Work functions, ionization potentials, and in-between: Scaling relations based on the image charge model

We revisit a model in which the ionization energy of a metal particle is associated with the work done by the image charge force in moving the electron from infinity to a small cut-off distance just outside the surface. We show that this model can be compactly, and productively, employed to study the size dependence of electron removal energies over the range encompassing bulk surfaces, finite clusters, and individual atoms. It accounts in a straightforward manner for the empirically known correlation between the atomic ionization potential (IP) and the metal work function (WF), IP/WF$\sim$2. We formulate simple expressions for the model parameters, requiring only a single property (the atomic polarizability or the nearest neighbor distance) as input. Without any additional adjustable parameters, the model yields both the IP and the WF within $\sim$10% for all metallic elements, as well as matches the size evolution of the ionization potentials of finite metal clusters for a large fraction of the experimental data. The parametrization takes advantage of a remarkably constant numerical correlation between the nearest-neighbor distance in a crystal, the cube root of the atomic polarizability, and the image force cutoff length. The paper also includes an analytical derivation of the relation of the outer radius of a cluster of close-packed spheres to its geometric structure.Comment: Original submission: 8 pages with 7 figures incorporated in the text. Revised submission (added one more paragraph about alloy work functions): 18 double spaced pages + 8 separate figures. Accepted for publication in PR

Ab initio studies of structures and properties of small potassium clusters

We have studied the structure and properties of potassium clusters containing even number of atoms ranging from 2 to 20 at the ab initio level. The geometry optimization calculations are performed using all-electron density functional theory with gradient corrected exchange-correlation functional. Using these optimized geometries we investigate the evolution of binding energy, ionization potential, and static polarizability with the increasing size of the clusters. The polarizabilities are calculated by employing Moller-Plesset perturbation theory and time dependent density functional theory. The polarizabilities of dimer and tetramer are also calculated by employing large basis set coupled cluster theory with single and double excitations and perturbative triple excitations. The time dependent density functional theory calculations of polarizabilities are carried out with two different exchange-correlation potentials: (i) an asymptotically correct model potential and (ii) within the local density approximation. A systematic comparison with the other available theoretical and experimental data for various properties of small potassium clusters mentioned above has been performed. These comparisons reveal that both the binding energy and the ionization potential obtained with gradient corrected potential match quite well with the already published data. Similarly, the polarizabilities obtained with Moller-Plesset perturbation theory and with model potential are quite close to each other and also close to experimental data.Comment: 33 pages including 10 figure

Electrochemical characteristics of xLi(2)MnO(3)-(1-x)Li(Mn0.375Ni0.375Co0.25)O-2 (0.0 <= x <= 1.0) composite cathodes: Effect of particle and Li2MnO3 domain size

We have investigated the electrochemical characteristics of a series of high capacity xLi(2)MnO(3)-(1-x)Li(Mn0.375Ni0.375Co0.25)O-2 (0.0 <= x <= 1.0) integrated cathodes. Among several interrelated factors (viz. nominal molar content of Li2MnO3 and Li(Mn0.375Ni0.375Co0.25)O2 constituents, activation of Li2MnO3 component, crystallinity of the particles etc) an optimum particle size is argued to be most critical to yield better electrochemical performance of the synthesized cathodes. Through X-ray diffraction in conjunction with micro-Raman spectroscopy and high resolution transmission microscopy analyses we have demonstrated that with the increase of nominal Li2MnO3 contents, the size of the ordered nano-domains (inside the active matrix) and the average size of the composite cathode particles increase systematically. The size of the ordered nano-domains, cathode particles, and electrochemically triggered layer to spinel phase transformation influence the electrochemical characteristics of these cathodes. The average particle size of 0.5Li(2)MnO(3)-0.5Li(Mn0.375Ni0.375Co0.25)O-2 particles have been systematically varied by tuning the calcination time temperature combination. The optimized cathode yields a discharge capacity similar to 300 mAhg(-1) with capacity retention about 96% after 50 charge-discharge cycles at 10 mAg(-1) rate. The cathode with optimal particle size also exhibits a decent rate capability with room temperature discharge capacity similar to 200 mAhg(-1) at 300 mAg(-1) rate. (C) 2014 Elsevier Ltd. All rights reserved

Performance of wet chemical synthesized xLi(2)MnO(3)-(1-x)Li(Mn0.375Ni0.375Co0.25)O-2 (0.0 <= x <= 1.0) integrated cathode for lithium rechargeable battery

In the present work, we have reported the electrochemical performance of xLi(2)MnO(3)-(1-x)Li(Mn0.375Ni0.375Co0.25)O-2 (0.0 = 0.5) integrated cathodes to economic environmentally benign manganese rich, high voltage, high capacity lithium ion batteries. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.081207jes] All rights reserved

Electrochemical performances of 0.9Li(2)MnO(3)-0.1Li(Mn0.375Ni0.375Co0.25)O-2 cathodes: Role of the cycling induced layered to spinel phase transformation

