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Separator - Binder Evaluation

The separator permits ion flow from one electrode to the other while preventing any electron flow, essentially separating the anode from the cathode.

The typical separator is made up of polyolefins, usually polypropylene and/or polyethylene, along with other polymers, ceramics, and ceramic/polymer blends.

Separators are highly porous, typically
>40% porosity, approximately 25 μm thick and exhibit low ionic resistivity. Layered or composite separators are used as safety devices to prevent thermal runaway
of the cell.

Binder materials are used to hold the active electrode material particles together and in contact with the current collectors, i.e. the Aluminum Foil of the cathode or the Copper Foil of the anode.

Specification of percent porosity is an important parameter in the acceptance criteria for the separator. The separator must have sufficient pore density to hold the liquid electrolyte that supports ionic movement between the anode and cathode. Higher porosity means less heat generated in the cell and greater energy density.

Uniform porosity is essential to avoid variations in ion flow. The more variation in ionic flow within the separator, the greater the effect at the surface of the electrode and the quicker it will fail with a significantly decreased cycle life. Excessive porosity hinders the ability of the pores to close, which is vital to allow the separator to shut down an overheating battery.

The separator pore size must be smaller than the particle size of the electrode components, i.e. the electrode active materials and any conducting additives. Most separator membranes contain submicron pore sizes that block the penetration of particles.

Uniform distribution and a tortuous structure of the pores are also a requirement. Uniform distribution prevents uneven current distribution throughout the separator and tortuosity suppresses the growth of
dendritic lithium.

To further understand the transport mechanisms of the separator membrane,
zeta potential can indicate the membranes electrolyte affinity. This can permit fine tuning of battery performance to improve cycle life.

Cycle life is extended when the separator has a low electrolytic resistance but high aqueous permeability. Zeta potential can also provide needed information about the membranes affinity with electrolyte additives.