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Two-Dimensional Vanadium Carbide (MXene) as
Positive Electrode for Sodium-Ion Capacitors
Yohan Dall’agnese, Pierre-Louis Taberna, Yury Gogotsi, Patrice Simon
To cite this version:
Yohan Dall’agnese, Pierre-Louis Taberna, Yury Gogotsi, Patrice Simon. Two-Dimensional Vanadium
Carbide (MXene) as Positive Electrode for Sodium-Ion Capacitors. Journal of Physical Chemistry Let-
ters, American Chemical Society, 2015, vol. 6 (n° 12), pp. 2305-2309. �10.1021/acs.jpclett.5b00868�.
�hal-01447700�
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This is an author-deposited version published in : http://oatao.univ-
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Eprints ID : 16802
To link to this article : DOI : 10.1021/acs.jpclett.5b00868
URL : http://dx.doi.org/10.1021/acs.jpclett.5b00868
To cite this version :
Dall’Agnese, Yohan and Taberna, Pierre-Louis
and Gogotsi, Yury and Simon, Patrice Two-Dimensional Vanadium
Carbide (MXene) as Positive Electrode for Sodium-Ion Capacitors.
(2015) The Journal of Physical Chemistry Letters, vol. 6 (n° 12). pp.
2305-2309. ISSN 1948-7185
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concerning this service should be sent to the repository
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Two-Dimensional Vanadium Carbide (MXene) as Positive Electrode
for Sodium-Ion Capacitors
Yohan Dall’Agnese,
†,‡,§
Pierre-Louis Taberna,
†,‡
Yury Gogotsi,
§
and Patrice Simon*
,†,‡
†
Universite
Paul Sabatier, CIRIMAT UMR CNRS 5085, 118 route de Narbonne, 31062 Toulouse, France
‡
Re
seau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, 33 rue Saint Leu, 80039 Amiens Cedex, France
§
Department of Materials Science and Engineering and A. J. Drexel Nanomaterials Institute, Drexel University, 3141 Chestnut Street,
Philadelphia, Pennsylvania 19104, United States
ABSTRACT: Ion capacitors store energy through intercalation of cations into an electrode
at a faster rate than in batteries and within a larger potential window. These devices reach a
higher energy density compared to electrochemical double layer capacitor. Li-ion capacitors
are already produced commercially, but the development of Na-ion capacitors is hindered by
lack of materials that would allow fast intercalation of Na-ions. Here we investigated the
electrochemical behavior of 2D vanadium carbide, V
2
C, from the MXene family. We
investigated
the mechanism of Na intercalation by XRD and achieved capacitance of ∼100
F/g at 0.2 mV/s. We assembled a full cell with hard carbon as negative electrode, a known
anode material for Na ion batteries, and achieved capacity of 50 mAh/g with a maximum
cell voltage of 3.5 V.
L
ithium-ion capacitors (Li-IC) are new promising energy
storage devices that bridge the gap between batteries and
supercapacitors.
1−3
Because of their high energy and power
densities, they are intended for use in a wide variety of
applications, such as transportation (electric and hybrid cars),
electronics (telephones and laptops), and storage of renewable
energy. Ion capacitor materials combine the high energy density
from the intercalation mechanism of batteries and the high
power of supercapacitor.
4−8
JM Energy and JSR Micro have
commercialized a graphite/activated carbon Li-IC; however, the
limited supply of lithium and quickly widening use of energy
storage devices justify replacement of lithium with cheap and
abundant sodium. Sodium ion batte ries emerged as an
alternative to Li-ion batteries in specific applications, such as
large-scale stationary storage, where lower cost can compensate
for the lower energy density.
9
Na-ion batteries use hard carbon
(HC) as negative electrode with a capacity up to 320 mAh/g at
C/10 when cycled in the 2 V window.
10
Similarly to Li-ion
batteries, layered metal oxides, such as NaMnO
2
, NaCoO
2
, or
V
2
O
5
, have been proposed as positive electrodes, with capacities
of 160, 95, and 250 mAh/g, respectively.
11−13
Recently, sodium-ion capacitors (Na-IC), where the Li
intercalation electrode is replaced with a low-cost Na ion
electrode,
14
were demonstrated. Most of the ongoing work on
these systems is focused on the development of negative
electrodes, and several anodes have been proposed, such as
carbon nanotubes, NiCo
2
O
4
, and sodium titanate nano-
tubes.
11−14
In 2012, Kuratani et al. investigated HC/activated
carbon Na-IC and showed that HC could be used as negative
electrode. Despite Chen et al. proposing V
2
O
5
, all other studies
included
activated carbons as positive electrodes,
15,16
and little
was done to develop alternative cathodes.
We report the electrochemical characterization of a new
positive electrode material, namely, V
2
C, that belongs to a large
family of 2D transition metal carbides called MXenes.
