Native film

Lithium foil is commercially available. Its surface is covered with a "native film" consisting of various lithium compounds [Li0H,Li20,Li3N, (Li20-C02) adduct, or Li2C03], These compounds are produced by the reaction of lithium with 02, H20, C02, or N2. These compounds can be detected by electron spectroscopy for chemical analysis (ESCA) [2], As mentioned below, the surface film is closely related to the cycling efficiency. 

Lithium foil is made by extruding a lithium ingot through a slit. A study of the influence of the extrusion atmosphere on the kind of native film produced showed that lithium covered with Li2CO, is superior both in terms of storage and discharge because of its stability and because a lithium anode has a low impedance [3, 4],... 

Tekehara and co-workers [67] tried to modify the native film of lithium by an acid-base reaction. HF, HI, H3P04, and HC1 were selected as acids, because of the... 

Li PC/UCIO4 XPS LiOH, Li2C03,Li20, (a little LiCl) (native film) no N compounds [179]... 

A1 is thermodynamically unstable, with an oxidation potential at 1.39 V. Its stability in various applications comes from the formation of a native passivation film, which is composed of AI2O3 or oxyhydroxide and hydroxide.This protective layer, with a thickness of 50 nm, not only stabilizes A1 in various nonaqueous electrolytes at high potentials but also renders the A1 surface coating-friendly by enabling excellent adhesion of the electrode materials. It has been reported that with the native film intact A1 could maintain anodic stability up to 5.0 V even in Lilm-based electrolytes. Similar stability has also been observed with A1 pretreated at 480 °C in air, which remains corrosion-free in LiC104/EC/ DME up to 4.2 However, since mechanical... 

Reactive electrodes refer mostly to metals from the alkaline (e.g., lithium, sodium) and the alkaline earth (e.g., calcium, magnesium) groups. These metals may react spontaneously with most of the nonaqueous polar solvents, salt anions containing elements in a high oxidation state (e.g., C104 , AsF6 , PF6 , SO CF ) and atmospheric components (02, C02, H20, N2). Note that ah the polar solvents have groups that may contain C—O, C—S, C—N, C—Cl, C—F, S—O, S—Cl, etc. These bonds can be attacked by active metals to form ionic species, and thus the electrode-solution reactions may produce reduction products that are more stable thermodynamically than the mother solution components. Consequently, active metals in nonaqueous systems are always covered by surface films [46], When introduced to the solutions, active metals are usually already covered by native films (formed by reactions with atmospheric species), and then these initial layers are substituted by surface species formed by the reduction of solution components [47], In most of these cases, the open circuit potentials of these metals reflect the potential of the M/MX/MZ+ half-cell, where MX refers to the metal salts/oxide/hydroxide/carbonates which comprise the surface films. The potential of this half-cell may be close to that of the M/Mz+ couple [48],... 

Alkaline, alkaline earth metals and aluminum are naturally covered with anodic films. The removal of these native films, even in the best glove box atmosphere, exposes the fresh metal to reactive atmospheric contaminants at a high enough concentration and quickly cover the metal with new surface films. As discussed above, even the glove box atmosphere of an inert gas containing atmospheric components at the ppm level should be considered as being quite reactive to active metals such as lithium. Therefore, anyone intending to study the intrinsic behavior of active metal electrodes in solution must prepare a fresh electrode surface in solution. 

There are studies in which the fact that active metal electrodes are covered with surface films is not so important, e.g., when these metals are used as counterelectrodes, or when they are studied as practical anodes in batteries. However, even in these cases, the native active metals as received may be covered with two thick films. It is therefore, necessary to remove the initial native film covering the active metal under an inert atmosphere. The passivating films of lithium and calcium can be scraped off with a stainless steel knife. In the case of harder active metals such as magnesium and aluminum, an abrasive cloth or... 

Figure 2 A schematic view of formation of multilayer surface films by secondary reactions of native films with solution species.

The Li surface preparation is very important. Immersion of Li electrodes covered by native films leads to complicated surface film replacement processes that may form a highly nonhomogeneous metal-solution interphase. In situ electrochemical surface preparation by dissolution or deposition may form very rough surfaces whose impedance spectra may be difficult to interpret properly. Hence, it seems that the most preferred way of studying the electrochemical behavior of a Li electrode in a specific solution is by using Li surfaces freshly and smoothly prepared in solutions. 

It must be appreciated, that surfaces of Nb, Ta, V and Zr will be immediately reoxidized or otherwise contaminated upon re-exposure to air, albeit with thitmer layers (or in the limit a monolayer) relative to thick native films often encountered before abrasion. As a minimum, monolayer adsorption of contaminants is almost certainly assured in all but the best ultra-high vacuum (< 1 x torr or <1 X 10 Pa) or in systems which simultaneously sputter substrate surfaces with argon or other inert ion during catalyst deposition. Some interfacial impurities between the substrate and the catalyst layer are tolerated in practice. However, the state of the substrate surface immediately prior to catalyst deposition is critical for wetting and adherence of the catalyst layers and for prevention of delamination. Theoretical flux maxima will not be achieved if thick impurity layers at the cata-lyst/substrate interface hinder hydrogen diffusion. 

When introduced to the solutions, active metals are usually already covered by native films (formed by reactions with atmospheric species), and then these initial layers are substituted by surface species formed by the reduction of solution components [47]. In most of these cases, the open circuit potentials of these metals reflect the potential of the M/MX/M half-cell, where MX refers to the metal salts/oxide/hydroxide/carbonates which comprise the surface films. The potential of this half-cell may be close to that of the M/M + couple [48]. 

It is well known that the less porous the polymer, the better is the barrier effect, and the better the metal is protected against corrosion [41,42]. Unfortunately, native conducting polymers prepared by electropolymerization of a monomer in aqueous media and deposited directly onto the metal are very porous [77]. Moreover, polymer synthesis in aqueous media leads to native films containing large amounts of water, which are detrimental [41,42]. In general, the films are dried in air, but, even after that, the water content remains high. Clearly, submitting the material to thermal treatment, in order to remove all the water inside the polymer and make it more compact, should improve the barrier effect. 

In conclusion, native films electrosynthesized in aqueous media are strongly hydrated, and are highly porous. Dehydration is essential to improve the barrier effect. 

Mn(ni)-like Q band blue shifted by 4nm compared to the native Mn(III) one. When the reductive electrolysis is stopped (curve 3), there is a complete vanishing of the band at 688 nm to the detriment of the Mn(III)-like one at 729 nm. The Q band for the native Mn(III) film is reported for comparison (dashed curve). These observations may be explained by the formation of the doubly reduced superoxo intermediate (steps 1-3) that exhibits a Q band at 688 nm, while the formulated Mn(III)-superoxide adduct exhibits a Q band at 729 nm. When molecular oxygen was added to the reduced poly[Mn(II)-32] film in a DCM solution containing benzoic anhydride, with or without stopping the reductive electrolysis, it appears that there is a total restitution of the native film within 10 min, as it can be seen in... 

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