Phase Formation Study of Solid-State LLZNO and LLZTO via Structural, Thermal, and Morphological Analyses

Preparation and characterization of Ga and Sr co-doped Li7La3Zr2O12 garnet-type solid electrolyte

With the advantageous features include suitable lithium insertion position and "zero strain" structure, Li4Ti5O12 has become one of the ideal anode materials for lithium-ion battery and battery-capacitor system. However, the obstacle of its poor electronic and Li+ conductivity restrict its rate capacity. Many researches proved that the one-pot preparation process, which is mixing the carbon precursor with lithium and titanium precursor and then thermal treating in an inert or reducing atmosphere together, not only can form a carbon coating layer to improve the electronic property of the materials easily, but also inhibits the particle size growth of LTO by a barrier effect of carbon. Among kinds of carbon source, graphene presents excellent unique properties, which has been considered as an ideal carbon source to improve the electrochemical performance of LTO and many other active materials. However, the intersheet π-π attractions of GS easily resulted in its restacking and agglomeration. Therefore, an oxygenated graphene (GO) with a special amphiphilic molecular structure is adopted as GS precursor to prepare GS modified composites. Due to the amphiphilic characteristic of GO, the choice of an appropriate Li precursor is extremely important to obtain a high quality product. In this study, we systematically investigated the influence of lithium precursor and calcination atmosphere on the reaction mechanisms, phase formation, particulate morphology, surface properties and electrochemical performance of graphene sheets-modified nano-Li4Ti5O12 composite. The results shown that the lithium precursor containing carboxyl anion such as LiAc and Li2CO3 would connect with oxygen groups of GO by strong hydrogen bonds to restrict the morphology of composites and the phase formation of pure spinel Li4Ti5O12. Furthermore, the oxygen ratio in the molecular structure of lithium compound is proportional to the consumption of graphene. In addition, the reducing atmosphere facilitates the partial reduction of Ti4+ to promote the interfacial charge transfer kinetics of the product. Through optimization the conditions of the reaction, the sample adopting LiOH precursor with adjustment GS ratio calcined under Ar/H2 (5 %) atmosphere delivers 172.8 mAh g-1 at 1 C and maintained a discharge capacity of 98.0 mAh g-1 at a high rate of 40 C. This sample also showed fairly stable cycling performance. After 800 deep cycles at a high charge/discharge rate of 40 C, the capacity can hold 97.7 % of its second discharge value. This study not only provides an optimization of Li precursor and calcination condition for GS modified LTO material, but also has significant reference value for any combination reaction with GO participation.

