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A simple and environmentally friendly self-assembly process of oppositely charged polymer PEI and inorganic oxide SiO2 was demonstrated for the construction of an ultrathin layer on the surface of PE separator. The XPS, FT-IR, SEM, and EDS characterizations give clear evidence of the successful self-assembly of PEI and SiO2 without significantly increasing the thickness and sacrificing the pristine porous structure of PE separator. This process improves a variety of crucial properties of PE separator such as the electrolyte wetting, the electrolyte uptake, the thermal stability, the ionic conductivity, Li+ transference number, the electrochemical stability and the compatibility with lithium electrode, endowing lithium-ion battery (Li as anode and LiCoO2 as cathode) with excellent capacity retention at high C-rates and superior cycling performance. At the current density of 5 C, the cell with PE separator almost loses all the capacity. In contrast, the cell with (PEI/SiO2)-modified PE separator still holds 45.2% of the discharge capacity at 0.2 C. The stabilized SEI formation and high Li+ transference number of (PEI/SiO2)-modified PE separator were interpreted to be the substantial reasons leading to the remarkably enhanced battery performance, rendering some new insights into the role of the separator in lithium-ion batteries.

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A simple layer-by-layer (LbL) self-assembly process of poly(acrylic acid) (PAA) and ZrO2 was applied to construct functional ultrathin multilayers on polyethylene (PE) separators without sacrificing the excellent porous structure of separators. Such PAA/ZrO2 LbL-modified PE separators possess good electrolyte wettability, excellent electrolyte uptake, high ionic conductivity and large Li(+) transference number. More importantly, the top layer of LbL self-assembly would affect the dissociation of electrolyte and the formation of solid electrolyte interphase (SEI) layer in half-cells. Compared with the pristine and (PAA/ZrO2)1PAA-modified PE separators, (PAA/ZrO2)3-modified PE separator shows a larger Li(+) transference number (0.6) and a faster tendency to form a stable SEI layer, endowing half-cells with excellent capacity retention at high C-rates and superior cycling performance. These fascinating characteristics will provide the LbL self-assembly with a promising method to improve the surface property of PE separators for high performance lithium-ion batteries.