Preparation and Characterization of ALD TiN Thin Films on Lithium Titanate Spinel (Li 4 Ti 5 O 12 ) for Lithium Ion Batt
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Preparation and Characterization of ALD TiN Thin Films on Lithium Titanate Spinel (Li4Ti5O12) for Lithium Ion Battery Applications Mark Q. Snyder1, Svetlana Trebukhova2, Boris Ravdel2, M. Clayton Wheeler1, Joseph DiCarlo2, Carl P. Tripp3,4, and William J. DeSisto1,4 1 Chemical & Biological Engineering, University of Maine, 5737 Jenness Hall, Orono, ME, 04469 2 Yardney Technical Products/Lithion, Inc., Pawcatuck, CT, 06379 3 Department of Chemistry, University of Maine, Orono, ME, 04469 4 Laboratory for Surface Science and Technology (LASST), University of Maine, Orono, ME, 04469 ABSTRACT Lithium titanate spinel (Li4Ti5O12, or LTS) has received an increasing level of attention as a nanopowder lithium-ion battery anode. Nanopowder electrodes may provide a higher energy density than currently available. Furthermore, the surface of the spinel nanopowder has been studied in air, under vacuum, and at varying temperatures with diffuse reflectance infrared Fourier transform spectroscopy revealing surface hydroxyls, carbonates and water. Applying a TiN thin film, a film that is both conducting and chemically inert to harmful reactions with the solvent/electrolyte, by atomic layer deposition (ALD) may enhance battery cycle life. A 200layer film was deposited at 500 ºC. We have characterized the influence of a TiN thin film on Liion battery performance. Total nitrogen content and transmission electron microscopy were used to verify the presence of nitrogen and formation of a thin film, respectively, on LTS. Modifying the powder with an ALD thin film coating produced an anode material with a voltage profile that demonstrated longer charge maintenance with shorter transient periods. It also held a more consistent charge capacity over varying discharge rates in coin cell testing than unmodified LTS. INTRODUCTION By synthesizing a lithium-ion battery anode from a nanoparticulate material, the potential exists to greatly enhance the energy density by increasing surface area of the anode while decreasing anode volume. A disadvantage of high surface area materials is that the greater availability of reaction sites leads to deactivation of the active mass at a faster rate and shortens the life of the electrode [1]. Bulk changes to the alloy may result as well [2]. Modification of the surface chemistry to influence reaction chemistry is under consideration as a means to improve Li-ion battery performance. Lithium titanate spinel (Li4Ti5O12, or LTS) is available as a nanomaterial, and has a Liinsertion potential of 1.5 V which is above the reduction potential of most organic electrolytes. Thus, a solid electrolyte interphase (SEI) will not be formed on the particle surface [3]. The reduction potential of LTS allows for examination of any effects associated with fabrication of an anode coating without the interference of SEI formation. The anodic intercalation process, representing the charge cycle [3, 4], is described in equation (1) Li4Ti5O12 + 2.7Li+ + 2.7e- ↔ Li6.7Ti5O12.
(1)
Atomic layer deposition (ALD) may
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