In the energy transmission chain of lithium batteries, the “compatibility” between the separator wettability and electrolyte directly determines the upper limit of battery performance. The core indicator for measuring this compatibility — wettability, which seems to be a simple “penetration ability”, actually profoundly affects the battery’s internal resistance, rate performance, cycle life, and even safety.
A separator with poor wettability will lead to slow electrolyte penetration and uneven distribution, which not only increases ion transport resistance but also may cause safety hazards such as lithium dendrite growth; while a separator with excellent wettability allows the electrolyte to spread and penetrate quickly, building an unobstructed channel for ion migration. This article will systematically dissect the essence, influencing factors, key effects on battery performance, and practical improvement strategies of separator wettability, providing precise guidance for scientific research and production.
1. What is Separator Wettability? Core Definition and Judgment Standards
Simply put, separator wettability refers to the spreading and penetration ability of the electrolyte when it comes into contact with the separator surface. Specifically, it refers to whether the electrolyte can quickly and uniformly wet the entire surface of the separator and penetrate into the internal pores to form a continuous ion transport channel.
The core basis for judging the quality of wettability is the contact angle: when a drop of electrolyte falls on the separator surface, the smaller the contact angle, the easier the electrolyte spreads, and the better the wettability; on the contrary, the larger the contact angle, the easier the electrolyte accumulates into water droplets, making it difficult to penetrate, and the worse the wettability. Ideally, a high-quality separator should allow the electrolyte to spread quickly on the surface with a contact angle close to 0°, achieving complete wetting.
2. Four Key Factors: Core Variables Determining Separator Wettability
Separator wettability is not determined by a single factor, but by the combined effect of the separator’s own characteristics and electrolyte properties. The core influencing factors can be summarized into four categories:
1. Separator Thickness: Thinner is Easier to Wet
Under the same material and structure, the thickness of the separator is negatively correlated with wettability. A thinner separator has a shorter path for electrolyte penetration and less resistance, so it is easier to be completely wetted; while a thicker separator not only increases the difficulty of penetration but also may lead to uneven distribution of electrolyte inside, resulting in local unwetted areas.
2. Microstructure: “Synergistic Effect” of Porosity and Pore Size
The microstructure of the separator is the core internal factor affecting wettability, mainly reflected in three aspects: porosity, pore size, and pore channel structure:
Porosity: The higher the porosity, the larger the “accommodation space” inside the separator, the larger the contact area with the electrolyte, which is more conducive to electrolyte penetration and storage;
Pore Size and Distribution: The pore size needs to match the electrolyte molecule size; too large or too small will affect the penetration efficiency; uniform pore size distribution can ensure uniform diffusion of electrolyte inside the separator, avoiding local aggregation or gaps;
Pore Channel Structure: Pore channels with good connectivity and no blockage can provide a smooth penetration path for the electrolyte. For example, nanofiber separators prepared by electrospinning have excellent wettability due to their high porosity and continuous pore channel structure.
3. Chemical Properties: “Attraction” of Surface Energy
The surface energy of the separator directly determines its molecular interaction with the electrolyte: the higher the surface energy, the stronger the intermolecular attraction, the easier the electrolyte spreads on the separator surface, and the better the wettability; on the contrary, a separator with low surface energy has weak intermolecular force with the electrolyte, making it difficult for the electrolyte to attach and spread, resulting in poor wettability.
This characteristic also explains the difference in wettability of separators made of different materials: for example, some polymer separators need to be modified to improve their affinity with the electrolyte due to their low surface energy.
4. Electrolyte Properties: “Regulatory Role” of Additives
The composition and characteristics of the electrolyte itself also affect wettability, among which additives play the most critical role. An appropriate amount of additives can improve the surface tension of the electrolyte, enhance its compatibility with the separator surface, and promote the spreading and penetration of the electrolyte on the separator; while inappropriate additives may increase the interface resistance between the electrolyte and the separator, reducing wettability.
3. Performance Linkage: How Does Wettability Affect Core Battery Indicators?
Separator wettability is not an isolated parameter, but is deeply bound to many core performance indicators of the battery, and its impact runs through the entire life cycle of the battery:
1. Internal Resistance and Rate Performance: Wettability Determines “Transmission Efficiency”
Good wettability allows the electrolyte to quickly penetrate and fill the separator pores, forming a continuous ion transport channel, which greatly reduces ion migration resistance and further reduces battery internal resistance; after the internal resistance is reduced, the polarization phenomenon of the battery during high-current charge and discharge is weakened, and the rate performance is significantly improved. On the contrary, a separator with poor wettability will lead to insufficient electrolyte penetration, discontinuous ion transport channels, increased internal resistance, and a significant decrease in charge and discharge rate at high rates.
