In the last two decades cathodic materials for rechargeable lithium batteries have been extensively studied because these batteries show elevated potential, high energy density, long shelf-life, and safety. 1,2 Among the cathodic materials investigated LiMn 2 O 4 -based spinels are promising candidates to replace LiCoO 2 , which is the material currently used in commercial lithium batteries. In fact, the low cost of the manganese compounds together with their acceptable environmental requirement make these compounds suitable for cathodic materials. [3][4][5][6][7][8][9][10][11][12][13][14][15] It is worth noting that the positive electrodes (cathodes) of the lithium batteries are normally composites in which, besides the active electrode material (LiCoO 2 , LiMn 2 O 4 , etc.), a polymer and a good electronic conductor are usually added. So, it seems that a detailed knowledge of the composites is needed for improving their electrochemical response as positive electrodes. Carbon black (CB) is generally chosen because of its good electronic conductivity which assures an efficient electronic transport through the composite. The efficiency in electronic transport is required because the deinsertion/insertion of Li ϩ ions in the active material is accompanied by oxidation/reduction of some of its elements (Mn, Co, etc.). Moreover, it seems that CB favors a good absorption of the liquid electrolyte by the composite, and hence, a good contact between Li ϩ ions and the particles of the active material. The polymer, which is normally an insulating material from the electrical point of view, can be used as a binder for improving handling of the composite. 1,2 In binary composites formed of a polymer and a good electronic conductor such as CB, graphite, etc., the relative content of the components and, specially, their distribution within the composite are responsible for the electrical conductivity found. [16][17][18][19][20][21][22] In ternary composites formed of LiMn 2 O 4 , CB, and a polymer, the relative content of the three compounds would be the parameter more strongly affecting the electrical conductivity as well as the electrochemical response of the composites. However, this point has not been analyzed in detail so far.In this work we have prepared composites from physical mixtures of LiMn 2 O 4 , CB, and polyvinylidenefluoride (PVDF). The content in PVDF was kept constant, and we analyzed how the electrical conductivity and the discharge capacity were changed as a function of the relative content in LiMn 2 O 4 and CB. The significant variation found in both conductivity and capacity is discussed in terms of the microstructure of the composites as followed by scanning electron microscopy (SEM).
ExperimentalComposites of different composition were prepared by mixing and grinding the three components, LiMn 2 O 4 , polyvinylidenefluoride (PVDF), and carbon black (CB), in an agate mortar. Cylindrical and parallelepiped pellets of the composites as well as of the individual components were obtained by cold pressing at ...
