Passivated films, liquid electrolytes

The lithium passivation in liquid electrolyte cells has been extensively investigated and its characteristics - in terms of nature and of growth rate of the passivation film, as well as its effect on the cyclability of the lithium electrode - have been well established. As a result of these studies, it is now clear that the efficiency of the lithium plating-stripping process greatly depends on the nature of the selected electrolyte solution. 

Since this is a new field, little has been published on the LiXC6 /electrolyte interface. However, there is much similarity between the SEIs on lithium and on LixC6 electrodes. The mechanism of formation of the passivation film at the interface between lithiated carbon and a liquid or polymer electrolyte was studied by AC impedance [128, 142]. Two semicircles observed in AC-impedance spectra of LiAsF6/EC-2Me-THF electrolytes at 0.8 V vs. Li/Li+ [142] were attributed to the formation of a surface film during the first charge cycle. However, in the cases of LiC104 or LiBF4 /EC-PC-DME (di-... 

Current collectors in Ni/MH batteries are usually made of nickel foam, passivated by a NiOOH layer at the positive side. Separators are usually microporous plastic films impregnated with liquid electrolyte. It is, therefore, necessary that the active masses be insoluble, or at least sparingly soluble, in order to avoid mixing of components via diffusion through the separator, thus leading to self-discharge. 

As already mentioned, salt-containing liquid solvents are typically used as electrolytes. The most prominent example is LiPF6 as a conductive salt, dissolved in a 1 1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as 1 molar solution. It should be mentioned that this electrolyte is not thermodynamically stable in contact with lithium or, for example, LiC6. Its success comes from the fact that it forms an extremely stable passivation layer on top of the electrode, the so-called solid-electrolyte interface (SEI) [35], Key properties of such SEI layers are high Li+ and very low e conductivity - that is, they act as additional electrolyte films, where the electrode potential drops to a level the liquid electrolyte can withstand [36],... 

The most diffused actuating configuration, in which these materials are used, is represented by the so-called unimorph bilayer bender. This kind of actuator consists of a film of active material coupled to a passive supporting layer. The bilayer structure is operated within an electrochemical cell, having a liquid electrolyte in which the device is immersed. The active polymeric layer of the actuator works as one electrode of the cell, while a counter electrode and a third reference electrode are separately immersed in the electrolyte. One end of the bilayer is constrained, while the other is free. The potential difference applied between the electrodes causes red-ox reactions of the conducting polymer. Since the CP and the passive layers are mechanically interlocked, when the polymer swells/shrinks the passive layer, which can not modify its dimensions, transforms the CP linear displacement into a bending movement of the structure [238-242]. Very similar is the bimorph structure. In this case the passive layer is substituted by a second CP film and they work in opposition of phase. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid electrolytes or PEs, 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 yetnot 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 Q anode/electrolyte interface in both liquid electrolyte and PE 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. 

When organic solvents are used for anodic fluorination, anode passivation (the formation of a nonconducting polymer film on the anode surface that suppresses faradaic current) takes place very often, which results in low efficiency. Moreover, depending on the substrates the use of acetonitrile can yield an acetoamidation byproduct. To prevent acetoamidation and anode passivation, Meurs and his coworkers used an EtaN 3HF ionic liquid as both a solvent and supporting electrolyte (also a fluorine source) for the anodic fluorination of benzenes, naphthalene, olefins, furan, benzofuran, and phenanthroline. They obtained corresponding partially fluorinated products in less than 50% yields (Scheme 8.2) [11]. 

Compound Semiconductors, Electrochemical Decomposition, Fig. 6 Passivating phosphazine film formed by anodizing n-InP in a liquid NH3 electrolyte under bandgap illumination (From Ref. [24])... 

The main research interests of Vas ko were related to the electrochemistry of refractory metals where he was a well-known expert. He developed the process of galvanic coating of W-Ni and Mo-Ni alloys from aqueous electrolytes. Trying to explain the mechanism of this process, he assumed two stages of the formation of such alloys first, the deposition of solid film consisting of low-valency compounds and, second, electrochemical reduction of the film at the inner surface by solid mechanism. Thus, the solid non-metal film on the electrode surface for the first time became an active participant of the electrochemical process rather than simple passivative layer. This was a breakthrough. Further on, this electrochemical film system (EFS) concept was usefully applied not only in the aqueous electrochemistry but in the electrochemistry of molten salts and ionic liquids as well (see [7] for more details). 

In another work [81] a silver thin-film pattern was covered with a polyimide protection layer with a 50 pm wide slit at the center of the pattern and the Ag AgCl layer was grown from there into the silver layer. A liquid junction was formed with a photocurable hydrophihc polymer. A silicone rubber passivation covered the entire area except for the pad and the end of the junction. The complete miniature liquid-junction reference electrode could maintain a stable level within 1 mV for time longer than 100 h with the aid of poly(vinyl pyrrolidone) matrix in the electrolyte layer. 

The polymer electrolytes discussed so far suffer from a number of disadvantages. Firstly, they exhibit low conductivities in comparison with liquid or solid (crystalline or glassy) electrolytes at or below room temperature. The best all-amorphous systems have conductivities less than 10 S cm at room temperature. These ambient temperature conductivities may be insufficient in some cases for the power required by a lithium battery. Secondly, the interfacial impedances present at both the lithium anode (passivation) and composite cathode (passivation, contact) are in addition to the ohmic losses in the electrolyte. Thirdly, the lowness of cation transference number, although similar to the values in liquid systems, is a major issue since the total conductivity is lower and could limit the use of solvent-free polymer electrolytes except in the form of extremely thin films or above room temperature. 

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