Lithium Vanadium Oxide and Carbon Nanotube Substrate Composite Battery Electrodes
Schaffer, Corey Paul
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Lightweight composite lithium trivanadate, derived from a low-temperature sol-gel synthesis, and carbon nanotube substrate electrodes showing a 45% reduction in electrode mass over typical foil coating electrodes were fabricated and characterized using materials and electrochemical techniques. First, the effects of two post-synthesis treatment conditions were studied using pure lithium trivanadate gel precursors. Post synthesis treatment conditions included freeze drying or separating the solid component from the gel, and annealing each of the previous conditions at 200°C or 300°C. Powder X-Ray diffraction data validated the synthesis of lithium trivanadate. Crystallite size was calculated and found to be in the range of 178.0 to 238.4 Angstroms. D-spacing was found to be in the range of 6.338 to 6.215 Angstroms. Scanning Electron Microscopy images showed needle and branch-like morphology of the lithium trivanadates, with very little observable differences between the amorphous gel precursors and crystalline annealed samples. Thermal gravimetric analysis and Inductively Coupled Plasma - Optical Emission Spectroscopy indicated the composition of the samples. Compositions were found to range from 0.04 < x < 0.29 and 0.96 2 O·Li y V 3 O<sub>8</sub>, where the water composition was highly dependent on annealing temperature as expected. Surface areas were found using a Brunauer, Emmett, and Teller (BET) theoretical gas adsorption method and found to not be influenced by temperature, but rather the post synthesis separation process. Surface areas for the freeze dried total gels were found to be 1.2 m 2 /g versus 4.9 m 2 /g for the solid component samples. Lithium trivandate and carbon nanotube subsrate electrodes were fabricated by directly integrating the subsrates with the gel and heat treating at 200°C and 300°C without the use of additional conductivity additives or polymeric binders. The electrodes were studied using X-Ray Diffraction, and the susceptibility of the lithium vanadium oxide to form a secondary phase during heat treatment in the presence of carbon was studied using X-Ray diffraction and thermal gravimetric analysis. The composite electrodes were studied using electrochemical methods and benchmarked against standard metal foil electrodes (utilizing the lithium trivanadate powders from the four post-synthesis conditions). Cyclic voltammetry was used to infer lithium insertion and de-insertion mechanisms associated with discharge and charge. The appropriate discharge window was also determined. First discharge curves indicated capacities in the range of 306–346 mAh/g for the foil electrodes and 206–303 mAh/g for the composite electrodes. Capacity was found to be higher in the solid component samples in addition to the 300°C annealing temperature. Charge-discharge cycling determined that the 200°C electrodes showed the best cycle life, and both composite electrodes outperformed the foil electrodes. The effect of potential window on cycle life was studied and found to favor windows that cycled the cells above 2.40 V. In terms of higher discharge rate performance, the freeze dried total gel foil electrodes performed better than both the composite electrodes and solid component foil electrodes. Using the total electrode mass in the cycling performance analysis shows a distinct improvement in capacity (by as much as 23 mAh/g electrode at the 100 th discharge cycle) that the composite electrodes afford over the foil coating benchmark electrodes.