Abstract
The electrochemical performance of composite electrode materials suffers strong kinetic limitations due to non-optimized electronic and ionic transport properties. Electrode materials consist of complex hierarchical architectures in which interfaces create junction resistances and capacitance causing electronic transfer limitations. Moreover, the tortuosity of the pore network within the electrodes gives rise to ionic transfer limitations. Ionic and electronic dynamics occur at widely different time scales: long times (i.e. low frequencies) for slow ionic motions and short times (i.e. high frequencies) for fast electronic motions. Electrochemical Impedance Spectroscopy technique, which is appropriated to determine ionic diffusion, cannot characterize electronic transfers within electrode materials. This requires thus the simultaneous measurement of ionic and electronic conductivities. The broadband dielectric spectroscopy (BDS) over a frequency range from 40 Hz to 10 GHz is a powerful technique to discriminate ionic and electronic motions at different scales of the materials.
Previous BDS studies have distinguished different types of interfaces inducing electric polarizations and disturbing charge transport at all scales of the materials [1-5]. Strong interactions between ions (Li+, PF6- ) of the electrolyte (LP30) and electrons were evidenced in active material (LiNi1/3Mn1/3Co1/3O2) and carbon black (CB) by conductivity measurements.
Comprehensive characterization of the multiscale electronic transport in LiNi0.5Mn0.3Co0.2O2 (hereafter called NMC532) by BDS is here detailed. This study is assumed without carbon-black addition to only understand the interactions of the electrolyte with mixtures based on NMC532 and PVdF. A systematic approach is followed, which consists of studying the materials at different temperatures, starting by a study of NMC532 with polyvinylidene fluoride (i.e. PVdF) first in the dry state, then filled with the solvent EC-DMC alone and finally the LP30 electrolyte (containing ions).
The PVdF content impacts the quality of the interfaces within the samples hampering the electronic transfer in NMC. When EC: DMC fills the pores of the mixture, it is shown that the dipoles of ethylene carbonate adsorbed on the NMC532 modify its electronic properties.
Moreover, strong interactions occur between the liquid electrolyte LP30 (i.e. dipoles and ions) and the active material, as well as between the electrolyte and the carbon black. Space charges, created on the surface of electronically conductive NMC532 generate electrical polarizations whose frequency responses are in the range of radio frequencies and microwaves. The presence of the electrolyte changes the intensities and dynamics of space charge polarizations due to ion-electron and dipole-electron interactions [3,4].
Acknowledgments
Financial funding from the ANR program no. ANR-15-CE05-0001-01 is acknowledged
References
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