Synthetic wood-pulp separators SWP

Synthetic wood-pulp separators (SWP). So-called synthetic wood-pulp (SWP) separators are used widely in Japan for automotive batteries. These are fleeces made from fibrillated polyethylene or polypropylene and small amounts of additional fibres (e.g., polyester fibres) sometimes, the fleeces are also filled with silica. The organic sheets are laminated with a glass mat, which serves to stabilize the positive active-material [39]. 

Within the ALABC programme, various new SWP separators have been evaluated with respect to their suitability for VRLA batteries [31,40]. These separators vary in their fibre composition, and some also contain silica powder and microglass fibres. The porosity is in the range of 85-88%, which is somewhat less than that of AGM separators. Due to the hydrophobic nature of the organic fibres, the electrical resistance is about 3-5 times higher than that of conventional AGM separators. On the other hand, the SWP separators exhibit lower compressibility, lower contraction on wetting, and better recovery compared with AGM separators. In a separate ALABC task [27], one of these SWP separators (referenced as SWP-7) was compared with a standard AGM the results are compiled in Table 7.14. 

figures in brackets represent percentage of initial thickness at lOkPa. Contraction = (dry thickness - acid-saturated thickness)/dry thickness x 100%. 

In battery cycle tests to a regular 72% DoD, the initial discharge capacities of the SWP-7 cells were 10% lower than for the AGM cell. This difference was attributed to the lower porosity of the SWP-7 separator and the attendant lower acid volume. Cells with SWP-7 separators achieved 300 cycles, but the AGM cell failed after 150 cycles (failure point 80% of initial capacity). It should be mentioned that whereas the SWP-7 cells were built with an assembly pressure of 48-58 kPa, the AGM cell used only 22 kPa and this may have contributed to its shorter cycle-life. 

In a different battery test with a simulated EV load pattern, a SWP-7 cell with an assembly pressure of 60 kPa achieved 450 cycles versus 270 cycles for an AGM cell with 73 kPa. The failure mode was found not to be the expansion of positive plate but, rather, sulfation of the negative plate. This led to the conclusion that the favourable mechanical properties of SWP-type separators suppress degradation of the positive active-material. 

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