Role-assigned electrolytes: Additives

Abstract

The role of liquid electrolytes in lithium-ion cells is to act as an ionic conductor to transport lithium ions back and forth between positive and negative electrodes as the cells are charged and discharged. Since the electrodes in lithium-ion cells are the porous composite electrodes, consisting of an active material [carbon in the negative electrode and lithium transition metal (Co, Ni, Mn) oxide in the positive electrode, respectively], a conductive material (carbon black), and a polymer binder as depicted in Fig. 4.1, the liquid electrolyte must seep into the porous electrodes and transfer lithium ions smoothly at the interfaces between the liquid and solid phases. Most lithium-ion cells available in the market utilize nonaqueous electrolyte solutions, where lithium salts are dissolved in aprotic organic solvents. The gelled electrolytes used in lithium-ion polymer cells also are regarded as a liquid electrolyte immobilized with a high molecular weight polymer. Therefore, the same functions are required for the liquid and gelled electrolytes to greater or lesser degrees. There are many books reviewing the liquid electrolytes for lithium or lithiumion cells, where the various properties of aprotic solvents, lithium salts, and their mixtures are described.1-11 This author also reviewed them mainly from the aspect of solution chemistry. 12, 13 However, recent researches in the liquid electrolytes are mainly focused on the electrolyte additives, which add extra functions to the liquid electrolytes in addition to a fundamental function as an ionic conductor. This author will attempt to outline the electrolyte additives for the first time by classifying them into several categories, 14, 15 although the fragmented information on the additives began to appear in the recent reviews. 8, 9, 16, 17 In addition to the information published in the literature during 1991-2004, some experimental results obtained in our laboratories also are included. © 2009 Springer Science+Business Media, LLC.