Characterization of microporous separators for lithium-ion batteries

Several properties including porosity, pore-size distribution, thickness va lue, electrochemical stability and mechanical properties have to be optimiz ed before a membrane can qualify as a separator for a lithium-ion battery. In this paper we present results of characterization studies carried out on some commercially available lithium-ion battery separators. The relevance of these results to battery performance and safety are also discussed. Poro sity values were measured using a simple liquid absorption test and gas per meabilities were measured using a novel pressure drop technique that is sim ilar in principle to the Gurley test. For separators from one particular ma nufacturer, the trend observed in the pressure drop times was found to be i n agreement with the Gurley numbers reported by the separator manufacturer. Shutdown characteristics of the separators were studied by measuring the i mpedance of batteries containing the separators as a function of temperatur e. Overcharge tests were also performed to confirm that separator shutdown is indeed a useful mechanism for preventing thermal runaway situations. Pol yethylene containing separators, in particular trilayer laminates of polypr opylene, polyethylene and polypropylene, appear to have the most attractive properties for preventing thermal runaway in lithium ion cells. (C) 1999 E lsevier Science S.A. All rights reserved.