Investigation of anode materials for lithium-ion batteries

Transcription

1 University of Wollongong Thesis Collections University of Wollongong Thesis Collection University of Wollongong Year 2006 Investigation of anode materials for lithium-ion batteries Ling Yuan University of Wollongong Yuan, Ling, Investigation of anode materials for lithium-ion batteries, PhD thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, This paper is posted at Research Online.

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3 CHAPTER 9. MESOPOROUS GOLD AS ANODE MATERIAL FOR LITHIUM ION BATTERIES 9.1 Introduction Mesoporous metallic surfaces may be prepared by leaching an active element from suitable intermetallic precursor compounds, but are in general not favored for technological applications (catalysts aside) because they are friable and readily oxidized. Gold is potentially an exception to this rule and stable, clean mesoporous surfaces and powders of it may be prepared (Van der Lingen, 2003; Cortie 2005). Despite their shortcomings, porous metal coatings have found actual or potential applications as electrodes in electrochemical devices (Conway, 1997) and in sensors of various kinds (Van Noort, 2000; Natan, 2001), with the main attraction being that their increased surface area facilitates both faradaic and capacitive processes. Recently, porous coppertin has been investigated for application in anodes of Li batteries (Shin, 2005). The properties desired from such anodes are that they should be stable after repeated cycling and that they should have a wide range of stable cell voltages and the lowest possible alloying/de-alloying potentials relative to the Li/Li + couple. This last point is important because any deviation from the potential of the Li/Li + couple will be at the expense of the overall cell voltage, and the net effect will be a reduction in the energy density of the battery (Huggins, 1989; Taillades, 2002). The first criterion has been largely addressed by use of an inert but conductive matrix material for the anode, into which the Li passes after reduction to form intermetallic compounds. 122

4 The second criterion is facilitated when the phase diagram of the anode alloy has a broad two phase region on the Li-rich side (Huggins, 1989). By consulting a standard compilation of phase diagrams (Baker, 1992), it can be shown that Li-Au is suitable, with the field Li+Li 15 Au 4 extending from 12 to 100 wt.% Li. In principle, voltage will be constant during alloying and de-alloying over this range since the activity of the Li would be maintained at that pertaining to Li 15 Au 4 as long as any of this phase is in contact with electrolyte. The two phase range in Li-Au is broader than in Li+Li 22 Sn 5 (20.5 to 100 wt. % Li) or Li-Ag (40 to 100 wt. % Li). The third criterion is related to the maximum theoretical capacity of the anode. Pure Li would yield 3854 mahg -1 of anode, whereas graphite, tin, and gold are expected to yield 372, 746, and 451 mahg -1 respectively (based on lithiated intermetallic compounds with the stoichiometries Li 2 C 11, Li 22 Sn 5, Li 15 Au 4 ). However, since the intermetallic compounds are significantly denser than the Li-C phases, their volumetric energy densities appear to be very competitive, with Li-Au for example reported at 3400 ma.h.cm -3, which is roughly midway between the 840 ma.h.cm -3 of Li-C and the 7200 ma.h.cm -3 of Li-Sn (Taillades, 2002; Shin, 2005). The fourth criterion, the cell voltage, is determined by thermodynamics, in particular by the activity of the Li in the anode. This should be as close to unity as possible to give as big a cell voltage with the cathode as possible. Li-Sn and Li(Cu,Sn) (with charging voltages that vary from 0.8 to 0 V vs Li/Li + (Huggins, 1989; Shin, 2005)) are inferior in 123

5 this regard to Li-Au (0.4 to 0 V), which in turn is theoretically inferior to Li-Ag or Li-Zn (0.25 to 0 V) (Taillades, 2002; Huggins, 1989). G. Taillades et al. reported that thin, solid, Au anodes showed a capacity of 3400 mah/cm 3 and a very negative and narrow voltage window (Taillades, 2002). However, there have been no reports to the best of our knowledge on the usefulness of mesoporous Au as an electrode material for lithium rechargeable batteries. The high surface area (including occluded pores) of a mesoporous gold mass, taken in combination with the high conductivity yet chemical stability of Au, the low voltage for alloying/de-alloying in Li-Au, and the form of the Li-Au binary phase diagram, suggested to the authors that mesoporous Au anodes might have applications in respect of lithium rechargeable batteries. 9.2 Synthesis and characterization of mesoporous Au Samples of mesoporous Au were prepared by the chemical removal of Al from thin films of the intermetallic compound AuAl 2. This intermetallic precursor material is commonly known as purple gold (Cretu, 1999) or purple glory (Cahn, 1998) on account of its attractive purple color. The thin films of Au x Al y were produced by simultaneously sputtering Au and Al from two elemental targets of 50 mm diameter using a high vacuum DC magnetron sputtering apparatus. The rates of sputtering were controlled by varying the power delivered, with the system having been pre-calibrated using a quartz microbalance. The necessary flux was determined using the relative densities of the elements and the desired stoichiometry of the target compound. Copper sheets of 1 mm thickness was used for the substrate and were heated to 400 C during 124

