ALD PROCESSES AND APPLICATIONS TO NANOSTRUCTURED ELECTROCHEMICAL ENERGY STORAGE DEVECES
Rubloff, Gary W
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Next generation Li-ion batteries (LIB) are expected to display high power densities (i.e. high rate performance, or fast energy storage) while maintaining high energy densities and stable cycling performance. The key to fast energy storage is the efficient management of electron conduction, Li diffusion, and Li-ion migration in the electrode systems, which requires tailored material and structural engineering in nanometer scale. Atomic layer deposition (ALD) is a unique technique for nanostructure fabrications due to its precise thickness control, unprecedented conformality, and wide variety of available materials. This research aims at using ALD to fabricate materials, electrodes, and devices for fast electrochemical energy storage. First, we performed a detailed study of ALD V2O5 as a high capacity cathode material, using vanadium tri-isopropoxide (VTOP) precursor with both O3 and H2O as oxidant. The new O3-based process produces polycrystalline films with generally higher storage capacity than the amorphous films resulting from the traditional H2O-based process. We identified the crucial tradeoff between higher gravimetric capacity with thinner films and higher material mass with thicker films. For the thickness regime 10-120 nm, we chose areal energy and power density as a useful metric for this tradeoff and found that it is optimized at 60 nm for the O3-VTOP ALD V2O5 films. In order to increase material loading on fixed footprint area, we explored various 3-dimentional (3D) substrates. In the first example, we used multiwall carbon nanotube (MWCNT) sponge as scaffold and current collector. The core/shell MWCNT/V2O5 sponge delivers a stable high areal capacity of 816 μAh/cm2 for 2 Li/V2O5 voltage range (4.0-2.1 V) at 1C rate (nC means charge/discharge in 1/n hour), 450 times that of a planar V2O5 thin film cathode. Due to low density of MWCNT and thin V2O5 layer, the sponge cathode also delivers high gravimetric power density in device level that shows 5X higher power density than commercial LIBs. In the other example, Li-storage paper cathodes, functionalized of conductivity from CNT and Li-storage capability from V2O5¬, presented remarkably high rate performance due to the hierarchical porosity in paper for Li+ migration. The specific capacity of V2O5 is as high as 410 mAh/g at 1C rate, and retained 116 mAh/g at high rate of 100C. We found V2O5 capacities decreased by about 30% at high rates of 5C-100C after blocking the mesopores in cellulose fiber, which serves to be the first confirmative evidence of the critical role of mesoporosity in paper fibers for high-rate electrochemical devices. Finally, we made high density well-aligned nanoporous electrodes (2 billion/cm2) using anodic alumina template (AAO). ALD materials were deposited into the nanopores sequentially - Ru or TiN for current collection, and V2O5 for Li-storage. Ru metal by ALD shows high conductivity and conformality, and serves best as the current collector for V2O5. The capacity of V2O5 reaches about 88% of its theoretic value at high rate of 50C. Such electrodes can be cycled for 1000 times with 78% capacity retention.