In-situ Coating of Cathode by Electrolyte Additive for High-voltage Performance of Lithium-ion Batteries

• In-situ formation of interface films on LiCoO2 surface by electrolyte additive.
• Thickness-tunable interface films is obtained by adding different concentrations of BMP additive.
• High-voltage cycling performance (4.5 V) is closely associated with the thickness of the interface film.
• 0.5% BMP electrolyte additive shows superior high-voltage cycleability.

We have previously demonstrated that N,N′-4,4′-diphenylmethane-bismaleimide (BMI) as an electrolyte additive enhances the high-voltage performance of lithium-ion batteries by electrochemically forming an interface film on cathode surface. In order to obtain a comprehensive understanding of the bismaleimide-based additives, 2,2′-Bis[4-(4-maleimidophenoxy) phenyl]propane (BMP), which is more compatible with electrolyte than BMI, is studied as a new electrolyte additive. LiCoO2 is chosen as the typical cathode material. Firstly, the structure of interface films on LiCoO2 surface is studied with different concentrations of BMP additive. The morphology, thickness and chemical composition of the interface film are characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) respectively. The oxidation potential of BMP is measured by linear sweep voltammetry (LSV). Secondly, how the interface films influence the high-voltage cycling performance of LiCoO2/Li batteries is studied. AC impedance measurements (EIS), X-ray diffraction (XRD) and discharge profile analysis are used to further clarify the mechanism. For the first time, we find that thickness-tunable interface films could be generated on LiCoO2 surface by adding different concentrations of BMP additives in electrolyte. Also, the high-voltage cycling performance of the corresponding LiCoO2/Li batteries is closely associated with the thickness of the interface film. Optimized amount of BMP additive (0.5% w/v in our work) presents superior high-voltage cycling performance of the corresponding LiCoO2/Li batteries.