(255m) 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.