Electrochemical intercalation

The interest in intercalation reactions stems from different motivations. From a preparation point of view they provide routes for the systematic synthesis of new solids with kinetic rather than thermodynamic stability that cannot be obtained by other preparation techniques [11, 13], Furthermore, they permit controlled systemic modifications of chemical as well as physical properties, including electronic, magnetic and optical properties. From an application viewpoint, they are of importance in supercapacitors, rechargeable batteries, non-emissive electrochromic displays, and so forth [4, 14], 

Remarkably, a large number of electronically conducting intercalation hosts are accessible today, including numerous transition-metal oxides with framework lattices, such as vanadium pentoxide and nickel oxide, and organic solids with molecular lattices. Additionally, a large variety of ionic guest species have been studied. 

The fundamental intercalation process can be defined as the bulk reaction of an electronically conducting host lattice (H) that contains an intracrystalline system of accessible vacancies ([ ]) with mobile guest ions (I) present in the electrolyte phase which is in contact with the solid driven by an applied electric potential. This transfer of ions between the electrolyte and the solid is accompanied by a compensating electron transfer [13] 

This double injection of ions and electrons maintains electroneutrality of the system. The majority of intercalation compounds are known to undergo electron/cation transfer reactions, but NiO, for example, also supports reversible electron/anion transfer processes. In particular, an increase in the specific surface area is generally expected to significantly reduce the characteristic diffusion length of intercalation ions, while simultaneously increasing the number of accessible intercalation sites [16]. 

Orthorhombic crystalline vanadium pentoxide is a typical intercalation compound as a result of its layered structure, see Fig. 5.2, which finds widespread use in lithium ion intercalation applications such as electrochromic cells [17], high energy density batteries [18], supercapacitors [19], and sensors [20], since it offers the essential advantages of low cost, abundant availability, easy synthesis, and high intercalation densities [15, 16]. 

In heterogeneous solid-state reactions where the composition of both solid reactants does not change, the electrode s eqnilibrinm potential depends only on the nature of the two phases, not on their relative amonnts. Hence, dnring the reaction the potential does not change. It also remains constant when the cnrrent is interrupted after partial reduction or oxidation. 

Another example for reactions with the insertion of protons is the cathodic reduction of manganese dioxide, which occnrs dnring discharge of the positive electrodes in zinc-manganese dioxide batteries. This reaction can be formulated as 

During the reaction, protons which have been produced from water molecules or from the hydroxonium (HjO ) ions of the solntion are inserted into the manganese dioxide lattice. At the same time, an eqnivalent nnmber of Mn ions of the lattice are rednced to Mn + ions by the electrons arriving through the external circuit. Hence, the overall balance of positive and negative charges in the lattice remains nnchanged. 

FIGURE 25.11 Discharge curves of batteries with oxides as the positive electrode (a) continuous discharge (b) interrupted discharge (1) MnOj, (2) AgjO. 

Protons are not the sole species that can be incorporated into the lattices of different host materials. At the beginning of the 1960s, Boris N. Kabanov showed that during cathodic polarization of different metals in alkaline solutions, intercalation of atoms of the corresponding alkali metal is possible. As a result of such an electrochemical intercalation, either homogeneous alloys are formed (solid solutions) or heterogeneous polyphase systems, or even intermetallic compounds, are formed. 

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The quality and quantity of sites which are capable of reversible lithium accommodation depend in a complex manner on the crystallinity, the texture, the (mi-cro)structure, and the (micro)morphology of the carbonaceous host material [7, 19, 22, 40-57]. The type of carbon determines the current/potential characteristics of the electrochemical intercalation reaction and also potential side-reactions. Carbonaceous materials suitable for lithium intercalation are commercially available in many types and qualities [19, 43, 58-61], Many exotic carbons have been specially synthesized on a laboratory scale by pyrolysis of various precursors, e.g., carbons with a remarkably high lithium storage capacity (see Secs. 

Whereas the electrochemical decomposition of propylene carbonate (PC) on graphite electrodes at potentials between 1 and 0.8 V vs. Li/Li was already reported in 1970 [140], it took about four years to find out that this reaction is accompanied by a partially reversible electrochemical intercalation of solvated lithium ions, Li (solv)y, into the graphite host [64], In general, the intercalation of Li (and other alkali-metal) ions from electrolytes with organic donor solvents into fairly crystalline graphitic carbons quite often yields solvated (ternary) lithiated graphites, Li r(solv)yC 1 (Fig. 8) [7,24,26,65,66,141-146],... 

Electrochemistry was at the sonrce of the cold-fusion boom, bnt then at hrst sight seemed to stand aside. However, as a matter of fact, the central point in the experiments concerning electrolysis at palladium has been a phenomenon which now is investigated more vigoronsly and persistently electrochemical intercalation. 

Palladium hydride is a unique model system for fundamental studies of electrochemical intercalation. It is precisely in work on cold fusion that a balanced materials science approach based on the concepts of crystal chemistry, crystallography, and solid-state chemistry was developed in order to characterize the intercalation products. Very striking examples were obtained in attempts to understand the nature of the sporadic manifestations of nuclear reactions, true or imaginary. In the case of palladium, the elfects of intercalation on the state of grain boundaries, the orientation of the crystals, reversible and irreversible deformations of the lattice, and the like have been demonstrated. 

