Soft carbons

Two types of carbon can be used as active anode materials [94, 95] soft carbon, also called graphitized carbon where crystallites are oriented almost in the same direction, and hard carbon where crystallites have disordered orientations. Both are used in lithium-ion batteries, but they do not have the same properties. 

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Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

TT-Aliylpalladium chloride reacts with a soft carbon nucleophile such as mal-onate and acetoacetate in DMSO as a coordinating solvent, and facile carbon-carbon bond formation takes place[l2,265], This reaction constitutes the basis of both stoichiometric and catalytic 7r-allylpalladium chemistry. Depending on the way in which 7r-allylpalladium complexes are prepared, the reaction becomes stoichiometric or catalytic. Preparation of the 7r-allylpalladium complexes 298 by the oxidative addition of Pd(0) to various allylic compounds (esters, carbonates etc.), and their reactions with nucleophiles, are catalytic, because Pd(0) is regenerated after the reaction with the nucleophile, and reacts again with allylic compounds. These catalytic reactions are treated in Chapter 4, Section 2. On the other hand, the preparation of the 7r-allyl complexes 299 from alkenes requires Pd(II) salts. The subsequent reaction with the nucleophile forms Pd(0). The whole process consumes Pd(ll), and ends as a stoichiometric process, because the in situ reoxidation of Pd(0) is hardly attainable. These stoichiometric reactions are treated in this section. 

The TT-allylpalladiLim complexes formed as intermediates in the reaction of 1,3-dienes are trapped by soft carbon nucleophiles such as malonate, cyanoacctate, and malononitrile[ 177-179). The reaction of (o-iodophenyl-methyl) malonate (261) with 1,4-cyclohexadiene is terminated by the capture of malonate via Pd migration to form 262. The intramolecular reaction of 263 generates Tr-allylpalladium, which is trapped by malononitrile to give 264. o-[odophenylmalonate (265) adds to 1,4-cyciohexadiene to form a Tr-allylpalladium intermediate via elimination of H—Pd—X and its readdition, which is trapped intramolecularly with malonate to form 266)176]. 

Arylation or alkenylation of soft carbon nucleophiles such as malonate is carried out by using a copper catalyst, but it is not a smooth reaction. The reaction of malononitrile, cyanoacetate, and phenylsulfonylacetonitrile with aryl iodide is possible by using a Pd catalyst to give the coupling products. 

Application of 7r-allylpalladium chemistry to organic synthesis has made remarkable progress[l]. As deseribed in Chapter 3, Seetion 3, Tt-allylpalladium complexes react with soft carbon nucleophiles such as maionates, /3-keto esters, and enamines in DMSO to form earbon-carbon bonds[2, 3], The characteristie feature of this reaction is that whereas organometallic reagents are eonsidered to be nucleophilic and react with electrophiles, typieally earbonyl eompounds, Tt-allylpalladium complexes are electrophilie and reaet with nucleophiles such as active methylene compounds, and Pd(0) is formed after the reaction. 

In addition, a catalytic version of Tt-allylpalladium chemistry has been devel-oped[6,7]. Formation of the Tr-allylpalladium complexes by the oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of the complex with soft carbon nucleophiles are the basis of catalytic allylation. After the reaction, Pd(0) is reformed, and undergoes oxidative addition to the allylic compounds again, making the reaction catalytic.-In addition to the soft carbon nucleophiles, hard carbon nucleophiles of organometallic compounds of main group metals are allylated with 7r-allylpalladium complexes. The reaction proceeds via transmetallation. These catalytic reactions are treated in this chapter. 

The stereochemistry of the Pd-catalyzed allylation of nucleophiles has been studied extensively[5,l8-20]. In the first step, 7r-allylpalladium complex formation by the attack of Pd(0) on an allylic part proceeds by inversion (anti attack). Then subsequent reaction of soft carbon nucleophiles, N- and 0-nucleophiles proceeds by inversion to give 1. Thus overall retention is observed. On the other hand, the reaction of hard carbon nucleophiles of organometallic compounds proceeds via transmetallation, which affords 2 by retention, and reductive elimination affords the final product 3. Thus the overall inversion is observed in this case[21,22]. 

The intramolecular allylation of soft carbon nucleophiles with allylic acetates as a good cyclization method has been extensively applied to syntheses of various three, four, five and six-membered rings, and medium and macrocyclic compounds[44]. Only a few typical examples of the cyclizations are treated among numerous applications. 

The allylic esters 189 and 191 conjugated with cyclopropane undergo regio-selective reactions without opening the cyclopropane ring. The soft carbon nucleophiles are introduced at the terminal carbon to give 190, and phenylation with phenylzinc chloride takes place on the cyclopropane ring to form 192[120]. 

The reaction of 2,3-butadienyl acetate (843) with soft carbon nucleophiles such as dimethyl malonate gives dimethyl 2,3-butadienylmalonate (844)[520]. On the other hand, the reaction of the 2,3-butadienyl phosphate 845 with hard carbon nucleophiles such as Mg and Zn reagents affords the 2-allcyl-1,3-butadiene 846[520,521]. The 3-methoxy-1,3-butadiene 848 is obtained by the reaction of the 2-methoxy-2,3-butadienyl carbonate 847 with organozinc reagent. 

No reaction of soft carbon nucleophiles takes place with propargylic acet-ates[37], but soft carbon nucleophiles, such as / -keto esters and malonates, react with propargylic carbonates under neutral conditions using dppe as a ligand. The carbon nucleophile attacks the central carbon of the cr-allenylpal-ladium complex 81 to form the rr-allylpalladium complex 82, which reacts further with the carbon nucleophile to give the alkene 83. Thus two molecules of the a-monosubstituted /3-keto ester 84, which has one active proton, are... 

Fig. 2. The master graph of reversible capacity for lithium plotted versus heat treatment temperature for a variety of carbon samples. The three regions of commereial relevance are marked. Solid symbols are data for soft carbons, open symbols are data for hard carbons.

Graphitic carbon normally refers to soft carbon heated above about 2100°C. The probability of finding turbostratie disorder begins to decrease as the heat-treatment temperature increases to above 2100°C. When the heating temperature reaehes above 3000°C, graphite forms. Coneeptually, graphite is a graphitic carbon with no or very little turbostratic disorder. 

Petroleum pitch was obtained from Kureha Company, Japan (designated here as KS pitch). A second petroleum pitch was obtained from the Crowley Tar Company, U.S.A. (designated here as CRO pitch). Polyvinyl chloride (PVC) was obtained from the Aldrich Chemical Company (U.S.A.). These samples are all soft carbon precursors. 

The single-point BET surface area measurement was used to check for open pores. The results for some soft and hard carbon samples heated at 700°C and 1000°C are presented in Table 2 for comparison. The hard carbon samples studied here have about ten times more open porosity than the soft carbons. 

Strueturally, the materials are grouped into two main classes, those (soft carbons) with predominantly staeked layers (CRO, KS and PVC) and those (hard earbons) which have significant amounts of single layer sheets (OXY and ENR). All the samples show similar values of L when heated to a given temperature. 

F igure 21 compares the voltage-capacity profiles for the second cycle of lithium/carbon electrochemical cells made from OXY, a representative hard carbon, and those for samples made from CRO, a representative soft carbon. 

Addition of carbon nucleophiles to vinylepoxides is of particular importance, since a new carbon-carbon bond is formed. It is of considerable tactical value that conditions allowing for regiocontrolled opening of vinyloxiranes with this type of nucleophiles have been developed. Reactions that proceed through fonnation of a rr-allyl metal intermediate with subsequent external delivery of the nucleophile, or that make use of a soft carbon nucleophile, generally deliver the SN2 product. In contrast, the Sn2 variant is often the major reaction pathway when hard nucleophiles are employed. In some methods a nucleophile can be delivered selectively at either the Sn2 or SN2 positions by changing the reaction conditions. 

Additions of carbon nucleophiles to vinylepoxides are well documented and can be accomplished by several different techniques. Palladium-catalyzed allylic alkylation of these substrates with soft carbon nucleophiles (pKa 10-20) proceeds under neutral conditions and with excellent regioselectivities [103, 104]. The sul-fone 51, for example, was cyclized through the use of catalytic amounts of Pd(PPh3)4 and bis(diphenylphosphino)ethane (dppe) under high-dilution conditions to give macrocycle 52, an intermediate in a total synthesis of the antitumor agent roseophilin, in excellent yield (Scheme 9.26) [115, 116]. 

There are many kinds of carbon materials, with different crystallinity. Their crystallinity generally develops due to heat-treatment in a gas atmosphere ("soft" carbon). However, there are some kinds of carbon ("hard" carbon) in which it is difficult to develop this cristallinity by the heat-treatment method. Both kinds of carbon materials are used as the negative electrode for lithium-ion batteries. 

Soft carbon is also classified by its crystallinity. For example, acetylene black and carbon black are regarded as typical carbon materials with low crystallinity. Coke materials are carbon materials with intermediate crystallinity. It is easy to obtain these materials because they are made from petroleum and coal and they were actively studied in the 1980s. In contrast, there are some graphite materials which have high crystallinity their capacity is greater than that of coke materials, and these materials have been studied more recently, in the 1990s [76-80]. 

Figure 61. Relationship between discharge capacity, initial efficiency, and L, of soft carbon materials.

Figure 61 shows the relationship between the discharge capacity, the initial efficiency, and the L of some soft carbon materials when ethylene carbonate was used as a solvent. Figure 62 shows the re-... 

Both hard and soft carbons are used as negative electrode materials for lithium-ion batteries. Hard carbon is made by heat-treating organic polymer materials such as phenol resin. The heat-treatment tempera-... 

The physicochemical properties of carbonaceous materials can be altered in a predictable manner by different types of treatments. For example, heat treatment of soft carbons, depending on the temperature, leads to an increase in the crystallite parameters, La and Lc and a decrease in the d(0 0 2) spacing. Besides these physical changes in the carbon material, other properties such as the electrical conductivity and chemical reactivity are changed. A review of the electronic properties of graphite and other types of carbonaceous materials is presented by Spain [3],... 

Thus it appears that the presence of two soft carbons on the palladium stabilizes the trans coordination of hard ligands and drives the selective coordination of ambidente ligands through their hardest atom. These results, as those described in the previous scheme, constitute other examples of the antisymbiotic effect which can be observed in soft palladium(II) complexes. 

Therefore, there is a wide spectrum of carbon materials suitable for the usage in Li-ion batteries the choice of a specific one determined by many factors. According to Ref. 7, the percentage of various carbon materials used in commercial Li-ion batteries was as follows graphites - 43 %, hard carbons - 52 %, soft carbons - 5 %. 

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