Electrode passivating layer

Corrosion of the positive grid [Eq. (28)1 occurs equivalent to about 1 mA/lOOAh at open-circuit voltage and intact passivation layer. It depends on electrode potential, and is at minimum about 40-80mV above the PbS04/Pb02 equilibrium potential. The corrosion rate depends furthermore to some extent on alloy composition and is increased with high anti-monial alloys,... 

In acidic electrolytes only lead, because it forms passive layers on the active surfaces, has proven sufficiently chemically stable to produce durable storage batteries. In contrast, in alkaline medium there are several substances basically suitable as electrode materials nickel hydroxide, silver oxide, and manganese dioxide as positive active materials may be combined with zinc, cadmium, iron, or metal hydrides. In each case potassium hydroxide is the electrolyte, at a concentration — depending on battery systems and application — in the range of 1.15 - 1,45 gem"3. Several elec-... 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

In the first papers dealing with SEI electrodes it was suggested that the passivating layer consists of one or two layers [1, 2], The first one (the SEI) is thin and compact the second (if it exists), on top of the SEI, is a more porous, or structurally open, layer that suppresses the mass transport of ions in the electrolyte filling the pores of this layer. 

Upon an increase of the anodic reverse potential finally up to 8 V versus Li the cyclic voltammogran corresponding to Fig. 9 remains unchanged, showing that the passivating layer at the electrode also protects the solvents (PC and DME) from being oxidized. Subsequent deposition and dissolution of lithium at the passivated electrodes remains possible when the electrode is passivated but the cycling efficiency decreases. 

The TFTs are made on transparent glass substrates, onto which gate electrodes are patterned. Typically, the gate electrode is made of chromium. This substrate is introduced in a PECVD reactor, in which silane and ammonia are used for plasma deposition of SiN as the gate material. After subsequent deposition of the a-Si H active layer and the heavily doped n-type a-Si H for the contacts, the devices are taken out of the reactor. Cr contacts are evaporated on top of the structure. The transistor channel is then defined by etching away the top metal and n-type a-Si H. Special care must be taken in that the etchant used for the n-type a-Si H also etches the intrinsic a-Si H. Finally the top passivation SiN, is deposited in a separate run. This passivation layer is needed to protect the TFT during additional processing steps. 

There is a number of essentially non-conducting metal oxides acting as passive layers on electrodes the best known example is A1203. Metals that... 

Mossbauer spectroscopy can be used for in situ study of electrodes containing nuclei capable of resonance absorption of y radiation for practical systems, primarily the 57Fe isotope is used (passivation layers on iron electrodes, adsorbed iron complexes, etc.). It yields valuable information on the electron density on the iron atom, on the composition and symmetry of the coordination sphere around the iron atom and on its oxidation state. 

XPS investigations of the composition of the anodically grown passive layer on Ti electrodes were performed by Armstrong and Quinn [123, 124], The formation of a suboxide layer between the underlying Ti metal substrate and the stoichiometric Ti02 on top was demonstrated using XPS, AES and electrochemical techniques. 

Figure 5. 7Li NMR spectra recorded during 3 cycles of reversible lithium insertion-deinsertion in the composite carbon electrode. The peak at 0ppm is due to ionic lithium (Li+PF6 and passivation layer). The peak of lithium at 263 ppm is not shown.

For example, the investigations of the current-generating mechanism for the polyaniline (PANI) electrode have shown that at least within the main range of potential AEn the "capacitor" model of ion electrosorption/ desorption in well conducting emeraldine salt phase is more preferable. Nevertheless, the possibilities of redox processes at the limits and beyond this range of potentials AEn should be taken into account. At the same time, these processes can lead to the fast formation of thin insulation passive layers of new poorly conducting phases (leucoemeraldine salt, leucoemeraldine base, etc.) near the current collector (Figure 7). The formation of such phases even in small amounts rapidly inhibits and discontinues the electrochemical process. 

At the end of these measurements, the electrode was polarized by sweeping the potential to -1.2 V, yielding a six-line spectrum corresponding to metallic iron with some contribution from Fe(0H)2 (curve c, Fig. 5). The potential was then scanned up to -0.3 V and a spectrum essentially identical to that recorded at -1.2 V was observed. This result clearly indicates that the iron metal particles formed by the electrochemical reduction are large enough for the contributions arising from the passivation layer to be too small to be clearly resolved. After scanning the potential several... 

