A. Van der Ven

LI CONDUCTIVITY IN LIX MPO 4 ( M = MN , FE , CO , NI ) OLIVINE MATERIALS

Materials with the olivine structure form an important class of rechargeable battery cathodes. Using first-principles methods, activation barriers to Li ion motion are calculated and an estimate for Li diffusion constants, in the absence of electrical conductivity constraints, is made. Materials with Fe, Co, Ni are considered. Li diffuses through one-dimensional channels with high energy barriers to cross between the channels. Without electrical conductivity limitations the intrinsic Li diffusivity is high.

Lithium Diffusion in Layered Li x CoO2

The results of a first principles investigation of lithium diffusion within the layered form of are presented. Kinetic Monte Carlo simulations predict that lithium diffusion is mediated through a divacancy mechanism between and and with isolated vacancies at infinite vacancy dilution. The activation barrier for the divacancy migration mechanism depends strongly on lithium concentration resulting in a diffusion coefficient that varies within several orders of magnitude. We also argue that the thermodynamic factor in the expression of the chemical diffusion coefficient plays an important role at high lithium concentration. ©2000 The Electrochemical Society

Lithium diffusion mechanisms in layered intercalation compounds

Abstract We investigate the mechanisms of lithium diffusion in layered intercalation compounds from first-principles. We focus on Li x CoO 2 and find that lithium diffusion in this compound occurs predominantly with a divacancy mechanism. First-principles calculations predict that the activation barrier is very sensitive to the lithium concentration due to the strongly varying c -lattice parameter of the host and the change in effective valence of the cobalt ions. This translates into a diffusion coefficient that varies by several orders of magnitude with state of charge.

First-principles evidence for stage ordering in Li{sub x}CoO{sub 2}

The authors have investigated phase stability in the layered Li{sub x}CoO{sub 2} intercalation compound for x < 0.4 from first principles. By combining a lattice model description of the Li-vacancy configurational degrees of freedom with first-principles pseudopotential calculations, the authors have calculated the free energy of the material as a function of Li concentration in three different host structures: (1) the rhombohedral form of Li{sub x}CoO{sub 2}, (2) the hexagonal form of CoO{sub 2}, and (3) a stage II compound of Li{sub x}CoO{sub 2} in which the host structure can be considered as a hybrid of the rhombohedral and hexagonal host structures. The first-principles free energies indicate that the stage II compound is the most stable of the three phases for Li concentrations ranging between 0.12 and 0.19. This result is consistent with the experimental observation by Ohzuku and Ueda and Amatucci et al. that the rhombohedral form of Li{sub x}CoO{sub 2} transforms to a new phase at Li concentrations around x = 0.15. The authors find that the calculated X-ray powder diffraction patterns of the stage II structure agree qualitatively with those observed experimentally at low Li concentration.

Phase Stability of Nickel Hydroxides and Oxyhydroxides

In this paper, we investigate the phase stability of nickel hydroxides from first-principles. We predict that the previously uncharacterized crystal structure of III‐NiOOH is actually derived from the P3 host. Furthermore, we identify a plausible crystal structure for the -NiO2H2O0.67K0.33Hx phase that is consistent with available experimental observations. The proposed crystal structure has a P3 host and the K ions reside exactly between adjacent trigonal prismatic sites of the intercalation layer. We have also calculated the topotactic voltage curves for the and phases, and predict the existence of a large step in voltage at -NiOOH, which effectively limits the capacity of the Ni-hydroxide compound to one electron per Ni ion. Methodology We combine first-principles electronic structure calculations with a cluster expansion approach for the disorder of protons in the materials to calculate phase stability and thermodynamic properties of the nickel hydroxide system. The electronic structure calculations

