AlPO₄-coated V₂ O₅ nanoplatelet and its electrochemical properties in aqueous electrolyte

Preparation of the V₂ O₅ nanoplatelet and its surface coating

V₂ O₅ nanoplatelets were synthesized by a sol-gel method using β-cyclodextrin (β-CD) as an exfoliation agent, as reported in our previous work [42]. Typically, 1.36 g β-CD (1 mmol) and 0.74 g NH₄ VO₃ (6 mmol) were dissolved in 200 ml distilled water at 70 °C forming a bright yellow solution. The color of the solution changed to clear blue after 24 ml H₂ C₂ O₄ solution (0.5 mol dm^–3) was added dropwise under magnetic agitation. The mixed solution was then continuously stirred at 80 °C until the precursor was obtained. The precursor was calcined at 400 °C for 10 h in air finally producing the bare V₂ O₅ sample.

Under strong mechanical agitation, a solution of NH₄ H₂ PO₄ in ethanol (0.1 mol dm^–3) was slowly dropped into the Al(NO₃)₃ suspension (0.1 mol dm^–3), to which the V₂ O₅ nanoplatelet had previously been added. The mass fraction of the formed AlPO₄ was controlled to about 0.8 %, 1.6 % and 3.2 % of the V₂ O₅ nanoplatelet. The precursor was obtained by evaporating the ethanol at 60 °C under continuous agitation for 2 h, and calcined at 300 °C for 5 h in air producing the AlPO₄-coated sample.

Ammonium metavanadate (NH₄ VO₃), β-cyclodextrin (β-CD), oxalic acid (H₂ C₂ O₄) Al(NO₃)₃·9H₂ O and NH₄ H₂ PO₄ as analytically pure agents were bought from National Pharmaceutical Group Corporation (Shanghai, China).

Characterizations of the samples

The crystal structure of the sample was determined from the X-ray diffraction (XRD, Bruker D8) pattern. The sample’s morphology was observed by using the scanning electron microscope (SEM, Hitachi S-4800) and the transmission electron microscope (TEM, Jeol JEM2010).

Electrochemical studies of the samples

A cyclic voltammetry experiment was carried out on the EG&G 273 potentiostat/galvanostat system in the potential range of –0.7∼0.7 V (vs. SCE) using a three-electrode system, which uses the V₂ O₅ electrode, Pt sheet electrode and saturated calomel electrode (SCE) as the working electrode, counter electrode and reference electrode, respectively. A galvanostatic charge – discharge measurement was performed on an Arbin2000 instrument using the three-electrode system. In all tests, 1 mol L^–1 Li₂ SO₄ solution was chosen as the electrolyte. V₂ O₅, acetylene black and poly-tetrafluoroethylene (PTFE) emulsion (30 % in ethanol) with a mass ratio of 8:1:1 were first mixed. The mixture was rolled into a 100 μm thick membrane with help of ethanol, and was pressed onto a stainless steel mesh current collector. The obtained membrane was dried at 120 °C overnight and cut into 1 × 1 cm squares obtaining the V₂ O₅ electrode. The loading of V₂ O₅ in one piece of the electrode was about 5.1 mg.

Morphology of the nanoplatelet

Figure 1 shows the SEM images of the bare (a) and the AlPO₄-coated (b) V₂ O₅ nanoplatelets. It is found that the bare V₂ O₅ nanoplatelet is smooth (Fig. 1a), and displays 500 nm in length and 200 nm in width. After being coated with AlPO₄, the V₂ O₅ nanoplatelet seems also smooth, and less nanoparticles can be found on their surface (Fig. 1b). TEM images of the bare (c) and the AlPO₄-coated (d–f) V₂ O₅ nanoplatelets are also shown in Fig. 1. It can be seen that the nanoplatelets are completely covered by the AlPO₄ layer whose thickness increases with the amount of AlPO₄. The nanoplatelets coated with 1.6 % AlPO₄ exhibit a layer about 20 nm in thickness (Fig. 1e). The AlPO₄-coating layer was confirmed to be homogeneous on the surface of the V₂ O₅ nanoplatelet by the energy dispersive X-ray analysis (EDX) shown in Fig. 2, in which the P, Al, V and O elements are well distributed throughout the whole region.