6 co-deposition using an in situ heater. The sheets had been previously cleaned by sandblasting followed by etching in a citric acid solution. After deposition, the Al was removed by treatment with 0.5 NaOH. The conditions used to produce the five types of coating mentioned in this paper are listed in Table 9-1. Au and Al (where applicable) were simultaneously co-deposited. The actual thickness of the Au x Al y coating prior to removal of the Al would have been the sum of the two thicknesses shown. However, the coatings densified considerably during and after removal of the Al. Table 9-1 Details of the five coating types discussed here. Sample Description Top layer Middle layer Bottom layer A single layer of mesoporous Au on Cu sheet 200 nm Au nm Al - - B C D E multiple layers of mesoporous Au on Cu sheet thin film of solid Au on Cu sheet mesoporous Au on silicon wafer single layer of mesoporous Au on Cu sheet 50 nm Au nm Al 80 nm Au nm Al 100 nm Au nm Au nm Al 30 nm Au + nm Al nm Au+160 nm Al A SEM image of the surface of a mesoporous Au coating is shown in Fig. 9-1(a) at high magnification of Sample A. In Fig. 9-1(b), we show a low magnification image of the surface of Sample D, while a high magnification cross-sectional view of that coating is shown in Fig. 9-1(c). In all cases, it is clear that the coatings exhibit porous microstructures with pore diameters of about 20 to 50 nm. However, while the composite coating in Sample D was of the order of 900 nm thick before chemical removal of the Al, the removal of that element caused the thickness of the remaining material to shrink down to 165 nm. Since the equivalent of 100 nm Au had been co- 125

7 Chapter 9. Mesoporous gold anode materials for lithium ion batteries deposited with the Al, it follows that at a thickness of 165 nm the gold must contain about 65% porosity and have an average density of about 12 g/cm3. Fig. 9-1(d) shows the cracks that develop on the surface of the mesoporous coatings after their first charge/discharge cycle. These cracks seem similar in nature to those found by Yang et al. (1996) on Sn electrodes. In that case, it was surmised that a drastic increase in volume occurred in the first alloying half cycle. However, lithium removal and further cycling caused only minor effects. (a) (b) (c) (d) Fig. 9-1 SEM images of mesoporous Au: (a) Sample A, single layer containing the equivalent of 200 nm Au; (b) Sample B single layer containing the equivalent of 100 nm of Au; (c) cross-section of Sample B showing that coating is actually 165 nm thickness (on a Si substrate) ;and (d) Sample E, surface of a coating containing the equivalent of 30 nm Au after first discharge. 126

8 9.3 Electrochemical properties of mesoporous Au The second alloying/de-alloying cycle of the three types of Au electrode are shown in Fig Volatage (V) Time (seconds) (a) Voltage(V) Time(seconds) (b) 127

9 Voltage(V) Time(seconds) (c) Fig. 9-2 The second alloying/de-alloying cycle of Au electrodes: (a) Sample A, single mesoporous layer, (b) Sample B, multilayer of mesoporous Au, (c) Sample C, single, solid layer of Au. Fig. 9-2(a) shows the results for Sample A, the single layer of mesoporous gold, Fig. 9-2(b) shows data for Sample B, the multilayered mesoporous coating, and Fig. 9-2(c) shows results for Sample C, the single layer of solid gold with a nominal thickness of 100 nm. The alloying and de-alloying reactions are especially visible in Fig. 9-2(c), and it can be seen that during the alloying process lithium reduction occurred at two extended plateaus at about 0.23 and 0.11 V, while during de-alloying lithium oxidation occurred at two plateaus at 0.15 and 0.40 V. These figures are comparable to the 0.15 and 0.10 V vs. Li reported previously for alloying in Au, and to the 0.18 and 0.40 V vs. Li reported for de-alloying on that element (Taillades, 2002). The voltage ranges associated with the single layer mesoporous electrode, Sample A, are similar, being

10 and 0.10 V for alloying, and 0.20 and 0.40 V for de-alloying. However, only reactions at 0.15 and 0.45 V are well developed on the curves for the multilayer Sample B. These results confirm that the electrochemical active potential of Li-Au is lower than Li- Sn (Taillades, 2002) and higher than Li-Ag (Hwang, 2001). In Ag, substantial insertion of Li occurs below 0.07 V, probably due to the formation of Li-Ag alloys (Hwang, 2001). As mentioned above, the lower the potential of the alloy vs. Li, the higher the energy density of the cell. It is known that there are potential applications for high voltage rechargeable batteries (4.5 V), while the voltage for commercial lithium batteries is 3.6V. The cyclic voltammograms for Samples A and B (Fig. 9-3) also confirm that the alloying/de-alloying processes occur in a very low voltage range (0.4 to 0 V), compared to the Li Sn system (0.2 to 1.0 V) (Choi, 2004), which minimizes the reduction in cell voltage. In the first scanning cycle, two lithiation peaks appear, which correspond to the formation of different Li x Au alloys as Au + xli+ + xe- Li x Au. From the second scanning cycle, only one pair of redox peaks occurs. 600 sample A sample B sample C Capacity (mah/g) Cyclelife (times) (a) 129

