TiO3 Solid Electrolyte Research Articles

The influence of crystal orientation on the change in Li chemical potential of LiCoO2 under mechanical stress

Change of the lithium chemical potential in a crystalline LiCoO2 with the application of a mechanical stress was discussed based on the results of electromotive force (emf) measurements between preferentially-oriented LiCoO2 films on Li0.33La0.55TiO3 solid electrolytes under mechanical stress. [00l]-oriented LiCoO2 films with different degrees of orientation were prepared, and it was investigated how the direction of mechanical stress to the crystal lattice affects emf. It is found that emf emerged when mechanical stress was applied. The magnitude of emf under stress was dependent not only on the magnitude of applied stress but also the degree of the film orientation. This clearly demonstrated that the crystal orientation, i.e. the direction of mechanical stress to the crystal, affects the lithium chemical potential.

Comparative study of solid-state reaction and sol-gel process for synthesis of Zr-doped Li0.5La0.5TiO3 solid electrolytes

We investigated the synergistic influences of synthesis methods (solid-state reaction vs. sol-gel process) and Zr4+ doping on the structure and ionic conductivity of perovskite-structured Li0.5La0.5TiO3 (LLTO) solid electrolytes. The lithium-ion conductivity of Li0.5La0.5Ti1 − x Zr x O3 ceramic specimens was evaluated as a function of x value and compared carefully between those two synthesis methods. Regarding the conductivity, sol-gel process is better for the synthesis of LLTO than solid-state reaction. As a result, the highest grain conductivity is obtained in the sol-gel-derived pure LLTO sample with x = 0, reaching 1.10 × 10−3 S · cm−1. Partial substitution of Zr4+ enlarges the LLTO’s grain aggregate size and increases the total superficial area of aggregates. Consequently, Zr4+ substitution not only affects the grain (bulk) conductivity, but more importantly, also improves the grain boundary conductivity and the total conductivity. The highest total conductivity is 5.84 × 10−5 S · cm−1 with x = 0.04 by sol-gel process.

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 (E a) 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.

Lithium ion conductivity of oriented Li0.33La0.56TiO3 solid electrolyte films prepared by a sol–gel process

Lithium lanthanum titanates (Li3xLa2/3−xTiO3, LLTOs) having a-axis orientations were synthesized via a simple sol–gel route on SrTiO3 and Nb-doped SrTiO3 substrates. The lithium ion conduction behaviors of the LLTO films were determined. Annealing the films above 1200°C yielded strong a-axis growth orientation, while heating samples above 1300°C led to the reduction of the Li transference number, TLi, which was attributed to Li evaporation. The optimized annealing temperature was 1200°C, where both the bulk conductivity, σdc, and the TLi exhibited their maxima. The bulk conductivity was 4.42×10−5S/cm for a 411-nm-thick film, which was almost 10-times-higher than that previously reported for sol–gel-derived films. A key reason for the enhancement in conductivity is the expanded bottleneck of the LLTO lattice because of a tensile stress induced by the substrate. Optimization of the annealing temperature and the film thickness further improved the conductivity.

Effects of synthesis conditions on the structural, stability and ion conducting properties of Li0.30(La0.50Ln0.50)0.567TiO3 (Ln=La, Pr, Nd) solid electrolytes for rechargeable lithium batteries

The structure, thermal stability, morphology and ion conductivity of titanium perovskites with the general formula Li3xLn2/3−xTiO3 (Ln=rare earth element; 3x=0.30) are studied in the context of their possible use as solid electrolyte materials for lithium ion batteries. Materials are prepared by a glycine–nitrate method using different sintering treatments, with a cation-disorder-induced structural transition from tetragonal to cubic symmetry, and are detected as quenching temperature increases. SEM images show that the average grain size increases with increasing sintering temperature and time. Slightly higher bulk conductivity values have been observed for quenched samples sintered at high temperature. Bulk conductivity decreases with the lanthanide ion size. A slight conductivity enhancement, always limited by grain boundaries, is observed for longer sintering times. TDX measurements of the electrolyte/cathode mixtures also show a good stability of the electrolytes in the temperature range of 30–1100°C.

A facile method for the synthesis of the Li(0.3)La(0.57)TiO₃ solid state electrolyte.

We report a facile method for the synthesis of Li(0.3)La(0.57)TiO3 by forming a coagulated precursor solution which contains Li(+), La(3+), and TiO2 nanoparticles mixed highly homogeneously. The grain and overall conductivities of the synthesized Li(0.3)La(0.57)TiO3 are comparable to the values in literature for the material prepared by other methods.

