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The First Report on Excellent Cycling Stability and Superior
Rate Capability of Na3V2(PO4)3 for Sodium Ion Batteries
Kuppan Saravanan, Chad W. Mason, Ashish Rudola, Kim Hai Wong, and Palani Balaya*
into high energy fields, such as electric
vehicles and grid storage puts into question the availability of readily-accessible
lithium in the environment. Though the
abundance of lithium resources is not an
immediate concern, an expanding market
for LIBs may induce strain on lithium
supply.[5] As expected, an increase in
demand will have a corresponding effect
on the price of lithium compounds, thus
alternative battery chemistries should be
explored.
The sodium (Na) ion battery (NIB) is
a promising alternative to the aforementioned battery technologies. In particular,
the abundance of easily accessible sodiumcontaining resources is significantly
greater than that of their lithium counterparts, which makes NIB a potentially
cost effective technology (Na2CO3 0.07–
0.37 ¤ kg−1 vs. Li2CO3 4.11–4.49 ¤ kg−1).[6]
Furthermore, the alkaline ion chemistry of
NIBs is similar to that of LIBs, and many
of the materials and design processes currently used for LIBs
can also be used in the production of NIBs.
Despite the numerous advantages of NIB technology, it still
remains in the nascent stage due to challenges in finding suitable electrode materials. Considerable effort has been dedicated to the design and fabrication of novel, high performance
NIB electrodes.[5–7] Different types of carbonaceous materials
have exhibited electrochemical activity towards Na+ ions at
lower voltages (as anode);[8,9] however their storage performance and cyclability does not perform equally to their lithium
counterparts, due to the bigger ionic radius of the Na+ ion (Li+
0.69Å vs. Na+ 0.98Å). This is also true for the cathode. Recently
Na2FePO4F/C,[10] Na0.85Li0.17Ni0.21Mn0.64O2,[11] Na4Mn9O18,[12]
Na0.6Fe0.5Mn0.5O2,[13] Fe(CN)64– doped polypyrrole[14] and
Na4Fe(CN)6/C[15] have been tested for hosting Na+ ions, but
their long term cycle life or rate capability do not meet necessary
requirements for use in stationary energy storage applications.
To overcome such performance and cyclability limits imposed
by sodium’s larger ionic radius, unique crystalline structures,
such as open framework or 3D structures with large tunnels,
have to be employed to accommodate these large sodium ions
for a lattice perturbation-free intercalation/de-intercalation
process.[5–7] In this context, NASICON (Na3M2(XO4)3); X = Si4+,
P5+, S6+, Mo6+, As5+) framework materials are well known for
their facile sodium ion conductivity but they have not yet been
significantly explored as NIB electrodes.[16,17] Amongst various
materials with the NASICON structure, the theoretical energy
Sodium ion batteries are attractive for the rapidly emerging large-scale energy
storage market for intermittent renewable resources. Currently a viable
cathode material does not exist for practical non-aqueous sodium ion battery
applications. Here we disclose a high performance, durable electrode material
based on the 3D NASICON framework. Porous Na3V2(PO4)3/C was synthesized using a novel solution-based approach. This material, as a cathode, is
capable of delivering an energy storage capacity of ∼400 mWh/g vs. sodium
metal. Furthermore, at high current rates (10, 20 and 40 C), it displayed
remarkable capacity retention. Equally impressive is the long term cycle life.
Nearly 50% of the initial capacity was retained after 30,000 charge/discharge
cycles at 40 C (4.7 A/g). Notably, coulombic efficiency was 99.68% (average)
over the course of cycling. To the best of our knowledge, the combination of
high energy density, high power density and ultra long cycle life demonstrated
here has never been reported before for sodium ion batteries. We believe our
findings will have profound implications for developing large-scale energy
storage systems for renewable energy sources.