Another factor also contributed to the appearance of new concepts in electrochemistry in the second half of the twentieth century The development and broad apphca-tion of hthium batteries was a stimulus for numerous investigations of dilferent types of nonaqueous electrolytes (in particular, of sohd polymer electrolytes). These batteries also initiated investigations in the held of electrochemical intercalation processes. 

Carbon electrodes are widely used in electrochemistry both in the laboratory and on the industrial scale. The latter includes production of aluminium, fluorine, and chlorine, organic electrosynthesis, electrochemical power sources, etc. Besides the use of graphite (carbons) as a virtually inert electode material, the electrochemical intercalation deserves special attention. This topic will be treated in the next paragraph. 

The electrochemical intercalation into graphite leads in the most simple case to binary compounds (graphite salts) according to the schematic equations ... 

Reactions (5.5.30) and (5.5.31) proceed prevailingly during intercalation from solid or polymer electrolytes (cf. Section 2.6) or melts. When using common liquid electrolyte solutions, a co-insertion of solvent molecules (and/or intercalation of solvated ions) very often occurs. The usual products of electrochemical intercalation are therefore ternary compounds of a general composition ... 

The electrochemical intercalation of HS04 anions together with H2S04 was described by Thiele in 1934. The composition of the product of prolonged anodic oxidation of graphite in concentrated sulphuric acid is... 

The electrochemical intercalation/insertion has not only a preparative significance, but appears equally useful for charge storage devices, such as electrochemical power sources and capacitors. For this purpose, the co-insertion of solvent molecules is undesired, since it limits the accessible specific faradaic capacity. 

The electrochemical intercalation/insertion is not a special property of graphite. It is apparent also with many other host/guest pairs, provided that the host lattice is a thermodynamically or kinetically stable system of interconnected vacant lattice sites for transport and location of guest species. Particularly useful are host lattices of inorganic oxides and sulphides with layer or chain-type structures. Figure 5.30 presents an example of the cathodic insertion of Li+ into the TiS2 host lattice, which is practically important in lithium batteries. 

The concept of electrochemical intercalation/insertion of guest ions into the host material is further used in connection with redox processes in electronically conductive polymers (polyacetylene, polypyrrole, etc., see below). The product of the electrochemical insertion reaction should also be an electrical conductor. The latter condition is sometimes by-passed, in systems where the non-conducting host material (e.g. fluorographite) is finely mixed with a conductive binder. All the mentioned host materials (graphite, oxides, sulphides, polymers, fluorographite) are studied as prospective cathodic materials for Li batteries. 

Naji A., Willmann P., and Billaud D. Electrochemical Intercalation of Lithium into Graphite Influence of the Solvent Composition and the Nature of the Lithium Salt. Carbon, 36, 1347-1352 (1998). 

ELECTROCHEMICAL INTERCALATION OF PFg AND BF4 INTO SINGLE-WALLED CARBON NANOTUBES... 

SWNTs, the stability of the (C, PF ) ionic compound should be lower than in flat graphite layers. Therefore, during the electrochemical intercalation a chemical de-intercalation (decomposition) may take place, which explains the low faradaic yield of the anodic intercalation. 

The electrochemical intercalation of Li was studied for carbon electrodes modified by the 2Co-Ni complex, which showed the best effect in the reaction of oxygen electroreduction. Galvanostatic charge-discharge technique (PC governed automatic bench) in 2016 coin type cells was used for this purpose. 

Abe, T., Kawabata, N., Mizutani, Y., Inaba, M., and Ogumi, Z., Correlation between cointercalation of solvents and electrochemical intercalation of lithium into graphite in propylene carbonate solution, J. Electrochem. Soc. (2003) 150 (3), A257-A261. 

B. Gao, A. Kleinhammes, X.P. Tang, C. Bower, L. Fleming, Y. Wu, 0. Zhou, Electrochemical intercalation of singie-waiied carbon nanotubes with lithium, Chem. Phys. Lett., vol. 307, pp. 153-157,1999. 

It has been known for some time that lithium can be intercalated between the carbon layers in graphite by chemical reaction at a high temperature. Mori et al. (1989) have reported that lithium can be electrochemically intercalated into carbon formed by thermal decomposition to form LiCg. Sony has used the carbon from the thermal decomposition of polymers such as furfuryl alcohol resin. In Fig. 11.23, the discharge curve for a cylindrical cell with the dimensions (f) 20 mm x 50 mm is shown, where the current is 0.2 A. The energy density for a cutoff voltage of 3.7 V is 219 W h 1 which is about two times higher than that of Ni-Cd cells. The capacity loss with cycle number is only 30% after 1200 cycles. This is not a lithium battery in the spirit of those described in Section 11.2. 

Figure 25. (top) Electrochemical intercalation of lithium into e-V0P04, ° and (bottom) relationship between the various structures in the VOPO4 system the building block for all these structures is shown at the lower right. 

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