Morphology of Deposited Lithium

The reason why lithium cycling efficiency is not 100 percent are generally considered to be as follows  

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. 

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

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. 

1) Lithium is deposited on a lithium anode under the protective fihn without serious damage to the film. 

2) The deposition points on the lithium electrode are the points at which the protective fihn has a higher hthium-ion conductivity. One example of these 

3) As hthium does not deposit uniformly for the reason mentioned above, mechanical stress is created in the lithium electrode under the protective film. 

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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. 

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.

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]. 

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.

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). 

Electrochemical deposition of lithium usually forms a fresh Li surface which is exposed to the solution phase. The newly formed surface reacts immediately with the solution species and thus becomes covered by surface films composed of reduction products of solution species. In any event, the surface films that cover these electrodes have a multilayer structure [49], resulting from a delicate balance among several types of possible reduction processes of solution species, dissolution-deposition cycles of surface species, and secondary reactions between surface species and solution components, as explained above. Consequently, the microscopic surface film structure may be mosaiclike, containing different regions of surface species. The structure and composition of these surface films determine the morphology of Li dissolution-deposition processes and, thus, the performance of Li electrodes as battery anodes. Due to the mosaic structure of the surface... 

There are reports that the surface chemistry of Li alloys is indeed largely modified, compared with Li metal electrodes [303], It appears that they are less reactive with solution species, as is expected. The morphology of Li deposition on Li alloys may also be largely modified and smooth, compared with Li deposition on Li substrates [302,304], A critical point in the use of Li alloys as battery anodes is the lithium diffusion rates into the alloys. Typical values of Li diffusion coefficient into alloys are 3-LiAl —> 7 16 9 cm2/s [305], Li44Sn —> 2 10 9 cm2/s [306], LiCd and LiZn —> 1010 cm2/s [307], It should be emphasized that it is very difficult to obtain reliable values of Li diffusion coefficient into Li alloys, and thus the above values provide only a rough approximation for diffusion rates of Li into alloys. However, it is clear that Li diffusion into Li alloys is a slow process, and thus is the rate-limiting process of these electrodes. Li deposition of rates above that of Li diffusion leads to the formation of a bulk metallic lithium layer on the alloy s surface which may be accompanied by mas-... 

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