17
MXenes are synthesized by selective etching of the A layer
from MAX phases.
18
Since discovery of the first MXene
(Ti
3
C
2
) in 2011,
19
more than 10 new members have been
successfully synthesized and many more predicted. Ti
2
C, Ti
3
C
2
,
Nb
2
C, and V
2
C have quickly drawn the attention as candidates
for energy storage due to the possibility of spontaneous
intercalation of a variety of cations between their layers.
17−27
For example, Ti
3
C
2
has been used in aqueous supercapacitors
and as electrode material of Li-, Na-, and K-ion batteries.
25−27
Two layers of Li or Na ions were predicted to intercalate
between MXene layers,
27
and the formation of a double-layer of
Na has been experimentally shown.
28
From density functional
theory (DFT) predictions,
27
V
2
C is one of the most promising
electrode materials for Li-ion batteries, but experimental results
show a very wide working potential window and a sloping
charge−discha rge;
17,27,29
however, the se materials show a
potential for assembling hybrid devices, and we previously
proposed Ti
2
C for Li-ion capacitors.
20
In early 2015, the first
experimental investigation of Na-IC using MXenes was
published.
28
Wang et al. investigated Ti
2
C as negative electrode
and alluaudite Na
2
Fe
2
(SO
4
)
3
as positive electrode. It showed a
good rate capability and high specific power of 1.4 kW/kg with
specific energy of 260 Wh per kg of Ti
2
C. Although promising
results were shown, previous sodium ion battery studies
demonstrated that Ti
2
C is not the best MXene in terms of
performance.
To create a Na-IC, we took into account the previous
experimental and theoretical research on MXene and selected
V
2
C as a promising material for Na-IC. In this work, we
investigate for the first time the sodiation of V
2
C in a half cell.
The energy-storage mechanism is studied by X-ray diffraction
(XRD) and electrochemical impedance spectroscopy (EIS). To
assemble a full cell, we selected HC as negative electrode.
Figure 1 shows a schematic view of the synthesis and
structure of V
2
C and its Na-intercalation mechanism during
cycling. From previous studies,
17,22
it is known that MXenes
synthesized using HF contain fluorinated and oxygenated
surface functional groups, such as −OH, −O, and −F,
17
and
their presence was noted by adding “T
x
” to the chemical
formula, V
2
CT
x
. Note that the effect of the surface chemistry
has not been studied here.
Figure 2a shows CVs at different scan rates, while the change
of the capacitance with the scan rate is described in Figure 2b.
High capacitance of 100 F/g or 170 F/cm
3
was obtained at
slow scanning, and 50 F/g was still measured at 50 mV/s,
evidencing a good power performance of V
2
CT
x
for Na
intercalation.
At low scan rates, two different regions can be
seen in the CV, corresponding to two different electrochemical
processes. From 1 to 2.2 V, the rectangular shape of the CV
describes pseudocapacitive behavior. A similar storage mecha-
nism has been previously demonstrated in other MXenes. For
example, Ti
3
C
2
cycled in aqueous electrolyte exhibits a
r
ectangular-shaped CV attributed t o redox reactions a nd
intercalation.
26,30
Redox peaks are identified at low scan rates,
with an oxidation peak at 3 V (peak A) and a reduction peak at
2.5 V versus Na
+
/Na, (peak B). As the scan rate increases, the
redox peaks tend to disappear, thus suggesting a diffusion
limitation at scan rate beyond 2 mV/s. The large potential
range and the absence of any 2-phase system plateau make V
2
C
less suitable for sodium ion battery electrodes, but such features
are attractive for sodium ion capacitors.
Characterization by EIS was made at different potentials
(Figure 3a). The constant charge-transfer resistance, as well as
the improvement in the capacitive region at low frequencies,
between 1 to 2.5 V correlates well with a pseudocapacitive
intercalation mechanism. The charge-transfer resistance (200
Ohm/cm
2
) associated with the Na
+
pseudointerca lation
reaction
explains the resistive behavior observed in the CV
curves. The increase in the charge-transfer resistance and the
semi-infinite diffusion limitation visible in the low-frequency
region at 3.2 V is associated with the full desodiation of V
2
CTx,
in
agreement with the redox peaks observed in the CV.
Figure 3b shows ex situ XRD patterns of V
2
CT
x
recorded at
different
voltages, where it can be observed that the (002) peak
shifts continuously and reversibly from 9 to 12° during cycling
between 1 and 3.5 V versus Na
+
/Na. In this potential range, the
change is perfectly reversible, thus demonstrating that there are
no undesired side reactions. During sodiation, c-lattice
parameter increases with the amount of Na
+
stored. This
demonstrates that V
2
CT
x
stores energy through intercalation of
Na ions in between layers in a similar way as that previously
demonstrated for both intercalation of Li
+
into Ti
2
C
20
or
Ti
3
C
2
31
and Na
+
into Ti
3
C
2
.