1. Introduction
Energy storage in utility power grids has been known for many
years; however, it is receiving more attention recently due to the
proliferation of intermittent renewable energy resources. As a
majority of nations continue to enhance their renewable energy
portfolio, the importance of load leveling in these power grids
is significantly elevated. The basic requirements for large-scale
storage systems are ultra long cycle life, high power capability,
high round trip energy efficiency (RTEE) and low cost. Unfortunately, existing battery technologies available for these largescale applications are either too costly (sodium-sulphur and
lithium-ion) or do not endure deep charge/discharge cycling
over a long period of time (lead-acid).[1–3]
In this context, lithium ion battery (LIB) technology has
been viewed by many in the industry as the best solution for
demanding energy storage applications.[4] However, LIBs have
continued to be primarily relegated to the electronics market
due to cost and material stability issues. The entrance of LIBs
K. Saravanan, C. W. Mason, A. Rudola,
K. H. Wong, P. Balaya
Department of Mechanical Engineering
National University of Singapore
9 Engineering Drive1, 117576, Singapore
E-mail: mpepb@nus.edu.sg
DOI: 10.1002/aenm.201200803
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density of Na3V2(PO4)3 (NVP) as a cathode material is impressive, at 400 Wh/kg (117.6 mAh/g × 3.4 V for the V3+/V4+ redox
couple). Further, the material was reported to possess good
thermal stability (450 °C) under charged state which could aid
safe battery operation.[18] Yet, limited reports of NVP’s performance as a battery electrode exist in literature. The first report by
Uebou et al.[19] on Na3V2(PO4)3 in NIBs showed electrochemical
activities at 3.4 V and 1.6 V, corresponding to the V3+/V4+ and
V3+/V2+ redox couples, respectively. The reversibility and cyclability were found to be poor, though. More recently Hu et al.[20]
showed improved performance at low current rates, with relatively stable cyclability (80 cycles).[21] However, the full theoretical capacity of Na3V2(PO4)3 has not been realized, even at
moderate C-rates.[19,20] This is because the synthetic procedures
reported so far typically involve high temperatures and lengthy
solid state methods (900 °C for 24-48 h) that yields large micronsized particles (>2 μm).[16,19,20,22–23] The long Na+ diffusion pathways present in these particles and poor electronic conductivity
of phosphate framework restrict the high rate operation and
cyclability of NVP. In this regard, a novel, low temperature solution process for the preparation of porous NVP nanostructures,
integrated within a conductive network, is attractive.
Herein, we report such an NVP architecture that demonstrates impressive storage and cycle performance in a manner
not previously reported. At moderate current rates (1C), nearly
full theoretical capacity and a high RTEE of 96% were achieved
in half cells. Furthermore, this material provided superior
sodium storage capacity of 62 mAh/g at a high current rate of
40 C (4.7 A/g) (90 sec of charge/discharge) and impressively
50% of this capacity is retained even after 30,000 cycles.
Figure 1. PXRD pattern of the NVP/C nanoparticles synthesized by the
soft template method (Rexp: 11.30, Rwp: 7.74, Rp: 6.12).
were indexed to the NASICON structure (Figure 2) with R-3c
space group (Rhombohedral unit cell). The lattice parameters
obtained for these NVP nanoparticles were a = 8.6217 Å and
c = 21.6924 Å. These values compare well with existing literature of Na3V2(PO4)3[22] and JCPDS card no. 53-0018. There is
no evidence in the PXRD pattern for the presence of crystalline
2. Results & Discussion
2.1. Synthesis of Na3V2(PO4)3/C (NVP/C)
A novel soft template approach was used to synthesize porous
NVP/C composite. This method involved a one pot synthesis,
in which cationic surfactant along with stoichiometric amounts
of precursors were dissolved in a mixture of water and ethanol.
This allowed the uniform mixing of the reactants at the atomic
level, and subsequently amorphous NVP precursors were precipitated. At the end of the precipitation step, the surfactant
(carbon source) used was localized within aggregates of amorphous NVP precursors. This was in turn decomposed during
the subsequent annealing process (650 °C/6 h) to form an integrated conductive carbon matrix for hosting the crystallized
NVP nanoparticles. This carbon matrix restricted the growth of
the primary grain during the sintering process and spread uniformly throughout the sample (Figure S1).
2.2. Structural Characterization of NVP/C
Figure 1 depicts the powder X-ray diffraction pattern (PXRD)
of the as-synthesized NVP. Rietveld refinement of the PXRD
pattern was performed to show the pure phase formation of
NVP, without any impurities. All peaks in the PXRD pattern
Adv. Energy Mater. 2013, 3, 444–450
Figure 2. View of the NVP NASICON structure on the ab plane.
