Energy storage technologies are critical in addressing the global challenge of clean sustainable energy. Major advances in rechargeable batteries for portable electronics, electric vehicles and large-scale grid storage will depend on the discovery and exploitation of new high performance materials, which requires a greater fundamental understanding of their properties on the atomic and nanoscopic scales. This review describes some of the exciting progress being made in this area through use of computer simulation techniques, focusing primarily on positive electrode (cathode) materials for lithium-ion batteries, but also including a timely overview of the growing area of new cathode materials for sodium-ion batteries. In general, two main types of technique have been employed, namely electronic structure methods based on density functional theory, and atomistic potentials-based methods. A major theme of much computational work has been the significant synergy with experimental studies. The scope of contemporary work is highlighted by studies of a broad range of topical materials encompassing layered, spinel and polyanionic framework compounds such as LiCoO2, LiMn2O4 and LiFePO4 respectively. Fundamental features important to cathode performance are examined, including voltage trends, ion diffusion paths and dimensionalities, intrinsic defect chemistry, and surface properties of nanostructures.

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Lithium and sodium battery cathode materials: computational insights into voltage, diffusion and nanostructural properties

Transition metal oxides are capable of a wide range of work functions. This quality allows them to be used in many applications that involve charge transfer with adsorbed molecules, for example as heterogeneous catalysts, as charge-injection layers in organic electronics, and as electrodes in fuel cells. Chemical and structural factors can alter transition-metal oxide work functions, often making their work functions difficult to control. Little is known about the effects of the cation oxidation state and point defects on the oxide work function. It is necessary to understand how such chemical and structural factors affect work functions in order to controllably tune transition metal oxides for desired applications. Here, a correlation between the oxide work function and cation oxidation state is demonstrated. This correlation is attributed to the change in cation electronegativity with oxidation state. A model is presented that relates the work function to the oxygen deficiency for d0 oxides in the limit of dilute oxygen vacancies. It is proposed that the rapid initial decrease in work function, observed for d0 oxides, is caused by an increase in the density of donor-like defect states. It is also shown that oxides tend to have decreased work functions near a metal/metal-oxide interface as a consequence of the relationship between defects and work function. These insights provide guidelines for tuning transition metal oxide work functions.

Transition Metal Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies

Charge and discharge of lithium ion battery electrodes is accompanied by severe volume changes. In a confined space, the volume cannot expand, leading to significant pressures induced by local microstructural changes within the battery. While volume changes appear to be less critical in batteries with liquid electrolytes, they will be more critical in the case of lithium ion batteries with solid electrolytes and they will be even more critical and detrimental in the case of all-solid-state batteries with a lithium metal electrode. In this work we first summarize, compare, and analyze the volume changes occurring in state of the art electrode materials, based on crystallographic studies. A quantitative analysis follows that is based on the evaluation of the partial molar volume of lithium as a function of the degree of lithiation for different electrode materials. Second, the reaction volumes of operating full cells (“charge/discharge volumes”) are experimentally determined from pressure-dependent open-circuit voltage measurements. The resulting changes in the open-circuit voltage are in the order of 1 mV/100 MPa, are well measurable, and agree with changes observed in the crystallographic data. Third, the pressure changes within solid-state batteries are approximated under the assumption of incompressibility, i.e. for constant volume of the cell casing, and are compared to experimental data obtained from model-type full cells. In addition to the understanding of the occurring volume changes of electrode materials and resulting pressure changes in solid-state batteries, we propose “mechanical” blending of electrode materials to achieve better cycling performance when aiming at “zero-strain” electrodes.

Chemo-mechanical expansion of lithium electrode materials – on the route to mechanically optimized all-solid-state batteries

The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-007233.pdf

First-principles calculations of electronic structure and spectra of strongly correlated systems: the LDA+U method

We theoretically elucidated the characteristics of the space–charge layer (SCL) at interfaces between oxide cathode and sulfide electrolyte in all-solid-state lithium-ion batteries (ASS-LIBs) and the effect of the buffer layer interposition, for the first time, via the calculations with density functional theory (DFT) + U framework. As a most representative system, we examined the interfaces between LiCoO2 cathode and β-Li3PS4 solid electrolyte (LCO/LPS), and the LiCoO2/LiNbO3/β-Li3PS4 (LCO/LNO/LPS) interfaces with the LiNbO3 buffer layers. The DFT+U calculations, coupling with a systematic procedure for interface matching, showed the stable structures and the electronic states of the interfaces. The LCO/LPS interface has attractive Li adsorption sites and rather disordered structure, whereas the interposition of the LNO buffer layers forms smooth interfaces without Li adsorption sites for both LCO and LPS sides. The calculated energies of the Li-vacancy formation and the Li migration reveal that subsurfa...

Space–Charge Layer Effect at Interface between Oxide Cathode and Sulfide Electrolyte in All-Solid-State Lithium-Ion Battery

LixCoO2 and LixNiO2 (0.5 < x < 1) are used as prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue when used as cathode material during electrochemical cycling. In order to analyze the change of the structure and the electronic structure of LixCoO2 and LixNiO2 as a function of Li content x in detail, we have performed X-ray diffraction studies, photoelectron spectroscopy (PES) investigations and band structure calculations for a series of compounds Lix(Co,Ni)O2 (0 < x⩽ 1). The calculated density of states (DOS) are weighted by theoretical photoionization cross sections and compared with the DOS gained from the PES experiments. Consistently, the experimental and calculated DOS show a broadening of the Co/Ni 3d states upon lithium de-intercalation. The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.

