Spectrum-Dependent Spiro-OMeTAD Oxidization Mechanism in Perovskite Solar Cells

We propose a spectrum-dependent mechanism for the oxidation of 2,2′,7,7′-tetrakis­(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) with bis­(trifluoromethane)­sulfonimide lithium salt (LiTFSI), which is commonly used in perovskite solar cells as the hole transport layer. The perovskite layer plays different roles in the Spiro-OMeTAD oxidization for various spectral ranges. The effect of oxidized Spiro-OMeTAD on the solar cell performance was observed and characterized. With the initial long-wavelength illumination (>450 nm), the charge recombination at the TiO₂/Spiro-OMeTAD interface was increased due to the higher amount of the oxidized Spiro-OMeTAD. On the other hand, the increased conductivity of the Spiro-OMeTAD layer and enhanced charge transfer at the Au/Spiro-OMeTAD interface facilitated the solar cell performance

Engineering Three-Dimensionally Electrodeposited Si-on-Ni Inverse Opal Structure for High Volumetric Capacity Li-Ion Microbattery Anode

Aiming at improving the volumetric capacity of nanostructured Li-ion battery anode, an electrodeposited Si-on-Ni inverse opal structure has been proposed in the present work. This type of electrode provides three-dimensional bi-continuous pathways for ion/electron transport and high surface area-to-volume ratios, and thus exhibits lower interfacial resistance, but higher effective Li ions diffusion coefficients, when compared to the Si-on-Ni nanocable array electrode of the same active material mass. As a result, improved volumetric capacities and rate capabilities have been demonstrated in the Si-on-Ni inverse opal anode. We also show that optimization of the volumetric capacities and the rate performance of the inverse opal electrode can be realized by manipulating the pore size of the Ni scaffold and the thickness of the Si deposit

Effect of Surface Modification on Nano-Structured LiNi_0.5Mn_1.5O₄ Spinel Materials

Fine-tuning of particle size and morphology has been shown to result in differential material performance in the area of secondary lithium-ion batteries. For instance, reduction of particle size to the nanoregime typically leads to better transport of electrochemically active species by increasing the amount of reaction sites as a result of higher electrode surface area. The spinel-phase oxide LiNi_0.5Mn_1.5O₄ (LNMO), was prepared using a sol–gel based template synthesis to yield nanowire morphology without any additional binders or electronic conducting agents. Therefore, proper experimentation of the nanosize effect can be achieved in this study. The spinel phase LMNO is a high energy electrode material currently being explored for use in lithium-ion batteries, with a specific capacity of 146 mAh/g and high-voltage plateau at ∼4.7 V (vs Li/Li⁺). However, research has shown that extensive electrolyte decomposition and the formation of a surface passivation layer results when LMNO is implemented as a cathode in electrochemical cells. As a result of the high surface area associated with nanosized particles, manganese ion dissolution results in capacity fading over prolonged cycling. In order to prevent these detrimental effects without compromising electrochemical performance, various coating methods have been explored. In this work, TiO₂ and Al₂O₃ thin films were deposited using atomic layer deposition (ALD) on the surface of LNMO particles. This resulted in effective surface protection by prevention of electrolyte side reactions and a sharp reduction in resistance at the electrode/electrolyte interface region

Modified Coprecipitation Synthesis of Mesostructure-Controlled Li-Rich Layered Oxides for Minimizing Voltage Degradation

Modified carbonate coprecipitation synthesis without addition of chelating agent is introduced to obtain mesostructure-controlled Li-rich layered oxides. The designed mesostructure for target material Li_1.2Ni_0.2Mn_0.6O₂ has uniformly dispersed spherical secondary particles with size around 3 μm. These micrometer-sized particles consist of self-assembled crystallites with size of ∼150 nm. This unique design not only decreases the surface area compared with the sample with dispersive particles, but also increases overall structural mechanical stability compared with the sample with larger dense secondary particles as observed by transmission X-ray microscope. As a result, the voltage decay and capacity loss during long-term cycling have been minimized to a large extent. Our findings clearly demonstrate that mesostructure design of Li-rich layered oxides play a key role in optimizing this class of cathode materials. Surprisingly, the voltage fading issue can be partially mitigated by such an approach

Improvement of the Cathode Electrolyte Interphase on P2-Na_2/3Ni_1/3Mn_2/3O₂ by Atomic Layer Deposition

