Saya Takeuchi

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.

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.

Editors' Choice Communication—Comparison of Nanoscale Focused Ion Beam and Electrochemical Lithiation in β-Sn Microspheres

The development of Li focused ion beams (Li-FIB) enables controlled Li ion insertion into materials with nanoscale resolution. We take the first step toward establishing the relevance of the Li-FIB for studies of ion dynamics in electrochemically active materials by comparing FIB lithiation with conventional electrochemical lithiation of isolated β-Sn microspheres. Samples are characterized by cross-sectioning with Ga FIB and imaging via electron microscopy. The Li-FIB and electrochemical lithiated Sn exhibit similarities that suggest that the Li-FIB can be a powerful tool for exploring dynamical Li ion-material interactions at the nanoscale in a range of battery materials.

Local degradation pathways in lithium-rich manganese–nickel–cobalt oxide epitaxial thin films

The electrochemical performance and microstructure of positive electrodes are intimately linked. As such, developing batteries resistance to capacity and voltage fade requires understanding these underlying structure–property relationships and their evolution with operation. Epitaxial films of a Li-rich manganese–nickel–cobalt oxide cathode material were deposited on (100)- and (111)-oriented SrRuO3/SrTiO3 substrates. Cyclic voltammetry and impedance spectroscopy tracked the response of these positive electrode materials, while the microstructure of the pristine and cycled films was characterized using transmission electron microscopy. Energy-dispersive X-ray spectroscopy identifies compositional fluctuations in as-deposited films. Phase transformations and dissolution were observed after electrochemical testing. There is a correlation between both local composition and substrate orientation (i.e., surface faceting) and what degradation pathways are active. Regions with comparatively higher concentrations of Ni and Co were more resistant to dissolution and unfavorable phase transformations than those with relatively more Mn. As such, a global composition metric may not be an accurate predictor of degradation and performance. Rather possessing the synthetic ability to engineer the chemical profile as well as characterizing it, pose a challenge and opportunity.

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.

Structural Characterization of Powders and Thin Films of Layered Li1.2Mn0.55Ni0.15Co0.1O2 Cathode Materials

Layered, lithium and manganese rich transition metal oxides are becoming the next generation high energy cathode material for lithium ion batteries with superior performance including high capacity (>200 mAh/g), low cost, and better thermal stability. A thorough atomic-level structural understanding of these materials is needed to understand and improve their electrochemical properties. To this end, we have studied a layer cathode material with a composition of Li1.2Mn0.55Ni0.15Co0.1O2 (HE5050, TODA Inc.) [1]. A commercially available powder can provide a common platform for study in a field where differences of compositions and processing methods can make comparisons between studies difficult. In addition to the powders, pulse laser deposition (PLD) was used to grow epitaxial thin films on singlecrystal SrTiO3 substrates. The targets for growth were made from the HE5050 powder.