Mechanical and electrochemical properties of cubic and tetragonal LixLa0.557TiO3 perovskite oxide electrolytes

Solid oxide electrolytes with high Li ion conductivity and mechanical stability are vital for all solid-state lithium ion batteries. The perovskite material Li_xLa_0.557TiO₃ with various initial Li (0.303 ≤ x ≤ 0.370) is synthesized by traditional solid-state reaction. The cubic and tetragonal structures are prepared with fast and slow cooling, respectively. The results reveal that the Li ion conductivity of the cubic structure is higher. In fact, the bulk conductivity of 1.65 × 10^?3 S cm^?1 is obtained at room temperature for x = 0.350. The crystal structure is not affected by the Li₂O quantity. In addition, Young's modulus, hardness, and fracture toughness are determined with indentation method for both structures. The Young's modulus increases with increasing Li₂O. However, hardness and fracture toughness keep a relatively stable value independent of Li₂O quantity.     

Deformation and bending strength of high-performance lead-free piezoceramics

Bending strength of commercially relevant lead-free piezoceramics – 0.935Na_0.5Bi_0.5TiO₃-0.065BaTiO₃ (NBT-6.5BT) with and without acceptor Zn-doping and 0.92(K_0.5Na_0.5)NbO₃-0.02(Bi_0.5Li_0.5)TiO₃-xBaZrO3; x = 0.06, 0.07 (KNN-BZ100x) have been quantified using 4-point bending tests and contrasted to that of commercial lead zirconate titanate (PZT, PIC151). The compressive and tensile strength probed using additional strain gauges under 4-point bending indicate negligible non-linear deformation, in stark contrast to that of commercial PIC151. The bending strength of KNN-BZ100x is about 100 MPa, comparable to PIC151, while that of NBT-6.5BT-based materials is about twice that of PIC151 at ∼160 MPa. These results are rationalized based on the intrinsic characteristics of the lead-free piezoceramics in terms of fracture toughness, coercive stress, and Young's modulus. Weibull statistics indicate a higher fracture probability for KNN-based materials, with NBT-6.5BT-based materials featuring Weibull modulus twice that of KNN-based materials.     

Electrochemical and mechanical stability of LixLa0.557TiO3-δ perovskite electrolyte at various voltages

Perovskite Li_xLa_0.557TiO₃ electrolytes in all-solid-state lithium batteries are operated under a voltage gradient, which potentially induces the electrochemical and mechanical instability. To simulate the properties of Li_xLa_0.557TiO₃ (LLTO) under application relevant conditions, samples are charged (or discharged) to 0.2 V, 3.2 V, 4.0 V, and 4.5 V, respectively. The Li ion conductivity is 9.55 × 10⁻⁵ S/cm at 3.2 V and decreases obviously to 2.54 × 10⁻⁵ S/cm as the voltage increases to 4.5 V, whereas the value of the LLTO-0.2 V is between that of LLTO-3.2 V and LLTO-4.0 V. In terms of mechanical behavior, elastic modulus (E), hardness (H), and fracture toughness (K_IC) of LLTO operated at different voltages are also tested using depth-sensitive indentation. The results can be used in the designing, monitoring and also improving of the battery cells.     

Influence of heat-treatment protocols on mechanical behavior of lithium silicate dental ceramics