11 Current(A) Voltage(V) (b) Fig. 9-3 The cyclic voltammogram curves of mesoporous Au electrode: (a) Sample A, (b) Sample B. The total charges passed in the second cycle of alloying and de-alloying the three types of electrodes are compared in Table 9-2. The first point to note is that the charge passed when alloying, as expected, is significantly larger than the charge passed when dealloying. This is an indication of the relative inefficiency of the de-alloying process of an anode material in comparison to the alloying process. However, it is very clear that the processes are more reversible on the mesoporous electrodes than on the solid gold film. So far as drawing power from a battery is concerned, it would be the charge passed during de-alloying of an anode that is important. The multilayer (Sample B) and solid gold (Sample C) electrodes start off far better than the single layer mesoporous film in this regard. The calculated volumetric parameters are of the same order of magnitude as those reported for Au by Taillades et al. (2002), but have evidently been effected by uncertainty regarding the actual density of the lithiated structures formed. They are reported here only to indicate the approximate magnitude of energy density that can be achieved. 130

12 Table 9-2. Alloying and de-alloying capacities in the second cycle (shown in Fig. 9-2) for each of the three samples. A notional film density of 12 g cm -3 has been used to obtain volumetric capacity. Sample A alloying Sample A, de-alloying Sample B, alloying Sample B, de-alloying Sample C, alloying Sample C, de-alloying Charge passed, C(coulomb) cm -2 Charge passed, mah cm -2 Charge passed, mahg -1 Charge passed, mah cm ~ ~ ~ ~ ~ ~8000 Fig. 9-4 shows the specific charge capacity and cycle life numbers for Sample A, Sample B, and Sample C, as determined during their de-alloying cycles. 600 sample A sample B sample C Capacity (mah/g) Cyclelife number (times) Fig. 9-4 The specific charge storage capacity and cycle life response for mesoporous Au (Sample A - single mesoporous layer, Sample B - multilayer of mesoporous gold, Sample C - single layer of solid gold, 100 nm thick.). 131

13 It is clearly evident that the capacity of the mesoporous Au on de-alloying is superior to that of the equivalent layer of solid Au. The available data also suggest that the method of synthesis of the mesoporous gold may affect this capacity, with that of the multilayer mesoporous sample being superior after 30 cycles to that of the single layer mesoporous sample. In our case, among the three samples, the multilayer mesoporous electrode demonstrates a significantly better electrochemical cyclability than the other types tested. The capacity decay of all three types of electrode is a problem shared with many other metallic alloy systems (Taillades, 2002). After 30 cycles the electrodes made of materials A, B and C retained 15%, 10% and 2% of their capacity (in mah.g -1 ) respectively. The fading of the capacity may be caused by two reactions: (1) insertion of Li atoms leads to significant volume expansion of host structure, especially in the beginning few cycles. According to a model (Yang, 1996) of lithium insertion into a loosely packed small particle size metallic matrix, even 100% volume expansion of individual particles does not necessarily crack them, as their absolute changes in dimensions are still small. Removal of lithium from the Li x Au particles does not affect the size of the expanded particles very much. So the discharge capacities of the electrodes can remain almost constant in subsequent cycles. (2) The formation of a passivating film, an electrically insulating layer on the battery electrodes also known as the solid electrolyte interphase (SEI) film, might also be effecting the capacity (Dolle, 2001). When the thickness of the SEI layer increases, the ionic impedance of the SEI increases (Meyer, 2005). Since the lithium insertion process occurs on an electrode 132

14 covered with the SEI, the characteristics of lithation/delithiation, and stability of the interface are effected (Ratnakumar, 2001). 9.4 Conclusion Thin films of mesoporous Au have been evaluated for use as the anode of lithium rechargeable batteries for the first time. The behavior was similar to that for thin, solid films of gold, with the alloying/de-alloying processes occurring in a very low voltage range vs. Li/Li + (0.25 to 0 V, and 0.15 to 0.40 V, compared to the 0.2 to 1.0 V of the Li Sn system). This would minimize the reduction in cell voltage in a battery that used this material as an anode. The multilayer mesoporous Au electrode showed superior discharge capacities and better cycling stability than the thin, solid gold film. The mesoporous structure of Au probably contributes to accommodating the large volume changes in the electrode that occur during alloying and de-alloying, which can reduce the fade in capacity to some extent. 133