Lithium-ion batteries (LIBs) cathode/electrolyte interfacial degradation during cycling is one of the key obstacles to upgrade cathode material for better cycling stability. In order to address the interfacial degradation issues, inherently conductive polymers (ICPs) surface coating is one of the effective approaches. To produce effective coating, conformal ICPs coating morphology is desired. Nevertheless, there is a lack of methods to produce ICPs conformally on LIBs cathode material particles due to the morphological complexity and high surface ratio barriers. This research reports a novel method for conformal ICPs coating on LIBs cathode materials. To carry out a systematic study, commercial LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) was chosen as a model LIBs material. Three most attractive ICPs, in...[ Read more ]

Lithium-ion batteries (LIBs) cathode/electrolyte interfacial degradation during cycling is one of the key obstacles to upgrade cathode material for better cycling stability. In order to address the interfacial degradation issues, inherently conductive polymers (ICPs) surface coating is one of the effective approaches. To produce effective coating, conformal ICPs coating morphology is desired. Nevertheless, there is a lack of methods to produce ICPs conformally on LIBs cathode material particles due to the morphological complexity and high surface ratio barriers. This research reports a novel method for conformal ICPs coating on LIBs cathode materials. To carry out a systematic study, commercial LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) was chosen as a model LIBs material. Three most attractive ICPs, including poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), and polyaniline (PANi) have been successfully coated on NCM by oxidative chemical vapor deposition (oCVD) for the first time, achieving significant improvement on the cycling performance. Furthermore, a model to understand the role of the conformal ICPs coating has been proposed and developed.

To obtain conformal ICPs coating, dedicate oxidant selection was performed. The oxidant sticking coefficient was found to govern the degree of conformality, with VOCl₃ identified as a suitable oxidant for oCVD process. The oCVD technique produces conformal PEDOT, PPy, and PANi thin film coating on NCM particles successfully, and demonstrates excellent thickness control capability. To improve the cycling stability, ~20 nm was identified to be an optimal thickness for PEDOT coating on NCM, which significantly improves the retention rate from 46.5 to 91.1% over 200 cycles, at 1C rate between 3.0-4.6 V. For PPy coating, the optimized thickness was ~12 nm with a retention rate of 93.4%. Without thickness optimization, PANi was coated on NCM for the first time, achieving a retention rate of 77.7% after 200 cycles at 1C. The results achieved establish oCVD to be a powerful and universal coating technique for many conductive polymers coating on LIBs material.

Based on the experimental observation, a model to understand the role(s) of ICPs coating in the material performance enhancement was proposed and developed for the first time. The ICPs coating is believed to be capable of HF chemical scavenging and providing physical protection against degradation, supported by experimental analysis. Furthermore, oCVD technique successfully produces conformal PEDOT coating on Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ and Li₂Mn₂O₄ cathode materials, broadening the application capability of the oCVD technique. To conclude, this research provides a high quality technique for ICPs coating on LIBs materials, and lays down a solid foundation for wide application of oCVD in LIBs area.