Cell Design and Materials: Li-Ion

Separator Introduction Across rechargeable battery systems, the separator is a porous, electrically insulating layer placed between the anode and cathode. Its primary function is to prevent direct electrical contact between electrodes while allowing ionic transport through the electrolyte. The separator plays a critical role in safety, power capability, cycle life, and thermal behavior, and is often a key determinant of a batteryʼs tolerance to abuse or failure conditions.Separator requirements vary by battery chemistry, operating temperature, and application, but all separators must balance mechanical integrity, ionic permeability, chemical stability, and manufacturability.

Separators in Lithium-Ion Batteries:

In lithium-ion batteries, separators are most commonly made from polyolefin polymers, such as polyethylene PE) and polypropylene PP, produced via dry or wet processes. These materials are chosen for their chemical compatibility with organic electrolytes, mechanical strength, and ability to be manufactured at scale.

Function of the Separator:

During normal operation, the separator:

Maintains physical separation of the electrodes to prevent short circuits

Allows ions to migrate through its porous structure during charge and discharge

Helps regulate electrolyte distribution and wetting within the cell

Under abnormal conditions, the separator may also contribute to safety mechanisms, such as pore

shutdown or dimensional stability at elevated temperatures.

Factors influencing Separator choice:

Safety & Thermal Stability: Resistance to shrinkage, melting, or puncture at elevated temperatures; contribution to shutdown behavior. Ionic Conductivity & Porosity: Ability to support ion transport without excessive resistance, impacting power and fast-charge capability. Mechanical Strength: Puncture resistance, tensile strength, and dimensional stability under swelling or stack pressure Electrolyte Compatibility: Chemical stability and wetting behavior with the chosen electrolyte system. Cost & Manufacturability: Scalability of production processes and compatibility with high-volume cell manufacturing

Source: Redwood, ANL, Nature-lithium-ion battery cathode chemistry, Battery Report 20232024, Battery Talk: Battery Application Break Down 12/31/2025 Version 3.0

Extracted from the PDF; charts and images may read imperfectly.