J. Phys. Chem. C 2007, 111, 14925-14931
14925
Pseudocapacitive Contributions to Electrochemical Energy Storage in TiO2 (Anatase) Nanoparticles John Wang, Julien Polleux, James Lim, and Bruce Dunn* Department of Materials Science and Engineering, UniVersity of California, Los Angeles, Los Angeles, California 90095 ReceiVed: June 8, 2007; In Final Form: July 31, 2007
The advantages in using nanostructured materials for electrochemical energy storage have largely focused on the benefits associated with short path lengths. In this paper, we consider another contribution, that of the capacitive effects, which become increasingly important at nanoscale dimensions. Nanocrystalline TiO2 (anatase) was studied over a dimensional regime where both capacitive and lithium intercalation processes contribute to the total stored charge. An analysis of the voltammetric sweep data was used to distinguish between the amount of charge stored by these two processes. At particle sizes below 10 nm, capacitive contributions became increasingly important, leading to greater amounts of total stored charge (gravimetrically normalized) with decreasing TiO2 particle size. The area normalized capacitance was determined to be well above 100 µF/cm2, confirming that the capacitive contribution was pseudocapacitive in nature. Moreover, reducing the particle size to the nanoscale regime led to faster charge/discharge rates because the diffusion-controlled lithium ion intercalation process was replaced by faradaic reactions which occur at the surface of the material. The charge storage and kinetics benefits derived from using nanoscale metal oxides provide an interesting direction for the design of materials that offer both power density and energy density.
I. Introduction In recent years, substantial efforts have been made toward the development of nanostructured materials for electrochemical energy storage. Nanostructured materials offer enhanced electrochemical performance because of the ability to access both bulk and surface properties.1,2 The advantages in using nanostructured materials are especially well documented for lithium ion batteries as high energy density, fast charge/discharge rates, and excellent cycling stability have been reported for both anode and cathode material systems.3 These remarkable results are generally attributed to the short path lengths for both electronic transport and Li ion transport, as well as the large active surface area of the nanomaterials.1 Another significant feature which occurs as electrochemically active materials approach nanoscale dimensions is that the charge storage of lithium ions from faradaic processes occurring at the surface of the material, referred to as the pseudocapacitive effect, becomes increasingly important.4,5 Because of its fast faradaic reactions at the surface of the material, the development of pseudocapacitance is of interest for high power density applications. This charge storage mechanism is different from that which occurs in lithium ion batteries where lithium ion intercalation reactions lead to high energy density. It would seem, therefore, that nanostructured materials offer a unique opportunity to tailor power density and energy density, enabling an intermediate state between Li ion batteries and supercapacitors to be realized.4 Nanocrystalline TiO2 (anatase) represents a model system in which to investigate materials that combine both pseudocapacitance and lithium intercalation. The material is considered to be a reasonably attractive lithium insertion compound for secondary lithium ion batteries because of its charge storage capacity and cycle life.6 A few research groups have begun to * Corresponding author. E-mail:
[email protected].
explore the fundamental question of how particle size and morphology influence the electrochemical properties of nanocrystalline TiO2.7-11 Decreasing the crystallite size to nanoscale dimensions leads to improved lithium capacity and extends the LixTiO2 solid-solution region.9,10 The kinetics of lithium ion intercalation in TiO2 are also enhanced with nanostructured materials.11,12 It is interesting to note that the capacitive effects arising from nanostructured TiO2 have received much less attention. Lindstrom et al.13 investigated the capacitive contribution associated with lithium ion insertion in TiO2 films containing nanodimensional (4 nm) pores. An important part of this work was to separate the capacitive behavior from the diffusioncontrolled intercalation behavior by fitting the voltammetric currents at various sweep rates to appropriate power law relationships. The large value for the area normalized capacitance (90-120 µF/cm2) reported in this study was attributed to the adsorption of Li ions on the surface of mesoporous TiO2 films. In another study, Sudant et al.9 quantified the capacitive contribution in nanocrystalline TiO2 films by comparing the total concentration of stored lithium ions measured electrochemically with the lithium content in the solid determined by a chemical titration method. TiO2 nanotubes also have pseudocapacitive contributions to the electrochemical lithium storage, with the relative amount dependent upon phase and morphology.14 Thus, while capacitive behavior has been observed with different forms of nanostructured TiO2, the lithium ion storage mechanism at the surface is, at present, not fully understood. Moreover, there is little insight concerning the nanoparticle dimension range where pseudocapacitive behavior begins to become important in comparison to intercalation processes. In this paper, we have used a detailed voltammetric analysis to quantify the dependence of the pseudocapacitance on the size of nanocrystalline TiO2. A nonhydrolytic sol-gel route was used to synthesize phase-pure TiO2 (anatase) with particle sizes
