Lithium Covered by Native Film

Among many polar aprotic solvents, including ethers, BL, PC, and ethylene carbonate (EC), methyl formate (MF) seems to be the most reactive toward hthium. It is reduced to lithium formate as a major product, which precipitates on the 

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The order of the interfacial resistance of the SEI on lithium covered by native film in 1 rnolL LiX/PC solutions was determined by Aurbach and Zaban [18, 23] from Nyquist plots. Eor the different salts, the order of I sei was LIPFg LiBp4 > LiSOjCFj LiAsP > LiNfSOj CFjIj > LiBr, LiCl04 [18]. The values for LiPFg/PC and LiN(S02CF3)2/PC were about 800 and 23 cm, respectively. The resistivity of the film was found to be directly proportional to the salt concentration, and the presence of CO2 in solutions considerably reduced the interfacial resistance. 

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],... 

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],... 

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. 

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