Lithium-ion Battery Separator

Among the various existing energy storage technologies, rechargeable lithium-ion batteries are considered as effective solution to the increasing need for high-energy electrochemical power sources. Rechargeable lithium-ion batteries offer energy densities 2-3 times and power densities 5-6 times higher than conventional Ni-Cd and Ni-MH batteries, and as a result, they weigh loss, take less space, and deliver more energy. In addition to high energy and power densities, lithium-ion batteries also have other advantages, such as high coulombic efficiency, low self-discharge, high operating voltage, and no “memory effect”.
Each lithium-ion battery consists of an anode and a cathode separated by an electrolyte containing dissociated lithium salts, which enables the transfer of lithium ions between the two electrodes, as illustrated in following figure. The electrolyte is typically contained in a porous separator membrane that prevents the physical contact between the anode and cathode. When the battery is being charged, an external electrical power source injects electrons into the anode. At the same time, the cathode gives up some of its lithium ions, which move through the electrolyte to the anode and remain there. During this process, electricity is stored in the battery in the form of chemical energy. When the battery is discharging, the lithium ions move back across the electrolyte to the cathode, enabling the release of electrons to the outer circuit to do the electrical work. Current lithium-ion batteries were developed mainly for portable applications (such as cell phones and lap-tops) and they depend on using active powder materials (such as graphite powder in the anode and LiFePO4 powder in the cathode) to store energy. However, powder materials have a long diffusion path for lithium ions and slow electrode reaction kinetics, and as a result, the performance of current lithium-ion batteries has not reached their potential.

Lithium_ion_Battery_Separator

Separator is a porous membrane placed between the anode and cathode of a battery. Its main function is to prevent physical contact of the electrodes while serving as the electrolyte reservoir to enable ionic transport. Currently, most lithium-ion batteries use conventional microporous polymer membranes as the separator. Microporous polymer membranes have good chemical stability, suitable thickness, and reasonable mechanical strength, but they have low thermal stability, low porosity, and poor wettability with polar liquid electrolyte which in turn lead to high cell resistance, reduced energy density, and low rate capability of rechargeable lithium-ion batteries. Recently, electrospinning technology has been used to fabricate novel nanofiber-based nonwoven membranes, which have a small pore size and large porosity, and can be directly used as separators in rechargeable lithium-ion batteries. The electrospun nanofiber separators enable high-rate charge/discharge of lithium-ion batteries because of their high porosity and desirable pathways for ion transport.

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References
Zhang, Xiangwu, Liwen Ji, Ozan Toprakci, Yinzheng Liang, and Mataz Alcoutlabi. "Electrospun nanofiber-based anodes, cathodes, and separators for advanced lithium-ion batteries." Polymer Reviews 51, no. 3 (2011): 239-264.