Separator materials

The separator between the anode and cathode in lithium-ion batteries plays a vital role in safety and performance.

Its primary function is to prevent electrical short circuits, by keeping the anode and cathode physically separated while allowing ions to move freely between them during charging and discharging.

Separators are mainly made of microporous materials consisting of polymeric membranes, or nonwoven fabric mats.

_{Battery with polymer separator. Tkarcher; CC BY-SA 3.0}

Although separators do not directly participate in the battery’s electrochemical reactions, their porous structure and other properties significantly influence the battery’s overall performance.

The separator must be strong enough to tolerate high tension during battery assembly and have sufficient porosity to absorb the electrolyte, which is crucial for ion transport (Arora & Zhang, 2004; Lee et al., 2014).

An ideal separator should fulfill the following specifications (Arora & Zhang, 2004):

• Excellent electrical insulation
• Robust physical strength and mechanical stability during assembly and repeated charge/discharge cycles
• Effective absorption of the electrolyte
• Low resistance to electrolyte flow
• Uniform thickness to avoid the migration of electrode particles
• Resistance to chemical degradation from the electrolyte, impurities, and electrode reactions/products

Microporous separators

Microporous separators are made from various inorganic, organic, and natural materials and typically feature pores ranging from 50–100 angstrom (Å) in diameter.

Materials used for these separators include nonwoven fibres such as nylon, cotton, and glass, as well as polymer films such as polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), and polyvinyl chloride (PVC). Natural substances such as rubber, asbestos, and wood are also sometimes used.

Microporous separators are suitable for batteries operating at temperatures below 100°C, and microporous polyolefins (PP, PE, or combinations of PP and PE) are commonly used in lithium-based nonaqueous batteries (Arora & Zhang, 2004).

Nonwoven separators

Nonwoven separators are textiles created directly from fibres and formed like sheets, webs or mats, arranged in specific orientations or randomly. The fibres can be either natural or synthetic, and they can be stapled fibres, continuous filaments, or formed during the production process.

Nonwoven fabrics usually can be made from single polyolefins or a mixture of polyolefins, such as polyethylene (PE), polypropylene (PP), polyamide (PA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and polyvinyl chloride (PVC).

Nonwoven separators have struggled to match the performance of microporous films in lithium-ion batteries due to their unsuitable pore structure and the challenges in producing thin (<25 micrometre) nonwoven materials with satisfactory physical properties (Arora & Zhang, 2004; Lee et al., 2014).

Research and share

Research the temperature limits of microporous separators and the potential issues when operating in higher temperature environments. What are some alternative separator materials for improved temperature tolerance? Share some suggestions in the comments below.

References

Arora, P., & Zhang, Z. (2004). Battery separators. Chemical reviews, 104(10), 4419-4462.

Kim, J., Choi, S., Jo, S., Lee, W., & Kim, B. (2004). Characterization and properties of P (VdF-HFP)-based fibrous polymer electrolyte membrane prepared by electrospinning. Journal of The Electrochemical Society, 152(2), A295.

Lee, H., Yanilmaz, M., Toprakci, O., Fu, K., & Zhang, X. (2014). A review of recent developments in membrane separators for rechargeable lithium-ion batteries. Energy & Environmental Science, 7(12), 3857-3886.

Battery separator. David Baillot/UC San Diego Jacobs School of Engineering; CC BY 2.0