Synthesis and properties of a vanadium oxide based lithium ion Cathode
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Mat. Res. Soc. Symp. Proc. Vol. 575 ©2000 Materials Research Society
counterparts, most vanadium oxides possess no extractable lithium ions. Therefore, the vanadium oxides must be lithiated, i.e., lithium must be inserted into the vanadium oxide to produce a LixVyOz. Once lithiated, the lithium vanadium oxide must cycle the lithium ions reversibly in order to function as a cathode within a lithium ion battery. Lithiation of vanadium oxides can be accomplished via two methods: reduction of a high (+5 or +4) oxidation state vanadium oxide to obtain a LixVyOz, where the average oxidation state of the V is +3 to +4, or lithium insertion into a low (+3) oxidation state vanadium oxide. Insertion into a low oxidation state has shown to be difficult; reduction of V (+5 or +4) is a simpler method to produce a lithiated vanadium oxide. The most obvious reducing agent is lithium metal. However, lithium metal can "over-reduce" the parent vanadium oxide. Thus, we embarked on a search to find a reducing agent that would lithiate but not over reduce the vanadium oxide. The reducing agent must comprise of two components: 1) lithium ions and 2) an oxidizable element. They can be combined into one compound, as in a lithium salt, or a combination of at least two components. Upon reaction of the starting materials, the lithium ion would be inserted into the vanadium oxide host. This would result in the reduction of the vanadium. The choice of reducible materials by V (+5) is rather limited. The agent of choice was sulfide, S2 . An example of this reaction type would be: V20 5 + Li 2S
--.
Li 2V 20 5 + S
Vanadium pentoxide, the starting material, is in its highest oxidation state (+5) and is reduced by
sulfide
(S2)
to V (+4).
EXPERIMENTAL SECTION All materials were used as received. LiV 3 07.9 was prepared by grinding V 20 5 (KerrMcGee Chemical Corporation, LLC), LiOHoH 20 (Aldrich Chemical Corporation) and NH 4CO 2CH 3 (Aldrich Chemical Corporation) in a 4:3:1 molar ratio, respectively, and thermally treating them in air at 585 °C for 16 hours. After cooling the material was reground and heated at 585 TC in air for an additional 16h. The resulting product was black in color. X-ray diffraction indicated that the material produced was LiV 3 07. 9 not LiV 30 8 which is reddish in color. The resulting solid was ground in a ball mill with 67% of the carbon, steel media and acetone. The material was then incorporated into a coating formulation with the following composition: 60% Active material, 30% acetylene black 100% compressed carbon (Chevron Chemical Corporation) and 10% Kynar 2801 (Elf-Atochem). This formulation is then coated from an acetone solution onto an aluminum foil substrate. The composite cathode was dried under vacuum at 100 TC prior to battery assembly. Using a Swagelok type cell, a test battery was 2 then constructed using a 2 cm piece of the above coating as a cathode, a Celgard 350OTM (Hoechst-Celanese), and lithium metal (FMC Corporation) as the anode. The electrolyte consisted of a 1M LiPF 6 in 50/50 weight per
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