Ultrathin Gel Electrolyte Layers for Interfacial Control of Lithium Insertion Electrodes

In lithium ion batteries, interfacial reactivity of the electrodes (both the anode and cathode) must be controlled to achieve high power capabilities and long cycle lives. The conventional lithium ion battery anode, graphite, forms a stable solid electrolyte interface (SEI) in initial cycles, which then kinetically hinders electrochemical reduction of the electrolyte in subsequent cycling. Lithium ion battery technology is moving towards higher capacity anodes (e.g. Si) and higher voltage cathodes (e.g. LiNi_0.5Mn_1.5O₄), and it appears that these materials do not naturally form stable SEIs. There is a need for engineered, synthetic solid electrolyte interfaces to accommodate the mechanical effects and electrochemical potentials associated with these new materials. We are developing polymer gel electrolyte coatings that provide mechanically and chemically stable interfaces for both anodes and cathodes. Ultrathin gel electrolyte layers were synthesized using a technique called initiated chemical vapor deposition (iCVD). iCVD provides exquisite control over composition and network structure in polymer thin films with thicknesses from 10 nm to several micrometers. Crosslinked poly(n-butyl acrylate) was synthesized directly on top of LiCoO₂ composite cathodes by iCVD. Adhesion promoters were used to covalently attach the polymer to the LiCoO₂ active material. The polymer layer imbibed standard liquid electrolyte (1M LiPF₆ in alkyl carbonate mixtures), and characterization by Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy and scanning electron microscopy showed that the gel remained attached to the electrode. The electrodes were assembled into half cells against Li metal and then cycled. The performance of the gel-coated electrodes compared to uncoated, state-of-the-art lithium ion battery electrodes will be presented.