1,721,139 research outputs found
Factors that affect Li mobility in layered lithium transition metal oxides
No full textThe diffusion constant of Li in electrode materials is a key aspect of the rate capability of rechargeable Li batteries. The factors that affect Li mobility in layered lithium transition metal oxides are systematically studied in this paper by means of first-principles calculations. In close packed oxides octahedral ions diffuse by migrating through intermediate tetrahedral sites. Our results indicate that the activation barrier for Li hopping is strongly affected by the size of the tetrahedral site and the electrostatic interaction between Li+ in that site and the cation in the octahedron that shares a face with it. The size of the tetrahedral site is determined by the c-lattice parameter which has a remarkably strong effect on the activation barrier for Li migration. The effect of other factors such as cation mixing and doping with nontransition metal ions can be interpreted quantitatively in terms of the size and electrostatic effect. A general strategy to design high rate electrode materials is discussed.This work was supported by the MRSEC Program of the National Science Foundation under Grant No. DMR 02-13282, by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098, Subcontracts No. 6517748
and No. 6517749 with the Lawrence Berkeley National Laboratory. Additional computer resources were provided by the National Partnership for Advanced Computing Infrastructure (NPACI)
The Li intercalation potential of LiMPO 4 and LiMSiO 4 olivines with M = Fe, Mn, Co, Ni
No full textThe Li intercalation potential of LiMPO4 and LiMSiO4 compounds with M = Fe, Mn, Co and Ni is computed with the GGA + U method. It is found that this approach is considerably more accurate than standard LDA or GGA methods. The calculated potentials for LiFePO4, LiMnPO4 and LiCoPO4 agree to within 0.1 V with experimental results. The LiNiPO4 potential is predicted to be above 5 V. The potentials of the silicate materials are all found to be rather high, but LiFeSiO4 and LiCoSiO4 have negligible volume change upon Li extraction
Synthesis, electrochemical properties, and phase stability of Li2NiO2 with the Immm structure
No full textThe electrochemical properties and phase stability of the orthorhombic Immm structure of composition Li2NiO2 are studied experimentally and with first principles calculations. The material shows a high specific charge capacity of about 320 mAh/g and discharge capacity of about 240 mAh/g at the first cycle. The experimental results and first principles calculations all indicate that the orthorhombic Immm structure is rather prone to phase transformation to a close-packed layered structure during the electrochemical cycling. The possibility of stabilizing the orthorhombic Immm structure during the electrochemical cycling by partial substitution of Ni is also evaluated. A detailed analysis of the crystal field energy difference between octahedral and square-planar coordinated Ni2+ indicates that crystal field effects may not be large enough to stabilize Ni2+ in a square planar environment when the cost of electron pairing is taken into account. Rather, we attribute the stability of Li2NiO2 in the Immm structure to the more favorable Li arrangement as compared to a possible Li2NiO2 structure with octahedral Ni.We thank Prof. Yang Shao Horn for the valuable discussion. This work was supported
by the MRSEC Program of the National Science Foundation under award DMR 02-13282, by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract DE-AC03-
76SF00098, Subcontract 6517748 with the Lawrence Berkeley National Laboratory, and in part by the Ministry of Education of Taiwan (EX-91-E-FA09-5-4). We are grateful to Dr. Dane Morgan, Fei Zhou, Dr.Anton Van der Ven, Dr. Dany Carlier, Chris Fischer,
and Tim Mueller for their advice
The electronic structure and band gap of LiFePO4 and LiMnPO4
No full textMaterials with the olivine LixMPO4 structure form an important new class of materials for rechargeable Li batteries. There is significant interest in their electronic properties because of the importance of electronic conductivity in batteries for high-rate applications. The density of states of LixMPO4 (x = 0, 1 and M = Fe, Mn) has been determined with the ab initio generalized gradient approximation (GGA) + U method, appropriate for these correlated electron systems. Computed results are compared with the optical gap of LiFePO4, as measured using UV-Vis-NIR diffuse reflectance spectroscopy. The results obtained from experiment (3.8-4.0 eV) and GGA + U computations (3.7 eV) are in very good agreement. However, standard GGA, without the same level of treatment of electron correlation, is shown to make large errors in predicting the electronic structure. It is argued that olivines are likely to be polaronic conductors with extrinsically determined carrier levels and that their electronic conductivity is therefore not simply related to the band gap. (C) 2004 Elsevier Ltd. All rights reserved.We gratefully acknowledge support from the Department of Energy under grant DE-FG02-96ER45571, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies
of the US Department of Energy via subcontract number 6517748, the National Science Foundation (MRSEC Program) under contract DMR-0213282, and computing
resources from the National Partnership for Advanced Computational Infrastructure (NPACI)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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