AVS2010 Session EN+EM-TuA: Electronic Materials for Energy Conversion & Storage

More recently we have focused our work on iron oxide nanostructures, as iron is an inexpensive, abundant and a non-toxic material. Furthermore, we have synthesized binder-free, high-rate capability electrodes. The electrodes contain Fe₃O₄ nanorods as the active lithium storage material and carbon single-wall nanotubes (SWNTs) as the conductive additive. The highest reversible capacity is obtained using 5 wt.% SWNTs, reaching 1000 mAh/g (~2000 mAh/cm³) at C rate when coupled with a lithium metal electrode, and this high capacity is sustained over 100 cycles. Furthermore, the electrodes exhibit high-rate capability and stable capacities of 800 mAh/g at 5C and ~600 mAh/g at 10C. Scanning electron microscopy indicates that this high-rate capability is achieved because Fe₃O₄ nanorods are uniformly suspended in a conductive matrix of SWNTs. Raman spectroscopy is employed to understand how the SWNTs function as a highly flexible conductive additive. We expect that this method can be used to achieve other binder-free anodes as well as cathodes with similar high-rate capability⁴.

(1) Lee, S.-H.; Kim, Y.-H.; Deshpande, R.; Parilla, P. A.; Whitney, E.; Gillaspie, D. T.; Jones, K. M.; Mahan, A. H.; Zhang, S. B.; Dillon, A. C. Adv. Mat. 2008, 20, 3627-3632.

(2) Riley, L. A.; Lee, S.-H.; Gedvilias, L.; Dillon, A. C. Journal of Power Sources 2010, 195, 588-592.

(3) Riley, L. A.; Cavanagh, A. S.; George, S. M.; Jung, Y.-S.; Yan, Y.; Lee, S.-H.; Dillon, A. C. ChemPhysChem 2010, in press.

(4) Ban, C.; Wu, Z.; Gillaspie, D. T.; Chen, L.; Yan, Y.; Blackburn, J. L.; Dillon, A. C. Advanced Materials 2010, in press.

The development of secondary lithium-ion batteries has been directed primarily at portable electronics applications. However, these batteries also have the potential to function as a power source for micro-systems through engineering of electrodes into 3D architectures based on high aspect ratio pillars. In order to utilize this potential, an ultra-thin and highly conformal solid electrolyte layer is required to coat the 3D electrode array. The solid electrolyte lithium aluminosilicate (LiAlSiO₄), is a promising candidate for this application due to high ionic conductivity along its c-axis resulting from channels formed by the alternating tetrahedra of aluminum-oxygen (Al-O) and silicon-oxygen (Si-O). The length of c-axis of lithium aluminosilicate can be adjusted by changing the crystallization temperature for desired conductivity characteristics.

Atomic layer deposition (ALD) was employed in this work to synthesize thin film lithium aluminosilicate. The self-limiting characteristic of ALD allows for precise control of thickness and composition of complex oxides and results in a highly conformal and pinhole-free coating suitable in 3D micro-battery applications. The metal precursors used in this work are tetraethyl orthosilicate (TEOS), trimethylaluminum (TMA) and lithium t-butoxide (LTB). We also investigated the use of tri-t-butoxy-hydroaluminate (LTBA) but found that the metal composition was difficult to control and high carbon contents. Using the three precursors mentioned above with water vapor as the oxidant, we deposited SiO₂, Al₂O₃ and Li₂O, at deposition rates in the range of 0.8~2Å/cycle. The overall deposition rate of stoichiometric LiAlSiO₄ was ~5Å/cycle using a chamber base pressure of 10^-2 Torr and substrate temperature of 300°C. The concentration of each metal element in Li_xAl_ySi_zO thin films is found to correlate closely to ALD cycles and the associated incubation times. The crystalline structures as well as the local environment of the Li-conducting channels are also affected by the ALD cycles and sequences, as indicated by ultraviolet photoelectron spectroscopy (UPS), transmission electron microscopy (TEM) imaging and nuclear magnetic resonance (NMR) analyses. The Li-ion conductivities of ALD Li_xAl_ySi_zO thin films were determined by impedance measurements using a four-point probe setup with contacts made to the film surface. The films have high ion conductivity and low electronic conductivity, the values of which are strongly influenced by the lithium content and distribution in the synthesized thin films.

To achieve the highest possible conversion efficiencies in multijunction photovoltaics, the individual layers of the device must both be lattice-matched and have optimal band-gap spacing. Lattice-matched or strain-compensated epitaxy is required for the growth of junctions thick enough to elicit high quantum efficiency. Ideally, for spectral matching, one would have an infinite number of junctions that are current-matched; however, fabrication of a large number of junctions is neither easy nor desirable because of problems that arise from series resistance. In the end, it becomes a balancing act where the optimal number of junctions for a high efficiency concentrator cell is 3-6 junctions, with the conversion efficiency directly linked to how well spacing of the band gaps of the cell are optimized for absorption of the solar spectrum. Unfortunately, many of the optimal lower band-gaps for these multijunction cells do not occur in the dominant materials system (i.e. Ge and mixtures of In, Ga, Al, As, and P). As such, of late there has been a strong push to characterize new materials in hopes of providing more design options for photovoltaic cells. GaTlP is one such material, theorized to be useful as one of the lower junctions of 3+-junction cells while still being lattice-matched to GaAs and Ge. In this research, the change in lattice constant and band gap of GaTlP with varying compositions are investigated first by computational simulation and then with physical devices. New efficiency records should be achievable by incorporating these new optimal junction materials into the design for multijunction cells. This development will help solar concentrator cells achieve grid parity, thereby becoming a viable renewable energy choice.

Thin films of lanthanum doped lead zirconate titatante (PLZT) have gained attention due to the large photostrictive response, and their possible use for contact less actuators and sensors. Variation in composition and doping are known to influence the photostrictive responses as well as ferroelectric behavior of these materials. PLZT is also a potential material for photovoltaic devices due to its high electro-optic coefficient and optical transparency.