Battery Separators ― A Closer Look at Oxidation Mechanisms and Oxygen Transport
Separators for lead–acid batteries are traditionally manufactured from the extrusion and extraction of ultrahigh-molecular-weight polyethylene / silica/ oil mixtures to form microporous ribbed sheets. A relatively large amount of process oil is left behind in the PE/SiO2 separator to protect the polymer chains from oxidative attack. In the ENTEK model, certain molecules within the process oil are preferentially oxidized and can become solubilized in sulfuric acid, while preventing polymer chain scission or crosslinking. Many battery manufacturers believe that it is important to limit the amount of oil in the separator to achieve lower electrical resistance or to prevent ‘black scum’ formation. This presentation dispels these myths and demonstrates the importance of controlling the process oil content in the 11–22wt.% range to maintain good elongation and mechanical properties. Furthermore, there will be report of an examination of separator mechanical properties in both a pristine and oxidized state that uses a new approach for measuring fatigue resistance in puncture and elongation modes. In addition to impacting separator properties, oxygen is also important to battery performance. Water loss has become a topic of great interest, particularly in Enhanced Flooded batteries (EFBs) batteries that are cycled at a partial state-of-charge. While it is generally accepted that valve-regulated lead–acid (VRLA) batteries rely on the crossover of oxygen generated at the positive electrode during charge to be reduced at the negative electrode, it is now believed that an oxygen cycle may also play a role in reducing water loss in flooded battery designs. In this respect, the presentation closes with an account of a laboratory test and its results for measuring oxygen transport through lead–acid battery separators that have different porosities and pore-size distributions.
Richard Pekala holds a Batchelor’s degree from Duke University (1981) and a ScD degree in from Massachusetts Institute of Technology (1984). He began his career at the Lawrence Livermore National Laboratory where he conducted research on sol-gel chemistry and organic-based aerogels. In 1996, he joined PPG Industries to advance precipitated silica for use in battery separators. Richard transferred to ENTEK in 1999 and is now the company’s Chief Technology Officer.