Hierarchical Cu₄V_2.15O_9.38 micro-/nanostructures: a lithium intercalating electrode material

Abstract

Hierarchical Cu₄V_2.15O_9.38 micro-/nanostructures have been prepared by a facile “forced hydrolysis” method, from an aqueous peroxovanadate and cupric nitrate solution in the presence of urea. The hierarchical architectures with diameters of 10–20 µm are assembled from flexible nanosheets and rigid nanoplates with widths of 2–4 µm and lengths of 5–10 µm in a radiative way. The preliminary electrochemical properties of Cu₄V_2.15O_9.38 have been investigated for the first time and correlated with its structure. This material delivers a large discharge capacity of 471 mA h g⁻¹ above 1.5 V, thus making it an interesting electrode material for primary lithium ion batteries used in implantable cardioverter defibrillators.

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Introduction

Primary and secondary batteries, which provide back-up for intermittent wind and solar energy and thus represent one of the most important energy storage and conversion devices nowadays, are playing more and more important roles in modern society.^1–11 They are currently being developed to power an increasingly diverse range of applications, from microchips to cars. One significant role that the batteries play in biomedical area is to power implantable cardioverter defibrillators (ICDs), which are widely applied to hundreds of millions of cardiovascular patients.^12–15 The cardiovascular disease was the number one cause of death and caused an estimated 17.5 million people deaths in 2005, representing 30% of the global deaths according to the World Health Organization.¹⁶ The ICDs, which are powered by primary lithium ion batteries, continuously monitor the heartbeats; when dangerous arrhythmias are detected, they deliver one or more high-energy electric shocks to return the heart to a normal rhythm; thus, they can effectively protect the cardiovascular patients against sudden cardiac death.¹²

Lithium/silver vanadium oxide (SVO) is the dominating type of cathode currently used in primary batteries for ICDs owing to its high energy density, high power performance and long-term stability.^12,15,17,18 However, from the standpoint of future applications, the batteries for ICDs are facing more stringent requirements, including larger discharge capacity, higher power capability, and longer lifespan.^12,19 Since the performances of lithium ion batteries are mainly cathode limited,²⁰ the seeking of alternative cathode materials for ICDs with improved performance is of significant scientific and technological importance. Recent studies on hybrid Ag₂V₄O₁₁-CF_x,²¹Ag₄V₂O₆F₂,^22–24SVO nanowires,^18,25 silver molybdenum oxyfluorides,^26,27silver vanadium phosphorus oxides,^28–30silver iron vanadates,³¹CuV₂O₆ micro-/nanostructures¹⁹ have shown exciting enhancement over discharge capacity, high rate capability, discharge voltage, or cycling performance.

Among the potential candidates, copper vanadium oxides (CVOs, also called copper vanadates) are of particular interest. The benefits of CVOs over SVO include not only the much lighter atomic weight and much lower cost of Cu when compared with Ag, but also the fact that Cu²⁺ takes a two-electron reduction process to Cu⁰ during the discharge rather than the single-electron reduction.¹⁹ Thus, it is reasonable to expect that CVOs may deliver higher energy density than that of SVO by taking advantage of the two-electron Cu²⁺/Cu redox couple. Indeed, according to a recent report by Ma et al.,¹⁹α-CuV₂O₆ mesowires and nanowires may deliver a discharge capacity as high as 447–514 mA h g⁻¹, much larger than that of SVO which has a capacity of 315 mA h g⁻¹. Moreover, CVOs have a wide variety of crystalline phases,^19,32–37 such as CuV₂O₆, Cu_2.33V₄O₁₁, ε-Cu_0.85V₂O₅, Cu₂V₂O₇, Cu₅V₂O₁₀, Cu_1.1V₄O₁₁, Cu₁₁V₆O₂₆, and Cu₄V_2.15O_9.38. Although most of the CVO phases have been demonstrated to be potential cathode materials for either primary or secondary lithium ion batteries, the electrochemical properties of Cu₄V_2.15O_9.38 have been ignored since its discovery.³² Therefore, it is of interest to investigate the electrochemical performance of this compound.

