Function Of Lithium Content Research Articles

Electronic Structure Changes As Source of Chemical and Interfacial Instability in LiCoPO4 As Positive Electrode for High Voltage Li-Ion Batteries

The energy storage capability of a battery scales with the potential difference between its electrodes. Yet operation of positive electrode materials at high potentials introduces challenges of stabilization of charged states. As of today, no positive electrode material has been demonstrated to durably and safely operate above 4.5 V vs. Li+/Li0. LiCoPO4-based electrodes theoretically offer high specific capacity and high potentials of operation, around 4.8V vs. Li+/Li0, but these electrodes are prone to failure during cycling. Failure occurs through chemical and structural degradation in the bulk of the active material or at its interfaces with cell components, especially the electrolyte. The development of Li-ion battery electrodes operating at high potential is indispensable to meet the specific energy target of 250 kWh/kg at the packaged cell level. Changes in the electronic structure and chemical stability of olivine-type LiCoPO4 and Fe-substituted LiCoPO4 were explored as both a function of ion substitution and oxidation state. Soft Ex situX-Ray absorption spectroscopy (XAS) made it possible to compare the changes in chemical bonding between electrode bulk and surface as a function of lithium content. This technique can probe the density of states at the transition metal and O levels. The evolution of these levels revealed changes in the metal-oxide covalence when lithium was deintercalated from the structure, and, thus, the material was oxidized. An increase in covalence can lead to the destabilization of the anions. If this process takes place in the bulk of the material, this destabilization can lead to thermal degradation via oxygen loss. At the surface, even small degrees of destabilization are sufficient to produce oxidizing species that attack the electron-rich solvent molecules in the electrolyte, leading to irreversible capacity loss. Increased metal-oxygen covalence was universally observed in the spectroscopy in the form of a rising pre-O K-edge peak at ~530 eV as a function of lithium deintercalation in both bulk and surface. However, accompanying changes in the Co spectroscopy were only observed in the Fe-substituted sample. These changes are also indicative of increased hybridization between Co 3d and O 2p orbitals. Co K-edge XANES and EXAFS experiments further corroborated these findings, in which virtually no changes in the Co K-edge were observed upon oxidation of unsubstituted LiCoPO4. Fe doping appears to play a substantial role in getting Co to participate in redox chemistry, and the mechanism by which it occurs is currently being explored using Density Functional Theory. Fe-substituted LiCoPO4 is an exciting new positive electrode material that may prove useful in advancing Li-ion battery technology.

