Multiscale Characterization By Dielectric Spectroscopy of Ionic and Electronic Transfers in Composite Electrodes for Lithium Batteries
Authors: Agrawal Anshuman , Badot Jean-Claude , Dubrunfaut Olivier , Lestriez Bernard 
Published: May 2020

Determine electron and ion transfers in composite porous electrodes is essential to improve their electrochemical performance. When mounting the battery, the composite electrode is soaked with a liquid electrolyte (EL) that completely fills its pores. An electrode has a double hierarchical structure: the first consists of agglomerates (secondary particles) of AM (primary) particles and the second of agglomerates of carbon black/binder (CB-B) mixture. From the nanometric level to the macroscopic level, this double structure leads to numerous interfaces: AM / AM, CB / CB, CB / AM, EL / AM, EL / CB, electrode / current collector.
The charge transport in composite electrodes of lithium batteries is generally explained by the percolation and tortuosity of the carbon black particle network for the electrons on the one hand, and the pore network for the ions on the other hand. In return, the contributions of the various interfaces within the composite electrodes have been little studied. Indeed, this requires the simultaneous measurement of ionic and electronic conductivities.
The development of an instrumentation taking into account both the constraints imposed by the nature of the samples and the dynamics of mobile species (ions, electrons) is necessary. In-situ and ex-situ dielectric spectroscopy achieves this goal over a wide frequency range from 40 Hz to 10 GHz. Many factors and parameters affect the electrical properties [1-5].
The nature and quality of the interfaces, which depend on morphologies of the different agglomerates and their distribution can strongly limit electronic and ionic transfers within composite electrodes. Contact resistance at interface between current collector and the electrode can partially hamper charge transfers. Moreover, strong interactions occur between the liquid electrolyte (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 materials (active material and carbon black), generate electrical polarizations whose frequency responses are in the range of radio frequencies and microwaves. The presence of an electrolyte influences the intensities and dynamics of space charge polarizations due to ion-electron and dipole-electron interactions [3,4].
In this presentation, recent discoveries are reported on influences of different interfaces on electronic and ionic conductions in composite electrodes composed of LiNi0.5Mn0.3Co0.2O2, Poly(vinylidene) fluoride and carbon black with an aluminium current collector.

Acknowledgments

Financial funding from the ANR program no. ANR-15-CE05-0001-01 is acknowledged.

References

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