Here we report the synthesis of hierarchical Cu₄V_2.15O_9.38 micro-/nanostructuresvia a facile “forced hydrolysis” method. The superstructures are assembled from flexible nanosheets and rigid nanoplates in a radiative way. The hierarchical micro-/nanostructures provide this material with a large discharge capacity of 471 mA h g⁻¹ at 5 mA g⁻¹ (above 1.5 V), thus making it an interesting candidate for primary lithium ion batteries used in ICDs.

Experimental section

Synthesis

In a typical synthesis, 5.0 mmol of vanadium pentoxide (V₂O₅) was dissolved in 100 mL of 3.0 wt% hydrogen peroxide (H₂O₂) solution at 25 °C to form a orange-colored clear solution. After stirring for 12 h at this temperature, 15 mmol (0.90 g) of urea (CO(NH₂)₂) and 5 mmol (1.2 g) of cupric nitrate trihydrate (Cu(NO₃)₂·3H₂O) were added to the above solution. The mixture solution was sealed in a Teflon-lined autoclave and hydrothermally treated at 200 °C for 24 h. The products were collected by filtration, washed with water thoroughly for 3 times, and dried at room temperature.

Characterization

X-Ray diffraction (XRD) pattern was recorded on a Bruker D4 X-Ray Diffractometer with Ni-filtered Cu Kα radiation at a voltage of 40 kV and a current of 40 mA. Scanning electron microscopy (SEM) images were obtained on a Philips XL30 microscope operated at 20 kV. Transmission electron microscope (TEM) experiments were conducted on a JEOL 2100 microscope with an accelerating voltage of 200 kV.

Electrochemical measurement

The electrochemical test was characterized in a CR2016-type coin cell. The working electrodes were prepared by mixing 80% active material (namely the as-prepared Cu₄V_2.15O_9.38), 10% carbon black, and 10% polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidinone (NMP). The slurries of the mixture were coated on stainless steel foil. After coating, the electrodes were dried at 80 °C for 10 min to remove the solvent before pressing. The electrodes were cut into sheets with 1 cm² in area, vacuum-dried at 100 °C for 24 h, and weighed before assembly. The weight of active material on each electrode was ∼8 mg. The cell assembly was operated in a glove box (model 100G, MBraun, Germany) filled with pure argon. The electrolyte solution was 1 M LiPF₆ in ethylene carbonate (EC)/diethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (1 : 1 : 1 by volume). The cells were assembled with the cathode as-prepared, lithium metal as an anode, and Celgard 2300 film as a separator. The capacity of the electrode was measured by a galvanostatic discharge method at 5–80 mA g⁻¹ to the cutoff voltage of 1.5 V, using a LAND CT2001A Battery Cycler (Wuhan, China) at room temperature. The capacity (above 1.5 V) was calculated based on the amount of the active material, excluding the weight of the additives in the electrode. The cyclic voltammogram (CV) was performed on a CHI660 electrochemical workstation (CHI, USA) at a scanning rate of 0.1 mV s⁻¹ and at room temperature.

Results

The Cu₄V_2.15O_9.38 hierarchical micro-/nanostructures were prepared by a “forced hydrolysis” method from an aqueous peroxovanadate and cupric nitrate solution in the presence of urea. Fig. 1 shows the XRD pattern of the as-synthesized Cu₄V_2.15O_9.38 hierarchical structures. All the diffraction peaks can be indexed to orthorhombic Cu₄V_2.15O_9.38 with the lattice parameters of a = 1.502 nm, b = 0.8564 nm, c = 0.6055 nm and a space group of P2₁2₁2₁ (Joint Committee on Powder Diffraction Standards, JCPDS Card No. 70-1696). No other peaks have been detected, suggesting the high purity of the sample. The relatively broad diffraction peaks demonstrate the nanocrystalline characteristic of the Cu₄V_2.15O_9.38 products.

Fig. 1 XRD pattern of the as-synthesized Cu₄V_2.15O_9.38 hierarchical structures.

