Surface of Lithium Coupled With Electrolytes

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], 

Lithium metal is chemically very active and reacts thermodynamically with any organic electrolyte. However, in practice, lithium metal can be dissolved and deposited electrochemically in some organic electrolytes [5]. It is generally believed that a protective film is formed on the lithium anode which prevents further reaction [6, 7]. This film strongly affects the lithium cycling efficiency. 

According to the solid electrolyte interphase (SEI) model presented by Peled [8], the reaction products of the lithium and the 

2MeTHF (119 Qcm2) or PC/2MeTHF (214 Qcm2). The cycle life increases with decreases in heat output and resistivity. They indicate that these measurements are effective in determining electrolyte stability. 

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Many studies have been undertaken with a view to improving lithium anode performance to obtain a practical cell. This section will describe recent progress in the study of lithium-metal anodes and the cells. Sections 3.2 to 3.7 describe studies on the surface of uncycled lithium and of lithium coupled with electrolytes, methods for measuring the cycling efficiency of lithium, the morphology of deposited lithium, the mechanism of lithium deposition and dissolution, the amount of dead lithium, the improvement of cycling efficiency, and alternatives to the lithium-metal anode. Section 3.8 describes the safety of rechargeable lithium-metal cells. 

XRR has been applied to the study of EEIs on several systems [201-205]. The technique was found to be sensitive not only to the formation of reaction layers but also to mass loss at the electrode surface due to processes of corrosion (dissolution) [201]. Of particular interest is the application of high energy synchrotron beams as sources, as their deep penetration capabilities enables the design of operando cells (Fig. 7.10a) [203], Therefore, uncertainty due to equilibration in the absence of an electrochemical potential is eliminated. The structural and chemical stability of EEIs during the lithium insertion/extraction processes have thus been evaluated (Fig. 7.10b) [201-204]. The dependence of these irreversible reactions on the crystal facet of the electrode material forming the EEI was established. It was found that electrolyte decomposition processes were coupled with the redox process occurring in the bulk of the electrode, which is a critical piece of information when designing materials that bypass such layer formation. 

The surface overpotential, O,is the deviation from the thermodynamic potential difference between the sohd and solution at the existing surface concentrations. U is the open-circuit potential of the sohd material evaluated at the surface concentration of the sohd with respect to a hypothetical lithium reference electrode in soluhonjust outside the diffuse part of the double layer, at the same local electrolyte concentrahon, and is a function of sohd concentration in insertion electrodes. Thus, U must be specified as a function of intercalant concentration but not as a function of electrolyte concentration. This equation is coupled to Equahons 3 and 4 for potenhal in the sohd and electrolyte, and sets the surface overpotential with respect to the local potential in solution and in the sohd required to force the reaction. 

First, we should consider the role of the tetraalkylammonium ion in the hydrodimerization reaction. Certainly the ions are essential to the process in their absence (but with for example a lithium or sodium salt as the electrolyte) the reduction of acrylonitrile leads only to propionitrile and a critical concentration of R4N is necessary to obtain a good yield of adiponitrile. This critical concentration decreases along the series (CH3)4N > (C2H5)4N > (C4H9)4N and with the latter can be as low at 0.01%. It may also be noted that the presence of these ions suppresses hydrogen evolution and there is evidence that they adsorb on the cathode surface. This led to the proposal that the adsorption of the tetraalkylammonium ion produces a layer at the electrode surface which is relatively aprotic. In this layer anionic coupling can occur because protonation is slow compared with that in the bulk solution. 

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