Spray pyrolysis synthesis of Ta_xLi_7-xLa₃Zr_2-xO₁₂ nanostructured ceramic

Abstract

Li_7-xLa₃Zr_2-xO₁₂-Tax (x =0, 0.1, 0.2, 0.3) nanostructured powders were successfully synthesized using an innovative wet chemical route via spray pyrolysis technique at 800 °C for 10 seconds, followed by post heat treatment at 800 °C for 30 minutes in a muffle furnace. X-ray diffraction (XRD) analysis confirmed that Ta-doping increased the cubic phase up to 99%. Scanning Electron Microscopy (SEM) showed spherical particles with sizes ranging from slightly below 200 nm to 3 µm. The powders were then pressed at 50 MPa and sintered at 850 °C for 7 minutes using Spark Plasma Sintering, followed by post heat-treatment at 900 °C for 2 hours in a muffle furnace. XRD indicated the majority predominance of the garnet cubic phase, and SEM images showed coalescence and longitudinal growth of the particles, forming layers inside the samples. This study reports a fast synthesis and sintering method to obtain cubic garnet crystalline structure in contrast to conventional methods that are widely used and published in the literature, which require longer parameters in the synthesis stages (~24 hours) and sintering stages (over 12 hours).

Keywords:
garnet structure; tantalum; spray pyrolysis; ceramic electrolyte; lithium batteries

INTRODUCTION

Lithium-ion batteries (LIBs) are one of the most promising technologies for energy storage to address the global increase in energy demand. They exhibit high stability with a lithium metal anode and a wide electrochemical window ^1)-(3. Solid-state LIBs are expected to overcome safety issues due to their inorganic crystalline lithium ceramic electrolyte. This eliminates the risk of explosions observed in conventional batteries that currently use liquid electrolyte flammable organic solvents. Ceramic electrolytes have good thermal and chemical stability, high ionic conductivity, and are easy to prepare. In recent years, some authors have studied the synthesis of nanostructured ceramic electrolytes, and notable materials include Li₇La₃Zr₂O₁₂ (LLZ), Li₅La₃Nb₂O₁₂ (LLN), Li₅La₃Bi₂O₁₂ (LLB) e o Li₅La₃Ta₂O₁₂ (LLT) ^4)-(8.

The Li₇La₃Zr₂O₁₂ (LLZ) is a ceramic sample that exhibits a garnet crystalline phase with both tetragonal and cubic structures. At room temperature, the un-doped LLZ primarily exists in the tetragonal structure, which has lower lithionic conductivity (10^-6 S.cm^-1) than the cubic crystal structure (10^-4 S/cm). The cubic crystal structure can be achieved through doping with multivalent ions. s (as Al^{3+ (12}, Nb^{5+ (13}, Ti^{4+ (14}, Ta^{5+ (15}, Ge^{4+ (16}, Ba^{2+ (17} and others). ^9)-(11

The substitutions by doping of Ta⁵⁺ on Li⁺ and Zr⁴⁺ sites will generate lithium vacancy to ensure charge neutrality in the system (1[IMG] = 4[IMG]), (1[IMG] = 1 [IMG]) and also decrease O^2- bonds sites. This decrease in the amount of lithium increases the ion disorder in the system thus influencing the stabilization of the cubic structure and the improvement of ionic conductivity ¹⁸. The LLZ-garnet has a Li⁺ framework interconnected into their 3D crystalline structure constituted by LaO₈ dodecahedral and ZrO₆ octahedral, where the lithium ions are located and conducted in the 24d site (Li1 position), 48g and 96h sites (L₂ e L₃ positions) which are surrounded by ZrO₆-octahedron, as demonstrated in Figure 1. The substitution of Zr⁴⁺ ions by Ta⁵⁺ leads to a variation in the Metal--O bond in the octahedral structure ¹⁹, increasing the lithium ion mobility, and in counterpoint the decrease of this bond forces the output of the lithium ion (ionic radius of 0.76 Å), being this effect observed when is carried out the substitution of Ta⁵⁺ (ionic radius of 0.64 Å) on Zr⁴⁺ sites (ionic radius of 0.72 Å), proving that it is efficient and necessary to regulate the size of the octahedral structure to improve the ionic transport.

Fig. 1:
Scheme of (a) Li-ion positions between Zr-octahedron sites in a Li₇La₃Zr₂O₁₂-structure and (b) for modifications of O₂-bonds compressing or widening the Li-site (Based on ¹⁹)

There are various methods for synthesizing and doping garnet materials. The Solid-State Reaction method, which is commonly reported in scientific journals, involves milling and calcination of precursor materials for up to 12 hours to ensure efficient mixing and reaction to obtain the desired garnet phase ^{20), (21}. However, besides the long steps required, this method has some drawbacks, such as the formation of non-reactive parts that negatively affect lithium ion conductivity. To address this issue, researchers have turned to the wet chemical route synthesis, which promotes a more homogeneous dispersion of doping ions into the ceramic matrix thus favoring the complete formation of the ZrO₆ octahedral and LaO₈ dodecahedral sites of the Garnet Structure, leading to a better lithium ionic mobility.

Other methods include the sol-gel and co-precipitation techniques. The sol-gel method involves solubilizing raw materials to form a molecular dispersion, followed by calcination at temperatures between 1000-1200 °C for about 6 hours ^22)-(24. The co-precipitation technique involves dissolving and precipitating reagents (usually with NH₄OH), followed by mixing the precipitates and drying at ~800°C to remove organic compounds ^25)-(27. While these methods can synthesize particles of nanometric sizes, the reduction in diameter size increases surface energy, resulting in the formation of agglomerated particles to reduce the total energy of the system.

Spray pyrolysis synthesis has recently been used to produce nanoparticulated powders that can be applied in various fields such as sensors, solar cells, batteries, supercapacitors, and others. This technique generates a highly dispersed powder, which makes it easier to sinter at a lower temperature ^28)-(34. In this method, a solution-based substance is subjected to an ultrasonic frequency (typically generated by piezoelectric ceramics) to form a nebulized mist that is carried into a tubular furnace by a carrier gas to promote calcination and obtain the oxide material. The parameters that influence the morphology of the synthesized particles, such as the composition and concentration of the solution, ultrasonic frequency intensity (which affects the diameter of the mist droplets), carrier gas flow rate (which influences the residence time in the oven) and calcination temperature can be adjusted.

