Cycling lithiated carbons

The excess charge consumed in the first cycle is generally ascribed to SEI formation and corrosion-like reactions of Li C6[19, 66, 118-120]. Like metallic lithium and Li-rich Li alloys, lithiated graphites, and more generally lithiated carbons are thermodynamically unstable in all known electrolytes, and therefore the surfaces which are exposed to the electrolyte have to be kinetically protected by SEI films (see Chapter III, Sec.6). Neverthe-... 

Since this is a new field, little has been published on the LiXC6 /electrolyte interface. However, there is much similarity between the SEIs on lithium and on LixC6 electrodes. The mechanism of formation of the passivation film at the interface between lithiated carbon and a liquid or polymer electrolyte was studied by AC impedance [128, 142]. Two semicircles observed in AC-impedance spectra of LiAsF6/EC-2Me-THF electrolytes at 0.8 V vs. Li/Li+ [142] were attributed to the formation of a surface film during the first charge cycle. However, in the cases of LiC104 or LiBF4 /EC-PC-DME (di-... 

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],... 

Finally, since there is always an attempt to increase performance, hybrid systems may offer some hope. These systems combine a battery electrode, such as a lithiated carbon graphite negative electrode, and a positive supercapacitor electrode such as porous carbon. The faradaic electrode provides a high capacity and the supercapacitor electrode maintains power performance. This approach is attractive because it can increase both the capacity and power by judiciously choosing the electrode materials. In practice, there are some challenges, such as the balancing of electrodes, the limited cycle life of the electrodes in the... 

Microbatteries are typically produced from multiple thin layers. An alternative design has been reported in which polypyrrole was proposed to be used as the cathode and lithiated carbon as the anode [152]. The performance targets specified were a cell voltage of 3—4 V a discharge rate of 0.1 mA/cm, and a 200 cycle lifetime (at 80% discharge). Although the conceptual design was published in 1999 and the properties of the carbon anode considered, no further work has been reported on this system and no CP microbatteries have yet been constructed. 

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 theoretical capacity of metallic lithium is much higher than that of lithiated carbon material having a composition of LiCg. The advantage of the higher capacity of metallic lithium is reduced because a three- to fivefold excess of lithium is required in rechargeable batteries having metallic lithium anodes to achieve a reasonable cycle life. The comparison is shown in Table 34.4. 

Fluorinated carbonates were also used by Smart et al. as low-temperature cosolvents (Table 12), in the hope that better low-temperature performances could be imparted by their lower melting points and favorable effects on SEI chemistry. Cycling tests with anode half-cells showed that, compared with the ternary composition with nonfluorinated carbonates, these fluorinated solvents showed comparable and slightly better capacity utilizations at room temperature or —20 °C, if the cells were charged at room temperature however, pronounced differences in discharge (delithiation) capacity could be observed if the cells were charged (lithiated) at —20 °C, where one of these solvents, ethyl-2,2,2-trifluoroethyl carbonate (ETFEC), allowed the cell to deliver far superior capacity, as Figure 63 shows. Only 50% of the capacity deliverable at room temperature was... 

Figure 27 shows a typical chronopotentiogram of the first lithiation-delithiation cycle of a petroleum coke electrode [357], It demonstrates the irreversible capacity loss due to the carbon s surface reactions (plateau around 1 V versus Li/Li+), the sloping potential profile, and the fact that the maximal reversible capacity is less than that of graphite. However, its structural disorder makes this electrode... 

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