Lithiated carbon electrodes

The last contaminant that we mention in this section is S02. As already proven, this species may be a desirable additive in organic electrolyte solutions for Li and Li ion batteries [221], It was found that the addition of a small amount of S02, even at a subpercent level, influences the surface chemistry of lithiated carbon electrodes. It was possible to detect Li2S204 surface species on lithiated carbon electrodes treated in alkyl carbonate solutions (e.g., EC-DMC) containing S02 [221],... 

As discussed in the next section, lithiated carbon electrodes are covered with surface films that influence and, in some cases, determine their electrochemical behavior (in terms of stability and reversibility). They are formed during the first intercalation process of the pristine materials, and their formation involves an irreversible consumption of charge that depends on the surface area of the carbons. This irreversible loss of capacity during the first intercalation/deintercalation cycle is common to all carbonaceous materials. However, several hard, disordered carbons exhibit additional irreversibility during the first cycle, in addition to that related to surface reactions with solution species and film formation. This additional irreversibility relates to consumption of lithium at sites of the disordered carbon, from which it cannot be electrochemically removed [346-351],... 

E. A. Levi, J. E. Fischer, and A. Claye [2001] On the Correlation Among Surface Chemistry, 3D Structure, Morphology, Electrochemical and Impedance Behavior of Various Lithiated Carbon Electrodes, Journal of Power Sources 97-98, 92-96. 

In case of an inerease in operating temperature with a strong current or because of poorly-eontrolled or uncontrolled environmental conditions, the phenomenon leading to thermal runaway is linked primarily to the deeomposition of the SEI of the lithiated carbon electrode, leaving exposed the electrode, whieh deeomposes by reaction between the inserted lithium and the eleetrolyte. Gas formation occurs, once again leading to an increase in the internal pressure of the element. 

Surface Studies of Lithium and Lithiated Carbon Electrodes by Scanning Probe Microscopy... 

Various approaches have been identified to reduce the extent of electrolyte decomposition and irreversible capacity loss at the carbon negative electrode. By adding additives to PC such as CO, N,0, CO, the self-discharge and cycling behavior of the lithiated carbon electrodes has improved. These additives affect the film properties by decreasing the low-frequency impedance, thus permitting a more rapid Li -ion transport. 

The complexity of SEI formation is topped off with reactions of the electrolyte with contaminants and additives. Because of different reaction rates of all reactive components with lithium, which yield surface films of different quality, additives can be used to modify the surface films to highly conductive lithium films, preventing the components of the electrolyte from further decomposition. There are many successful examples of this approach in the open and patent literature, both for lithium anodes and also for lithiated carbon electrodes. 

Typical additives include, for example, 2-methylfuran and KOH in DIOX or THF based solutions [359, 360], methyl formate (MF) in 2-Me-THF (THF, low content) [361], CO2 in PC [362, 363], DMC, or EC at lithium electrodes [72, 336], and CO2 at lithiated carbon electrodes [62, 63]. It is also interesting that even some gases (O2, SO2) at low pressures form thin primary films in fast reactions [364-368]. 

We should note that in addition to the above-described surface chemistry, there are reports in the literature on the formation of polymeric species on lithiated carbon electrodes in alkyl carbonate solution. These polymers may include polyethylene (due to polymerization of the ethylene formed by EC reduetion), and polycarbonates (due to polymerization of cyclic alkyl carbonates such as EC) [58,63]. 

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