Conforming the synthesized oxides into films or pellets, followed by heat treatment, is an important stage for various applications. The sintering process aims to confer mechanical strength by promoting mass transport and coalescence between particles. Different techniques can be used for ceramic processing, such as a conventional muffle oven ^{35), (36}, which requires long times (>10 hours) and high temperatures (≥ 1100 °C); a microwave oven ^{37), (38}, which offers faster (<1 hour) and more homogeneous sintering at a lower temperature (approximately 200 °C lower than conventional ovens); and Spark Plasma Sintering (SPS), a very fast technique (≤30 minutes) that uses pulsed direct electrical current (pulsed DC) and uniaxial compaction pressure to consolidate powders. SPS has been studied for its application in obtaining lithium ceramic electrolytes for solid-state batteries ^39)-(41. SPS can refine grains and purify grain boundaries, leading to an increase in lithium ion conductivity in ceramic electrolytes. However, some manufacturing challenges still need to be overcome. The process uses dense graphite punches and matrices due to their electrical conductivity and thermal resistance.

In this research, we investigated the synthesis of Li₇La₃Zr₂O₁₂ (LLZ) and doping it with tantalum ions via a wet chemical route combined with spark plasma sintering. This is the first time such a technique has been employed, and it results in increased defect concentration, including Li-ion vacancies and crystal lattice distortions. This promotes the formation of a cubic garnet structure and significantly contributes to higher lithium ionic mobility. LLZ samples with tetragonal crystalline structure exhibit ionic conductivity of 10^-7 - 10^-6 S cm^-1, while samples with a cubic structure exhibit ionic conductivity of 10^-4 - 10^-3 S cm^-1. The higher ionic conductivity is due to the number and charge of carriers and the mobility of lithium through unoccupied sites ^{19), (22), (42}. In our methodology, we combine fast calcination (about 10 seconds) followed by 30 minutes of heat treatment at 800°C to obtain a powder free of secondary phases. Additionally, we are developing a methodology to obtain dense pellets with SPS combined with post-heat treatment to avoid the presence of pyrochlore phase and impurities and obtain a sintered ceramic with a cubic garnet crystalline structure.

MATERIALS AND METHODS

Synthesis process: LiNO₃ (Alphatec, 95 wt.-%), La(NO₃)₃.6H₂O (Sigma-Aldrich, 99.9 wt.-%), ZrO(NO₃).5H₂O (Sigma-Aldrich, 99.99 wt.-%) and TaCl₅ (Sigma-Aldrich, 99.9 wt.-%) were used as raw materials with analytical purity. From our methodology previously described ⁶ the systems of Li₇La₃Zr₂O₁₂ (LLZ), Li_6.9La₃Zr_1.9Ta_0.1O₁₂ (LLZ-Ta_0.1), Li_6.8La₃Zr_1.8Ta_0.2O₁₂ (LLZ-Ta_0.2) and Li_6.7La₃Zr_1.9Ta_0.7O₁₂ (LLZ-Ta_0.3) were synthesized with 10 wt% excess LiNO₃ (to compensate for the loss of lithium during synthesis and sintering steps) through the use of stoichiometric amounts from raw materials dissolved in a solution of distilled water and ethanol (20% v/v). All calculations were performed considering the purity of each reagent and are presented in detail in supplementary material S1. Each reagent was dissolved separately in 200 mL of distilled water and then the solutions were mixed. Finally, the volume was completed with ethanol until 1 L of the final solution.

The aerosols in Spray Pyrolysis (Figure 2) were produced by an ultrasonic vibration from piezoelectric ceramic at 1.7 MHz, transported at 5 L min^-1 into the tubular furnace by compressed air as carrier gas and it is calcined at 800 °C for 5 seconds (this is the time it takes for the nebulized sample to pass through the tubular oven). The resulting powders were deposited in a metal collector by electrostatic attraction from 7 kV applied by a high voltage source (HIPOT EH-510PCC model), and then it was kept in a warm house at 110 °C to prevent the adsorption of humidity on the particle surfaces.

Fig. 2:
Scheme of Spray Pyrolysis used for the synthesis step.

Spark plasma sintering: the as-synthesized powders were used to obtain sintered ceramic pellets (diameter of 10 mm and thickness of 2 mm) in a Spark Plasma Sintering SPS (GT Advanced Technologies 10-4 model), by applying pulsed DC simultaneously with a uniaxial compression pressure to consolidate the powders. The sintering process was carried out at 850 °C per 7 min with 50 MPa, where the LLZ-based powder was placed into a graphite matrix with a diameter of 10 mm using a graphite sheet on the mold walls and at the interface between the powder and the punches. After sintering, all the samples were submitted to a thermal treatment in a muffle furnace at 800 °C per 1 hour with an air atmosphere to remove residual carbon from surface pellets. Dense graphite punches and matrices are used in the process due to their electrical conductivity and thermal resistance.

Characterizations: the SEM images were obtained by JEOL 7500F model-field emission scanning electron microscope, thus making a qualitative morphological analysis (size and shape) of the powders (particles) and the pellets (grain and grain boundary) sintered by Spark Plasma Sintering. By using the AxionVision SE64 software an area of 20 μm x 20 μm was selected for analysis and determination of diameter size for the particles.

The Garnet compositions for powders and pellets were characterized by XRD (X-ray diffraction) measurements obtained from Bruker equipment D2 Phaser 2^nd Gen model, with the experimental condition: 2θ = 10 to 80° of range with increment of Δ2θ = 0.02° and Cu-Kα1, 40 kV, 20 mA. The results were compared to Garnet structure patterns analyzed in Diffrac.Suite Eva software with ICDD PDF-2 2015 card (International Center for Diffraction Data) database. The data obtained were treated using the Topas software (Bruker, version 5), based on the CIF file, for analyses of Rietveld refinement, to provide us with information about unit cell and allow us to quantify the crystalline structures (cubic and tetragonal)

The Archimedes method was used to determine the relative density (Equation 1) of sintered-pressed pellets by the liquid displacement and consists of measuring the mass of the dry sample, the wet sample, and the submerged sample, considering the theoretical density of cubic LLZ (5.1 g cm^-3) phase. The ceramic pellet was oven-dried at 110 °C overnight to obtain the dry mass (m_d) and then was submerged in distilled water for 12 hours after that period the excess water in the pellet was removed with absorbent paper to obtain the wet mass (m_w).