Atomic layer deposition (ALD) is a commonly used coating technique for lithium ion battery electrodes. Recently, it has been applied to sodium ion battery anode materials. ALD is known to improve the cycling performance, Coulombic efficiency of batteries, and maintain electrode integrity. Here, the electrochemical performance of uncoated P2-Na_2/3Ni_1/3Mn_2/3O₂ electrodes is compared to that of ALD-coated Al₂O₃ P2-Na_2/3Ni_1/3Mn_2/3O₂ electrodes. Given that ALD coatings are in the early stage of development for NIB cathode materials, little is known about how ALD coatings, in particular aluminum oxide (Al₂O₃), affect the electrode–electrolyte interface. Therefore, full characterizations of its effects are presented in this work. For the first time, X-ray photoelectron spectroscopy (XPS) is used to elucidate the cathode electrolyte interphase (CEI) on ALD-coated electrodes. It contains less carbonate species and more inorganic species, which allows for fast Na kinetics, resulting in significant increase in Coulombic efficiency and decrease in cathode impedance. The effectiveness of Al₂O₃ ALD coating is also surprisingly reflected in the enhanced mechanical stability of the particle which prevents particle exfoliation

Self-Assembled Framework Formed During Lithiation of SnS₂ Nanoplates Revealed by in Situ Electron Microscopy

ConspectusLithium-ion batteries (LIBs) commercially dominate portable energy storage and have been extended to hybrid/electric vehicles by utilizing electrode materials with enhanced energy density. However, the energy density and cycling life of LIBs must extend beyond the current reach of commercial electrodes to meet the performance requirements for transportation applications. Carbon-based anodes, serving as the main negative electrodes in LIBs, have an intrinsic capacity limitation due to the intercalation mechanism. Some nanostructured carbon materials offer very interesting reversible capacities and can be considered as future anode materials. However, their fabrication processes are often complicated and expensive. Theoretically, using a lithium metal anode is the best way of delivering high energy density due to its largest theoretical capacity of more than 3800 mAh g^–1; however, lithium metal is highly reactive with liquid electrolytes. Alternative anodes are being explored, including other lithium-reactive metals, such as Si, Ge, Zn, V, and so forth. These metals react reversibly with a large amount of Li per formula unit to form lithium–metal alloys, rendering these materials promising candidates for next-generation LIBs with high energy density. Though, most of these pure metallic anodes experience large volume changes during lithiation and delithiation processes that often results in cracking of the anode material and a loss electrical contact between the particles.Nanosized metal sulfides were recently found to possess better cycling stability and larger reversible capacities over pure metals. Further improvements and developments of metal sulfide-based anodes rely on a fundamental understanding of their electrochemical cycling mechanisms. Not only must the specific electrochemical reactions be correctly identified, but also the microstructural evolution upon electrochemical cycling, which often dictates the cyclability and stability of nanomaterials in batteries, must be clearly understood. Probing these dynamic evolution processes, i.e. the lithiation reactions and morphology evolutions, are often challenging. It requires both high-resolution chemical analysis and microstructural identification. In situ transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS) has recently been raised as one of the most powerful techniques for monitoring electrochemical processes in anode materials for LIBs.In this work, we focus on elucidating the origin of the structural stability of SnS₂ during electrochemical cycling by revealing the microstructural evolution of SnS₂ upon lithiation using in situ TEM. Crystalline SnS₂ was observed to undergo a two-step reaction after the initial lithium intercalation: (1) irreversible formation of metallic tin and amorphous lithium sulfide and (2) reversible transformation of metallic tin to Li–Sn alloys, which is determined to be the rate-determining step. More interestingly, it was discovered that a self-assembled composite framework formed during the irreversible conversion reaction, which has not been previously reported. Crystalline Sn nanoparticles are well arranged within an amorphous Li₂S “matrix” in this self-assembled framework. This nanoscale framework confines the locations of individual Sn nanoparticles and prevents particle agglomeration during the subsequent cycling processes, therefore providing desired structural tolerance and warranting a sufficientelectron pathway. Our results not only explain the outstanding cycling stability of SnS₂ over metallic tin anodes, but also provide important mechanistic insights into the design of high-performance electrodes for next-generation LIBs through the integration of a unique nanoframework

Lithium Lanthanum Titanium Oxides: A Fast Ionic Conductive Coating for Lithium-Ion Battery Cathodes

