Gwenaëlle Toussaint

Air Electrodes for Aqueous Lithium Air Batteries

The aqueous rechargeable lithium battery such as the one developed by EDF and its partners, uses an aqueous electrolyte as opposed to the anhydrous lithium air concept which uses an organic electrolyte. Typical electrolytes used are saturated aqueous solutions of lithium hydroxide or lithium chloride. Since the negative electrode (lithium metal) is not stable in aqueous media, it needs to be separated from the positive aqueous compartment by using a watertight, lithium–ion conducting, separator such as a Lisicon ceramic membrane [1]. This separator is therefore the interface between the negative compartment and the aqueous electrolyte. The negative compartment needs to be hermetically sealed from the external environment, that is, both from the air and the aqueous electrolyte. The air electrode provides the other interface, between the aqueous electrolyte and the external environment. It must therefore be open to allow oxygen from the air to access the positive electrode. The air electrode not only needs to be sufficiently porous for the air to penetrate the electrode from outside the cell, but it also needs to act as a barrier to contain the aqueous electrolyte inside the cell, and prevent it from leaking through the electrode. This compromise between water tightness and porosity to air is typically obtained by the addition of a hydrophobic agent such as PTFE.

Ni(OH)2 and NiO Based Composites: Battery Type Electrode Materials for Hybrid Supercapacitor Devices

Nanocomposites of Ni(OH)2 or NiO have successfully been used in electrodes in the last five years, but they have been falsely presented as pseudocapacitive electrodes for electrochemical capacitors and hybrid devices. Indeed, these nickel oxide or hydroxide electrodes are pure battery-type electrodes which store charges through faradaic processes as can be shown by cyclic voltammograms or constant current galvanostatic charge/discharge plots. Despite this misunderstanding, such electrodes can be of interest as positive electrodes in hybrid supercapacitors operating under KOH electrolyte, together with an activated carbon-negative electrode. This study indicates the requirements for the implementation of Ni(OH)2-based electrodes in hybrid designs and the improvements that are necessary in order to increase the energy and power densities of such devices. Mass loading is the key parameter which must be above 10 mg·cm−2 to correctly evaluate the performance of Ni(OH)2 or NiO-based nanocomposite electrodes and provide gravimetric capacity values. With such loadings, rate capability, capacity, cycling ability, energy and power densities can be accurately evaluated. Among the 80 papers analyzed in this study, there are indications that such nanocomposite electrode can successfully improve the performance of standard Ni(OH)2 (+)//6 M KOH//activated carbon (−) hybrid supercapacitor.