Phase transformations and volume changes in spinel LixMn2O4

Abstract First-principles methods have been used to calculate the phase diagram and volume expansion of spinel Li x Mn 2 O 4 as a function of lithium content. The calculations confirm the experimentally observed two phase region between x =1 and 2 and the ordered phase at x =1/2. In addition, the expected large step in voltage at x =1 is obtained. It is shown that these phenomena are qualitatively determined by the interactions of Li with each other and the Mn 2 O 4 host and only quantitatively influenced by the more subtle electronic effects such as Jahn–Teller distortions, charge ordering and magnetic excitations. The two-phase region between x =1 and 2 is found to be driven by strong repulsive interactions between lithium ions occupying adjacent tetrahedral 8a and octahedral 16c sites. The origin of the large volume change upon transforming from LiMn 2 O 4 to Li 2 Mn 2 O 4 is also investigated from first principles. The possible sources of the volume change are identified to be the intercalated lithium, the Jahn–Teller distortion, and the introduction of anti-bonding e g electrons into the Mn d-orbitals. The latter effect is found to be dominant. Some speculation is offered on how the large volume change upon lithiation of manganese dioxide can be prevented.

The Role of Coherency Strains on Phase Stability in LixFePO4: Needle Crystallites Minimize Coherency Strain and Overpotential

We investigate the role of coherency strains on the thermodynamics of two-phase coexistence during Li (de)intercalation of LixFePO4. We explicitly account for the anisotropy of the elastic moduli and analytically derive coupled chemical and mechanical equilibrium criteria for two-phase morphologies observed experimentally. Coherent two-phase equilibrium leads to a variable voltage profile of individual crystallites within the two-phase region as the dimensions of the crystallite parallel to the interface depend on the phase fractions of the coexisting phases. With a model free energy for LixFePO4, we illustrate the effect of coherency strains on the compositions of the coexisting phases and on the voltage profile. We also show how coherency strains can stabilize intermediate solid solutions at low temperatures if phase separation is restricted to Li diffusion along the b-axis of olivine LixFePO4. A finite element analysis shows that long needlelike crystallites with the long axis parallel to the a lattice vector of LixFePO4 minimize coherency strain energy. Hence, needlelike crystallites of LiFePO4 reduce the overpotential needed for Li insertion and removal and minimize mechanical damage, such as dislocation nucleation and crack formation, resulting from large coherency strain energies.

Linking the electronic structure of solids to their thermodynamic and kinetic properties

Predicting measurable thermodynamic and kinetic properties of solids from first-principles requires the use of statistical mechanics. A major challenge for materials of technological importance arises from the fact that first-principles electronic structure calculations of elementary excited states are computationally very demanding. Hence statistical mechanical averaging over the spectrum of excited states must rely on the use of effective Hamiltonians that are parameterized by a limited number of first-principles electronic structure calculations, but nevertheless predict energies of excited states with a high level accuracy. Here we review important effective Hamiltonians that account for vibrational and configurational degrees of freedom in multi-component crystalline solids and show how they can be used to predict phase stability as a function of composition and temperature as well as kinetic transport constants such as diffusion coefficients in non-dilute crystalline solids.

Designing the next generation high capacity battery electrodes

Much of current research in electrochemical energy storage is devoted to new electrode chemistries and reaction mechanisms that promise substantial increases in energy density. Unfortunately, most high capacity electrodes exhibit an unacceptably large hysteresis in their voltage profile. Using a first-principles multi-scale approach to examine particle level dynamics, we identify intrinsic thermodynamic and kinetic properties that are responsible for the large hysteresis exhibited by many high capacity electrodes. Our analysis shows that the hysteresis in the voltage profile of high capacity electrodes that rely on displacement reactions arises from a difference in reaction paths between charge and discharge. We demonstrate that different reaction paths are followed (i) when there is a large mismatch in ionic mobilities between the electrochemically active species (e.g. Li) and displaced ionic species and (ii) when there is a lack of a thermodynamic driving force to redistribute displaced ions upon charging of the electrode. These insights motivate the formulation of design metrics for displacement reactions in terms of fundamental properties determined by the chemistry and crystallography of the electrode material, properties that are now readily accessible with first-principles computation.

First-principles alloy theory in oxides

The physical mechanisms which may contribute to the energy and entropy of mixing in oxide systems are identified and discussed. Ionic size, magnetism and electrostatics can all contribute to the configurational energy dependence of transition-metal oxides. While the many sources of substitutional disorder make configurational entropy an essential contribution to the free energy of oxides, electronic and magnetic entropy may be of the same order of magnitude. This is illustrated with some first-principles results on LiCoO2 and LiMnO2.