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Separators play a vital role in electronic insulation and ionic conduction in lithium-ion batteries. The common improvement strategy of polyolefin separators is mostly based on modifications with a coating layer, which is simple and effective to some extent. However, the improvement is often accompanied by negative effects such as the increase of the thickness and the blockage of the porous structure, resulting in the decrease of energy density and power density. The porous structure of the separators serves as a conduction path for ions to travel back and forth between the anode and cathode, which has an important impact on the performance of lithium-ion batteries. If the porous structure of the separators can be modified, it will essentially affect the ionic transport behavior through the whole conduction path. Herein, we provide a simple and effective method to functionalize the porous polyolefin separator via the γ-ray co-irradiation grafting process, where high-energy γ-ray is used to generate active sites on the polymer chain to initiate the grafting polymerization of chosen monomers with selected functional groups. In this work, 4,4,5,5-tetramethyl-2-vinyl-1,3,2-dioxaborolane, a kind of borane molecule with an electron-deficient group, was chosen as the grafting monomer. After the γ-ray co-irradiation grafting process, both the surface and pores of the polyolefin separators were functionalized by electron-deficient groups in the borane molecule and the whole electrolyte conduction path within the separator was activated. Due to the electron-deficient effect of the B atom, the lithium-ion conduction is promoted and the lithium-ion transference number can be increased to 0.5. As a result, the half-cell assembled with the functionalized separator shows better cycle stability and better capacity retention under high current rate.

The solid electrolyte interphase (SEI) film is vital to the plating/stripping behavior of lithium metal, while the formation mechanism has not been explained clearly. Here, the formation process of the SEI film is first classified into chemical and electrochemical degradation routes by introducing a LiZn alloy, a chemically inert but electrochemically reactive interphase, as a distinguished research substrate. In a carbonate electrolyte, it is found that the common inorganic matters (such as Li 2 O and LiF) mainly originate from the chemical degradation of the electrolyte; meanwhile, the electrochemical degradation mainly generates organic species such as C-OR and COOR. Based on this understanding, the SEI film can be further accurately regulated, and a heterojunction-type SEI film with a well-defined composition and structure is constructed. This offers us a new perspective to understand and regulate the formation of the SEI film for applicable lithium metal batteries.