge rate, and low-temperature (-20℃) discharge capacity retention rate.
2. Core Finding 1: Key Correlation Laws of Separator Physical Properties
The experiment first clarified the internal relationship between the separator’s own parameters, laying the foundation for subsequent performance analysis:
1. Porosity and Air Permeability: A Linear Negative Correlation
For wet-process separators with the same thickness (A, B, C), porosity and air permeability show an extremely strong linear negative correlation (fitting degree R²=0.9998): as porosity increases from 37% to 52%, air permeability decreases from 195s/(100mL) to 89s/(100mL). This is because higher porosity means more abundant ion transport channels and lower gas permeability resistance.
2. Thickness and Air Permeability: A Linear Positive Correlation
In wet-process separators with similar porosity and tortuosity (B, D, E), thickness and air permeability show a linear positive correlation (fitting degree R²=0.9545): as thickness increases from 12μm to 25μm, air permeability increases from 155s/(100mL) to 286s/(100mL). Increased thickness extends the ion transport path, leading to increased air permeability resistance.
3. Physical Property Differences Between Dry and Wet Processes
Microstructure: Wet-process separators are biaxially stretched, with uniform and round pore sizes (about 40nm); dry-process separators are uniaxially stretched, with narrow and long pore sizes (about 25nm);
Wettability: The contact angle between wet-process separators and electrolyte (38°-40°) is significantly smaller than that of dry-process separators (54°), indicating that wet-process separators have better electrolyte wettability;
Air Permeability: Under the same thickness and similar porosity, the air permeability of dry-process separators (375s/(100mL)) is significantly higher than that of wet-process separators (286s/(100mL)), which is consistent with the smaller pore size characteristic.
3. Core Finding 2: Impact of Separators on Battery Internal Resistance
Battery internal resistance directly determines charge-discharge efficiency and power performance. The experiment revealed a clear correlation between separator parameters and internal resistance:
1. Impact of Porosity: Higher Porosity, Lower Internal Resistance
In wet-process separators with the same thickness (A, B, C), as porosity increases from 37% to 52%, the ACR and DCR of the battery continue to decrease. Higher porosity provides smoother ion transport channels, effectively reducing ion migration resistance.
2. Impact of Thickness: Higher Thickness, Higher Internal Resistance
In wet-process separators with similar porosity (B, D, E), as thickness increases from 12μm to 25μm, battery internal resistance shows an upward trend. Increased thickness not only extends the ion transport path but also may increase the difficulty of electrolyte wetting, leading to higher internal resistance.
3. Impact of Process: Dry-Process Separators Have Lower Internal Resistance
Under the same thickness (25μm) and similar porosity (43%), the ACR and DCR of batteries assembled with dry-process separator F are significantly lower than those with wet-process separator E. Although dry-process separators have smaller pore sizes, their narrow and long pore structure may reduce the tortuosity of ion transport, thereby lowering internal resistance.
4. Core Finding 3: Regulation Law of Separators on Battery Self-Discharge
Self-discharge rate is a key indicator to measure battery storage performance. The experiment confirmed that the impact of separator parameters on self-discharge is linearly correlated:
1. Room-Temperature Self-Discharge (25℃, 30 Days)
Impact of Porosity: For wet-process separators with the same thickness (A, B, C), as porosity increases from 37% to 52%, the battery voltage drop increases from 8.33mV to 9.40mV (linear fitting degree R²=0.9995), indicating that higher porosity leads to more serious self-discharge;
Impact of Thickness: For wet-process separators with similar porosity (B, D, E), as thickness increases from 12μm to 25μm, the voltage drop decreases from 8.67mV to 7.80mV (linear fitting degree R²=0.9998), and increased thickness can inhibit self-discharge;
Impact of Process: Under the same thickness, the room-temperature voltage drop of dry-process separator F (8.20mV) is slightly higher than that of wet-process separator E (7.80mV), which may be related to the straight pore structure of dry-process separators.
2. High-Temperature Self-Discharge (55℃, 7 Days)
Impact of Porosity: The higher the porosity of the wet-process separator, the higher the battery physical self-discharge rate (linear fitting degree R²=0.9832). For example, the self-discharge rate of battery C with 52% porosity (1.11%) is higher than that of battery A with 37% porosity (0.88%);
Impact of Thickness: The thicker the wet-process separator, the lower the high-temperature self-discharge rate (linear fitting degree R²=0.9819). The self-discharge rate of battery E with 25μm thickness (0.90%) is lower than that of battery B with 12μm thickness (0.99%);
Impact of Process: The high-temperature self-discharge rate of dry-process separator F (1.00%) is slightly higher than that of wet-process separator E (0.90%), which is consistent with the room-temperature self-discharge law.
5. Core Finding 4: Impact of Separators on Low-Temperature Performance (-20℃)
In low-temperature environments, ion transport efficiency decreases, and the impact of separator parameters on battery discharge performance is more significant:
1. Impact of Porosity: Higher Porosity, Lower Low-Temperature Capacity Retention Rate
In wet-process separators with the same thickness (A, B, C), battery A with 37% porosity has the highest capacity retention rate (33.92%), and battery C with 52% porosity has the lowest (31.59%). This is because separators with low porosity have higher battery internal resistance, and more Joule heat generated during discharge can alleviate the problem of insufficient electrolyte activity at low temperatures.
2. Impact of Thickness: Higher Thickness, Higher Low-Temperature Capacity Retention Rate
In wet-process separators with similar porosity (B, D, E), the capacity retention rate of battery E with 25μm thickness (33.30%) is higher than that of battery B with 12μm thickness (32.33%), which is related to the Joule heat effect generated by internal resistance.
3. Impact of Process: Wet-Process Separators Have Better Low-Temperature Performance
Under the same thickness, the low-temperature capacity retention rate of wet-process separator E (33.30%) is slightly higher than that of dry-process separator F (30.15%), which benefits from the better electrolyte wettability of wet-process separators, providing a more stable medium for ion transport at low temperatures.
For more in-depth research on separator parameters, processes and lithium battery performance, you can refer to the research published by the Journal of Power Sources. Our previous articles on lithium battery separator selection guide and aramid lithium battery separator safety design further elaborate on battery material performance and modification technologies. For detailed industry standards and separator performance test specifications, refer to the report released by theInstitute of Electrical and Electronics Engineers (IEEE).