Deposited lithium

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. 

Deposited lithium is isolated from the base anode [30, 31]. When a cell is charged, lithium is deposited on the lithium substrate of the anode. Sometimes, the plated lithium is not flat but fiber-like. When the cell is discharged, the lithium anode dissolves, and sometimes the fiber-like lithium is cut and becomes isolated from the anode substrate [31]. This isolated lithium is called "dead lithium", and it is electochemically inactive but chemically active. During cycling,... 

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. 

After the fiber-like lithium has grown, lithium is still deposited on the lithium substrate that is not at the tip of the fiber-like lithium. If the deposition continues for a long time, the lithium electrode becomes covered with long, fiber-like lithium. In this situation, lithium-ion transport in the electrolyte to the lithium electrode surface is hindered by the fiber-like lithium. Then, lithium begins to be deposited on the tip and on kinks of the fiber-like lithium, where there are crystalline defects. The morphology of the deposited lithium is particle-like or amorphous. As there are many kinks, the current density of the lithium deposition becomes very low. This low current density may create particle-like, rather than fiber-like, lithium. Thus the morphology of the lithium as a whole becomes mushroom-like [31]. 

An electrochemical quartz crystal microbalance (EQCM or QCM) can be used to estimate the surface roughness of deposited lithium [43],... 

When the discharge current is large, delocalized pits formed in the anode are shallow, so the deposited lithium whiskers can easily emerge from the pits and stack pressure can be applied to them, as mentioned in Sec.3.7.3. 

Naoi and co-workers [55], with a QCM, studied lithium deposition and dissolution processes in the presence of polymer surfactants in an attempt to obtain the uniform current distribution at the electrode surface and hence smooth surface morphology of the deposited lithium. The polymer surfactants they used were polyethyleneglycol dimethyl ether (molecular weight 446), or a copolymer of dimethylsilicone (ca. 25 wt%) and propylene oxide (ca. 75 wt%) (molecular weight 3000) in LiC104-EC/DMC (3 2, v/v). 

Figure 4. Morphology of deposited lithium on lithium, after five cycles with 1 mAcm 2,2 mAh cm 2 in 1 mol L-1 LiPF6 - PC. ...

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. 

The first electrodeposition of lithium from an ionic liquid was reported in 1985 by Lipsztajn and Osteryoung [2], They were able to deposit lithium from a 1-ethyl-3-methyl-imidazolium chloride/aluminum trichloride ionic liquid. They noticed that a neutral ionic liquid, a neutral basic ionic liquid (neutral + small excess of RC1) and a neutral acidic ionic liquid (neutral + small excess of AICI3) each have unique features. Both the basic and the neutral acidic ionic liquids show an extension of 1.5 V of the electrochemical window, making them interesting for electrochemical applications. 

Solvent/lithium interfaces are formed by sequentially depositing lithium onto the clean substrate and, following rapid AES characterization, condensing the solvent onto it, as shown schematically in Figure 5. An array of techniques, including XPS, AES, TPD and FTIR, are then used to examine their electronic, structural and vibrational properties. Additional insight into the effects induced by Li was also obtained from studies of the behavior of THF and PC condensed on bare, clean, nominally unreactive substrates such as Ag and Au. 

A solution of 2-fluoroplienyllitliium (6 mmol, freshly prepared from 1 -bromo-2-fluorobenzene and butyllithium at —78°C) in anhydrous diethyl ether was fed through a small Teflon tube into a stirred suspension of triphenylbismuth dichloride (3 mmol) in the same solvent (20 ml) cooled to —78°C. The resulting suspension was gradually warmed to room temperature under stirring. After decanting from deposited lithium chloride, the clear red-violet solution was carefully evaporated. Crystallization at 0°C afforded bis(2-fluorophenyl)triphenylbismuth (45%), which is readily soluble in ether, but insoluble in pentane and sensitive to hydrolysis [90AG(E)213]. 

A lilhium-drifted detector is formed by vapor-depositing lithium on the surface of a p-doped silicon crystal. When the crystal is heated to 4(X) C to 5(X) C the lithium diffuses into the crystal. Because lithium easily loses electrons, its presence converts the p-lype region to an n-type region. While still at an elevated temperature, a dc voltage applied across the crystal causes withdrawal of Ihe electrons from the lithium layer and holes from Ihe p-type layer. Current across thepn junction causes migration, or drifting, of lithium ions into the p layer and formation of the intrinsic layer, where the lithium ions replace the holes lost by conduction. When the crystal cools, this central layer has a high resistance relative to the other layers because the lithium ions in this medium are less mobile than the holes they displaced. 

The lithium-propylene carbonate system is of particular interest in high energy density batteries and it has been claimed " that the metal can be deposited with 100% current efficiency. However, that electro-deposited lithium displays a much less stable open-circuit potential than lithium ribbon electrodes, can be taken as evidence that the former contains impurities. Kinetic studies of the LilLi" couple in PC have been made in order to understand the nature of the considerable polarisation of that electrode, ... 

Figure 5. Morphology of deposited lithium on lithium after immersion of lithium in I molL" LiPFf,-PC with HF (3 vol. %) for three days five cycles with 1 mAh cm ini molL LiPF -PC.

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