Influence of La³⁺ Substitution on Electrical and Photocatalytic Behavior of Complex Bi₂Sn₂O₇ Oxides

Pyrochlore-type Bi_2–<i>x</i>La_<i>x</i>Sn₂O₇ (0.0 ≤ <i>x</i> ≤ 0.20) system has been explored for its facile synthesis and electrical and photocatalytic functionalities. The highly distorted α-polymorph of Bi₂Sn₂O₇ was synthesized, and successive La³⁺ substitutions were found to increase the lattice symmetry. Electric field dependent polarization measurements showed the ferroelectric hysteresis loop for pure Bi₂Sn₂O₇ for the first time, and La³⁺ substitution was found to have a bearing on the ferroelectric properties with concomitant increase in leakage current. Temperature-dependent polarization studies were also performed on various nominal compositions. Diffuse reflectance spectroscopy established the tenability of the band gap as the function of La³⁺ content (2.5–3.0 eV). In order to explore the multifunctionality of this unique bismuth-containing system, the photocatalytic dye degradation of rhodamine B was investigated with Bi_2–<i>x</i>La_<i>x</i>Sn₂O₇ (0.0 ≤ <i>x</i> ≤ 0.20) system in both UV and visible regions. The variation in band gap introduced La³⁺ substitution significantly enhanced the photocatalytic behavior of bismuth stannate for rhodamine B degradation. The rate constant for the nominal composition Bi_1.85La_0.15Sn₂O₇ (17.6 × 10^–2 min^–1) is 6-fold the value for the pure Bi₂Sn₂O₇ (2.75 × 10^–2 min^–1). The degradation profiles of rhodamine B in the UV and visible regions showed that dye degradation proceeds through different mechanisms which are discussed. The present study attempts to give an insight into the variation in electrical and photocatalytic properties and relate them to changes in structure. Bi_2–<i>x</i>La_<i>x</i>Sn₂O₇ (0.0 ≤ <i>x</i> ≤ 0.20) system is being proposed here as multifunctional candidates for lead-free electrical materials and efficient photocatalyst in the UV and visible regions

Sequential Evolution of Different Phases in Metastable Gd_2–<i>x</i>Ce_<i>x</i>Zr_2–<i>x</i>Al_<i>x</i>O₇ (0.0 ≤ <i>x</i> ≤ 2.0) System: Crucial Role of Reaction Conditions

The Gd_2–<i>x</i>Ce_<i>x</i>Zr_2–<i>x</i>Al_<i>x</i>O₇ (0.0 ≤ <i>x</i> ≤ 2.0) series was synthesized by the gel combustion method. This system exhibited the presence of a fluorite-type phase, along with a narrow biphasic region, depending upon the Ce/Gd content in the sample. Thermal stability of these new compounds under oxidizing and reducing conditions has been investigated. The products obtained on decomposition of Gd_2–<i>x</i>Ce_<i>x</i>Zr_2–<i>x</i>Al_<i>x</i>O₇ in oxidizing and reducing conditions were found to be entirely different. It was observed that in air the fluorite-type solid solutions of Gd_2–<i>x</i>Ce_<i>x</i>Zr_2–<i>x</i>Al_<i>x</i>O₇ composition undergo phase separation into perovskite GdAlO₃ and fluorite-type solid solutions of Gd–Ce–Zr–O or Ce–Zr–Al–O depending upon the extent of Ce and Al substitution. On the other hand, Gd_2–<i>x</i>Ce_<i>x</i>Zr_2–<i>x</i>Al_<i>x</i>O₇ samples on heating under reducing conditions show a phase separation to CeAlO₃ perovskite and a defect-fluorite of Gd₂Zr₂O₇. The extent of metastability for a typical composition of Gd_1.2Ce_0.8Zr_1.2Al_0.8O₇ (nano), Gd_1.2Ce_0.8Zr_1.2Al_0.8O_6.6 (heated under reduced conditions), Gd_1.2Ce_0.8Zr_1.2Al_0.8O₇ (heated in air at 1200 °C) has been experimentally determined employing a high temperature Calvet calorimeter. On the basis of thermodynamic stability data, it could be inferred that the formation of a more stable compound in the presence of two competing cations (i.e., Gd³⁺ and Ce³⁺) is guided by the crystallographic stability

Curious Case of Positive Current Collectors: Corrosion and Passivation at High Temperature

In the evaluation of compatibility of different components of cell for high-energy and extreme-conditions applications, the highly focused are positive and negative electrodes and their interaction with electrolyte. However, for high-temperature application, the other components are also of significant influence and contribute toward the total health of battery. In present study, we have investigated the behavior of aluminum, the most common current collector for positive electrode materials for its electrochemical and temperature stability. For electrochemical stability, different electrolytes, organic and room temperature ionic liquids with varying Li salts (LiTFSI, LiFSI), are investigated. The combination of electrochemical and spectroscopic investigations reflects the varying mechanism of passivation at room and high temperature, as different compositions of decomposed complexes are found at the surface of metals

Understanding the Surface Regeneration and Reactivity of Garnet Solid-State Electrolytes