Fig. 2 and S1† represent the SEM images of the as-synthesized Cu₄V_2.15O_9.38 products. Large quantities of Cu₄V_2.15O_9.38 hierarchical architectures with diameters of 10–20 µm and high purity can be observed in Fig. S1†. As can be seen from higher magnification SEM images (Fig. 2), the superstructures are assembled from thin and flexible nanosheets (indicated by black arrows) and relatively thick and rigid nanoplates (indicated by white arrows) in a radiative way. The nanosheets and nanoplates have similar lengths (5–10 µm) and widths (2–4 µm). Both the nanosheets and the nanoplates have sharp tips and smooth surfaces.

Fig. 2 SEM images of the as-synthesized Cu₄V_2.15O_9.38 hierarchical structures.

To understand the detailed structural and morphological characteristics of the as-synthesized products, TEM technology was employed. A typical TEM image of a well-developed single Cu₄V_2.15O_9.38nanosheet broken from the superstructures by ultrasonication is shown in Fig. 3a. Instead of being perfectly flat, the nanosheet displays some intrinsic out-of-plane wrinkles which indicate the high crystallinity and the ultrathin nature of the nanosheet. The corresponding selected area electron diffraction (SAED) pattern, as shown in Fig. 3b, shows a set of diffraction spots which indicate the single-crystalline characteristic of the nanosheet, and it can be indexed to the [100] zone axis of orthorhombic Cu₄V_2.15O_9.38. What's worth mentioning is that, besides the strong diffraction spots such as 020* and 002*, some weak diffraction spots can also be found at those forbidden sites (such as 010* and 001*) as indicated by white arrows in Fig. 3b. A possible reason for those weak spots is the high-order Laue zone caused by the combination of ultrathin nature of the nanosheet (elongation of the spots along the normal of the nanosheet) and the large {100} spacing (narrowed Laue zones along [100] direction). Similar phenomenon has also been found in the case of WO₃·0.33H₂O nanosheets in our previous study.³⁸ A combination of the TEM and the corresponding ED pattern suggests that the nanosheet grows along the [001] direction, and the top and bottom surfaces are {100} planes, while the other surfaces of the nanosheet are surrounded by the {014}, {010} and {011} planes. The high resolution TEM (HRTEM) image of the nanosheet and the corresponding simulated one are shown in Fig. 3c and d, respectively. Two sets of atomic spacings can be distinguished from Fig. 3c clearly, which correspond to the {020} (0.43 nm) and {002} (0.30 nm) lattice fringes.

Fig. 3 TEM image (a), ED pattern (b), HRTEM (c) and simulated HRTEM images (d) of a single Cu₄V_2.15O_9.38nanosheet.

A typical TEM image of an individual nanoplate and the corresponding SAED pattern are shown in Fig. 4a and b, respectively. The diffraction spots shown in Fig. 4b indicate the single-crystalline nature of the nanoplate, which can be indexed to the [120] zone. Again, besides the allowed diffraction spots, some forbidden spots can also be observed in Fig. 4b such as 001* and 001̄* as indicated by white arrows. However, in this case (relatively thick nanoplate), the unusual 001* and 001̄* spots are due to double diffraction caused by the dynamic scattering of the strong electron beam rather than “high-order Laue zone” in the nanosheet case. By comparing Fig. 4a and b, it is confirmed that the nanoplate also grows along the [001] direction, the top and bottom surfaces of the nanoplate are {120} planes, and the other surfaces are enclosed by the {210} and {211} planes. Fig. 4c and d represent the HRTEM image and the simulated HRTEM image of the nanoplate, respectively. The {002} (0.30 nm) and {210} (0.56 nm) spacings can be observed clearly from Fig. 4c.

Fig. 4 TEM image (a), ED pattern (b), HRTEM (c) and simulated HRTEM images (d) of a single Cu₄V_2.15O_9.38 nanoplate.

From Fig. 3 and 4, it has been confirmed that both the nanosheets and nanoplates grow along the [001] direction; however, they may have different normals, i.e. [100] or [120]. It should be mentioned that not all the nanoplates have a normal of [120] direction, and vice versa. However, one can distinguish the normals by measuring the angle of the tips of the nanosheets or nanoplates, since the dihedral angle between the (011) and (011̄) planes is 70.5° (or 180° − 70.5° = 109.5°, see Fig. 3a), and the dihedral angle between the (21̄1) and (2̄11) planes is 94° (or 180° − 94° = 86°, see Fig. 4a).

