{V2O5 as magnesium cathode material with extended cyclic stability}
http://dx.doi.org/10.5599/jese.769 257
J. Electrochem. Sci. Eng. 10(3) (2020) 257-262; http://dx.doi.org/10.5599/jese.769
Open Access: ISSN 1847-9286
www.jESE-online.org
Original scientific paper
V2O5 as magnesium cathode material with extended cyclic
stability
Charalampos Drosos1,, Benjamin Moss2, Andreas Kafizas2,3, and
Dimitra Vernardou4,
1Delta Nano – Engineering Solutions Ltd., Cyprus
2Department of Chemistry, Molecular Science Research Hub, Imperial College London, White City
Campus, London, W12 0BZ, U.K.
3Grantham Institute for Climate Change, Imperial College London, South Kensington, London, SW7
2AZ, U.K.
4Department of Electrical & Computer Engineering, School of Engineering, Hellenic Mediterranean
University, 710 04 Heraklion, Crete, Greece
Corresponding authors: info@delta-nano.com; a.kafizas@imperial.ac.uk; dvernardou@hmu.gr
Received: December 24, 2019; Revised: February 19, 2020; Accepted: February 22, 2020
Abstract
In this work, the electrochemical performance of aerosol-assisted chemical vapour deposited
vanadium pentoxide cathodes at 600 °C, is presented. The as-grown oxides indicate specific
discharge capacity of 300 mA h g-1 with capacity retention of 92 % after 10000 scans,
coulombic efficiency of 100 %, noble structural stability and high reversibility. The present
study shows the possibility to grow large-area magnesium cathode material with extended
cycle stability via utilization of an aqueous electrolyte under a corrosive environment. This
enhanced performance may be a combination of electrode morphology and adherence, when
compared to previous work employing electrode growth temperature at 500 °C.
Keywords
Magnesium ion batteries; chemical vapor deposition; electrode morphology; coating
adherence; corrosive environment.
Introduction
Aqueous metal-ion batteries (AIBs) are auspicious alternatives for large-scale energy storage
since the water-based battery system is low cost, environmentally friendly, safe and does not
require rigorous manufacturing conditions (elimination of glove box in the production line) [1].
Additionally, AIBs have no special operational and maintenance requirements and do not require
regular charge because of low self-discharge [2].
http://dx.doi.org/10.5599/jese.769
http://dx.doi.org/10.5599/jese.769
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mailto:info@delta-nano.com
mailto:a.kafizas@imperial.ac.uk
mailto:dvernardou@hmu.gr
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258
In this perspective, multivalent-ion technologies (Mg2+, Ca2+, Al3+) have been proposed as
replacements for Li-ion technology because they can store more charge per ion at the cathode, leading
to larger energy density than the univalent ion batteries [3]. Among them, magnesium-ion batteries
have attracted attention due to the high abundance of magnesium (1.94 %) [4], high volume specific
capacity (3833 mAh cm-3) [5] and low reduction voltage (-2.37 V vs. SHE) [6].
In our previous work [7], the viability of a technology that is proven to be industrially competitive
– that of cold-wall aerosol assisted chemical vapor deposition was established for the growth of
vanadium pentoxide (V2O5) with excellent performance as cathodes for aqueous magnesium-ion
batteries (specific discharge capacity 427 mA h g-1 with capacity retention of 82 % after 2000 scans).
The cathodes presented an increase of current intensity as the scan number proceeded to 500,
remaining however unchanged after 1000 scans. Inspired by the respectable results of Mg2+ ion
battery chemistry, herein we would like to report that this path can be further exploited and
enhanced for high-performance cathode materials. In this work, we demonstrate that a mixed phase
of α-V2O5 and β-V2O5 deposited at 600 °C (i.e. higher substrate temperature than before [7]) is
capable of reversibly inserting magnesium ions showing a specific discharge capacity of 300 mAh g-
1 with capacity retention of 92 % after 10000 scans, which may be a combination of electrode
morphology and adherence. This particular cathode offers a safe and economical energy storage
solution to large-scale applications with extended cyclic stability.
Experimental
Synthesis of V2O5 cathodes
Thin layers of V2O5 were deposited at 600 °C, following the same procedure of aerosol-assisted
chemical vapor deposition (AACVD) as previously published [7], having similar characteristics in
color and stability.
Physical characterization
X-ray diffraction (XRD) was implemented to study the structure of V2O5 coatings on a modified
Bruker-Axs D8 diffractometer. Field-emission scanning electron microscopy (FE-SEM) and x-ray
photoelectron spectroscopy (XPS) were utilized to evaluate the surface morphology and chemistry of
the material in its initial state, after Mg2+intercalation and 10000 continuous Mg2+ intercala-
tion/deintercalation scans. The processing parameters and the instruments employed were the same
as those reported in [7].
