Morphology 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. 

Figure 2. A possible mechanism for lithium deposition (left) based on dissolution (right) lithium morphology observations in LiAsF6 - EC/2MeTHF electrolyte.

A possible mechanism for lithium deposition based on our observations of lithium morphology in LiAsFb-EC/ 2MeTHF electrolyte is described below [31], Figure 2 (left-hand panel) shows our image of the mechanism. 

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]. 

The lithium morphology at the beginning of the deposition was measured by in-situ atomic force microscopy (AFM) [42], When lithium was deposited at 0.6 C cm2, small particles 200-1000 nm in size were deposited on the thin lines and grain boundaries in LiC104-PC. Lump-like growth was observed in LiAsF6-PC along the line. 

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. ...

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.

In 2003, Nohira et al. (2003) demonstrated the electrochemical reduction of silica by calcium in a molten salt bath at 850 C. This process could generate porous silicon deposits. Lithium is another potential reductant, as shown by Yasuda et al. (2005) in 2005. They were able to use a molten salt temperature of only 500 C and the silicon product had a spongy morphology with a particle size below 50 nm. 

One of the most important factors determining whether or not secondary lithium metal batteries become commercially viable is battery safety, which is affected by many factors insufficient information is available about safety of practical secondary lithium metal batteries [88]. Vanadium compounds dissolve electrochemically and are deposited on the lithium anode during charge-discharge cycle. The low reactivity of the vanadium-deposited lithium anode has been observed by calorimetry a chemical-state analysis and morphological investigation of the lithium anode suggest that the improvement in stability is primarily due to a passivation film [89]. 

There have been many reports on the morphology of the lithium that is electrochemically deposited in various kinds of organic electrolyte [32-39]. 

Figure 1. Morphology of lithium deposited on stainless steel, 3 mAcm 2, 3 mAh cm 2, 1.5 molL 1 LiAsF6 - EC/2MeTHF (1 1), v/v.

Figure 1 shows a typical lithium deposition morphology. Here, the lithium is deposited on stainless steel at 3 mA cm 2 for 1 h with 1.5 mol L1 LiAsF6 -EC/2MeTHF (1 1, v/v). 

Koshina et al. have reported that there are three kinds of morphology [40] dendritic, granular and mossy. Mossy lithium is formed when the deposition current is small and the salt concentration is high. This mossy lithium provides a high cycling efficiency. 

Sulfur is known to be easily reducible in nonaqueous solvents and its reduction products exist at various levels of reduction of polysulfide radical anions (S . ) and dianions (Sm2 ) 173], Recently Be-senhard and co-workers [74] have examined the effect of the addition of polysulfide to LiC104-PC. Lithium is cycled on an Ni substrate with Qc=2.7 C cm 2 and cycling currents of 1 mA cm. The cycling efficiency in PC with polysulfide is higher than that without an additive. The lithium deposition morphology is compact and smooth in PC with added polysulfide, whereas it is dendritic in PC alone. 

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