Capacitive behavior of Ag doped V2O5 grown by aerosol assisted chemical vapour deposition
Introduction
Vanadium pentoxide (V2O5) has attracted a lot of attention as a lithium intercalation host due to its layered structure and consequently its ability to intercalate ions between the adjacent layers [1]. Electrical energy is stored in the form of a chemical potential during intercalation and released in the form of electrical charge during deintercalation [2]. Hence, V2O5 is a promising cathode for rechargeable lithium ion batteries [3] and electrode material for electrochemical pseudocapacitors [4], [5]. However, the low diffusion coefficient of Li+ (≈10−12 cm−2 s−1) and the poor electronic conductivity (10−2–10−3 S cm−1) result in low specific capacity and poor rate capability [6], [7].
The doping of metal oxides for lithium ion battery applications has been extensively reviewed by Reddy et al. [8]. More specifically for the improvement of the capacitive behaviour of the oxide with respect to its current density, reversibility, ion storage capacity and cyclic stability, several researchers have employed vanadium bronzes of the general stoichiometry MxV2O5 (where M is for example Na, Nb, Ta, K, Cu and Ag). Sakunthala et al. employed a polymer precursor method to generate Nb or Ta doped V2O5 with excellent cycling stability and a discharge capacity of up to 260 mAhg−1 [9].Takeuchi et al. have reviewed the preparation, characterization and reactivity of silver vanadate bronzes (Ag-V2O5) and their application in rechargeable lithium ion batteries [10]. Coustier et al. have investigated the growth of doped V2O5 with a doping ratio M/V (M = Ag and Cu) ranging from 0.01 to 0.5 [11]. The electronic conductivity of doped V2O5 was increased by two to three orders of magnitude compared with the undoped with good reversibility and no capacity fading. In addition, Ag-V2O5 composite has been fabricated by 355 nm pulsed reactive deposition on stainless steel substrates indicating 2–3 orders of magnitude higher electronic conductivities than the pure V2O5 [12]. The effect of silver co-sputtering on the characteristics of amorphous V2O5 films grown by dc reactive sputtering has been studied showing a better cyclic performance than the undoped one [13], [14]. Smyrl and co-workers reported that cyclic capacity and life increased with the addition of metallic elements (Ag and Cu) [15], [16]. The electrical conductivity of the sol-gel doped vanadium oxides was up to three orders of magnitude higher than that of the nominally undoped material, furthermore, the doped films exhibited a dramatic enhancement in the lithium ion diffusion coefficient [7].
In this work, the atmospheric pressure solution-based deposition method aerosol assisted chemical vapour deposition (AACVD) has been chosen for the materials growth because it has certain advantages over the standard atmospheric pressure CVD and physical vapour deposition (PVD) processes [17], [18]. First of all, vapour pressure generation and gas delivery are simpler compared to conventional CVD, which utilizes bubblers. The use of a single source provides good molecular mixing of chemical precursors, which enables the synthesis of multi-component materials with controlled stoichiometry. In addition, the method does not involve expensive and delicate high-vacuum apparatus, thus it is cost effective and compatible with high-volume manufacturing.
The growth of V2O5 by AACVD has previously been reported on glass using both vanadium (III) and (IV) acetylacetonate [19], and on fluorine doped tin dioxide (FTO) coated glass [19]. In this in this paper we explore the influence of the addition of silver on the process and the resultant properties of the grown materials.
Section snippets
Materials growth
Silver-doped AACVD films were grown using a modified CVD system consisting of a tube furnace and a quartz reactor containing a silicon carbide coated graphite susceptor held at temperatures ranging from 350–450 °C. Vanadyl (IV) acetylacetonate (VO(acac)2) and Silver Trifluoroacetate (CF3COOAg) (STREM) were employed as precursors with methanol (Sigma Aldrich) as the solvent. VO(acac)2 (0.1 moldm−3) solutions were made, containing 0, 5, and 15 molar percent silver. The aerosol was generated using a
Structure
The XRD patterns of the annealed samples grown at 350, 375, 400° C and 450° C are given in Fig. 1(a)–(d) respectively. Silver loaded samples grown at 450° C are displayed as Fig. 1(e) and (f) for 5 and 15%. In all scans the prominent peaks at 27.1, 34.3 and 38.1 ° correspond to the Miller indices [110], [101] and [200] of the FTO coated glass substrate [19]. The peak at 45.1° is attributed to the aluminium sample holder. At 350° C the film exhibit a single weak peak at 12.5° corresponding to the
Conclusions
Silver vanadate bronzes with good stability under environmental conditions were successfully fabricated by AACVD at 450 °C for 0, 5 and 15% silver content. The morphology of the coatings changed from rod- to pellet-like structures after the addition of silver indicating a significant effect of the metallic ion. The bronze with 15% silver content presented a specific discharge capacity of 230 mAhg−1, which is higher than the nominally pure V2O5 (22.5 mAhg−1). Additionally, the 15% Ag sample gave
Acknowledgements
The Tyndall authors would like to acknowledge financial support from Science Foundation Ireland under grant 11/PI/1117.
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