High Rate Li-Ion Batteries with Cation-Disordered Cathodes

All-solid-state Li-ion batteries (ASSLBs) using oxide based solid electrolytes have been considered as a promising candidates for the next generation batteries due to their excellent thermal and electrochemical stability. Especially, garnet-type Li7La3Zr2O12 (LLZO) has gained much attention due to its good conductivity and large electrochemical window. Since the electrochemical window of LLZO allows usage of high voltage cathodes such as NMC (LiNixMnyCo1-x-yO2) and Li anode, it is possible to design high voltage, high capacity and safe energy storage medium using the eletrolyte.However, current attempts on exploiting LLZO as a solid electrolyte have been hindered by high resistance at the interface, especially between cathode and the solid electrolyte. Even though the importance of the issue is well known, there has not been a comprehensive understanding yet on the topic. The lack of understanding comes from the difficulty of characterizing buried interface. It is difficult to study thin interfacial region between cathode and solid electrolyte with conventional techniques since the signal from the interface is often overwhelmed by the bulk, and destructive characterization techniques ruin the signal itself.We developed model system by sputtering thin-film cathode on top of pre-prepared solid state electrolyte pellets to overcome this problem. Samples were annealed afterwards to simulate sintering process. Since the interfacial region is near surface, we could use XANES and EXAFS which are surface-sensitive and non-destructive. Those techniques gave us valuable information regarding oxidation state and local chemical environment of each element, which were used to evaluate migration tendency. Moreover, those results can be used to precisely characterize secondary phases by complementing complex XRD results. Results obtained from those characterization techniques were correlated with electrochemical analysis techniques such as EIS and chronovoltammetry to evaluate the effect of the interfacial degradation on cell performance.In this work, we aimed to obtain fundamental understanding on the instability of cathode|electrolyte interface during two different stages: manufacturing stage and operation stage. We systematically varied variables related to manufacture (Sintering temperature, sintering time, gas environment during sintering), and operation (Electrochemical potential, current density). We chose NMC622 (LiNi0.6Mn0.2Co0.2O2) as a cathode, and Al-doped cubic LLZO (Li7La3Zr2O12) as a solid electrolyte.To study degradation of NMC622|LLZO during manufacturing stage, we varied temperature condition (300°C, 500°C, 700°C for 4h respectively) and gas environment (air, O2, humidified O2, N2, CO2) during annealing. Samples annealed in air showed high interfacial resistance and poor performance, which are signs for interfacial degradation. At 700°C, formation of detrimental phase such as La2Zr2O7 and La(Ni,Co)O3 led to complete blockage of Li ions. XANES and EXAFS showed severe change of oxidation state and chemical environment of Ni and Co, which indicated that those species escaped out of the cathode and participate in the secondary phase formation. In contrast, NMC622|LLZO interface remained chemically stable up to 700°C with no formation of detrimental phase when the sample was annealed in O2. On the other hand, when the sample was annealed in humidified O2 at 700°C, detrimental phase(La2Zr2O7) could be found. In addition, we could characterize substantial amount of Li2CO3 from samples annealed in humidified O2, which could have formed by reaction between LiOH and CO2 during sample characterization done in air. Annealing in CO2 condition led to the crystallization of detrimental phases such as Li2CO3, La2O2CO3, NiCO3 at 500°C. In addition, we could see formation of La2Zr2O7 and La2(Ni,Co)O4 from the sample annealed at 700°C. In both air and CO2 environment, we could find La-(Ni,Co)-O secondary phase from annealed samples, which indicates interdiffusion at the interface. However, extent of the degradation was much higher in CO2 environment. This could be seen by the intensity of XRD peaks corresponding to secondary phases, which overwhelmed the ones corresponding to bulk LLZO. Co L-edge XANES and XRD data for samples annealed at 700°C in different gas conditions could be seen in the attached figure.These findings suggest that interfacial degradation is strongly dependent on the sintering temperature and gas environment. Sintering temperature should be kept as low as possible to avoid formation of detrimental phase. In terms of gas environment, O2 environment is the most promising since the sample remained stable up to 700°C. On the other hand, we found that CO2 and H2O(vapor) are detrimental to the sample. This could be because that they could both form phases which could extract Li out of the system, which led to formation of delithiated secondary phases such as La2Zr2O7. Findings from the work can be used to design optimal conditions to manufacture and operate all solid Li-batteries using LLZO with minimal interfacial degradation. Figure 1

Lanthanide doping of Li7La3-xMxZr2O12 (M=Sm, Dy, Er, Yb; x=0.1–1.0) and dopant size effect on the electrochemical properties

The effects of the dopants, Mg 2+ , Sr 2+ , Sc 3+ , Yb 3+ , Gd 3+ , La 3+ , Ti 4+ , Zr 4+ , Ce 4+ , and Nb 5+ , on the grain boundary mobility of dense Y 2 O 3 have been investigated from 1500° to 1650°C. Parabolic grain growth has been observed in all cases over a grain size from 0.31 to 12.5 μm. Together with atmospheric effects, the results suggest that interstitial transport is the rate‐limiting step for diffusive processes in Y 2 O 3 , which is also the case in CeO 2 . The effect of solute drag cannot be ascertained but the anomalous effect of undersized dopants (Ti and Nb) on diffusion enhancement, previously reported in CeO 2 , is again confirmed. Indications of very large binding energies between aliovalent dopants and oxygen defects are also observed. Overall, the most effective grain growth inhibitor is Zr 4+ , while the most potent grain growth promoter is Sr 2+ , both at 1.0% concentration.

Bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT) is considered to be one of the most promising lead-free alternatives to piezoelectric lead zirconate titanate (PZT). However, the effect of dopants on the material has so far received little attention from an atomic point of view. In this study we investigated the effects of cobalt-doping on the formation of additional phases and determined the preferred lattice site of cobalt in BNT. The latter was achieved by comparing the measured x-ray absorption near-edge structure (XANES) spectra to numerically calculated spectra of cobalt on various lattice sites in BNT. (Bi0.5Na0.5)TiO3 + x mol% Co (x = 0.0, 0.5, 1.0, 2.6) was synthesized via solid state reaction. As revealed by SEM backscattering images, a secondary phase formed in all doped specimens. Using both XRD and SEM-EDX, it was identified as Co2TiO4 for dopant levels >0.5 mol%. In addition, a certain amount of cobalt was incorporated into BNT, as shown by electron probe microanalysis. This amount increased with increasing dopant levels, suggesting that an equilibrium forms together with the secondary phase. The XANES experiments revealed that cobalt occupies the octahedral B-site in the BNT perovskite lattice, substituting Ti and promoting the formation of oxygen vacancies in the material.

Garnet-type Li7La3Zr2O12 (LLZO) solid Li ion conductor is a promising solid electrolyte for the all-solid-state Li-ion batteries (ASSLIB) because of its appropriate ionic conductivity and its good electrochemical stability. The previous research on B doping into LLZO structure proven to decrease the sintering temperature because B serve as a sintering agent. Meanwhile, Al doping into LLZO structure is known to increase the ionic conductivity of this solid electrolyte. Consider those previous results, this research combined B and Al doping into LLZO which was conducted through solid state reaction under a various composition of B and Al. The synthesis result was then analyzed by XRD, SEM/EDX to understand its characteristic of the diffraction pattern, crystal structure, surface morphology of the sintered- pellets and also the elements content inside. The result found that the B-Al doped-LLZO are still in similar cubic structure with a space group of Ia-3d. The cubic phase is the main composition with mol percentage of around 82 %, and the rest are secondary phases. The pellet of materials can be sintered well, however due to its hygroscopic properties, the pellets turned into powder after 12 h store under room condition. Therefore, the dry atmosphere is required to further analyzed such as to measure the electrical conductivity and to assembly the materials into a battery prototype.

Suppressed secondary phase generation in thermoelectric higher manganese silicide by fabrication process optimization

The aim of this work is to show the effect of the iron ion doping in LaCoO3 perovskite, both in powders and in sintered samples obtained from combustion reaction and solid state route. The phase formation and particle morphology and particle size distribution of the powders were analysed by XRD, SEM and sedimentation techniques, respectively. Relative density, microstructure (secondary phases and grain size) and pore size distribution of LaCo1-xFexO3 sintered ceramics were investigated by SEM/EDS and Hg porosimetry analysis. Although LaCo1-xFexO3 powders obtained from the combustion reaction exhibited smaller grain sizes when sintered at high temperatures, they showed a higher volume fraction of secondary phases. The presence of these crystalline phases in addition to the desired perovskite affected the microstructure acting as grain growth inhibitors by grain boundary pinning. It is believed that by observing three grain junction pores that the LaFeO3 phase has a smaller dihedral angle than LaCoO3. This fact would explain why LaFeO3 presented a smaller driving force for sintering with a higher tendency of pore and inclusion coarsening at higher temperatures (1400°C).

