Charge-Transfer Perturbations Due to Interfaces on Electronic and Ionic Conductions within Composite Electrodes for Li-Ion Batteries

Abstract

The understanding of electron and ion transfers in composite porous electrodes is essential to improve their electrochemical performance. Composite porous electrodes are generally made of an active material (AM), a carbon black conducting additive (CB) and a polymeric binder (B). At battery assembly, it is infiltrated by a liquid electrolyte (EL). An electrode has a double hierarchical structure: the first one consists of agglomerates of particles of AM and the second is of agglomerates of CB - B mixture. From the nanometric to the macroscopic level, this double structure is therefore a hierarchy of interfaces: AM/AM, CB/CB, CB/AM, EL/AM, EL/CB, electrode/current collector. The charge transports in composite electrodes are generally explained by considering the percolation and the tortuosity of the CB particles network for electrons on one hand, and of the pores network for ions on the other hand. The contributions of the different interfaces within composite electrodes are however scarcely studied. Indeed, this requires the simultaneous measurement of ionic and electronic conductivities. To do so, the development of an instrumentation that takes into account both the constraints imposed by the nature of the samples and the mobilities of mobile species (ions, electrons) is required. The in- and ex-situ dielectric spectroscopy enables this objective to be fulfilled over a wide frequency range from 40 Hz to 10 GHz between 200 and 300 K [1-5]. The nature and the quality of the interfaces depend on the shape and the dimensionality of the particles of the active material (for example, platelets, cuboids, etc.), which can strongly influence electronic transfers within the composite electrode [5]. Strong interactions between the liquid electrolyte and the active material, and between the electrolyte and the carbon black have been highlighted [3,4]. The binder (B) also affects the properties of these interfaces. Space charges, which are created on the surface of electronically conductive materials (AM and CB), generate electrical polarizations whose frequency responses are located in the range of radio frequencies and microwaves. The presence of an electrolyte modifies the intensities and dynamics of space charge polarizations because of ion-electron and dipole-electron Coulomb interactions [3,4]. On the other hand, ionic double-layer capacitances are created at the solid/liquid interfaces on the electrolyte side. The latter also generate electric polarizations at lower frequencies than those of electronic space charges because of the lower mobility of the ions. These different interactions modify the electronic mobility of the active material and the carbon black as well as the diffusion of the ions (cations and anions) of the confined electrolyte into the porous network of the electrode [4]. In this presentation, we will report our recent discoveries of the perturbations due to interfaces on electronic and ionic conductions within composite electrodes for Li-Ion batteries.