Electrochimica Acta 66 (2012) 306–312 Contents lists available at SciVerse ScienceDirect Electrochimica Acta j ourna l ho me pag e: www.elsevier.com/locate/electacta Solution synthesis of nanometric layered cobalt oxides for electrochemical applications Xavier Pétrissans a , Angélique Bétard a , Domitille Giaume a , Philippe Barboux a,∗ , Bruce Dunn b , Lorette Sicard c , Jean-Yves Piquemal c a Chimie de la Matière Condensée de Paris, CNRS UMR 7574, Chimie-ParisTech, 11 rue Pierre et Marie Curie, 75005 Paris, France b Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, CA 90095, USA c Laboratoire ITODYS, Université Paris-Diderot, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France a r t i c l e i n f o Article history: Received 12 December 2011 Received in revised form 24 January 2012 Accepted 28 January 2012 Available online 5 February 2012 Keywords: NaxCoO2 nanoparticles Colloidal oxide Ionic exchange Pseudocapacitor Cyclic voltammetry a b s t r a c t Dispersed Na 0.6 CoO 2 ·yH 2 O and Li 0.5 CoO 2 powders have been obtained at room temperature by rapid precipitation in aqueous solutions of LiOH or NaOH in the presence of a strong oxidizer. The precipitates are well crystallized and consist of nanoscale platelets with high specific area (above 100 m 2 /g). The Li 0.5 CoO 2 phase is stable in aqueous electrolytes whereas the Na 0.6 CoO 2 ·yH 2 O rapidly converts to CoOOH in neutral electrolytes or pure water. It also transforms to anhydrous Na 0.6 CoO 2 upon drying at moderate temperatures. Electrochemical studies show that at slow sweep rates the Na 0.6 CoO 2 ·yH 2 O can store large amounts of charge in 10 M NaOH from a combination of both faradic and capacitive reactions. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction The large scale application of batteries to electrical vehicles requires an improvement in their power density in order to reach an acceptable driving range. Moreover, the energy storage of these systems decreases considerably when the power demand increases [1] while safety problems arise due to heat dissipation and out- of-equilibrium electrochemical side reactions [2]. To address the problem of peak power management, batteries can be coupled with complementary devices such as electrochemical capacitors, also known as supercapacitors, that provide high power but for short durations because of low energy density. These devices are based on the development of electrochemical double-layers that store charge at the solid-electrolyte interface and do not involve chemical reactions as in the case of batteries. Electrochemical capacitors pos- sess a number of attractive features compared to batteries including higher power, shorter charging times and much longer cycle life. One method of increasing the energy density of electrochemical capacitors is to have reversible redox processes at the surface of high surface area oxide films or nanoparticles [3,4]. The resulting pseudocapacitance increases the level of energy storage by an order of magnitude or more, with the best properties exhibited by ∗ Corresponding author. Tel.: +33 1 53 73 79 25. E-mail address: philippe-barboux@chimie-paristech.fr (P. Barboux). hydrated RuO 2 oxides [5,6]. The cost of RuO 2 is prohibitive and for that reason, a number of less expensive candidates such as tran- sition metal oxides or hydroxides have been investigated (MnO 2 [7–9], Ni(OH) 2 [10], Co 3 O 4 [11,12], CoOOH [13], FeOOH [14], CuO [15], V 2 O 5 [16–18], TiO 2 [19,20], Zn 2 CoO 4 [21], LiCoO 2 [22], etc.). Manganese based oxides have probably attracted the largest inter- est due to their low cost, and low toxicity. Lamellar MnO 2 structures such as birnessite have been the subject of many studies due to their ability to reversibly intercalate monovalent cations in aque- ous electrolytes [8]. To achieve significant capacity (on the order of hundreds of F/g or C/g in capacity or charge storage respectively), it is necessary to optimize the synthesis, chemistry and microstruc- ture so that high surface area can be combined with favorable diffusion pathways for both ions and electrons to enable redox reactions to occur at the interface [7,19,23].More generally lamel- lar structures can achieve good ionic mobility and good exchange properties. Lamellar MnO 2 structures such as birnessite have been the subject of many studies due to their high ability to reversibly intercalate monovalent cations in aqueous electrolytes [8]. Layered hydroxides also demonstrated large specific capacitance and good cycle reversibility [24,25]. In this paper, we particularly focus on layered cobalt oxides, especially on the Na 0.6 CoO 2 phase [26] and on its hydrated polymorphs [27]. The good electronic conductivity of this material associated to its fast ion exchange properties make it a good candidate as a model material to study the effect of ionic mobility on the distribution of pseudocapacity between double 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.01.104