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that these particles are formed by the agglomeration of NVP
nano-grains of 50–300 nm. TEM analysis revealed that the
NVP nano-grains were well dispersed in the carbon matrix
and high resolution TEM showed an ∼8 nm coating of carbon
on the surface of these nano-grains (Figure 3c–e). Raman
spectroscopy (Figure S2) showed that this carbon matrix was
composed primarily of graphene clusters (SP2) with some
disordered carbon, which is known to enhance the electronic
conductivity.[26] In addition, the Brunauer, Emmett, and Teller
(BET) surface area of the NVP/C particles was found to be
40 m2/g with peak pore size distribution around 4-20 nm (refer
Figure S3). The relatively high surface area and the electronically interconnected pores of the material were instrumental in
retaining higher capacity at increasing current rates (discussed
later).
Upon inspection of HRTEM images, clear lattice fringes indicating single crystallinity of the nanoparticles were also detected
(Figure 3f). The width of 6.1 and 3.1 Å between neighboring
lattice fringes corresponds to the (0 1 2) and (0 2 4) planes of
NVP, respectively, thus confirming a pure crystalline phase of
these nanoparticles (Figure 3f). Spots on the selected area electron diffraction pattern (SAED) (Figure 3e inset) are consistent
with the PXRD pattern results shown in Figure 1.
2.4. Electrochemical Behavior
Figure 3. Morphological analysis of NVP/C. FESEM images at (a) low,
and (b) high magnification. (c-e) TEM images at different magnification.
(f) HRTEM image. Circle in (d) shows the NVP particle anchored on the
carbon matrix. Rectangle shows the amorphous carbon matrix (diffused
rings in the SAED confirms the amorphous nature of carbon).
or amorphous carbon. However, confirmation for a uniform
carbon coating over NVP nanoparticles is shown in the transmission electron microscopy (TEM) (Figure 3c, d) and Raman
spectroscopy (discussed later).
Figure 2 demonstrates the NASICON framework, rhombohedral form of NVP, which is built up from isolated VO6
octahedra and PO4 tetrahedral units interlinked via corners
to establish the framework anion [V2(PO4)3]3−. The two independent sodium atoms are located in the voids/channels of the
framework with two different oxygen environments, with six
fold coordination for the first and eightfold for the second Na+
cation.[24] The open, 3D nature of the NASICON framework is
known for facile sodium ion migration. This outstanding ionic
mobility has been well documented.[25]
2.3. Morphological Analysis of NVP/C
Morphology of the as-synthesized NVP powder was characterized by electron microscopy. Low magnification field emission
scanning electron microscope (FESEM) images (Figure 3a,b)
showed a network of NVP particles with an irregular morphology (particle and sheet) and various sizes ranging from
500–900 nm. At higher magnification (Figure 3b), it was noted
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To gauge the electrochemical activity, [NVP/C || 1 M NaClO4
in (EC/PC) || Na] cells were constructed. Slow scan cyclic voltammetry (CV) (refer Figure S4) was performed on the half
cells to find out the electrochemically active regions of NVP/C.
Two sharp redox peaks were observed in the low voltage and
high voltage regions. The mid-composition reaction potential at
1.64 V vs. Na+/Na in the low voltage region is attributed to the
V3+/V2+ redox couple and high voltage peak at 3.37 V vs. Na+/
Na corresponds to the V4+/V3+ redox couple. These redox peaks
are in good agreement with previous studies.[19,20]
The galvanostatic cycling curves obtained for porous NVP/C
as a positive electrode in a NIB cell are presented in Figure 4a.
Up to two Na+ ions per formula unit were extracted during
charging. The voltage profile had a flat charge plateau at 3.39 V
before it rose steeply to the cutoff voltage of 3.90 V. The discharge curve commenced from 3.90 V, with a direct voltage drop
to 3.36 V followed by a very flat voltage plateau that spreads
over a long range of sodium composition (up to 1.88 mole of
Na+) and then falls steeply to the cutoff voltage (2.30 V), leading
to the storage capacity of 116 mAh/g. The flat voltage profile
for a wide range of sodium ion molar concentrations observed
for NVP/C is much higher than any other known sodium positive electrode material. During subsequent cycles, the observed
voltage profiles remained unaltered, demonstrating excellent
reversibility of the cycling process (refer Figure S5 for detailed
cycling).