In this work, three different commercial lithium silicate (LS) glass-ceramics for computer aided design/computer aided machining systems, CeltraDuo-Dentsply (LS-C), E-MaxCAD-Ivoclar (LS-E), and Suprinity-Vita (LS-S), were comparatively characterized. Following the protocols recommended by the manufacturers, the glass-ceramics were heat-treated under low vacuum and characterized by X-ray diffraction, scanning electron microscopy, hardness, fracture toughness, Young's modulus, and flexural strength. Rietveld refinement indicated that the materials “as-received” present mostly amorphous phase and Li₂SiO₃ as secondary crystalline phase in LS-E and LS-S specimens, while LS-C specimens also present Li₂Si₂O₅ and Li₃PO₄ as crystalline phases. All “as-received” glass-ceramics present hardness, fracture toughness, and Young's modulus of around 647-678 HV, 1.15-1.40 MPa.m^1/2, and 82-92 GPa, respectively. After heat treatment, the LS-C and LS-S specimens presented decreasing of amorphous phase associated to Li₂SiO₃ and Li₂Si₂O₅ grains with low aspect ratio, while LS-E indicates a reduction of amorphous phase and Li₂Si₂O₅ elongated grains. Fracture toughness and Young's modulus increase about 10% due to the crystallization of residual amorphous phase for all materials. Moreover, crystallographic and microstructural characteristics are responsible for the higher flexural strength of LS-E (327 MPa), regarding LS-C and LS-S. However, the glass-ceramics LS-E present lower Weibull modulus (m = 5.4) comparatively to LS-C (m = 9) and LS-S (m = 6).     

Structure,stability, and ionic conductivity of perovskite Li2x-ySr1-x-yLayTiO3 solid electrolytes

Solid electrolytes with high lithium-ion conductivity and superior stability are key components in the development of all-solid-state lithium-ion batteries. In this study, novel quaternary solid electrolytes Li_2x-ySr_1-x-yLa_yTiO₃ (x = 3y/4, y = 1/7, 2/7, 3/7, 1/2, 15/28, and 4/7) were synthesized by conventional solid-state reaction approach. X-ray diffraction analysis revealed that with the increase in La³⁺ content, Li_2x−ySr_1−x−yLa_yTiO₃ structure changes from cubic to tetragonal perovskite-type structure. Electrochemical impedance spectroscopy revealed that with the increase in y-value, enhanced conductivity was initial observed, followed by a decrease. Li_15/56Sr_1/16La_15/28TiO₃ electrolyte exhibited optimal total Li-ion conductivity of 4.84 × 10⁻⁴ S cm⁻¹, electronic conductivity of 6.84 × 10⁻¹⁰ S cm⁻¹, and activation energy of 0.29 eV. On the other hand, cyclic voltammetry revealed unstable Li_1/8Sr_1/8La_1/2TiO₃, Li_15/56Sr_1/16La_15/28TiO₃, and Li_2/7La_4/7TiO₃ specimens at voltages of less than ~2 V, indicative of their incompatibility with lithium metal or Li₄Ti₅O₁₂ in all-solid-state batteries. Charge-discharge tests confirmed the utility of electrolytes as solid separators with good performance in semi-solid-state batteries. Overall, these results are beneficial for future research on solid electrolytes and their applications in all-solid-state lithium-ion batteries.     

Evolution of mechanical properties of polypropylene separator in liquid electrolytes for lithium‐ion batteries

Knowledge of the mechanical behaviors of polymeric separators immersed in liquid electrolytes is of great significance for predicting the long‐term performance of lithium batteries with high performance and safety. In terms of tensile tests, heating shrinkage, and dynamic mechanical analysis as well as the essential work of fracture method, the study reported here encompasses a systematic investigation of the mechanical properties of a typical commercial polypropylene separator in mixtures of ethylene carbonate and dimethyl carbonate and lithium hexafluorophosphate (LiPF₆), comparing with the results in ionic liquid (IL) electrolyte composed of lithium tetrafluoroborate (LiBF₄) and 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIBF₄) and dry condition. It has been found that liquid electrolytes have obvious negative effect on the dimensional stability at elevated temperature and mechanical properties, especially on crack resistance of the polymer separator. LiBF₄‐BMIBF₄ has much smaller damage on the strength, Young's modulus and fracture toughness of separator than the organic solution except the dynamic modulus at high temperature. Notably, the maximum tensile stress, Young's modulus and the reciprocal of relaxation time of the polymer separator are linearly dependent with strain rate under quasi‐static condition, and the relaxation time has clarified the coupling effect mechanism of liquid electrolyte and loading rate. Moreover, the non‐dimensional viscoelastic constitute equation could perfectly track the tensile behavior of wet and dry separators at different strain rate, and a property model could well characterize the temperature‐dependent storage modulus of polymer separators from rubbery to viscous state. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46441.     

