135,677 research outputs found
Structural and electrochemical characterization of xLi[Li1/3Mn2/3]O2·(1 - x)Li[Ni1/3Mn1/3Co1/3]O2 (0 ≤ x ≤ 0.9) as cathode materials for lithium ion batteries
No full textA series of cathode materials with molecular notation of xLi[Li1/3Mn2/3]O2·(1 - x)Li[Ni1/3Mn1/3Co1/3]O2 (0 ≤ x ≤ 0.9) were synthesized by combination of co-precipitation and solid state calcination method. The prepared materials were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques, and their electrochemical performances were investigated. The results showed that sample 0.6Li[Li1/3Mn2/3]O2·0.4Li[Ni1/3Mn1/3Co1/3]O2 (x = 0.6) delivers the highest capacity and shows good capacity-retention, which delivers a capacity ∼250 mAh g-1 between 2.0 and 4.8 V at 18 mA g-1. © 2008 Elsevier B.V. All rights reserved
A new bound for the smallest x with ?(x) > li(x)
No full textWe reduce the leading term in Lehman's theorem. This improved estimate allows us to refine the main theorem of Bays and Hudson [2]. Entering 2,000,000 Riemann zeros, we prove that there exists x in the interval [exp (727.951858), exp (727.952178)] for which ?(x) - li(x) > 3.2 × 10151. There are at least 10154 successive integers x in this interval for which ?(x) > li(x). This interval is strictly a sub-interval of the interval in Bays and Hudson, and is narrower by a factor of about 12
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
Measurements of K S 0 - K L 0 asymmetries in the decays Λ c + → p K L , S 0 , p K L , S 0 π + π − and p K L , S 0 π 0
No full textAbstract Using e + e − annihilation data sets corresponding to an integrated luminosity of 4.5 fb −1, collected with the BESIII detector at center-of-mass energies between 4.600 and 4.699 GeV, we report the first measurements of the absolute branching fractions B Λ c + → p K L 0 = (1.67 ± 0.06 ± 0.04)%, B Λ c + → p K L 0 π + π − = (1.69 ± 0.10 ± 0.05)%, and B Λ c + → p K L 0 π 0 = (2.02 ± 0.13 ± 0.05)%, where the first uncertainties are statistical and the second systematic. Combining with the known branching fractions of Λ c + → p K S 0 , Λ c + → p K S 0 π + π − , and Λ c + → p K S 0 π 0 , we present the first measurements of the K S 0 - K L 0 asymmetries R Λ c + K S , L 0 X = B Λ c + → K S 0 X − B Λ c + → K L 0 X B Λ c + → K S 0 X + B Λ c + → K L 0 X in charmed baryon decays: R Λ c + p K S , L 0 = − 0.025 ± 0.031 , R Λ c + p K S , L 0 π + π − = − 0.027 ± 0.048 and R Λ c + p K S , L 0 π 0 = − 0.015 ± 0.046 . No significant asymmetries with statistical significance are observed
Advancing knowledge of electrochemically generated lithium microstructure and performance decay of lithium ion battery by synchrotron X-ray tomography
No full textAn intrinsic knowledge gap between current understandings obtained experimentally and the underlying working or degradation mechanisms of rechargeable lithium batteries still remains, giving direct rise to application challenges, e.g., safety issues, predicaments in identifying performance-aging factors and dilemmas in guiding further research directions. Against this background, non-destructive and three-dimensional (synchrotron) X-ray tomography that guarantees a direct visual access to inner electrodes has been employed herein to: in-situ record the evolution of internal short circuits; characterize the behaviors of widely employed separators; investigate the morphological evolution of Li electrodes under different cycling conditions; and study the degradation mechanisms of Li/carbon cells. By incorporating the currently presented results with the previously published studies on those topics, a complete picture of the degradation mechanism of rechargeable lithium batteries has been painted. This advancement of mechanistic understanding supplies the missing pieces of information to bridge fundamental R&D research activities and practical applications
A Study of the Thermodynamics and Kinetics of LiₓFePO₄ as a Cathode Material for Li Batteries
No full textOlivine-type LiFePO4 has been recognized as one of the most promising cathode materials for rechargeable Li batteries. Its advantages include high capacity, high stability, nontoxicity, and low cost. Our methods for synthesizing nanocrystalline LixFePO4 with the olivine structure are described. Solid-state reactions and precipitation reactions were both successful, and ball milling was especially effective at reducing crystallite sizes. Diffractometry and microscopy were used to characterize these materials, and results of impurity phases, excess Fe3+, and internal stresses are reported for the different types of synthesis.
