Lithium electrodes

Effect of Electrolyte Composition on SEI Properties 16.4.3.1 Lithium Electrode 

The properties of SEI electrodes, the growth rate of the SEI, the mechanism of dissolution and deposition, and the effects of various factors on SEI conductivity have been addressed elsewhere [1,2] space limitations do not permit their repetition here. 

The order of the interfacial resistance of the SEI on lithium covered by native film in 1 rnolL LiX/PC solutions was determined by Aurbach and Zaban [18, 23] from Nyquist plots. Eor the different salts, the order of I sei was LIPFg LiBp4 LiSOjCFj LiAsP LiNfSOj CFjIj LiBr, LiCl04 [18]. The values for LiPFg/PC and LiN(S02CF3)2/PC were about 800 and 23 cm, respectively. The resistivity of the film was found to be directly proportional to the salt concentration, and the presence of CO2 in solutions considerably reduced the interfacial resistance. 

The interfacial properties of gel electrolytes containing EC immobilized in a polyacrylonitrile (PAN) matrix with a lithium (bis)trifluoromethane sulfonimide (LiTFSl) salthavebeen studied [137]. SEl stability appeared to be strongly dependent on the LiTFSl concentration. A minimum value of Rsei of about 1000 cm was obtained after 200 h of storage of an electrolyte containing 14% salt. This value was 

In hthium-ion batteries, with carbonaceous anodes, Qjr can be lowered by decreasing the true surface area of the carbon, using pure carbon and electrolyte, applying high current density at the beginning of the first charge, and using appropriate electrolyte combinations. 

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Dioxolane-l, 2-dimethoxyethane-Li2 B1()C11() exhibited chemical stability towards the components of a lithium-titanium disulfide cell and showed promise as an electrolyte in such cells [98], Among various systems composed of an ether-based solvent and a lithium salt, THF-LiAsF6 was the least reactive to lithium at elevated temperature and gave the best cycling efficiency [99, 100], Tetrahydrofu-ran-diethyl ether-LiAsF(i afforded lithium electrode cycling efficiency in excess of 98% [101],... 

The dendritic growth of lithium was suppressed on a lithium electrode surface modified by an ultrathin solid polymer electrolyte prepared from 1,1—difluoro-ethane by plasma polymerization [114]. 

Li[LivMn2 y]02 (0 < y < 1/3)) shows an operating voltage above 3.5V with respect to a lithium electrode. 

The deposition points on the lithium electrode are the points at which the protective film has a higher lithium-ion conductivity. One example of these deposition points are the pits on the lithium anode caused by discharge. Crystalline defects and the grain boundaries in lithium may also initiate deposition. 

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

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]. 

In practical cases, however, the excess weight and volume due to the use of alloys may not be very far from those required with pure lithium electrodes, for one generally has to operate with a large amount of excess lithium in rechargeable cells in order to make up for the capacity loss related to the filament growth problem upon cycling. 

The superiority of LiAsF6 in ether based solvents (2-Me-THF, THF, MeF) at lithium electrodes is an example of the formation of useful protecting films (As, Li2 As, Li (AsFv) allowing uniform lithium deposition [195], According to Aur-bach and co-workers, LiAsF6/2 - Me -THF is a highly suitable electrolyte for rechargeable lithium batteries. However, as 2-Me-THF is one of the least reactive sol-... 

The corrosion resistance of lithium electrodes in contact with aprotic organic solvents is due to a particular protective film forming on the electrode surface when it first comes in contact witfi tfie solvent, preventing further interaction of the metal with the solvent. This film thus leads to a certain passivation of lithium, which, however, has the special feature of being efiective only while no current passes through the external circuit. The passive film does not prevent any of the current flow associated with the basic current-generating electrode reaction. The film contains insoluble lithium compounds (oxide, chloride) and products of solvent degradation. Its detailed chemical composition and physicochemical properties depend on the composition of the electrolyte solution and on the various impurity levels in this solution. 

The intercalation compounds of lithium with graphite are very different in their behavior from intercalation compounds with oxides or halcogenides. Intercalation processes in the former compounds occur in the potential region from 0 to 0.4 V vs. the potential of the lithium electrode. Thus, the thermodynamic activity of lithium in these compounds is close to that for metallic lithium. For this reason, lithium intercalation compounds of graphite can be used as negative electrodes in batteries rather than the difficultly of handling metallic lithium, which is difficult to handle. 

A point meriting attention is the voltage difference above. Doped polymers are rather electropositive (up to more than 4 V vs. a lithium electrode in the same solution), so much so that charging may have to be limited in order not to exceed the stability limits of the electrolyte (typically, propylene carbonate or acetonitrile as aprotic nonaqueous solvents). 

Electrochemical noise studies have also been beneficial in lithium battery research. The lithium electrode sitting in the aprotic electrolyte is covered by a passivating film... 

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