Underpotential Deposition of Lithium on Platinum Single Crystal

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2007, 111, 9016-9018 Published on Web 06/05/2007

Underpotential Deposition of Lithium on Platinum Single Crystal Electrodes in Tetrahydrofuran Christopher A. Paddon and Richard G. Compton* Physical and Theoretical Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford, OX1 3QZ, United Kingdom ReceiVed: April 30, 2007; In Final Form: May 22, 2007

We present the first electrochemical observation/studies of nucleation and underpotential deposition (upd) of lithium in tetrahydrofuran (THF) on platinum single crystals. Electrochemical data using platinum (100) [Ep,c ) -3.3 V, and Ep,a ) -2.6 V], (110) [Ep,c ) -3.0 V, and Ep,a ) -2.2 V], and (111) [Ep,c ) -1.6 V, and Ep,a ) -1.3 V] single crystal electrodes are reported against a standard reference electrode, Fc/Fc+PF6- for use in THF. The study has shown that lithium upd occurs at different characteristic potentials on well-defined crystal planes of platinum electrodes. These studies have significance for the advancement of industrial applications of lithium, for example, in battery/reactor technologies and in organic synthesis.

1. Introduction The electrochemical deposition and dissolution of lithium in aprotic solvents has attracted much attention because of the industrial significance with regards to lithium secondary batteries,1 lithium reactor technologies2,3 and as a reducing agent in organic synthesis.4-6 Previous studies of the (underpotential) deposition (upd)7-9 and dissolution of lithium salts in aprotic solvents10 have been focused upon surface films generated on aluminum,11 copper,12,13 gold,2,8,14 mercury,15 nickel,16-20 and silver21 electrodes and in solvents such as propylene carbonate (PC),22-25 dimethoxyethane, tetrahydrofuran (THF),26 2-MeTHF, γ-butyrolactone, methyl formate, 1,3-dioxolane,27 dimethyl sulfoxide,28 and their mixtures.29-34 In addition, Scherson et al. has studied the upd of lithium on polycrystalline gold from a LiClO4/poly(ethylene oxide) solid polymer electrolyte in ultrahigh vacuum.9 However, no investigations into the deposition/ dissolution of lithium upon single crystal electrodes, specifically platinum, in aprotic solvents have been reported. Herein, we report the first results of a preliminary study into the electrodeposition/dissolution of lithium upon platinum single crystal electrodes in THF. We will show that platinum single crystal plane electrodes ((100), (110), and (111))35 display characteristic upd/ups peaks prior to bulk electrochemical deposition and multilayer formation of lithium layers upon the electrode surface. Analyses are performed in THF using LiAsF6 in 0.1 M tetra-n-butylammonium perchlorate (TBAP). Comparisons are made with a platinum polycrystalline electrode that displays a range of upd/ups peaks corresponding to the different crystal planes. To the best of our knowledge this is first electrochemical observation of upd of lithium in THF at different platinum single crystal planes. In addition, the electrode potentials reported are obtained using a standard reference electrode (Fc/Fc+PF6-) for use in tetrahydrofuran (THF). These * To whom correspondence should be addressed: E-mail: [email protected]. Tel: +44 (0) 1865 275 413. Fax: +44 (0) 1865 275 410.

