Novel Nanofiber-based Membrane Separators for Lithium-ion Batteries (2015)

YANILMAZ, MELTEM.Novel Nanofiber-based Membrane Separators for Lithium-ion Batteries.(Under the direction of Professor Xiangwu Zhang).Lithium-ion batteries have been widely used in electronic devices including mobile phones, laptop computers, and cameras due to their high specific energy, high energy density, long cycling lifetime, and low self-discharge rate.Nowadays, lithium-ion batteries are finding new applications in electric/hybrid vehicles and energy storage for smart grids.To be used in these new applications, novel battery components are needed so that lithiumion batteries with higher cell performance, better safety, and lower cost can be developed.A separator is an important component to obtain safe batteries and its primary function is to prevent electronic contact between electrodes while regulating cell kinetics and ionic flow.Currently, microporous membranes are the most commonly used separator type and they have good mechanical properties and chemical stability.However, their wettability and thermal stabilities are not sufficient for applications that require high operating temperature and high performance.Due to the superior properties such as large specific surface area, small pore size and high porosity, electrospun nanofiber membranes can be good separator candidate for highperformance lithium-ion batteries.In this work, we focus our research on fabricating nanofiber-based membranes to design new high-performance separators with good thermal stability, as well as superior electrochemical performance compared to microporous polyolefin membranes.To combine the good mechanical strength of PP nonwovens with the excellent electrochemical properties of SiO 2 /polyvinylidene fluoride (PVDF) composite nanofibers, SiO 2 /PVDF composite nanofiber-coated PP nonwoven membranes were Voltage: the electrical driving force and it is equal to the electric potential difference between two points in an electric circuit.Active mass: the materials that create electrical current as a result of chemical reaction.Open-circuit voltage: is the voltage of the cell when no external current flows.Current: the rate of flow of electric charge.Discharge: process in which battery delivers electrical energy to an external load.Cutoff voltage: the battery voltage at the point at which discharge is complete.Charge: process in which current flow reversed and battery is restored to its original charged condition.Capacity: the product of the total amount of charge or a total number of ampere hours that can be obtained in discharging.Energy density: the amount of energy stored per unit mass or volume.Power: the energy that can be derived from battery per unit time.Cycle life: the number of charge and discharge cycles that a battery can achieve before its capacity is consumed.C-rate: Charge/discharge rate, the speed at which the batteries charge and discharge.Polarization: a lack or excess of electrode potential at equilibrium.Overpotential: the potential difference between the actual potential and the equilibrium potential.It is used as a measure of the extent of polarization.Internal resistance: resistance or impedance that battery offers to current flow.Thermal runaway: event that occurs when the battery electrode's reaction with the electrolyte becomes self-sustaining and this may cause safety incidents and fires [6, 9].