Tailoring NaVO3 as a novel stable cathode for lithium rechargeable batteries

Article abstract of DOI:10.1016/j.electacta.2019.03.164

Vanadium-based compounds hold great promise as high capacity cathode candidate for future lithium rechargeable batteries. However, developing highly stable vanadium-based cathode materials with long cycle life remains a great challenge. Herein, we report a novel layered sodium vanadium oxide, NaVO₃, as a promising cathode electrode contender. This material is capable of delivering a capacity of 224.8 mAh g^?1 at the current density of 150 mA g^?1, and a high rate capability of 85 mAh g^?1 even at a high current density of 3 A g^?1. Moreover, outstanding capacity retention of 77% after 1000 cycles is achieved. Ex situ characterizations verify that the excellent electrochemical performance of NaVO₃ is attributed to superior structural stability and electrochemical reversibility upon long-term cycling. Furthermore, the lithium ion de/intercalation mechanism for NaVO₃ is also revealed involving one electron transfer reaction between V⁵⁺ and V⁴⁺ redox couple. Considering the low cost and material sustainability as well as the outstanding electrochemical performances, we believe that NaVO₃ is a highly promising cathode material for lithium rechargeable batteries and our findings may help to pave the way for developing vanadium-based layered structure materials for high-performance alkali and alkaline-earth ion batteries.

Full text of DOI:10.1016/j.electacta.2019.03.164

Accepted Manuscript
Tailoring NaVO as a novel stable cathode for lithium rechargeable batteries
3
Long Chen, Honglun Wu, Huiming Wang, Liang Chen, Xiangjun Pu, Zhongxue Chen
PII:
S0013-4686(19)30587-0
https://doi.org/10.1016/j.electacta.2019.03.164
EA 33880
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 6 February 2019
Revised Date: 23 March 2019
Accepted Date: 23 March 2019
Please cite this article as: L. Chen, H. Wu, H. Wang, L. Chen, X. Pu, Z. Chen, Tailoring NaVO as
3
a novel stable cathode for lithium rechargeable batteries, Electrochimica Acta (2019), doi: https://
doi.org/10.1016/j.electacta.2019.03.164.
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ACCEPTED MANUSCRIPT
Tailoring NaVO3 as a novel stable cathode for lithium
rechargeable batteries
Long Chen a, Honglun Wu a, Huiming Wang a, Liang Chen b, **, Xiangjun Pu a,
Zhongxue Chen a, *
a Key Laboratory of Hydraulic Machinery Transients, Ministry of Education, School of Power and
Mechanical Engineering, Wuhan University, Wuhan 430072, China. Email:
zxchen_pmc@whu.edu.cn.
b School of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology,
Yueyang 414006, China. Email: clvilance@163.com.
Abstract
Vanadium-based compounds hold great promise as high capacity cathode
candidate for future lithium rechargeable batteries. However, developing highly stable
vanadium-based cathode materials with long cycle life remains a great challenge.
Herein, we report a novel layered sodium vanadium oxide, NaVO3, as a promising
cathode electrode contender. This material is capable of delivering a capacity of 224.8
mAh g−1 at the current density of 150 mA g−1, and a high rate capability of 85 mAh
g−1 even at a high current density of 3 A g−1. Moreover, outstanding capacity retention
of 77% after 1000 cycles is achieved. Ex situ characterizations verify that the
excellent electrochemical performance of NaVO3 is attributed to superior structural
stability and electrochemical reversibility upon long-term cycling. Furthermore, the
lithium ion de/intercalation mechanism for NaVO3 is also revealed involving one
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electron transfer reaction between V5+ and V4+ redox couple. Considering the low cost
and material sustainability as well as the outstanding electrochemical performances,
we believe that NaVO3 is a highly promising cathode material for lithium
rechargeable batteries and our findings may help to pave the way for developing
vanadium-based layered structure materials for high-performance alkali and
alkaline-earth ion batteries.
Keywords: Sodium vanadium oxide, Long life, Structural stability, Cathode material,
Lithium rechargeable battery
1. Introduction
The ever growing demands for portable electronics, electric vehicles (EVs) and
renewable energy power generation have driven the rapid progression of improved
batteries technologies related to energy storage and conversion. As the most advanced
power supplier for future EVs and sustainable energy systems, lithium rechargeable
battery (LRBs) has gained considerable recognition in the past three decades [1, 2].