Fig. 1

SEM images of the bare (a) and the 1.6 % AlPO₄-coated (b) V₂ O₅ nanoplatelets. TEM images of the bare (c), 0.8 % (d), 1.6 % (e) and 3.2 % (f) AlPO₄-coated V₂ O₅ nanoplatelets.

Fig. 2

TEM image (a) and the corresponding EDX elemental mappings of (b) V, (c) O, (d) Al, and (e) P for the 1.6 % AlPO₄-coated V₂ O₅ nanoplatelet.

Crystal structure of the nanoplatelet

Figure 3 shows the powder XRD patterns of the bare and coated V₂ O₅ nanoplatelets. For the bare V₂ O₅ nanoplatelet, the diffraction peaks that located at 15.4, 20.3, 21.7, 26.1, 31.1, 32.4, 33.4, 34.5, 41.2, 42.1, 45.5, 47.4, 47.9, 48.9, 51.3, 52.2, 55.8, 56.4, 58.6, 59.0, 61.1 and 62.2 °, which can be ascribed to the crystalline planes of (200), (001), (101), (110), (400), (011), (111), (310), (002), (102), (411), (600), (302), (012), (020), (601), (021), (121), (611), (412) (420) and (710), respectively. These characteristic diffraction peaks are in good agreement with those of the standard values (JCPDS No.77-2418), indicating the bare sample have well-symmetric orthogonal structure (space group: Pmmn). After being coated with the AlPO₄ layer, the nanoplatelet shows similar diffraction peaks to the bare V₂ O₅ sample except the intensity of diffraction peak decreases. The results suggest that the AlPO₄ coating layer is amorphous, and the crystal structure of the V₂ O₅ nanoplatelet has no significant change with the surface modification.

Fig. 3

XRD patterns of the bare and AlPO₄-coated V₂ O₅ nanoplatelets.

Cylic voltammetry of the nanoplatelet

Figure 4 shows the cyclic voltammograms (CV) of the bare (a) and the coated (b) V₂ O₅ samples in the Li₂ SO₄ aqueous solution at various scan rates at 25 °C. It is clear that there are two pairs of redox peaks in each curve. In the case of 2 mV s^–1, we can see that one redox peak locates at 0.019/–0.124 V and another one at 0.217/0.095 V (vs. SCE) for the bare V₂ O₅ nanoplatelet. The scan potential range is 0.7∼–0.7 V (vs. SCE), which is equivalent to 3.94∼2.54 V (vs. Li/Li⁺), indicating that the V₂ O₅ nanoplatelet took place one-electron reaction per formula [46–48]. Therefore, the V₂ O₅ nanoplatelet could be considered to experience the phase transitions from α to ε at 0.095 V, and ε to δ at –0.124 V, respectively, when intercalated with Li⁺ ion. The corresponding electrochemical reactions can be assigned as following:

Fig. 4

Cyclic voltammograms of the bare (a) and the AlPO₄-coated (b) V₂ O₅ nanoplatelets at various scan rates at room temperature (25 °C).

The surface-coating sample displays the similar redox peaks to the bare one (Fig. 4b), which located at 0.016/–0.114, 0.213/0.097 V (vs. SCE). However, the potential differences of the redox couples are 0.143 and 0.122 V for the bare sample, and 0.130 and 0.116 V for the coated sample. It is obvious that the bare sample possesses larger potential differences than the coated sample, implying that the latter exhibits more reversible electrode reaction than the former [49].

Figure 5 shows the relationship between the scan rates and peak currents of the CV curves for the bare (a) and AlPO₄-coated (b) samples. It can be seen that all the i_p∼v^1/2 plots appear linear indicating that the Li⁺ intercalation is limited by solid-state diffusion. It is well known that the relationship between the peak currents and the scan rates in the CV plots could be used to calculate the diffusion coefficient of Li⁺ (D_Li) according to the Randles – Sevcik equation if the rate-limiting step is Li⁺ diffusion in electrode [49].