Empirical Evidence for A‐Site Order in Perovskites

Models for composition–structure relationships are useful in both the lab and industry, yet few exist for perovskites‐containing extrinsic defects or cation ordering. In this work, an empirical model is used to predict the existence of A‐site cation ordering. Specifically, four compositions in the Na_(1?3x)/2La_(1+x)/2TiO₃ system (x = 0.0, 0.0533, 0.1733 and 0.225) were synthesized using a conventional solid‐state mixed‐oxide method. The structure of the x = 0 end‐member (Na_0.5La_0.5TiO₃) has been reported in various space groups, but always with a random distribution of Na⁺ and La³⁺ on the A site; however, empirical modeling suggests that it is not only ordered but also that a small volume increase accompanies the ordering process. While no evidence of long‐range A‐site ordering is observed in this composition via X‐ray or neutron diffraction, electron‐diffraction data indicate short‐range ordering of Na⁺ and La³⁺ ions, with the degree of cation ordering decreasing (but the scale of ordered domains and degree of vacancy ordering generally increasing) with increasing x. First‐principles calculations via density functional theory support both conclusions that short‐range ordering in Na_0.5La_0.5TiO₃ is stable and that it results in a volume increase with respect to the disordered analog. A similar analysis has been conducted for the Li_(1?3x)/2La_(1+x)/2TiO₃ and Na_(1?3x)/2La_(1+x)/2(Mg_0.5W_0.5)O₃ solid solutions. These systems provide additional validation of the accuracy and versatility of the empirical modeling method used.     

Preparation of transparent LLZTO electrolyte and its application in the observation of Li dendrite

The application of solid-state electrolytes endows the distinctive features of high safety and high energy density for lithium (Li) metal batteries. Among all kinds of solid-state electrolytes, garnet-type Li₇La₃Zr₂O₁₂ (LLZO) with high Young's modulus is a promising candidate. Nevertheless, Li metal can still grow inside LLZO and lead to short circuit. But it is difficult to determine the failure mechanism caused by Li dendrite due to the opacity of the common LLZO. Herein, a transparent Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) electrolyte is fabricated for the observation of Li growth inside the electrolyte directly. The factor that influences the transparency of LLZTO is investigated thoroughly, which is crucial for studying the behavior of Li metal during the plating process inside LLZTO. Results demonstrate that the amount of residual Li-containing compound at grain boundaries is the dominant factor influencing the transparency of LLZTO. The formation of Li filament and its growth process inside transparent LLZTO are also observed directly.     

Thermodynamical evaluation,microstructural characterization and mechanical properties of B4C–TiB2 nanocomposite produced by in-situ reaction of Nano-TiO2

This work discusses the pressureless sintering of a boron carbide-titanium diboride (B₄C– TiB₂) nanocomposite via in-situ reaction of the boron carbide/titanium dioxide/carbon system. Attempting to sinter pure boron carbide leads to poor mechanical properties. In this work, the effect of adding TiO₂ to B₄C on mechanical properties of the boron carbide was investigated. Thermodynamic simulations were performed with HSC chemistry software to determine the phases which were most likely to form during the sintering process. The reaction thermodynamics suggested that during the sintering process, formation of TiB₂ occurs preferentially over formation of TiC. For examination of the microstructural evolution of the samples, Scanning Electron Microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized. The density, porosity, Young's modulus, microhardness and fracture toughness of the specimens were compared. Optimum properties were achieved by adding 10 wt% TiO₂. In the sample possessing 10 wt% TiO₂, the relative density, Young's modulus, hardness and fracture toughness were 94.26%, 428 GPa, 23.04 GPa and 5.19 MPa m^0.5, respectively, and the porosity was decreased to 5.73%. Furthermore, phase analysis via XRD confirmed that the final product was free of unreacted TiO₂ or carbon.     