$\mathrm{\rho }=\frac{{\mathrm{m}}_{\mathrm{s}}}{{\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}}=\frac{{\mathrm{m}}_{\mathrm{d}}{\mathrm{\rho }}_{{\mathrm{H}}_2\mathrm{O}}}{{\mathrm{m}}_{\mathrm{w}}-{\mathrm{m}}_{\mathrm{s}}}$ (1)

where m_d = dry mass, m_w = wet mass, m_i = submerged mass e IMG = water density.

Impedance Spectroscopy was carried out at room temperature (22 °C), in an air atmosphere, by applying a frequency range of 10^-2-10⁶ Hz and 50 mV of sinusoidal amplitude in LLZ-based pellets with a gold layer (thickness of ~55 nm) deposited by the evaporation technique to produce both top and bottom electrodes. Zview program software (Scribner Associates Inc.) was used to analyze and perform the fit to obtain total resistance determination according to the Nyquist diagram.

RESULTS AND DISCUSSION

The morphology of particles has an important function in the ionic conductivity property to improve the interaction between the electrode and ceramic electrolyte. The as-synthesized LLZ-based powders were morphologically characterized using Scanning Electron Microscopy. Through SEM in Figure 3, it was possible to observe nanometric particles with a spherical shape and average diameter size of 597 nm (standard deviation of ±46 nm), in general, without observing a linear behavior between the samples based on the added amount of tantalum. But eventually, it was founded big particles (diameter size of ~910 nm, radius size ~455 nm) and small particles (diameter size of ~330 nm, radius size ~170 nm), as marked in Fig.3 for LLZ-Ta_0.2 powder sample. This determination was performed by the AxionVision SE64 software, used to analyze and determine the diameter of various particles after statistical calculations. Due to the small size of the particles, there is a tendency for particles to form between them to reduce the entropy of the system. To verify the effectiveness of the introduction of Ta-ions into the solid solution, Energy Dispersive X-ray spectroscopy (EDS) was performed. This technique enables qualitative analysis of the chemical composition and was used to determine the uniform distribution of the dopant within the ceramic matrix. As shown in Figure 4, tantalum (indicated in green) was uniformly distributed in the powder, and the color intensity was proportional to the amount of Ta added during the synthesis step.

Fig. 3:
SEM micrographs of LLZ-based as-synthesized powders.

Fig. 4:
EDS analysis at LLZ-Ta-based powders.

The crystalline structure was analyzed by XRD (Figure 5) and after comparing the results with the ICDD database, the peaks in the diffractograms obtained matched to PDF card no. 00-017-0450, indexed to a La₂Zr₂O₇ cubic structure with spacial group Fd-3m. We determine the particle size by carrying out an analysis of integrated area from significant intensity peak(s) in the Diffract Suite Eva software, using the Scherrer formula (Equation 2) ^43)-(46. Considering the area around the peak(s) at 28.4°, 32.7°, 47.1° and 55.8° for calculation, it was found an average crystallite size of 19.59 nm.

$\mathrm{D}=\mathrm{k}.\mathrm{\lambda }/\mathrm{\beta }.\mathrm{cos\theta }$ (2)

Fig. 5:
XRD patterns of (a) LLZ, (b) LLZ-Ta_0.1, (c) LLZ-Ta_0.2, and (d) LLZ-Ta_0.3 powders.

where D is the mean size of particle ordered domains (Å), k is a shape factor (0.94 considering spherical shape domains), λ is X-ray wavelength (nm), β (in radians) is the line broadening at half the maximum intensity (FWHM) and θ is the Bragg angle/peak position (degree/°).

As the amount of Ta-doping was below the detection limit from XRD equipment, the results (Fig. 4) show diffractograms free of secondary phase containing tantalum. The La₂Zr₂O₇ is an intermediate compound (pyrochlore phase) and has a disordered structure that can be described from cubic ZrO₂ lattice with La³⁺ and Zr⁴⁺ ions randomly distributed on Zr-sites and 7/8^th occupancy of oxygen ions in the anionic sublattice ⁴⁷, being necessary more energy to promote to Li-Garnet phase desired.

So, to obtain Li₇la₃Zr₂O₁₂, the powders were submitted to a heat treatment in a muffle furnace at 800 °C during 30 minutes, with 5 °C min^-1 as heating rate, based on an efficient methodology to transform the pyrochlore phase into garnet structure, as it was showed by Botros et al. ⁴⁸, which we also observed a growth of the particles (Fig 6), with formation of coalescence/interconnections between the grains after annealing of the powders, a similar behavior as the beginning of the sintering process. This new faceted and irregular morphology makes it difficult to determine an average particle size, but it was less than 1 µm.

Fig. 6:
SEM micrographs of LLZ-based powders after post heat treatment at 800 °C/30 min.

In addition, as LLZ-garnet can show both tetragonal and cubic structures, the characterization was carried out for 4 hours to improve the accuracy of the obtained data to allow us to quantify these phases on our samples through refinement. The Rietveld method refinement is a powerful technique used to obtain detailed information about the crystallographic structure of a material. In this research, the refinement was performed on the XRD data obtained from the LLZ-based powders to determine the proportion of the tetragonal and cubic crystalline structures present in the sample (as an example, the analysis for LLZ-Ta_0.1 was shown in Fig. 8).

Fig. 7:
XRD patterns of as-synthesized powders after treatment at 800 °C/30 min. Patterns of Cubic (card nº 01-080-4947) and Tetragonal (card nº 00-064-0140) structures.

Fig. 8:
XRD patterns of LLZ-Ta­_0.1 powder refined by the Rietveld method.

