10 research outputs found

    Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics

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    Cerium substituting gadolinium in Ca<sub>2</sub>Gd<sub>8</sub>(SiO<sub>4</sub>)<sub>6</sub>O<sub>2</sub> occupies two intrinsic sites of distinct coordination. The coexistence of an ionic bonding at a 4F site and an ionic–covalent mixed bonding at a 6H site in the same crystalline compound provides an ideal system for comparative studies of ion–ligand interactions. Experimentally, the spectroscopic properties and photoluminescence dynamics of this white-phosphor are investigated. An anomalous thermal quenching of the photoluminescence of Ce<sup>3+</sup> at the 6H site is analyzed. Theoretically, ab initio calculations are conducted to reveal the distinctive properties of the Ce–O coordination at the two Ce<sup>3+</sup> sites. The calculated eigenstates of Ce<sup>3+</sup> at the 6H site suggest a weak Ce–O covalent bond formed between Ce<sup>3+</sup> and one of the coordinated oxygen ions not bonded with Si<sup>4+</sup>. The electronic energy levels and frequencies of local vibrational modes are correlated with specific Ce–O pairs to provide a comparative understanding of the site-resolved experimental results. On the basis of the calculated results, we propose a model of charge transfer and vibronic coupling for interpretation of the anomalous thermal quenching of the Ce<sup>3+</sup> luminescence. The combination of experimental and theoretical studies in the present work provides a comprehensive understanding of the spectroscopy and luminescence dynamics of Ce<sup>3+</sup> in crystals of ionic–covalent coordination

    Disproportionation in Li–O<sub>2</sub> Batteries Based on a Large Surface Area Carbon Cathode

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    In this paper we report on a kinetics study of the discharge process and its relationship to the charge overpotential in a Li–O<sub>2</sub> cell for large surface area cathode material. The kinetics study reveals evidence for a first-order disproportionation reaction during discharge from an oxygen-rich Li<sub>2</sub>O<sub>2</sub> component with superoxide-like character to a Li<sub>2</sub>O<sub>2</sub> component. The oxygen-rich superoxide-like component has a much smaller potential during charge (3.2–3.5 V) than the Li<sub>2</sub>O<sub>2</sub> component (∼4.2 V). The formation of the superoxide-like component is likely due to the porosity of the activated carbon used in the Li–O<sub>2</sub> cell cathode that provides a good environment for growth during discharge. The discharge product containing these two components is characterized by toroids, which are assemblies of nanoparticles. The morphologic growth and decomposition process of the toroids during the reversible discharge/charge process was observed by scanning electron microscopy and is consistent with the presence of the two components in the discharge product. The results of this study provide new insight into how growth conditions control the nature of discharge product, which can be used to achieve improved performance in Li–O<sub>2</sub> cell

    Exploring Stability of Nonaqueous Electrolytes for Potassium-Ion Batteries

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    Recently nonaqueous potassium-ion batteries (KIBs) have attracted tremendous attention, but a systematic study about the electrolytes remains lacking. Here, the stability of a commonly used electrolyte (KPF<sub>6</sub> in ethylene carbonate (EC) and diethyl carbonate (DEC)) at the anodes (e.g., graphite, solid K, and liquid Na–K alloy) was studied. Interesting results show that the linear DEC is unstable. Possibly attributed to stronger reducibility against the anodes for KIBs, the decomposition of DEC is initiated by the C­(H<sub>2</sub>)–O bond breaking of the solvent molecule. This study shows that a systematic study to look for a more stable electrolyte is critically important for KIBs

    Deciphering the Formation and Accumulation of Solid-Electrolyte Interphases in Na and K Carbonate-Based Batteries

