奈米結構氧化錳電極合成及其在鋰離子電池的應用計畫類別： 個別型計畫 計畫編號： NSC94-2218-E-151-004- 執行期間： 94 年 10 月 01 日至 95 年 07 月 31 日 執行單位： 國立高雄應用科技大學化學工程與材料工程系 計畫主持人： 吳茂松 報告類型： 精簡報告 處理方式： 本計畫可公開查詢
中 華 民 國 95 年 9 月 18 日
▓ 成 果 報 告
執行期間：94 年 10 月 01 日至 95 年 07 月 31 日
奈米結構氧化錳電極合成及其在鋰離子電池的應用Synthesis of nano-structured manganese oxide electrodes for lithium-ion batteries
計畫編號：NSC 94-2218-E-151-004 執行期限：94年10月01日 至 95年07月31日
Nanowires of manganese oxide are electrochemically synthesized on nickel substrate by manganous acetate solution at room temperature without any template or catalyst. The synthesized electrode of high porosity is composed of 8-12 nm nanowires after 300oC annealing. These manganese oxide nanowires show low crystallization. The synthesized anode material has a much higher capacity than the traditional graphite materials for lithium storage. The electrode’s reversible capacity reaches 1160 mAh g-1 at the first cycle; the specific capacity retention after 50 charge-discharge cycles remains 61% of its initial capacity. During cyclic voltammetric measurements, the linear relationship between peak current and square root of the scan rate suggests that the reaction is controlled by a semi-infinite diffusion of lithium.
Keywords: manganese oxide, lithium
storage, anode material, diffusion coefficient, and lithium-ion batteries.
Lithium-ion battery materials of high specific capacity and high rate dischargeability have attracted great attention. Graphite materials are currently the dominating anode materials because graphite offers high capacity, high safety, high electronic conductivity and low electrochemical potential with respect to lithium metal . Nevertheless, graphite
materials can hardly meet the
high-energy-density demand of the electronic devices today; therefore researches on alternative anodes of higher lithium storage capacities continue. Transition-metal oxides have attracted attention due by their advantageously high capacities [2-10]. Poizot et al. proposed new anode materials, the nanosized transition-metal oxides, for lithium-ion batteries [2,3]. Electrodes made of nanoparticles of transition-metal oxides (MO, where M is Co, Ni, Cu or Fe) demonstrate high electrochemical capacities (about 700 mAhg-1), great capacity retention, and high discharging rates. More importantly, the mechanism of Li reactivity in such electrodes
differs from the classical Li
insertion/deinsertion in graphite anodes or Li-alloying processes in alloy electrodes . The mechanism involves the formation and decomposition of Li2O, accompanying the
reduction and oxidation of metal nanoparticles respectively . Wang et al. reported that the nickel oxide showed a high reversible capacity similar to that of anode materials (transition-metal oxides) for lithium-ion batteries [4,5]. The nanocrystalline NiO thin-film electrode prepared by pulsed laser ablation showed a reversible capacity of 700 mAh g-1 in the range of 0.01 –3.0 V vs. Li/Li+. A high capacity retention up to 100 cycles may be achieved by optimizing the NiO films . Improved specific capacity, discharge rate, and cycling performance of the NiO thin-film electrode have all been attributed to its nanosized characteristics .
Wang et al. reported that cobalt oxides (Co3O4)
synthesized by chemical decomposition of cobalt octacarbonyl in toluene at low temperature is nanosized and demonstrates a stable reversible lithium storage capacity of 360 mAh g-1 within 30 cycles . On the other hand, a much higher capacity of nano-Co3O4 electrode was found to be
approximately 700 mAh g-1. Morales et al. reported an excellent method in obtaining Cu2O films for lithium-ion batteries; also
indicating that for films of the same thickness, the discharge capacity increased with a decrease in particle size . Gao et al. showed that the fine and polycrystalline CuO nanorods as anode material for lithium-ion battery exhibit a high electrochemical capacity of 766 mAh g-1 compared to 416 mAh g-1 for the nanorods with single crystalline structure due to their large surface area and numerous structural defects . Therefore, the particle size of anode materials has a crucial effect on the electrochemical behavior of these transition-metal oxides toward lithium, i.e. nanosized transition-metal oxides play an important role in facilitating the reduction and oxidation between Li2O and metal
nanoparticle during charging/discharging. Therefore, synthesis of nanosized anode materials has become important. Until now, nanosized manganese oxide material has rarely been reported as an anode material for rechargeable lithium-ion batteries. In this work, we synthesized the nanowires of manganese oxide electrochemically as a new anode material without any template or catalyst, in which the production cost is low and fabrication is easy.
