Dendrites formation

In a constitutionally imdercooled melt, the primary dendrites will be rich in the first-to-freeze composition and the interdendritic fluid will be rich in the last-to-freeze composition. The microstructure of the resulting solid will then have regions of the first-to-freeze composition interspersed with the last-to-freeze composition and the microstructure of the resulting structure will be determined by the spacing of the dendrite arms. For this reason, much theoretical and experimental work has been done to understand the morphology of the dendrites, their growth rate, and their arm spacing as a function of their thermal and solutal environment. 

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Solid Electrolyte Systems. Whereas there has been considerable research into the development of soHd electrolyte batteries (18—21), development of practical batteries has been slow because of problems relating to the low conductivity of the soHd electrolyte. The development of an all sohd-state battery would offer significant advantages. Such a battery would overcome problems of electrolyte leakage, dendrite formation, and corrosion that can be encountered with Hquid electrolytes. 

Coin and Button Cell Commercial Systems. Initial commercialization of rechargeable lithium technology has been through the introduction of coin or button cells. The eadiest of these systems was the Li—C system commercialized by Matsushita Electric Industries (MEI) in 1985 (26,27). The negative electrode consists of a lithium alloy and the positive electrode consists of activated carbon [7440-44-0J, carbon black, and binder. The discharge curve is not flat, but rather slopes from about 3 V to 1.5 V in a manner similar to a capacitor. Use of lithium alloy circumvents problems with cycle life, dendrite formation, and safety. However, the system suffers from generally low energy density. 

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]. 

A remedy could be achieved by a decrease in the zinc solubility in the electrolyte or by suppression of dendrite formation cadmium-, lead-, or bismuth oxide,... 

A third type of problem, that is often mistakenly confused with dendrite formation, is due to the presence of a reaction-product layer upon the growth interface if the electrode and electrolyte are not stable in the presence of each other. This leads to filamentary or hairy growth, and the deposit often appears to have a spongy character. During a subsequent discharge step the filaments often become disconnected from the underlying metal, so that they cannot participate in the electrochemical reaction, and the rechargeable capacity of the electrode is reduced. 

The simulated short-circuit test was developed to characterize the response of the separator to a short circuit without the complications of battery electrodes. The separator was spirally wound between lithium foils and placed in an AA-size can. To avoid lithium dendrite formation, an alternating voltage was applied to the cell. The cell current and can temperature were monitored. Figure 6 shows the behavior of Celgard membranes. 

Wang et al. [96] constructed a Na/S battery with a sodium metal anode, liquid electrolyte, and a sulfur (dispersed in polyacrylonitrile) composite cathode and tested its electrochemical characteristics at room temperature. The charge/discharge curves indicated that sodium could reversibly react with the composite cathode at room temperature. Average charge and discharge voltage was 1.8 and 1.4 V, respectively. Similar to lithium batteries, dendrite formation was noted as a critical problem for these cells. 

It must be emphasized here that the study of dendrite formation has important practical implications. In energy-storage devices (batteries), dendrites often rupture the membranous separators and go over to touch the other, electron-sink electrode (anode), which leads to a disastrous short circuit of the cell. In substance producers... 

In lithium rechargeable batteries carbon materials are used that function as a lithium reservoir at the negative electrode. Reversible intercalation, or insertion, of lithium into the carbon host lattice avoids the problem of lithium dendrite formation and provides a large improvement in terms of cycleability and safety (111). 

The phase-field simulations reproduce a wide range of microstructural phenomena such as dendrite formation in supercooled fixed-stoichiometry systems [10], dendrite formation and segregation patterns in constitutionally supercooled alloy systems [11], elastic interactions between precipitates [12], and polycrystalline solidification, impingement, and grain growth [6]. 

The effect of the stack pressure on the morphology of Li electrodes was studied by Wilkinson et al. [106], This issue is highly important, as applying stack pressure on Li electrodes considerably suppresses dendrite formation during Li deposition. 

A great deal of effort was dedicated to the study of Li electrodes in polymeric electrolyte systems [112-115], These can serve as alternatives for the liquid electrolyte solutions in which dendrite formation is such a severe problem. 

It should be noted that in practical batteries such as coin cell (parallel plate configuration) or AA, C, and D (jelly-roll configuration), there is a stack pressure on the electrodes (the Li anodes are pressed by the separator), and the ratio between the solution volume and the electrode s area is usually much lower than in laboratory testing. Both factors may considerably increase the Li cycling efficiency obtained in practical cells, compared with values measured for the same electrolyte solutions in the Li half-cell testing described above. It has already been proven that stack pressure suppresses Li dendrite formation and thus improves the uniformity of Li deposition-dissolution processes [107], The low ratio between the solution volume and the electrode area in practical batteries decreases the detrimental effects of contaminants such as Lewis acids, water, etc., on Li passivation. 

It should be emphasized that Li dendrite formation is not prevented by surface Li2C03 formation in C02-containing solutions. However, once the dendrites are formed, they become highly passivated, due to the Li2C03. Thus, the corrosion of dendrites in these solutions, which is a major cause for low Li cycling efficiency, is largely prevented. 

It is assumed that a solid state electrolyte system should suppress dendrite formation upon Li deposition. This is also supposed to contribute to enhanced safety and prolonged cycle life. 

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