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    The continuous solid-electrolyte interphase (SEI) accumulation has been blamed for the rapid capacity loss of carbon anodes in Na and K ethylene carbonate (EC)/diethyl carbonate (DEC) electrolytes, but the understanding of the SEI composition and its formation chemistry remains incomplete. Here, we explain this SEI accumulation as the continuous production of organic species in solution-phase reactions. By comparing the NMR spectra of SEIs and model compounds we synthesized, alkali metal ethyl carbonate (MEC, M = Na or K), long-chain alkali metal ethylene carbonate (LCMEC, M = Na or K), and poly(ethylene oxide) (PEO) oligomers with ethyl carbonate ending groups are identified in Na and K SEIs. These components can be continuously generated in a series of solution-phase nucleophilic reactions triggered by ethoxides. Compared with the Li SEI formation chemistry, the enhancement of the nucleophilicity of an intermediate should be the cause of continuous nucleophilic reactions in the Na and K cases

    Molecular Sieve Induced Solution Growth of Li<sub>2</sub>O<sub>2</sub> in the Li–O<sub>2</sub> Battery with Largely Enhanced Discharge Capacity

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    The formation of the insulated film-like discharge products (Li<sub>2</sub>O<sub>2</sub>) on the surface of the carbon cathode gradually hinders the oxygen reduction reaction (ORR) process, which usually leads to the premature death of the Li–O<sub>2</sub> battery. In this work, by introducing the molecular sieve powder into the ether electrolyte, the Li–O<sub>2</sub> battery exhibits a largely improved discharge capacity (63 times) compared with the one in the absence of this inorganic oxide additive. Meanwhile, XRD and SEM results qualitatively demonstrate the generation of the toroid Li<sub>2</sub>O<sub>2</sub> as the dominated discharge products, and the chemical titration quantifies a higher yield of the Li<sub>2</sub>O<sub>2</sub> with the presence of the molecular sieve additive. The addition of the molecular sieve controls the amount of the free water in the electrolyte, which distinguishes the effect of the molecular sieve and the free water on the discharge process. Hence, a possible mechanism has been proposed that the adsorption of the molecular sieves toward the soluble lithium superoxides improves the disproportionation of the lithium superoxides and consequently enhances the solution-growth of the lithium peroxides in the low donor number ether electrolyte. In general, the application of the molecular sieve triggers further studies concerning the improvement of the discharge performance in the Li–O<sub>2</sub> battery by adding the inorganic additives

    Dendrite-Free Potassium–Oxygen Battery Based on a Liquid Alloy Anode

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    The safety issue caused by the dendrite growth is not only a key research problem in lithium-ion batteries but also a critical concern in alkali metal (i.e., Li, Na, and K)–oxygen batteries where a solid metal is usually used as the anode. Herein, we demonstrate the first dendrite-free K–O<sub>2</sub> battery at ambient temperature based on a liquid Na–K alloy anode. The unique liquid–liquid connection between the liquid alloy and the electrolyte in our alloy anode-based battery provides a homogeneous and robust anode–electrolyte interface. Meanwhile, we manage to show that the Na–K alloy is only compatible in K–O<sub>2</sub> batteries but not in Na–O<sub>2</sub> batteries, which is mainly attributed to the stronger reducibility of potassium and relatively more favorable thermodynamic formation of KO<sub>2</sub> over NaO<sub>2</sub> during the discharge process. It is observed that our K–O<sub>2</sub> battery based on a liquid alloy anode shows a long cycle life (over 620 h) and a low discharge–charge overpotential (about 0.05 V at initial cycles). Moreover, the mechanism investigation into the K–O<sub>2</sub> cell degradation shows that the O<sub>2</sub> crossover effect and the ether–electrolyte instability are the critical problems for K–O<sub>2</sub> batteries. In a word, this study provides a new route to solve the problems caused by the dendrite growth in alkali metal–oxygen batteries

    Achieving Low Overpotential Lithium–Oxygen Batteries by Exploiting a New Electrolyte Based on <i>N</i>,<i>N</i>′‑Dimethylpropyleneurea

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    Recently, the lithium–oxygen (Li–O<sub>2</sub>) battery has attracted much interest due to its ultrahigh theoretical energy density. However, its potential application is limited by an unstable electrolyte system, low round-trip efficiency, and poor cyclic performance. In this study, we present a new electrolyte based on <i>N</i>,<i>N</i>′-dimethylpropyleneurea (DMPU) applied for the Li–O<sub>2</sub> battery. This electrolyte possesses high ionic conductivity and achieves a low discharge/charge voltage gap of 0.6 V, which is mainly due to the possible one-electron charge transfer mechanism. The introduction of the antioxidant butylatedhydroxytoluene (BHT) as an additive stabilizes the superoxide radical by chemical adsorption and improves the cyclic performance remarkably. Thus, this new electrolyte system may be one of the candidates for Li–O<sub>2</sub> batteries

