provided are separators used in power accumulators such as lithium ion secondary batteries and a preparation method thereof . the said separators are obtained through following steps : providing a polymer colloidal emulsion through a polymerization reaction of polyvinyl alcohol , hydrophobic monomer and hydrophilic monomer in water solution initiated by an initiator ; coating a plastic substrate with the said polymer colloidal emulsion using tape - casting method ; drying the plastic substrate coated with the polymer colloidal emulsion , and then obtaining the said separators by delaminating them from the substrate . the said separators have good liquid absorbability , high liquid absorption rate and retention , low resistivity , good mechanical strength and good thermal stability as well as electrochemical stability . the prepared lithium ion batteries have good cycle stability and long service life .