Recent Achievements on High Voltage Olivine-Type Cathode Materials

The development of high power and high energy density cathode materials for rechargeable Li-ion batteries suitable with high operation voltage is a challenge. In this regard, the fundamental understanding of the thermodynamic stability and performance limits of high voltage cathode materials is essential, since one allows us to predict/determine the intrinsic voltage limit of the cathode material [1, 2] and tailor the components of the battery cell to provide a high Li+ diffusion rate in the bulk material and across the interface. The main reasons which limit to use high voltage LiMPO4 (LMPO, M=Co, Ni) cathode materials for electric vehicle applications are low electronic conductivity, which results from the olivine structure where the valence electrons of the transition metal are isolated from each other by the polyanion (PO3- 4), and an instability of the electronic structure of LMPO at the electrode-electrolyte interface caused by the decomposition of liquid electrolytes at high voltage potential. Low electronic conductivity of the Co- and Ni based olivine-type cathode materials makes it difficult to use electronic spectroscopy techniques which give unambiguous information on the density of the occupied and unoccupied states near the Fermi level (E F), on the oxidation and spin states of the transition metals, as well as on the chemical composition at the electrode-electrolyte interface [3-5]. In this presentation, we demonstrate our latest achievements on the study of the electronic structure of the layered- and olivine- type film cathode materials and its correlation with the electrochemical performance, Fig. 1. We demonstrate that the electronic structure of the bulk cathode material and the cathode-electrolyte interface is a key parameter which governs the thermodynamic stability of the whole battery cell. By plotting the experimentally obtained energy diagrams of the LiCoO2 (LCO) layered structure and LCPO-olivine structure cathode materials, we demonstrate the role of the inductive effect [7] which leads to the increase of the voltage window by the substitution of oxygen (O2-) by the polyanion. We discuss the role of morphology of the LiMPO4 film cathode materials on the Li-ion rate and demonstrate that high electronic conductivity can be achieved without using the carbon additives to the active material, Figs. 1(b). On the example of a LCPO film with different lithium content, x, we demonstrates the gradual shift of E F to the valence band maximum due to the increase of the work function of the cathode material in accordance with the rigid band model. By the analysis of the evolution of the core-levels and valence band structure as a function of lithium content in LCPO, we demonstrate that the charge compensation caused by Li+ removal takes place at the Co site resulting in the change of Co2+ ions (x=1.0) with the high spin (HS) state to the Co3+state in the accordance with DFT calculations [8]. The possible physicochemical mechanisms contributed to the degradation of the LMPO batteries are also discussed. [1] J. B. Goodenough, Y. Kim, Challenges for Rechargeable Li Batteries. Chem. Mat. (2010) 22, 587. [2] D. Ensling, G. Cherkashinin, S. Schmid, S. Bhuvaneswari, A. Thissen, W. Jaegermann, Nonrigid Band Behavior of the Electronic Structure of LiCoO2 Thin Film during Electrochemical Li Deintercalation. Chem. Mater. (2014) 26, 3948. [3] G. Cherkashinin, D. Ensling, W. Jaegermann, LiMO2 (M=Ni, Co) Thin Film Cathode Materials: a Correlation Between the Valence State of Transition Metals and the Electrochemical Properties. J. Mater. Chem. A (2014) 2, 3571. [5] D. Becker, G. Cherkashinin, R. Hausbrand, W. Jaegermann, Adsorption of Diethyl Carbonate on LiCoO2 Thin Films: Formation of the Electrochemical Interface. J. Phys. Chem. C (2014) 118,962. [6] G. Cherkashinin, M. Motzko, N. Schulz, T. Späth, W. Jaegermann, Electron Spectroscopy Study of Li[Ni,Co,Mn]O2/Electrolyte Interface: Electronic Structure, Interface Composition, and Device Implications. Chem. Mater. (2015) 27, 2875. [7] A. K. Padhi, K. S. Nanjundaswami, J. B. Goodenough, Phospho- Olivines as Positive- Electrode Materials for Rechargeable Lithium Batteries, J. Electrochem. Soc. (1997) 144, 1188. [8] F. Zhou, M. Cococcioni, C. A. Marianetti, D. Morgan, G. Ceder, First- Principles Prediction of Redox Potentials in Transition-Metal Compounds with LDA+U. Phys. Rev. B (2004) 70, 235121. Figure 1

Role of the Transition Metal in Intermetallic Anodes

Intermetallic anodes are a small but important subclass of lithium-ion battery anode materials. In general materials in this class represent a compromise in terms of capacity and voltage compared to commercial materials but also tend to be crystallographically denser and have more stable cycling profiles. Common examples include Cu2Sb, Cu6Sn5, and MnP [1-5]. They operate by inserting and storing lithium at low voltages using a variety of mechanisms that combine aspects of the main storage mechanisms. Intercalation anodes (e.g. graphite) insert lithium into existing crystallographic vacancies, conversion anodes store lithium in binary compounds formed by a disproportionation reaction, and Zintl phases react with lithium to form distinct crystallographic phases that undergo large bonding and volume changes as a function of lithium content [6]. In this presentation, a variety of intermetallic anodes will be assessed to better understand the role of the transition metal in allowing these materials to utilize multiple mechanisms to store lithium. Knowledge gained from this analysis will be used to predict and better understand new high density anode materials. [1] A. N. Jansen, J. A. Clevenger, A. M. Baebler, J. T. Vaughey J. Alloys Compounds 509 4457–4461 (2011) [2] Keith D. Kepler, John T. Vaughey, M. M. Thackeray Electrochem. Sol State Lett., 2 307-309 (1999). [3] Seung-Wan Song, R. P. Reade, E. J. Cairns, J. T. Vaughey, M. M. Thackeray, K. A. Striebel J. Electrochemical Soc., 151 (2004) A1012-A1019. [4] L.M. L. Fransson, J. Vaughey, R. Benedek, K. Edstrom, J. O. Thomas, M. Thackeray Electrochem. Comm. 3 (2001) 317-323. [5] D. C. S. Souza, V. Pralong, A. J. Jacobson, L. F. Nazar Science 296 (2002) 296, 2012. [6] V. Aravindan , Y.-S. Lee, S. Madhavi Adv. Energy Mater. (2015) 1402225. Figure 1

