How to Choose the Right Battery Separator for Your Application: Part 2

If you are in battery development, you may be wondering what you should consider in identifying the right separator for your application. In part 1 of this blog series we described six different types of separators – namely microporous separators, nonwoven, ion-exchange membranes, supported liquid membranes, polymer electrolytes and solid ion conductors – each with its set of advantages and disadvantages. The latter two combine the properties of electrolytes and separators. In this blog, we list six things for you to ponder when looking into the best separator choice for your battery. 

What Are the Most Important Features of a Separator?

Listed below are several important separator requirements, most of them described in detail in a review paper authored by Arora and Zhang. Even the length of the list and the complexity of several requirements denote the need for finding a reasonable trade-off for the desirable features of the separator, selected for a certain battery application. The priority of these features changes with your targeted battery application.

  1. Chemical stability –  the separator material must be chemically stable over an extended time period; it should be inert to both strong reducing and strong oxidizing conditions, and should not degrade or lose mechanical strength or produce impurities, which may interfere with the battery operation. Chemical stability of the separator is essential for all possible battery applications. For example, glass fiber-based separators do not last long in strongly alkaline media. Consequently, they will not secure sufficiently long shelf life.                                                                                                            
  2. Thickness the separator must be thin and its thickness should be uniform, so that it adds the lowest possible mass and volume to the battery. Low mass translates in high specific energy [Ah/kg], while smaller volume yields greater energy density [Ah/L]. In addition, thin film separators enable greater power than thick separators.                                                                                         
  3. Permeability – the separator must not limit cell performance. The presence of a separator typically increases the effective resistance of the electrolyte by a factor of 6-7.                                      
  4. Porosity – this is implied in the permeability requirement. Porosity allows the separator to hold liquid or gel electrolyte. Porosity and its control are important criteria for separator choice and acceptance.                                                                                                                                                        
  5. Pore size – they must be smaller than the smallest particle size of the electrode materials, so that the separator can prevent passage across the separator of particles possibly detached from the electrodes. Once such particles pass from one side to another of the separator, they may cause a chemical short circuit. Ideally, the pores should be uniformly distributed and should create a tortuous open structure. Even pore distribution enables a uniform current distribution throughout the separator, which is vital for the cycle life of rechargeable batteries. As an example, separators for lithium ion batteries should have submicron pore size.                                                                          
  6. Wettability the separator should wet quickly and completely in the battery electrolyte. If your battery operates with an organic electrolyte and you need a very thin separator, you can choose a regular membrane separator. Given that these membranes are typically hydrophobic, they can be wet easily with organic solvents. By contrast, should you need a very thin separator for an aqueous electrolyte, you have to proceed with a surface-modified membrane, namely, use a separator, which has been rendered hydrophilic via the adsorption of a surfactant.                                                                                                                                                            
  7. Electrolyte absorption and retention – electrolyte absorption and retention are prerequisites for ion transport. For example, in a micro battery or an ultrathin battery the separator serves as the reservoir of electrolytes; these cells typically operate with electrolytes absorbed in the separator. For rechargeable batteries it is important that the electrolyte should wet the separator permanently; thus dry out is prevented, or at least delayed, and extended cycle life enabled.                                                                                                                                                           
  8. Electrical resistance – this is a comprehensive measure of permeability, which impacts the electrical performance of the separator. The ionic resistivity of a porous membrane represents the resistivity of the electrolyte contained in the pores of the separator.                                                   
  9. Mechanical strength or tensile strength, Puncture strength, and Mechanical integrity – the separator must withstand the mechanical pressure experienced during battery assembly, such as the tension of the winding for jelly roll type cells, and stresses encountered during the lifetime of the battery. High puncture strength is needed, so that particulate materials from the electrodes do not penetrate the separator. As an important requirement, the separator must be free of any type of defects, such as pinholes, wrinkles, fissures, or contaminants. As many electrodes expand during discharge, they create additional compression of the separator. Therefore, the separator needs to withstandthe buildup of mechanical stress as well.                                                                                         
  10. Dimensional stability and skew – a strip of separator should stay flat, without curling at the edges when unrolled; if it curls, the cell assembly may become more demanding. Upon being laid out separator should be straight, rather than bow or skew. When skew is observed, it may cause misalignment between the electrodes and the separator. In addition, the separator should not shrink when exposed to the electrolyte.                                                                                                         
  11. Thermal stability and Thermal shutdown – the separator should be stable over a wide temperature range specific to the battery application. The separator is not allowed to shrink significantly and, particularly, wrinkle at the high or low temperatures limits of battery operation. An important safety feature built into lithium-ion batteries is that the separators should shut down cell operation, when the temperature rises slightly to a value close to the thermal runaway. This is accomplished by the material’s ability to self-close its own pores when exposed to severe internal heat.                                                                                                                                                                                  
  12. Electrode interface – the separator is expected to form an intimate and steady interface with the electrodes for providing good electrolyte flow.

Comparison of Separator Specifications

Type

Material

Wettability

Permeability

Selectivity [%]

Cost [US$/m2]

Microporous

Organic or inorganic

Hydrophobic or Hydrophilic*

High

N/A

Costly

Nonwoven

Various polymers

 Hydrophobic or Hydrophilic*

 High

N/A

Cheap

Ion-exchange**

Polymers with ion-exchange groups

Hydrophilic

Low

≤ 90

Expensive

Supported liquid membranes

Polymer membrane having pores filled with organic liquid and carrier

Mostly hydrophobic

Low

N/A

Expensive

Polymer electrolytes

Solid electrically conducting polymer / salt complex

Hydrophobic or Hydrophilic

Low

≤ 90

Costly to Expensive

* Precipitated silica renders mcroporous separators acid-wettable

**Poly (perfluoro olefinsulfonic acid)

Clearly, as explained in Parts 1 and 2 of this blog series on battery separators, there are several factors to consider when selecting the appropriate separator material for your battery application. Each of them plays a role in the performance, operating conditions, and safety of your product. As always, feel free to contact us for battery development questions or inquiries. You can also check out the battery fundamentals or the battery resources sections of our blog for useful information.

Topics in this article: Battery Technology

About the Author

John

Hi, I'm John, editor-in-chief of an Flexel Battery online magazine!

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