ABSTRACT
As shown in Figure 13.1, an electrochemical process for a battery needs a concurrent gain or loss of electrons and ions for the charge or discharge process to continue. However, electronic conductivities for electrode active materials are generally low. In order to promote the required electron gain and loss processes, electronic conductive agents are added. In some cases, the addition of these agents can enhance the apparent diffusion of lithium ions during the charge and discharge processes. Some common conductive agents
CONTENTS
13.1 Electronic Conductive Agents .................................................................. 461 13.2 Binders ......................................................................................................... 467
13.2.1 F-Containing Polymers as Binders ..............................................468 13.2.2 SBR ................................................................................................... 469 13.2.3 Sodium Carboxymethyl Cellulose .............................................. 470 13.2.4 Polyacrylates ................................................................................... 471 13.2.5 Other Binders ................................................................................. 472
13.3 Solvents ........................................................................................................ 474 13.4 PTC Materials ............................................................................................. 474 13.5 Current Collectors ...................................................................................... 475 13.6 Case Materials ............................................................................................477 References ............................................................................................................. 478
are graphite, carbon black, ultrafine carbon suspension (UFC), and teflonized acetylene black (TAB). UFC has a more suitable particle size and less resistance than TAB. Its addition to positive electrode materials can improve capacity by 5-10% for pellets with the same size when other conditions are not changed. Structure models for acetylene black or carbon black, chopped acetylene black, and UFC are shown in Figure 13.2. Carbon black and acetylene black are made by gas-phase carbonization, as discussed in Chapter 7, for example, by decomposition of hydrocarbons. These materials have hydrophobic surfaces, and are not well dispersed in solvents such as water or N-methylpyrrolidone (NMP) during the mixing process. When they are more finely divided or chopped into uniform particles of 0.1-0.2 μm diameter and attached or dispersed in hydrophilic polymers, UFC is obtained. In the case of lithium-ion batteries, the polymers used include poly(vinyl pyrrolidone) and polyvinylidene fluoride (PVDF). UFC is also a useful material for application in other batteries such as Ni-MH. The diameter of UFC particles is usually about 0.03 μm. The good conductivity of UFC can be ascribed to the following three reasons:
1. More electrochemically active material can be added. From the mechanisms of action of graphite, carbon black, and UFC as conductive agents (Figure 13.3), it can be seen that UFC can cover the total surface of electrochemically active electrode materials and thus provide a conductive network. In the case of graphite or carbon black, larger amounts are needed to ensure total coverage. Using the same mass of conductive agent, the amount of electrochemically active material covered is about 20-25% higher when UFC is used, compared with graphite or carbon black.
