New Insights in the Hydrothermal Synthesis of Rare-Earth Carbonates

Mechanistic Insights into the Early-Stage Crystallization and Nanophase Formation of Metastable Light Rare-Earth Carbonates

Rare-earth element (REE) carbonates play a crucial role in geochemistry due to their prevalence in carbonatite ore deposits, which are extensively mined globally for REE extraction. Two primary mineral groups of interest are lanthanites (REE2(CO3)3·8H2O) and bastnäsites (REECO3(OH,F)), typically enriched in light REE. This study aims to elucidate the mechanisms and kinetics of La-, Ce-, Pr-, and Nd-carbonate crystallization reactions at the earliest stages. REE-carbonates were synthesized through homogeneous crystallization by combining CO3 2- and REE-bearing solutions at temperatures ranging from near-ambient to low hydrothermal conditions (5-80 °C). The crystallization processes were monitored in situ and in real-time using UV-vis spectrophotometry and synchrotron-based wide-angle X-ray scattering (WAXS). The characterization and quantification of the newly formed phases were conducted using a combination of conventional powder X-ray diffraction, high-resolution scanning electron microscopy with energy-dispersive spectroscopy and Fourier transform infrared spectroscopy. Our findings reveal a complex, multistep crystallization pathway specific to each REE, influenced by factors such as temperature, solution concentration and ratio, phase stability, and REE ionic potential. Additionally, the REE-carbonate crystallization pathways align with a progressive dehydration sequence involving multiple intermediate nanophases and reversible reactions. Notably, a reversible reaction between lanthanite and nanotengerite was observed at ambient temperature, involving structural rearrangements and hydration-dehydration processes. Our findings emphasize the importance of nanophase formation during the initial stages of REE-carbonate crystallization, with implications for the development of more efficient REE extraction methods.

High-Rate Layered Cathode of Lithium-Ion Batteries through Regulating Three-Dimensional Agglomerated Structure

LiNixCoyMnzO2 (LNCM)-layered materials are considered the most promising cathode for high-energy lithium ion batteries, but suffer from poor rate capability and short lifecycle. In addition, the LiNi1/3Co1/3Mn1/3O2 (NCM 111) is considered one of the most widely used LNCM cathodes because of its high energy density and good safety. Herein, a kind of NCM 111 with semi-closed structure was designed by controlling the amount of urea, which possesses high rate capability and long lifespan, exhibiting 140.9 mAh·g−1 at 0.85 A·g−1 and 114.3 mAh·g−1 at 1.70 A·g−1, respectively. The semi-closed structure is conducive to the infiltration of electrolytes and fast lithium ion-transfer inside the electrode material, thus improving the rate performance of the battery. Our work may provide an effective strategy for designing layered-cathode materials with high rate capability.

Entropy-Stabilized Oxides owning Fluorite Structure obtained by Hydrothermal Treatment

Entropy-Stabilized Oxides (ESO) is a modern class of multicomponent advanced ceramic materials with attractive functional properties. Through a five-component oxide formulation, the configurational entropy is used to drive the phase stabilization over a reversible solid-state transformation from a multiphase to a single-phase state. In this paper, a new transition metal/rare earth entropy-stabilized oxide, with composition Ce0.2Zr0.2Y0.2Gd0.2La0.2O2-, was found after several investigations on alternative candidate systems. X-Ray Diffraction (XRD) analyses of calcined powders pointed out different behavior as a function of the composition and a single-phase fluorite structure was obtained after a specific thermal treatment at 1500 °C. Powders presented the absence of agglomeration, so that the sintered specimen exhibited sufficient densification with a small porosity, uniformly distributed in the sample.