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Commercial polyolefin separator cannot guarantee the safety of lithium-ion batteries at high temperatures due to their poor thermal stability. All kinds of nanofiber membranes with high thermal stability become the focus. In this paper, we have prepared PVDF nanofiber membrane by electrospinning method and have studied the relationship between the electrochemical performance and nanofiber morphology in detail. It is found that the morphology of PVDF is transformed from ball-like to various nanofiber with increasing the concentration of spinning solution. As a result, the PVDF nanofiber membrane (24 wt%-HP) prepared from spinning solution with the concentration of 24 wt% is composed of coarse nanofiber and fine nanofiber, which exhibits better electrochemical properties, such as cycling stability (300 cycles) and ionic conductivity (1.65 mS cm À1 ). LiCoO 2 /Li cells based on PVDF nanofiber separators have higher discharge capacity even at the current density of 5 C-rate, superior than the cells based on PE separator. It is inspiring for high-energy-density battery safety to use this PVDF nanofiber separator.

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Commercial polyolefin separators often cause explosions and thermal runaways in lithium-ion batteries due to their poor thermal stability. Therefore, various polymer separators with high thermal stability have attracted the attention of researchers. In this paper, a kind of special nano-TiO2/Polyimide (TiO2@PI) composite separator was prepared by adding nano-TiO2 particles with certain content to the PAA solution before electrospinning. It is found that the added TiO2 ceramic particles greatly improve the flame retardant performance of the separator, and at the same time, they inhibit lithium dendrites and prevent the internal short circuit of the battery. As a result, the TiO2@PI nanofiber separator (5.0-TiO2@PI) prepared by a spinning solution with a TiO2 concentration of 5.0 wt % is formed by connecting spherical beads with a diameter of about 500 nm, which greatly inhibits lithium dendrites. It has better properties, such as electrolyte uptake (721%) and ionic conductivity (2.52 mS cm–1). The LiCoO2/Li battery based on 5.0-TiO2@PI nanofiber separator has a higher discharge capacity even at a current density of 4 C, which is better than that of PE separator. The use of this 5.0-TiO2@PI nanofiber separator has enlightening significance for the safety of high energy density batteries.

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