Separator – Ensuring battery safety and reliability
Acting as a mechanical barrier between the cathode and anode, the separator serves an important cell safety function and allows maximum ionic exchange in the Li-ion battery cell.
A separator, as the name suggests, separates the two building blocks of a battery, the cathode, and the anode while enabling the exchange of lithium-ions from one side to another. Although separators are electrochemically inactive components in a Li-ion cell, they play an active role in determining cell safety.
The separator is typically a non-conductive, permeable membrane that allows ions to pass through from cathode to anode when the battery is being charged and the reverse when the battery is being discharged. The permeable pores of the membrane allow ions to travel freely, but it also acts as an electric insulator which prevents cathode and anode from short-circuiting.
Separator characteristics and their role
The Li-ion separator must have a pore size ranging from 30 to 100nm (Nm stands for nano-meter, 10-9, which is one-millionth of a millimeter or about 10 atoms thick). The recommended porosity is 30–50 percent. This holds enough liquid electrolyte and enables the pores to close should the cell overheat (Battery University).
At present, commercially available Li-ion cells use polyolefin as a separator. Polyolefin has excellent mechanical properties, good chemical stability, and is low-cost. It is a class of polymer that is produced from olefin by polymerizing olefin ethylene. Ethylene comes from a petrochemical source; polyolefin could be made from polyethylene, polypropylene, or laminates of both materials. At present, commercialized separators are synthetic resin such as polyethylene (PE) and polypropylene (PP).
The different polypropylene layers give different functionality to the separator. The layers have break-down mechanisms that shut the pores in an event of thermal runaway thus insulating the cell. Shutdown results from the 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. At least one layer gives this mechanical strength and stability.
Li-ion separator cost and leading players
Typically, Li-ion separator accounts for 10-15 percent of cell component costs strongly depending on the specific cell design.
At present, the Asia-Pacific region dominates the Li-ion battery separator market and the market is fragmented with several players.