The Application Of Conductive Polymers PPy And PAn In The Positive Electrode For Lithium Ion Batteries

Conductive reagents must be added in the preparation of positive electrode films due to the low conductivities of positive electrode active materials for lithium ion batteries (LIB). Owing to the low conductivity of conductive reagent Acetylene black (AB)(7.77S/cm), conductive electroactive polymers Polypyrrole (PPy) and (?)aniline (PAn) were used to replace AB as conductive reagent.(?) conditions of chemical oxidative method to synthesize PPy and PAn were optimized by single factor method. The changing trends of the conductivities of p(?)tive electrode films were discussed when different contents of PPy or PAn were used to substitute AB as conductive reagents. And the content of conductive reagents was optimized by the percolation theory. The influences of PPy and PAn on the electrochemical performances of the positive electrode active materials LiCoO2, LiMn2O4and LiFePO4/C were systematically investigated. The experimental results were as follows:1. The effects of the polymerization conditions of chemical oxidative method on the conductivities and yields of PPy and PAn were researched.(1) When FeCl3is used as the oxidant, the highest conductivity of PPy reaches2.14S/cm and its corresponding yield is66.71%. When0.09mol/L surfactant Naphthalene (?)onic acid (NSA) is added as the dopant, the conductivity increases to8.76S/cm and its corresponding yield reaches112.60%. When (NH4)2S2O8is adopted as the (?)dant, the conductivities of PPy before and after adding the surfactant are0.73S/cm (?)3.80S/cm, respectively. The chains of PPy will be warped by the doping of (?) or SO42-anions and its regularity will decrease.(?) The optimum conductivity of PAn is18.39S/cm and its corresponding yield is (?)06%when (NH4)2S2O8is used as the oxidant.2. The traditional instrument used for determining the conductivities was remoulded, by which the conductivities of the positive electrode films constituted by the binder, conductive reagents and positive electrode active materials were determined.3. The positive electrode films are made up of binder polytetrafluoroethylene (?). conductive reagents (AB, PPy or PAn) and positive electrode active materials (?)2LiMn2O4or LiFePO4/C). Their conductivities were systematically studied.(?)he experimental results show that the changes of conductivities of the three components positive electrode films with the increase of the volume contents of conductive reagents are also in accordance with the percolation theory applied in the two components conductive polymers. Therefore, the content of the conductive reagents in positive electrodes can be optimized by the percolation theory.(1) When the content of PPy is15wt%, the conductivity of LiCoO2positive electrode film is3.17×10-1S/cm, which is very close to that of the film with the same content of AB. However, when15wt%PAn is used, the conductivity of LiCoO2positive electrode film reaches4.02×10-1S/cm which is25%larger than that of the film with the same content of AB.(2) The conductivity of LiMn2O4positive electrode film containing15wt%PPy reaches5.07×10-1S/cm, which does not change obviously compared with the addition of the same content of AB. However, when15wt%PAn is used, the conductivity of the LiMn2O4positive electrode film is8.19×10-1S/cm, which improves evidently compared with the addition of the same content of AB.(3) When using15wt%PPy as conductive reagent, the conductivity of LiFePO4/C positive electrode film is6.29×10-1S/cm. The difference is very small when PPy is used to replace the same content of AB. However, when15wt%PAn is applied, the conductivity of the LiFePO4/C positive electrode film increases obviously from6.34×10-1S/cm to1.01S/cm.Therefore, when different conductive reagents are used with the same kind of positive electrode active material and15wt%PAn is used as conductive reagent, the conductivity of positive electrode film reaches the maximum value.4. The influences of conductive reagents PPy and PAn on the electrochemical performances of the positive electrode active materials were investigated.(1) The discharge capacities of PPy and PAn themselves in the discharge potential range of positive electrode are57.2mAh/g and60.8mAh/g.(2) When15wt%AB substitutes with the same content of PPy, the discharge capacities of LiCoO2, LiMn2O4and LiFePO4/C positive electrodes at the current density of15mA/g increase from159.2mAh/g,115.4mAh/g and155.0mAh/g to166.1mAh/g,122.0mAh/g and161.9mAh/g, respectively. The increases of the discharge capacities are contributed by the discharge capacity of PPy itself. The rate performances of positive electrodes are not improved evidently, since the conductivities of PPy and AB are nearly the same. The charge transfer resistances of positive electrodes in the20th cycle increase rapidly, because the partial destruction of the conductive networks occurs. (3) When15wt%PAn is used to replace the same content of AB, the rate performances of positive electrodes are enhanced obviously, because PAn possesses discharge capacity itself and the conductivities of positive electrode films increase greatly. The discharge capacities of LiCoO2, LiMn2O4and LiFePO4/C positive electrodes with the current density of170mA/g increase from58.3mAh/g,66.3mAh/g and110.5mAh/g to88.3mAh/g,95.9mAh/g and140.1mAh/g, respectively. The charge transfer resistances of positive electrode at various Depth of discharge (DOD) are lower than those of positive electrodes using AB and PPy. The elasticity of polymer PAn can buffer the expansion and shrink of the conductive network in the charge and discharge process. Hence, the destruction of conductive network is depressed, the change of charge transfer resistance (Rct) of positive electrode after20cycle is very small.