Nanostructured manganese oxide electrodes were deposited onto nickel foils (1
) by applying a current density of 0.25 mA cm-2 in a solution bath of 0.1 M manganous acetate and 0.1 M sodium sulfate for 10 min at room temperature [11-13]. A saturated calomel electrode (SCE) was used as the reference electrode and a platinum foil with dimension 2 2 cm2 was used as the counter electrode. The plating solution was stirred continuously by a Teflon stirrer during the entire deposition. After deposition, plated foils were rinsed several times in deionized water and dried at 300oC for 1 h in air. The amount of deposited manganese oxide was measured by a microbalance. Surface morphology of the electrochemically deposited electrode was examined with a scanning electron microscope (FE-SEM, JEOL JSM-6500F) with an accelerating voltage of 10 keV. Nanostructure of the manganese oxide was examined with a transmission electron microscopy (FE-TEM, JEOL JEM-2100F) with an accelerating voltage of 200 kV. TEM specimen were prepared by the following procedure: nanowires were stripped off and dispersed in anhydrous ethanol with ultrasonic vibration for 5 minutes, a drop of the supernatant was then transferred onto a standard holey carbon-covered-copper TEM grid. An X-ray photoelectron spectroscopy (XPS, Perkin-Elmer, PHI Quantera SXM) with a focused monochromatic Al K radiation (1486.6 eV) was used to analyze the composition of the deposited manganese oxide electrode. Crystal structures of the deposited manganese oxide were identified by X-ray powder diffraction (XRD, Philips PW1700) with a Cu Ktarget (wavelength = 1.54056 Å). Samples were stripped from a nickel substrate. Diffraction data were collected for 1 s at each 0.04ostep width over
2, ranging from 30o
to 80o. Batteries were assembled in a dry room (temperature 25oC, relative humidity 0.22%). The electrochemical cell comprised a working electrode (manganese oxide electrode), a counter electrode (lithium metal), and a reference electrode (lithium metal). A working electrode wrapped with separator (Celgard 2320, 20m in thickness, USA) was placed between two counter electrodes and then inserted into an aluminum-plastic laminated film case. Electrolyte was injected into the case and the case was then sealed off at a reduced pressure of 60 mmHg. The electrolyte was 1 M lithium hexafluorophosphate (LiPF6, Tomiyama Pure
Chemical, Japan) in ethylene carbonate / propylene carbonate / diethylene carbonate (EC/PC/DEC = 1/1/2, by volume). Electrochemical tests were performed on a charge/discharge unit (Maccor model series 4000). Cells were charged at a constant current (85 mA g-1) to a cutoff voltage of 10 mV vs Li/Li+. Discharge was performed at the same rate to a cutoff voltage of 3.0 V vs Li/Li+. Cyclic voltammetric (CV) measurements were taken by means of a potentiostat/galvanostat (Schlumberger SI 1286). The voltage was cycled in the range of 0.01 to 3.0 V vs Li/Li+, with different scan rates.
4. Results and discussion
According to Preisler, -manganese dioxides, which exhibit a marked growth orientation and a fibrous structure as well as cleavability, are deposited on the anodes during electrolysis of manganous salt solutions in sulfate bath [14,15]. As a result, the-manganese dioxides are fibrous or wires in nature. Figure 1a shows the SEM micrograph of a manganese oxide electrode after 300oC annealing. Electrode is highly porous and is of nanowire structure.
Nanowires are 8-12 nm in diameter as shown in Figure 1b (TEM image). In general, the electrochemically deposited manganese oxide contains structural water in the solid phase of manganese oxide . According to Preisler, only a small percentage of water in electrodeposited manganese dioxide is volatile at 120oC, and a predominant portion of water is desorbed in a relatively smooth manner up to 350oC . The electrodeposited manganese oxide decomposes rapidly to form Mn2O3 at temperatures beyond 500oC .
The water content within is known to affect the electrochemical reactivity and the thermodynamic stability of various manganese oxide phases, as it causes variations in crystal lattice and consequently in electrical conductivity and electrode potential [17,18]. Moreover, water in the structure creates challenges in lithium battery application because of the instability of lithium with water and the decomposition of water during charge/discharge processes. Figure 2 shows the XRD pattern of manganese oxide nanowires after 300oC annealing. The electrochemically deposited manganese oxide resembles closely to -Mn(O,OH)2
(-manganese oxide hydroxide, JCPDS 17-0510). In order to avoid the structural destruction by water contamination, the annealing temperature has been set at 300oC. However, from Figure 2, the XRD characteristic peaks are broader. Such a widening indicates a poor crystallinity of the electrochemically deposited manganese oxide. The mean grain size of the deposited manganese oxide was calculated using Scherrer’s equation with diffraction peak at 2 =37.1o: D = 0.9/(cos), where is the X-ray wavelength, is the full width at half maximum (FWHM), and is the Bragg angle.
The calculated grain size is about 3.0 nm. In general, as an inorganic oxide composed of multivalent ions, manganese oxide possesses such important characteristics as inherent nonstoichiometric character . This natural phenomenon makes the real composition identification difficult. Figure 3 shows the XPS spectra of manganese oxide nanowires after annealing at 100 and 300oC. The two significant peaks of binding energies correspond to Mn 2p1/2 and Mn 2p3/2
respectively, which identify the major chemical composition being MnO2; therefore
these electrochemically synthesized manganese oxide nanowires are similar to the sol-gel-derived MnO2 nanoparticles in
composition . However, the broadened peaks in Figure 3 imply the existence of a multivalent state of manganese ions. Although it is not easy to differentiate the valence states from the XPS spectrum, a shift in binding energy of the Mn 2p3/2electron for the oxides
annealed at different temperatures has been observed. The peaks of Mn 2p3/2 electron in
the oxides annealed at 100 and 300oC are 642.4 eV and 642.6 eV respectively, suggesting that the mean oxidation state of manganese is increased with the annealing temperature. A higher oxidation state of manganese in the oxide gives a higher binding energy of the Mn 2p3/2electron .