The refinement was done by fitting the observed diffraction pattern to a calculated diffraction pattern, considering the atomic positions and thermal parameters of the crystal structure. The range of 16° < 2θ < 54° was selected for the analysis, as it contains the characteristic peaks of the tetragonal and cubic structures. In the tetragonal structure, the peak of greatest intensity is observed at 16.63°, with a secondary peak of lower intensity at 16.93°. On the other hand, in the cubic structure, there is only a single intense peak at 16.71°. Another important characteristic region for the analysis is between 25° < 2θ < 26°, where the tetragonal structure has three medium intensity peaks at 25.44°, 25.64°, and 25.97°, while the cubic structure has a single peak at 2θ = 25.65°. This analysis allows for the quantification of the proportion of the two structures present in the sample, which is important for understanding the material’s properties and behavior. It is worth mentioning the region between 30° < 2θ < 32° where, with the addition of the doping cations, the main peak of the cubic structure at 2θ = 30.77° becomes more defined (as well as the secondary peak at 2θ = 33.79°), whereas the peaks referring to the tetragonal structure (30.66°, 30.44° and 31.33°) has decreased their intensities, as the same time the peak at 2θ = 34.24° also become smaller. It is noticeable the decrease of the peaks referring to the tetragonal structure about the peaks of significant intensity from the cubic structure so that the peaks related to the cubic structure are observed almost exclusively.

Figure 8 shows the refinement graph for LLZ-Ta_0.1, very similar to results for other samples with Ta-doping, and it can be observed the X-ray diffractogram (blue curve) overlaps with the adjustment calculated curves (red color). The Bragg peaks relative to the tetragonal (blue peaks) and cubic (black peaks) structure are identified below the refined XRD, and thereafter the residuals of the calculated settings. It is observed that the calculated curves have an excellent fit over the diffractogram and that overlap them along the same, observed according to the lower values of GoF (goodness-of-fit). The quantification of the crystalline structures in samples and the unit cell parameters are shown in Table I.

Table I
Quantification of tetragonal and cubic structures and determination of crystal lattice parameters (Ǻ), volume (Ǻ³), and density (g/cm³) of the unit cell for LLZ powders.

Samples	Structures1 (%)	Crystal Lattice Parameters (Å)	V (Å3)	Density (g cm-3)	Gof²
Li7La3Zr2O12	T = 56.35	a = 13.12; c = 12.69	T = 2183.98	5.108	2.57
	C = 43.65	a = 13.02	C = 2210.38	5.0.54
Li6,9La3Zr2O12-Ta0,1	T = 0.99	a = 13.099; c = 12.698	T = 2178.80	5.120	9.48
	C = 99.01	a = 13.055	C = 2225.27	5.175
Li6,8La3Zr2O12-Ta0,2	T = 0.66	a = 13.108; c = 12.673	T = 2177.81	5.116	7.93
	C = 99.34	a = 13.027	C = 2210.58	5.209
Li6,7La3Zr2O12-Ta0,3	T = 0	a = -- ; c = --	--	--	6.57
	C = 100	a = 13.052	C = 2223.61	5.178

• ¹Tetragonal (T) and Cubic (C) phases
• ²A low value of Goodness-of-Fit indicates the quality of refinement

After analyzing the XRD it is possible to infer that, by the intensity and definition of the peaks from the diffractogram of LLZ-based powders, the addiction of tantalum as a dopant into LLZ Garnet structure has been shown to significantly affect the crystal structure. Undoped LLZ primarily exhibits a mix of tetragonal (56.35%) and cubic (43.65%) phases. However, incorporating Ta into the lattice promotes most of the cubic structure (over 99%). This transition can be attributed to the substitution of Ta⁵⁺ ions for Zr⁴⁺. The ionic radius of Ta⁵⁺ (0.64 Å) compared with Zr⁴⁺ (0.76 Å) distorts the surrounding lattice, leading to the rearrangement of oxygen ions and a shift towards a more symmetric cubic structure. It also observed an expansion of the unit cell volume, directly proportional to the amount of Ta doping, while the volume of the tetragonal phase decreased. This can be attributed to a higher coefficient of thermal expansion for Ta-doped LLZ and also can influence the ionic conductivity by facilitating the movement of oxygen ions through the lattice. However, further research is needed to fully understand the complex interplay between tantalum doping, crystal structure, and materials properties for specific applications and evaluate their performance under realistic conditions.

For the sequence of this work, the pellets were prepared with a thickness of 2 mm and diameter of 10 mm in a graphite mold and sintered by SPS at 850 °C for 7 minutes. It was used as an intermediate step with a hold at 450 °C for 10 minutes. During the process, the heating and cooling rate was 50 °C min^-1. After sintering, the graphite sheets were mechanically removed with the aid of sandpaper, and then the density of the pellets was determined by the Archimedes Method. According to the procedure described and using Equation 3 it was found densities of 4.42, 4.45, 4.49, and 4.51 g cm^-3 respectively for LLZ (86.8%), LLZ-Ta_0.1 (87.2%), LLZ-Ta_0.2 (88.1), LLZ-Ta_0.3 (88.4%), less than theoretical density (5.1 g cm^-3) ⁴⁹. The addition of tantalum has been shown to increase the density of sintered ceramic pellets. This improvement can be attributed to several factors, such as the ionic radius of Ta ions that promote grain growth during the sintering process (since Ta ions create a local lattice strain which increases the diffusion rate of other ions and facilitates the formation of stronger bonds between grains). The Ta doping can reduce the pores and other defects within sintered pellets, enhancing the mobility of grain boundaries, and allowing them to move more freely during the SPS process leading to a rearrangement of grains and the formation of a denser microstructure. The sintering also can be influenced by the creation of oxygen vacancies by Tantalum ions that substitute Zirconium into the lattice, enhancing the ions diffusion and facilitating the sintering. The lower densification of undoped LLZ can be attributed to a more volatilization of lanthanum oxide at high sintering temperatures, the Ta ions can help to prevent this by stabilizing the La₂O₃ in the lattice, further contributing to densification.

The diffractogram obtained by structural characterization of LLZ without Ta-doping after sintering by SPS (Figure 9.a) showed no formation of garnet structure (cubic or tetragonal), indication La₂Zr₂O₇ pyrochlore phase (blue color) with the characteristic’s peaks at 28.7°, 33.3°, 47.6° and 56.5°. Considering the same principle used in the methodology applied to powder samples, the LLZ sintered pellet was subjected to an additional heat treatment in a muffle furnace at 900 ° C/2 hours. After analyzed by XRD it was observed, in Figure 9.b, a residual part of the pyrochlore phase, but it was also possible to identify a major transformation directly to the cubic phase, based on the indexation of the main peaks (red color) in this sample that has no doping to promote cubic crystalline structure.

