Ultra-fast high temperature sintering (UHS) of Li1.5Al0.5Ge1.5P3O12 electrolyte: A rationalization of the heating schedule

The renewed interest on Fast Firing has been motivated by the suppressed Li, Na volatilization during the short sintering cycle (a few seconds). Ultra-fast high temperature sintering represents the latest development in Fast Firing with heating rates approaching 10⁴ °C/min. At present, there is no clear indication on how heating rate should be modulated to target a certain physical property. In this work, we compared different heating schedules to consolidate Li_1.5Al_0.5Ge_1.5P₃O₁₂ electrolyte. Ultra-fast high temperature sintering carried under a single step current mode promoted the formation of radial cracks around the edge of the sample as a result of the differential shrinkage resulting from the inward heat fluxes. Multistep processing, based on the hybrid combination of conventional sintering (pre-sintering step) followed by UHS resulted in crack free samples. Under a discharge time as short as 160 s, hybrid UHS resulted in a relative density as high as 94.62% and conductivity of 1.72 × 10^?4 S/cm, surpassing conventional sintered counterparts.     

Cold sintering process of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte

The recently developed technique of cold sintering process (CSP) enables densification of ceramics at low temperatures, i.e., <300°C. CSP employs a transient aqueous solvent to enable liquid phase‐assisted densification through mediating the dissolution‐precipitation process under a uniaxial applied pressure. Using CSP in this study, 80% dense Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) electrolytes were obtained at 120°C in 20 minutes. After a 5 minute belt furnace treatment at 650°C, 50°C above the crystallization onset, Li‐ion conductivity was 5.4 × 10^?5 S/cm at 25°C. Another route to high ionic conductivities ~10^?4 S/cm at 25°C is through a composite LAGP ‐ (PVDF‐HFP) co‐sintered system that was soaked in a liquid electrolyte. After soaking 95, 90, 80, 70, and 60 vol% LAGP in 1 M LiPF₆ EC‐DMC (50:50 vol%) at 25°C, Li‐ion conductivities were 1.0 × 10^?4 S/cm at 25°C with 5 to 10 wt% liquid electrolyte. This paper focuses on the microstructural development and impedance contributions within solid electrolytes processed by (i) Crystallization of bulk glasses, (ii) CSP of ceramics, and (iii) CSP of ceramic‐polymer composites. CSP may offer a new route to enable multilayer battery technology by avoiding the detrimental effects of high temperature heat treatments.     

Microstructure and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte prepared by spark plasma sintering

In this paper, the microstructure and ionic conductivity of Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) solid electrolytes prepared by spark plasma sintering (SPS) were investigated by XRD, SEM, TEM and EIS, respectively. The results showed that as the sintering temperature was increased, both the relative density and the ionic conductivity of the sintered LAGP samples first increased and then decreased, achieving a maximum value of 97% and 2.12 × 10⁻⁴ S cm⁻¹ simultaneously at 700 °C. At the same time, the crystallinity of the sintered samples was improved, while a few impurity phases, such as AlPO₄ and GeO₂, appeared in the samples. It was also found that carbon contamination and oxycarbide gas was be brought in during SPS. Carbon contamination could produce an extra grain boundary impedance to the samples and could be removed by annealing at 500 °C in an air atmosphere. Oxycarbide gas could affect the relative density of the sintered LAGP samples and could be mitigated by choosing a suitable SPS process. Moreover, the shear modulus of the sintered LAGP was measured to be 49.6 GPa, which exceeded the minimum value of 8.5 GPa that was necessary to suppress Li dendrite growth.     

Preparation and ionic conduction of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte using inorganic germanium as precursor

NASION-type Li_1.5Al_0.5Ge_1.5(PO₄)₃(LAGP) is prepared by a novel sol-gel method using low cost inorganic germanium (GeO₂) as the precursor. The composition and phase transformation during the heating of the LAGP precursors are analyzed using thermogravimetric-differential scanning calorimetry (TG-DSC) and X-ray diffraction (XRD). The structures and morphologies of the LAGP are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the LAGP annealed at 900 ℃ is partially crystallized and consists of a large number of nanoscale grains surrounded by amorphous regions. The LAGP particles present an irregular morphology with a large size distribution over a range of 0.2–1 μm. In addition, ionic conductivities of the prepared LAGP first increase and then decrease with an increase in the sintering temperature and time. A high ionic conductivity (4.18 × 10⁻⁴Scm^-1) with an activation energy of 0.30 eV are obtained for the LAGP sample sintered at 900 °C for 8 h.     

