Polarity-tuned Gel Polymer Electrolyte Coating of High-voltage LiCoO_2 Cathode Materials

초록

We demonstrate a new surface modification of high-voltage lithium cobalt oxide (LiCoO_2) cathode active materials for lithium-ion batteries. This approach is based on exploitation of a polarity-tuned gel polymer electrolyte (GPE) coating. Herein, two contrast polymers having different polarity are chosen: polyimide (PI) synthesized from thermally curing 4-component (pyromellitic dianhydride/biphenyl dianhydride/phenylenediamine/oxydianiline) polyamic acid (as a polar GPE) and ethylene-vinyl acetate copolymer (EVA) containing 12 wt% vinyl acetate repeating unit (as a less polar GPE). The strong affinity of polyamic acid for LiCoO_2 allows the resulting PI coating layer to present a highly-continuous surface film of nanometer thickness. On the other hand, the less polar EVA coating layer is poorly deposited onto the LiCoO_2, resulting in a locally agglomerated morphology with relatively high thickness. Based on the characterization of GPE coating layers, their structural difference on the electrochemical performance and thermal stability of high-voltage (herein, 4.4 V) LiCoO_2 is thoroughly investigated. In comparison to the EVA coating layer, the PI coating layer is effective in preventing the direct exposure of LiCoO_2 to liquid electrolyte, which thus plays a viable role in improving the high-voltage cell performance and mitigating the interfacial exothermic reaction between the charged LiCoO_2 and liquid electrolytes.

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