Fig. 9:
XRD patterns of LLZ pellet (a) as sintered and (b) with additional heat treatment. Insert peaks of Li₇La₃Zr₂O₁₂ (PDF card n° 01-080-4947) and La₂Zr₂O₇ (PDF card n° 00-017-0450).

Based on this result, the post heat treatment in a muffle furnace at 900 °C per 2 hours after the sintering step was applied for LLZ-Ta pellets, and the XRD analysis for these samples is shown in Figure 10. Due to operational limitations, such as the difference in sizes between sample and sample holder, it would not be possible to perform a Rietveld refinement with a lower GOF value (goodness-of-fit), as the peaks in diffractograms are displaced. The CIF file for the La₂Zr₂O₇ intermediate phase was not found, which would make it difficult to carry out the Rietveld refinement to quantify the crystalline structures.

Fig. 10:
XRD patterns of (a) LLZ-Ta_0.1, (b)LLZ-Ta_0.2 and (c) LLZ-Ta_0.3. Pellets were sintered at 850 °C per 7 minutes and post heat treated at 900 °C per 2 hours. Insert peaks of Li₇La₃Zr₂O₁₂ cubic phase (PDF card n° 01-080-4947).

The XRD analysis of the sintered samples indicated that there was a complete transformation of the LLZ phase to cubic garnet structure (PDF card n° 01-080-4947), as shown in Fig. 10. However, the morphological characterization of the sintered samples showed an unexpected behavior of uniaxial growth with the formation of plaques, which can decrease the lithium ionic conductivity in the grain boundary region. Therefore, the researchers are developing an alternative methodology to overcome this issue ⁶, which includes the use of intermediate sintering temperature levels and a gradual application of pressure up to the final stage of 50 MPa. These parameters are expected to result in a nanostructured ceramic electrolyte with high ionic conductivity, similar to that observed for the cubic phases of LLZ with tantalum doping that promotes structural defects and increases the mobility of lithium ions.

The electrical characterization (Fig. 12) shows a typical Nyquist diagram, a complex plane plot of -Z” imaginary part vs Z’ real part. Our samples exhibit a single semicircle at high and medium frequencies related to particle conduction mechanisms (grain + grain boundary) and a linear tail related to the Au-blocking electrode. Equation 3 was used to determine the lithionic conductivity (ϭ_Li) ⁴⁴.

${\mathrm{ϭ}}_{\mathrm{T}}=\mathrm{L}/\mathrm{R}.\mathrm{A}$ (3)

where L is the thickness of the ceramic electrolyte (cm), R is the resistance determined by the diameter of the semicircle from the Nyquist plot (ohm) and A is the electrode area (cm²).

Fig. 11:
SEM micrographs at the cross-sectional LLZ-Ta sintered pellets samples with post heat treatment at 900 °C/2 h.

Fig. 12:
Nyquist diagram fitted of (A) LL-Ta_0.1 and (B) LLZ-Ta_0.2 sintered pellets.

The results from impedance characterizations are present in Figure 12. For LLZ-based pellets showed high values of resistance, to other garnet materials cited in the literature. This result can be related to the larger average grain size after the sintering step, leading to a smaller contribution of the grain contour to the total electrical resistivity, as constated by Matsuki and co-authors ⁵⁰. The equivalent electrical circuit for the total system was inserted into Fig. 12 and it was identified using Zview software and is represented by a work resistance R_W (cables and connections in equipment) in series with a parallel combination of a grain resistance R₁ and a constancy phase element CPE₁ followed in series by parallel R₂CPE₂ (grain boundary contribution) and by CPE_El, a constancy phase element from Au-electrodes, characterizing the entire system as a mixed conductor (ionic for the electrolyte and electronic for the gold layer ⁵¹. Due to operational limitations, the individual contributions of the grain and the grain boundary weren’t identified, only a semicircle representing the entire system was obtained. Extrapolating the graph to the intersection with the axis was performed to represent the actual part of the graph, which corresponds to both contributions of grain and grain boundary resistance (R = R₁+ R₂). Through fit performed in Zview software, determined values of resistance of 8.12 x 10⁸ Ω for LLZ, 7.47 x 10⁷ Ω for LLZ-Ta_0.1, 2.52 10⁵ Ω for LLZ-Ta_0.2 and 9.87 x 10⁵ Ω for LLZ-Ta_0.3. Then, the lithionic conductivity (ϭ_Li) for the pellets was calculated as 1.55 x10^-9 S.cm^-1 for LLZ, 1.69 x10^-8 S.cm^-1 for LLZ-Ta_0.1, 5.02 x10^-6 S.cm^-1 for LLZ-Ta_0.2 and 1.28 x 10^-5 S.cm^-1 for LLZ-Ta_0.3. It’s clear the influence of tantalum addition on increasing the sample’s conductivity, as well as the peaks from XRD (Fig. 10), become more defined in its cubic crystalline structure.

CONCLUSIONS

This research successfully synthesized, in a continuous flow, a particulate powder with a spherical shape with an average diameter size of 597 nm by a solution-based chemical route using the innovation Spray Pyrolysis technique where the nebulized solution was calcined by only 5 seconds. The crystalline structure of the LLZ-powders was characterized by XRD indicating obtaining in a first step to the La₂Zr₂O₇ pyrochlore phase, which was fully transformed by doping with Ta-ions to garnet cubic structure (over 99%) after applying an additional heat treatment. All samples doped with Tantalum were less resistive than undoped LLZ (~8.2 x 10⁸ Ω). The LLZ-Ta_0.3 pellet showed the better lithionic conductivity (~1.3 x 10^-5 S.cm^-1). Improvements in the parameters of SPS equipment for sintering are being carried out to improve the densification of the tablets without the longitudinal growth of the grains, which will be an important factor in increasing the ionic conductor behavior of our ceramic pellets to be applied in lithium solid-state batteries. Through continuous research and exploration, Ta-doped LLZ can emerge as a versatile and high-performance material with broad potential for various technological advancements.

ACKNOWLEDGEMENT

Special thanks to our graphical designer Ariane R. Pokes for the design of the figures. Funding: Financial support by CNPq Brazilian Research Funding Agency (Process n° 400801/2016-7).

