Topotactic insertion

The second way in which an electroactive species such as lithium can be incorporated into the structure of an electrode is by a topotactic insertion reaction. In this case the guest species is relatively mobile and enters the crystal structure of the host phase so that no significant change in the structural configuration of the host lattice occurs. 

Insertion (intercalation) compounds. Insertion compounds are defined as products of a reversible reaction of suitable crystalline host materials with guest molecules (ions). Guests are introduced into the host lattice, whose structure is virtually intact except for a possible increase of some lattice constants. This reaction is called topotactic. A special case of topotactic insertion is reaction with host crystals possessing stacked layered structure. In this case, we speak about intercalation (from the Latin verb intercalare, used originally for inserting an extra month, mensis intercalarius, into the calendar). 

These qualitative trends can be rationalized and put on a quantitative basis by using a simple bond model for the topotactic insertion of hydrogen into It is assumed that the product is an oxy-hydroxide... 

Originally, this referred specifically to layered structures that can undergo insertion of ions or molecules between the van der Waals gaps, but is now commonly used for any structure that undergoes topotactic insertion reactions. For Li-ion battery materials, it refers specifically to compounds that undergo reductive insertion of lithium ions, such as graphite (used as an anode) or LiCo02 (used as a cathode). 

In this case a two-step reaction was observed. The Fe " " to Fe + reaction mechanism was found to consist of a two-phase transformation followed by a topotactic insertion reaction as suggested by XRD and electrochemical characterization ... 

Furthermore, the topotactic insertion/extraction reactions may occm by diSusion in onedimensional channels as in the hexagonal tungsten bronze, in two-dimensional as in layered transition metal dichalcogenides, or in three-dimensional as in the close-packed spinel structure. There are a large number of candidate materials which can be used in lithimn insertion... 

The thermodynamic properties of topotactic insertion reaction materials with selective equilibrium are quite different from those of materials in which complete equilibrium can be assumed, and reconstitution reactions take place. Instead of flat plateaus related to polyphase equihbria, the composition-dependence of the potential generally has a flat S-type form. 

The reactions of these insertion materials, except for LiMnO, [19, 20], consist of electron and lithium-ion insertion into, or extraction from, each matrix without the destruction of its core structure this is called a topotactic reaction. A series of LiCovNi, v02 (0 < y < 1)) and... 

Bruce PG, Irvine ITS (2001) Insertion Compounds. In Encyclopedia of Materials Science and Technology, Elsevier Science Ltd Oxford ISBN 0-08-0431526, pp. 4115-4121 SchoUhom R (1980) Reversible topotactic redox reactions of solids by electron/ion transfer. Angew Chem Int Ed Engl 19 983-1003... 

Two types of transformations can be very broadly distinguished. The first is the formation of a solid solution, in which solute atoms are inserted into vacancies (lattice sites or interstitial sites) or substitute for a solvent atom on a particular sublattice. Many types of synthetic processes can result in this type of transformation, including ion-exchange reactions, intercalation reactions, alloy solidification processes, and the high-temperature ceramic method. Of these, ion exchange, intercalation, and other so-called soft chemical (chimie douce) reactions produce no stmctural changes except, perhaps, an expansion or contraction of the lattice to accommodate the new species. They are said to be under topotactic, or topochemical, control. 

The SEM photographs for NMO-2/7 and NMO-1/2 samples are shown in Figure 4. All these samples have similar sheety crystal morphology as that of NaBir precursor [21]. This result suggests that the hydrothermal insertion of Ni(OH)2 is a topotactic reaction, in which almost no dissolution-deposition of NaBir or NiBir occurs. The sheety crystal morphology also suggests that the Ni(OH)2 inserted samples have a layered structure. 

In the weak basic solution, Ni(OH)2 is preferentially formed in the interlayer space of blmessite rather than as free Ni(OH)2 from Ni containing solution. Hydrothermal intercalation method is favored the formation of a mixed layered Ni(OH)2-manganese oxide, in which the Ni(OH)2 insertion into the bimessites is topotactic reaction. 

In a-U03, hydrogen insertion proceeds topotactically up to a-Hi.i7U03. Evidence for the formation of—OH2 groups comes from the INS spectrum, which shows a large peak at 909 cm In the IR spectrum, the uranyl bands decrease in intensity and eventually disappear at high hydrogen contents . 

Insertion and intercalation electrode materials are electroactive and have composition, chemical and physical properties that change with oxidation state. The materials have a common characteristic that they serve as a host solid into which guest species are inserted from an electrolyte, and the insertion process is reversible. In particular, the intercalation process is described as the reversible insertion of guest species into a host lattice without structural modification of the host. The term topotactic or topochemical is also used to describe the insertion reaction. 

The ionic conductivity of transition metal oxides is due to ion conducting channels that exist in one (e.g., olivine), two (layered), or three (spinel) dimensions (Figure 36.2). In order to maintain the integritjr of these channels, lithium insertion reactions are generally desired to be topotactic, that is, the lithiated and delithiated states should be as crystallographically similar as possible. 

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