2. Discharge Capacity: Wettability Ensures “Energy Output”
The essence of discharge capacity is the degree of ion participation in the electrode reaction. A separator with excellent wettability can ensure full contact between the electrolyte and the positive and negative electrodes, provide sufficient channels for ion migration, and allow more ions to participate in the electrochemical reaction, thereby improving the actual discharge capacity of the battery; while poor wettability will cause some electrode areas to be unable to participate in the reaction due to lack of electrolyte wetting, resulting in reduced discharge capacity.
3. Liquid Retention Capacity: Wettability Extends “Cycle Life”
A separator with good wettability can firmly lock the electrolyte through intermolecular forces, improve liquid retention capacity, and avoid performance degradation caused by electrolyte loss during long-term battery cycles; at the same time, sufficient electrolyte can continuously provide guarantee for ion transport, slow down the electrode aging rate, and extend the battery cycle life.
4. Lithium Dendrite Growth: Wettability Guards “Structural Safety”
In lithium metal batteries, wettability is crucial for inhibiting lithium dendrites. Good wettability improves the current distribution on the surface of the lithium metal anode, allowing uniform deposition and extraction of lithium ions, avoiding lithium dendrite growth caused by local current concentration; while a separator with poor wettability will cause uneven current distribution, and lithium ions are likely to accumulate locally to form dendrites, which may pierce the separator and cause battery short circuit.
5. Battery Safety: Wettability Builds the “Safety Bottom Line”
Problems such as uneven electrolyte distribution and lithium dendrite growth caused by poor wettability are the sources of battery safety hazards. Excellent wettability reduces the safety risks such as battery short circuit and fire from multiple dimensions by optimizing current distribution, inhibiting lithium dendrites, and ensuring the stability of ion transport, thereby improving the overall safety performance of the battery.
4. Practical Optimization Strategies: Four Methods to Improve Separator Wettability
Aiming at the core factors affecting wettability, the industry has formed a series of mature and implementable improvement methods, and suitable schemes can be selected according to actual needs:
1. Optimize Preparation Process to Improve Porosity
By adjusting the key parameters in the separator preparation process (such as stretching conditions, pore-forming agent dosage, etc.), the porosity of the separator is maximized on the premise of ensuring mechanical strength. Higher porosity means more electrolyte contact sites and storage space, which can significantly improve wettability and liquid retention capacity.
2. Precisely Control Pore Size and Optimize Distribution Uniformity
By improving the film-forming process (such as parameter adjustment of electrospinning, optimization of phase separation conditions, etc.), the pore size of the separator is precisely controlled to match the electrolyte molecule size; at the same time, the pore size distribution is ensured to be uniform, avoiding uneven electrolyte penetration caused by local excessive or too small pore size, and promoting uniform diffusion of electrolyte inside the separator.
3. Surface Coating Modification to Enhance Affinity
Coating hydrophilic materials (such as PVDF, ceramic particles, etc.) on the separator surface is a common and effective method to improve wettability. For example, PVDF coating can improve the compatibility between components such as DMC in the electrolyte and the separator, making the wetting and bonding between the separator and the electrode sheet closer, without affecting the pore structure and mechanical properties of the separator.
4. Surface Pretreatment to Improve Surface Energy
Surface pretreatment technologies such as plasma treatment and ultraviolet irradiation are used to increase the roughness and surface energy of the separator surface, and strengthen the intermolecular force between the separator and the electrolyte. For the pretreated separator, the spreading resistance of the electrolyte on the surface is reduced, the wettability is significantly improved, and this method is easy to operate and has little impact on the main performance of the separator.
Conclusion
Separator wettability is the “invisible cornerstone” of lithium battery performance and safety, and its quality is directly related to the battery’s internal resistance, rate performance, capacity, cycle life, and safety. In essence, wettability is the combined effect of separator thickness, microstructure, chemical properties, and electrolyte characteristics. Through practical strategies such as optimizing preparation processes, regulating microstructure, and surface modification, precise improvement of wettability can be achieved.
For researchers and producers, it is necessary to selectively choose wettability optimization schemes according to the battery’s application scenarios (such as consumer electronics, power batteries, energy storage batteries) and performance requirements: for example, power batteries have high requirements for rate performance and safety, so the combined scheme of surface coating and porosity optimization can be preferred; energy storage batteries focus on cycle life, so the wettability design related to improving liquid retention capacity can be focused on.
In the future, as lithium batteries develop towards high energy density, high rate, and long life, the optimization of separator wettability will become more refined, and the collaborative design with other properties (such as thermal stability, mechanical strength) will also become the core R&D direction, providing solid support for the high-quality development of the lithium battery industry.
For more in-depth research on separator wettability optimization and performance testing, you can refer to the research published by the Journal of Power Sources. Our previous articles on ceramic-coated 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 modification technologies, refer to the report released by the Institute of Electrical and Electronics Engineers (IEEE).