Electrochemical Formation of Lithiated Carbons

The electroinsertion reaction of mobile lithium ions into a solid carbon host proceeds according to the general reaction scheme 

During electrochemical reduction (charge) of the carbon host, lithium cations from the electrolyte penetrate into the carbon and form a lithiated carbon Li rCn. The corresponding negative charges are accepted by the carbon host lattice. As for any other electrochemical insertion process, the prerequisite for the formation of lithiated carbons is a host material that exhibits mixed (electronic and ionic) conductance. 

The reversibility of this so-called intercalation reaction can be demonstrated by a subsequent electrochemical oxidation (discharge) of IixC , that is, the de-intercalation of Ii+. This is considered to be a special type of intercalation in that, unusually, a layer of guest ions slides between the sheets of a layered host matrix, while the host broadly retains its structural integrity. This occurs in the case of the insertion of lithium ions into graphite. In most cases, however, a strict differentiation between insertion and intercalation is a formal question and both terms are used inter-changeably. Following historical conventions, the terms intercalation and lithium/carbon intercalation compounds will be used in this review, even though only a small fraction of layered structure units may be present in a specific carbon material (see also Refs [2, 6]). 

The electrochemical performance of lithiated carbons depends basically on the electrolyte, the parent carbonaceous material, and the interaction between the two (see also Part III, Chapter 17). As far as the lithium intercalation process is concerned, interactions with the electrolyte, which Hmit the suitabihty of an electrolyte system, will be discussed in Sections 15.2.2.3, 15.2.3 and 15.2.4 of this chapter. First, several properties of the carbonaceous materials will be described. 

The quahty and quantity of sites which are capable of reversible lithium accommodation depend in a complex manner on the crystaUinity, the texture, the (micro)structure, and the (micro)morphology of the carbonaceous host material [7,19, 22, 39-56]. The type of carbon determines the current/potential characteristics of the electrochemical intercalation reaction and also potential side-reactions. Carbonaceous materials suitable for lithium intercalation are commercially available in many types and qualities [19, 42, 57-60]. Many exotic carbons have been specially synthesized on a laboratory scale by pyrolysis of various precursors. 

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Carbon Materials. In the lithium-ion cell, carbon materials, which can reversibly accept and donate significant amounts of lithium (Li C = 1 6) without affecting their mechanical and electrical properties, can be used for the anode instead of metallic lithium. Carbon material is used as an anode in lithium-ion cells since the chemical potential of lithiated carbon material is close to that of metallic lithium, as shown in Fig. 34.2. Thus an electrochemical cell made with a lithiated carbon material will have almost the same open-circuit voltage as one made with metallic lithium. In practice, the lithium-ion cell is manufactured in a fully discharged state. Instead of using lithiated carbon material, which is air-sensitive, the anode is made with carbon and lithiation is carried out by subsequent formation of the cell. 

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],... 

Lithium ion can be intercalated, more or less, into most kinds of carbon, and the resulting lithiated carbons show extremely negative electrochemical potentials close to that of the metallic lithium electrode. The reversible intercalation/deintercalation reactions overcome the problem of dendrite formation of lithium and provide dramatic improvements in safety and cycleability. The carbon anodes are combined with non-aqueous electrolyte solutions and lithium-transition metal oxides such as LiCoOg as cathodes to fabricate 4 V-class LIBs. Only lithium ion moves back and forth between the cathode and the anode upon charging and discharging, which give rise to a potential difference of about 4 V between the two electrodes. The name, "lithium-ion" batteries came from this simple mechanism. 

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