FY2010 - Oak Ridge National Laboratory

Director’s R&D Fund—
Science for Extreme Environment: Advanced Materials and Interfacial Processes for Energy
05838
Improving the Performance of Lithium Ion Batteries by Tuning the
Graphite/Carbon Electrode Surface
Xiao-Guang Sun
Project Description
Rechargeable lithium ion batteries that have been proposed for applications in electric vehicles (EVs)
should meet safety requirements and have long calendar lives (>10 years). These, to a larger extent, are
determined by the quality of the solid electrolyte interface (SEI) formed on the surface of graphite/carbon
electrodes. However, the SEI layer on graphite/carbon electrodes grows with cycling and storage, which
deteriorates the battery performance and calendar life. Therefore designing desirable interfaces/
interphases (SEI) are major scientific challenges that must be met to achieve truly innovative
breakthroughs in future chemical energy storage devices. The overarching goal of this project is targeted
on tuning the surface of graphite/carbon electrodes by pre-deposition of mixtures of different inorganic or
organic lithium salts or their combinations with single ion conductors. This approach entails a precision
control over formation of SEI layers and opens up a new avenue to solve the critical problems associated
with SEI layers.
Mission Relevance
This project involves the synthesis of a series of new lithium borate salts having good SEI formation
ability that cannot only be used to form an artificial SEI layer on a graphite/carbon electrode surface
(tuned up in this project) but also can be evaluated as promising lithium salts in conventional carbonate
solvents to replace lithium hexafluorophosphate (LiPF 6 ), which is not stable at high temperatures. This
will benefit DOE’s other battery-related programs and offices, such as Energy Efficiency Renewable
Energy (EERE), Vehicle Technology, and Batteries for Advanced Transport Technology, etc.
Results and Accomplishments
We have used two different approaches to modify the surface of graphite electrode—direct deposit
lithium malonate (LM) on the surface of graphite electrode and use of salt as an additive to carbonate
electrolyte mixtures. Both approaches have successfully reduced the initial irreversible capacity loss in
the graphite||lithium half cells and improved the corresponding coulombic efficiencies. However, the
capacities of the half cells, with and without surface-modified electrodes, continually decrease with
cycling, which is due to the poor quality of SEI formed on the bare graphite electrode surface and the lack
of mechanical strength of the artificial SEI on the modified graphite electrode surface.
To increase the mechanical strength of the surface coating, we then synthesized single ion conductors
based on allyl-group-containing lithium borate salt grafted on allyl-group-containing polymethacrylate
polymers via hydrosilylation reaction. It is found that the surface coating with single ion conductors on
both carbon and graphite particles could successfully reduce the initial electrolyte decomposition and
improve the corresponding coulombic efficiencies; however, they still could not prevent the eventual
capacity loss with cycling. This observation primarily resulted from the un-removed platinum catalyst
within the crosslinked membrane, which can induce electron tunneling and thus induce more electrolyte
decomposition, and therefore capacity loss. Future experiments need to focus on removing the catalyst
after the film crosslinking reaction.
We have been successful in securing following-on funding to continue this work under two sponsorships:
the DOE Vehicle Technology Program (Hard Carbon Materials for High-Capacity Li-ion Battery Anodes)
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