Figure 4 shows the charge-discharge curves of a manganese oxide electrode after 300oC annealing at different cycles. Capacity is measured between 0.01 and 3.0 V vs Li/Li+. During the first charge process (cathodic process), cell voltage decreases slowly from 3.0 V to a wide plateau (at about 0.40 V), then decreases continuously to 0.01V. In the first discharge process (anodic process), cell voltage increases with no significant plateau,
and the capacity is much lower than its corresponding charge capacity. The reversible capacity is approximately 1160 mAh g-1, and irreversible capacity 600 mAh g-1. After the initial charge/discharge cycle, an irreversible capacity exists still throughout each of the following sequential cycle, but the irreversibility is rather small. According to the charge-discharge behaviors of transition-metal oxide reported by Poizot et al., the voltage curve of the second charge process differs considerably from the first because some drastic, lithium-driven, structural or textural modifications have occurred in the first charge process . The large irreversible capacity occurs only in the first cycle may be caused by decomposition reactions of the electrolyte and formation of the SEI (solid-electrolyte interphase) film onto the surface of manganese oxide electrode .
After 50 charge-discharge cycles, the reversible capacity decreases to 710 mAh g-1, 61 % of the initial reversible capacity value (Figure 4). The Coulombic efficiency of manganese oxide electrode is high except for the first cycle; the high efficiency indicates that almost all the charged capacity can be discharged. Reversible capacity decay indicates a decrease of active material with cycle number. A possible cause to the decay is an isolation of catalytic manganese metal by non-conducting materials such as Li2O and the
passive SEI film, during the charge-discharge processes. An ideal metallic nano-particle formation is in which the network is interconnected during the first charge/discharge cycle; for the network facilitates both charge-transfer reaction and electron conduction.
Figure 5 shows the cyclic voltammetric curves of the manganese oxide electrode at different
scan rates. A broad anodic peak locates at about 1.22 V vs Li/Li+, and a fairly sharp cathodic peak at 0.22 V vs Li/Li+. These two peaks might be attributed by the reversible electrochemical oxidation and reduction of manganese oxide with lithium in the electrode. Compared with other transition metal oxides in literatures, manganese oxide electrode with nanowire structure has lower reduction and oxidation voltages (0.22 V and 1.22 V vs Li/Li+) than that of NiO (0.90 V and 2.30 V vs Li/Li+) [4,5]. In general, lower reduction and oxidation voltages vs Li/Li+ deliver higher energy density; an example is graphite.
The synthesized anode is composed of manganese oxide nanowires. The diameter of the synthesized nanowires is about 8-12 nm and these nanowires have poor crystallinity after 300oC annealing. The electrode reversible capacity in the first cycle is approximately 1160 mAh g-1; and the large irreversible capacity of 600 mAh g-1is caused by structural or textural modifications, decompositions of electrolyte, and formation of SEI film. The reaction mechanism of manganese oxide with lithium is the formation/decomposition of Li2O facilitated
by metallic manganese. Coulombic efficiency of manganese oxide electrode is high except for the first cycle; almost all the charged capacity can be discharged. Decrease in cycle capacity indicates a decrease of active material with increasing cycle number; a possible cause is that the catalytic manganese metal may be isolated by the non-conducting materials such as Li2O and passive SEI film.
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Fig.1(a) SEM micrograph, (b) TEM image of a manganese oxide electrode.
30 35 40 45 50 55 60 65 70 75 80 (401) (511) (222) In te n s it y / a rb . u n it s 2/ degree (220)
Fig. 2 XRD pattern of manganese oxide nanowires. 660 655 650 645 640 635 630 Mn 2p 3/2 1/2 C o u n ts / a rb . u n it s Binding energy / eV 300o C 100o C
Fig. 3 XPS spectra of manganese oxide nanowires annealed at 100 and 300oC.
0 200 400 600 800 1000 1200 1400 1600 1800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 V o lt a g e / V v s . L i/ L i + Capacity / mAh g-1 1st discharge 1st charge 2nd discharge 2nd charge 10th discharge 10th charge 50th discharge 50th charge
Fig. 4 Charge-discharge curves of a manganese oxide electrode annealing at different cycles.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 -6 -5 -4 -3 -2 -1 0 1 2 C u rr e n t / m A Voltage / V vs. Li/Li+ scan rate: 0.5 mVs-1 1.0 mVs-1 2.0 mVs-1 3.0 mVs-1 4.0 mVs-1
Fig. 5 Cyclic voltammetric curves of the manganese oxide electrode at different scan rates.