Charge carbon/graphite

Although the high specific charge carbons exhibit a multiple of the specific charge that is severafold that of graphite, there are still some problems to solve ... 

The reaction at the anode in Li-Ion cells is given in Equation 1. During charge the lithium ions approach the surface of the carbon where they accept an electron and enter the lattice. On discharge, the opposite reaction occurs. The electrochemical reaction is thought to occur on the edge planes and not the basal plane of the carbon/graphite particles. 

The electric current enters and leaves the electrolyte through electrodes, which are usually made of unreactive metals such as platinum or of the non-metal carbon (graphite). These are said to be inert electrodes because they do not react with the products of electrolysis. The names given to the two electrodes are cathode, the negative electrode which attracts cations (positively charged ions), and anode, the positive electrode which attracts anions (negatively charged ions). 

The cycle life within one cycle-set depends strongly on the nature and properties of the carbon or graphite additives used. These materials differ in particle size, structure and affinity to lead and to the expander. Of special importance is the interface between carbon and lead particles, and its area as it determines the resistance that electrons have to overcome when transferred between these two phases and thus affects the potential and the rate of the electrochemical reactions at the carbon/solution interface. Only a limited number of carbon and graphite materials have optimum structural characteristics and may improve substantially the cycle life performance of the cells. It is of crucial importance to identify the most effective carbon (graphite) additives, i.e. with most beneficial effect on the parallel mechanism of charge of the negative plates. 

When PC is used as a solvent, the electrolyte continues to decompose at about 0.9 V Vi. LP/Li, as shown in Fig. 4.4, and graphitic (crystalline) carbons cannot be charged due to their exfohation caused by the solvent co-intercalation. This is the reason why EC is dominantly used instead of PC, which was adopted for the first commercial lithium-ion cells with anongraphitic (amorphous) carbon anode. However, the addition of some compounds such as vinylene carbonate (VC) prevents the graphite exfoliation and enables the charge of graphitic carbons, as shown in Fig. 4.4. 

Values estimated from DSC and ARC measurements of cell components at TIAX, for charged active (graphite and LiCo02) in contact with standard carbonate electrolyte (LiPFg in carbonate solvents)... 

Fig. 5 The top left snapshot shows a typical supercapacitor simulation cell. In this example the electrolyte is a EMIM-BF4 ionic liquid (red EMIM, green BF4"), which is represented using a coarse-grained model as illustrated on the right panel. The electrodes, which are made of graphite, are held at constant potential by allowing the charges of the carbon atoms to fluctuate. An illustration of the instantaneous charges is shown on the bottom left snapshot, where green and red colors are respectively employed for negatively and positively charged carbon atoms,...

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