The ceramic-coated lithium battery separator has become a key choice for improving lithium battery performance due to its excellent thermal stability, electrolyte wettability, and mechanical strength. The performance of ceramic separators depends not only on coating materials and processes but also closely on the preparation process (dry/wet method) of the base separator.
To address the pain points of traditional polyolefin separators, such as poor low-temperature resistance, insufficient porosity, and difficulty in electrolyte wetting, the research team prepared Al₂O₃ nano-ceramic coated separators using dry and wet polyolefin separators as substrates. Through systematic physical performance tests and battery performance verification, the advantages and disadvantages of ceramic separators with the two processes were comprehensively compared. This article will detail the experimental process and results, providing precise data support for separator selection in high-performance LiCoO₂/C system lithium-ion batteries.
Experimental Design: Preparation and Testing of Ceramic Separators Under Fair Comparison
To ensure the objectivity of the comparison results, the experiment adopted a standardized process to prepare two types of ceramic separators and established a comprehensive testing system covering both the separator itself and battery application performance:
1. Preparation of Ceramic Separators
Using distilled water and N,N-dimethylacetamide (volume ratio 5:95) as solvents, Al₂O₃ powder and polyvinyl alcohol binder were dispersed at a mass ratio of 1:1 to form a uniform Al₂O₃ slurry. Taking 16μm thick dry-process polyolefin microporous membrane (Separator 1) and wet-process polyolefin microporous membrane (Separator 2) as substrates respectively, a double-sided coating process was adopted: drying at room temperature for 30 minutes, then drying overnight in a vacuum oven at 80℃, and finally obtaining ceramic composite separators. The coating amounts of the two separators were 0.16mg/cm² (Separator 1) and 0.19mg/cm² (Separator 2) respectively.
2. Construction of Testing System
Separator Performance Testing: The surface morphology was observed by Scanning Electron Microscope (SEM); physical parameters such as porosity, air permeability, and liquid absorption rate were tested; Electrochemical Impedance Spectroscopy (EIS) testing was performed using an electrochemical workstation to calculate ionic conductivity.
Battery Performance Testing: Using LiCoO₂ as the positive electrode and graphite as the negative electrode, 57mm×57mm 1Ah soft-pack lithium-ion batteries were assembled. Formation capacity, rate discharge (1.00C, 20.00C, 50.00C), low-temperature performance (-20℃), and cycle performance (300 cycles of 1.00C charge-discharge) tests were carried out to comprehensively evaluate the actual application effect of the two separators.
Physical Performance Duel: Structural Advantages of Wet-Process Base Separator Highlighted
The differences in physical properties between the two ceramic separators directly determine their ion transport efficiency. Test results show that Separator 2 with wet-process substrate is comprehensively leading in core parameters:
Separator TypeThickness (μm)Porosity (%)Air Permeability (s·(100ml)⁻¹)Liquid Absorption Rate (mg·cm⁻²)Dry-Process Substrate (Separator 1)19.446.9244.32.89Wet-Process Substrate (Separator 2)20.856.2134.03.81
Porosity and Liquid Absorption Rate: The porosity of Separator 2 reaches 56.2%, much higher than 46.9% of Separator 1; the liquid absorption rate reaches 3.81mg·cm⁻², 31.8% higher than that of Separator 1. Higher porosity means more ion transport channels, and stronger liquid absorption capacity can improve the electrolyte retention, providing sufficient “carriers” for ion migration.
Air Permeability: The air permeability of Separator 2 is only 134.0s·(100ml)⁻¹, less than 60% of Separator 1 (244.3s·(100ml)⁻¹), indicating lower resistance to gas passage, which indirectly reflects that its pore structure is more unobstructed, facilitating rapid ion transport.
Micro morphology: SEM observation shows that both separators have Al₂O₃ particles smaller than 500nm uniformly distributed on the surface, with gaps between the particles. However, the pore distribution of Separator 2 is more uniform and has better connectivity, further enhancing its ion transport advantage.
