Carbon Composite Positive Electrode for

Oct 22, 2015 - Rechargeable batteries with high capacity, high safety, and low cost catch much attention because of recent requirement of high-perform...
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Amorphous Vanadium Oxide/Carbon Composite Positive Electrode for Rechargeable Aluminum Battery Masanobu Chiku,* Hiroki Takeda, Shota Matsumura, Eiji Higuchi, and Hiroshi Inoue Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan S Supporting Information *

ABSTRACT: Amorphous vanadium oxide/carbon composite (V2O5/C) was first applied to the positive electrode active material for rechargeable aluminum batteries. Electrochemical properties of V2O5/C were investigated by cyclic voltammetry and charge−discharge tests. Reversible reduction/oxidation peaks were observed for the V2O5/C electrode and the rechargeable aluminum cell showed the maximum discharge capacity over 200 mAh g−1 in the first discharging. The XPS analyses after discharging and the following charging exhibited that the redox of vanadium ion in the V2O5/C active material occurred during discharging and charging, and the average valence of V changed between 4.14 and 4.85. KEYWORDS: rechargeable aluminum battery, multivalent cation battery, amorphous vanadium oxide, aluminum, dipropylsulfone, rechargeable battery, vanadium oxide

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reported that amorphous V2O5/carbon composite (V2O5/C) could successfully intercalate/deintercalate Mg2+ ion because H2O molecules existing between V2O5 layers weakened the interaction between V2O5 layers and the Mg2+ ion.1 This is a fascinating result and may enable the reversible intercalation/ deintercalation of some other multivalent ions including Al3+. In this paper, we investigated the applicability of the amorphous V2O5/C composite as the positive electrode material for rechargeable Al batteries. To get out of a suspicion that the stainless steel current collector was electroactive, we replaced it with a Mo current collector and the ionic liquid electrolyte with a mixture of AlCl3, dipropylsulfone and toluene (1:10:5 in mole ratio). The amorphous V2O5/C composite11,12 used in this study was prepared without any waste just by mixing raw materials under atmospheric pressure and mild temperature conditions, which is low energy consumption and zero emission technique. We characterized electrochemical Al3+ intercalation/deintercalation properties of the amorphous V2O5/C positive electrode by cyclic voltammetry (CV) and evaluated the performance of rechargeable Al batteries with the amorphous V2O5/C positive electrode by charge/discharge cycling tests. Figure 1 shows SEM images of KB and pristine V2O5/C. As shown in Figure 1a, KB had a porous structure formed by the aggregation of spherical particles with about 50 nm in diameter, whereas Figure 1b indicated V2O5 uniformly covered the spherical KB particle surface, which was consistent with the

echargeable batteries with high capacity, high safety, and low cost catch much attention because of recent requirement of high-performance power sources for electric vehicles and mobile computing devices consuming high electric energy. Employing metals oxidized to multivalent cations as the negative electrode materials is a promising strategy. For example, magnesium is oxidized to a divalent cation, Mg2+, and the volumetric capacity of Mg (3.84 Ah cm−3) estimated from its oxidation is higher than that of Li (2.06 Ah cm−3), so substituting Mg for Li makes us expect higher energy density rechargeable cells.1−3 Aluminum is oxidized to a trivalent cation, Al3+, and its volumetric capacity (8.04 Ah cm−3) is 4-fold as high as that of Li. Moreover, Al is one of the most abundant metals on the earth and more tolerant to water and air than Li. So far, there are limited researches on Al primary batteries4−6 and rechargeable batteries.7−10 Various intercalation compounds such as LiCoO2, LiMn2O4 and so on have been used as positive electrode active materials for lithium-ion batteries. In the case of rechargeable multivalent cation batteries, V2O5 can be a promising candidate for positive electrode material.8,12−14 However, it is concerned that the intercalating Al3+ ions are strongly fixed between the V2O5 layers and their diffusion through the V2O5 interlayers is very slow, so reversible charge/ discharge cycling is inhibited. To overcome this issue, Archer et al. used V2O5 nanowires as the positive electrode material in ionic liquid electrolyte, AlCl3-dissolved 1-ethyl-3-methylimidazolium chloride.8 However, Menke et al. suggested the V2O5 nanowire was electrochemically inactive, and iron and chromium in the stainless steel current collector worked as the positive electrode active material instead.13 It has been © XXXX American Chemical Society

Received: July 16, 2015 Accepted: October 22, 2015

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DOI: 10.1021/acsami.5b06420 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

