The rapid development of portable electronic devices and the efforts to find alternatives to fossil fuels have triggered the rapid development of battery technology. The conventional lithium-ion batteries have reached a high degree of sophistication. However, improvements related to specific capacity, charge rate, safety and sustainability are still required. Solid state batteries try to answer these demands by replacing the organic electrolyte of the standard battery with a solid (crystalline, but also polymer and hybrid) electrolyte. One of the most promising solid electrolytes is Li_3xLa_2/3−xTiO₃ (LLTO). The material nevertheless presents a set of key challenges that must be resolved before it can be used for commercial applications. This review discusses the synthesis methods, the crystallographic and the ionic conduction properties of LLTO and the main limitations encountered through a number of selected studies on this material.

The aim of this paper was to investigate the structure and thermal properties of zirconia ceramics co-doped with rare earth (RE) elements in equimolar concentrations. We prepared (1 − x)ZrO₂ − x(yLa₂O₃ + yNd₂O₃ + ySm₂O₃ + yGd₂O₃ + yYb₂O₃) (x = 0.2; y = 0.2) powders by a hydrothermal method in mild conditions (200 °C, 2 h, 60–100 atm.) The powder was analyzed by XRD, SEM-EDAX, BET, and FT-IR after synthesis and heat treatments at 1200 °C and 1500 °C. The samples exhibit good thermal stability, with a single cubic phase presented after heat treatment at 1500 °C. The compound exhibits a low thermal conductivity (0.61 W·m⁻¹·K⁻¹), a low heat capacity (0.42 J·g⁻¹K⁻¹), and a low thermal diffusivity (0.34 mm²·s⁻¹). The values are lower than reported for conventional RE-doped zirconia.

Zirconium dioxide (ZrO₂) is one of the ceramic materials with high potential in many areas of modern technologies. ZrO₂ doped with 8 wt.% (~4.5 mol%) Y₂O₃ is a commercial powder used for obtaining stabilized zirconia materials (8 wt.% YSZ) with high temperature resistance and good ionic conductivity. During recent years it was reported the co-doping with multiple rare earth elements has a significant influence on the thermal, mechanical and ionic conductivity of zirconia, due complex grain size segregation and enhanced oxygen vacancies mobility. Different methods have been proposed to synthesize these materials. Here, we present the hydrothermal synthesis of 8 wt.% (~4.5 mol%) YSZ co-doped with 4, 6 and 8 wt.% La₂O₃, Nd₂O₃, Sm₂O₃ and Gd₂O₃ respectively. The crystalline phases formed during their thermal treatment in a large temperature range were analyzed by X-ray diffraction. The evolution of phase composition vs. thermal treatment temperatures shows as a major trend the formation at temperatures >1000 °C of a cubic solid solutions enriched in the rare earth oxide used for co-doping as major phase. The first results on the thermal conductivities and impedance measurements on sintered pellets obtained from powders co-doped with 8 wt.% Y and 6% Ln (Ln = La, Nd, Sm and Gd) and the corresponding activation energies are presented and discussed. The lowest thermal conductivity was obtained for La co-doped 8 wt.% YSZ while the lowest activation energy for ionic conduction for Gd co-doped 8 wt.% YSZ materials.

Microencapsulation of sodium nitrate (NaNO₃) as phase change material for high temperature thermal energy storage aims to reduce costs related to metal corrosion in storage tanks. The goal of this work was to test in a prototype thermal energy storage tank (16.7 L internal volume) the thermal properties of NaNO₃ microencapsulated in zinc oxide shells, and estimate the potential of NaNO₃–ZnO microcapsules for thermal storage applications. A fast and scalable microencapsulation procedure was developed, a flow calorimetry method was adapted, and a template document created to perform tank thermal transfer simulation by the finite element method (FEM) was set in Microsoft Excel. Differential scanning calorimetry (DSC) and transient plane source (TPS) methods were used to measure, in small samples, the temperature dependency of melting/solidification heat, specific heat, and thermal conductivity of the NaNO₃–ZnO microcapsules. Scanning electron microscopy (SEM) and chemical analysis demonstrated the stability of microcapsules over multiple tank charge–discharge cycles. The energy stored as latent heat is available for a temperature interval from 303 to 285 °C, corresponding to onset–offset for NaNO₃ solidification. Charge–self-discharge experiments on the pilot tank showed that the amount of thermal energy stored in this interval largely corresponds to the NaNO₃ content of the microcapsules; the high temperature energy density of microcapsules is estimated in the range from 145 to 179 MJ/m³. Comparison between real tank experiments and FEM simulations demonstrated that DSC and TPS laboratory measurements on microcapsule thermal properties may reliably be used to design applications for thermal energy storage.