This work introduces Li–La–Ti–O (LLTO), which is a fast lithium-ion conductor, as an effective coating material for cathode materials used in rechargeable lithium-ion batteries. This fast Li-ion conductor is characterized by first-principles calculations showing low activation barrier for lithium diffusion at various different lithium concentrations. The morphology and the microstructure of the pristine electrode and coated electrode materials are characterized systematically, and we show clear evidence of the presence of the coating after electrochemical cycling. The coated electrodes show significantly improved rate capabilities and cycling performance, compared to the pristine electrodes. The possible reasons for such enhancements are explored experimentally using potentiostatic intermittent titration technique (PITT), electrochemical impedance spectroscopy (EIS). Because of the high lithium conductivity in the LLTO coating material, the chemical Li⁺ diffusion coefficient is one magnitude of order higher in the coated samples than that in the uncoated samples. In addition, the impedances of both interfacial charge transfer and Li⁺ transportation in the solid-electrolyte-interphase (SEI) layer are reduced up to 50% in the coated samples. Our findings provide significant insights into the role of coating regarding the improvements of electrochemical properties, as well as the potential use of solid electrolyte as an effective coating material

Investigating the Energy Storage Mechanism of SnS₂‑rGO Composite Anode for Advanced Na-Ion Batteries

Tin sulfide–reduced graphene oxide (SnS₂-rGO) composite material is investigated as an advanced anode material for Na-ion batteries. It can deliver a reversible capacity of 630 mAh g^–1 with negligible capacity loss and exhibits superb rate performance. Here, the energy storage mechanism of this SnS₂-rGO anode and the critical mechanistic role of rGO will be revealed in detail. A synergistic mechanism involving conversion and alloying reactions is proposed based on our synchrotron X-ray diffraction (SXRD) and <i>in situ</i> X-ray absorption spectroscopy (XAS) results. Contrary to what has been proposed in the literature, we determined that Na₂S₂ forms instead of Na₂S at the fully discharge state. The as-formed Na₂S₂ works as a matrix to relieve the strain from the huge volume expansion of the Na–Sn alloy reaction, shown in the high resolution transmission electron microscope (HRTEM). In addition, the Raman spectra results suggest that the rGO not only assists the material to have better electrochemical performance by preventing particle agglomeration of the active material but also coordinates with Na-ions through electrostatic interaction during the first cycle. The unique reaction mechanism in SnS₂-rGO offers a well-balanced approach for sodium storage to deliver high capacity, long-cycle life, and superior rate capability

Reusable Oxidation Catalysis Using Metal-Monocatecholato Species in a Robust Metal–Organic Framework

An isolated metal-monocatecholato moiety has been achieved in a highly robust metal–organic framework (MOF) by two fundamentally different postsynthetic strategies: postsynthetic deprotection (PSD) and postsynthetic exchange (PSE). Compared with PSD, PSE proved to be a more facile and efficient functionalization approach to access MOFs that could not be directly synthesized under solvothermal conditions. Metalation of the catechol functionality residing in the MOFs resulted in unprecedented Fe-monocatecholato and Cr-monocatecholato species, which were characterized by X-ray absorption spectroscopy, X-band electron paramagnetic resonance spectroscopy, and ⁵⁷Fe Mössbauer spectroscopy. The resulting materials are among the first examples of Zr­(IV)-based UiO MOFs (UiO = University of Oslo) with coordinatively unsaturated active metal centers. Importantly, the Cr-metalated MOFs are active and efficient catalysts for the oxidation of alcohols to ketones using a wide range of substrates. Catalysis could be achieved with very low metal loadings (0.5–1 mol %). Unlike zeolite-supported, Cr-exchange oxidation catalysts, the MOF-based catalysts reported here are completely recyclable and reusable, which may make them attractive catalysts for ‘green’ chemistry processes

Role of Amines in Thermal-Runaway-Mitigating Lithium-Ion Battery

Benzylamine (BA), dibenzylamine (DBA), and trihexylamine (THA) are investigated as thermal-runaway retardants (TRR) for lithium-ion batteries (LIBs). In a LIB, TRR is packaged separately and released when internal shorting happens, so as to suppress exothermic reactions and slow down temperature increase. THA is identified as the most efficient TRR. Upon nail penetration, 4 wt % THA can reduce the peak temperature by nearly 50%. The working mechanisms of the three amines are different: THA is highly wettable to the separator and immiscible with the electrolyte, and therefore, it blocks lithium-ion (Li⁺) transport. BA and DBA decrease the ionic conductivity of electrolyte and increase the charge transfer resistance. All three amines react with charged electrodes; the reactions of DBA and THA do not have much influence on the overall heat generation, while the reaction of BA cannot be ignored