    Graphene-Directed Formation of a Nitrogen-Doped Porous Carbon Sheet with High Catalytic Performance for the Oxygen Reduction Reaction

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    A nitrogen (N)-doped porous carbon sheet is prepared by in situ polymerization of pyrrole on both sides of graphene oxide, following which the polypyrrole layers are then transformed to the N-doped porous carbon layers during the following carbonization, and a sandwich structure is formed. Such a sheet-like structure possesses a high specific surface area and, more importantly, guarantees the sufficient utilization of the N-doping active porous sites. The internal graphene layer acts as an excellent electron pathway, and meanwhile, the external thin and porous carbon layer helps to decrease the ion diffusion resistance during electrochemical reactions. As a result, this sandwich structure exhibits prominent catalytic activity toward the oxygen reduction reaction in alkaline media, as evidenced by a more positive onset potential, a larger diffusion-limited current, better durability and poison-tolerance than commercial Pt/C. This study shows a novel method of using graphene to template the traditional porous carbon into a two-dimensional, thin, and porous carbon sheet, which greatly increases the specific surface area and boosts the utilization of inner active sites with suppressed mass diffusion resistance

    Interfacial Effects on Lithium Superoxide Disproportionation in Li-O<sub>2</sub> Batteries

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    During the cycling of Li-O<sub>2</sub> batteries the discharge process gives rise to dynamically evolving agglomerates composed of lithium–oxygen nanostructures; however, little is known about their composition. In this paper, we present results for a Li-O<sub>2</sub> battery based on an activated carbon cathode that indicate interfacial effects can suppress disproportionation of a LiO<sub>2</sub> component in the discharge product. High-intensity X-ray diffraction and transmission electron microscopy measurements are first used to show that there is a LiO<sub>2</sub> component along with Li<sub>2</sub>O<sub>2</sub> in the discharge product. The stability of the discharge product was then probed by investigating the dependence of the charge potential and Raman intensity of the superoxide peak with time. The results indicate that the LiO<sub>2</sub> component can be stable for possibly up to days when an electrolyte is left on the surface of the discharged cathode. Density functional calculations on amorphous LiO<sub>2</sub> reveal that the disproportionation process will be slower at an electrolyte/LiO<sub>2</sub> interface compared to a vacuum/LiO<sub>2</sub> interface. The combined experimental and theoretical results provide new insight into how interfacial effects can stabilize LiO<sub>2</sub> and suggest that these interfacial effects may play an important role in the charge and discharge chemistries of a Li–O<sub>2</sub> battery

    Raman Evidence for Late Stage Disproportionation in a Li–O<sub>2</sub> Battery

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    Raman spectroscopy is used to characterize the composition of toroids formed in an aprotic Li–O<sub>2</sub> cell based on an activated carbon cathode. The trends in the Raman data as a function of discharge current density and charging cutoff voltage provide evidence that the toroids are made up of outer LiO<sub>2</sub>-like and inner Li<sub>2</sub>O<sub>2</sub> regions, consistent with a disproportionation reaction occurring in the solid phase. The LiO<sub>2</sub>-like component is found to be associated with a new Raman peak identified in the carbon stretching region at ∼1505 cm<sup>–1</sup>, which appears only when the LiO<sub>2</sub> peak at 1123 cm<sup>–1</sup> is present. The new peak is assigned to distortion of the graphitic ring stretching due to coupling with the LiO<sub>2</sub>-like component based on density functional calculations. These new results on the LiO<sub>2</sub>-like component from Raman spectroscopy provide evidence that a late stage disproportionation mechanism can occur during discharge and add new understanding to the complexities of possible processes occurring in Li–O<sub>2</sub> batteries
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