Pressure Effect on the Electrochemical Performance of Composite Electrodes

Electrochemical capacitors store charges in an electric double layer set up by ions at the interface between a high-surface area material and a liquid electrolyte [1]. Many works have focused on the high surface exposed to the electrolyte in relation with the capacity. However, the best figure of merit requires the lowest electrical resistance to be combined with the largest capacity. And this is a real issue for devices based on oxides where the conductivity remains low because it is limited by intergranular transport.

 For instance, cyclic voltammetry curves of supercapacitors should have a rectangular shape. However, for real systems, this rectangular shape is hardly observed, and a delay to the steady-state regime appears. By analogy with an equivalent electrical RC circuit, this delay can be attributed to a characteristic time τ. This is the consequence of the presence of the macroscopic resistance in the circuit which depends on ion diffusion into the material and on electronic percolation in the system.

This work focuses on the analysis of the source of such resistances by electrochemical measurements. Samples were prepared under different applied pressures to follow the effect of powder packing, grain connectivity and grain boundary surface. Focus was given to the nanometric lamellar ionic conductor M_xCoO₂ with M = H, Li, Na [2], as for specific x values, this material should also be electronically conductive [3], and mixtures with carbon were also investigated.

Upon applying a load up to 25 MPa, the compactness of the electrodes irreversibly increases from 20% to 60%. The conductivity increases by two orders of magnitude but all the oxides studied in this work remain with a very high resistivity as compared to carbon. Then, the electronic and ionic conductivity has been investigated by electrochemical impedance spectroscopy as a function of the powder packing and the content of carbon additive. The addition of carbon is characteristic of the percolation of a high conducting system in a low conductivity material. The total resistance of mixtures of nanometric Na_xCoO₂and carbon at low frequencies drastically decreases with application of a moderate pressure. Carbon also acts as a plasticizer.

Complete symmetrical capacitors have been made in swagelock cells and studied under pressure. The cycles have been recorded at a moderate rate (5mV/s), which is widely used to characterize supercapacitor electrode. Again, there is a strong effect of the pressure on the electrochemical behavior. Before applying a pressure, the cycle is closed and characteristic of a high resistance in series with a capacitor. Upon pressure of 20MPa, the cycle opens, and appears more characteristic of a true capacitor. This behavior is still clearly present after pressure release, and the link with the resistivity behavior depending on the pressure can be made (Figure 1). Impedance spectroscopy shows a decrease of the transfer impedance associated to an increase of the capacity.

Thus, the enhanced connectivity between grains allows a lower series resistance and the charging of the capacity within a shorter time. In the range of pressures used in this work there is no collapse of the porosity and the ionic conductivity of the electrolyte is nearly constant.

Based on this study, different electrodes formulations have been investigated. The best results have been obtained with direct casting of a slurry (active material, carbon and binder) onto an aluminum foil which is further pressed and heated.

As a conclusion, optimizing both electronic and ionic conductivity of an oxide system are the key elements to maximize the total measured capacity of thick electrodes. Moreover special attention on the electrode formulation drastically increases the measured capacity.

[1] P. Simon, Y. Gogotsi, Nature Materials7 (2008) 845

 [2] X. Pétrissans, A. Bétard, D. Giaume, P. Barboux, B. Dunn,  L. Sicard, J.-Y. Piquemal, Electrochimica Acta66 (2012) 306.

[3] F. Tronel, L. Guerlou-Demourgues, M. Basterreix, C. Delmas, Journal of Power Sources 158 (2006) 722.