Measurements of stress and fracture in germanium electrodes of lithium-ion batteries during electrochemical lithiation and delithiation

We measure stresses that develop in sputter-deposited amorphous Ge thin films during electrochemical lithiation and delithiation. Amorphous LixGe electrodes are found to flow plastically at stresses that are significantly smaller than those of their amorphous LixSi counterparts. The stress measurements allow for quantification of the elastic modulus of amorphous LixGe as a function of lithium concentration, indicating a much-reduced stiffness compared to pure Ge. Additionally, we observe that thinner films of Ge survive a cycle of lithiation and delithiation, whereas thicker films fracture. By monitoring the critical conditions for crack formation, the fracture energy is calculated using an analysis from fracture mechanics. The fracture energies are determined to be Γ = 8.0 J m−2 for a-Li0.3Ge and Γ = 5.6 J m−2 for a-Li1.6Ge. These values are similar to the fracture energy of pure Ge and are typical for brittle fracture. Despite being brittle, the ability of amorphous LixGe to flow at relatively small stresses during lithiation results in an enhanced ability of Ge electrodes to endure electrochemical cycling without fracture.

A simple technique for measuring the fracture energy of lithiated thin-film silicon electrodes at various lithium concentrations

We have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration using a bending test. First, silicon thin-films on copper substrates were lithiated to various states of charge. Then, bending tests were performed by deforming the substrate to a pre-defined shape, producing a variation of the curvature along the length of the electrode. The bending tests allow determination of the critical strains at which cracks initiate in the lithiated silicon. Using the substrate curvature technique, we also measured the elastic moduli and the stresses that develop in the electrodes during electrochemical lithiation. From these measurements, the fracture energy was calculated as a function of lithium concentration using a finite element simulation of fracture of an elastic film on an elastic–plastic substrate. The fracture energy was determined to be Γ = 12.0 ± 3.0 J m−2 for amorphous silicon and Γ = 10.0 ± 3.6 J m−2 for Li3.28Si, with little variation in the fracture energy for intermediate Li concentrations. These results provide a guideline for the practical design of high-capacity lithium ion batteries to avoid fracture. The experimental technique described in this paper also provides a simple means of measuring the fracture energy of brittle thin-films.

Experimental and Theoretical Characterization of Electrode Materials That Undergo Large Volume Changes and Application to the Lithium-Silicon System

The life and performance of lithium-ion batteries is related to the mechanical expansion and contraction of the active materials, particularly for silicon-enhanced negative electrodes. In this work, we develop a theory and commensurate equations to describe how lithium diffuses within lithiated silicon, and we include the influence of active material expansion (upon lithiation) and contraction (upon delithiation). We employ thin-films of vapor-deposited Si to construct our electrodes. This allows us to isolate our analysis to the active material and avoids the necessity of treating binders, conductive diluents, and complicated geometries associated with conventional porous electrodes used in most practical lithium-ion batteries and in the construction and modeling of a Li-Si porous electrode. The model is shown to compare favorably with experimental results. We have derived an equation system for the treatment of electrode materials that undergo large volume changes during operation. The approach is applied to a thin-film Li-Si system, wherein a volume change of about 270% is observed relative to pure Si and Li3.75Si. Good agreement is obtained when the model calculations are compared with experimental data obtained from Li-Si thin film electrodes (67-nm thickness) subjected to linear-sweep voltammetry experiments over current and potential ranges of interest for many applications, including electrified vehicles. We ignore the influence of stress on the thermodynamics and transport of the Li-Si thin films. Instead, we use a one-dimensional model which assumes that stress does not impact the chemical potential, because the stresses are negligible within the infinitely thin film that is unconstrained at its interface with the electrolyte. However, the local expansion and contraction of the film, as a function of lithium content, is taken into consideration, and this leads to a moving boundary value problem, where the position of the film surface must be calculated.

