ABSTRACT
The growth of crystalline Li-based oxide thin films on silicon substrates is essential for the integration of next-generation solid-state lithionic and electronic devices including on-chip microbatteries, memristors, and sensors. However, growing crystalline oxides directly on silicon typically requires high temperatures and oxygen partial pressures, which leads to the formation of undesired chemical species at the interface compromising the crystal quality of the films. In this work, we employ a 2 nm gamma-alumina (γ-Al2O3) buffer layer on Si substrates in order to grow crystalline thin films of Li4Ti5O12 (LTO), a well-known active material for lithium-ion batteries. The ultrathin γ-Al2O3 layer enables the formation of a stable heterostructure with sharp interfaces and drastically improves the LTO crystallographic and electrochemical properties. Long-term galvanostatic cycling of 50 nm LTO films in liquid-based half-cells demonstrates a high capacity retention of 91% after 5000 cycles at 100 C. Rate capability tests showcase a specific charge of 56 mA h g-1 at an exceptional C-rate of 5000 C (15 mA cm-2). Moreover, with sub-millisecond current pulse tests, the reported thin-film heterostructure exhibits rapid Li-ion (de)intercalation, which could lead to fast switching timescales in resistive memory devices and electrochemical transistors.
ABSTRACT
Thin films of Y2Zr2O7 were grown via pulsed laser deposition (PLD) on substrates of MgO(110), Al2O3(0001) and Al2O3(11[combining macron]02). Electrical properties were investigated via electrical impedance spectroscopy. Unexpectedly, the ionic conductivity is not affected by the microstructure; only minor differences in conductivities and activation energies were measured between epitaxial thin films (on MgO) and textured thin films (on Al2O3, both orientations). This indicates the grain boundaries of such a material to only marginally block the oxygen vacancy transport. Starting from these results, epitaxial multilayers of Y2Zr2O7 and 8 mol% yttria-stabilized zirconia with same overall thickness (between 60 and 70 nm) and different number of interfaces (from 1 up to 9) have been deposited on MgO(110) and the role of the residual compressive strain on the electrical properties has been investigated by means of XRD analysis and impedance spectroscopy. The results, showing no effect of the strain field on the ionic conductivity, indicate the negligible effect of the compressive strain on the ionic transport properties of the material.
ABSTRACT
Multilayered heterostructures of Ce0.85Sm0.15O2-δ and Y0.16Zr0.92O2-δ of a high crystallographic quality were fabricated on (001)-oriented MgO single crystal substrates. Keeping the total thickness of the heterostructures constant, the number of ceria-zirconia bilayers was increased while reducing the thickness of each layer. At each interface Ce was found primarily in the reduced, 3+ oxidation state in a layer extending about 2 nm from the interface. Concurrently, the conductivity decreased as the thickness of the layers was reduced, suggesting a progressive confinement of the charge transport along the YSZ layers. The comparative analysis of the in-plane electrical characterization suggests that the contribution to the total electrical conductivity of these interfacial regions is negligible. For the smallest layer thickness of 2 nm the doped ceria layers are electrically insulating and the ionic transport only occurs through the zirconia layers. This is explained in terms of a reduced mobility of the oxygen vacancies in the highly reduced ceria.
ABSTRACT
The systematic study of the ionic transport properties of epitaxial Y2Zr2O7 films with defective fluorite structure reveals an enhanced oxygen vacancy conductivity at the interface between the films and the MgO(110) substrate, which is characterized by a high density of misfit dislocations. This beneficial effect is discussed in terms of space-charge and mobility effects.