Electrochemical performance
The AACVD coating, Pt foil (with surface area 1 cm2) and Ag/AgCl/KCl (0.1 M) were acted as
working, counter and reference electrodes [7,8], respectively. An aqueous solution of 0.075 M MgCl2
was conducted as the electrolyte in all measurements for the scan rate of 10 mV s-1 and potential
ranging -1.5 V to +1.0 V. Cyclic voltammograms, potentiometric measurements and electrochemical
impedance spectroscopy (EIS) studies were performed in an electrochemical workstation
(PGSTAT302N) [7].In particular, EIS curves were performed for alternating current (AC) amplitude
of 5.0 mV and set potential of -1.0 V over the frequency range of 100 kHz - 10 mHz.
Results and discussion
The crystal structure of the coating presents three peaks at 12.6, 21 and 30.7o with respective Miller
indices (200), (001) and (301) (Figure 1a). The first one is consistent with the formation of β-V2O5 [9],
C. Drosos et al. J. Electrochem. Sci. Eng. 10(3) (2020) 257-262
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while the other two with α-V2O5[10]. Regarding the rest of the peaks (■) are due to the FTO glass
substrate [11]. To further investigate the surface chemistry, XPS indicates in Figure 1b binding energies
at 517.3 (V 2p3/2), 524.9 (V 2p1/2) and 530.2 eV (O 1s), which correspond to V5+ and O2- of V2O5 [12,13].
Additionally, there is a peak at 532.6 eV associated with the presence of chemisorbed OH groups on
the surface of the coating [12]. Increasing the substrate temperature from 500 °C [7] to 600 °C, leads
to the improvement in the crystallinity (i.e. (001) peak intensity increases) as expected for a
conventional activated process. Higher temperatures enhance surface mobility, which in turn
improves coating crystallization. The co-existence of α- and β-V2O5 is supported by SEM analysis
indicating the grain structure recorded at magnification of ×50k (inset of Fig. 1a). It is clearly observed
that the coating exhibits a compact structure with well distributed grains as compared with pure α-
V2O5 [7]. The grains have irregular shape presenting some degree of agglomeration.
Figure 1.XRD of V2O5 coating grown on FTO glass substrate by AACVD at 600 °C (squares represent
reflections from FTO glass substrate) (a). XPS depth profile and FE-SEM image
at ×50000 of V2O5 coating as inset (b).
The electrochemical intercalation of Mg2+ into V2O5 coatings was studied in a three-electrode
electrochemical cell using an aqueous solution of 0.075 M MgCl2 as electrolyte. To examine if Mg2+
ions intercalate into the lattice of the host material, the state of the vanadium was examined by
XPS. Figure 2a inset shows Mg 1s spectrum, which presents one peak at 1304 eV [14].
Figure 2. XPS depth profiles (a) and FE-SEM image (b) at ×50000 after Mg2+ intercalation into
the host of V2O5 coating.
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The clear magnesium signal indicates that metallic Mg is indeed intercalated from the electrolyte.
The core level XPS spectrum of O 1s indicates two peaks, which are located at 530.1 and 532.5 eV
due to O2- in V2O5 and OH groups on the surface of V2O5 [12], respectively. After Mg2+ intercalation
in the host of V2O5, the XPS spectrum resembles that of V2O5 in Figure 1a and only minor
morphological changes observed (Figure 2b). The stability of V2O5 was evaluated via cyclic
voltammograms for 10000 continuous Mg2+ intercalation/deintercalation scans (Figure 3a) in the aqueous
MgCl2 0.075 M electrolyte sweeping the potential from -1.5 V to +1.0 V (vs. Ag/AgCl/KCl(0.1 M)). The
voltammograms show one oxidation peak at -0.61 V with the reverse reaction occurring, as presented by the
corresponding reduction peak at -1.10 V. These peaks are observed after the first scan due to the activation
of intercalation/deintercalation processes into V2O5 host cathode material. The current density was found to
decrease from the first to the 2000th scan and remained constant afterwards, presenting an extended cyclic
stability compared to the previously published work [7].
The specific capacity of V2O5 cathode was evaluated under the constant specific current of 15 A g-
1 and applied potentials ranging from -1.5 V to +0.7 V for the first and the 10000th scan (Figure 3b).
A peak at approximately -1.12 V is indicated, employing the one-step deintercalation (discharge)
process for the specific current applied. Additionally, a faint plateau is shown at approximately +0.5
V suggesting the one-step intercalation (charge) process. The specific discharge capacity was
estimated to be 300 mA h g-1 with capacity retention of 92 % after 10000 scans and coulombic
efficiency of 100 %. The capacity performance of this electrode is enhanced as compared to AACVD
V2O5 at 500 °C [7], FeVO4 in 1 M Mg SO4 (300 mA h g-1, efficiency: 100 % after 50 cycles [15]) and
magnesium manganese oxide sieves in 0.5 M MgCl2 (300 mA h g-1, efficiency: 100 % after 300 cycles
[16]). Finally, the shape of the curves remained unchanged for the first to the 10000th proposing
good electrode stability.