Due to the limited availability of raw materials, sodium-ion batteries (SiB) are a viable alternative to lithium-ion batteries. However, liquid-electrolyte-based SiBs have a comparably low energy density and suffer from safety issues similar to their lithium-ion-based counterparts. To address these issues, solid-state SiBs are a promising option and sodium super ionic conductors (NaSICON) are suitable solid electrolytes for such systems. They exhibit good mechanical properties and chemical stability, high ionic conductivity and compatibility with sodium metal based anodes enabling high energy density [1]. One of the best studied NaSICON materials is Na3Zr2Si2PO12 (NZSP) with a high ionic conductivity in the order of 10-4 –10-3 S·cm-1 at room temperature [2]. Conventional synthesis methods of this class of materials such as solid-state reaction and liquid-phase synthesis have several drawbacks due to time-consuming process steps such as milling, high temperature sintering, precipitation, washing and drying steps to obtain the final product.We present spray flame synthesis (SFS) as a new approach for the synthesis of nanosized NaSICON materials. Recent studies have shown that sintering of nanoparticular NZSP precursors offers several advantages: A high specific surface area, which increases the sintering activity, and short atomic diffusion paths, allowing high homogeneity and phase purity to be achieved at comparatively low sintering temperatures [3]. In SFS, metal salts dissolved in organic solvents are combusted resulting in fine metal oxide particles. They are characterized by transmission electron microscopy (TEM), X-Ray diffraction (XRD) and Raman-Spectroscopy for structural and morphological investigation. Elemental information is obtained via energy-dispersive X-Ray spectroscopy (EDX). Ionic conductivities of sintered NZSP pellets are measured by impedance spectroscopy.In our approach, nanoparticles with a median diameter of around 5–9 nm are obtained. The pristine particles consist of crystalline ZrO2, homogeneously covered with an amorphous layer consisting of the elements Na, Si, P and O. After a short annealing step for 1h at 1000°C, this mixture can be converted almost quantitatively into the desired rhombohedral NZSP phase. Moreover, aliovalent dopants were successfully added for the synthesis of Na3+2xAxZr2−xSi2PO12 with A = Mg or Ca. Pressed pellets sintered at 1100°C for 3h to a relative density of ~92% showed - for a material sintered for such a short period of time - a surprisingly high ionic conductivity of up to 7.9×10-4 S·cm-1 (Mg-doped).In conclusion, a novel approach for the preparation of NZSP allowing the phase formation at relatively low temperatures is demonstrated. Spray flame synthesis is an elegant and promising possibility for the scalable production of solid electrolytes, which also holds great potential, especially with regard to further improvement of ionic conductivity through targeted doping.[1] Zhang et al., ACS Appl. Energy Mater., 3 (2020) 7427; doi.org/10.1021/acsaem.0c00820[2] Narayanan et al., Solid State Ion., 331 (2019), 22; doi.org/10.1016/j.ssi.2018.12.003[3] Jalalian-Khakshour et al. J.Mater.Sci., 55, (2019) 2291; doi.org/10.1007/s10853-019-04162-8

Preparation of single crystalline Sr 0.5Ba 0.5Nb 2O 6 particles

The interplay between aliovalent CuO doping and nonstoichiometry on the development of defect structures and the formation of secondary phases of antiferroelectric NaNbO${}_{3}$ ceramics has been investigated by means of x-ray diffraction (XRD), first-principles calculations using density functional theory (DFT), and electron paramagnetic resonance spectroscopy. The results indicate that, for stoichiometric 0.25 mol% CuO-doped NaNbO${}_{3}$, as well as for 2.0 mol% Nb-excess sodium niobate, the Cu${}^{2+}$ functional centers are incorporated at the Nb site (${\mathrm{Cu}}_{\mathrm{Nb}}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}$). For reasons of charge compensation, two kinds of mutually compensating defect complexes ${({\mathrm{Cu}}_{\mathrm{Nb}}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}\ensuremath{-}{V}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}})}^{\ensuremath{'}}$ and ${({V}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}}\ensuremath{-}{\mathrm{Cu}}_{\mathrm{Nb}}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}\ensuremath{-}{V}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}})}^{\ifmmode\bullet\else\textbullet\fi{}}$ are formed where, for the niobium-excess compound, additionally, ${V}_{\mathrm{Na}}^{\ensuremath{'}}$ contribute to the mechanism of charge compensation. In contrast, for 2.0 mol% Na-excess sodium niobate, a Na${}_{3}$NbO${}_{4}$ secondary phase has been detected by XRD, and only part of the Cu${}^{2+}$ forms these types of defect complexes. The major part of the Cu${}^{2+}$ is incorporated in a fundamentally different way by forming Cu${}^{2+}$-Cu${}^{2+}$ dimeric defect complexes.

