ABSTRACT
Lithium metal-based solid-state batteries (SSBs) have attracted much attention due to their potentially higher energy densities and improved safety compared with lithium-ion batteries. One of the most promising solid electrolytes, garnet-type Li7La3Zr2O12 (LLZO), has been investigated intensively in recent years. It enables the use of a lithium metal anode, but its application is still challenging because of lithium dendrites that grow at voids, cracks, and grain boundaries of sintered bodies during cycling of the battery cell. In this work, glass-ceramic Ta-doped LLZO produced in a unique melting process was investigated. Upon cooling, an amorphous phase is generated intrinsically, whose composition and fraction are adjusted during the process. Herein, it was set to about 4 wt % containing Li2O and a Li2O-SiO2 phase. During sintering, it was shown to segregate into the grain boundaries and decrease porosity via liquid phase sintering. Sintering temperature and sintering time were found to be reduced compared with the LLZO fabricated by a solid-state reaction while maintaining high density and ionic conductivity. The glass-ceramic sintered at 1130 °C for 0.5 h showed a room-temperature ionic conductivity of 0.64 mS cm-1. Most importantly, the evenly distributed amorphous phase along the grain boundaries effectively hinders lithium dendrite growth. Besides mechanically blocking pores and voids, it helps to prevent inhomogeneous distribution of current density. The critical current density (CCD) of the Li|LLZTO|Li symmetric cell was determined as 1.15 mA cm-2, and in situ lithium plating experiments in a scanning electron microscope revealed superior dendrite stability properties. Therefore, this work provides a promising strategy to prepare a dense and dendrite-suppressing solid electrolyte for future implementation in SSBs.
ABSTRACT
Crystallization via metastable phases plays an important role in chemical manufacturing, biomineralization, and protein crystallization, but the kinetic pathways leading from metastable phases to the stable crystalline modifications are not well understood. In particular, the fast crystallization of amorphous intermediates makes a detailed characterization challenging. To circumvent this problem, we devised a system that allows trapping and stabilizing the amorphous intermediates of representative carbonates (calcium, strontium, barium, manganese, and cadmium). The long-term stabilization of these transient species enabled a detailed investigation of their composition, structure, and morphology. Total scattering experiments with high-energy synchrotron radiation revealed a short-range order of several angstroms in all amorphous intermediates. From the synchrotron data, a structural model of amorphous calcium carbonate was derived that indicates a lower coordination number of calcium compared to the crystalline polymorphs. Our study shows that a multistep crystallization pathway via amorphous intermediates is open to many carbonates. We could isolate and characterize these transient species, thereby providing new insights into their crystallization mechanism.
ABSTRACT
Amorphous intermediates play a crucial role during the crystallization of alkaline earth carbonates. We synthesized amorphous carbonates of magnesium, calcium, strontium, and barium from methanolic solution. The local environment of water and the strength of hydrogen bonding in these hydrated modifications were probed with Fourier transform IR spectroscopy, 1H NMR spectroscopy, and heteronuclear correlation experiments. Temperature-dependent spin-lattice (T1) relaxation experiments provided information about the water motion in the amorphous materials. The Pearson hardness of the respective divalent metal cation predominantly determines the strength of the internal hydrogen-bonding network. Amorphous magnesium carbonate deviates from the remaining carbonates, as it contains additional hydroxide ions, which act as strong hydrogen-bond acceptors. Amorphous calcium carbonate exhibits the weakest hydrogen bonds of all alkaline earth carbonates. Our study provides a coherent picture of the hydrogen bonding situation in these transient species and thereby contributes to a deeper understanding of the crystallization process of carbonates.
ABSTRACT
Calcium sulfate is one of the most important construction materials. Today it is employed as high-performance compound in medical applications and cement mixtures. We report a synthesis for calcium sulfate nanoparticles with outstanding dispersibility properties in organic solvents without further functionalization. The nanoparticles (amorphous with small γ-anhydrite crystallites, 5-50 nm particle size) form long-term stable dispersions in acetone without any sign of precipitation. 1H NMR spectroscopic techniques and Fourier-transform infrared spectroscopy (FTIR) reveal absorbed 2-propanol on the particle surfaces that induce the unusual dispersibility. Adding water to the nanoparticle dispersion leads to immediate precipitation. A phase transformation to gypsum via bassanite was monitored by an in situ kinetic FT-IR spectroscopic study and transmission electron microscopy (TEM). The dispersibility in a volatile organic solvent and the crystallization upon contact with water open a broad field of applications for the CaSO4 nanoparticles, e.g., as nanogypsum for coatings or the fabrication of hybrid composites.