During cycling at 1C rate, a capacity of 114 mAh/g was
observed, and 92.2% of this capacity (106 mAh/g), was retained
during cycling at 10 C. Even at much higher rates of 20 and
40 C, nearly 80% and 54% of the capacity was retained, respectively (Figure 4b). This capacity retention at high current rates
compares well with reports on LiFePO4,[27] a well studied LIB
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Figure 4. Electrochemical characterization of NVP/C as a cathode. (a) Galvanostatic cycling profile of NVP/C at different current rates between
2.3–3.9 V. (b) The capacity retention of the as-synthesized NVP/C at different current densities in comparison with the best positive hosts recently
tested for the Na+ intercalation (Na2FePO4F/C,[10] Na0.85Li0.17Ni 0.21Mn0.64O2,[11] Na4Mn9O18,[12] Na0.6Fe0.5Mn0.5O2,[13] Fe(CN)64– doped polypyrrole,[14]
Na4Fe(CN)6/C).[15] (c) Long term cycle life and coulombic efficiency for 30,000 cycles at 40 C. (d) The ΔV of NVP/C at different current densities (measured at the middle of cycling profile) in comparison with Na4Fe(CN)6/C[15] and Fe(CN)64−-Ppy[14] (only two other NIB systems with charge/discharge
profiles have been reported at different rates).
cathode, and to the best of our knowledge, has never been
reported on a non-aqueous NIB.
In addition to the tremendous capacity retention at high current rates, ultra long cycle life of a non-aqueous NIB electrode
is displayed here for the first time (Figure 4c). Nearly 50% of
the initial capacity was retained after 30,000 discharge cycles at
40 C (4.7 A/g) rate and coulombic efficiency averaged 99.68%
throughout, with a standard deviation of 0.09%. At other rates
(1 C, 10 C and 20 C) (Table 1 and Figure S5), more than 85%
of the initial capacity could be retained after long-term cycling
(>1000). Note that large-scale energy storage mainly requires
long cycle life and high rate capability, which we have conclusively demonstrated here. This excellent long term cyclability
compares well with the commercial LIBs and exceeds most
other rechargeable batteries, including those used in grid
storage applications (lead-acid, Na-S). Although the theoretical
capacity of NVP/C is less than the recently reported layered
Table 1. Capacity retention after long term cyclability at 1, 10, 20 C.
Rate & cycle
Initial capacity
(mAh/g)
Final capacity
(mAh/g)
Capacity retention
(%)
1000 cycles at 1 C
113
98
86.7
1000 cycles at 10 C
106
90
84.9
3500 cycles at 20 C
91
82
89.7
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positive electrodes for NIBs, the capacity retention at high current densities and cyclability achieved here surpassed them
significantly.[13,28]
The voltage difference (ΔV) between the charge and discharge plateaus of an electrode significantly affects energy
efficiency. To benchmark our porous NVP/C, the ΔV of other
existing NIB positive electrodes were plotted at different current rates (Figure 4d, Figure S6). The observed ΔV values are
superior to the other non-aqueous NIB electrodes tested so far
and also compare well with the best LIB electrodes (LFP).[27]
Energy and power density of NVP/C at different current densities (Figure 5) clearly distinguish it from the existing cathode
materials for NIBs. If a suitable anode with a stable capacity
(∼120 mAh/g) at low insertion potential is achieved (e.g. hard
carbon[29] or Na2Ti3O7 (NTO)),[30] then an energy density close
to 400 mWh/g is achievable with the as-prepared NVP cathode.
