Most conventional batteries use separators, which are essentially membranes made up of a gelatinous material, to prevent short-circuiting of the electrodes. Micro-porous, polymer-based separators are used to block the physical contact between the anode and cathode laminates in a cell. Polyethylene (PE) or polypropylene (PP) membranes are used as separators. The desired characteristics of a separator include its thickness uniformity (in micron scale), adequate mechanical strength during cell fabrication, and chemical as well as electrochemical stability when in contact with the electrolyte and during charge-discharge cycling respectively.
Smart separator materials have recently been developed which melts in situations like an accidental short circuit, over-charging/discharging or thermal runaway (all these leads to the generation of excessive heat), only to block the physical contact between the electrodes.
In lithium-ion batteries, dendritic formations are unavoidable and this leads to the phenomenon of growing electrodes. When the dendrites from the electrodes (after growing considerably) meet each other the electrodes become essentially short-circuited. This leads to sparking which further ignites the flammable electrolyte. To avoid this situation most batteries incorporate a separator, which has small openings to allow for the diffusion of ions. This separator acts as a barrier for dendrites and hinders their growth in unwanted directions. The downside of the use of a separator is the hindrance created for diffusion of ions between the electrodes. This reduces the response time and increases the charging time. Efforts are on the way to eliminate the use of separators by using solid-state systems (like solid electrolytes) to create more efficient batteries.
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All commercial separators so far have been made of polyolefins but they provide only limited heat resistance. Research is now focusing on separators made of different materials which would offer superior heat resistance. These include heat-resistant rubber such as silicone rubber and fluororubber, aromatic polyamide resin, liquid crystalline polyester resin, heat-resistant resin containing polyoxyalkylene, and resin with cross-linked groups. Separators made of such materials are expected to demonstrate not only high temperature stability and safety but also superior ion transportation for better rate capability at high current discharge.
The total market for separators for all applications of Li-ion batteries was approximately 900 mm2 in 2015 and the compound annual growth rate (CAGR) amounted to 15% in the period between 2005 and 2015. It is expected that the separator market for Li-ion batteries will continue to grow steadily with CAGR of 12% reaching ca. 2700 mm2> in 2025 and a major contribution to this growth is expected to come from the needs of electric vehicles and buses.