Film 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-... 

The extent of the irreversible charge losses due to film formation depends to a first approximation on the surface area of the lithiated carbon which is wetted by the electrolyte [36, 66, 120-124]. Electrode manufacturing parameters influencing the pore size distribution within the electrode [36, 121, 124, 125] and the coverage of the individual particles by a binder [124, 126] have an additional influence on the carbon electrode surface exposed to the electro-... 

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-... 

Kinetic stability of lithium and the lithiated carbons results from film formation which yields protective layers on lithium or on the surfaces of carbonaceous materials, able to conduct lithium ions and to prevent the electrolyte from continuously being reduced film formation at the Li/PC interphase by the reductive decomposition of PC or EC/DMC yielding alkyl-carbonates passivates lithium, in contrast to the situation with DEC where lithium is dissolved to form lithium ethylcarbonate [149]. EMC is superior to DMC as a single solvent, due to better surface film properties at the carbon electrode [151]. However, the quality of films can be increased further by using the mixed solvent EMC/EC, in contrast to the recently proposed solvent methyl propyl carbonate (MPC) which may be used as a single sol-... 

Table 8. Composition of surface films at lithium and lithiated carbons...

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],... 

In this chapter we deal with four major electrode surfaces active metals, carbons, non-active metals (e.g., noble metals), and composite electrodes comprising lithiated transition metal oxide powders as the active mass, plus polymeric binder and conductive additives (usually carbon black or graphite powders at low percentage). In terms of general surface chemistry, we find that the surface reactions on lithium, lithiated carbons, carbon, and noble metals polarized to low potentials in non-aqueous Li salt solutions are very similar. All of these electrodes are covered by surface films comprising insoluble Li salts, which are formed by reduction of solution species. Upon anodic polarization of carbon or noble metal electrodes in non-aqueous solutions, solution species are oxidized. Here, the impact of the cations is negligible. It seems that the species that determine the anodic stability of non-aqueous solutions are the solvents. For instance, ether may be oxidized at potentials below 4 V, while alkyl carbonates may apparently be stable up to 5 V (Li/Li ). However, it should be noted that some minor oxidation reactions of alkyl carbonate solvents on noble metal electrodes (e.g., Pt, Au) can be detected even at a potential below 4 V. The... 

The subject of surface films on electrodes in non-aqueous solutions is mostly important for the field of batteries. The performance of both Li and Li-ion batteries depends strongly on passivation phenomena that relate to surface film formation on both the anodes and the cathodes. Lithium and lithiated carbon anodes reduce all the solvents and salt anions in electrolyte solutions relevant to Li batteries. The products of these surface reactions always contain insoluble Li salts that precipitate on the electrodes as surface films. All charge transfer processes of Li, Li-C, and Li alloy anodes in Li batteries involve the critical step of Li-ion migration through the surface films. Thereby, the composition, structure, morphology, and electrical properties of surface films on Li, Li-C, and Li alloy electrodes were smdied very intensively over the years. In contrast, reversible magnesium electrodes can function only in surface film-free conditions. ... 

The above-described gradual surface reaction processes also form multilayer surface films, as is illustrated schematically in Figure 7. As the electrode reaches the very low potentials, and/or fully lithiated carbon is formed, the surface layer close to the electrode can be further reduced to form species of very low oxidation states (Li20, LiF, Li-C, LiH, LisN, etc.). Hence, we can... 

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 electrochemically active electrode materials in Li-ion batteries are a lithium metal oxide for the positive electrode and lithiated carbon for the negative electrode. These materials are adhered to a metal foil current collector with a binder, typically polyvinylidene fluoride (PVDF) or the copolymer polyvinylidene fluoride-hexafluroropropylene (PVDF-HFP), and a conductive diluent, typically a high-surface-area carbon black or graphite. The positive and negative electrodes are electrically isolated by a microporous polyethylene or polypropylene separator film in products that employ a liquid electrolyte, a layer of gel-polymer electrolyte in gel-polymer batteries, or a layer of solid electrolyte in solid-state batteries. 

---