Research On CMC Spunlaced Composite Separator For Ni-MH Batteries

Nickel metal hydride (Ni-MH) battery was one of the most promising future power systems,which was the preference of vehicular battery in the near future and medium-term because ofadvantages such as high specific energy, good safety performance, and great improvement ofrecharge/discharge performance. As a key component of Ni-MH batteries, the quality of theseparator had a great influence on discharge capacity, discharge voltage, self-discharge, etc. Inaddition, the domestic production technology of battery separator was still in the primary stage,and most of the separators used in high capacity batteries and high power batteries were basicallyrelied on imports, so it was of great significance to research and develop high quality separators.In the paper, hydrophilic melt-blown nonwovens were firstly prepared by melt-blendingmethod that commercial hydrophilic addictive (CHA) were fully mixed with the polypropylene(PP) granules, and then the mixture was fed into the melt-blown system. The differences ofperformance between hydrophilic melt-blown nonwovens and commercial pure PP melt-blownnonwovens were analyzed. Based on differential capillary effect of nonwoven materials, spunlaceand hot-pressing process were used to prepare CMC composite separator, which was compositedwith two layers of PP staple fibers, sandwiched with one layer of melt-blown nonwoven, forminga compact structure that the coarse fibers and melt-blown fibers entangled each other, and thestructure was benefit to improve the electrolyte absorption of the separator.Finally the propertiesof three sorts of separators were analyzed, which mainly concluded tensile strength, pore size,chemical stability, electrolyte absorption and retention, and the recharge/discharge performanceand storage performance were studied as well.This paper obtained the following results from the above studies:(1) Contrasted with commercial pure PP melt-blown nonwovens, the hydrophilic melt-blownnonwovens with fluffy structure had lower tensile strength, better wettability property and betterelectrolyte absorption.(2) The tensile strength of CMC composite separators at first increased and then decreasedwith the maximum water pressure increasing, which all met the requirement of battery separator. The thickness, air permeability and electrolyte absorption of CMC composite separators alldecreased with the maximum water pressure increasing, the electrolyte absorption of them wereover500%and better than commercial separators. The suitable conditions of spunlace processwere as following: the distance of the injectors and the conveyor belt is20mm, and the speed ofconveyor belt is always2m/min, the maximum water pressure should range from40bar to60bar.(3) After hot-pressing, the thickness, the porosity, maximum pore size and average pore sizeof the CMC composite separator obviously reduced and all comply with the requirement of batteryseparator, in addition, the air permeability and electrolyte absorption decreased with hot-pressingtemperature increasing,the hot-pressing pressure increasing,or hot-pressing time increasing.Hot-pressing temperature was the key factor affecting the comprehensive performance of CMCcomposite separator and was suitable the range of80℃to90℃. The hot-pressing pressure andhot-pressing time were respectively1MPa and15s, taking into account the economic cost.(4) Contrasted with commercial sulfonated separator, the CMC composite separator exhibitedbetter chemical stability, and the mass loss of them was less than1%. The electrolyte absorptionof CMC composite separator with pure PP nonwovens was similar to commercial sulfonatedseparators, while was lower compared to CMC composite separators with hydrophilic nonwovens.It showed that the structure of the CMC composite separators is benefit to improve the electrolyteabsorption.(5) The batteries assembled with CMC composite separators with pure PP nonwovens hadlowest discharging capacity,202.6mAh/g; and the batteries assembled with commercial sulfonatedseparator had highest discharging capacity,282.6mAh/g. The discharging capacity of the batteriesincreased with the increasing of the content of CHA in hydrophilic nonwovens. The batteries withCMC composite separators had better retention of discharging capacity and lower self-dischargingrate after storage for30days, exhibiting good storage performance.Through the above analysis, it could be found that the CMC composite separators withhydrophilic nonwovens exhibited good chemical stability and electrolyte absorption, and that thebatteries assembled with them had good storage performance and recharge/discharge performance.Taking into account the cost and the similar cell performance of the batteries assembled with threekinds of CMC composite separators with hydrophilic nonwovens, the best content of CHA was5%.