and KB are applied as the active materials of the electric double-layer capacitors, so if the surface area of V2O5/C was high, it will show the capacitive characteristics. We applied the BET gas adsorption method to KB and V2O5/C and determined their surface area as 1430 m2 g−1 and 319 m2 g−1, respectively. Our V2O5/C had low surface area for electrical double-layer capacitor, so V2O5/C will not act like capacitors. Figure S1 shows the TEM images of V2O5/C before and after three charge/discharge cycles. The lattice stripes for KB were clearly observed in high-resolution images (Figure S1b, d), thus we can say these TEM images have enough resolution to identify V2O5 particles. In Figure S1b, d, some particlelike circles were observed on KB, but the lattice stripes for V2O5 were not clearly observed, strongly suggesting that the V2O5 particles in this study were not nanocrystalline but amorphous. Figure 2a shows a CV of a pellet-type V2O5/C electrode and KB electrode in the mixed electrolyte solution. A couple of broad oxidation and reduction peaks were observed with V2O5/ C electrode, suggesting that the V2O5/C is electrochemically active as the positive electrode for rechargeable Al battery. Archer et al. reported that in the CV of the V2O5 nanowire electrode cathodic and anodic peaks were observed at 0.45 and 0.95 V vs Al/Al3+, respectively.8 In contrast, Menke et al. suggested the V2O5 nanowire was electrochemically inactive and charge−discharge behavior was caused by stainless steel current collector.13 As shown in Figure 2b, the Mo current collector used in this study showed large electrochemical window between 0 and 2 V vs Al/Al3+, suggesting that the Mo current collector is very stable in the mixed electrolyte solution. In the case of Li+ and Mg2+ insertion, two pairs of redox peaks were observed in CVs due to the insertion to two distinct Li+ sites between the V2O5 layers.11 Almost the same features were observed for the Mg2+ ion. In the CV of the amorphous V2O5/ C electrode, cathodic and anodic peaks were observed at 0.8

Figure 1. SEM images of (a) KB and (b) pristine V2O5/C, and (c) XRD spectra of V2O5/C before and after heat treatment at 200 °C.

report by Kudo et al.12 In the XRD spectrum of the pristine V2O5/C (Figure 1c), a large broad peak assigned to KB was observed at 2θ = 25° and any other diffraction peaks were not observed. The XRD pattern was maintained even after heat treatment at 120 °C as shown in Figure 1c, suggesting that the resultant V2O5 had an amorphous-like structure.12 Both V2O5

Figure 2. (a) CV of the V2O5/C electrode and KB electrode in a mixed electrolyte solution. Scan rate was 2 mV s−1. (b) CV of the Mo plate electrode with the potential range between 0 and 2.5 V in the same electrolyte solution. Scan rate was 1 mV s−1. (c) CV of the Mo plate electrode with the potential range between −1.5 and 2.0 V in the same electrolyte solution. Scan rate was 10 mV s−1. (d) SEM and EDX image of deposited Al metal. Al was deposited with constant current density at 0.5 mA cm−2 for 10 h. B

DOI: 10.1021/acsami.5b06420 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. (a) Charge and discharge curves and (b) charge and discharge capacities of a rechargeable Al cell with the V2O5/C positive electrode in the mixed electrolyte solution. Charge and discharge rates are C/20 and C/20, respectively. (c) Cycle performance of a rechargeable Al cell with the V2O5/C positive electrodes at C/40, C/20, and C/10 discharge rates. Charge rate is the same as discharging rate. (d) Charge and discharge curves of KB electrode with the current density at 15 mA g−1.

As shown in Figure 3b, the discharge capacity at the first cycle (C1) was 150 mAh (g of V2O5)−1, whereas that at the 15th cycle (C15) was below 100 mAh (g of V2O5)−1. Charge and discharge capacities were almost the same, suggesting that the rechargeable Al cell showed high Coulombic efficiency. Figure 3c shows discharge capacities with different charge/discharge current densities. When discharge current was decreased to C/ 40, C1 was increased to 200 mAh (g of V2O5)−1, whereas C15 was below 100 mAh (g of V2O5)−1, which was similar to discharging at the C/20 rate. The discharge capacity was decreased to 60 mAh (g of V2O5)−1 with discharge current at C/10 but the discharge capacity was kept at above 50 mAh (g of V2O5)−1 for 30 cycles. Figure 3d shows charge/discharge curves of the KB electrode. In the first discharge, the KB electrode showed discharge capacity of 27 mAh g−1 and the charge/discharge capacities became less than 10 mAh g−1 in the second discharge. Taking into account the change of the surface area of KB before and after V2O5 modification (from 1430 to 319 m2 g−1), the double-layer capacitance of KB in V2O5/C active materials will be less than 10 mAh g−1, indicating the capacity of V2O5/C electrode is due to the V2O5 but not KB. During charging/discharging, the valence of the V component will be changed, so its valences after charging and discharging were determined by XPS. The average valence of V component was calculated as follows