High temperature electrode‐electrolyte interface formation between LiMn1.5Ni0.5O4 and Li1.4Al0.4Ge1.6(PO4)3

All‐solid‐state lithium‐ion electrolytes offer substantial safety benefits compared to flammable liquid organic electrolytes. However, a great challenge in solid electrolyte batteries is forming a stable and ion conducting interface between the electrolyte and active material. This study investigates and characterizes a possible solid‐state electrode‐electrolyte pair for the high voltage active cathode material LiMn_1.5Ni_0.5O₄ (LMNO) and electrolyte Li_1+xAl_xGe_2‐x(PO₄)₃ (LAGP). In situ X‐ray diffraction measurements were taken on pressed pellets comprised of a blend of LMNO and LAGP during exposure to elevated temperatures to determine the product materials that form at the interface of LMNO and LAGP and the temperatures at which they form. In particular, above 600°C a material consistent with LiMnPO₄ was formed. Scanning electron microscopy and energy‐dispersive X‐ray spectroscopy were used to image the morphology and elemental compositions of product materials at the interface, and electrochemical characterization was performed on LMNO‐coated LAGP electrolyte pellet half cells. Although the voltage of Li/LAGP/LMNO assembled batteries was promising, thick interfacial phases resulted in high electrochemical resistance, demonstrating the need for further understanding and control over material processing in the LAGP/LMNO system to reduce interfacial resistance and improve electrochemical performance.     

Structural evolution at short and medium range distances during crystallization of a P2O5-Li2O-Al2O3-SiO2 glass

Li₂O-Al₂O₃-SiO₂ (LAS) glass-ceramics have important industrial applications and bulk nucleation is usually achieved by using nucleating agents. In particular, P₂O₅ is an efficient agent in glasses containing a low level of Al₂O₃ but its role in the first stages of nucleation is not well established. In this study, we combine structural investigations from local to mesoscales to describe the structural evolution during crystallization of LAS glass-ceramics. Local environment is probed using ²⁹Si and ³¹P MAS-NMR, indicating organization of P in poorly crystallized Li₃PO₄ species prior to any crystallization. To better understand the detailed nanoscale changes of the glass structure, ³¹P-³¹P DQ-DRENAR homonuclear correlation experiments have been carried out, revealing the gradual segregation of P atoms associated with the formation of disordered Li₃PO₄. Small-angle neutron scattering data also show the apparition of nanoscale heterogeneities associated with Li₃PO₄ species upon heating treatments and allow the determination of their average sizes. These new structural information enhance our understanding of the role of P in nucleation mechanisms. Nucleation is initiated by gradual change in P environment implying P segregation upon heating treatments, forming disordered Li₃PO₄ heterogeneities. The segregation of P atoms enables the precipitation of meta- and disilicate phases.     

Novel B-site scheelite structure ceramic Bi(Ge0.5Mo0.5)O4 and its dielectric properties

Scheelite structure phase inorganic oxides show their irreplaceable role in numerous application areas due to their clear structure and superior properties, especially in dielectrics. Scheelite structure phase BiVO₄ has been permanently studied but substitutions, modifications, and explorations of novel phases persist hitherto and inspire more interest. In this work, we report a novel Scheelite structure phase of Bi(Ge_0.5Mo_0.5)O₄ and a detailed study of both structural analysis and dielectric properties investigation. Bi(Ge_0.5Mo_0.5)O₄ adopts the monoclinic Scheelite structure, identical to BiVO₄, with a dielectric permittivity of ∼ 35, Qf value of ∼20 000 GHz, and TCF value of ‒46 ppm/°C. No secondary ferroelastic transition was seen in Bi(Ge_0.5Mo_0.5)O₄ till 600°C, close to its synthetical temperature. The results indicate the success of discovering a new Scheelite structure phase and its prior engineering potential in modifying and substituting BiVO₄ over the dielectric area, photocatalyst, ion conductor, and so forth.     

Structure and lithium-ion mobility in Li1.5M0.5Ge1.5(PO4)3 (M = Ga,Sc, Y) NASICON glass-ceramics

This work reports structural and lithium-ion mobility studies in NASICON single- or multiple phase Li_1+xM_xGe_2−x(PO₄)₃ (M = Ga³⁺, Sc³⁺, Y³⁺) glass-ceramics using solid-state NMR techniques, X-ray powder diffraction, and impedance spectroscopy. X-ray powder diffraction data show the successful incorporation of Ga³⁺ and Sc³⁺ into the Ge⁴⁺ octahedral sites of the NASICON structure at the levels of x = 0.5 and 0.4, respectively. The glass-to-crystal transition was further characterized by multinuclear NMR and electrical conductivity measurements. Among the studied samples, the gallium-containing glass-ceramic presented the highest DC conductivity, 1.1 × 10⁻⁴ S/cm at room temperature, whereas for the Sc-containing samples, the maximum room temperature conductivity that could be reached was 4.8 × 10⁻⁶ S/cm. No indications of any substitution of Ge⁴⁺ by Y³⁺ could be found.     