Electrochemical Performance Competition: Wet-Process Base Separator Leads in All Scenarios
The differences in physical properties are directly transformed into gaps in battery electrochemical performance. In the three core tests of rate, low temperature, and cycle, Separator 2 with wet-process substrate shows an overwhelming advantage:
1. Ionic Conductivity: Lower Internal Resistance, Faster Transmission
EIS test results show that the internal resistance of Separator 1 is 1.32Ω, and the ionic conductivity is only 0.731mS/cm; while the internal resistance of Separator 2 is reduced to 0.82Ω, and the ionic conductivity is increased to 1.260mS/cm, which is 1.72 times that of Separator 1. Lower internal resistance and higher ionic conductivity mean smaller migration resistance of Li⁺ inside the battery, laying the foundation for high-rate discharge.
2. Rate Performance: High Capacity Retention Under High Current
The discharge capacities of batteries assembled with the two separators are close at 1.00C rate (0.948Ah for Separator 1 and 0.956Ah for Separator 2), but the gap is significant in high-rate scenarios:
At 20.00C rate, the discharge capacity of the battery assembled with Separator 2 reaches 0.935Ah, with a capacity retention rate of 97.80% compared to 1.00C capacity; while the capacity of Separator 1 is only 0.854Ah, with a capacity retention rate of 90.08%;
At 50.00C ultra-high rate, the advantage of Separator 2 is more obvious, with a capacity retention rate of 30.86%, much higher than 25.53% of Separator 1.
This result proves that the pore structure of the ceramic separator with wet-process substrate can better adapt to the rapid Li⁺ migration demand under high current.
3. Low-Temperature Performance: Stable Output at -20℃
Low-temperature environment will significantly increase ion transport resistance, testing the low-temperature adaptability of the separator. At -20℃ and 1.00C rate:
The discharge capacity of the battery assembled with Separator 2 is 80.01% of that at room temperature, still maintaining stable output capacity;
The capacity of the battery assembled with Separator 1 is only 64.64% of that at room temperature, with obvious insufficient low-temperature performance.
This is due to the higher porosity and liquid absorption rate of Separator 2. Even at low temperatures, sufficient electrolyte and unobstructed pores can ensure the effective transmission of Li⁺.
4. Cycle Performance: High Capacity Retention After 300 Cycles
Cycle stability is the core indicator of battery life. The 1.00C charge-discharge cycle test results show:
After 200 cycles, the capacity retention rate of the battery assembled with Separator 1 is 92.05%, while that of Separator 2 reaches 94.72%;
After 300 cycles, the capacity retention rate of Separator 2 is still as high as 91.45%, while that of Separator 1 drops to 88.31%.
The faster cycle performance attenuation of Separator 1 is mainly due to its low porosity. Micropores are easily blocked by side reaction products during long-term cycles, leading to increased internal resistance and aggravated polarization; while the high porosity and unobstructed pore structure of Separator 2 can effectively delay micropore blocking and maintain long-term cycle stability.
Core Conclusion: Wet-Process Base Ceramic Separator is the Preferred Choice for High-Performance Batteries
Experimental results clearly show that the ceramic separator with wet-process polyolefin separator as the substrate is comprehensively superior to the ceramic separator with dry-process substrate in both physical and electrochemical properties:
With higher porosity, better liquid absorption, and more unobstructed pore structure, the wet-process base separator achieves lower internal resistance and higher ionic conductivity;
Reflected in battery performance, its rate discharge capacity, low-temperature stability, and long-term cycle life are all better. The capacity retention rate is still over 91% after 300 cycles, 30.86% at 50.00C ultra-high rate, and 80.01% of room temperature capacity output at -20℃ low temperature.
This conclusion provides a clear direction for separator selection in high-performance LiCoO₂/C system lithium-ion batteries: for application scenarios with high requirements for rate performance, low-temperature adaptability, and cycle life (such as high-end consumer electronics and power batteries), choosing ceramic separators with wet-process polyolefin separators as substrates can give full play to the battery performance potential. In the future, by further optimizing the coating process and coating materials, it is expected to achieve greater breakthroughs in the performance of wet-process base ceramic separators, providing more solid support for the high-end development of lithium batteries.
For more in-depth research on ceramic-coated separator preparation and performance testing, you can refer to the research published by the Journal of Power Sources. Our previous articles on polyolefin lithium battery separators and high-performance lithium-ion battery separators further elaborate on the development of separator materials and processes. For detailed industry standards and testing methods, refer to the report released by theInstitute of Electrical and Electronics Engineers (IEEE).