On the Origin of the Pre-plasticizer Effect of the Composite Electrode for Lithium Batteries

Abstract

Most studies in the field of rechargeable batteries deal with the search of new structures of active electrode materials or with the optimization of already known composition. These battery active materials are routinely evaluated via the electrochemical performance of their composite electrodes that are prepared with standard formulations and routine processing conditions. The engineering and fundamental issues of the composite electrode are, however, of a growing interest. 1,2 A few studies report that the organization of the conducting agent and binder dispersion and/or morphology within the composite electrode has an influence on the electrode performance and cyclability. 3-14 Less work deals with the relationships between the composite electrode morphology and the processing conditions. 15-21 In our previous work, we obtained a 50% increase of the cycling capacity of Li1.1V3O8-based electrodes after using a PEO binder which had been pre-plasticized by the solvent ethylene carbonate EC + propylene carbonate PC of the liquid electrolyte or the liquid electrolyte itself EC + PC + LiTFSI during the fabrication step of the composite electrode. 11,12 The increased cycled capacity was interpreted qualitatively as a more efficient carbon black CB distribution in the electrode due to the presence of EC + PC molecules at the time of the composite processing. 20 In this study, we generalize the pre-plasticization effect of the binder with EC + PC to another binder of interest for the positive electrode of advanced Li-ion batteries, polymethyl methacrylate PMMA, 22 and we study the origin of such an unexpected phenomenon. To achieve our goal, we couple particle size distribution measurement in different model slurries, optical characterization of various dispersion obtained from these slurries, electronic conductivity measurement of various composite electrodes, and viscosity measurement of various binder solutions. We show that a 50% performance improvement of Li1.1V3O8-based electrodes is achieved with the PMMA/EC+PC combination. We emphasize, however, that the plasticizer content must be adjusted to a very critical optimal quantity to be effective. Our results demonstrate the key role of the solvent to control the characteristics of the solid/liquid dispersion in the slurry used as electrode precursor, in order to obtain a further uniform distribution of both active material and CB grains within the electrode after solvent evaporation and the thinnest binder layers in between the active material and CB grains.