Using first-principles calculations, we investigate the surface energies, equilibrium morphology, and surface redox potentials for in the olivine structure. Low-energy surfaces are found in the [100], [010], [011], [101], [201], and [301] directions of the orthorhombic structure. With the calculated surface energies, we provide the thermodynamic equilibrium shape for the crystal through a Wulff construction. The dominating surfaces in the Wulff shape are (010), (011), and (201). Most of the surfaces in the Wulff shape have lower Li extraction potentials than the bulk, except for the (100) and (011) surfaces.
First-principles investigations of the thermodynamics of binary alloys using a cluster expansion have so far neglected the presence of vacancies. Here, we invoke a local cluster expansion as a perturbation to the standard binary cluster expansion to model the equilibrium vacancy concentration in a binary alloy as a function of temperature and alloy composition. We apply this approach to a first-principles investigation of the fcc ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Li}}_{x}$ alloy (for $x$ less than 0.3) which at $x=0.25$ exhibits $\mathrm{L}{1}_{2}$ superstructure ordering. The equilibrium vacancy concentration is predicted to be sensitive to the bulk alloy composition $x$ in the ordered $\mathrm{L}{1}_{2}$ phase, varying by more than an order of magnitude in a narrow interval of $x$ at intermediate temperatures. Both in the solid solution and in the ordered $\mathrm{L}{1}_{2}$ phase, the vacancy prefers a nearest neighbor shell rich in aluminum. In the $\mathrm{L}{1}_{2}$ ordered phase, the vacancy predominantly occupies the lithium sublattice. The type of short-range order around a vacancy should affect the mobility of the constituents of the alloy and hence its interdiffusion coefficient.
We argue that surface segregation can be substantially modified by the presence of adsorbates and present a first-principles method that allows us to equilibrate segregation and adsorption simultaneously on surfaces with fixed topology. The method is based on a cluster expansion theory to write the state of the system in terms of adsorbate and surface layer occupation variables. This model can be parametrized with density functional theory calculations and equilibrated at finite temperature with Monte Carlo simulation. The method is applied to surface ordering and segregation at a (111) surface of ${\mathrm{Pt}}_{(1\ensuremath{-}x)}{\mathrm{Ru}}_{x}$ alloys in the presence of adsorbing oxygen. While Pt segregates under vacuum conditions, the strong binding between oxygen and Ru couples the segregation energy of the Ru to the oxygen chemical potential. As a result, we find that variations in oxygen chemical potential can dramatically alter the segregation and surface ordering tendency of dilute Ru in Pt.
Read moreMonolithically integrating the energy supply unit on a silicon integrated circuit (IC) requires the development of a thin-film solid-state battery compatible with silicon IC fabrication methods, materials, and performance. We have envisioned materials that can be processed in a silicon fabrication environment, thus bringing local stored energy to silicon ICs. By incorporating the material directly onto the silicon wafer, the economic parallelism that silicon complementary metal-oxide-semiconductor (CMOS) technology has enjoyed can be brought to power incorporation in each IC on a processed wafer. It is natural to look first towards silicon CMOS materials, and ask which materials need enhancement, which need replacement, and which can be used “as is.” In this study, we begin by using two existing CMOS materials and one unconventional material for the construction of a source of electric power. We have explored the use of thermally grown silicon dioxide (SiO2) as thin as 9nm acting as an electrolyte material candidate in a solid-state power cell integrated on silicon. Other components of the thin-film cell consisted of rf-sputtered lithium cobalt oxide (LiCoO2) as the cathode and highly doped n-type polycrystalline silicon (polysilicon) grown by low-pressure chemical-vapor deposition as the anode. All structures were fabricated using conventional microelectronics fabrication technology. The charge and discharge behaviors of the LiCoO2∕SiO2∕polysilicon cells were studied. On the basis of the impedance measurements an equivalent circuit model of an ultrathin cell was inferred, and its microstructure was characterized by electron microscopy imaging. In spite of its high series resistance (∼4×107Ω), we have shown that an ultrathin layer of an as-deposited Li-free SiO2 is an interesting candidate for an electrolyte or controllable barrier layer in lithium-ion-based devices.
Read moreA general continuum model has recently been proposed for the dynamics of ion intercalation in a single crystal of rechargeable-battery electrode materials [1]. When applied to strongly phase-separating, highly anisotropic materials such as LiFePO4, phase-transformation waves are predicted between the lithiated and unlithiated portions of a crystal. In this paper, we extend the analysis of the wave dynamics, and we describe a new mechanism for current capacity fade through the interactions of these waves with defects in the material.
Read moreThe layered P2-K4Co7O14 oxide has been prepared and characterized by means of X-ray diffraction, electrical conductivity, thermopower, and magnetic measurements. The crystal structure of K4Co7O14 (P6(3)/m space group, Z=2, a=7.5171(1) A, and c=12.371(1) A) consists of a stacking of slabs of edge-shared CoO6 octahedra with K+ ions occupying ordered positions in the interslab space, leading to a a0 radical7xa0 radical7 supercell. Potential energy calculations at 0 K are in good agreement with the ordered distribution of potassium ions in the (ab) plane. This oxide is metallic, and the magnetic susceptibility is of Pauli-type, which contrasts with the Curie-Weiss behavior of the homologous NaxCoO2 (x approximately 0.6) oxide with close alkali content. The thermopower at room temperature is about one-third that of polycrystalline Na0.6CoO2.
Read moreElectrochemical Properties of Nonstoichiometric LiNi0.5Mn1.5O4− Thin-Film Electrodes Prepared by Pulsed Laser Deposition H. Xia, Y. S. Meng,* L. Lu, and G. Ceder* Advanced Materials for Microand Nano-System, Singapore-MIT Alliance, Singapore 117576, Singapore Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore
Read moreIn this work, a computational study of the dielectric absorption and Q-factor at microwave frequencies in rocksalt oxides is presented. This work is performed within a theory for the two-phonon anharmonic absorption process in terms of third-order force constant matrices, phonon eigendata, and Born effective charges. The complex dielectric permittivity is expressed by means of Green’s functions. This theory is used on model systems that are described using empirical Buckingham potentials with shells. The Q-factor and its temperature dependence in AO (A=Mg, Ca, Ba, Sr) oxides with the rocksalt structure is calculated. For MgO, the calculated Q-factor at room temperature agrees relatively well with experiments, and the temperature dependence is in qualitative agreement. The Q-factor in a model “AO” system is also calculated to determine the effects of cation mass and size on the Q-factor. Increased cation size is found to lower the Q-factor, whereas increasing cation mass is found to increase the Q-factor when the cation mass is higher than that of Ca.
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Read moreA combined computational/experimental study on LiNi1/3Co1/3Mn1/3O2 is presented. Both density functional theory and experiments are used to probe the active redox pairs and changes in electronic structure of LiNi1/3Co1/3Mn1/3O2 during intercalation or deintercalation of Li. The phase stability and voltage curve of this material are also shown in this paper. Both the experimental and computational data show that LiNi1/3Co1/3Mn1/3O2 material is a high-capacity stable electrode for advanced rechargeable lithium ion batteries.
Read more10.1149/1.2741157
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