The preliminary electrochemical properties of the as-prepared hierarchical Cu₄V_2.15O_9.38 micro-/nanostructures have also been studied. Fig. S2† shows the cyclic voltammogram (CV) of the electrode made from the as-synthesized Cu₄V_2.15O_9.38 product for the first cycle at a scan rate of 0.1 mV s⁻¹ in the potential window of 3.0–1.0 V. In the cathodic polarization process, a strong peak at ∼1.9 V vs.Li⁺/Li can be observed. Since Cu₄V_2.15O_9.38 is composed of V⁵⁺ and Cu²⁺, this peak can be assigned to the reduction of V⁵⁺ and Cu²⁺ due to the lithium intercalation. During the following anodic polarization, no peaks can be observed, which means the lithium intercalation process is irreversible. Similar irreversible lithium insertion behavior has also been reported in Ma et al.'s¹⁹ study on α-CuV₂O₆ nano/mesowires.

Fig. 5 displays the discharge profiles of the electrode made from as-prepared Cu₄V_2.15O_9.38 micro-/nanostructures between 3.0 and 1.5 V at various current densities. At a current density of 5 mA g⁻¹, a flat plateau at ∼2.2 V can be identified in the first discharge profile. The initial discharge capacity of the as-synthesized Cu₄V_2.15O_9.38 product to a cutoff voltage of 1.5 V is 471 mA h g⁻¹, which is equivalent to an intercalation of approximately 9 Li⁺ per formula unit according to the Faraday equation. By increasing the current density from 5 mA g⁻¹ to 80 mA g⁻¹, both the discharge voltage plateau and discharge capacity drop slightly. At a current density of 80 mA g⁻¹, the capacity retains 348 mA h g⁻¹, 74% of that under 5 mA g⁻¹. In agreement with the CV results (irreversible lithium insertion behavior), the capacity fades rapidly during the following cycles (Fig. S3†).

Fig. 5 Discharge profiles of the cell made from Cu₄V_2.15O_9.38 micro-/nanostructures at various current densities of 5–80 mA g⁻¹ between 3.0 and 1.5 V.

Discussion

The utilization of urea in the “forced hydrolysis”^39–42 plays an essential role in the successful preparation of Cu₄V_2.15O_9.38 phase. It has been reported that a direct hydrothermal treatment of a peroxovanadate solution leads to the formation of single crystalline V₂O₅ nanobelts via a layered vanadium oxide hydrate (V₂O₅·nH₂O) intermediate phase.⁴³ In order to investigate the effects of urea, a controlled experiment (the synthesis conditions were identical with those of hierarchical Cu₄V_2.15O_9.38 in this case except no urea was used) was performed. When cupric nitrate trihydrate was introduced into the aqueous peroxovanadate solution and hydrothermally treated at 200 °C for 24 h, rather than the Cu₄V_2.15O_9.38 phase, layered vanadium oxide hydrate phase^43–45 was obtained as confirmed by XRD and SEM characterizations (Fig. S4 and S5†, respectively). The basal distance deduced from the position of the first diffraction peak, d = 1.36 nm, is a little smaller than that reported by Liet al.⁴³ (1.42 nm), while larger than that shown by Alonso and Livage⁴⁵ (1.2 nm). Only when urea (10–20 mmol) is introduced simultaneously with cupric nitrate trihydrate, hierarchical Cu₄V_2.15O_9.38 micro-/nanostructures can be obtained (Fig. 1).