Applications of lithium-ion batteries, including automotive applications, require fast kinetics and high conductivity of ions and electrons. Unfortunately, LixFePO4 has the electronic structure of an insulator, an entirely unsatisfactory situation if it is to be used as a battery electrode. Electrical conductivity in LixFePO4 occurs by the motion of small polarons, which are valence electrons at Fe atoms plus their distorted local environments. Electrical conductivity of LixFePO4 is interpreted in terms of small polaron hopping. There are other factors of importance in these measurements, such as impurities or defects that block the one-dimensional conduction channels of the olivine structure of LixFePO4.
We studied the polaron hopping directly, which allows us to understand the intrinsic electrical conductivity, and how it depends on microstructure and composition of LixFePO4. The experimental technique was Mossbauer spectrometry, which has been used for many years as a means for determining the fractions of Fe2+ and Fe3+ in a material. Usually the spectral signatures of Fe2+ and Fe3+ are distinct. When valence electrons hop between Fe2+ and Fe3+ at a frequency of 108 Hz or higher, however, the valence changes during the timescale of the Mossbauer measurement and the spectrum is blurred. By measuring Mossbauer spectra at elevated temperatures, we can determine the fractions of Fe atoms participating in polaron hopping, and determine the activation energy of the process. From this we estimate intrinsic electrical conductivities of 10-7S/cm at room temperature for nanocrystalline Li0.5FePO4, for example. We find a comparable conductivity for LixFePO4 prepared as a solid solution, but the conductivity of conventional LixFePO4 is much lower.
There has been much discussion about how surface area might thermodynamically stabilize the solid solution phase of nanocrystalline LixFePO4. In a series of X-ray diffraction measurements, some at elevated temperatures, we found the solid solution phase of LixFePO4 to be especially robust at room temperature when the material was prepared in nanocrystalline form. Moreover, the consistent phase transition temperature around 200°C was observed, as evidence for the unchanged equilibrium phase diagram by crystallite size. This is consistent with our evaluation on the boundaries of the two-phase mixture of triphylite and heterosite during Li insertion and extraction. Profiles of entropy and enthalpy changes were evaluated by open-circuit voltage measurements. The boundaries were found at x=0.05 and 0.95 in the LixFePO4 with crystal size of 70 nm, similar to the reported values on bulk-LixFePO4. These are important in practice, because electrochemical lithiation and delithiation at room temperature should remain as a two-phase transformation, even if a solid solution of lithium is present in the initial electrode material.</p
A three-dimensional TiO2-Graphene architecture with superior Li ion and Na ion storage performance
No full textIn this work, a three-dimensional TiO2-graphene composite with large specific surface area is designed by freeze drying. In this architecture, primary TiO2 nanoparticles (less than 10 nm in size) are wrapped with graphene homogeneously, forming spherical secondary particles (≈100 nm), and the spherical TiO2 particles further agglomerate into platelet-like particles with several micrometers in size. The TiO2-graphene composite delivers high de-lithiation and de-sodiation capacities of 312 mAh g−1 and 280 mAh g−1 at 17 mA g−1 as negative electrode materials in lithium and sodium cells, respectively, and excellent cycling performance with negligible capacity loss after 500 cycles at a specific current of 85 mA g−1. When coupled with an activated carbon positive electrode, it demonstrates high capacitance and long cycle life in sodium ion capacitors (81% after 2000 cycles at 1 A g−1) and lithium ion capacitors (95% after 5000 cycles at 1 A g−1). Its superior performance benefits from the designed 3D architecture, which combines the advantages of small primary particle size, a homogeneous carbon coating and an adequate contact area with the liquid electrolyte. The storage mechanisms of the superior TiO2-graphene composite in lithium and sodium cells are investigated by operando X-ray diffraction studies
Aster jiangkouensis X. L. Yu & X. Li 2021, sp. nov.