REFERENCES

• ¹
  Kim KH, Iriyama Y, Yamamoto K, Kumazaki S, Asaka T, Tanabe K, et al. Characterization of the interface between LiCoO2 and Li7La3Zr2O12 in an all-solid-state rechargeable lithium battery. J Power Sources. 2011; 196: 764. doi:10.1016/j.jpowsour.2010.07.073.
  » https://doi.org/10.1016/j.jpowsour.2010.07.073
• ²
  Huang M, Shoji M, Shen Y, Nan CW, Munakata H, Kanamura K. Preparation and electrochemical properties of Zr-site substituted Li7La3(Zr2−xMx)O12 (M = Ta, Nb) solid electrolytes. J Power Sources . 2014; 261: 290. doi:10.1016/j.jpowsour.2014.03.070.
  » https://doi.org/10.1016/j.jpowsour.2014.03.070
• ³
  Suzuki Y, Kami K, Watanabe K, Watanabe A, Saito N, Ohnishi T, et al. Transparent cubic garnet-type solid electrolyte of Al2O3-doped Li7La3Zr2O12. Solid State Ionics. 2015; 282: 51. doi:10.1016/j.ssi.2015.06.009.
  » https://doi.org/10.1016/j.ssi.2015.06.009
• ⁴
  Langer F, Glenneberg J, Bardenhagen I, Kun R. Synthesis of single phase cubic Al-substituted Li7La3Zr2O12 by solid state lithiation of mixed hydroxides. J Alloys Compd. 2015; 640: 340. doi:10.1016/j.jallcom.2015.03.209.
  » https://doi.org/10.1016/j.jallcom.2015.03.209
• ⁵
  Xiang X, Liu Y, Chen F, Yang W, Yang J, Ma X, et al. A combined ultrasonic B-mode and color Doppler system for the classification of breast masses using neural network. J Eur Ceram Soc. 2020; 40: 2795. doi:10.1016/j.jeurceramsoc.2020.02.054.
  » https://doi.org/10.1016/j.jeurceramsoc.2020.02.054
• ⁶
  Lustosa GMMM, Franchetti MGS, de Souza A, da Conceição L, Berton MAC. Fast synthesis and sintering of Li5La3Nb2O12 garnet ceramic. Mater Phys. 2021;255:123848. doi:10.1016/j.matchemphys.2020.123848.
  » https://doi.org/10.1016/j.matchemphys.2020.123848
• ⁷
  Peng H, Zhang Y, Li L, Feng L. Effect of quenching method on Li ion conductivity of Li5La3Bi2O12 solid state electrolyte. Solid State Ionics . 2017;302:128. doi:10.1016/j.ssi.2017.03.030.
  » https://doi.org/10.1016/j.ssi.2017.03.030
• ⁸
  Zhao P, Xiang Y, Xu Y, Wen Y, Zhang W, Zhu X, et al. Dense garnet-like Li5La3Nb2O12 solid electrolyte prepared by self-consolidation method. Ceram Int. 2018;44:10290. doi:10.1016/j.ceramint.2018.02.118.
  » https://doi.org/10.1016/j.ceramint.2018.02.118
• ⁹
  Wolfenstine J, Rangasamy E, Allen JL, Sakamoto J. Effect of substitution (Ta, Al, Ga) on the conductivity of Li7La3Zr2O12. J Power Sources . 2012;208:193. doi:10.1016/j.jpowsour.2012.02.031.
  » https://doi.org/10.1016/j.jpowsour.2012.02.031
• ¹⁰
  Awaka J, Kijima N, Hayakawa H, Akimoto J. Synthesis and crystallographic studies of garnet-related lithium-ion conductors Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12. J Solid State Chem. 2009;182:2046. doi:10.1016/j.jssc.2009.05.020.
  » https://doi.org/10.1016/j.jssc.2009.05.020
• ¹¹
  Larraz G, Orera A, Sanjuan ML. Cubic phases of garnet-type Li7La3Zr2O12: the role of hydration. J Mater Chem A. 2013;1:11419. doi:10.1039/C3TA11996C.
  » https://doi.org/10.1039/C3TA11996C
• ¹²
  Hubaud AA, Schroeder DJ, Ingram BJ, Okasinski JS, Vaughey JT. Thermal expansion in the garnet-type solid electrolyte (Li7−Al/3)La3Zr2O12 as a function of Al content. J Alloys Compd . 2015;645. doi:10.1016/j.jallcom.2015.05.067.
  » https://doi.org/10.1016/j.jallcom.2015.05.067
• ¹³
  Imagawa H, Ohta S, Kihira Y, Asaoka T. Garnet-type Li6.75La3Zr1.75Nb0.25O12 synthesized by coprecipitation method and its lithium ion conductivity. Solid State Ionics . 2014;262:625. doi:10.1016/j.ssi.2013.10.059.
  » https://doi.org/10.1016/j.ssi.2013.10.059
• ¹⁴
  Shao C, Yu Z, Liu H, Zheng Z, Sun N, Diao C. Structure and ionic conductivity of cubic Li7La3Zr2O12 solid electrolyte prepared by chemical co-precipitation method. Electrochim Acta. 2017;228:332. doi:10.1016/j.electacta.2016.12.140.
  » https://doi.org/10.1016/j.electacta.2016.12.140
• ¹⁵
  Guo H, Su J, Zha W, Xiu T, Song Z, Badding ME, et al. Achieving high critical current density in Ta-doped Li7La3Zr2O12/MgO composite electrolytes. J Alloys Compd . 2021;847:157222. doi:10.1016/j.jallcom.2020.157222.
  » https://doi.org/10.1016/j.jallcom.2020.157222
• ¹⁶
  Huang M, Xu W, Shen Y, Lin YH, Nan CW. X-ray absorption near-edge spectroscopy study on Ge-doped Li7La3Zr2O12: enhanced ionic conductivity and defect chemistry. Electrochim Acta . 2014;116:430. doi:10.1016/j.electacta.2013.11.020.
  » https://doi.org/10.1016/j.electacta.2013.11.020
• ¹⁷
  Hofstetter K, Samson AJ, Thangadurai V. Characterization of lithium-rich garnet-type Li6.5La2.5Ba0.5ZrTaO12 for beyond intercalation chemistry-based lithium-ion batteries. Solid State Ionics . 2018;320:35. doi:10.1016/j.ssi.2017.09.005.
