Haiyan Tan

Epitaxial LiCoO2 Films as a Model System for Fundamental Electrochemical Studies of Positive Electrodes

Epitaxial LiCoO2 (LCO) thin films of different orientations were fabricated by pulsed laser deposition (PLD) in order to model single-crystal behavior during electrochemical reaction. This paper demonstrates that deposition of conductive SrRuO3 between a SrTiO3 (STO) substrate and an LCO film allows (1) epitaxial growth of LCO with orientation determined by STO and (2) electrochemical measurements, such as cyclic voltammetry and impedance spectroscopy. Scanning transmission electron microscopy (S/TEM and SEM) has demonstrated an orientation relationship between LCO and STO of three orientations, (111), (110) and (100), and identified a LCO/electrolyte surface as consisting of two crystallographic facets of LCO, (001) and {104}. The difference in the orientation of LCO accounts for the difference in the exposed area of {104} planes to the electrolyte, where lithium ions have easy access to fast diffusion planes. The resistance for lithium ion transfer measured by electrochemical impedance spectroscopy had inverse correlation with exposed area of {104} plane measured by TEM. Chemical diffusivity of lithium ions in LCO was measured by fitting electrochemical impedance spectroscopy data to a modified Randles equivalent circuit and allowed us to determine its dependence on film orientation.

Self-terminated electrodeposition of iridium electrocatalysts

A simple electrochemical process for submonolayer deposition of ultrathin catalytic Ir films is demonstrated. This method enables effective utilization of one of natures rarest elements while different substrates facilitate the exploration of promising bimetallic catalysts for a sustainable hydrogen economy. Semi-coherent Ir films were deposited on Au, Pt and Ni substrates using K3IrCl6–Na2SO4–H2SO4 electrolytes operated between 40 °C and 70 °C. However, the deposition reaction is quenched at the onset of H2 production where adsorbed H blocks the reduction of IrCl6−xH2Oxx−3 to Ir. The electrode can be reactivated for further deposition by pulsing the potential to more positive values where adsorbed H is oxidized. The electrocatalytic activity of ultrathin Ir and Pt films, and combinations thereof, were examined as function of the number of self-terminating deposition pulses. The ultrathin films match or exceed the best reported activity metrics for hydrogen oxidation in alkaline media and oxygen evolution in acid.

Microscopy Study of Structural Evolution in Epitaxial LiCoO2 Positive Electrode Films during Electrochemical Cycling

The evolution of interface between the epitaxial thin film LiCoO2 (LCO) electrode and liquid electrolyte and inside the LCO film during electrochemical cycling has been analyzed by high resolution scanning transmission electron microscopy. Relaxation of sharp translational domain boundaries with mismatched layers of CoO2 octahedra occurs during cycling and results in formation of continuous CoO2 layers across the boundaries. The original trigonal layered structure of LiCoO2 tends to change into a spinel structure at the electrode/electrolyte interface after significant extraction of Li from LCO. This change is more pronounced at 4.2 V peak of CV, indicating lower stability of the layered LCO structure near its surface after Li is extracted above 60%. The transformed structure is identified to be close to Co3O4, with Co both on tetrahedral and octahedral sites, rather than to LiCo2O4 as it was suggested in earlier publications. Electron energy-loss spectroscopy measurements also show that Co ions oxidation state is reduced to mixed valence state Co(2+)/Co(3+) during the structure changes to spinel rather than oxidized.

Effect of oxygen pressure on structure and ionic conductivity of epitaxial Li0.33La0.55TiO3 solid electrolyte thin films produced by pulsed laser deposition

We report on the ionic conductivity of Li0.33La0.55TiO3 (LLTO) epitaxial films grown on the (100) and (111) surfaces of single crystal SrTiO3 (STO) substrates at different oxygen partial pressures (from 1.33 to 26.66 Pa). The films are intended for use as solid electrolytes for all-solid-state Li-ion batteries, and the epitaxial growth for modeling the electrolyte single crystal properties. The LLTO films overall exhibit formation of the perovskite-based orthorhombic structure with the epitaxial cube-on-cube orientation for both (100)STO and (111)STO substrates. Room temperature ionic conductivity of the LLTO films measured by impedance spectroscopy slightly decreases with the oxygen partial pressure changing from 1.33 to 26.66 Pa and is in the range of 10−4 to 10−5 S cm−1. Complex impedance plots at different temperatures indicate that the conductivity in these epitaxial films is predominantly an intrinsic bulk property and exhibits distribution of relaxation time. Activation energies (Ea) for all the films were calculated employing the Arrhenius relationship and are between 0.30 eV and 0.40 eV, agreeing well with the reported values of bulk materials. Systematic difference in ionic conductivity between the (100)STO and (111)STO films is understood as being related to the difference in distribution of a “bottleneck” diffusion path. The measured conductivity of LLTO films indicates that these films can be used as a solid electrolyte in all-solid-state batteries.

Evolution of grain boundary conduction with increasing temperature in pure and Ti doped Co ferrite materials

We report on the structural, temperature, and frequency dependent impedance studies of Ti doped cobalt ferrite material (CoFe1.95Ti0.05O4) in comparison with the pure CoFe2O4. XRD and Raman spectroscopy studies confirm the inverse spinel crystallization of the materials with space group of Fd-3 m. Scanning electron microscope images shows the microcrystalline nature of the particles. Homogeneity, stoichiometry, and ionic states of the ions in the composition were confirmed by energy dispersive X-ray analysis and X-ray photoelectron spectroscopic studies. Temperature and frequency dependent real (Z′) and imaginary (Z″) part of the impedance shows the existence of relaxation processes and their distribution in CoFe2O4 and CoFe1.95Ti0.05O4 materials. Complex impedance spectroscopy studies at low temperatures shows that the conductivity in these materials is predominantly due to the intrinsic bulk grains. With increasing the temperature, evolution of grain boundary conduction is clearly seen through the appea...

Structural Evolution During Electrochemical Cycling of Epitaxial LiCoO2 Films Studied by S/TEM

LiCoO2 (LCO) has been the most important and most studied positive electrode material for lithium-ion batteries; thus for this work we have selected LCO as a model material for studying electrochemical property of single orientation, binder-free cathodes in a form of epitaxial thin films [1]. In order to capture the effect of crystallographic orientation of a cathode/electrolyte interface and diffusion anisotropy, the different orientation films were obtained by deposition on single-crystal substrates of different orientations. SrTiO3 (STO) substrates with 111, 110, and 100 surfaces were used to induce 001, 110, and 104 out-of-plane orientation of LCO, respectively. In the course of this study it was realized that a layer of highly conductive SrRuO3 (SRO) between LCO and STO is essential to (a) remove a rectifying Schottky barrier between LCO and STO, (b) act as high-conductivity current collector, and (c) preserve the intended orientation of LCO films. Both SRO and LCO films were deposited sequentially by pulsed laser deposition (PLD) at 600 °C temperature of a substrate, 200 mTorr oxygen pressure with a KrF laser (wavelength 248 nm) using repetition rate 0 Hz and the laser energy 100 mJ per pulse.