32
There is no new phase appearing
at 3.5 V versus Na
+
/Na, and thus the redox process identified
by peaks A and B in the CV does not modify the
crystallographic structure of the material. A 4.6 Å change in
c-lattice parameter was observed, as calculated from Bragg’s law.
Taking into account the fact that there are two interlayer gaps
in a lattice unit, there is a 2.3 Å expansion or shrinkage during
sodiation and desodiation, respectively. This is a larger change
than expected for a single layer of Na
+
ions, which indicates that
Figure 1. Schematic illustration of the synthesis of V
2
CT
x
and its sodium intercalation.
Figure 2. (a) Cyclic voltammetry of V
2
CT
x
at different scan rate and
(b) summary of rate performance.
a second layer of Na
+
could be intercalated, as shown for Ti
3
C
2
intercalated by Na
+
.
32
The peak at 13° corresponds to V
2
AlC
from incomplete synthesis reaction. This peak does not move
during cycling, demonstrating that the MAX phase is not
electrochemically active and that the capacity could be
increased by increasing V
2
CT
x
yield. Nevertheless, the presence
of this peak is useful as a reference for the other peaks.
Differently from Ti
2
C and Ti
3
C
2
, which can only be used as
n
egative electrodes because of their op erating potential
window, V
2
CT
x
shows a potential window ranging from 1 to
3.5 V versus Na
+
/Na, being attractive as a positive electrode in
Na sodium ion capacitors. V
2
CT
x
was cycled at current density
from 30 mA/g to 1 A/g, corresponding to the rate from C/3 (3
h discharge) to 20C (3 min discharge), as shown in Figure 4b.
The objective was to assemble a full cell using V
2
CT
x
as
positive electrode and HC as negative electrode for sodium
intercalation.
10
Galvanostatic charge−discharge cycling of HC
electrodes was done at the same C rate. The expected key
features of a carbon intercalation electrode were observed, with
an intercalation potential below 1 V versus Na
+
/Na and a
capacity beyond 200 mAh/g at low rates (C/3, Figure 4b).
Figure 4b shows the cycling stability at different C-rates. In
this example, the positive electrode/negative electrode mass
ratio was 1:2, to keep each electrode potential in their working
potential window. At low charge/discharge rates, the faradic
efficiency decreases, thus leading to a capacity fade with cycling.
At low rate (C/3), the capacity fading is more pronounced in
the first 40 cycles. Good capacity up to 70 mAh/g was
obtained, with remarkable stability at discharge rates beyond
3C. The performance achieved experimentally is lower than
that predicted by first-principles simulation corresponding to
the maximum theoretical capacity for a bare V
2
C monolayer
(335
mAh/g).
27
Experimentally, the capacity is limited by the
presence of MAX phase residue, functionalized layers, and
stacked layers. Thus, there is much room for further increase in
capacitance of this material.
On the basis of half-cell results, hybrid Na-ion capacitor cells
were assembled. By anticipating the capacity decrease in V
2
CT
x
during the first
cycles, we calculated a HC/V
2
CT
x
weight ratio
of 1:2. In such conditions, the overcapacitive HC electrode
allows a better potential stability for the negative electrode.
Before assembling a full cell, each electrode was pretreated as
described in the Experimental Methods. The full cells were
tested from C/3 to 20C rate. Figure 5 shows the electro-
chemical performance obtained in a full cell configuration. All
gravimetric capacities are calculated based on the total weight
of both positive and negative electrodes to focus on the
performances of the device. Because the mass ratio of positive
to negative electrode is 1:2, the equivalent capacities based on
the mass of V
2
CT
x
are three times higher than those presented
in
Figure 5.
The charge/discharge galvanostatic profiles are presented in
Figure 5a. During discharge, a sharp potential drop occurred
from 3.5 V down to 2.6 V, followed by a small plateau at 2.5 V
due to the redox reaction peaks observed in Figure 2a, so that
the practical cell voltage was 2.6 V. Figure 5b shows that high-
power performance could be achieved, with 40% of the total
capacity obtained at 20C, despite the use of a Na-ion
intercalation HC negative electrode. The capacity decrease
during the first cycles at the low rate (C/3) is associated with a
decrease in the Coulombic efficiency due to redox reaction
beyond 3 V.
Figure 5c shows the cycle life of the full cell at a high rate
(20C). After 300 cycles, the capacity retention was 70%.
Interestingly, the capacity slightly increased during the first 70
cycles. Afterward, the capacity decrease was associated with the
Figure 3. (a) Nyquist plot from EIS. (b) XRD patterns at different
potentials. (*) Peak of unreacted V
2
AlC.
Figure 4. (a) Charge−discharge profiles of V
2
CT
x
(2 mg) and hard
carbon (2 mg) and (b) cycle life from galvanostatic charge−discharge
at different rates.