and 1.6 V vs Al/Al3+, respectively. Smyrl et al. exhibited in their research on the Al3+ insertion into the V2O5 aerogel that in its CV only a pair of broad oxidation/reduction peaks were observed,14 which was consistent with our result. With KB electrode, the anodic peak was not observed and cathodic current was observed around 0.5 V vs Al/Al3+, indicating that although the KB has higher surface area (1430 m2 g−1), the double layer capacitance of KB was quite small and it will not contribute to the charge/discharge performance of the V2O5/C electrode. Figure 2c shows the CV of Mo electrode with the potential range between −1.5 and 2.0 V vs Al/Al3+ in the mixed electrolyte. The potential range was extended to negative potential, and clear reduction current and oxidation peak were observed, suggesting the Al deposition and dissolution were occurred in the mixed electrolyte. Figure 2d shows an SEM and EDX images of deposited Al on a Mo electrode. EDX measurement indicated the deposits were Al, and grainlike structures were found on the deposited Al surface but it was not dendritic form. CVs of an Al plate electrode measured at different sweep rates are shown in Figure S2. Each CV was a straight line, which passed the origin of the graph, because in discharging Al was not exhausted, indicating that the Al metal was reversibly reduced and oxidized on the Al plate electrode. To investigate the property of the rechargeable Al cell with the V2O5/C positive electrode, we performed charge/discharge tests. Figure 3a, b shows charge/discharge curves and charge/ discharge capacities of the rechargeable Al cell with the V2O5/C positive electrode at C/20 charge/discharge rate (1 C rate = 442 mAh (g of V2O5)−1), respectively. The theoretical discharge capacity was evaluated with the electrochemical reaction described below Al3 + + 3e− + V(V)2 O5 ⇄ AlV(IV)V(III)O5

average valence = 5x[V2O5] + 4x[VO2 ] + 3x[V2O3]

(2)

Figure 4 shows V 2p spectra for the V2O5/C electrode before and after the first discharging at C/40 rate and after the following charging at the same rate. The as-prepared V2O5/C consisted of purely V5+ ion. After the first discharging, the content of V5+ was decreased to 40 at %, whereas that of V3+ and V4+ was increased, clearly indicating that V2O5 was reduced to V3+ and V4+ during discharging. The contents of V5+, V4+, and V3+ are indicated in Table 1. The average valence after the

(1) C

DOI: 10.1021/acsami.5b06420 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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battery during charging/discharging.16,17 The valence of V was consecutively changed from V5+ to V3+ via V4+ with the Li+ insertion in the potential range between 4.0 and 1.6 V vs Li/Li+ (2.6 and 0.2 V vs Al/Al3+). In this study such the change in the valence of V occur together with the Al3+ insertion during charge/discharge measurements. If the positive electrode reaction is described as eq 1, the theoretical capacity of V2O5 is evaluated to be 442 mAh (gV2O5)−1. In the eq 1, the valence of V changes between 5 and 3.5. After the first discharging, it was charged from 5 to 4.38. Since the valence of V was changed by 0.62 during the first discharging, the capacity of V2O5 is calculated as 183 mAh g−1, which is quite close to the capacity of the present rechargeable Al cell with the V2O5/C positive electrode. It means Mo current collector does not worked as positive electrode materials, and the V2O5 charge/discharge reactions with the Al3+ intercalation/deintercalation were first observed in the rechargeable Al battery. Amorphous vanadium oxide/carbon composite was applied to the positive electrode material for rechargeable Al battery. The amorphous V2O5 uniformly covered the KB showed reversible oxidation/reduction reaction in the mixed electrolyte solution of aluminum chloride, dipropylsulfone and toluene (1:10:5 in mass ratio). Charge/discharge cycle tests revealed that the rechargeable Al cell with the V2O5/C positive electrode showed maximum discharge capacity over 200 mA g−1 with the discharge rate at C/40. XPS analysis after discharging and the following charging indicated that the redox of vanadium ion in the V2O5/C active material occurred and the average valence of V changed between 4.38 and 4.91 during discharging and the following charging.

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S Supporting Information *

Figure 4. V 2p spectra of the V2O5/C positive electrode (a) asprepared, (b) after first discharging at C/40 rate, and (c) after the following charging at C/40 rate.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b06420. Experimental section for preparation and characterization of V2O5/C and electrochemical cells, TEM images of V2O5/C, and CV of Al plate electrode (PDF)

Table 1. Average Valence of Vanadium in V2O5 Electrode As-Prepared, After First Discharging at C/40 and after the Following Charging at C/40

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contents (%) V as-prepared after first discharge after following charge

5+

100 62.2 91.0

V4+

V3+

average valence

0 13.5 9.0

0 24.3 0

5.00 4.38 4.91

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

first discharging was 4.38. Then average valence was increased to 4.91. After the following charging, the content of V3+ and V4+ was significantly decreased and that of V5+ was increased again, indicating that V3+ and V4+ were oxidized to V5+. Aurbach et al. attempted Li+ intercalation/deintercalation with the V2O3 particle.15 In its CV, the reversible redox peaks between V4+ and V3+ accompanied by the Li+ intercalation/ deintercalation were observed at 2.5−2.8 V vs Li/Li+ in the cathodic sweep and 2.6−2.9 V vs Li/Li+ in the anodic sweep, which corresponded to 1.1−1.4 V vs Al/Al3+ and 1.2−1.5 V vs Al/Al3+, respectively. In the present study the V2O5/C electrode also showed the cathodic and anodic peaks at 0.8 and 1.6 V vs Al/Al3+ (Figure 2a), respectively, suggesting that the reversible redox reactions between V4+ and V3+ occurred. Mansour et al. revealed the valence changes of amorphous V2O5·nH2O (n = 0.5) ambigel positive electrode for Li-ion

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Shigeo Mori (Osaka Prefecture University) for the TEM measurement. REFERENCES

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DOI: 10.1021/acsami.5b06420 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX