M. K. Aydinol

Identification of cathode materials for lithium batteries guided by first-principles calculations

Lithium batteries have the highest energy density of all rechargeable batteries and are favoured in applications where low weight or small volume are desired — for example, laptop computers, cellular telephones and electric vehicles. One of the limitations of present commercial lithium batteries is the high cost of the LiCoO2 cathode material. Searches for a replacement material that, like LiCoO2, intercalates lithium ions reversibly have covered most of the known lithium/transition-metal oxides, but the number of possible mixtures of these is almost limitless, making an empirical search labourious and expensive. Here we show that first-principles calculations can instead direct the search for possible cathode materials. Through such calculations we identify a large class of new candidate materials in which non-transition metals are substituted for transition metals. The replacement with non-transition metals is driven by the realization that oxygen, rather than transition-metal ions, function as the electron acceptor upon insertion of Li. For one such material, Li(Co,Al)O2, we predict and verify experimentally that aluminium substitution raises the cell voltage while decreasing both the density of the material and its cost.

Ab initio calculation of the intercalation voltage of lithium-transition-metal oxide electrodes for rechargeable batteries

Abstract A first-principles method to predict the intercalation voltage for lithium in metal oxides is presented. Although the intercalation voltage for lithium is often related to aspects of the electronic structure, this voltage can only be accurately calculated from the lithium chemical potential in the anode and cathode. Using the pseudopotential technique the average intercalation voltage of LiMO2 cathodes is computed for M = Ti, V, Co, Ni and Cu. Although no experimental data are used as input, agreement with experiment is good, indicating the potential to use this method to predict the properties of new compounds.

Application of first-principles calculations to the design of rechargeable Li-batteries

Abstract Rechargeable Li batteries consist of an anode, electrolyte, and cathode. The cathode is typically an oxide that intercalates Li at very low chemical potential ensuring a large open-cell voltage for the battery. We show how first-principles pseudopotential calculations can be used to predict the intercalation voltage for these materials. By means of a series of computational experiments on virtual structures, we identify the parameters that are important in determining the intercalation voltage of a compound. We found that Li intercalation causes significant electron transfer to the oxygen ions in the structure. Results are presented for LiTiO2, LiVO2, LiMnO2, LiCoO2, LiNiO2, and LiZnO2.

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.

First‐Principles Prediction of Insertion Potentials in Li‐Mn Oxides for Secondary Li Batteries

First-principles methods have started to be widely used in materials science for the prediction of properties of metals, alloys, and compounds. In this study, we demonstrate how first-principles methods can be used to predict the average open-circuit voltage that can be obtained from a lithium battery with spinel or layered manganese oxides used as the cathode. For this purpose we combine a basic thermodynamical model with the ab initio pseudopotential method. The good agreement between the computed and experimental average output potentials suggests that first-principles methods can be an important tool to design novel battery materials.

A comparative ab initio study of the ferroelectric behaviour in KNO3 and CaCO3

Potassium nitrate exhibits a reentrant phase transformation, where a metastable ferroelectric phase (γ -KNO3) is formed upon cooling from high temperature. The layered structure of this ferroelectric phase is composed of alternating layers of potassium ions and nitrate groups; wherein, a central nitrogen atom is coordinated by three equilateral triangular oxygen atoms. The group layer is located less than midway between the cation layers, giving rise to a polar structure. From a structural perspective, the calcite phase of calcium carbonate looks quite similar to this ferroelectric phase; however; it does not exhibit a ferroelectric transition. In this work we have performed an ab initio computational analysis to study the: structural stability, electronic characteristics, and bonding of various phases and ferroelectric properties of CaCO3 and KNO3. We find that both material systems have mixed covalent and ionic bonding. The covalent interactions are within the group atoms of carbonate and nitrate atoms while the ionic interactions occur between the negatively charged (carbonate or nitrate) group and the calcium or potassium cations. For the low temperature stable phase of CaCO3 (calcite), however, there is a slight covalency between the cations and the oxygen atoms of the group. This latter interaction results in the crystallization of CaCO3 in the calcite form and prevents a ferroelectric transition. We suggest that, in analogy to KNO3, a metastable form of CaCO3 may also exist, similar to the phase of γ -KNO3 that should have a spontaneous polarization equal to 30.6 μ Cc m −2 , twice that of γ -KNO3. Moreover, our analysis indicates that this material should have a coercive field smaller than that of γ -KNO3. (Some figures in this article are in colour only in the electronic version) 4 Author to whom any correspondence should be addressed.

Phase Separation Tendencies of Aluminum-Doped Transition-Metal Oxides (LiAl 1-x M x O 2 ) in the a-NaFeO 2 Crystal Structure

First-principles methods are used to calculate the miscibility of eight aluminum-doped transition-metal oxides in the layered α-NaFeO 2 structure. This study finds that for all Li(Al,M)O 2 compounds investigated (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) the enthalpy of mixing is positive. In addition, detailed analyses were performed on LiAl 1- xCoxO2 and LiAl 1-x Cr x O 2 by calculating full temperature-composition phase diagrams. For the Li(Al,Co)O 2 system, we find regions of immiscibility below - 173°C and above 600°C. For both Li(Al,Co)O 2 and Li(Al,Cr)O 2 above 600°C, Al-doping is limited by the formation of γ-LiAlO 2 .

Phase separation tendencies of aluminum-doped transition-metal oxides (LiAl{sub 1{minus}x}M{sub x}O{sub 2}) in the {alpha}-NaFeO{sub 2} crystal structure

First-principles methods are used to calculate the miscibility of eight aluminum-doped transition-metal oxides in the layered α-NaFeO 2 structure. This study finds that for all Li(Al,M)O 2 compounds investigated (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) the enthalpy of mixing is positive. In addition, detailed analyses were performed on LiAl 1- xCoxO2 and LiAl 1-x Cr x O 2 by calculating full temperature-composition phase diagrams. For the Li(Al,Co)O 2 system, we find regions of immiscibility below - 173°C and above 600°C. For both Li(Al,Co)O 2 and Li(Al,Cr)O 2 above 600°C, Al-doping is limited by the formation of γ-LiAlO 2 .

The electrochemical stability of lithium-metal oxides against metal reduction

Abstract The possibility of metal reduction during the charging of secondary lithium batteries with Li x MO 2 cathodes is investigated. Loss of active material due to metal reduction can be one of the causes of capacity decay in these batteries after repeated charging. First principles methods are used to calculate the metal reduction potentials in layered Li x MO 2 compounds where M=Ti, V, Mn, Fe, Co or Ni. It is found that, for several of these compositions, the metal ions may preferably reduce before the lithium ion during charging.

Atomistic simulation of self-diffusion in Al and Al alloys under electromigration conditions

The effect of alloying elements on the self-diffusion behavior of Al under electromigration conditions was investigated using nonequilibrium molecular dynamics. The corresponding defect structures were also characterized energetically by Mott–Littleton approach. Pd, Cu, Mn, and Sn were found to be the most effective alloying elements that may retard the electromigration failure by increasing the activation energy for self-diffusion of Al. This increase in the activation energy is believed to be either because of the dragging effect that may be experienced in a coupled substitutional-vacancy defect structure or the energy penalty for the separation of this couple.