Rate capability of the electrodes was studied at specific currents of 3.9 A g-1 up to 15 A g-1, and
then returned to 15 A g-1 (Figure 3 c). The electrode shows the highest specific discharge capacity at
15 A g-1 (300 mA h g-1), which could still deliver the same value when the specific current returned
to 15 A g-1 exhibiting capacity retention of 100 %. This performance confirms the upright structural
stability and high reversibility of the cathode. The specific discharge capacity shows a rising trend
with increasing specific currents, which is similar with the AACVD V2O5 at 500 °C [7].
Impedance responses of AACVD V2O5 cathode were evaluated by Nyquist plots for 1, 4000 and
8000 scans (Figure 3d). The experimental data were fitted by the electrical equivalent circuit (EEC)
shown in Figure 3d inset. EEC consists of two parallel R-C circuits in series, for which a discussion still
exists on the attribution of different elements to different processes in intercalation battery
electrodes. Here, R1 is ascribed to the electrolyte and FTO resistances, R2 to the contact resistance
between FTO and V2O5, which could possibly be influenced with artefacts due to presence of CE and
RE in three-electrode cell, and R3 to the charge transfer reaction resistance at V2O5/redox electrolyte
interface [17,18].The first semi-circle (high-frequency) slightly changes upon different scan
numbers, while the second semi-circle (low-frequency)is larger from the first to the 8000th scan
showing however the percentage increase is smaller after 4000 scans (26 %). The impedance shape
of the curves is not the same with those reported in [7] due to the differentiation of materials
structure and morphology at higher growth temperature.
Ragone plot is used to relate the power density with the energy density using the total weight of
the active material (0.0028 g), the cell voltage and the capacity based on charge/discharge curves
at various current densities. The vanadium pentoxide cathode achieved a high value of 448 Wh kg-1
at 24200 W kg-1. Assuming that the cathode takes up 50 wt.% of a cell, the magnesium-ion cell with
C. Drosos et al. J. Electrochem. Sci. Eng. 10(3) (2020) 257-262
http://dx.doi.org/10.5599/jese.769 261
V2O5 cathode can deliver a 224 W h kg-1, which is comparable with Li-ion batteries (<300 W h kg-1cell)
and Li-S batteries (<500 W h kg-1cell) [19].
Figure 3. Cyclic voltammetry curves of AACVD V2O5 for 1, 1000, 2000, 4000, 8000 and 10000 scans in
0.075 M MgCl2 at scan rate of 10 mV s
-1(a). Specific capacity for V2O5 cathode for a potential ranging -1.5 V
to +0.7 V under constant specific current of 15 A g-1 for 1 and 10000 scans (b).Rate capability of the
electrode at specific current values of 3.9 A g-1 up to 15 A g-1 and then back to 15 A g-1(c). Nyquist plot of
measured (symbols) and fitted (solid lines) of V2O5 electrode after 1, 4000 and 8000 scans. Electrical
equivalent circuit is also indicated as inset (d). Ragone plot based on the mass of the active material, cell
voltage and capacity values obtained in Figure 3b (e). XPS depth profiles and FE-SEM image at ×50000 of
V2O5 as inset, after 10000 continuous Mg
2+ intercalation/deintercalation scans (f)
Finally, the surface morphology and chemistry of V2O5 cathodes were studied after 10000 conti-
nuous intercalation/deintercalation scans. The grains appear elongated due to changes associated
with Mg2+ intercalation/deintercalation processes in V2O5 (Figure 3f, inset). The presence of V2O5 is
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still supported after 10000 scans (Figure 3f) with lower intensity as compared with Figure 1b),
whereas the peak due to chemisorbed OH groups at 532.6 eV disappears upon cycling due to the
weak interaction of water with the metal oxide.
Conclusions
This paper demonstrates the utilization of cold-wall AACVD to grow V2O5 electrodes as cathodes
for magnesium ion batteries. Furthermore, the growth temperature increase to 600 from 500 °C
(reported previously) is found decisive forV2O5 cathode with extended cyclic life. This enhancement
may be a combination of the co-existence α- and β-V2O5 along with the adherence of the coating. In
particular, the electrode indicated a specific discharge capacity of 300 mA h g-1 with capacity retention
of 92 %, coulombic efficiency of 100 %, good structural stability and comparable specific energy values
with Li-ion and Li-S batteries under a corrosive environment. This was obtained for the first time to
the best of our knowledge.
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