Native surface film

The formation of a native surface film on lithium is unavoidable. It arises from storage and laboratory handling of the lithium metal in the gaseous atmosphere of the dry... 

Figure 1 A schematic view of replacement of native surface films on active metals by new ones in solutions. It should be noted that in reality the borders between the layers which comprise the surface films are not distinctive, and their microstructure is mosaiclike. This, and the subsequent figures, are attempts to emphasize the fact that the composition and structure of the surface films change as a function of their distance from the active metal.

As discussed in Ref. 84, Li/Hg amalgam cannot be a model system for solid Li surfaces, because reduction of solution species on the liquid Li/Hg interface does not produce stable surface films. Thus, a massive solvent reduction may occur on Li/Hg in which each solvent molecule reacts directly with the bare active surface. In such a situation, PC and EC are indeed reduced directly to Li2C03 [84,131], However, R0C02Li species are major reduction products of PC and EC on Li/Hg as well. It should be noted that when the Li is initially covered by native surface films (Li20, Li2C03), the situation is more complicated. Only part of the native surface films may be replaced upon storage in the solutions thus, in such a case the nature of the surface films remains more inorganic than in the case of fresh Li surfaces [101-105], In any event, upon Li deposition or dissolution, the replacement of the native surface films by solution reduction products is fast and pronounced, and the above-described surface chemistry is very relevant to practical Li anodes in batteries. 

Li is always covered by native surface films that are replaced in solutions by layers originating from reduction of solution species. These replacement processes may have prolonged time constants and are accompanied by secondary reactions between surface species and solution components. Hence, it may be difficult to measure these systems at a true steady state condition. 

In conclusion, there are still many unanswered questions regarding Mg deposition-dissolution processes in these systems that call for further work in this area. It is not at all clear whether Mg deposition in Grignard/ether systems occurs under surface film free conditions or whether there are mechanisms of ion transport (e.g., RMg+ or Mg2+) through surface films on Mg electrodes. It is not clear whether the native surface films on Mg electrodes dissolve in the Grignard solutions, allowing the interaction of a fresh Mg surface with the solution species (which may explain the high reversibility of the Mg electrode), or whether, even in this case, Mg dissolution occurs via the breakdown and repair of surface films (due to reaction of the active metal with solution species). 

The native surface films can be composed of CaO, Ca(OH)2 and CaC03. These films are replaced by solvent and salt anion reduction products during storage of Ca electrodes in solutions. 

Fig. 14. Schematic illustration for native surface film on lithium foil.

Figure 7.2 XPS spectra for copper film exposed to 1 vol% NH OH slurry for 10 minutes after polishing to remove native surface films.

SEI 2-5nm thick. When lithium is cut while immersed in the electrolyte, the SEI forms almost instantaneously (in less than 1ms [15,16]). On continuous plating of lithium through the SEI during battery charge, some electrolyte is consumed in each charge cycle in a break-and-repair process of the SEI [1,2] and this results in a faradaic efficiency lower than 1. When a battery is made with commercial lithium foil, the foil is covered with a native surface film. The composition of this surface film depends on the environment to which the lithium is exposed. It consists of Li20, LiOH, Li2C03, U3N, and other impurities. When this type of lithium is immersed in the electrolyte, the native surface film may react with the solvent, salts, and impurities to form an SEI, whose composition may differ from that of elec-trodeposited lithium in the same electrolyte. The formation of SEI on carbonaceous anodes is discussed in Sec. 6.3. 

Highly interesting, and also complicated, is the surface chemistry of reactive metals in nonaqueous solutions. When active metals (e.g., Li, Mg, Ca), which are always covered by native surface films, are introduced into nonaqueous, polar aprotic solutions, a large variety of surface reactions takes place, which form highly complicated, multilayer and laterally non-uniform surface films. Active metals seem to be stable in a large variety of nonaqueous solutions because of their passivation by these complicated surface films. In the next sections of this chapter, the surface films on active metals and related phenomena are rigorously dealt with in detail. 

Active metals (Li, Mg, Ca, etc.) react spontaneously with the main atmospheric gases (N2, O2, H2O, CO2) and with most relevant polar aprotic solvents and salt anions. All active metals are covered initially by native surface films formed during their production by their reaction with atmospheric gases. It should be noted that even a usual glove box atmosphere that officially contains less than 1 ppm of H2O and O2 (but may contain hundreds of ppm of CO2 and N2) should be considered as reactive towards lithium or magnesium surfaces prepared freshly in the glove box. Active metals are usually covered by bilayer surface films. The inner layer is comprised of metal oxide, while the outer layer contains mostly carbonates and hydroxides. When an active metal is introduced into a polar aprotic electrolyte solution, several processes take place in parallel. These include dissolution of part of the initial surface species, nucleophilic reactions between metal oxide and hydroxide and electrophilic solvents such as esters and alkyl carbonates, and diffusion of solvent molecules towards the active metal-native film interference and their reduction by the active metal. 

Figure 12 shows families of impedance spectra (Nyquist plots) of two Mg electrodes in Grignard salt/TFlF solutions, one of which was initially covered by native films (MgO-MgCOs), and the other was prepared freshly in solution. The electrode covered by the native surface films has an initially high impedance, which decreases upon storage, while the impedance of the freshly prepared... 

Active metals are always covered by native surface films formed by reactions between the metal and atmospheric components. The native surface films on active metals usually have a bilayer stmcture. The inner layer comprises... 

An active metal covered by native surface films is introduced into the solution. 

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