Surface films formed

The treated water contains sufficient concentration of surface film-forming agents if cold water spends about 12 min and warm water at least 20 min in the tank [19]. Sudden temperature variations over 10°C must be prevented because the active form of Al(OH)3 is sensitive to them [20]. If mixing with cold water or subsequent warming cannot be avoided, a short-term electrolytic aftertreatment must be provided in a small reaction tank. The development of undisturbed protective films in the tubing assumes continuous water flow with forced circulation by pumps [20]. 

Electrical characteristics of surface films formed electrochemically can be analysed using frequency response analysis (FRA) (sometimes called electrochemical impedance spectroscopy, or This technique is... 

Tin when made anodic shows passive behaviour as surface films are built up but slow dissolution of tin may persist in some solutions and transpassive dissolution may occur in strongly alkaline solutions. Some details have been published for phosphoric acid with readily obtained passivity, and sulphuric acid " for which activity is more persistent, but most interest has been shown in the effects in alkaline solutions. For galvanostatic polarisation in sodium borate and in sodium carbonate solutions at 1 x 10" -50 X 10" A/cm, simultaneous dissolution of tin as stannite ions and formation of a layer of SnO occurs until a critical potential is reached, at which a different oxide or hydroxide (possibly SnOj) is formed and dissolution ceases. Finally oxygen is evolved from the passive metal. The nature of the surface films formed in KOH solutions up to 7 m and other alkaline solutions has also been examined. 

As the hydrogen ions replace alkali (R) ions a surface film forms which has properties different from the massive glass. This film swells, acting as a barrier to further diffusion of ions into, and out of, the surface, inhibiting further attack. If this layer dries out, the thin film gives characteristic irridescent interference colours. 

Parker, J. G. and Roscow J. A., Method for the Assessment of the Quality of Surface Films Formed on the Cooling Water Side of Copper-Based Alloy Condenser Tubes , Br. Corros. J., 16, 2, 107-110(1981)... 

While the initial surface species formed on lithium in alkyl carbonates consist of ROC02Li compounds, these species react with water to form Li,CO, C02, and ROH. This reaction gradually changes the composition of the surface films formed on... 

According to the depth profile of lithium passivated in LiAsF6 / dimethoxyethane (DME), the SEI has a bilayer structure containing lithium methoxide, LiOH, Li20, and LiF [21]. The oxide-hydroxide layer is close to the lithium surface and there are solvent-reduction species in the outer part of the film. The thickness of the surface film formed on lithium freshly immersed in LiAsF /DME solutions is of the order of 100 A. 

Levi E, Lancry E, Gofer Y, Aurbach D (2006) The crystal structure of the inorganic surface films formed on Mg and Li intercalation compounds and the electrode performance. J Solid State Electrochem (2006) 10 176-184... 

Figure 9. Schematic illustrations of the surface film formed on lithium in nonaqueous electrolytes based on LiBp4 solutions and the subsequent reactions. (Reproduced with permission from ref 222 (Figure 12). Copyright 1995 The Electrochemical Society.)...

The presence of a protective SEI or surface layer prevents those irreversible reactions of electrolytes on anode/cathode surfaces that are otherwise favored by thermodynamics. Like the chemical process in the bulk electrolyte, the reactivity of the surface films formed in state-of-the-art electrolytes is negligible at room temperature. However, during long-term storage and cycling, their stability is still under question. 

Irreversible Capacity. Because an SEI and surface film form on both the anode and cathode, a certain amount of electrolyte is permanently consumed. As has been shown in section 6, this irreversible process of SEI or surface layer formation is accompanied by the quantitative loss of lithium ions, which are immobilized in the form of insoluble salts such as Li20 or lithium alkyl carbonate. Since most lithium ion cells are built as cathode-limited in order to avoid the occurrence of lithium metal deposition on a carbonaceous anode at the end of charging, this consumption of the limited lithium ion source during the initial cycles results in permanent capacity loss of the cell. Eventually the cell energy density as well as the corresponding cost is compromised because of the irreversible capacities during the initial cycles. 

Figure 9. Electron micrograph of brain lipid surface film formed in presence of ATP alone, X 22,500...

The hollow nano-objects may be synthesized by the joint evaporation of carbon and a metal component in liquid. These nano-objects are formed when products get into the zone of high plasma temperatures repeatedly. This process may be presented as that proceeding in two stages. At the first stage a metal crystal with the more thermally stable surface film forms. At the second stage the product gets repeatedly into the zone of high temperatures (up to 12000 K) due to the turbulent movement of liquid with this product within the reactor (Fig. 3). This results in evaporation of metal from the thermally stable shell and formation of the nano-scaled hollow structure (Fig. 4). 

The next section reviews some work on the identification of surface films formed on nonactive metal electrodes in different solutions of importance. Most of the work devoted so far to these nonaqueous systems relates to Li salt solutions. However, it can be speculated that an effect similar to that of the cations on the surface chemistry found for Li ions (compared with tetraalkyl ammonium cations) is expected for other cations of alkaline and alkaline earth metals (e.g., Na+, Ca2+, Mg2+). 

IV. IDENTIFICATION OF SURFACE FILMS FORMED ON NONACTIVE METAL ELECTRODES IN NONAQUEOUS SOLUTIONS USING SURFACE SENSITIVE SPECTROSCOPIC TECHNIQUES... 

Thus, the aged surface films formed in nonactive metals in alkyl carbonates contain a mixture of ROC02Li and Li2C03 (demonstrated in Figures 11-15). 