  » https://doi.org/10.1016/j.ssi.2017.09.005
• ¹⁸
  Zhang B, Tan R, Yang L, Zheng J, Zhang K, Mo S, et al. Mechanisms and properties of ion-transport in inorganic solid electrolytes. Energ Storage Mater. 2018;12:56. doi:10.1016/j.ensm.2017.08.015.
  » https://doi.org/10.1016/j.ensm.2017.08.015
• ¹⁹
  Zhang Y, Chen F, Li J, Zhang L, Gu J, Zhang D, et al. Regulation mechanism of bottleneck size on Li+ migration activation energy in garnet-type Li7La3Zr2O12. Electrochim Acta . 2018;275:199. doi:10.1016/j.electacta.2017.12.133.
  » https://doi.org/10.1016/j.electacta.2017.12.133
• ²⁰
  Thangadurai V, Weppner W. Development of all-solid-state lithium batteries. J Solid State Chem . 2006;179:974. doi:10.1016/j.jssc.2005.12.025.
  » https://doi.org/10.1016/j.jssc.2005.12.025
• ²¹
  Truong L, Thangadurai V. Soft-Chemistry of Garnet-Type Li5+xBaxLa3−xNbxO12 (x = 0, 0.5, 1): Reversible H+ ↔ Li+ Ion-Exchange Reaction and Their X-ray, 7Li MAS NMR, IR, and AC Impedance Spectroscopy Characterization. Chem Mater. 2011;23:3976. doi:10.1021/cm2015127.
  » https://doi.org/10.1021/cm2015127
• ²²
  Rosenkiewitz N, Schuhmacher J, Bockmeyer M, Deubener J. Nitrogen-free sol-gel synthesis of Al-substituted cubic garnet Li7La3Zr2O12 (LLZO). J Power Sources . 2015;275:769. doi:10.1016/j.jpowsour.2014.12.066.
  » https://doi.org/10.1016/j.jpowsour.2014.12.066
• ²³
  Li Y, Han JT, Wang CA, Vogel SC, Xie H, Xu M, et al. Ionic distribution and conductivity in lithium garnet Li7La3Zr2O12. J Power Sources . 2012;209:278. doi:10.1016/j.jpowsour.2012.02.100.
  » https://doi.org/10.1016/j.jpowsour.2012.02.100
• ²⁴
  Jin Y, McGinn PJ. Al-doped Li7La3Zr2O12 synthesized by a polymerized complex method. J Power Sources . 2011;196:8683. doi:10.1016/j.jpowsour.2011.05.065.
  » https://doi.org/10.1016/j.jpowsour.2011.05.065
• ²⁵
  Zhang X, Fergus JW. Phase Content and Conductivity of Aluminum- and Tantalum-Doped Garnet-Type Lithium Lanthanum Zirconate Solid Electrolyte Materials. ECS Trans. 2017;77:509. doi:10.1149/07711.0509ecst.
  » https://doi.org/10.1149/07711.0509ecst
• ²⁶
  Allen JL, Wolfenstine J, Rangasamy E, Sakamoto J. Effect of substitution (Ta, Al, Ga) on the conductivity of Li7La3Zr2O12. J Power Sources . 2012;206:315. doi:10.1016/j.jpowsour.2012.01.131.
  » https://doi.org/10.1016/j.jpowsour.2012.01.131
• ²⁷
  Ramanujam P, Vaidhyanathan B, Binner J, Anshuman A, Spacie C. A comparative study of the synthesis of nanocrystalline yttrium aluminium garnet using sol-gel and co-precipitation methods. Ceram Int . 2014;40:3791. doi:10.1016/j.ceramint.2013.08.075.
  » https://doi.org/10.1016/j.ceramint.2013.08.075
• ²⁸
  da Conceição L, Lustosa GMMM, Miranda GCC, Franchetti MGS, Ribas RM, Vicente AA. Ion conduction in ceramics. In: XVI European Ceramic Society Conference; 2019. p. 825. doi:10.1142/9789813233898_0004.
  » https://doi.org/10.1142/9789813233898_0004
• ²⁹
  Lustosa GMMM, Franchetti MGS, Miranda GCC, da Conceição L. High ionic conductivity lithium garnet oxides of Li7−xLa3Zr2−xTaxO12 compositions. ECS Trans . 2019;16:831. doi:10.1149/2.024205esl.
  » https://doi.org/10.1149/2.024205esl
• ³⁰
  Djenadic R, Botros M, Benel C, Clemens O, Indris S, Choudhary A, et al. Nebulized spray pyrolysis of Al-doped Li7La3Zr2O12 solid electrolyte for battery applications. Solid State Ionics . 2014;262:674. doi:10.1016/j.ssi.2014.05.007.
  » https://doi.org/10.1016/j.ssi.2014.05.007
• ³¹
  Maheswari S, Karunakaran M, Annalakshmi V, Kasirajan K. Effect of gadolinium on tin oxide thin films prepared by nebulizer spray pyrolysis method. Results Mater. 2020;4:100130. doi:10.1016/j.rinma.2020.100130.
  » https://doi.org/10.1016/j.rinma.2020.100130
• ³²
  Wittawat R, Rittipun R, Jarasfah M, Nattporn B. Synthesis of ZnO/TiO2 spherical particles for blue light screening by ultrasonic spray pyrolysis. Mater Today Commun. 2020;23:101126. doi:10.1016/j.mtcomm.2020.101126.
  » https://doi.org/10.1016/j.mtcomm.2020.101126
• ³³
  Khateeb SA, Sparks TD. JMR volume 34 issue 10 Cover and Front matter. J Mater Res. 2019;34:208. doi:10.1557/jmr.2019.208.
  » https://doi.org/10.1557/jmr.2019.208
• ³⁴
  Kim JK, Yoo Y, Kang YC. Scalable green synthesis of hierarchically porous carbon microspheres by spray pyrolysis for high-performance supercapacitors. Chem Eng J. 2020;389:122805. doi:10.1016/j.cej.2019.122805.
  » https://doi.org/10.1016/j.cej.2019.122805
• ³⁵
  Matsuda Y, Sakaida A, Sugimoto K, Mori D, Takeda Y, Yamamoto O, et al. Sintering behavior and electrochemical properties of garnet-like lithium conductor Li6.25M0.25La3Zr2O12 (M: Al3+ and Ga3+).Solid State Ionics . 2017;313:18. doi:10.1016/j.ssi.2017.09.014.