Kinetic Analysis of Crystallization in Li1.3Al0.3Ti1.7(PO4)3 Glass Ceramics

The crystallization mechanisms for Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) glass ceramics were studied using thermophysical property characterization techniques. Differential scanning calorimetry (DSC) revealed two separate exothermic events that were ascribed to the initial growth and growth to coherency of a dendritic phase. It was found that the commonly used Johnson‐Mehl‐Avrami is not a suitable kinetic model for this material. Rather, the Sestak‐Berggren (SB) autocatalytic kinetic model was used to analyze the DSC data and the activation energy for initial growth (259 kJ/mol) and coherency (272 kJ/mol) was calculated using isoconversional methods. The calculated parameters for the SB model were used to compare experimental and calculated values for heat flow during the crystallization of LATP and good fits were found for both exothermic events.     

Orthorhombic‐pseudocubic phase transition and piezoelectric properties of (Na0.5K0.5)(Nb1−xSbx)‐SrZrO3 ceramics

0.96(Na_0.5K_0.5)(Nb_1?xSb_x)‐0.04SrZrO₃ ceramics with 0.0≤x≤0.06 were well sintered at 1060°C for 6 hours without a secondary phase. Orthorhombic‐tetragonal transition temperature (T_O‐T) and Curie temperature (T_C) decreased with the addition of Sb₂O₅. The decrease in T_C was considerable compared to that in T_O‐T, and thus the tetragonal phase zone disappeared when x exceeded 0.03. Therefore, a broad peak for orthorhombic‐pseudocubic transition as opposed to that for orthorhombic‐tetragonal transition appeared at 115°C‐78.2°C for specimens with 0.04≤x≤0.06. An orthorhombic structure was observed for specimens with x≤0.03. However, the polymorphic phase boundary structure containing orthorhombic and pseudocubic structures was formed for the specimens 0.04≤x≤0.06. Furthermore, a specimen with x=0.055 exhibited a large piezoelectric strain constant of 325 pC/N, indicating that the coexistence of orthorhombic and pseudocubic structures could improve the piezoelectric properties of (Na_0.5K_0.5)NbO₃‐based lead‐free piezoelectric ceramics.     

不同热处理方式对Li1.3Al0.3Ti1.7(PO4)3薄膜性质的影响研究

采用快速退火和常规退火两种不同热处理方式制备Li1.3Al0.3Ti1.7(PO4)3薄膜,用X射线衍射、扫描电子显微镜、循环伏安及交流阻抗等技术分析检测薄膜物相、形貌、电化学窗口、离子电导率及电导活化能。结果表明,两种退火方式制备的薄膜均为纯相Li1.3Al0.3Ti1.7(PO4)3,制备的薄膜均匀、无龟裂,但采用快速退火制备的薄膜晶粒比采用常规退火制备的薄膜要小,薄膜更光滑更致密。两种退火方式制备的薄膜电化学窗口都超过了2.4 V,薄膜离子电导率分别为2.7×10-6 S/cm和1.4×10-6 S/cm。采用快速退火制备的Li1.3 Al0.3 Ti1.7(PO4)3薄膜离子电导活化能比采用常规退火制备的薄膜要小。     

Nanostructural characterization of Al(OH)3 formed during the hydration of calcium sulfoaluminate cement

Ye'elimite (Ca₄Al₆SO₁₆) is a main mineral in calcium sulfoaluminate cements. Aluminum hydroxide is one of the products formed by hydration of ye'elimite. To characterize this phase, various aluminum hydroxides were synthesized from their chemical constituents using sol‐gel processing and compared with the aluminum hydroxide formed during the hydration of ye'elimite. The nanostructure of aluminum hydroxide formed during the hydration of ye'elimite was investigated in detail using X‐ray diffraction, thermogravimetric analysis, scanning electron microscopy, inductively coupled plasma optical emission spectroscopy, and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy. No evidence was observed that indicated the existence of amorphous aluminum hydroxide in the hydration of ye'elimite. The pH values, thermogravimetric analysis, particle morphology, and Al‐O coordination indicated that the aluminum hydroxide from the hydration of ye'elimite had a crystal‐like structure. The X‐ray diffraction analysis, particle sizes, and ion activity product showed that the aluminum hydroxide from the hydration of ye'elimite had a microcrystalline structure.     

