Separators

Let us focus on an important component of Li-ion batteries: the separator. The separator is a very thin porous membrane that is made of an electrically insulating material and prevents the electrodes from being in direct contact with each other. Important factors that influence the performance of a separator as part of the Li-ion battery are:

• porosity,
• pore size and pore distribution, which also affects the tortuosity,
• the material the separator is made of, in particular its electrical and mechanical properties, and
• the thickness.

Requirements

There are a number of requirements that a separator material must meet for practical application.

Technical requirements

The separator material must fulfil a number of technical requirements. The separator:

• should allow a good wettability for the chosen electrolyte solution,
• must be chemically inert in contact with the electrolyte solution and electrode reactants,
• should be mechanically and dimensionally stable and tolerate the temperature window that is given by the final application,
• should be an electronic insulator,
• should minimise the loss of effective conductivity to maximise power density,
• should be thin and light to maximise energy density.

Economic requirements

The separator material:

• should be commercially available,
• should be available from several sources,
• should be available at a reasonable price.

Ecological requirementss

The separator material:

• should ideally be recycleable,
• should be accessible by an energy-efficient synthesis strategy.

Common materials

Battery separators in commercial use are most typically porous poly(ethylene) or poly(propylene). Thin coatings (ca. 2 µm) of aluminium oxide (Al₂O₃) or boehmite (AlO(OH)) particles on one or both sides are common, to enhance mechanical, thermal and electrochemical stability.

Key metrics

• MacMullin number. A measure of the reduction in effective ionic conductivity of the electrolyte layer caused by the separator. Read more about MacMullin number measurement.

Supporting literature

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