The rapid growth of energy storage technologies has placed Lithium-Ion Batteries (LiBs) at the cutting edge of innovation, powering everything from smartphones to electric vehicles. As demand for higher performance and safety in LiBs continues to rise, the role of the battery separator—an often overlooked but critical component becomes increasingly important. The separator maintains the physical barrier between the anode and cathode while allowing for ionic transport (Figure 1). The effectiveness of the separator directly impacts the battery’s overall efficiency, safety, and lifespan.
Figure 1. Separators for Lithium-ion Batteries: Innovative Separator LIELSORT®by Teijin (a) Laminated-type battery (b) Cylindrical type battery
While the separator does not engage in cell reactions, its structure and properties impact battery performance. Over the years, separators have evolved from laminated type and cylindrical battery casings to cellulosic papers, wood, cellophane, nonwoven fabrics, foams, ion exchange membranes, and microporous flat sheet membranes made from polymeric materials (Figure 1).
Issues with the Most Common Separator Materials in Lithium Ion Batteries
Lithium-ion battery (LiB) separators are critical components that ensure the safe and efficient functioning of the battery by physically isolating the cathode and anode while allowing the free flow of ions. However, commonly used separator materials, such as polyethylene and polypropylene, face several challenges.
Polypropylene (PP) is a popular choice due to its excellent chemical stability and mechanical strength. It is resistant to the electrolyte solutions used in lithium-ion batteries, which helps prevent degradation over time. PP also has a high melting point, which makes it suitable for applications where thermal stability is important.
Polyethylene (PE) is valued for its flexibility and low cost. PE separators often have a slightly lower melting point compared to PP, which can be advantageous in some designs where a lower temperature threshold is desired.
Together, these plastics are designed to be porous, allowing lithium ions to pass freely between the anode and cathode during the charge and discharge cycles. The porous structure ensures that the flow of lithium ions is not obstructed, which is crucial for maintaining the battery’s efficiency and capacity.
One major issue is their limited thermal stability, which can lead to shrinkage or melting under high-temperature conditions, such as during thermal runaway or overcharging. This poses a significant safety risk, as it can result in internal short circuits and battery failure.
Another common problem is their low wettability and electrolyte uptake, which negatively affects ionic conductivity and overall battery performance. Many polyolefin-based separators also exhibit poor mechanical strength, especially under repeated cycling or external stresses, making them prone to tearing or degradation. These limitations have driven research toward advanced separator materials and coatings, such as those incorporating ceramic particles or high-purity alumina, which enhance thermal stability, mechanical robustness, and electrolyte compatibility.
High Purity Alumina Coatings to Improve Separator Performance and Stability
Lithium-ion cell manufacturers are increasingly incorporating ceramic-layered separators into their designs to enhance both safety and performance. Ceramic coated-separators offer significant advantages over traditional plastic separators, particularly in terms of thermal stability and safety.
High-purity Alumina (HPA) is one of the most promising ceramic coatings for LiB separators. High-purity alumina (HPA) is a highly refined form of aluminum oxide (Al₂O₃) known for its exceptional purity and outstanding thermal, chemical, and mechanical properties. Widely used in advanced applications, HPA plays a critical role in industries such as lithium-ion batteries.
The HPA/ceramic coating performs several critical functions for lithium-ion battery separators. One of the most important roles is to improve the separator’s thermal stability, thereby mitigating the risk of thermal runaway. A study of Dong-Won Lee et al. demonstrated that thermal shrinkage was of a α-Al₂O₃ coated PE separator was considerably lower than that of an uncoated PE separator (Figure 2).
Figure 2. Thermal Shrinkage results from Research Study: Preparation of a high-purity ultrafine α-Al2O3 powder and characterization of an Al2O3-coated PE separator for lithium-ion batteries by Dong-Won Lee, Sang-Hun Lee, Yong-Nam Kim, and Jong-Min Oh
HPA coatings also contribute to the essential electrical insulation needed for the separator’s function. Additionally, they provide structural support and protect against dendrite formation in the gel electrolyte on either side of the separator (Figure 3). Furthermore, HPA coatings maintain permeability for lithium ions through the membrane, even during repeated charge and discharge cycles, ensuring efficient battery operation. Overall, HPA-coated separators enable higher internal battery temperatures and support designs with greater power density.
Grades of High-purity Alumina Coatings for LiB Separators
Impurities in the separator, particularly metal and non-metallic foreign particles, can cause defects. Metal particles might damage the separator membrane, leading to short circuits, or create local defects that result in hot spots of high electrochemical activity. These hot spots can cause local lithium deposition and dendrite growth, which may penetrate the separator and cause short circuits. Minor internal shorts caused by contaminants can lead to thermal run-away, especially as battery cell energy density increases. With separators as thin as 20-25 μm, even small metallic dust particles can have devastating effects. As such, the alumina coating must be highly pure (typically 99.99% purity) to minimize metallic cation impurities and metal impurities, which should be less than a few ppm.
Figure 4. Polar Performance Materials High Purity Alumina (HPA) Products
Currently, 4N grade HPA is utilized as a ceramic coating in common industrial inorganic polyvinyl insulating membrane, which serves as a separator. The grade delineation on high purity HPA is 99.99+%, also identified as 4N+. Higher grades, 5N or 6N, are offered and carry premium pricing over 4N. Virtually all HPA is manufactured today by chemical companies such as Polar Performance Materials (Canada) , utilizing high purity, expensive aluminum metal as feedstock and a chemical process. Various process techniques allow producers to consistently meet 4N+ grade spec, but at relatively high overall product cost. At 4N grade, HPA has under 100 ppm of total impurities.
| Property | Typical | Range |
| Specific surface Area BET, m2/g | 4-12 | 4-30 |
| D10 Particle size, µm | 0.1-0.2 | 0.1-5 |
| D50 Particle size, µm | 0.4-0.8 | 0.1-40 |
| D90 Particle size, µm | 1.5-2 | 1.5-200 |
| Moisture, % | <0.15 | 0.1-0.2 |
As batteries become more advanced, the demands on the separator function have increased. However, the mechanism may fail due to dendrite growth puncturing the separator or total shrinkage of the separator caused by rising temperatures in the cell due to higher energy density. By improving the purity and performance of alumina coatings on separators, the overall safety and efficiency of lithium-ion batteries can be significantly enhanced, supporting the development of more powerful and reliable energy storage solutions.
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