Garnet-type Li7La3Zr2O12 (LLZO) and its derivatives are considered promising candidate materials for all-solid-state Li-ion batteries as solid electrolytes due to their high Li+ conductivities of around 10-4 S cm-1 at room temperature and wide electrochemical window. Despite the larger number of detailed studies utilizing various experimental and theoretical approaches, the experimentally evaluated Li+ conductivities did not precisely match the computationally predicted values. The discrepancy between the ion conductivities can be attributed to the ambivalent properties of grain boundaries (GBs). GB structure is a critical parameter that controls the macroscopic properties of materials. However, various technological limitations of both the experimental and theoretical approaches utilized for elucidating the Li+ conduction behavior of GBs have restricted our ability of reaching a deeper understanding of the processes occurring at the atomic level. In this study, we investigated the Li+conducting behaviors of various tilted GBs of cubic LLZO with a garnet framework by using molecular dynamics approaches. Nine stoichiometric equilibrium GB models were used to determine Li+conductivity. It was found that the GB conductivity was smaller than the bulk one regardless of the orientation. In particular, the conductivities perpendicular to the GBs were characterized by the lowest values. Some factors affecting Li conductivity near the tilted GBs may be related to their stability. The relationship between the structural distortions at the GBs and the Li+ conducting characteristics were evaluated in terms of the GB energies and radial distribution function for the La-La, Zr-Zr, and O-O interactions in the LLZO bulk and GBs. We found that the Li+ conductivities are highly correlate with the structural distortions at the GBs. In order to elucidate the Li+ diffusion characteristics at the GBs in more detail, the variations of the atomic concentration near the GBs were calculated along the axis perpendicular to the GB surface. The atomic population at bulk region was constantly modulated, while that at GB region was anomalously dispersed. The average atomic concentration at the GBs was lower than that at the bulk. In particular, the observed decrease in the Li+ concentration suggests the formation of Li-deficient sites across the GB center, which represents the primary reason for the degraded ionic conductivity in the GB region. In fact, the Li ionic conductivity perpendicular to the GBs decreased with decreasing Li+ concentration at the GBs, which was caused by the slower diffusion of Li+. Thus, preventing the distortion of the GB crystallographic structure and the related drop in the Li+concentration should be considered one of the top priorities when designing GBs with desirable properties. Acknowledgement This work was partially supported by CREST, JST Grant Number JPMJCR1322, Japan.

Garnet-type Li7La3Zr2O12 (LLZO) with its competitive ionic conductivity (~ 1 mS cm-1), wide electrochemical stability (above 5.0 V vs. Li/Li+), and compatibility with Li-metal, is a promising electrolyte for solid-state Li-metal batteries. Despite the promise, manufacturing all-solid-state battery architectures faces challenges, mainly due to missing robust contact and high interfacial resistance on the cathode-electrolyte interface due to the evolution of secondary phases during processing. Simultaneously achieving high cathode active material loadings and low interfacial impedances remain to be shown in devices with relevant architectures. In this project, we investigate the scalable powder-to-device fabrication for a cathode/electrolyte half- and full-cell with LiCoO2 as the active material and LLZO as the ion conducting electrolyte phase, combined with a Li-metal anode. We report the electrical and ionic resistance of LLZO-LiCoO2 composite cathodes being reduced by three orders of magnitude by manipulating the sintering atmosphere. Raman spectroscopy mapping reveals the secondary phase evolution inside the LLZO due to Cobalt diffusion being absent in composite cathodes sintered in a lithium-rich atmosphere as compared to ambient conditions. Further investigations lead to the conclusive assumption that the formation of secondary phases can be blocked by stopping lithium volatilization during high-temperature co-firing. Testing the dense composite cathodes in a liquid electrolyte battery, very low initial impedances of >30 Ω and high areal capacities of 3.5 mAh cm-2 with almost full utilizations can be achieved. Such batteries can be operated at 0.25 mA cm-2 at room temperature for 50 cycles without failing. Furthermore, we demonstrate tape-cast composite cathodes with thicknesses of < 100 µm being built into such battery setups, showcasing their mechanical integrity and density. Combining these results with recently developed dense-porous Li-garnet bilayers, we give an outlook to manufacture an all-solid-state, co-fired battery architecture that operates without the need of stack pressure.