This is nearly 10 times higher than the commercial lead acid
batteries currently employed in the electrical grid, demonstrating the feasibility of this material to meet demanding
requirements for grid applications.[3]
The round-trip energy efficiency (RTEE) is a critical parameter
for stationary energy storage systems due to the large quantity
of energy being transferred through the system. RTEE is the
product of voltage efficiency and coulombic efficiency (refer
Figure S7). Since the porous NVP/C cathode exhibited outstanding coulombic efficiency and very minimal ΔV values, it
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Figure 5. Comparison of energy and power density of NVP/C electrodes against other cathode materials. (a) Energy density, (b) power density of the assynthesized NVP/C at different current densities in comparison with the best positive hosts recently tested for the Na+ intercalation Na0.6Fe0.5Mn0.5O2,[13]
Na2FePO4F/C,[10] Na0.85Li0.17Ni0.21Mn0.64O2,[11] Na4Fe(CN)6/C,[15] Fe(CN)64– doped polypyrrole.[14] (The mid-composition reaction potential was carefully
extracted from the galvanostatic profiles of the above reported positive hosts) (Energy and power densities were calculated based on the active material weight in the cathode)
is expected to also deliver high RTEE values. Based on the coulombic efficiency (∼99%) and voltage efficiency (∼98%) observed
at 1C for the as-prepared porous NVP/C, one can expect ∼96%
full cell RTEE for a corresponding NIB constructed with a
reversible anode exhibiting low ΔV values and high coulombic
efficiency. This full cell RTEE would easily exceed that of conventional rechargeable batteries (60–85%).[31] Moreover, even
at higher rates (10 C), NVP/C can deliver ∼91% RTEE. This
enhancement in RTEE greatly reduces expensive energy losses.
2.4. Electrochemical Characterization of NVP/C as Anode
NVP also displayed electrochemical activity at a lower voltage
and at this juncture, its properties as an anode were also evaluated (Figure S4). Figure 6a shows the galvanostatic profiles
of NVP/C at different current rates in the voltage window of
1.3–2.0 V. Again, a flat voltage profile was observed during
the charge/discharge process. The initial charge capacity of
50 mAh/g was observed at 0.2C and 85% of this capacity
(42 mAh/g) was retained at 10 C, along with plateau behavior
and minimal ΔV throughout cycling at various C-rates. Interestingly, in addition to the excellent properties displayed when
used as a cathode, exceptional cyclability, rate capability, capacity
retention and coulombic efficiency were also observed in the
anode region (Figure 6b). Hence by combining the properties
of NVP as both an anode and cathode, a symmetrical cell with
an output voltage of 1.70 V was built (refer Figure S8).
Interestingly, we observed a well-defined step in the discharge voltage profile of both NVP as a cathode (3.27 V) and as
an anode (1.56 V), especially at higher current rates (Figure 4a
and 6a). This was not observed during extraction and this feature corresponds well with a split in the cathodic peak of the
corresponding cyclic voltammograms (Figure S9). It was previously reported that the transfer of Na ions from the Na(1) to
the Na(2) sites is commonly observed with an increase in temperature. This rearrangement triggers a number of small but
considerable changes in the NASICON Na3M2(PO4)3 (M) Sc,
Cr, Fe) framework.[32] Since higher charge/discharge rates may
induce local heating, we believe similar structural rearrangements could happen in our porous NVP/C system at higher
current rates. Synchrotron X-ray studies are being pursued to
gain further insight into the origin of plateau splitting.
2.5. Ex-Situ XPS studies of NVP/C Electrodes
Ex-situ XPS studies were carried out to examine the Na insertion/extraction mechanism. The transition from Na3V2(PO4)3 to
NaV2(PO4)3 during charging, and then back to Na3V2(PO4)3 after
Figure 6. Electrochemical characterization of NVP/C as anode. (a) Cycling profile of NVP/C at different current rates in the anode region. (b) Rate
capability of NVP/C (voltage window 1.3–2.0 V).
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can offer capabilities nearly matching those of LIB materials.
The material’s excellent electrochemical properties are attributed
to facile sodium ion diffusion in the nano-sized NVP particles,
embedded in a conductive matrix. In light of our breakthrough,
we anticipate our findings to have profound implications in the
field of grid-connected energy storage.
4. Experimental Section
4.1. Synthesis and Physicochemical Characterization
Figure 7. Ex-situ XPS studies of NVP/C electrodes. (a) Pristine sample
(b) Charged (c) Discharged.
discharging, is expected to be accompanied by the oxidation and
reduction of vanadium from V3+ to V4+ and back to V3+. XPS
(Figure 7) was used to probe the oxidation states of the vanadium in the as-prepared, charged and discharged states. It was
found that the as-prepared material displayed a peak located at
binding energy of 515.5 eV, corresponding to V3+.[33–36] Notably,
after charge, the binding energy of V3+ increased significantly
(1 eV) from its original value, indicating the formation of a
higher oxidation state. In this state, only V4+, without V3+ or V5+
species,[34–37] was detected. Upon discharge, the V4+ species was
fully reduced to V3+. The above results showed that the intercalation of two Na+ ions in the as-prepared NVP is possible, thus
accounting for the close-to-theoretical capacity achieved.
