102,524 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)
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 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 and electrochemical properties of layered LiNi2/3Sb1/3O2
No full textLiNi2/3Sb1/3O2 has been prepared by ion-exchange from NaNi2/3Sb1/3O2. Powder X-ray diffraction (XRD) and single crystal electron diffraction by transmission electron microscopy (TEM) of the material indicate that it is isostructural with alpha-NaFeO2. Electrochemical test shows that the reversible capacity for Li intercalation and deintercalation drops rapidly with the number of times cycled. Both experimental evidences and first principles calculations point to the migration of nickel as the reason for the poor capacity retention. (C) 2007 Published by Elsevier B.V.This work was supported by the Assistant Secretary for Energy Efficiency, Office of FreedomCAR and Vehicle Technologies
of the US Department of Energy under contract No.DE-AC02-05CH11231, via subcontract No. PO 6806960. We also acknowledge the support by the Center for Materials Science and Engineering atMITunder contract No. DMR-0213282. Valuable discussion with Prof. C.P. Grey is acknowledged. X.H. Ma would like to thank Y. Hinuma and B. Kang for their fruitful
discussions
Phase transitions in the LiNi0.5Mn0.5O2 system with temperature
No full textWe investigate the phase transformations of layered LiNi0.5Mn0.5O2 at finite temperature with a combined computational and experimental approach. The detailed changes in the ionic configurations with temperature are investigated by Monte Carlo simulations on the basis of a coupled cluster expansion that describes the dependence of the energy on the arrangement of Li+, Ni2+, and Mn4+ in the lithium layer and transition metal layer. First-principles energies in the GGA+U approximation were used to fit the Hamiltonian, as we find that GGA+U better represents magnetic interactions than standard GGA. The simulation results suggest two phase-transition temperatures at approximately 550 and 620 degrees C. Below the first phase-transition temperature, a structure with almost no Li/Ni disorder in the Li layer is energetically favorable. Between the two temperatures, a partially disordered flower structure with about 8-11% Li/Ni disorder is found. Above the second phase transition, a structure that is more disordered but still consistent with a root 3 x root 3 honeycomb model with 8-11% Li/Ni disorder is stable. The results from these simulations are corroborated with DSC, TEM, and XRD measurements on a recently synthesized LiNi0.5Mn0.5O2 with negligible Li/Ni disorder.The work was supported 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, via subcontracts 6517748 and 6517749 with the Lawrence Berkeley National Laboratory. We also acknowledge the support by the
Center for Materials Science and Engineering, MIT, and the financial support by the Materials Research Science and
Engineering Centers program of the National Science Foundation under award DMR 02-13282. Valuable discussion with Prof. Clare Grey and Prof. Yang Shao-Horn is acknowledged.
Y.H. thanks Tim Mueller for extensive help with preparing the cluster expansion and Monte Carlo simulation code. Y.S.M.thanks
Prof. Heike Gabrisch from the University of New Orleans for helpful discussion on the TEM part of the work. Some figures
were created with VICS software in the VENUS package
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)
Challenges for Na-ion Negative Electrodes
No full textNa-ion batteries have been proposed as candidates for replacing Li-ion batteries. In this paper we examine the viability of Na-ion negative electrode materials based on Na alloys or hard carbons in terms of volumetric energy density. Due to the increased size of the Na atom compared to the Li atom, Na alloys would lead to negative electrode materials with roughly half the volumetric energy density of their Li analogs. Volumetric energy densities obtainable with sodiated hard carbons would also be significantly less than those obtainable with lithiated graphite. These findings highlight the need of novel ideas for Na-ion negative electrodes.United States. Dept. of Energy (Contract No. DE-FG02-96ER45571)United States. Dept. of Energy. Batteries for Advanced Transportation Technologies (BATT) Progra
Investigation of the Effect of Functional Group Substitutions on the Gas-Phase Electron Affinities and Ionization Energies of Room-Temperature Ionic Liquids Ions using Density Functional Theory
No full textThe cathodic and anodic stabilities of room-temperature ionic liquids (ILs) are important factors in their applications in electrochemical devices. In this work, we investigated the electron affinities of cations and ionization energies of anions for ionic liquids by density functional theory (DFT) calculations at the B3LYP/6-311+G(2d,p)//B3LYP/6-31+G(d) level. Over 200 unique cations and anions, formed from a set of six base cation structures, three base anion structures, and seven functional groups, were investigated. We find the trends in calculated EAs of alkylated cations and IEs of alkylated anions to be in good agreement with observed experimental trends in relative cathodic and anodic stabilities of various ILs. In addition, we also investigated the effect that functional group substitution at distinct positions in the ions have on the EA of the 1,2,3-trimethylimidazolium cation and the IE of the PF5CF3 anion. The overall impact on the EA or IE can be explained by the known electron-donating and electron-withdrawing inductive and resonance effects of the attached functional group, and the relative strength of the effect depends on the substitution position.DuPont MIT AllianceNational Science Foundation (U.S.) (TeraGrid resouces
High Rate Micron-Sized Ordered LiNi0.5Mn1.5O4
No full textOrdered LiNi[subscript 0.5]Mn[subscript 1.5]O[subscript 4] was synthesized through a solid-state reaction. Even though the material has a particle size of 3–5μm , it shows very high rate capability and excellent capacity retention. The capacity is as high as ≈78mAh/g at a 167C discharge rate. This high discharge rate performance is consistent with first-principles calculations of the activation barrier for lithium motion, which predict the lithium diffusivity in this material to be around 10[superscript −9]–10[superscript −8]cm[superscript 2]/s . We also systematically investigated the effect of several cell components and electrode construction on the measured rate performance and conclude that care has to be taken to remove all other rate limitations from the cell to measure the rate performance of an electrode material.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (grant no. DMR-0819762)United States. Dept. of Energy. Batteries for Advanced Transportation Technologies (BATT) Program (contract no. 6806960
Electrochemical Performance of LiMnPO[subscript 4] Synthesized with Off-Stoichiometry
No full textLiMnPO[subscript 4] was synthesized from an off-stoichiometric mix of starting materials with nominal composition LiMn[subscript 0.9]P[subscript 0.95]O[subscript 4−δ]. Stoichiometric LiMnPO[subscript 4] with particle size <50 nm was found with X-ray diffraction even with the large overall deviation from stoichiometry in the sample, indicating that other noncrystalline compounds are present. The off-stoichiometric sample had a discharge capacity of 145 mAh/g at C/10 and ∼100mAh/g at 2C after a constant current constant voltage charge. Capacity retention was excellent without a significant capacity loss at 1C. The performance of LiMn[subscript 0.9]P[subscript 0.95]O[subscript 4−δ] could not be significantly improved by diluting the electrode with more carbon, indicating a more intrinsic kinetic limitation of the material for LiMnPO[subscript 4] in contrast to LiFePO[subscript 4] [Nature (London), 458, 190 (2009)]. This is further corroborated by the difference in the overpotential between the two materials. Although LiMnPO[subscript 4] has a sloping voltage profile, LiFePO[subscript 4] tends to have a constant overpotential at high rate.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Contract DMR-0819762)United States. Dept. of Energy. Batteries for Advanced Transportation Technologies (BATT) Program (Contract DE-AC02-05CH1123, Subcontract PO6806960
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