Electrochemical behavior of anatase TiO2 in aqueous lithium hydroxide electrolyte

The electrochemical behavior of titanium dioxide (TiO₂) in aqueous lithium hydroxide (LiOH) electrolyte has been investigated. Cyclic voltammetry shows that electroreduction results in the formation of a number of products. X-ray diffraction of the electroreduced TiO₂ shows that Li _x TiO₂, Ti₂O₃, Ti₂O and TiO are formed. The formation of Li _x TiO₂ is confirmed through X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies of the electroreduced TiO₂. The formation of Li _x TiO₂ is electro reversible. In this respect, the electrochemical behavior of TiO₂ in concentrated aqueous lithium hydroxide electrolyte is similar to that for lithium perchlorate (LiClO₄) non-aqueous media.     

Lithium Ion Diffusion Mechanism in Lithium Lanthanum Titanate Solid‐State Electrolytes from Atomistic Simulations

Perovskite‐structured lithium lanthanum titanate (LLT, La_2/3–xLi_3xTiO_3, 0 <  x < 0.16) is a promising solid electrolyte with high lithium ion conductivity and a good model system to understand lithium ion diffusion behaviors in solids. Molecular dynamics (MD) and related atomistic computer simulations were used to study the diffusion behavior and diffusion mechanism as a function of composition in LLT solid‐state electrolytes. The effect of defect concentration on the structure and lithium ion diffusion behaviors in LLT was systematically studied using MD simulations and molecular static calculations with the goal to obtain fundamental understanding of the diffusion mechanism of lithium ions in these materials. The simulation results show that there exists an optimal vacancy concentration at around x = 0.067 at which lithium ions have the highest diffusion coefficient and the lowest diffusion energy barrier. The lowest energy barrier from dynamics simulations was found to be around 0.22 eV, which compared favorably with 0.19 eV from static nudged elastic band calculations. It was also found that lithium ions diffuse through bottleneck structures made of oxygen ions, which expand in dimension by 8%–10% when lithium ions pass through. By designing perovskite structures with larger bottleneck sizes can be a means of further improving lithium ion conductivities in these materials.     

Effect of TiO2 on crystallization and mechanical properties of MgO–Al2O3–SiO2 glasses containing P2O5

MgO–Al₂O₃–SiO₂ glass-ceramics have been widely used in military, industrial, and construction applications. The nucleating agent is one of the most important factors in the production of glass-ceramics as it can control the crystallization temperature or the grain size. In this study, we investigated the effect of replacing P₂O₅ with different amounts of TiO₂ on the crystallization, structure, and mechanical properties of an MgO–Al₂O₃–SiO₂ system. The crystallization and microstructure were investigated by differential scanning calorimetry, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The mechanical properties were investigated by measuring the Vickers hardness, Young's modulus, and fracture toughness. The results showed that adding TiO₂ favored the precipitation of fine grains and significantly increased the Vickers hardness, Young's modulus, and fracture toughness of the glasses. Introducing an appropriate amount of TiO₂ can make a glass structure more compact, promote crystallization, and improve the mechanical properties of MgO–Al₂O₃–SiO₂ glass-ceramics.     

Influence of porosity on the mechanical behaviour of single phase β-TCP ceramics

Processing and mechanical behaviour of fine grained (diameter ≈0.5–3 µm) and pure β-TCP materials with different levels of porosity (up to 19%) is described. Pores with diameters, d₅₀ ≈13–14 µm were formed fromcorn starch during sintering. Comprehensive mechanical characterisation –Young´s modulus, strength and toughness– has been done paying special attention to toughness determined in stable fracture tests. The dependence of Young´s modulus and strength with porosity was well fitted to the minimum solid area models while toughness values did not. The competitive processes occurring during fracture impede the degradation of toughness associated to the decrease in Young´s modulus as porosity increases. Materials present similar values of the critical energy release rate G_IC, which describes crack initiation. A maximum of the specific fracture energy, G_F, which averages crack propagation, has been obtained for the material with the highest porosity.     

