Nanostructured Li Ion Insertion Electrodes. 2. Tin Dioxide Nanocrystalline Layers and Discussion on “Nanoscale Effect” P. R. Bueno,* E. R. Leite, T. R. Giraldi, L. O. S. Bulho jes, and E. Longo Interdisciplinary Laboratory of Electrochemistry and Ceramics, Department of Chemistry, Federal UniVersity of Sa ˜ o Carlos, C. Postal 676, 13565-905 Sa ˜ o Carlos, SP, Brazil ReceiVed: February 27, 2003; In Final Form: June 6, 2003 Relaxation processes occurring during electroinsertion into pure SnO 2 and electrochromic SnO 2 /Sb were interpreted on the basis of frequency-dependent response models. Within the framework of the classic theory of porous electrodes, the results indicated that, in the case of nanosized particle-based electrodes, the overall kinetic aspects of the insertion process can be controlled by the transport of ionic and electronic species in the liquid and solid phases, respectively. Therefore, if both the electronic and ionic transport are fast in both the solid and liquid phases, or if the state-of-charge is high, a relaxation process corresponding to the insertion of Li + in specific sites inside the nanosized particles is clearly identifiable, as foreseen by the model discussed in part 1. As a result, in the SnO 2 /Sb samples, we have successfully separated the frequency of slow charging of transport effects due to the nonfaradaic capacitance-related process from that of faradaic related capacitances, because these two processes do not overlap and can, in such specific situations, be separated in complex capacitance diagrams. Moreover, our interpretation of the results indicates that the participation of a large amount of inserted charge in the Sb-doped sample is possible due to the “nanoscale factor”, which, allied to the high electronic conductivity in the solid phase, causes rapid charging of the faradaic capacitance-related process. In our interpretation, the faradaic capacitance related process is linked to the insertion of Li + into the solid-state phase of nanosized particles and can be interpreted as an ion immobilization or trapping process, as discussed earlier in part 1. I. Introduction Nanocrystalline metal oxide electrodes have been incorporated in electrochromic windows 1 and in ion-insertion batteries 2 with significant technological and commercial potentiality. During their operation, electrochromic windows and ion-insertion batteries undergo a common event originating from the insertion of Li + ions into the solid-state structure of the host materials, for example, metal oxide electrodes. Despite the geometrical porous effect, this event can be evaluated by frequency- dependent techniques. Nanoscale range materials have been widely studied for application in energy storage devices due to their distinctive properties, which originate from the presence of large numbers of interfaces, a characteristic that can be exploited to design more efficient devices, including those used for conversion and energy storage. These features allow for several technological applications, mainly in semiconductors and ceramics, depending on the features of the interfaces. Despite these important technological applications, the complex geometry of the elec- trodes involved in such devices renders any modeling of the transport of Li + ions difficult in terms of nanostructured insertion electrodes. This complex situation has only recently begun to be exploited from every standpoint to offer a predictable way to design a wide range of new Li + ion insertion electrodes, which might lead to improvements, particularly in the rate capabilities of the electrodes used in communication and remote sensing device batteries. 3 Still in regard to the use of nanoscale materials in the design of electrodes for Li + insertion, it is important to mention specifically the effects of the nanoporous geometry of the electrodes, to which the classic theory of porous electrodes can be applied, as stated in our previous article, Part 1. In an approach based on a situation of porous geometry, the nano- structured material and nanoporous geometry of lithium ion electrodes can dramatically improve the rate capabilities of thin- film dense materials because the distance that Li + diffuses in nanostructured electrodes is restricted to the radius of the host material’s nanocrystals or to the fibers in nanofiber-based electrodes. 4-6 The prevailing idea regarding the kinetic reaction of elec- troinsertion, from the standpoint of Li + insertion into the solid state, is that the transport of mobile guests inside the host lattice is the predominant step, with the guestssusually Li + ionss moving by ordinary diffusion under semi-infinite conditions. Thus, diffusion is considered the current- and power-limiting step in Li ion insertion electrodes. In the present paper, we analyze two distinct SnO 2 layers having different nanocrystalline structures and electronic con- ductivities with the specific purpose of evaluating the influence of these features on the Li + charge kinetics, as was stated in part 1 of this article. The charge modes were analyzed on the basis of impedance and complex capacitance responses. Our interpretation of the results indicates that the average size of nanocrystalline particles improves the overall kinetics, because Li + diffusion occurs rapidly through the bulk of SnO 2 -Sb nanocrystal-based electrodes whose particle sizes are smaller than those of the pure SnO 2 larger size nanoparticle-based * To whom correspondence should be addressed. E-mail: paulo@ iris.ufscar.br. 8878 J. Phys. Chem. B 2003, 107, 8878-8883 10.1021/jp034512m CCC: $25.00 © 2003 American Chemical Society Published on Web 08/06/2003