The role of carbon black distribution in cathodes for Li ion batteries
Introduction
To increase the poor electronic conductivity of oxide cathodes, a certain amount of electronic conductor, such as carbon black, is usually added to the active particles. Ideally, the particles of electronic conductor should be available at every spot on the surface of active particle in order to allow for simultaneous insertion/deinsertion on the whole surface (Fig. 1), thus maximizing the current density and minimizing the local stress and heating due to inhomogeneous mass and electrical transport.
In practice, it turns out that it is very difficult to control and design the carbon black arrangement in cathode composites. Actually, the problem seems to be underestimated or even ignored. Recently, as the particles size is decreasing to sub-micron dimensions, these problems are becoming even more relevant. For example, we have shown in a previous paper [1] that the conventional technology of cathode preparation, in which the constituents are simply mixed together, yields quite non-uniform distribution of carbon black in the final cathode composite. Consequently, the polarisation is much higher than if the active particles surface is pre-treated to induce a very uniform distribution of carbon black around active particles (we patented such a procedure as a novel coating technology (NCT) [1]). In our case, the pre-treating agent has been gelatin. After pre-treatment, the thin film of gelatin around each active particle serves as a sort of glue which, in dispersions, catches smaller particles, such as carbon black. At the end of the procedure, carbon black is uniformly deposited all over the surface of each active particle. Hence, the arrangement of carbon black becomes close to the ideal one.
In the present paper, we investigate the impact of carbon black distribution on cathode kinetics using three different active materials: LiMn2O2-spinel, LiCoO2, and LiFePO4. These materials are not only different in composition but also in particle geometry, size and surface morphology. We expected that the diversity of active materials would help us evaluate (a) if NCT is generally applicable to various cathode active materials and (b) if a more uniform carbon black distribution always—i.e. regardless of the type of active material—leads to a faster electrochemical kinetics.
Section snippets
Experimental
- (a)
Conventional procedure of cathode preparation:
The active materials were Merck SP30 LiMn2O4 (average particle size 30 μm), Merck SC20 LiCoO2 (average particle size 7 μm), and LiFePO4 (particle size 0.5–1.0 μm) delivered kindly by M. Armand, J. Thomas and N. Ravet. Each active material was mixed with a Teflon dispersion and carbon black (Printex XE2). The slurry was then deposited on an aluminium substrate to obtain an electrode of a thickness of 80 μm (5–7 mg of active material).
- (b)
NCT procedure:
The
Results and discussion
SEM micrographs show that the novel coating technology (NCT) gives much more uniform distribution of carbon black in the cathode material when compared to the distribution obtained using the conventional mixing procedure (Fig. 1). This feature was observed with all three types of active materials, indicating that NCT is an effective procedure, which quite generally leads to a more uniform carbon black distribution, i.e. regardless of the type of active material used. A more uniform carbon black
Conclusions
Using three different active materials, we have shown that the distribution of carbon black in cathode composites is a crucial parameter determining the cathode performance. A simple model has shown that in order to minimize the polarisation due to insertion, carbon black (or alternative electron conductor) must be uniformly distributed around each active particle. In terms of cathode performance, the lower polarisation leads to a higher reversible capacity at all C-rates and at all cycle
Acknowledgements
This research was sponsored by NATO’s Scientific Affairs Division in the framework of the Science for Peace Programme. The financial support from the Ministry of Science and Technology of Slovenia is also fully acknowledged. The authors thank to Profs. M. Armand and J. Thomas and Dr. N. Ravet for providing LiFePO4.
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