10.1021/jp073304h CCC: $37.00

results provide a basis for the understanding of the use of the Li/Li+ couple in both synthetic chemistry and in industrial electrochemical applications and also further our investigations into the kinetics and thermodynamics of the Li/Li+ couple in aprotic solvents and at low temperatures.5 The physical and chemical properties of an electrode surface will significantly affect the electrochemical properties of the lithium electrode. For example, nonuniformity of the surface film such as thickness distribution induces nonuniform current distribution on a lithium electrode and is reported to cause the growth of dendrites.11,16,22,30,34 “Mossy” or “dendritic” lithium deposition during charging reportedly creates “dead lithium”, which can cause problems in the development of industrial lithium batteries and in related applications. A “breakdown and repair mechanism” has been proposed to explain the behavior of the surface film during the dissolution of lithium.36 In particular, these studies have used surface probe techniques such as scanning tunneling microscopy (STM) and atomic force microscopy as a means to characterize the films formed.12,14,16,22,23,29,37 Aurbach et al. has suggested that lithium deposition preferentially proceeds at a specific site upon an electrode surface.12 Such studies are inherently limited to the order of micrometers or submicrometers at most with the preparation of a well-ordered lithium surface reportedly difficult to attain. Therefore, the use of well-defined, single crystal planes of noble metals and transition metals10,13,29 could be advantageous in lithium deposition/dissolution studies. Previous investigations using a gold (111) electrode in PC containing 0.1 M LiClO4 have shown by STM imaging that Au(111) electrodes are covered by “ultrathin” films of lithium.14 In addition, further analyses performed at more negative potentials have revealed further reduction processes including upd of lithium. 2. Experimental Section 2.1. Reagents. Lithium hexafluoroarsenate (LiAsF6) (V) (98%), (Aldrich) and tetra-n-butylammonium perchlorate, (Flu© 2007 American Chemical Society

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Figure 1. LiAsF6 (4.7 mM) and 0.1 M TBAP in THF at 295 K. Cyclic voltammogram obtained using a platinum polycrystalline electrode.

ka), were used as received without any further purification. Anhydrous THF was purified by filtration through two columns of activated alumina (grade DD-2) as supplied by Alcoa, employing the method of Grubbs et al.38 2.2. Cyclic Voltammetry (CV). Voltammetric measurements were carried out on an Autolab PGSTAT 20 (Eco-Chemie, Utrecht, Netherlands) potentiostat. A three-electrode arrangement was used in an airtight, three-necked electrochemical cell. The cell with solid electrolyte was dried in vacuo overnight before solvent addition and electrochemical experiments. The working electrodes used were a platinum macrodisk electrode housed in Teflon (area, A ) 0.0104 cm2), and the single crystal platinum electrodes: [(100), A ) 0.07 cm2]; [(110), A ) 0.11 cm2]; and [(111), A ) 0.03 cm2]. A large area, shiny platinum wire (Goodfellow Cambridge Ltd, Cambridge, UK) was employed as the counter electrode. The working electrodes were carefully flame annealed for 30 s before being sealed within the cell under a pressure of nitrogen. A Fc/Fc+PF6- (both in equimolar concentration; 4 mM) reference electrode was developed for use in THF and at low temperatures and has been described previously.39 The temperature was monitored and controlled by an external system (Julabo FT902, JULABO Labortechnik GmbH, D-77960 Seelbach/Germany) accurately ((1 K). Typically, the solutions were degassed vigorously for 5 min using impurity-free argon (BOC gases, Guildford, Surrey, UK) to remove any trace oxygen, and an inert atmosphere was maintained throughout all analyses. All solutions were prepared under an atmosphere of argon using oven-dried glassware such as syringes and needles used for the transfer of moisture sensitive reagents. All voltammetric measurements were performed inside a faraday cage to minimize any background noise. 3. Results and Discussion: Electrochemical Characteristics 3.1. Voltammetric Characteristics of Lithium at a Platinum Polycrystalline Electrode in THF. Figure 1 shows a cyclic voltammogram (CV) of the platinum polycrystalline electrode in THF solution containing 0.1 M TBAP and 4.7 mM LiAsF6 recorded between 0 and -3.5 V at a scan rate of 10 mV s-1 versus the standard reference electrode Fc/Fc+PF6- in THF. The temperature was maintained at 295 K ( 1 K. On the forward sweep, three voltammetric peaks can be observed, which are labeled A (Ep,c ) -1.6 V), B (Ep,c ) -3.0 V), and C (Ep,c ) -3.3 V). We have shown previously using platinum microdisk electrodes that scanning further into the negative potential region results in bulk electrochemical lithium deposition (i.e., multilayer lithium deposition).5 Our model40 allowed formal potential, Ef, and kinetic data to be extracted from the voltammetric data. In this study we were concerned with only the “pre-peaks” observed before bulk deposition begins.

Figure 2. LiAsF6 (3.8 mM) and 0.1 M TBAP in THF at 295 K. Cyclic voltammogram obtained using a platinum (100) single crystal electrode.