Structural dependence of crystallization in glasses along the nepheline (NaAlSiO4) ‐ eucryptite (LiAlSiO4) join

Lithium and sodium aluminosilicates are important glass‐forming systems for commercial glass‐ceramics, as well as being important model systems for ion transport in battery studies. In addition, uncontrolled crystallization of LiAlSiO₄ (eucryptite) in high‐Li₂O compositions, analogous to the more well‐known problem of NaAlSiO₄ (nepheline) crystallization, can cause concerns for long‐term chemical durability in nuclear waste glasses. To study the relationships between glass structure and crystallization, nine glasses were synthesized in the Li_xNa_1‐xAlSiO₄ series, from x = 0 to x = 1. Raman spectra, nuclear magnetic resonance (NMR) spectroscopy (Li‐7, Na‐23, Al‐27, Si‐29), and X‐ray diffraction were used to study the quenched and heat‐treated glasses. It was found that different LiAlSiO₄ and NaAlSiO₄ crystal phases crystallize from the glass depending on the Li/Na ratio. Raman and NMR spectra of quenched glasses suggest similar structures regardless of alkali substitution. Li‐7 and Na‐23 NMR spectra of the glass‐ceramics near the endmember compositions show evidence of several differentiable sites distinct from known Li_xNa_1‐xAlSiO₄ crystalline phases, suggesting that these measurements can reveal subtle chemical environment differences in mixed‐alkali systems, similar to what has been observed for zeolites.     

Uranium brannerite with Tb(III)/Dy(III) ions: Phase formation,structures, and crystallizations in glass

Uranium brannerite phases with terbium(III) or dysprosium(III) ions have been investigated. The precursors with molar ratio of 0.5:0.5:2 (Ln: U: Ti with Ln = Tb or Dy) were prepared and calcined at 750°C in argon. Sintering the pelletized samples in argon at 1200°C led to the formation of pyrochlore phases with TiO₂ rutile and U-rich oxides while sintering in air led to the formation of brannerite phases with the nominal composition close to Ln_0.5U_0.5Ti₂O₆ together with trace amounts of TiO₂ rutile and LnUO₄. Incorporating an excess of TiO₂ (20 wt%) and sintering at higher temperature (1300°C) resulted in no obvious change to the phase equilibrium. As designed, pentavalent uranium has been proven to be dominant in these brannerite phases with diffuse reflectance spectroscopy. The relationships between the cell parameters and the ionic radii of the A-site cations have been explored and rationalized from the structure point of view for a range of titanate brannerite phases (ATi₂O₆). In addition, the crystallization of Ln_0.5U_0.5Ti₂O₆ brannerite in glass has been achieved via heat treatment at 1200°C and confirmed with X-ray diffraction, scanning electron microscopy-energy dispersive spectroscopy and transmission electron microscopy–selected area electron diffraction.     

A Novel Sol–Gel Method for Large‐Scale Production of Nanopowders: Preparation of Li1.5Al0.5Ti1.5(PO4)3 as an Example

Sol–gel synthesis is an extensively used method for the preparation of nanopowders. However, complicated or expensive precursors, and the necessity of using organic solvent and/or heat assistance limit the method to laboratory‐scaled level. An aqueous‐based sol–gel method with spontaneous sol and gel formation is developed in this study. It can be applied on a large scale to synthesize compounds with Ti⁴⁺ and PO₄^3? as major components with low cost. Al‐substituted LiTi₂(PO₄)₃ (LATP) has been widely investigated as a promising candidate for solid electrolyte in Li‐ion or Li‐air batteries. Major challenges such as unsatisfactory phase purity, low sintering activity, and high production costs are faced during the fabrication of LATP. In this study, as a sample application, Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP05) is conveniently prepared on a large scale by the novel sol–gel method with high phase purity, active densification behavior and high conductivity.     

