Initiated Chemical Vapor Deposition of Ultrathin Polymer Coatings at Graphite Electrodes for Enhanced Performance in Li-Ion Batteries

Abstract

The solid-electrolyte interphases (SEIs) that form in situ at Li-ion battery electrodes play a critical role in determining cell- and system-level characteristics, but may not contribute optimized performance. Power, cycle life, and safety are improved when the electrodes are deliberately modified with nanoscale, Li+-conducting coatings. Herein, we use initiated chemical vapor deposition (iCVD) to apply ultrathin (<40 nm) polymer coatings, based on the monomer, 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3D3), at the surfaces of powder-composite graphite electrodes. The conformal nature of the polymer deposition is verified by scanning electron microscopy (SEM), while X-ray photoelectron spectroscopy (XPS) confirms the expected chemical structure and nanoscale nature of the polymer. Poly(V3D3)-coated graphite electrodes are electrochemically evaluated in coin cells that include conventional liquid Li-ion electrolytes and LiCoO2-based cathodes. Galvanostatic tests on these cells demonstrate significant improvements in specific power, coulombic efficiency, cycle life, and tolerance to overcharge conditions vs equivalent cells containing unmodified graphite anodes, while impedance analysis shows reduced charge-transfer resistance and more consistent cell-to-cell behavior with polymer-coated graphite anodes. Finally, we use XPS with depth profiling to demonstrate that some conventional SEI products do form within/beneath the poly(V3D3) coating after extensive cycling, but in speciations such as LiF, that are favorable for electrochemical performance.