Oxide Electrolytes for Lithium Batteries

As the most promising candidate of the solid electrolyte materials for future lithium batteries, oxide electrolytes with high–lithium‐ion conductivity have experienced a rapid development in the past few decades. Existing oxide electrolytes are divided into two groups, i.e., crystalline group including NASICON, perovskite, garnet, and some newly developing structures, and amorphous/glass group including Li₂O–MO_x (M = Si, B, P, etc.) and LiPON‐related materials. After a historical perspective on the general development of oxide electrolytes, we try to give a comprehensive review on the oxide electrolytes with high–lithium‐ion conductivity, with special emphasis on the aspect of materials selection and design for applications as solid electrolytes in lithium batteries. Some successful examples and meaningful attempts on the incorporation of oxide electrolytes in lithium batteries are also presented. In the conclusion part, an outlook for the future direction of oxide electrolytes development is given.     

Optimization of Lithium Content and Sintering Aid for Maximized Li+ Conductivity and Density in Ta‐Doped Li7La3Zr2O12

Ta‐doped cubic phase Li₇La₃Zr₂O₁₂ (LLZ) lithium garnet received considerable attention in recent times as prospective electrolyte for all‐solid‐state lithium battery. Although the conductivity has been improved by stabilizing the cubic phase with the Ta⁵⁺ doping for Zr⁴⁺ in LLZ, the density of the pellet was found to be relatively poor with large amount of pores. In addition to the high Li⁺ conductivity, density is also an essential parameter for the successful application of LLZ as solid electrolyte membrane in all‐solid‐state lithium battery. Systematic investigations carried out through this work indicated that the optimal Li concentration of 6.4 (i.e., Li_6.4La₃Zr_1.4Ta_0.6O₁₂) is required to obtain phase pure, relatively dense and high Li⁺ conductive cubic phase in Li_7?xLa₃Zr_2?xTa_xO₁₂ solid solutions. Effort has been also made in this work to enhance the density and Li⁺ conductivity of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ further through the Li₄SiO₄ addition. A maximized room‐temperature (33°C) total (bulk + grain boundary) Li⁺ conductivity of 3.7 × 10^?4 S/cm and maximized relative density of 94% was observed for Li_6.4La₃Zr_1.4Ta_0.6O₁₂ added with 1 wt% of Li₄SiO₄.     

The synergistic effect of dual substitution of Al and Sb on structure and ionic conductivity of Li7La3Zr2O12 ceramic

Li₇La₃Zr₂O₁₂ (LLZO) with cubic garnet type structure is a promising solid electrolyte. In this work, Li_6.925-3xAl_xLa₃Zr_1.925Sb_0.075O₁₂ (0 ≤ x ≤ 0.1) electrolytes were prepared by conventional solid-state reaction. The influence of Sb-Al cosubstitution on the structure, microstructure and conductivity of Li₇La₃Zr₂O₁₂ were investigated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and impedance spectroscopy. Single cubic phase has been achieved for Li_6.925-3xAl_xLa₃Zr_1.925Sb_0.075O₁₂ (x = 0–0.075). Suitable amount of Al-Sb cosubstitution accelerates densification and improves the ionic conductivity. Li_6.775Al_0.05La₃Zr_1.925Sb_0.075O₁₂ exhibits highest relative densities (96.7%) and total ionic conductivity (4.10 × 10^?4 S/cm at 30 °C).     