It is well-known that vanadium forms a wide variety of polyanions under different pH conditions.^46,47 In our system, the pH value of the solution is controlled in situ by the hydrolysis of urea. Thus, the dosage of urea also affects the formation of vanadium species. It is therefore not surprising that the Cu/V ratio in the products (4/2.15) differs significantly from the initial Cu/V ratio (1/2) in the solution since not all of the vanadium species precipitate in the reaction. In other words, some vanadium may exist in the solution after reaction in the form of polyanions. Considering that the final pH value of the supernatant after hydrothermal treatment is between 8.5 and 9.0, most of the vanadium may exist in the forms of VO₃OH₂⁻, V₄O₁₂⁴⁻ and V₃O₉³⁻ in the solution.^46,47 Similar phenomenon has also been observed in the synthesis of β-AgVO₃ as reported by Song et al.⁴⁸

With regard to CVOs, some phases can electrochemically react with Li⁺ reversibly, thus can be fabricated into secondary batteries;^35,36,49,50 while others cannot, and they can only be used in primary batteries.¹⁹ As we mentioned above, the electrochemical reaction between Cu₄V_2.15O_9.38 and Li⁺ is irreversible. Two major reasons account for this irreversibility. First, as shown in the structure of Cu₄V_2.15O_9.38 in Fig. S6†, a majority of the vanadium atoms sit in [VO₄] tetrahedral, and the other vanadium atoms sit in [VO₅] trigonal bipyramids, while after the V⁵⁺/V⁴⁺ reduction process, the V⁴⁺ ions are not stable in a tetrahedral environment.^22,35 Second, due to the intrinsic large Cu/V ratio of Cu₄V_2.15O_9.38, the [VO_x] polyhedrals cannot maintain the original and relatively rigid structure after the Cu²⁺/Cu⁰ reduction process. Thus, this Cu rich Cu₄V_2.15O_9.38 phase can only find applications in primary lithium ion batteries.

Based on the formula Cu₄V_2.15O_9.38, and assuming the complete reduction of Cu²⁺ to Cu and V⁵⁺ to V³⁺, it may be expected that up to 12.3 Li⁺ can be accommodated by this material. This would result in a capacity of 644 mA h g⁻¹. If the reduction of Cu²⁺ to Cu is only followed by the reduction of V⁵⁺ to V⁴⁺, the theoretical capacity would be 531 mA h g⁻¹. A practical capacity of 471 mA h g⁻¹ has been achieved in the current study. When compared Cu₄V_2.15O_9.38 to the commercial SVO cathode (Ag₂V₄O₁₁) for ICDs, the former has a much larger theoretical and practical capacity over 1.5 V, but lower discharge potential. For Cu₄V_2.15O_9.38, the discharge plateau is at ∼2.2 V; while for Ag₂V₄O₁₁, there are two plateaus, an initial one at ∼3.25 V, and a secondary one at roughly 2.5 V. The extended capacity of Cu₄V_2.15O_9.38 can be attributed to the two-electron reduction of Cu²⁺ and the lighter atomic weight of Cu compared to that of Ag. The lower potential is arisen from the lower redox potential of Cu²⁺/Cu (0.34 V vs.NHE) compared to that of Ag⁺/Ag (0.80 V vs.NHE). Considering the relatively low discharge voltage of this material, further improvement of the discharge potential by methods such as fluoride incorporation²² is required.

Conclusions

In summary, we have successfully prepared hierarchical Cu₄V_2.15O_9.38 micro-/nanostructuresvia a facile “forced hydrolysis” method. Preliminary electrochemical characterization demonstrates that this material delivers a much larger discharge capacity (471 mA h g⁻¹ above 1.5 V), but lower discharge potential than that of commercial SVO cathode for ICDs. Our contribution provides insights for the synthesis of novel electrode materials with extended capacity. Nevertheless, the Cu₄V_2.15O_9.38 material in the current study cannot fulfil all the requirements for ICD cathodes, further improvement of the discharge voltage of this material should be carried out by methods such as fluoride incorporation.²²

Acknowledgements

The authors thank Prof. Jin Zou for helpful discussions. This work is supported by the 973 Program (2010CB226901), Science & Technology Commission of Shanghai Municipality (0852nm01500) and SLADP (B108, B113).

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Footnote

† Electronic supplementary information (ESI) available: SEM images of hierarchical Cu₄V_2.15O_9.38, CV curves of the electrode and discharge profiles of the cell made from Cu₄V_2.15O_9.38 hierarchical structures, XRD pattern and SEM images of layered vanadium oxide hydrate, structure model of Cu₄V_2.15O_9.38. See DOI: 10.1039/c0nr00657b

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