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Abstract:After an introduction to lithium insertion compounds and the principles of Li-ion cells, we present a comparative study of the physical and electrochemical properties of positive electrodes used in lithium-ion batteries (LIBs). Electrode materials include three different classes of lattices according to the dimensionality of the Li + ion motion in them: olivine, layered transition-metal oxides and spinel frameworks. Their advantages and disadvantages are compared with emphasis on synthesis difficulties, electrochemical stability, faradaic performance and security issues.

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The electronic structure of LiFePO 4 and delithiated FePO 4 is revisited in the light of the previous calculations taking into account the coulomb correlation potential for d-electrons. The nature of the optical transitions across the energy gap is investigated. In LiFePO 4 , these are intra-atomic Fe 2+ -Fe 3+ transitions suffering a strong Franck-Condon effect due to the local distortion of the lattice in FePO 4 , which is indirect evidence of the formation of a small polaron. This situation contrasts with that met in the much more covalent delithiated phase, where the optical transition across the energy gap is associated with a transfer of an electron from the p-states of the oxygen to the d-states of iron ions. The small polarons in LiFePO 4 are associated with the presence of Fe 3+ ions introduced by native defects in relative concentration [Fe 3+ ]/[Fe 2+ +Fe 3+ ] ) 3 × 10 -3 in the samples known to be optimized with respect to their electrochemical properties. The nearest iron neighbors around the central polaron site are spin-polarized by the indirect exchange mediated by the electronic charge in excess. These small magnetic polarons are responsible for the interplay between electronic and magnetic properties that are quantitatively and self-consistently analyzed.

International audienceThe success of lithium-ion batteries in small-scale applications translates to large-scale applications, with an important impact in the future of the environment by improving energy efficiency and reduction of pollution. In this review, we present the progress that allows lithium-insertion compounds with the spinel structure to become the active cathode element of a new generation of Li-ion batteries, namely the 5 V cathodes, which promise to improve the technologies of energy storage and electric transportation, thereby addressing the replacement of the gasoline engine and the increasing demand for green energy power sources. The compounds considered here include the spinel LiNi0.5Mn1.5O4 and its related Cr-doped structure. Emphasis is placed on the control of physical properties that is needed to guarantee the reliability and the optimum electrochemical performance of these materials as the active cathode element of Li-ion batteries. We also report the structural evolution of the spinel phase in both charge (Li extraction) and discharge (Li insertion) reactions

Doping is generally used to tune and enhance the properties of metal oxides. However, their chemical composition cannot be readily modified beyond low dopant amounts without disrupting the crystalline atomic structure. In the case of anatase TiO 2 , we introduce a new solution-based chemical route allowing the composition to be significantly modified, substituting the divalent O 2− anions by monovalent F − and OH − anions resulting in the formation of cationic Ti 4+ vacancies (□) whose concentration can be controlled by the reaction temperature. The resulting polyanionic anatase has the general composition Ti 1−x−y □ x+y O 2−4(x+y) F 4x (OH) 4y , reaching vacancy concentrations of up to 22%, i.e., Ti 0.78 □ 0.22 O 1.12 F 0.4 (OH) 0.48 . Solid-state 19 F NMR spectroscopy reveals that fluoride ions can accommodate up to three different environments, depending on Ti and vacancies (i.e. Ti 3 -F, Ti 2 □ 1 -F, and Ti 1 □ 2 -F), with a preferential location close to vacancies. DFT calculations further confirm the fluoride/vacancy ordering. When its characteristics were evaluated as an electrode for reversible Li-ion storage, the material shows a modified lithium reaction mechanism, which has been rationalized by the occurrence of cationic vacancies acting as additional lithium hosting sites within the anatase framework. Finally, the material shows a fast discharging/charging behavior, compared to TiO 2 , highlighting the benefits of the structural modifications and paving the way for the design of advanced electrode materials, based on a defect mediated mechanism. ■ INTRODUCTIONTransition-metal oxides are an important class of materials, whose properties are dependent on many factors, including their composition, structure, and morphology. Among them, titanium dioxide (TiO 2 ) is a multifunctional material used for a broad range of applications. The low toxicity and the abundance of titanium have favored the emergence of Tibased compounds for photocatalytic hydrogen production by water splitting, rechargeable batteries/supercapacitors, dyesensitized solar cells, sensors, and biomedical devices. 1−6 Over the years, several approaches have been developed to improve its properties. For instance, crystal facets engineering and structural modifications have been widely employed. 7−9 The latter approach comprises the introduction of reduced titanium or heteroatoms within the lattice. Although the use of dopants has been proved as an effective way to tune this material's properties, changing the chemical composition of TiO 2 with a degree of substitution that goes beyond the doping level appears as a challenging and promising way to tailor its properties. On more general grounds, knowing how strongly a metal oxide can be modified while maintaining its original network is of fundamental interest.With the goal of modifying the composition of anatase, which is one of the polymorphs of TiO 2 , we propose an approach where divalent oxide anions are substituted by monovalent ones such as fluoride and hydroxide. The anatase structure...