  » https://doi.org/10.1016/j.ssi.2017.09.014
• ³⁶
  Wang XJ, Tian YM, Hao JY, Wang YY, Bai PB. Nickel(II)-Catalyzed Addition of Aryl-, Alkenyl-, and Alkylboronic Acids to Alkenylazaarenes. J Eur Ceram Soc . 2020;40:12408. doi:10.1016/j.jeurceramsoc.2020.07.008.
  » https://doi.org/10.1016/j.jeurceramsoc.2020.07.008
• ³⁷
  Lustosa GMMM, da Costa JPC, Perazolli LA, Stojanovic BD, Zaghete MA. Potential barrier of (Zn, Nb)SnO2-films induced by microwave thermal diffusion of Cr3+ for low-voltage varistor. J Am Ceram Soc. 2016;99:127. doi:10.1111/jace.13924.
  » https://doi.org/10.1111/jace.13924
• ³⁸
  Testoni GO, Amoresi RAC, Lustosa GMMM, da Costa JPC, Nogueira MV, Ruiz M, et al. Increased photocatalytic activity induced by TiO2/Pt/SnO2 heterostructured films. Solid State Sci. 2018; 78: 30. doi:10.1016/j.solidstatesciences.2017.12.006.
  » https://doi.org/10.1016/j.solidstatesciences.2017.12.006
• ³⁹
  Xue J, Zhang K, Chen D, Zeng J, Luo B. Spark plasma sintering plus heat-treatment of Ta-doped Li7La3Zr2O12 solid electrolyte and its ionic conductivity. Mater Res Express. 2020;7:036520. doi:10.1088/2053-1591/ab7618.
  » https://doi.org/10.1088/2053-1591/ab7618
• ⁴⁰
  Zhu H, Liu J. Emerging applications of spark plasma sintering in all solid-state lithium-ion batteries and beyond. J Power Sources . 2018;394:54. doi:10.1016/j.jpowsour.2018.04.054.
  » https://doi.org/10.1016/j.jpowsour.2018.04.054
• ⁴¹
  Castillo A, Charpentier T, Rapaud O, Pradeilles N, Yagoubi S, Foy E, et al. Bulk Li mobility enhancement in spark plasma sintered Li7−3xAlxLa3Zr2O12 garnet. Ceram Int . 2018;44:11708. doi:10.1016/j.ceramint.2018.07.119.
  » https://doi.org/10.1016/j.ceramint.2018.07.119
• ⁴²
  Li Y, Han JT, Wang CA, Xie H, Goodenough JB. Optimizing Li+ conductivity in a garnet framework. J Mater Chem. 2012;22:15357. doi:10.1039/C2JM31413D.
  » https://doi.org/10.1039/C2JM31413D
• ⁴³
  Shan K, Yi ZZ, Yin XT, Dastan D, Garmestani H. Conductivity and mixed conductivity of a novel dense diffusion barrier and the sensing properties of limiting current oxygen sensors. Dalton Trans. 2020;49:8341. doi:10.1039/D0DT01159B.
  » https://doi.org/10.1039/D0DT01159B
• ⁴⁴
  Yin XT, Zhou WD, Li J, Lv P, Wang Q, Wang D, et al. Tin dioxide nanoparticles with high sensitivity and selectivity for gas sensors at sub-ppm level of hydrogen gas detection. J Mater Sci Mater Electron. 2019;30:10312. doi:10.1007/s10854-019-01840-w.
  » https://doi.org/10.1007/s10854-019-01840-w
• ⁴⁵
  Jafari A, Alam MH, Dastan D, Ziakhodadadian S, Shi Z, Garmestani H, et al. Statistical, morphological, and corrosion behavior of PECVD derived cobalt oxide thin films. J Mater Sci Mater Electron . 2019;30:8303. doi:10.1007/s10854-019-02492-6.
  » https://doi.org/10.1007/s10854-019-02492-6
• ⁴⁶
  Zhou WD, Dastan D, Li J, Yin XT, Wang Q. Discriminable sensing response behavior to homogeneous gases based on n-ZnO/p-NiO composites. Nanomater. 2020;10:785. doi:10.3390/nano10040785.
  » https://doi.org/10.3390/nano10040785
• ⁴⁷
  Paul B, Singh K, Jaron T, Roy A, Chowdhury A. AES in partially reconfigurable CGRAs. J Alloys Compd . 2016;656:73. doi:10.1109/tencon.2016.7848368.
  » https://doi.org/10.1109/tencon.2016.7848368
• ⁴⁸
  Botros M, Djenadic R, Clemens O, Möller M, Hahn H. Field assisted sintering of fine-grained Li7−3La3Zr2AlO12 solid electrolyte and the influence of the microstructure on the electrochemical performance. J Power Sources . 2016;320:271. doi:10.1016/j.jpowsour.2016.01.086.
  » https://doi.org/10.1016/j.jpowsour.2016.01.086
• ⁴⁹
  Buannic L, Naviroj M, Miller SM, Zagorski J, Faber KT, Llordés A. Dense freeze-cast Li7La3Zr2O12 solid electrolytes with oriented open porosity and contiguous ceramic scaffold. J Am Ceram Soc . 2019;102:3194. doi:10.1111/jace.15938.
  » https://doi.org/10.1111/jace.15938
• ⁵⁰
  Matsuki Y, Noi K, Suzuki K, Sakuda A, Hayashi A, Tatsumisago M. Microstructure and conductivity of Al-substituted Li7La3Zr2O12 ceramics with different grain sizes. Solid State Ionics . 2019;344:115047. doi:10.1016/j.ssi.2019.115047.
  » https://doi.org/10.1016/j.ssi.2019.115047
• ⁵¹
  Zuo H, Fu W, Fan R, Dastan D, Wang H, Shi Z. Bilayer carbon nanowires/nickel cobalt hydroxides nanostructures for high-performance supercapacitors. Mater Lett. 2020;263:127217. doi:10.1016/j.matlet.2019.127217.
  » https://doi.org/10.1016/j.matlet.2019.127217

Edited by

AE:

F. M. B. Marques

Publication Dates

• Publication in this collection
  25 Oct 2024
• Date of issue
  2024

History

• Received
  28 July 2023
• Reviewed
  13 Dec 2023
• Accepted
  19 Jan 2024