Thermal Stability of Sb Anodes in Li-Ion Batteries and the Effects of Incorporating into Intermetallics and Composite Electrodes

Introduction As efforts are made to expand the application of lithium-ion cells into the realm of transportation and stationary storage, safety becomes one of the primary limiting concerns. Lithium-ion cells store a significant amount of energy, are composed of a number of flammable components, and battery systems based on lithium-ion technology can pose a significant flammability hazard if subjected to various abuse conditions or not properly designed. The primary cause of these safety issues is a condition known as thermal runaway, in which a cell undergoes self-sustaining exothermic reactions beginning at temperatures as low as 80 °C that results in uncontrolled temperature increase and eventual cell rupture or explosion. This is especially problematic in the large-scale applications mentioned above as such systems involve the close packing of a large number of cells, i.e., the heat output of one failed cell could result in a catastrophic chain reaction. While cathode reactions certainly contribute to this issue, it has been shown that the vast majority of the heat generated at low temperatures – that is the heat that initiates thermal runaway – arises from the anode.1 SEI layer degradation and the reaction of intercalated lithium with electrolyte are two primary sources of these exothermic reactions. While the composition of the SEI layer and subsequent thermal decomposition reactions have been well studied for graphite anodes, the similar SEI composition and reactions for alternative anodes are much less studied. Antimony is one such material that has been studied as a promising anode material in both lithium-ion and sodium-ion battery applications. It has a higher reaction potential of ~ 0.8 V that allows for fast charge and upon simple enthalpic analysis should be more thermally stable than lower potential anodes like graphite and silicon. The goal in our current study is to characterize the thermal stability performance of Sb-based electrodes using DSC and the corresponding SEI layer composition with SEM, XRD, and XPS. To achieve an acceptable cycle life, Sb is often incorporated into an intermetallic and/or an active/inactive composite system. As such, in addition to the pure Sb anode, Cu2Sb, Cu2Sb-Al2O3-C, Cu2Sb-TiC, and Cu2Sb-TiC-C electrodes are also tested to determine the effect of these common cycle-life-enhancing techniques on the thermal stability of Sb-based anodes. Results and Discussion Jiang and Dahn have demonstrated the dependence of the heat of reaction between lithiated anode materials and organic electrolytes to be dependent upon reaction potential of the electrode.2 This decreased reactivity of lithium is not observed in our materials as both graphite and antimony show roughly equal heat generation of ~ 70 kJ per mole of Li. However, as shown in Figure 1, the greater storage of lithium per mass of antimony (lithiated phase of Li3Sb vs. LiC6) yields a lower rate of heat generation per gram for antimony as a function of lithium content. In other words there is less heat generation per unit gravimetric capacity. For other lithium alloy anode materials like silicon, which demonstrates reduced heat generation relative to graphite, only a fraction of the difference in heat generated was determined to be from the difference in electrode potential.3 The bulk of the variation was attributed to difference in the SEI layer morphology and composition between the two electrode materials. The potential presence of antimony compounds in the SEI layer or as reaction products to heating could alter the thermal stability of Sb in unforeseen ways, necessitating SEI composition study via XRD and/or XPS. Figure 2 shows that the DSC profiles are similar for both lithiated graphite and antimony, which suggests similar decomposition routes for the two electrode materials. Both materials demonstrate a strong exothermic peak initiating near 150 °C with a tapering tail. While the primary safety enhancement from Sb arises from greater lithium storage per unit mass, Cu2Sb shows a decreased reactivity of lithium with a heat generation of only 60 kJ per mole of Li, a 15 % decrease relative to both Sb and graphite. The decreased heat generation from Cu2Sb offers further hope that the incorporation of additional inactive material in the form of TiC or Al2O3 could further enhance the thermal stability of Sb electrodes.

Electrochemical Characterization of Lithiation and De-Lithiation of Ultra-Thin Amorphous TiO2 Films