Garnet solid-electrolyte-based Li-metal batteries can be used in energy storage devices with high energy densities and thermal stability. However, the tendency of garnets to form lithium hydroxide and carbonate on the surface in an ambient atmosphere poses significant processing challenges. In this work, the decomposition of surface layers under various gas environments is studied by using two surface-sensitive techniques, near-ambient-pressure X-ray photoelectron spectroscopy and grazing incidence X-ray diffraction. It is found that heating to 500 °C under an oxygen atmosphere (of 1 mbar and above) leads to a clean garnet surface, whereas low oxygen partial pressures (i.e., in argon or vacuum) lead to additional graphitic carbon deposits. The clean surface of garnets reacts directly with moisture and carbon dioxide below 400 and 500 °C, respectively. This suggests that additional CO2 concentration controls are needed for the handling of garnets. By heating under O2 along with avoiding H2O and CO2, symmetric cells with less than 10 Ωcm2 interface resistance are prepared without the use of any interlayers; plating currents of >1 mA cm–2 without dendrite initiation are demonstrated

Low Temperature Epitaxial LiMn₂O₄ Cathodes Enabled by NiCo₂O₄ Current Collector for High-Performance Microbatteries

Epitaxial cathodes in lithium-ion microbatteries are ideal model systems to understand mass and charge transfer across interfaces, plus interphase degradation processes during cycling. Importantly, if grown at <450 °C, they also offer potential for complementary metal–oxide–semiconductor (CMOS) compatible microbatteries for the Internet of Things, flexible electronics, and MedTech devices. Currently, prominent epitaxial cathodes are grown at high temperatures (>600 °C), which imposes both manufacturing and scale-up challenges. Herein, we report structural and electrochemical studies of epitaxial LiMn2O4 (LMO) thin films grown on a new current collector material, NiCo2O4 (NCO). We achieve this at the low temperature of 360 °C, ∼200 °C lower than existing current collectors SrRuO3 and LaNiO3. Our films achieve a discharge capacity of >100 mAh g–1 for ∼6000 cycles with distinct LMO redox signatures, demonstrating long-term electrochemical stability of our NCO current collector. Hence, we show a route toward high-performance microbatteries for a range of miniaturized electronic devices

Does Heat Play a Role in the Observed Behavior of Aqueous Photobatteries?

Light-rechargeable photobatteries have emerged as an elegant solution to address the intermittency of solar irradiation by harvesting and storing solar energy directly through a battery electrode. Recently, a number of compact two-electrode photobatteries have been proposed, showing increases in capacity and open-circuit voltage upon illumination. Here, we analyze the thermal contributions to this increase in capacity under galvanostatic and photocharging conditions in two promising photoactive cathode materials, V2O5 and LiMn2O4. We propose an improved cell and experimental design and perform temperature-controlled photoelectrochemical measurements using these materials as photocathodes. We show that the photoenhanced capacities of these materials under 1 sun irradiation can be attributed mostly to thermal effects. Using operando reflection spectroscopy, we show that the spectral behavior of the photocathode changes as a function of the state of charge, resulting in changing optical absorption properties. Through this technique, we show that the band gap of V2O5 vanishes after continued zinc ion intercalation, making it unsuitable as a photocathode beyond a certain discharge voltage. These results and experimental techniques will enable the rational selection and testing of materials for next-generation photo-rechargeable systems

Does Heat Play a Role in the Observed Behavior of Aqueous Photobatteries?

Light-rechargeable photobatteries have emerged as an elegant solution to address the intermittency of solar irradiation by harvesting and storing solar energy directly through a battery electrode. Recently, a number of compact two-electrode photobatteries have been proposed, showing increases in capacity and open-circuit voltage upon illumination. Here, we analyze the thermal contributions to this increase in capacity under galvanostatic and photocharging conditions in two promising photoactive cathode materials, V2O5 and LiMn2O4. We propose an improved cell and experimental design and perform temperature-controlled photoelectrochemical measurements using these materials as photocathodes. We show that the photoenhanced capacities of these materials under 1 sun irradiation can be attributed mostly to thermal effects. Using operando reflection spectroscopy, we show that the spectral behavior of the photocathode changes as a function of the state of charge, resulting in changing optical absorption properties. Through this technique, we show that the band gap of V2O5 vanishes after continued zinc ion intercalation, making it unsuitable as a photocathode beyond a certain discharge voltage. These results and experimental techniques will enable the rational selection and testing of materials for next-generation photo-rechargeable systems

Forced Disorder in the Solid Solution Li₃P–Li₂S: A New Class of Fully Reduced Solid Electrolytes for Lithium Metal Anodes

All-solid-state batteries based on non-combustible solid electrolytes are promising candidates for safe energy storage systems. In addition, they offer the opportunity to utilize metallic lithium as an anode. However, it has proven to be a challenge to design an electrolyte that combines high ionic conductivity and processability with thermodynamic stability toward lithium. Herein, we report a new highly conducting solid solution that offers a route to overcome these challenges. The Li–P–S ternary was first explored via a combination of high-throughput crystal structure predictions and solid-state synthesis (via ball milling) of the most promising compositions, specifically, phases within the Li3P–Li2S tie line. We systematically characterized the structural properties and Li-ion mobility of the resulting materials by X-ray and neutron diffraction, solid-state nuclear magnetic resonance spectroscopy (relaxometry), and electrochemical impedance spectroscopy. A Li3P–Li2S metastable solid solution was identified, with the phases adopting the fluorite (Li2S) structure with P substituting for S and the extra Li+ ions occupying the octahedral voids and contributing to the ionic transport. The analysis of the experimental data is supported by extensive quantum-chemical calculations of both structural stability, diffusivity, and activation barriers for Li+ transport. The new solid electrolytes show Li-ion conductivities in the range of established materials, while their composition guarantees thermodynamic stability toward lithium metal anodes