In the present work, we have reported that as compared to Li2MnO3 lean compositions, the electrochemical properties of Li2MnO3 rich integrated cathodes (viz. 0.9Li(2)MnO(3)-0.1Li(Mn0.375Ni0.375Co0.25)O-2) are singular in three different ways. First, unlike its Li2MnO3 lean counterparts, the oxygen and concomitant lithium extraction in Li2MnO3 rich composition (viz. Li(Lio31Mn(0.646)Ni(0.026)Co(0.017))O-2) continues beyond first charging cycle. Second, for Li2MnO3 rich compositions, layered to spinel phase transformation seems to be unavoidable with repeated charge-discharge cycling. Formation of spine] phase triggers capacity fading in these. cathodes. We have found that for these cathodes, galvanostatic cycling at relatively higher rate (>= 20 mA g(-1)) retards the spinel formation with concomitant improvement of capacity retention with cycling. Finally, for Li2MnO3 rich integrated cathodes the major capacity contribution is found to be due to the reversible manganese redox (-3 V and -2.6 V). Therefore, the energy densities of these cathodes are less as compared to Li2MnO3 lean compositions. (C) 2013 Elsevier B.V. All rights reserved

Effect of structural integration on electrochemical properties of 0.5Li(2)MnO(3)-Li-0.5(Mn0.375Ni0.375Co0.25)O-2 composite cathodes for lithium rechargeable batteries

In the present work, we have investigated the effect of the structural integration between Li2MnO3 and Li(Mn0.375Ni0.375Co0.25)02 components on the electrochemical properties of the resultant compounds. We have adopted three processing methodologies to attain the structural integration of Li2MnO3 component in Li(Mn(0.375)Ni(0.375)Co0 025)02 layered cathode. The best electrochemical properties are achieved in the composites where Li2MnO3 ``type nano-domains were formed in situ in Li(Mn(0.37)sNi(0.375)Co(0.25))02 particles. For these composites the 1st cycle discharge capacity (at 10 mAg(-1) rate) was measured to be 300 mAhg(-1) with a capacity retention 220 mAhg(-1) after 50 cycles. These cathodes exhibit decent rate capability with typical discharge capacity 120 mAhg(-1) at 300 mAg(-1) rate. One of the remarkable finding of the present work is to demonstrate that in order to yield attractive electrochemical performance of such integrated cathode; structural integration between the Li2MnO3 and Li(Mn-0.375 Ni-0.375 Co0,25)02 is essential. To achieve better structural integrity in the physically mixed composites, finer particle size of the individual (Li2MnO3 and Li(Mn0.375Ni0.375Co0.25)02) components, through intermixing between them, and optimization of calcinations temperature and time are recommended. (C) 2013 The Electrochemical Society

Composite film processing

Under the broad umbrella of chemical solution depositions (CSD), synthesis of thick films (>1 μm) using a combination of sol and particles (consisting of particles >100 nm size) has first been reviewed. Here the sol is used to both enhance the sintering and performance of conventional powder films as well as being integral to the formation of true powder-sol composite films where the sol forms in integral part of the deposited ink. Advantages and limitation of these composite sol-gel processing techniques are considered and deposition routes explored. The subsequent sections are devoted to outline the novel concept of composite thin film synthesis using molecular precursors. Based on the authors' own experience and existing literature, the perspective, potential and possibilities of the electro-ceramic thin films synthesized using sub 100 nm particulate precursor sols has been outlined

Composite film processing

Under the broad umbrella of chemical solution depositions (CSD), synthesis of thick films (>1 μm) using a combination of sol and particles (consisting of particles >100 nm size) has first been reviewed. Here the sol is used to both enhance the sintering and performance of conventional powder films as well as being integral to the formation of true powder-sol composite films where the sol forms in integral part of the deposited ink. Advantages and limitation of these composite sol-gel processing techniques are considered and deposition routes explored. The subsequent sections are devoted to outline the novel concept of composite thin film synthesis using molecular precursors. Based on the authors' own experience and existing literature, the perspective, potential and possibilities of the electro-ceramic thin films synthesized using sub 100 nm particulate precursor sols has been outlined

Improvement of the Electrochemical Characteristics of Lithium and Manganese Rich Layered Cathode Materials: Effect of Surface Coating

Surface coating with electrochemically inert materials are found to be fruitful to improve the cycleability and rate capability characteristics of lithium and manganese rich composite cathode materials. In order to understand the structure-property relation between the nature of coating and the electrochemical performance, surface modification of composite cathodes was carried out either by a thin layer of carbon or zirconia particles. Zirconia coating helps to sustain 86% capacity retention after 50 cycles as compared to bare composite which exhibits 68% capacity retention when cycled at 10 mAg(-1). Among 1 wt%, 2.5 wt% and 5 wt% zirconia coated cathode materials, 2.5 wt% zirconia coating exhibits best rate capability. We have demonstrated that the porous particulate ZrO2 coating improved the capacity retention of the composite cathodes by suppressing the impedance growth at the electrodes-electrolyte interface. (C) 2015 The Electrochemical Society