However, the market share of EVs is still far lower than expected due to the high
battery cost and short endurance mileage limited by the battery performance [3-5].
The cathode for lithium rechargeable batteries in EVs is the heaviest and most
expensive ingredient, thus it has a critical impact on battery cost and energy density [6,
7]. For this reason, cathode materials with high capacity, long-term cycle life and low
cost are permanently pursued. Motivated by this goal, plenty of new materials have
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been discovered, such as lithium-rich transition-metal oxides [8, 9], Ni-rich NCM (or
NCA) [10], polyanionic compounds (silicates, fluorophosphates, borates, and
fluorosulfates) [11, 12], etc. One of the most promising classes of cathode materials
for LRBs is the vanadium oxides, with a high theoretical capacity (often higher than
200 mAh g−1), intrinsic safety characteristic and relatively high crust abundance [13,
14]. Furthermore, their diverse oxidation states and V−O coordination numbers bring
out a variety of vanadium-based compounds with open framework to afford reversible
lithium insertion and extraction.
Among vanadium oxides families and their derivatives, V2O5 is the earliest
reported cathode and has received the most attentions in last two decades. However,
the bulk vanadium oxides suffer from rapid capacity decay due to low electronic
conductivity, poor structural stability during the ion de/intercalation [15-18].
Interlayer metal ions (MxVnOm, M = metal ion) have been reported to act as pillars to
increase the stability of layered vanadium oxides, and greatly improve the ion
diffusion rate as well as electronic conductivity of these host vanadium oxides [19,
20]. Therefore, a variety of LixVnOm derivate have been extensively explored as
potential cathode materials for LRBs. Layered LiV3O8, built up of VO6 octahedra and
VO5 pyramids, can intercalate two Li+ cations in its tetrahedral sites. The
de/intercalation process involves only one redox couple V5+/V4+ with a high
–
1
theoretical capacity of 253 mAh g . However, the electrochemical performance of
LiV3O8 relies strongly on the nanostructural tailoring that severely hinders its
large-scale production [21, 22]. Recently, LiVO3 was reported with electro-activity
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towards lithium storage by Pralong and co-workers. The lithium
intercalation/deintercalation process of LiVO3 was also revealed involving one
electron transfer reaction between V5+ and V4+ redox couple, this merit therefore may
favor the structural reversibility and integrity upon cycling [23]. Encouraged by these
preliminary findings, researchers endeavored to understand its lithium storage
mechanism and explore feasible strategies to promote the electrochemical
performances. Despite a high reversible capacity of over 250 mAh g– was released
1
for LiVO3, the sluggish Li+ diffusion kinetics results in unsatisfactory rate capability,
and the vanadium dissolution and irreversible phase transformation during cycling
often lead to severe capacity decay [24-26].
As a matter of fact, the fast fading of capacity is still the common drawback for
most vanadium-based cathode materials. Aiming to address this issue, partial or full
substitution of lithium with other alkali elements, such as Na+ and K+ is developed,
and is widely recognized as an alternatively effective way to improve the cyclability
and rate performance [15, 27-29]. The Na+ (or K+) substituted counterparts could
supply larger channels for transport of Li+ and facilitate rapid diffusion of Li+ within
active materials due to the fact that the substitution of larger radius of Na+ (or K+)
enlarges the cell volume of pristine lithium compounds. Furthermore, the Na+ (or K+)
substituted counterparts may exhibit a different lithium storage mechanism and
improved structural stability against lithium de/intercalation. Based on this conception,
Na+ is introduced into various vanadium-based cathodes towards lithium storage [30].
Kawakita studied the lithium insertion behavior of Na1+xV3O8, and investigated the
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potential and structural changes of sodium trivanadate upon lithiation [31]. They
revealed that NaV3O8 has lower stabilization energy for site occupation of inserted Li
ion than LiV3O8; a larger layer distance leads to a relaxation of the interaction
between inserted Li+ ions. Liu et al. reported Na0.33V2O5 nanosheet-graphene hybrids
with high discharge capacity, good cycling stability, and superior rate performance as
cathodes for lithium storage, and they found that the overall structural changes of
β-Na0.33V2O5 were negligible, since its 3D structure could diminish the structural
stress induced by lithium insertion [15]. Wu et al. developed hierarchical
NaV6O15 flower-like structure with a high lithium storage capacity of 255 mAh g−1
and good cycling performance. The improved cyclability of NaV6O15 was attributed
to the immobilized Na ions in the structure that act as a pillar between the [V6O15]
layers, and prevent the structural collapse of NaV6O15 during cycling [32].
