Lithium surface

Fukunishi Y and Nakatsu] H 1992 Modifications for ab initio calculations of the moderately large-embedded-cluster model. Hydrogen adsorption on a lithium surface J. Chem. Phys. 97 6535-43... 

Secondary lithium-metal batteries which have a lithium-metal anode are attractive because their energy density is theoretically higher than that of lithium-ion batteries. Lithium-molybdenum disulfide batteries were the world s first secondary cylindrical lithium—metal batteries. However, the batteries were recalled in 1989 because of an overheating defect. Lithium-manganese dioxide batteries are the only secondary cylindrical lithium—metal batteries which are manufactured at present. Lithium-vanadium oxide batteries are being researched and developed. Furthermore, electrolytes, electrolyte additives and lithium surface treatments are being studied to improve safety and recharge-ability. 

Films on lithium play an important part in secondary lithium metal batteries. Electrolytes, electrolyte additives, and lithium surface treatments modify the lithium surface and change the morphology of the lithium and its current efficiency [93],... 

The addition of some metal ions, such as Mg2+,Zn2+, In3+,orGa3+, and some organic additives, such as 2-thiophene, 2-methylfuran, or benzene, to propylene carbonate-LiC104 improved the coulombic efficiency for lithium cycling [112]. Lithium deposition on a lithium surface covered with a chemically stable, thin and tight layer which was formed by the addition of HF to electrolyte can suppress the lithium dendrite formation in secondary lithium batteries [113]. 

Lithium is consumed by reaction with the electrolyte which forms a protective film [6]. During the deposition and stripping of lithium, the surface shape changes and a fresh lithium surface is formed, with a new protection film on it lithium is consumed in the process. 

We believe that (3) is the main reason for the low cycling efficiency. The thermal stability of lithium-metal cells decreases with cycling [30] and the dead lithium may be the cause of this reduction. This indicates that the cycling efficiency is strongly affected by the morphology of the lithium surface. 

The protective film is broken in certain places on the lithium surface by the stress. Fiber-like lithium grows, like an extrusion of lithium, through these broken holes in the film. If the deposition current is small enough and the stress is therefore small, the protective film will probably not break. In this case, the deposited lithium may be particle-like or amorphous. 

Carbon dioxide has been proposed as an additive to improve the performance of lithium batteries [60]. Aurbach et al. [61] studied the film formed on lithium in electrolytes saturated with C02, and using in situ FTIR found that Li2C03 is a major surface species. This means that the formation of a stable Li2C03 film on the lithium surface may improve cyclability [62], Osaka and co-workers [63] also studied the dependence of the lithium efficiency on the plating substrate in LiC104-PC. The addition of C02 resulted in an increase in the efficiency when the substrate was Ni or Ti, but no effect was observed with Ag or Cu substrates. 

It is generally considered that the lithium surface film is produced by a reaction between the lithium and the electrolyte materials. However, by XPS we have detected vanadium on a lithium anode surface cy-... 

Little work has been done on bare lithium metal that is well defined and free of surface film [15-24], Odziemkowski and Irish [15] showed that for carefully purified LiAsF6 tetrahydrofuran (THF) and 2-methyltetrahydrofuran 2Me-THF electrolytes the exchange-current density and corrosion potential on the lithium surface immediately after cutting in situ, are primarily determined by two reactions anodic dissolution of lithium, and cathodic reduc-... 

According to the depth profile of lithium passivated in LiAsF6 / dimethoxyethane (DME), the SEI has a bilayer structure containing lithium methoxide, LiOH, Li20, and LiF [21]. The oxide-hydroxide layer is close to the lithium surface and there are solvent-reduction species in the outer part of the film. The thickness of the surface film formed on lithium freshly immersed in LiAsF /DME solutions is of the order of 100 A. 

Among many polar aprotic solvents, including ethers, BL, PC, and ethylene carbonate (EC), methyl formate (MF) seems to be the most reactive towards lithium. It is reduced to lithium formate as a major product which precipitates on the lithium surface and passivates it [24], The presence of trace amounts of the two expected contaminants, water and methanol, in MF solutions does not affect the surface chemistry. C02 in MF causes the formation of a passive film containing both lithium formate and lithium carbonate. 

The structure and composition of the lithium surface layers in carbonate-based electrolytes have been studied extensively by many investigators [19-37], High reactivity of propylene carbonate (PC) to the bare lithium metal is expected, since its reduction on an ideal polarizable electrode takes place at much more positive potentials compared with THF and 2Me-THF [18]. Thevenin and Muller [29] found that the surface layer in LiC104/PC electrolyte is a mixture of solid Li2C03 and a... 

For electrode reactions at corroding electrodes the purity requirements are even more stringent a water content of 2x10 2 ppm suffices to produce a monolayer of LiOH on a lithium surface of 1 cm in contact with 1 cm electrolyte [1], However, despite good purification procedures [84-86], equipment, and purity control, even recent publications are based on materials used as received without (at least) purity control. As a consequence, results disagree among various authors. 

However, even if electrolytes have sufficiently large voltage windows, their components may not be stable (at least ki-netically) with lithium metal for example, acetonitrile shows very large voltage windows with various salts, but is polymerized at deposited lithium if this reaction is not suppressed by additives, such as S02 which forms a protective ionically conductive layer on the lithium surface. Nonetheless, electrochemical stability ranges from CV experiments may be used to choose useful electrolytes. 

In contrast to DEC/LiC104 solutions, where typical reaction products of carbonate solvents including alkyl carbonates, alkoxides, and Li2C03 are formed at the lithium surface, DEC/LiPF6 solutions yield LiF and Li20 only [168]. 

Scheme 1. Reduction of PC on a Lithium Surface One-Electron Process...

Almost immediately after lithium metal was found to be stable in nonaqueous electrolytes, researchers suggested that the passivation of the lithium surface by electrolytes is the origin of this unexpected stability, because the reduction potentials of these organic solvents are far above that of lithium. - Peled was the first author to formally introduce the concept of a protective interface between lithium and elec-... 

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