Changes in the crystal and electronic structure of LiCoO2 and LiNiO2 upon Li intercalation and de-intercalation

The electronic properties of ${\text{LiCoO}}_{2}$ have been studied by theoretical band-structure calculations (using density functional theory) and experimental methods (photoemission). Synchrotron-induced photoelectron spectroscopy, resonant photoemission spectroscopy (ResPES), and soft x-ray absorption (XAS) have been applied to investigate the electronic structure of both occupied and unoccupied states. High-quality PES spectra were obtained from stoichiometric and highly crystalline ${\text{LiCoO}}_{2}$ thin films deposited ``in situ'' by rf magnetron sputtering. An experimental approach of separating oxygen- and cobalt-derived (final) states by ResPES in the valence-band region is presented. The procedure takes advantage of an antiresonant behavior of cobalt-derived states at the $3p\text{\ensuremath{-}}3d$ excitation threshold. Information about the unoccupied density of states has been obtained by $\text{O}\text{ }K$ XAS. The structure of the $\text{Co}\text{ }L$ absorption edge is compared to semiempirical charge-transfer multiplet calculations. The experimental results are furthermore compared with band-structure calculations considering three different exchange potentials [generalized gradient approximation (GGA), using a nonlocal Hubbard $U$ $(\text{GGA}+U)$ and using a hybrid functional (Becke, three-parameter, Lee-Yang-Parr [B3LYP])]. For these different approaches total density of states and partial valence-band density of states have been investigated. The best qualitative agreement with experimental results has been obtained by using a $\text{GGA}+U$ functional with $U=2.9\text{ }\text{eV}$.

Electronic structure of LiCoO 2 thin films: A combined photoemission spectroscopy and density functional theory study

In this work the electronic structure of V2O5, reduced V2O5−x (V16O39) and sodium intercalated NaV2O5 has been studied by both theoretical and experimental methods. Theoretical band structure calculations have been performed using density functional methods (DFT). We have investigated the electron density distribution of the valence states, the total density of states (total DOS) and the partial valence band density of states (PVBDOS). Experimentally, amorphous V2O5 thin films have been prepared by physical vapour deposition (PVD) on freshly cleaved highly oriented pyrolytic graphite (HOPG) substrates at room temperature with an initial oxygen understoichiometry of about 4%, resulting in a net stoichiometry of V2O4.8. These films have been intercalated by sodium using vacuum deposition with subsequent spontaneous intercalation (NaV2O5) at room temperature. Resonant V3p–V3d photoelectron spectroscopy (ResPES) experiments have been performed to determine the PVBDOS focusing on the calculation of occupation numbers and the determination of effective oxidation state, reflecting ionicity and covalency of the V–O bonds. Using X-ray absorption near edge spectra (XANES) an attempt is made to visualize the changes in the unoccupied DOS due to sodium intercalation. For comparison measurements on nearly stoichiometric V2O5 single crystals have been performed. The experimental data for the freshly cleaved and only marginally reduced V2O5 single crystals and the NaV2O5 results are in good agreement with the calculated values. The ResPES results for V2O4.8 agree in principle with the calculations, but the trends in the change of the ionicity differ between experiment and theory. Experimentally we find partly occupied V 3d states above the oxygen 2p-like states and a band gap between these and the unoccupied states. In theory one finds this occupation scheme assuming oxygen vacancies in V2O5 and by performing a spin-polarized calculation of an antiferromagnetic ordered NaV2O5.

Theoretical and experimental determination of the electronic structure of V2O5, reduced V2O5−x and sodium intercalated NaV2O5

Using first-principles band structure methods, the electronic structure and local geometry of oxygen impurities in GaN have been investigated as a function of oxygen concentration. The concentrational variation has been simulated using various supercells. In order to test the validity of ``rigid band'' models usually employed to describe the effect of impurities on the electronic structure and associated properties of wide band gap semiconductors, the spatial localization of donor states associated with oxygen atoms at the nitrogen site has been carefully explored. For all the supercells studied in this work, the energy bands near the bottom of the conduction band are found to be severely perturbed. The donor electrons are found to occupy extended states. However, with increasing dilution of impurities, the donor electron tends to localize near the impurity ion. This localization monotonically progresses with decreasing oxygen concentration.

Stefan Laubach

Papers

Changes in the crystal and electronic structure of LiCoO2 and LiNiO2 upon Li intercalation and de-intercalation

Electronic structure of LiCoO 2 thin films: A combined photoemission spectroscopy and density functional theory study

Theoretical and experimental determination of the electronic structure of V2O5, reduced V2O5−x and sodium intercalated NaV2O5

Localization of oxygen donor states in gallium nitride from first-principles calculations

Theoretical and Experimental Determination of the Electronic Structure of V2O5, Reduced V2O5-x and Sodium Intercalated NaV2O5.