In this work, we report a novel cathode: NaVO3 synthesized via a facile solid-state
method and investigate the relationship between its structure and electrochemical
property. The NaVO3 electrode delivers a high discharge capacity, good rate
capability and excellent cycling stability in LRBs. The electrochemical reaction
mechanism of such NaVO3 electrode is further investigated systematically through a
series of measurements including ex situ X-ray diffraction (XRD), ex situ X-ray
photoelectron spectroscopy (XPS), ex situ Raman spectrum. The capability of NaVO3
to maintain its monoclinic structure upon repeated discharge and charge is revealed,
which is demonstrated beneficial for the prolonged cycling lifespan in comparison
with its lithium counterpart (LiVO3). Considering the low cost and material
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sustainability as well as the outstanding electrochemical performances, our work
poses the promising application of NaVO3 cathode in the field of LRBs.
2. Experimental
2.1 Material preparation: The NaVO3 sample was synthesized by a one-step
solid-state reaction as follows: the stoichiometric Na2CO3 (99.9%) and V2O5 (99.9%)
powders were put in the agate jar as precursors, and then the precursors were
ball-milled in a planetary ball mill at 300 rpm for 8 h. After ball milling, the mixture
was heated at 400 °C for 10 h in muffle furnace to obtain the final product.
The control sample LiVO3 was synthesized by the same solid-state method as
above, by using Li2CO3 (99.9%) and V2O5 (99.9%) as the metal ion resources. The
preparation procedure was the same as that for NaVO3.
2.2 Characterization: X-ray diffraction (XRD) of both samples was performed
on a Bruker D8 advanced powder X-ray diffractometer with Cu Kα radiation. To
obtain XRD patterns of the cycled electrodes, the test cells were disassembled in an
argon-filled glove box, and the electrodes were taken out. The morphologies of the
samples were observed with scanning electron microscopy (SEM, Hitachi S-4800)
and transmission electron microscopy (TEM, JEM-2100F). Raman spectroscopic
analysis was performed with a Labram-010 Raman micro spectroscopy (JY, FRA)
system utilizing a 514.5 nm incident radiation in the range from 40 to 2000 cm−1.
X-ray photoelectron spectroscopy (XPS) measurements were carried out with an
ESCALAB250 XPS spectrometer.
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2.3 Electrochemical measurements: The cathode used in this work was prepared
by mixing 70 wt. % NaVO3 (or LiVO3) powders, 20 wt. % Super P, and 10 wt. %
PVDF together and dissolving the mixture into N-methyl-2-pyrrolidone (NMP) to
make slurry, and then coating the slurry onto Al foil. The electrode was dried at
100 °C overnight in vacuum oven. The mass loading of the active material for each
cathode electrode is around 2 mg cm−2. The charge/discharge measurements were
performed in a coin cell, the electrode and lithium foil (counter electrode) was
separated by a microporous membrane (Celgard 2400), and the electrode was 1 M
LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC),
and ethylene methyl carbonate (EMC) (1:1:1 by volume). R2032 type coin cells were
assembled in an argon-filled glove box and galvanostatically charged and discharged
within the voltage range of 1.5 to 3.5 V at room temperature. Cyclic voltammetry
measurement was also carried out with the three-electrode cell at the scan rate of 0.1
mV s−1.
3. Results and discussion
NaVO3 was successfully synthesized using a facile solid-state method. The
structural characterization based on Rietveld refinement of X-ray diffraction (XRD) in
Figure 1a reveals that NaVO3 has the C2/c space group with a = 10.5507 Å, b =
9.4739 Å, c = 5.8814(11) Å, and β = 108.369°, which is consistent with the standard
XRD pattern of monoclinic NaVO3 (C2/c space group, JCPDS no. 70-1015). No
contamination or impurities are detected in the XRD pattern, indicating this material
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is stable in the air or humid condition without any structural changes, unlike LiVO3.