The measurement of potentials in electrolytes is not as easy as it is for solid-state devices. Depending on the composition of the electrolyte and the electrode material a monolayer of adsorbates or a thin passivation layer may be formed on the electrode, and can significantly shift the electrode potential. These effects have to be taken into account for the working as well as for the counter electrode. The potential at the latter becomes irrelevant if a reference electrode is used. The reference electrode should be placed as close as possible to the Si electrode or it can access the Si electrode via a capillary. The size of the reference electrode is not rel-... 

An electric field in the semiconductor may also produce passivation, as depicted in Fig. 6.1c. In semiconductors the concentration of free charge carriers is smaller by orders of magnitude than in metals. This permits the existence of extended space charges. The concept of pore formation due to an SCR as a passivating layer is supported by the fact that n-type, as well as p-type, silicon electrodes are under depletion in the pore formation regime [Ro3]. In addition a correlation between SCR width and pore density in the macroporous and the mesoporous regime is observed, as shown in Fig. 6.10 [Thl, Th2, Zh3, Le8]. 

Figure 14 Cyclic voltammetric responses for a reversible one-electron process expected at (a) a perfectly clean electrode surface (b) an electrode surface covered by a passivating layer...

The thickness of the passive layer formed on Zn electrode in alkaline solutions changes with operating conditions and reaches about 50 nm [244, 251]. Recently, it was found that the rate of oxide growth in the passive region increases with decreasing concentration of borate and increases with the imposed current density [252]. 

Zinc dissolution was also investigated in phosphate solutions over a wide pH range of 4.5-11.7 [258], in aerated neutral perchlorate [259], and in sulfate solutions [260, 261]. In the phosphate solutions [258], zinc phosphates were present in a passive layer of zinc electrode, while for sulfate solutions a kinetic model of spontaneous zinc passivation was proposed [261]. 

The electrochemical behavior of the cadmium electrodes in alkaline solutions was intensively studied [313-318]. It was suggested [314-318] that during anodic dissolution of the Cd electrode in alkaline solutions, a passive layer consisting of Cd(OH)2 and CdO is formed, and Cd(II) soluble species are also generated. The composition of the anodically formed layer on cadmium in alkaline solution was dependent on the electrolyte cation [319]. In 1 M NaOH and KOH solutions, both / -Cd(OH)2 and y-Cd(OH)2 were formed, while in 1 M LiOH, /J-Cd(OH)2 was the only product. 

The passivating layer formed on Cd electrode in alkaline solution in the presence of Na2S was studied voltammetri-cally [323]. At low Na2S concentrations, CdO, Cd(OH)2, and CdS layer were produced during the anodic oxidation of the Cd electrode. At higher Na2 S content, a few monolayers of thick CdS film were formed. 

An important feature of the positive electrode discharge concerns the nature of the PbS04 deposit since the formation of dense, coherent layers can lead to rapid electrode passivation. Lead dioxide exists in two crystalline forms, rhombic (a-) and tetragonal (/3-), both of which are present in freshly formed electrode structures. Since PbS04 and a-Pb02 are iso morphic, crystals of lead dioxide of this modification tend to become rapidly covered and isolated by lead sulphate, and their utilization is less... 

The properties of lithium metal were described in Chapter 4, where particular note was made of its high specific capacity and electrode potential. However, because of its highly electropositive nature, it is thermodynamically unstable in contact with a wide variety of reducible materials. In particular, lithium reacts with components of most electrolytes to form a passivating layer. Film formation of this type ensures long shelf life for primary lithium cells, but causes severe problems when the electrode is cycled in a secondary cell. 

Work has therefore been devoted by a number of developers to improving the cyclability of the lithium metal electrode. Since passivation of lithium is an unavoidable phenomenon, one approach has been directed to the promotion of uniform and smooth surface passivation layers, for example by selecting the most appropriate combination of solvents and electrolyte salts. An example is the inclusion of 2-methyltetrahydrofuran (2-Me-THF), since the presence of the methyl group slows down the reactivity towards the lithium metal. The selection of fluorine-based elec-... 

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