Multi-Faceted Approach for Evaluating Lithium-Ion Battery Separators

Introduction

Lithium-ion secondary batteries (in this report, simply referred to as "lithium-ion batteries") are widely used as power sources in cell phones, information devices and other small electronic devices because of their high energy density and high cell voltage. In recent years, the development of hybrid vehicles and electric vehicles has been proceeding at an accelerated rate, fueled by a heightening awareness for environmental conservation. Lithium-ion batteries are vital for their power systems, too. Moreover, a demand has emerged for large- capacity secondary batteries as a flexible response to the demand for power even in the field of natural energy usage (solar cells and wind power generation). On the other hand, however, short-circuiting, overcharging, physical impact, and other factors sometimes cause lithium-ion batteries to become unstable. So, various evaluations and measures for ensuring safety are being performed also at battery component level. This Application News introduces some attempts as examples of physical evaluation of separators, one of the component parts of lithium-ion batteries whose increased application is anticipated for the above reasons. Separators perform a dual role of preventing short-circuiting between the plus and minus electrodes and allowing lithium-ions to pass through smoothly. They also have a function for shutting off current when heat is generated, for example, by short-circuiting of the battery. Currently, polyolefin (PE) micro-porous film is generally used as the separator film. PE has a shutdown mechanism that functions when the battery heats up, whereby the porous structure closes near the melting point of the material to prevent ions from passing through.

December 2, 2009 GMT