Upon reversing the potential scan, three oxidation peaks were observed: A (Ep,a ) -1.3 V), B (Ep,a ) -2.2 V), and C (Ep,a ) -2.6 V). Previous investigations into the electrochemistry of noble metal electrodes in aprotic organic solvents containing lithium salts10 have reported that several separate film forming processes are responsible for the peaks observed at gold and silver polycrystalline electrodes. These included the retention of solvent, the salt, and traces of oxygen and water. In our system, the exclusion of oxygen was verified by “blank” voltammetric scans (0.1 M TBAP in THF) in the negative potential region showing no oxygen reduction signals prior to the addition of the lithium salt. In addition, the procedures used to dry the solvent and prepare the solutions (as described above) limited any trace water from our electrochemical analyses. Intentionally added water actually acted to narrow the potential window relative to Fc/Fc+PF6-, and any peaks corresponding to those observed in Figure 1 obtained under anhydrous conditions became “swamped” by large background currents rendering the voltammogram featureless. We proposed that these peaks shown in Figure 1 were the result of lithium upd at different planes upon the platinum electrode surface. Li upd has been reported to be controlled by the nature of the surface of the electrode.29 Uosaki et al.14 have also shown that gold polycrystalline and (111) electrodes display upd characteristics in PC. To test our theory, a series of experiments involving platinum single crystal electrodes was performed. 3.2. Voltammetric Characteristics of Lithium at a Platinum Monocrystalline Electrodes in THF: (100), (110), and (111) Planes. Figures 2-4 show CVs of the platinum monocrystalline electrodes: (100), (110), and (111) in THF solution, respectively, containing 0.1 M TBAP and LiAsF6. All scans were recorded between 0 and -3.5 V at a scan rate of 10 mV s-1 versus the standard reference electrode Fc/Fc+PF6- in THF. Figures 2-4 each showed a single pair of upd/ups peaks. As a standard reference electrode was used, a comparison to the data obtained using the polycrystalline electrode could be made with confidence in the peak potential, Ep, values. Figure 2 showed that the upd/ups peak pair, C (Ep,c ) -3.3 V and Ep,a ) -2.6 V versus Fc/Fc+PF6-) (Figure 1) can be attributed to the (100) plane. Further assignments were also made; Figures 3 and 4 upd/ups peak pairs were attributed to the upd/ups of lithium at the (110) (Ep,c ) -3.0 V and Ep,a ) -2.2 V versus Fc/Fc+PF6-) and (111) (Ep,c ) -1.6 V and Ep,a ) -1.3 V versus

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Figure 3. LiAsF6 (3.8 mM) and 0.1 M TBAP in THF at 295 K. Cyclic voltammogram obtained using a platinum (111) single crystal electrode.

Figure 4. LiAsF6 (4.3 mM) and 0.1 M TBAP in THF at 295 K. Cyclic voltammogram obtained using a platinum (110) single crystal electrode.

Fc/Fc+PF6-) planes, respectively. To the best of our knowledge, this is the first study to report the upd/ups characterization of lithium at different platinum crystal planes in THF. Various schemes and mechanisms have been reported recently concerning the exact morphology of the surface films generated.14,22 Further investigations using in situ surface microscopic techniques are now required to quantify the exact morphology of these “thin” films. 4. Conclusions The preliminary electrochemical observations presented in this paper provide a unique insight into the underpotential deposition/stripping of Li at platinum polycrystalline and monocyrystalline [(100), (110), and (111)] electrodes in THF. All electrochemical data are reported against a standard reference electrode, Fc/Fc+PF6- for use in THF. Acknowledgment. C.A.P. thanks EPSRC for joint project studentship (GR/T05011/01). We thank Professor Dr. G. A. Attard (Cardiff University, Department of Chemistry) for loan of the platinum single crystal electrodes. References and Notes (1) Peled, E. Lithium Batteries; Academic Press: London, 1983. (2) Farias, D.; Braun, K.-F.; Folsch, S.; Meyer, G.; Reider, K.-H. Surf. Sci. 2000, 470, L93.

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