Fig. 5

Relationship of the peak current (i_p) and the square root of scan rate (v^1/2) for the bare (a) and the AlPO₄-coated (b) V₂ O₅ nanoplatelets.

where i_p is the peak current (A), n is the charge-transfer number, A is the geometric area of electrode (cm²), C_Li^* is the concentration of Li⁺ in the cathode (mol cm^–3) (the molar volume of V₂ O₅ is 54.06 cm³ mol^–1), and v is the potential scan rate (V s^–1). The calculated D_Li values for all the phase transitions of the V₂ O₅ nanoplatelet are summarized in Table 1. It is clear that all the diffusion coefficients of the sample decrease slightly with surface-coating of the AlPO₄ amorphous layer, which is not electrochemically active. Each sample has a larger D_Li value of Li⁺ insertion during the phase transition from α to ε than that from ε to δ, suggesting the previously inserted Li would hinder its supervenient insertion in the layered structure [50].

Battery performance of the nanoplatelets

Figure 6 displays the cycling performances of the bare (a) and 1.6 % AlPO₄-coated (b) V₂ O₅ nanoplatelets at various rates, (c) cycling performances of the bare and 0.8 %, 1.6 %, 3.2 % AlPO₄-coated V₂ O₅ nanoplatelets at various rates (0.5, 1.5, 3 C), (d) the galvanostatic charging – discharging curves of the 1.6 % AlPO₄-coated sample. It is found that the bare sample delivers the capacities of 136, 127, 113 and 105 mAh g^–1 at 0.1, 0.3, 1.5, and 3 C rates in the first cycle, respectively. (Here the 1 C rate equals to 147 mA g^–1.) The corresponding capacities are 128, 119, 113 and 104 mAh g^–1 for the 1.6 % AlPO₄-coated sample. One can see that two potential plateaus at 0.07 and –0.13V (vs. SCE) appear in the discharging curves (Fig. 6d), which agree with those cathodic potential peaks in the CV plots (Fig. 5b). The result suggests the surface-coating layer scarcely changes the rate capability of the V₂ O₅ nanoplatelet because the AlPO₄-coated sample has similar D_Li+ values to the bare sample. However, after 50 cycles, the bare sample only remains at 70, 62, 58, and 49 % of their own initial capacities at 0.1, 0.3, 1.5, and 3 C rates, respectively, while the AlPO₄-coated sample maintains 99, 98, 92, and 87 % of their own initial capacities.

Fig. 6

Cycling performances of the bare (a) and the 1.6 % AlPO₄-coated (c) V₂ O₅ nanoplatelets at various rates (0.1, 0.3, 1.5, and 3C). The insets in (a) and (c) show the digital photos of the test cells cycled at 1.5C for 50 times. (b) Cycling performance of bare V₂ O₅ and 0.8 %, 1.6 %, 3.2 % AlPO₄-coated V₂ O₅ at various rates (0.3, 1.5, 3C) for 20 cycles. (d) Galvanostatic charging – discharging curves of the 1.6 % AlPO₄-coated V₂ O₅ nanoplatelet at various rates.

The detailed data of discharge capacities and capacity retentions of the samples at various rates are summarized in Table 2. It is found that the AlPO₄-coating layer is beneficial to capacity retention but adverse to rate capability of the V₂ O₅ nanoplatelets. The capacity retention is very much enhanced for the 3.2 % AlPO₄-coated sample, but the rate capability declines. It is obvious that the AlPO₄-coated sample possesses a superior cycling performance to the bare sample while the former exhibits an inferior rate capability to the latter. The main reason is that the physical barrier from the AlPO₄-coating layer prevents the V₂ O₅ nanoplatelet from contacting the aqueous electrolyte thus decreasing its dissolution on the one hand, which was confirmed by the digital photographs of the test cells after 50 cycles shown in Fig. 6 insets. From the pictures, one can see that the electrolyte in the test cell using the AlPO₄-coated sample displays much lighter color than that using the bare sample. The concentration of the VO²⁺ for the former (Fig. 6a) is 58.9 ppm whereas that for the latter (Fig. 6c) is 5.8 ppm, which measured by using the atomic absorption spectrophotometer (AAS, Shimadzu AA-6800). On the other hand, the AlPO₄-coating layer inhibits diffusion of Li⁺ ions into V₂ O₅ nanoplatelet resulting in a bad rate capability. The results suggest the 1.6 % AlPO₄-coated V₂ O₅ is superior in consideration of both rate capability and capacity retention.