Ultrathin inorganic-nanoshell encapsulation: TiO2 coated polyimide nanofiber membrane enabled by layer-by-layer deposition for advanced and safe high-power LIB separator

Separators with superior thermal stability, excellent electrolyte uptake and high ionic conductivity are imperative for developing high-power lithium ion batteries (LIBs). Yet it is not easy to build a separator possessing excellent performances in every aspect. For addressing this issue, herein, a novel in-situ growth-bonded titania (TiO2) nanolayer is coated on the polyimide (PI) nanofiber membrane via a novel layer-by-layer deposition (LBLD) strategy. Despite the tiny TiO2 content (2.2%), the resulting hybrid nano-fabric demonstrates improved performance and great potential for high-power LIBs. Benefited from the advantages of LBLD, the TiO2 nanolayer coated on PI nanofibers is ultrathin with neither sacrificing the porosity nor increasing the total thickness, which indicates that the strategy reported herein is a tremendous progress in comparison with conventional coating methods. After coated by the TiO2 nanolayer, the nanofibrous PI membranes exhibit superb heating-endurance, great flame-retardance, excellent wettability and high Li ion transmission, which would enhance the security of LIBs. More importantly, a unique bonding nanostructure is introduced into the nanofibrous PI membrane accompanied by TiO2 nanolayer, leading to strong interactions among PI nanofibers and promoted mechanical performance, which would improve the operability of the separators in LIBs assembly. The electrochemical tests show that cells assembled with the TiO2 coated PI separators have better high-rates performance, higher discharge capacity and great long-term running stability even at 120 degrees C. These characters suggest that the TiO2 coated PI membrane might play an important role in the forthcoming advanced power LIBs.