3. Conclusion
We show for the first time a sodium ion battery material
(cathode and anode) that demonstrates the durability and rate
performance needed for demanding applications, with over
30,000 cycles vs. sodium metal, capable of charge and discharge
in a matter of minutes. NVP/C was produced using a novel,
scalable aqueous precipitation method, with a relatively low
annealing temperature (650 °C) and short crystallization time
(6 hr). Combined with this facile synthesis procedure, NVP/C
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Carbon-coated porous Na3V2(PO4)3 was synthesized using the
soft template method.[38] Cationic surfactant (CTAB) was taken
in a round-bottomed flask and was dissolved in a mixture of
de-ionized water and absolute ethanol. To this solution stoichiometric mixture of sodium acetate, vanadium (III) acetylacetonate and ammonium dihydrogen phosphate (1:2/3:1 millimolar
ratio) were added with continuous vigorous stirring. The
resulting precipitate was then continuously stirred for several
hours and dried using rotor evaporator at 70 °C. Annealing was
done in Ar-H2 (95:5) atmosphere at 650 °C for 6 h to remove the
organic template and to increase the cross-linking of the inorganic framework. 6 wt% carbon was found from the elemental
analyses on NVP/C.
Powder X-Ray diffraction (PXRD) patterns were recorded
using a D5005 Bruker X-ray diffractometer equipped with Cu
Kα radiation. The accelerating voltage and current were 40 kV
and 40 mA, respectively. A scan speed of 0.015°/s was used to
record the PXRD patterns. Lattice parameters were obtained
using TOPAS-R (version 3.0) software. The morphology of the
product was examined using field emission scanning electron
microscope (FESEM) model Jeol JSM-7000F operated at 15 kV
and 20 mA and high resolution transmission electron microscope (JEOL JEM-2010). For SEM examination, the sample surface was sputtered with a thin gold coating. For TEM studies,
the sample was dispersed in ethanol by sonication, a drop was
loaded on Cu-grid and dried. XPS studies were performed with
a Kratos AXIS UltraDVD (Kratos Analytical Ltd) at a base pressure of 1 × 10−9 Torr and a working pressure of 5 × 10−9 Torr
with a mono Al Kα radiation.
4.2. Electrochemical Characterization
For energy storage studies, composite electrodes were fabricated
with the active material, Super P carbon black (CB) and binder
(Kynar 2801) in the weight ratio 75:15:10 using N-methyl-pyrrolidone (NMP) as solvent. Electrodes with a thickness of 10 μm
were prepared using an etched aluminium foil (20 μm thick) as
current collector by using the doctor-blade technique. Sodium
metal foil, 1 M NaClO4 in ethylene carbonate (EC) and propylene
carbonate (PC) (1:1 v/v) (Merck) and Whatman Glass Microfibre
Filter (Grade GF/F) were used as counter electrode, electrolyte
and separator, respectively, to assemble coin-type cells (size
2016) in an Ar-filled glove box (MBraun, Germany). The geometrical area of the electrode was 2.0 cm2. The active material
content in the electrode was around 3.0 mg. Charge–discharge
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cycling at a constant current mode was carried out using a
computer-controlled Arbin battery tester (Model, BT2000, USA)
and cyclic voltammetric and galvanostatic intermittent titration
technique studies were carried out at room temperature using a
computer controlled VMP3 (Bio-logic, France).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
Authors thank NRF (R-265-000-367-281) and DSO (R265-000-393-592)
for funding. A.R. thanks NUS for Ph.D. research studentship, K.H.W.
thanks NRF (Clean Energy Program Office) for Ph.D. scholarship. We
thank Srirama Hariharan for useful discussion. K.S. thanks World Future
Foundation (WFF) for PhD prize in Environmental and Sustainability
Research.
Received: October 5, 2012
Published online: November 29, 2012
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Adv. Energy Mater. 2013, 3, 444–450