Electrostatic Layer-by-Layer Deposition and Electrochemical Characterization of Thin Films Composed of MnO 2 Nanoparticles in a Room-Temperature Ionic Liquid Ta ˆnia M. Benedetti, ² Fernanda F. C. Bazito, ‡ Eduardo A. Ponzio, ²,§ and Roberto M Torresi* ,² Instituto de Quı ´mica, UniVersidade de Sa ˜ o Paulo, CP 26077, 05513-970 Sa ˜ o Paulo, SP, Brazil, and Departamento de Cie ˆ ncias Exatas e da Terra, UniVersidade Federal de Sa ˜ o Paulo, Campus Diadema, 09972-270, Sa ˜ o Paulo, SP, Brazil ReceiVed August 1, 2007. In Final Form: December 14, 2007 Thin films of MnO 2 nanoparticles were grown using the layer-by-layer method with poly(diallyldimetylammonium) as the intercalated layer. The film growth was followed by UV-vis, electrochemical quartz crystal microbalance (EQCM), and atomic force microscopy. Linear growth due to electrostatic immobilization of layers was observed up to 30 bilayers, but electrical connectivity was maintained only for 12 MnO 2 /PPDA bilayers. The electrochemical characterization of this film in 1-butyl-2,3-dimethyl-imidazolium (BMMI) bis(trifluoromethanesulfonyl)imide (TFSI) (BMMITFSI) with and without addition of a lithium salt indicated a higher electrochemical response of the nanostructured electrode in the lithium-containing electrolyte. On the basis of EQCM experiments, it was possible to confirm that the charge compensation process is achieved mainly by the TFSI anion at short times (<2 s) and by BMMI and lithium cations at longer times. The fact that large ions like TFSI and BMMI participate in the electroneutrality is attributed to the redox reaction that occurs at the superficial sites and to the high concentration of these species compared to that of lithium cations. Introduction Rechargeable Li-ion batteries have acquired a high commercial importance in recent years, being incorporated mainly in small mobile equipment such as cellular phones. Usually, the cathode materials for Li-ion batteries are transition metal oxides due to the high electrochemical potentials reached during reversible lithium insertion/expulsion. 1-3 Among these compounds, con- siderable research has been focused on manganese oxides, due to their low cost, limited environmental impact, and excellent voltage profile attributes. Despite these attractive properties, in comparison with other layered oxides (LiCoO 2 and LiNiO 2 ), the most used cathodic material based on manganese, LiMn 2 O 4 , presents problems related to capacity fading and limited cyclability when the potential upper limit reaches 4 V. 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Langmuir 2000, 16, 8850. 3602 Langmuir 2008, 24, 3602-3610 10.1021/la702347x CCC: $40.75 © 2008 American Chemical Society Published on Web 02/22/2008