The refinement shows the low structural R-factors (Rwp = 4.76%, Rp = 3.31%). Figure
1b shows the schematic crystal structure of NaVO3, long chains of corner sharing VO4
tetrahedral, which are interconnected through NaO6 octahedral, build up the
unidimensional structure. Compared to the previously known LiVO3, NaVO3 has
isostructure except that all lithium atoms are fully substituted by sodium atoms. Note
that Na+ (1.02 Å) has a larger ionic radius than Li+ (0.76 Å), the interlayer spacing
will be expanded in NaVO3 that provides highly accessible Li+ ion diffusion
pathways.
The morphology of the NaVO3 sample is examined using scanning electron
microscopy (SEM) and transmission electron microscopy (TEM). The SEM image in
Figure 2a shows the particles grow with uniform rod morphology, though some of
them aggregate and align in parallel stacks. The enlarged SEM image in Figure 2b
clearly reveals well defined nanorods with diameters of 50–150 nm and a length of
1–2 μm. TEM image in Figure 2c also confirms the shape and size of the nanorods.
The architecture of a single nanorod is further identified by high resolution TEM
(HRTEM). As shown in Figure 2d, the clear lattice fringes with an interplanar placing
of 0.499 nm correspond to the (200) plane of monoclinic NaVO3.
The Li-ion insertion-extraction properties of NaVO3 are initially studied by cyclic
voltammetry (CV). Figure 3a shows the CV curves of NaVO3 conducted at a scan
rate of 0.1 mV s−1 in the voltage range of 1.5 to 3.5 V (vs. Li/Li+). During the first
cathodic scan, four main reduction peaks around 2.90, 2.71, 2.44 and 1.75 V,
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accompanying several weak peaks are observed, corresponding to the insertion
process of lithium ions at different sites. In the first anodic scan, several oxidation
peaks correspondingly appear, implying a complex multi-phase transition mechanism
upon lithium extraction process. The main redox peaks are generally preserved during
the second scan, except that some small reflections that cannot be clearly identified
become stronger and more distinguishable. The following cycles exhibit quite similar
CV behaviors except for the gradually intensified peak intensity and narrowed voltage
gap between each redox couple, suggesting steadily improved electrochemical
reversibility of NaVO3 cathode. This variation is further evidenced by its
charge-discharge curves. As displayed in Figure 3b, the initial discharge curve of
NaVO3 exhibits a smooth slope, most of which center in the potential range of 2.0 to
1.5 V. After the first cycle, the discharge slope becomes more inclined and the average
discharge voltage is raised to 2.0 V. This change is attributed to the small structural
rearrangement upon the initial lithium intercalation. In the subsequent cycles, the
average discharge voltage of the battery gradually elevates with the increase in cycle
number, implying a typical activation process. Interestingly, a flat discharge voltage
plateau at ~3.0 V is observed after 50 cycles, and it becomes longer and more
pronounced on prolonged cycles. We assume that the appearance of the long voltage
plateau at ~3.0 V is associated with the gradually activated sites in the crystal to
accommodate lithium ions intercalation. The average discharge voltage keeps above
2.52 V after 500 cycles, indicative of a stable structure and lithium de/intercalation
property.
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Figure 4a shows the rate performance tested at different current densities, and
corresponding cycle profile is given in Figure 4b. As displayed in Figure 4a, the cell
exhibits a reversible discharge capacity of 224.8 mAh g−1 at the current density of 150
mA g−1, which is very close to the theoretical capacity of NaVO3. The electrode also
demonstrates excellent rate capability that delivers a capacity close to 215, 194, 171,
153, 133, 103 and 85 mAh g−1 at a current density of 0.3, 0.6, 0.9, 1.2, 1.8, 2.4 and 3.0
A g−1. The capacity of the NaVO3 electrode does not show severe losses in capacity
when cycling current density is brought back to 0.15 A g−1, suggesting outstanding
capacity durability against high current attack. Except for the different capacities, the
corresponding cycling profile in Figure 4b displays similar charge and discharge
curves at different current densities. Even at the high current density of 3 A g−1, the
cell still can provide an average operating voltage of over 1.95 V, corresponding to a
power density of 5850 W kg−1 (although the power density calculated based on the
results of half cells with Li metal as an anode is meaningless), which is among the
best rate performances reported to date in this field. Cycle stability of the NaVO3
electrode is evaluated at a current density of 300 mA g−1. It can be seen in Figure 4c
that the cathode delivers a capacity of 165 mAh g−1 for 1000 cycles, affording notable
capacity retention of 77%. To the best of our knowledge, the cycling stability and rate
capability of NaVO3 is comparable with the best electrochemical performance
achieved for other vanadium-based cathodes in previous literatures. The SEM image
in Figure S1 (left, Supplementary Materials) after the 500th cycle indicates the
electrode maintains the mechanical integrity throughout the battery operation. The
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enlarged image on the right reveals that the NaVO3 can retain its rod shape and size
upon long-term cycling without any collapse or splitting. The maintenance of the
stable structure of the NaVO3 is probably associated with its lithium
intercalation/deintercalation character. The lithium storage mechanism of this new
cathode therefore is invesitgated by a combination of ex situ XPS, XRD and Raman
measurements.