Elasticity,hardness, and fracture toughness of sodium aluminoborosilicate glasses

Due to an increasing demand for oxide glasses with a better mechanical performance, there is a need to improve our understanding of the composition-structure-mechanical property relations in these brittle materials. At present, some properties such as Young's modulus can to a large extent be predicted based on the chemical composition, while others—in particular fracture-related properties—are typically optimized based on a trial-and-error approach. In this work, we study the mechanical properties of a series of 20 glasses in the quartenary Na₂O–Al₂O₃–B₂O₃–SiO₂ system with fixed soda content, thus accessing different structural domains. Ultrasonic echography is used to determine the elastic moduli and Poisson's ratio, while Vickers indentation is used to determine hardness. Furthermore, the single-edge precracked beam method is used to estimate the fracture toughness (K_Ic) for some compositions of interest. The compositional evolutions of Vickers hardness and Young's modulus are in good agreement with those predicted from models based on bond constraint density and strength. Although there is a larger deviation, the overall compositional trend in K_Ic can also be predicted by a model based on the strength of the bonds assumed to be involved in the fracture process.     

Effect of sintering temperature on microstructure and transport properties of Li3xLa2/3−xTiO3 with different lithium contents

Li_3xLa_2/3−xTiO₃ (LLTO) powder with different lithium contents (nominal 3x = 0.03–0.75) was synthesized via a simple sol–gel route and then calcination of gel-derived precursor at 900 °C which was much below the calcination temperature required for synthesizing the LLTO powder via solid state reaction route. The LLTO powder of sub-micron sized particles, derived from such sol–gel method, showed almost no aggregation. Starting from the sol–gel-derived powder, the LLTO ceramics with different lithium contents were prepared at different sintering temperatures of 1250 and 1350 °C. It demonstrated that our sol–gel route is quite simple and convenient compared to the previous sol–gel method and requires lower temperature for the LLTO. Our results also illustrated that lithium content significantly affects the structure and ionic conductivity of the LLTO ceramics. The dependence of the ionic conductivity on the lithium content, lattice structure, microstructure and sintering temperature was investigated systematically.     

Temperature Compensating Microwave Dielectric Based on the (Mg0.95Ni0.05)TiO3–(La0.5Na0.5)TiO3 Ceramic System

The microstructures and microwave dielectric properties of the novel temperature-compensated composite ceramic (1−x)(Mg_0.95Ni_0.05)TiO₃–x(La_0.5Na_0.5)TiO₃ were investigated. Dense and mixed phase samples were produced in the range of 0.05≤x≤0.9. When the x value increased, the quality factor (Q×f) decreased nonlinearly while the dielectric constant (ɛ_r) steadily increased almost linearly due to the compensation effect. The temperature coefficient of resonant frequency (τ_f) was remarkably improved with an increase of (La_0.5Na_0.5)TiO₃ content. With x=0.13, a dielectric constant (ɛ_r) of 23.22, a Q×f value of 86,500 GHz, and a τ_f value of 2.8 ppm/°C was obtained for 0.87(Mg_0.95Ni_0.05)TiO₃–0.13(La_0.5Na_0.5)TiO₃ ceramics sintered at 1275°C for 4 h. Moreover, a band-pass filter with traditional hairpin resonators with a center frequency of 2.4 GHz was designed and fabricated using the proposed dielectric ceramic to study its performance.     

Properties of ion-conducting lanthanum lithium titanate based ceramics

Samples of lanthanum lithium titanate with the general formula La_{2/3 − x} Li_3x TiO₃ with a different vacancy content, which is varied by varying the ratio of lanthanum to lithium, are investigated. Methods of synthesis, neutron diffraction data for the samples obtained, measurements of the lithium-ion conductivity of equilibrium and quenched samples, the results of nuclear magnetic resonance on ⁷Li nuclei, the dynamic characteristics of heat release (heat absorption), and the magnetic susceptibility of some samples are presented. __________ Translated from Steklo i Keramika, No. 4, pp. 16–20, April, 2007.