Separators are an integral part of the performance, safety, and cost of lithium batteries. The term “lithium batteries” refers to both (1) non-rechargeable, lithium metal-based batteries and (2) rechargeable lithium-ion batteries which are widely used in portable electronic devices. During normal operation, the principal functions of the separator are to prevent electronic conduction (i.e., shorts or direct contact) between the anode and cathode while permitting ionic conduction via the electrolyte.

For small commercial cells under abuse conditions, such as external short circuit or overcharge, the separator is required to shut down at temperatures well below where thermal runaway can occur. Shutdown results from collapse of the pores in the separator due to melting and viscous flow of the polymer, thus slowing down or stopping ion flow between the electrodes. Nearly all Li-ion battery separators contain polyethylene as part of a single- or multi-layer construction so that shutdown begins at ~135 °C, the melting point of polyethylene. Battery failures in the field, however, have demonstrated that shutdown is not a guarantee of safety. The principal reason is that, after shutting down, the internal temperature of the battery is often high enough to cause residual stress and reduced mechanical properties leading to shrinkage, tearing, or pinhole formation.

Separator 101

Separator Manufacturing

Separators for the lithium battery market are usually manufactured via a “wet” or “dry” process. In the “dry” process, polypropylene (PP) or polyethylene (PE) is extruded into a thin sheet and subjected to rapid drawdown. The sheet is then annealed at 10-25 °C below the polymer melting point such that crystallite size and orientation are controlled. Next, the sheet is rapidly stretched in the machine direction to achieve slit-like pores or voids at 35-45% porosity. A PP/PE/PP trilayer separator can also be produced in this fashion.

Polyolefin separators based upon UHMWPE are usually produced in a “wet” or “gel” process involving extrusion of a plasticizer/polymer mixture at elevated temperature, followed by phase separation, biaxial stretching, and extraction of the pore former (i.e., plasticizer). The resultant separators have elliptical or spherical pores and porosity in the 40-50% range. Because of the biaxial orientation, good mechanical properties are achieved in both the machine and transverse directions. The separators also have strong chemical resistance, abrasion resistance, and good wettability with organic solvents. As such, UHMWPE separators have found wide use in lithium batteries.

ENTEK - Energy Storage Lithium

Structure-Property Relationships

​​ENTEK manufactures lithium-ion separators using a “wet” process. The molecular weight distribution of polyethylene, the percentage and type of plasticizer, extraction and drying conditions, biaxial stretch ratios, and annealing temperature are all factors that impact the final structure and properties of the separator. ENTEK works with battery manufacturers to customize key separator characteristics such as thickness, air permeability, and % porosity. Figure 1 compares the morphology of an ENTEK separator to a “dry” process separator.

Schematic drawing of a lithium-ion battery showing the separator and electrode arrangement.

Battery Integration

Separator manufacturers produce master rolls that must be subsequently slit to the width required for a particular battery design. Typically, slit rolls are supplied on plastic cores in lengths that range from 1000 -2000 meters. After delivery to the lithium battery manufacturer, separator rolls are loaded onto an un-winding station along with individual rolls of cathode and anode. Two separator rolls are required so that the separator is interspersed between the anode and cathode while all 4 layers are wound around a pin to form a “jellyroll”. The “jellyroll” is then removed from the pin and transported down the production line for further inspection followed by filling with electrolyte. The electrolyte-filled battery then undergoes an electrochemical formation step prior to final inspection and shipment. A schematic diagram of a lithium-ion battery is shown in Figure 2.

ENTEK Lithium Prismatic Battery

Schematic drawing of a lithium-ion battery showing the separator and electrode arrangement.

Translate »