Graphite positive electrode, conductive additives

Perhaps the first practical application of carbonaceous materials in batteries was demonstrated in 1868 by Georges Le-clanche in cells that bear his name [20]. Coarsely ground MnO, was mixed with an equal volume of retort carbon to form the positive electrode. Carbonaceous powdered materials such as acetylene black and graphite are commonly used to enhance the conductivity of electrodes in alkaline batteries. The particle morphology plays a significant role, particularly when carbon blacks are used in batteries as an electrode additive to enhance the electronic conductivity. One of the most common carbon blacks which is used as an additive to enhance the electronic conductivity of electrodes that contain metal oxides is acetylene black. A detailed discussion on the desirable properties of acetylene black in Leclanche cells is provided by Bregazzi [21], A suitable carbon for this application should have characteristics that include (i) low resistivity in the presence of the electrolyte and active electrode material, (ii) absorption and retention of a significant... 

One of possible ways of improving the specific characteristics of LIB electrodes is the use of thermally expanded graphites (TEG) as electrically conductive additives of positive active mass. The efficiency of TEG s use may be illustrated by analyzing a simple theoretical model proposed by us AM - electrically conductive additive (Figure 4). 

However, it can undergo self-reductive dissolution (loss of active material) accompanied by oxygen evolution [349]. The active material of the positive electrode (in pocket plate cells) consists of nickel hydroxide mixed with small additions of cobalt and barium hydroxides to improve the capacity and charging/discharging performance and graphite to improve conductivity [348]. 

Carbon is used in lithium-ion cells for different functions conductive carbon black and/or graphite additives are applied in both the negative and the positive electrode to improve the electronic conductivity of the electrodes. These conductive additives constitute a fraction of up to about 10% of the total carbon consumption. The major fraction is represented by the active carbon materials which are electrochemically reduced and oxidized in the negative electrode during the battery charge and discharge process, respectively. 

In modem commercial lithium-ion batteries, a variety of graphite powder and fibers, as well as carbon black, can be found as conductive additive in the positive electrode. Due to the variety of different battery formulations and chemistries which are applied, so far no standardization of materials has occurred. Every individual active electrode material and electrode formulation imposes special requirements on the conductive additive for an optimum battery performance. In addition, varying battery manufacturing processes implement differences in the electrode formulations. In this context, it is noteworthy that electrodes of lithium-ion batteries with a gelled or polymer electrolyte require the use of carbon black to attach the electrolyte to the active electrode materials.49-54 In the following, the characteristic material and battery-related properties of graphite, carbon black, and other specific carbon conductive additives are described. 

When applied as conductive additive in the positive electrode, graphite and carbon black show complementary properties which are summarized in Table 7.3. The decision which carbon type should be selected depends on the cell requirements and the type of active electrode materials used in the electrodes. The TEM pictures in Figure 7.7 compare the morphology of a typical conductive carbon black and a graphite powder and illustrate the dimensional differences of the primary particles of a factor of about 10. 

FIGURE 7.8 SEM pictures of a LiCo02 positive electrode (left) and of a surface-treated graphite negative electrode (right) both containing TIMREX KS6 graphite and SUPER P Li carbon black conductive additives. 

In addition to the beneficial effect on the electrochemical performance, the graphite conductive additives influence the electrode density positively. Typically, the graphitized mesocarbon or... 

The active substance of the positive electrode is the polymer of fluorinated carbon with the overall formula of (CFj ) . As a rule, subscript x in this formula is close to unity, polymerization degree n exceeds 1(X)0. The polymer of fluorinated carbon is a layered compound obtained by fluorination of carbon (graphitized or nongraphitized) in the form of a powder, fibers, or even fabrics by elementary fluorine at the temperatures of 350 - 600°C. As polyfluorocarbon is characterized by negligible electron conductivity, a certain amount of a conductive additive (carbon black) is introduced into the active mass of cathodes. Elementary carbon is formed in the course of discharge and the overall conductivity of the cathode increases. 

The types of graphite carbon powders, which primarily are applied as conductive additives in the positive and negative electrodes of lithium batteries, belong to the family of highly crystalline graphite materials. These graphite materials show real densities of 2.24-2.27 g cm- (values based on the xylene density according to DIN... 

Fig, 5.22 Capacity retention of half-cells containing a surface-treated graphite-negative electrode top) and a positive LiCoO, respectively, with different fractions of TIMREX KS graphite and Super P Li as conductive additive (bottom) (electrode porosity ca. 35%, electrolyte 1-M LiPF in ethylene carbonate/ethyl methyl carbonate 1 3 (v v)) ... 

Another example of the use of a graphite as an additive to improve the electronic conductivity of an electrode can be found in the discussion of the Fe/NiOOH cell developed by Edison in the early 19(X)s [25]. The positive electrode which contained graphite (20-30% graphite flake) degraded rapidly during charge because of oxidation and swelling. This experience led to the development of electrolytic nickel flakes and eventually to the porous nickel plaque for use in NiOOH electrodes. 

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