Hetero-epitaxial growth of Li0.17La0.61TiO3 solid electrolyte on LiMn2O4 electrode for all solid-state batteries

A Li0.17La0.61TiO3(111)/LiMn2O4(111) solid electrolyte/electrode hetero-epitaxial system with applications in all solid-state batteries has been synthesized on a SrRuO3(111)/SrTiO3(111) substrate using pulsed laser deposition. XRD patterns provided evidence for the epitaxial growth of Li0.17La0.61TiO3(111) on LiMn2O4 at a low deposition temperature of 750 °C. X-ray reflectivity analysis confirmed that the low temperature deposition resulted in intimate contact between LiMn2O4 and Li0.17La0.61TiO3, although small amounts of oxygen and titanium were found to have diffused between the materials. The Li0.17La0.61TiO3/LiMn2O4 film exhibited reversible electrochemical activity between 3.0 and 4.5 V. No significant structural changes were observed in the cycled film, indicating that the Li0.17La0.61TiO3/LiMn2O4 interface is chemically and electrochemically stable during the battery reaction. The integration of electrode and solid electrolyte oxides in epitaxial multi-layer structures is expected to allow the design of new configurations for all solid-state batteries.

Effect of the valence states of titanium on the lattice structure and ionic conductivity of Li0.33La0.55TiO3 solid electrolyte

Li0.33La0.55TiO3 solid electrolytes with a pure phase were synthesized by the citric acid-supported sol-gel method and then sintered under controlled redox atmospheres of air, argon and hydrogen. Although of similar morphology and relative density, Li0.33La0.55TiO3 samples sintered under reduction atmosphere such as argon and hydrogen exhibited a tetragonal structure with lattice distortion. The distortion and volume expansion of the crystal lattice was identified as originating from the transformation of the Ti valence state, and this was more clearly observed under sintering of the hydrogen atmosphere. The qualitative analysis of the Ti valence state using an X-ray Photoelectron Spectroscopy experiment was performed and indicated that the largest amount of Ti3+(18.6%) was formed in the Li0.33La0.55TiO3 samples sintered under hydrogen atmosphere. The relationship between the lattice distortion and the lithium ion conductivity of the Li0.33La0.55TiO3 sintered under different atmospheres is also discussed on the basis of the lattice distortion.

Synthesis and orientation control of Li-ion conducting epitaxial Li0.33La0.56TiO3 solid electrolyte thin films by pulsed laser deposition

Epitaxial thin films of the Li-ion conducting solid electrolyte Li0.33La0.56TiO3 were grown by pulsed laser deposition. The growth temperature and oxygen partial pressure required for crystallization were higher than 900°C and 5Pa, respectively. Further optimization of growth conditions led to the formation and orientation control of Li-ion conducting epitaxial Li0.33La0.56TiO3 thin films.

Lithium-ion conducting La2/3−xLi3xTiO3 solid electrolyte thin films with stepped and terraced surfaces

La2/3−xLi3xTiO3 (LLT, x = 0.1 and 0.167) thin films were grown on (001)-oriented SrTiO3 (STO) substrates by pulsed laser deposition. The resultant LLT films were heteroepitaxially grown with atomically flat surfaces. Although the films considered to be single crystalline, they were composed of 90°-rotated multi domains (∼5 nm in size) with epitaxial relationships of (100) [001] LLT || (001) [100] STO and (100) [010] LLT || (001) [100] STO. The bulk conductivity of the LLT (x = 0.1) films was 2.5 × 10−2 S cm−1 at 190 °C, comparable to that of a bulk single crystalline LLT. Single crystalline LLT thin films with atomically flat surfaces should help clarify interfacial lithium-ion conductivity. At the request of the authors, this article is being retracted effective 1 March 2013.

Fabrication of Li0.35La0.55TiO3 solid electrolyte with two-layered structure for all-solid-state Li battery by a colloidal crystal templating method

2-layered all-solid-state battery, which is made up of 3DOM (3dimensionally ordered macroporous) porous layer and dense layer, is one of the promising structures for all-solid-state Li battery with Li metal anode. In this paper, an influence of precursor powder size of solid electrolyte on formation of 2-layered electrolyte which is a framework of all-solid-state battery was studied using Li0.35La0.55TiO3 (LLT) electrolyte. The particle size strongly affected on morphology of prepared 2-layered structure. In the case of mixture of milled powder with below 1 µm diameter and non-milled powder with sub µm in weight ratio = 3:1, completely sintered 2-layered LLT could be obtained. This would be due to high bulk density of the mixture. Li ion conductivities of obtained 2-layered LLT at bulk and grain boundary were 8.0 × 10−4 and 2.6 × 10−5 S cm−1, respectively, which were well consistent with reported values. These findings imply the 2-layered structure can be prepared by many kinds of ceramics if size of precursor powder would be controlled properly.