No full textAster jiangkouensis X. L. Yu & X. Li, sp. nov. (Figs. 1 & 2) Diagnosis: —The species is morphologically similar to the adjacently distributed A. huangpingensis W. P. Li & Z. Li (Li et al. 2020: 242), but mainly differs in the following characters: leaves inconspicuously triplinerved (vs. conspicuously veined), leaf blades ovate to oblong-lanceolate (vs. narrowly oblanceolate or spatulate), involucre cylindric-campanulate (vs. campanulate), peduncle and pedicels glabrous (vs. sparsely strigillose), achenes 4-ribbed (vs. 2- or 3-ribbed). Type: — CHINA. Guizhou Province: Tongren city, Jiangkou county, Zhenjiang village [27°42’N, 108°48’E], elevation ca. 490 m, on the moist rock of cliff near river of karst valley, 2 November 2019, X. Li 19110201 (holotype CSFI!; isotype CSFI!). FIGURE 3. Distribution map of Aster jiangkouensis (★). Herbs perennial, 20–60 cm tall, rhizomes robust. Stem solitary, pendent or ascending, branched only in inflorescence, slender, glabrous. Leaves basal and cauline, lustrous, veins triplinerved, inconspicuous, thinly leathery, margins villous-ciliate, adaxially villous-strigose, abaxially glabrous or sparsely strigose, obviously when young. Basal leaf blades ovate, usually present at anthesis, 2–7 × 0.8–4 cm, base gradually narrowing, margin serrate with 3–5 pair of teeth, base attenuate, apex acute; lower cauline leaf blades ovate to lanceolate 3–7 × 2–4 cm, petiole 2–6 cm, base cuneate, apex acuminate; middle leaves lanceolate to oblong-lanceolate, 3–5 × 1–2 cm, shortly petiolate; upper leaves oblong-lanceolate, margin often entire, 2.5–3 × 0.5–1 cm, petiole nearly sessile, apex long acuminate. Capitula 3–7 (occasionally to 18) in terminal lax corymbiform to paniculate-corymbiform inflorescences, 2–2.5 cm in diameter; involucre cylindric-campanulate, 5.8–8 × ca. 3.8 mm; phyllaries 4–5-seriate, herbaceous, green, margins more or less scarious, fimbriate-ciliate, apex acute, receptacle slightly convex, abaxially sparsely puberulous; outermost phyllaries ovate-lanceolate, 0.6–0.9 × 2.4–2.8 mm; second-row phyllaries oblong-lanceolate, 0.7–1 × 3.5–4.2 mm; middle phyllaries oblong, 0.9–1.3 × 4.8–5.5 mm; fourth-row phyllaries narrowly oblong, 0.9–1.3 × 6–7 mm. Ray florets 9–13, female, glabrous, tube 3.4–4.2 mm long, ligules white, 9.5–11 × 1.3–1.7 mm, apex slightly 2–3-lobed. Disk florets hermaphrodite, many, tube base flared, limb campanulate, corolla yellow, 4.6–6 mm long, tube 3–3.7 mm long, sparsely strigillose at base, 5-lobed, lobes narrowly triangular; stigmatic appendage (style branch appendage) lanceolate, ca. 0.9 mm long. Anthers lanceolate, ca. 6.2 mm long (excluding collar), Achenes (mature) oblong, 2.5–2.8 × 0.6–0.8 mm, densely strigose, 4-ribbed. Pappus reddish-brown, short than disc corolla at anthesis, unequal, barbellate bristles (mature) 4.5–6 mm long. Phenology: —Flowering from September to early November and fruiting from November. Etymology: — Aster jiangkouensis is named after its type locality, Jiangkou county, which is situated in the southwestern Wuling Mountain. The Chinese name is given as “ 江口Ãẃ ”. Distribution and habitat: —The new species is presently recorded from the karst area of Jiangkou county, Tongren city, Guizhou Province, China. All individuals are restricted to crevices of damp limestone cliffs and rocks near the edge of the river at the elevation range of 400– 500 m. Preliminary conservation status: —So far, only three populations with a total of approximately 300 individuals were observed in a relatively narrow area (ca. 7.5 km 2). Moreover, Aster jiangkouensis appears to require