Research on TiO2 as an electrode for lithium-ion batteries is mainly focused on the anatase polymorph, as high capacities can be achieved in bulk, i.e. ~200 mAh/g1–3. However, due to limited diffusion of lithium inside anatase, these capacities are only achieved either by nanostructuring the material, or simply using slow charging rates (<<1C). In contrast, amorphous TiO2 has only recently gained attention as a high-power insertion electrode4–6; it was shown that next to its ease of fabrication, it shows a high capacity, good rate capability and a stable cyclic performance. Amorphous TiO2 undergoes an electrochemically driven phase transformation into a face-centered-cubic structure upon lithiation after the first cycle5. It was furthermore shown that this cubic phase has a capacity of 225 mAh/g at 2C which was stable up to 600 cycles —better than any other known nanostructured TiO2 polymorph (i.e. anatase and TiO2(B))5.In order to create a well-defined model system, we deposited ultra-thin (35 nm) amorphous TiO2 films by atomic layer deposition. Such thin films are interesting since they possess the same length scale as nanostructured particles or other nanostructures, such as nanotubes, but they offer a better defined geometry allowing more precise determination of the underlying lithium insertion/extraction kinetics. Furthermore, such ultra-thin electrodes might find application in 3D micro-batteries7.In order to "activate" amorphous TiO2 we performed a few potentiostatic cycling steps between 3.0 V and 0.1 V vs Li+/Li, which significantly increased the lithium insertion kinetics of 35 nm amorphous TiO2films, as probed by galvanostatic charge/discharge, cyclic voltammetry and impedance spectroscopy.Cyclic voltammetry performed at 10 mV/s showed a decrease in the peak potential difference between Li+ insertion and extraction of 0.36 V to 0.2 V after potentiostatic activation. Furthermore, galvanostatic charge/discharge experiments showed 106% increase in the specific capacity from 1.8 µAh cm-2 to 3.7 µAh cm-2 at a charging rate of 1C after activation.Impedance spectroscopy as function of lithium content before and after potentiostatic cycling showed that the overall impedance is greatly diminished, reflecting improved kinetics which lead to the increased rate-performance. Figure 1 shows complex plane plots of the potential dependent impedance measurements. The impedance response was fitted using a small-signal equivalent circuit incorporating, amongst others, a finite length Warburg impedance and charge transfer resistance.These results offer new electrochemical insights on the effect of the electrochemically driven phase transition of amorphous TiO2 showing its promises as a potential high-rate and high capacity electrode for lithium-ion batteries.

Measurements of the Fracture Energy of Lithiated Silicon Electrodes of Li-Ion Batteries

We have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration. To this end, we have constructed an electrochemical cell capable of testing multiple thin-film electrodes in parallel. The stress in the electrodes is measured during electrochemical cycling by the substrate curvature technique. The electrodes are disconnected one by one after delithiating to various states of charge, that is, to various concentrations of lithium. The electrodes are then examined by optical microscopy to determine when cracks first form. All of the observed cracks appear brittle in nature. By determining the condition for crack initiation, the fracture energy is calculated using an analysis from fracture mechanics. In the same set of experiments, the fracture energy at a second state of charge (at small concentrations of lithium) is measured by determining the maximum value of the stress during delithiation. The fracture energy was determined to be Γ = 8.5 ± 4.3 J/m(2) at small concentrations of lithium (~Li0.7Si) and have bounds of Γ = 5.4 ± 2.2 J/m(2) to Γ = 6.9 ± 1.9 J/m(2) at larger concentrations of lithium (~Li2.8Si). These values indicate that the fracture energy of lithiated silicon is similar to that of pure silicon and is essentially independent of the concentration of lithium. Thus, lithiated silicon demonstrates a unique ability to flow plastically and fracture in a brittle manner.

In situ NMR observation of the lithium extraction/insertion from LiCoO2 cathode

Rechargeable lithium-ion batteries (LIBs) are currently accepted to be one of the most suitable energy storage resources in portable electronic devices because of their high gravimetric and volumetric energy density. To understand the behavior of Li+ ions on electrochemical lithium extraction/insertion process, we performed in situ7Li nuclear magnetic resonance (NMR) measurements for LiCoO2 cathode in a plastic cell battery, and the spectral evolutions of the 7Li NMR signal of LixCoO2 (0≤x≤1) were well investigated. Very narrow solid solution region of LixCoO2 (∼0.99≤x<1) was explicitly defined from the large intensity reduction of LiCoO2 signal at ∼0ppm, which is related to the localized nature of the electronic spin of paramagnetic Co4+ ion formed at the very early delithiation stage. With further decreasing the signal intensity of LiCoO2, a Knight-shifted signal corresponding to an electrically conductive LixCoO2 phase emerged at x=0.97, which then monotonously decreased in intensity for x<0.75 in accordance with the electrochemical lithium de-intercalation from LixCoO2. These observations acquired in situ fully confirm the earlier studies obtained in ex situ measurements, although the present study offers more quantitative information. Moreover, it was shown that the peak position of the NMR shift for LixCoO2 moved as a function of lithium content, which behavior is analogous to the change in its c lattice parameter. Also, the growth and consumption of dendritic/mossy metallic lithium on the counter electrode was clearly observed during the charge/discharge cycles.