Mechanism of the interfacial reaction between cation-deficient La0.56Li0.33TiO3 and metallic lithium at room temperature

We used x-ray diffractometry (XRD), x-ray photoelectron spectrometry (XPS), and secondary-ion mass spectrometry (SIMS) to investigate the mechanism of the interfacial room-temperature (RT) chemical reaction between cation-deficient La0.56Li0.33TiO3 solid electrolytes and metallic lithium anodes in all-solid-state lithium batteries. A stoichiometric mixture of La2O3, Li2CO3, and TiO2 powders was calcined at 1250 °C for 8 h to obtain a single perovskite structure of La0.56Li0.33TiO3. When this La0.56Li0.33TiO3 sample and lithium were placed in contact at room temperature for 24 h, the phase of the La0.56Li0.33TiO3 remained unchanged. The XPS results indicate that 12% of the tetravalent Ti4+ ions were converted into trivalent Ti3+ ions. The valence conversion and degree of conversion were limited by the structural rigidity of the host crystal. Our SIMS analysis suggests the existence of a local electric field near the contact surface and indicates that the 6Li+ isotope ions were inserted into the specimen through the effect of this field. The change in the electrical properties of La0.56Li0.33TiO3 supports this mechanism for the interfacial reaction. The ionic conductivities of the grain and total grain boundary decreased and increased, respectively, after the insertion of Li+, and the total electronic conductivity increased as a result of the presence of intervalence electron hopping between mixed Ti3+/Ti4+ states. The mechanism of the lithium-activated RT interfacial reaction is associated with the reduction of Ti4+ transition metal ions from tetravalent to trivalent states and the local-electric-field-induced Li+ insertion into La3+/Li+-site vacancies of La0.56Li0.33TiO3.

Tetragonal superstructure and thermal history of Li0.3La0.567TiO3 (LLTO) solid electrolyte by neutron diffraction

High-resolution time-of-flight neutron powder diffraction data (ISIS facility, Rutherford Appleton Laboratory, U. K.) were collected on sample I, prepared by slow cooling from 1300 °C, of the Li0.3La0.567TiO3 (LLTO) lithium ion conductor, and on sample II quenched in liquid N2. Patterns were obtained at room temperature (RT), 400 and 700 °C on sample I, and at 550 °C on sample II, studied at RT in previous work. The structure was resolved and Rietveld refined in the tetragonal P4/nbm space group with √2ap × √2ap × 2ap unit-cell [a = 5.48162(1), c = 7.74646(1) Å at RT, Z = 4; ap is the cell edge of cubic perovskite] in all cases except 700 °C, where a P4/mmm (ap × ap × 2ap) structure was determined and refined [a = 3.89827(2), c = 7.8002(1) Å, Z = 2]. In both space groups lanthanum ordering over two La1 and La2 sites is present, but only in P4/nbm can an additional anti-phase octahedral tilting of type a0a0c− be observed, similar to what occurs in the I4/mcm RT metastable phase of the quenched sample. The two distortions are associated to hkl superlattice reflections with h + k even, l odd, and h + k odd, l odd, respectively. Lithium atoms were located in highly disordered sites within the less populated La1 hollows, with short Li–Li′ distances accounting for the high ionic mobility. Possible ordering schemes indicate a mainly (001) two-dimensional mechanism of ion hopping among Li sites.

Effect of Li0.5La0.5TiO3 solid electrolyte films on electrochemical properties of LiCoO2 thin film cathodes with different rapid-thermal annealing conditions

The structural and electrochemical properties of LiCoO2 (LCO) cathode films grown at 500°C and Li0.5La0.5TiO3 (LLTO) solid electrolyte films at room temperature by pulsed laser deposition were investigated by a rapid-thermal annealing (RTA) treatment. The LCO films annealed for 1min at 700°C showed the most superior crystallinity and the LLTO films grown on LCO at room temperature maintained an amorphous phase after RTA treatment for 5min up to 700°C. The highest initial discharge capacity and capacity retention were obtained in Li∕LiCoO2 cells annealed for 1min at 700°C. The RTA treatment is effective in improvement of an interface property and surface morphology of the LLTO films grown at room temperature on LCO. The LCO/Pt cells exhibit an initial discharge capacity of approximately 53μAhcm−2μm and a capacity retention of 73% at 100cycles. The LLTO/LCO/Pt cells treated at 700°C for 5min in an oxygen ambient by RTA exhibited an improvement of a capacity retention as high as 96.1% at 100cycles.