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).
One emerging solution to improve separator performance is the incorporation of coated High Purity Alumina (HPA). Well known for its exceptional thermal stability, chemical resistance, and mechanical strength, HPA offers a boost in the durability and efficiency of LiB separators. By applying a thin layer of HPA to the separator surface, manufacturers can significantly enhance the battery’s overall safety, energy density, and cycle life. This enables a significant growth potential in the emerging market for High Purity Alumina (HPA) in lithium-ion batteries.
The HPA/ceramic coating performs several critical functions for lithium-ion battery separators. It contributes to the essential electrical insulation required for the separator’s role. Additionally, it offers structural support and protects against dendrites that may form in the gel electrolyte on either side of the separator (Figure 2). This coating significantly enhances the thermal resistance of the membrane separator, allowing for higher internal battery temperatures and supporting designs with higher power density. Moreover, it maintains permeability for lithium ions through the membrane, even during repeated charge and discharge cycles, ensuring efficient battery operation.
Why High Purity Alumina in LiB Separator?
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 20-25μm, even small metallic dust particles can have devastating effects.
Figure 3. 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
To enhance separator performance and mitigate thermal run-away, ceramic particles, mainly alumina, are used to coat PE/PP membranes. These alumina-coated layers prevent failure at elevated temperatures and protect against dendrite damage. In the study of Dong-Won Lee et al., it shows that thermal shrinkage in various elevated temperatures was greatly minimize when polyethylene (PE) separator was coated with α-Al2O3 powder compared to the bare PE separator (Figure 3). The alumina must be highly pure (typically 99.99% purity) to minimize metallic cation impurities and metal impurities, which should be less than a few ppm. Impurities can leach into the electrolyte, form dendrites, or serve as nuclei that accelerate dendrite formation. Metals in the ceramic layer, introduced either through raw materials or the manufacturing process, are a source of short-circuits due to their proximity to the polymer membrane.
HPA Grades for LiB Separator
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 Sumitomo (Japan) and 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.
Ready to improve your battery separator with optimized HPA grades? CAPLINQ can help. Our team specializes in tailored solutions to boost performance and efficiency. Reach out today to improve your battery devices!
