Engineering Carboxyl Content in Aqueous Core‐Shell Emulsions for Efficient Inorganic Coated Separators Enhancing Lithium‐Ion Battery Safety Performance

Abstract Polypropylene separators (PP) are widely used in lithium‐ion batteries due to good electrochemical stability and low cost. However, PP separators are prone to thermal shrinkage at high temperatures, resulting in short circuit of positive and negative electrode contacts and thermal runaway. In this work, a waterborne core‐shell emulsion binder rich in carboxyl and ester groups with both strength and adhesion is designed and coated with alumina (Al 2 O 3 ) as a composite coating on the PP separator. Due to the good adhesion of the emulsion binder to the Al 2 O 3 and the PP separator, the separator has excellent dimensional stability at 120 °C, while the thickness of the separator only increases by 2.5 μm. With the help of the dissociation effect of the ester group on the lithium salt and the lithium ion conduction characteristics, the composite separator improves the ionic conductivity (0.82 mS/cm) by 25 % compared with the PP separator and the lithium ion transference number reaches 0.47. The cycling capacity of the lithium‐ion battery with the composite separator is 8.62 % higher than that of the PP separator after 100 cycles. The performance changes of acrylic acid as a functional monomer on emulsion binders and composite separators are further investigated.