Thermodynamic investigations of copper oxides used as conversion type electrodes in lithium ion batteries

Copper oxides, which exhibit the conversion mechanism with lithium during electrochemical cycling, are promising electrode materials for next-generation lithium ion batteries. To better understand phase formations and equilibria in the Li–Cu–O system, the LiCu2O2 phase was synthesized using the solid-state method and characterized with X-ray powder diffraction technique and Rietveld analysis as well as inductively coupled plasma—optical emission spectrometry (ICP–OES). The phase stabilities in argon atmosphere and in a mixture of argon and oxygen were determined between 200 and 900 °C and between 200 and 950 °C, respectively. Based on this data as well as on other literature data, a thermodynamic description of the Li–Cu–O system valid in the battery relevant temperature regime was developed using the CALPHAD method. From this thermodynamic description, titration curves for CuO and Cu2O cathodes, which give the equilibrium cell voltage as a function of lithium content along a selected composition path, were calculated at different temperatures.

ChemInform Abstract: Material Problems and Prospects of Li‐Ion Batteries for Vehicles Applications

AbstractReview: 35 refs.

MATERIAL PROBLEMS AND PROSPECTS OF Li-ION BATTERIES FOR VEHICLES APPLICATIONS

This paper reviews material issues of development of Li -ion batteries for vehicles application. The most important of them is safety, which is related to application of nonflammable electrolyte with large electrochemical window and possibility of forming protective SEI (solid/electrolyte interface) to prevent plating of lithium on carbon anode during fast charge of the batteries. The amount of electrical energy, which a battery is able to deliver, depend on the electromotive power of the cell as well as on its capacity — both these factors are related to the chemistry of electrode materials. Nanotechnology applied to electrode materials may be a breakthrough for Li -batteries performance due to extreme reactivity of nanoparticles in relation to lithium. The electrode-electrolyte interface phenomena are decisive for a cell lifetime. Review of physicochemical properties of intercalated transition metal compounds with layered, spinel or olivine-type structure is provided in order to correlate their microscopic electronic properties, i.e. the nature of electronic states, with the efficiency of lithium intercalation process, which is controlled by the chemical diffusion coefficient of lithium. Data concerning cell voltage and character of discharge curves for various materials are correlated with the nature of chemical bonding and electronic structure. Proposed electronic model of the intercalation process allow for prediction and design of operational properties of intercalated electrode materials. Proposed method of measuring the Li x M a X b potential on the basis of the measurement of the electromotive force of the Li / Li +/ Li x M a X b electrochemical cell is a powerful tool of solid state physics allowing for direct observation of the Fermi level changes in such systems as a function of lithium content.

Factors affecting Li mobility in spinel LiMn 2O 4—A first-principles study by GGA and GGA+ U methods

The structural changes and voltages of LiMn 2O 4 spinel as a function of lithium content were investigated with density functional theory (DFT) in the generalized gradient approximation (GGA) and in the GGA with Hubbard U correction (GGA+ U). The GGA+ U approximation can distinguish the charge separation between Mn 3+ and Mn 4+, which GGA fails to capture. Therefore with this method the effects of local environments on Li diffusion activation energy barriers in the Li-rich phase could be systematically investigated. Our results showed that the different valences states of Mn ions and their arrangement surrounding the lithium ions have a profound effect on the activation barrier of lithium diffusion in the spinel structure.

A delithiated LiNi 0.65Co 0.25Mn 0.10O 2 electrode material: A structural, magnetic and electrochemical study

A crystalline LiNi 0.65Co 0.25Mn 0.10O 2 electrode material was synthesized by the combustion method at 900 °C for 1 h. Rietveld refinement shows less than 3% of Li/Ni disorder in the structure. Lithium extraction involves only the Ni 2+/Ni 4+ redox couple while Co 3+ and Mn 4+ remain electrochemically inactive. No structural transition was detected during cycling in the whole composition range 0 < x < 1.0. Furthermore, the hexagonal cell volume changes by only 3% when all lithium was removed indicating a good mechanical stability of the studied compound. LiNi 0.65Co 0.25Mn 0.10O 2 has a discharge capacity of 150 mAh/g in the voltage range 2.5–4.5 V, but the best electrochemical performance was obtained with an upper cut-off potential of 4.3 V. Magnetic measurements reveal competing antiferromagnetic and ferromagnetic interactions – varying in strength as a function of lithium content – yielding a low temperature magnetically frustrated state. The evolution of the magnetic properties with lithium content confirms the preferential oxidation of Ni ions compared to Co 3+ and Mn 4+ during the delithiation process.