LijPOyFz, and Li BO F types. The latter two species result from partial hydrolysis of the BF3 or PF5 species (which may also be present in these salt solutions) with trace water, followed by electrochemical reduction in the presence of Li+. d. It should be emphasized that a critical parameter for the nature of the surface films formed on nonactive electrodes and the properties of the electrode passivation due to these surface films is the ratio between the electrode surface and the solution volume. The lower this ratio, the more pronounced is the rate of the above secondary reactions between the surface species initially formed and contaminants such as H20 and HF. 

We examined the representative esters, y-butyrolactone (BL), methyl formate (MF), and methyl acetate (MA). Figures 16 and 17 show FTIR spectra measured (ex situ) from noble metal electrodes polarized to low potentials in LiC104 solutions of BL and MF, respectively [30,39], As shown in these figures, at the onset reduction potential of around 1.3-1.2 V (Li/Li+), stable surface films precipitate on the electrode surfaces. Table 1 shows the spectral analysis for the surface films formed on noble metals at low potentials in BL. The conclusion drawn from the spectroscopic study is that the major surface compound formed is the dilithiated cyclic P-keto ester, which is similar to the electrolysis product of BL in TAA salt solutions (Scheme 2). 

Flence, aged surface films formed on nonactive electrodes at low potentials in alkyl carbonate solutions of these two salts contain LiF and other salt reduction products of the Li PF, Li BFy,... 

Most of the commonly used salts in nonaqueous systems comprise anions that are reactive and may be reduced at noble metal electrodes at low potentials. In the presence of cations such as Li+, salt anion reduction may precipitate insoluble surface species on the electrodes and thus become the dominant surface film forming process. The criteria chosen here for the reactivity of the various salt anions used are the onset potential of their reduction on noble metal electrodes and to what extent their reduction on the electrodes dominates the surface film chemistry. In this respect, the commonly used salt anions can be divided into three... 

It should be emphasized that all possible surface films formed on electrodes in Li salt solutions of polar aprotic solvents are permeable to water because all the above-described surface species are hygroscopic. Thus, water hydrates any surface species formed in these systems, diffuses to the metal surface, and may be reduced close to the electrode surface film interface at low potentials. Consequently, despite the apparent passivation of nonactive metal electrodes polarized... 

The previous section dealt with spectroscopic identification of surface films formed on nonactive metal electrodes polarized to low potentials in a variety of important polar aprotic systems and with the related voltammetric behavior. The... 

This is the case for magnesium and calcium electrodes whose cations are bivalent. The surface films formed on such metals in a wide variety of polar aprotic systems cannot transport the bivalent cations. Such electrodes are blocked for the metal deposition [28-30], However, anodic processes may occur via the breakdown and repair mechanism. Due to the positive electric field, which is the driving force for the anodic processes, the film may be broken and cracked, allowing metal dissolution. Continuous metal dissolution creates an unstable situation in the metal-film and metal-solution interfaces and prevents the formation of stable passivating films. Thus, once the surface films are broken and a continuous electrical field is applied, continuous metal dissolution may take place at a relatively low overpotential (compared with the high overpotential required for the initial breakdown of the surface films). Typical examples are calcium dissolution processes in several polar aprotic systems [31]. 

We can assume that as the surface films formed on active metals in solutions reach a certain thickness, they become electronic insulators. Hence, any possible electrical conductance can be due to ionic migration through the films under the... 

For a classical SEI electrode such as lithium, the surface films formed on it in most of the commonly used polar aprotic systems conduct Li ions, with a transference number (t+) close to unity. As stated earlier the surface films on active metals are reduction products of atmospheric and solution species by the active metal. Hence, these layers comprise ionic species that are inorganic and/or organic salts of the active metal. Conducting mechanisms in solid state ionics have been dealt with thoroughly in the past [36-44], Conductance in solid ionics is based on defects in the medium s lattice. Figure 8 illustrates the two common defects in ionic lattices interstitial (Frenkel-type) defects [37] and hole (Schottky-type) defects [38],... 

In the former case, the ions migrate among the interstitial defects, which may be relevant only to small ions such as Li+. This leads to a transference number close to 1 for the cation migration. In the other case, the lattice contains both anionic and cationic holes, and the ions migrate from hole to hole [39], The dominant type of defects in a lattice depends, of course, on its chemical structure as well as its formation pattern [40-43], In any event, it is possible that both types of holes exist simultaneously and contribute to conductance. It should be emphasized that this description is relevant to single crystals. Surface films formed on active metals are much more complicated and may be of a mosaic and multilayer structure. Hence, ion transport along the grain boundaries between different phases in the surface films may also contribute to conductance in these systems. 

Intensive work was carried out in the seventies by Dey et al. [69-71], Koch et al. [72-74], Jansta et al. [75,76], and Thevenin et al. [77-80] on the chemical analysis of the surface films formed on Li electrodes in solutions. These efforts, however, were based on XPS, AES, and some indirect methods. 

Judicious application of micropolarization techniques [81], as well as ellipsometry [82] and XRD [83], enabled analysis of the multilayer structure of the surface films formed on lithium in solutions. 

Introduction of FTIR spectroscopy for the analysis of the surface films formed on Li in electrolyte solutions by Yeager et al. [84,85], and further intensive use of this technique by Aurbach et al. [86,87], enabled more precise identification of the chemical composition of the surface films formed on lithium electrodes in a large variety of electrolyte solutions. 

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