Composite materials consisting of several interpenetrated and percolating phases are widely used as electrodes for a variety of electrochemical cells. Such electrodes are generally mixed ionic−electronic conductors, in which ions and electrons are confined to transport within the specific conducting network. In particular, for electrochemical storage and energy transformation devices, the transport properties play critical roles in determining their performance, which is pivotal to improve them for service as power sources. To reach this goal, an increased fundamental understanding of the factors that play on the transport properties within such composite materials is still required.

The transport of the electrons and ions through Li-ion battery electrode and their transfer to the active matter depend on the electrode architecture (nano- and microstructure) through the interfacial areas, materials connectivity, and the transport length scales. Novel opportunity for progressing in this direction and exploring relationships between electrode architecture, charge transport and electrochemical performance is implementation of X-ray computed tomography (XRCT) and focused ion beam in combination with scanning electron microscopy (FIB-SEM) techniques to characterize and quantify the 3D architecture of composite electrodes. In particular, tortuosity, a critical parameter with respect to ionic transport, has been exhaustively studied. Comparatively, factors that play on the electrons transport and wiring to the active mass have been much less considered. Moreover, attempts to correlate quantified 3D geometries with measured electrochemical power performance or electrical transport properties of same real electrodes remain rare.

We intend to fill this gap in this presentation. We have undertaken exhaustive characterizations of 3D geometries of LiNi_1/3Mn_1/3Co_1/3O₂ (NMC), LiFePO₄ (LFP), and NMC/LFP blended electrodes and rationally interpreted their measured electrical properties and electrochemical performance.

A combination of X-Ray tomography and FIB-SEM tomography are used for a multiscale analysis of electrodes 3D geometries, in particular the distribution of the conductive additive and the binder that perform as electronic wires, the tortuous pores distribution, which after electrolyte impregnation perform as ionic wires, and the inter-connectivity between the active matter phase and the electronic and the ionic wires [1].

The electrical properties (permittivity and conductivity) are measured from low (a few Hz) to microwave (a few GHz) frequencies by the broadband dielectric spectroscopy technique [2]. This one is very sensitive to the different scales of the electrode architecture involved in electronic transport, from interatomic distances to macroscopic sizes, as well as to the morphology at these scales, coarse or fine distribution of the constituents [2,3]. In addition to the geometry of the electrode architecture, the interfaces (adsorption of ions to the surface of active material and carbon nanoparticles), and interactions at such interfaces lead to a reciprocal influence of ionic and electronic transfers at the different scales of the electrode [4].

Finally, discharge rate performance is analyzed by simple, yet efficient methods. This approach allows discriminating between the contributions of the electronic and ionic wiring networks as the performance limiting factors, depending on the electrode geometry and discharge rate. [5]

Acknowledgements

Financial funding from the ANR Program No. ANR-15-CE05-0001-01 (PEPITE) is acknowledged. We also thank the CLYM (Centre Lyonnais de Microscopie) supported by the CNRS, the “Grand Lyon” and the Rhône-Alpes Region for use of the Zeiss NVision40 FIB/SEM. The IMD (Institut de la Mobilité Durable – Sustainable Mobility Institute) is also acknowledged for financial support.

References

[1] A. Etiemble et al., “Multiscale morphological characterization of process induced heterogeneities in blended positive electrodes for lithium-ion batteries”, J. Mater. Sci., 52, 2017, 3576–3596

[2] K.A. Seid et al., “Multiscale electronic transport mechanism and true conductivities in amorphous carbon-LiFePO₄ nanocomposites”, J. Mater. Chem., 2012, 22, 2641-2649.

[3] K. A. Seid et al., “Multiscale electronic transport in Li_1+xNi_1/3-uCo_1/3-vMn_1/3-wO₂: a broadband dielectric study from 40 Hz to 10 GHz”, Phys. Chem. Chem. Phys., 2013, 15, 19790-19798.

[4] E. Panabière et al., “Electronic and ionic dynamics coupled at solid-liquid electrolyte interfaces in carbon black-polyvinylidene fluoride-g alumina porous composites”, J. Phys. Chem. C, 2017, 121, 8364–8377.

[5] N. Besnard et al., “Multiscale morphological and electrical characterization of charge transport limitations to the power performance of positive electrode blends for lithium-ion batteries”, Adv. Energy Mater, 2017, 1602239.