Phase evolution and microstructure of new Sr0.5Zr2(PO4)3-NdPO4 composite ceramics prepared by one-step microwave sintering

Sodium Zirconium Phosphate (NaZr₂(PO₄)₃, hereinafter NZP) and monazite are both potential materials for immobilization of nuclear waste. In this work, novel (1-x)Sr_0.5Zr₂(PO₄)₃-xNdPO₄ composite ceramics (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) for simultaneously immobilizing the simulated fission product (FP) Sr and trivalent minor actinide (MA) Nd were prepared by one-step microwave sintering technique, in which Sr and Nd were immobilized into NZP and monazite type structures, respectively. The phase evolution and microstructure of the samples were investigated by X-ray diffraction (XRD), Raman, and backscattering scanning electron microscopy (BSE). The results showed that the expected composite ceramics were successfully obtained by one-step microwave sintering at 1050 °C for 2 h. The as-prepared samples consisted of Sr_0.5Zr₂(PO₄)₃ and NdPO₄ phases, and the content of the two phases varied regularly as x changed, generally conforming to the designed nominal chemical composition. Importantly, the composite ceramics presented the homogenous and dense microstructure. The relative density of the composite ceramics was more than 95%, meanwhile, the Vickers-hardness of the samples was higher than 600 MPa. It was indicated that NZP-monazite type composite ceramics could be a potential matrix for the simultaneous immobilization of actinide and fission product.     

Effect of sintering temperature on the depolarization behavior of (Bi0.5Na0.5)TiO3-based ceramics

(Bi_0.5Na_0.5)TiO₃ (BNT)-based ferroelectric ceramics have drawn extensive attention because of their excellent electrical properties and interesting depolarization behavior. However, the poor thermal stability of electrical properties limits their practical application. In this work, the effect of sintering temperature (T_s) on the depolarization behavior of BNT-based ceramics was systematically investigated. It is found that the depolarization temperature T_d determined from pyroelectric measurement tends to decrease with increasing T_s, which indicates that lower T_s defers the ferroelectric-relaxor (FE-RE) phase transition. However, for the samples sintered at higher T_s (such as 1180°C), although the T_d is reduced, the thermal stability is better compared with the sample sintered at lower T_s (1100°C) because the diffuse behavior of the FE-RE phase transition is suppressed. According to these results, we propose that the thermal stability of electrical properties for BNT-based ceramics is not only related to high T_d, but also to the diffuse degree of phase transition.     

Li3PO4助熔剂添加对Li1.3Al0.3Ti1.7(PO4)3性质的影响研究

采用溶胶-凝胶法合成Li1.3Al0.3Ti1.7(PO4)3粉末,向Li1.3Al0.3Ti1.7(PO4)3粉末中添加不同摩尔分数的Li3PO4助熔剂烧结制备锂离子固体电解质Li1.3Al0.3Ti1.7(PO4)3烧结片。通过X射线衍射仪、扫描电子显微镜研究合成产物的结构与形貌,采用循环伏安及交流阻抗技术研究合成产物的氧化-还原电位、离子电导率和活化能。结果表明,添加与未添加Li3PO4助熔剂的Li1.3Al0.3Ti1.7(PO4)3烧结片具有相似的X射线衍射结果。添加Li3PO4的Li1.3Al0.3Ti1.7(PO4)3烧结片空隙率较小,更为致密。添加Li3PO4对Li1.3Al0.3Ti1.7(PO4)3的氧化-还原电位影响不大。在所有添加Li3PO4助熔剂的Li1.3Al0.3Ti1.7(PO4)3烧结片中,添加摩尔分数1%Li3PO4的烧结片具有最高的离子电导率6.15×10-4S.cm-1和最低的活化能0.314 2 eV。     

表面粗糙度对Li_(1.4)Al_(0.4)Ge_(1.6)(PO_4)_3玻璃陶瓷交流阻抗的影响

采用玻璃晶化法制备了Li1.4Al0.4Ge1.6(PO4)3玻璃陶瓷。利用X射线衍射仪、扫描电镜、电化学工作站等现代测试手段,研究了表面粗糙度对样品的电学性能的影响。结果表明:玻璃晶化后其主晶相为Li Ge2(PO4)3,并含有少量的Al PO4以及其它未知相。Li1.4Al0.4Ge1.6(PO4)3玻璃晶化后离子电导率显著增加,同时随着样品粗糙度增加,交流阻抗图半圆逐渐被压扁甚至消失。     

Lithium conducting solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 obtained via solution chemistry

NaSICON-type lithium conductor Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) is synthesized with controlled grain size and composition using solution chemistry. After thermal treatment at 850 °C, sub-micronic crystallized powders with high purity are obtained. They are converted into ceramic through Spark Plasma Sintering at 850–1000 °C. By varying the processing parameters, pellet with conductivities up to 1.6 × 10^?4 S/cm with density of 97% of the theoretical density have been obtained. XRD, FEG-SEM, ac-impedance and Vickers indentation were used to characterize the products. The influence of sintering parameters on pellet composition, microstructure and conductivity is discussed in addition to the analysis of the mechanical behavior of the grains interfaces.