Multiscale Interfacial Characterisation of Transport Properties in Composite Li-Ion NMC532 Electrodes
Authors: Agrawal Anshuman , Badot Jean-Claude , Franger Sylvain , Lestriez Bernard , Dubrunfaut Olivier , Besnard Nicolas , Assaud Loïc 
Published: February 2020

Good ionic and electronic conductivities are required in battery electrodes to facilitate redox reactions and thus efficient charge and discharge of the battery. However, limitations of charge transport properties in state of the art lithium-ion positive composite electrodes greatly restrain the electrode thickness. This is largely attributed to: (i) the multitude of interfaces/contacts that arise with the different materials, and at different scales inside the composite electrode; and (ii) to the degree of percolation and tortuosity of the different phases.

The active material studied, LiNi0.5Mn0.3Co0.2 O2 (NMC532), consists of 1µm size grains (or particles) that agglomerate to form 4µm clusters. The binding polymer PVDF is used with Carbon Black (CB) to impart higher electronic conductivity.

The aim of this project is to study the effect of the electrode morphology and the various interfaces on electronic and ionic limitations. By changing the morphology of the electrode (i.e. composition and porosity) the aim is to minimize the transport limitations in the electrode [1]. In doing so it is hoped to be possible to increase electrode thickness and thus minimize the non-electroactive part in the battery electrodes, reducing cost and increasing efficiency. This research may also help in developing a faster method to optimize new active materials and thus reduce its time to market.

Broadband Dielectric Spectroscopy (BDS) apply AC perturbations, between 20Hz to 18GHz in our case, across a metal coated sample at the end of a coaxial waveguide. Complex permittivity and conductivity (resistivity) are then calculated from admittance (low-frequency) and the coefficient of reflection (high-frequency). The inferred electric polarisations (or susceptibilities) at each interface are additive in nature. It provides information about multiscale charge transfer phenomenon across interfaces. This allows for a multi-hierarchical study of the composite. The interfacial characterisation of resistive and capacitive responses is carried out using Nyquist plots.

Measurements are undertaken using dry nitrogen, from room temperature to -100°C. The conductivity of NMC532 is thermally activated and activation energies are derived using Arrhenius plots. To characterise the NMC532 active material alone, samples with no carbon black were referenced. Electrode metal contact, clusters and particles of NMC532 were characterised for dry samples. Samples were then impregnated with solvent and electrolyte to study their effect on the active material. Both conductivity and interfacial capacitance were seen to increase for the impregnated electrodes.

Industrially fabricated NMC532 composite electrodes with varying CB content and porosity, coated on aluminium, were also characterised. It was seen that the electrochemical performance was better for electrodes with a non-percolated CB-polymer network The results obtained bring to question the necessity to have percolated electrodes when using an active material with relatively high conductivity. This is investigated with the help of electrochemical impedance spectroscopy (1mHz-50kHz) characterising the charge-transfer resistance as well as operando BDS, allowing for interfacial characterisation of electrodes at different levels of charge/discharge.

The research is part of the project ANR Pepite comprising researchers conducting further characterisations such as FIB-SEM X-Ray tomography and PF-NMR to allow for a complete characterisation of the composite electrode and a better understanding on the effect of the electrode architecture on its performance.

Acknowledgement

We are grateful to the ANR for the funding of the Pepite project (ANR-15-CE05-0001).

References

[1] Besnard N., Tran-Van P., Etiemble A., Maire E., Douillard T., Dubrunfaut O., Gautier L., Franger S., Badot J.-C. and Lestriez B.. “Multiscale Morphological and Electrical Characterization of Charge Transport and Charge Transfer Limitations to Power Performance of - Positive Electrode for Li-Ion Batteries.” Jounral of the Electrochemical Society, Charge Transfer Electrons 2 (2018)

[2] Kalid-Ahmed Seid , Jean-Claude Badot ,Cristian Perca , Olivier Dubrunfaut, Patrick Soudan , Dominique Guyomard , and Bernard Lestriez " An In Situ Multiscale Study of Ion and Electron Motion in a Lithium-Ion Battery Composite Electrode." Advanced Energy Materials 5.2 (2015).

[3] Badot, J.-C., Ligneel, É., Dubrunfaut, O., Guyomard, D. and Lestriez, B., “A Multiscale Description of the Electronic Transport within the Hierarchical Architecture of a Composite Electrode for Lithium Batteries.” Adv. Funct. Mater., 19 (2009): 2749–2758.