undisturbed habitats. Two populations in narrow canyons, which are nearly inaccessible to the humans, are quite dense in both reproductive and immature individuals. On the contrary, only several weak individuals were found in disturbed habitats near roads and residential areas. Unfortunately, with the further local tourism development, these populations will be undoubtedly impacted. Therefore, based on its small population size, very limited distribution and threatening factors, A. jiangkouensis should be evaluated as Endangered (EN B2ab (iii) & D) (IUCN 2016). Notes: — Aster jiangkouensis is mainly characterized by inconspicuously triplinerved leaves, lax corymbiform to paniculate-corymbiform inflorescences, cylindric-campanulate involucre and reddish-brown pappus. In addition, the habitat of the new species also deserves attention; it grows in crevices of damp limestone cliffs and rocks near the edge of the river, especially favoring shady narrow canyons. The members of Aster occurring in such limestone habitats consist of only a few species in China, including A. jishouensis W. P. Li et S. X. Liu (2002: 455), A. saxicola W. P. Li & Z. Li (Xiong et al. 2019: 6), A. ageratoides var. pendulus W. P. Li & G. X. Chen and the new one. There are some morphological characters shared by these species, i.e. robust rhizome, lanceolate leaves, long and usually pendent stem, only basal leaves during the vegetative period.Published as part of Li, Xiong, Wu, Lei & Yu, Xun-Lin, 2021, Aster jiangkouensis (Asteraceae), a new species from Guizhou, China, pp. 75-79 in Phytotaxa 483 (1) on pages 77-79, DOI: 10.11646/phytotaxa.483.1.4, http://zenodo.org/record/542022
Generalized Marshall-Olkin Distributions, and Related Bivariate Aging Properties
No full textNational Natural Science Foundation of China [10771090]A class of generalized bivariate Marshall-Olkin distributions, which includes as special cases the Marshall-Olkin bivariate exponential distribution and the Marshall-Olkin type distribution due to Muliere and Scarsini (1987) [19] are examined in this paper. Stochastic comparison results are derived, and bivariate aging properties, together with properties related to evolution of dependence along time, are investigated for this class of distributions. Extensions of results previously presented in the literature are provided as well. (C) 2011 Elsevier Inc. All rights reserved
Mapping Heterogeneity of Pristine and Aged Li- and N-Mnhcf Cathode by Synchrotron-Based Energy-Dependent Full Field Transmission X-ray Microscopy
No full textManganese hexacyanoferrate is a promising cathode material for lithium and sodium ion batteries, however, it suffers of capacity fading during the cycling process. To access the structural and functional characteristics at the nanometer scale, fresh and cycled electrodes are extracted and investigated by transmission soft X-ray microscopy, which allows chemical characterization with spatial resolution from position-dependent x-ray spectra at the Mn L-, Fe L- and N K-edges. Furthermore, soft X-rays prove to show superior sensitivity toward Fe, compare to hard X-rays. Inhomogeneities within the samples are identified, increasing in the aged electrodes, more dramatically in the Li-ion system, which explains the poorer cycle life as Li-ion cathode material. Local spectra, revealing different oxidation states over the sample with strong correlation between the Fe L-edge, Mn L-edge, and N K-edge, imply a coupling between redox centers and an electron delocalization over the host framework
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