Atomic and Electronic Bulk versus Surface Structure: Lithium Intercalation in Anatase TiO2

Lithium intercalation in anatase TiO2 results in structural and electronic changes. X-ray absorption at the Ti K edge as a function of lithium content is interpreted as a sum of two coexisting crystallographic phases (anatase and Li-titanate), both in the case of chemically lithiated microcrystalline samples and electrochemically intercalated thin film samples. The changes in the near edge region of the Ti K edge (XANES region) indicate that the charge-compensating electron of the Li-ion mainly occupies the crystal field split Ti 3d t2g orbitals located at the bottom of the conduction band. Surface sensitive reflection mode XAFS indicates that intercalation leads to a 3−4 nm surface region characterized by an effective Ti oxidation state close to 3+. The composition is determined to be close to Li1TiO2, and appears to be the reason for a poor cycle-lifetime of the anatase TiO2 electrode material.

Lattice vibrations of materials for lithium rechargeable batteries: II. Lithium extraction–insertion in spinel structures

Lithiated spinel manganese oxides with various amounts of lithium have been prepared through solid-state reaction and electrochemical intercalation and deintercalation. Local structure of the samples are studied using Raman scattering and Fourier transform infrared spectroscopy. We report vibrational spectra of lithiated manganese oxides Li x Mn 2O 4 as a function of lithium concentration in the range 0.1≤ x≤2.0. Raman and Fourier transform infrared (FTIR) spectral results indicated multiple-phase reactions when the lithium content is modified in the spinel lattice. Lattice dynamics of lithiated spinel manganese oxides have been interpreted using either a classical factor-group analysis or a local environment model. The structural modifications have been studied on the basis of vibrations of LiO 4 tetrahedral and MnO 6 octahedral units when Li/Mn≤0.5, and LiO 4, LiO 6, and MnO 6 structural units when Li/Mn>0.5.

Experimental and theoretical studies on dynamic properties of Li ions in Li xMn 2O 4

In order to study the dynamic properties of Li x Mn 2O 4, potential relaxation techniques (PRT) is used to measure the chemical diffusion coefficient of Li x Mn 2O 4. Results are presented for x ranges from x=0.1 to 0.9. They show that the chemical diffusion coefficient at the two-phase coexistent stage near x=0.3 and 0.7 is higher than at the single-phase stage during the insertion and extraction process. Monte Carlo (MC) simulations are also used to simulate the ionic conductivity σ of Li ions in Li x Mn 2O 4 and its dependence as a function of lithium concentration x. The results show an M shaped curve in the plot of ionic conductivity σ versus x when the simulation temperature is 293 K, which confirms the experimental PRT results. The voltage profiles of Li x Mn 2O 4/Li cells were also simulated with different boundary conditions.

Structural and Electronic Properties of the Layered LiNi0.5Mn0.5O2 Lithium Battery Material

Computational studies based upon density functional theory (DFT) have been carried out on the LixNi0.5Mn0.5O2 system, a promising cathode material for rechargeable lithium batteries. Electronic structure calculations suggest that the nominal valence state distribution is given by the formula . Possible Ni−Mn cation ordering schemes in the layered structure have been examined including intralayer and interlayer configurations. The results on lithium deintercalation of LixNi0.5Mn0.5O2 indicate that the electrochemical behavior is linked to the oxidation of Ni2+. Our calculated cell voltage range as a function of lithium content (x) is compatible with electrochemical measurements that generally show sloping voltage profiles. The calculated Mn−O bond length shows relative invariance with Li extraction, whereas the Ni−O bond shortens significantly, which accords well with the available structural data.

On the Origin of the Monoclinic Distortion in LixNiO2

Using first-principles calculation and thermally equilibrated Li-vacancy configurations, we have calculated the monoclinicity, am/bm, as a function of lithium content in LixNiO2. In agreement with experimental data, maxima in am/bm are predicted around Li0.75NiO2 and Li0.4NiO2. We explain the monoclinicity maxima at Li0.75NiO2 in terms of a Jahn−Teller distortion assisted by the presence of lithium-vacancy ordering. The subsequent removal of lithium causes a decrease in the number of Ni3+ ions and therefore in the monoclinicity. However, the strong lithium-vacancy ordering at Li0.4NiO2 enables new maxima of monoclinicity despite the low Ni3+ content.