To better understand the high rate discharge nature of the NaVO3 cathode, cyclic
voltammetry experiment at different scan rates was conducted. Figure 5a shows the
CV curves obtained at 2, 5 and 10 mV s−1 between 3.5 and 1.5 V. Compared to the CV
curves tested at 0.1 mV s−1 in Figure 3a, the main redox peaks were still retained. The
potential gaps between the corresponding anodic and cathodic peaks did not widen
very much as the scan rates increased, demonstrating superior Li-ions de-intercalation
kinetics. It is well-known that the relation between peak currents and scan rates
indicates the different electrochemical reaction characteristics, including solid phase
diffusion-controlled or surface-confined charge-transfer processes [33]. Following
previously reported research; we can use the voltammetric sweep rate dependence to
determine quantitatively the capacitive contribution to the current response [34-36].
The current response (i) at a fixed potential (V) can be described as the combination
of capacitive effects (k1v) and diffusion-controlled insertion (k2v1/2) according to the
below equation.
i = k1v + k2v1/2
where v is the sweep rate. It is thus possible to distinguish the fraction of the current
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arising from Li+ insertion and that from capacitive processes by determining both k1
and k2 at specific potentials. The relation between i and v at the strong reduction peak
(2.4 V, vs. Li/Li+) in the cathodic process was calculated to be i = 0.0143 v + 0.1914
v1/2 (Figure 5b), indicating the current response is primarily from diffusion-controlled
Li+ intercalation and few from pseudocapacitive contributions.
Figure 6 shows the ex situ XRD pattern of NaVO3 electrode at different depths of
discharge and charge. Except for the weakened peak intensity due to the lowered
crystallinity, all the main peaks of pristine NaVO3 preserve during discharge and
charge, suggesting that the monoclinic structure remains almost unchanged after Li
insertion or removal, as opposed to the phase transition observed in LiVO3. To figure
out the benefits of structural stability on the cycling performance, monoclinic LiVO3
as a control sample was also synthesized via the same solid-state method. Figure S2
shows that the sample has a well-crystallized monoclinic phase (C2/c space group,
JCPDS no. 33-0835). Figure S3 reveals that LiVO3 also has a rod shape with similar
diameter and length to that of NaVO3. The battery tested (Figure S4) at a current
density of 300 mA g−1 in the voltage range of 1.5 to 3.5 V (vs. Li/Li+) exhibits an
initial discharge capacity of 247 mAh g−1 with capacity retention of 68% in 300 cycles.
Note that LiVO3 has the same morphology and particle size as NaVO3, but presents
significantly inferior cycling performance, which is primarily ascribed to its unusual
lithium storage mechanism involving a phase transition from monoclinic LiVO3 to
cubic Li2VO3 [25]. The locally magnified XRD pattern clearly reveals that the main
peaks, such as those at 17.7° and 24.7°, gradually shift towards lower angle with the
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insertion of lithium ions during discharge, indicative of a widened interlayer spacing
for the lithiated product NaLiVO3. These peaks gradually move back to its initial
position while charging to 3.5 V, suggesting high structural reversibility. Ex situ XRD
measurements of NaVO3 electrode after 50, 100, 300 and 500 cycles have also been
performed and presented in Figure S5. The XRD pattern of the electrode cycled after
500 cycles is also the same as that after initial cycle, indicating NaVO3 electrode can
maintain its monoclinic structure upon long-term cycling, and thus demonstrating
remarkable cycling stability.
To gain further insight into the valence state changes of V, Na and Li upon lithium
insertion and extraction, XPS of the discharged and charged NaVO3 are inspected.
Figure 7 shows the binding energy of V 2p signals in the pristine NaVO3, lithiated
phase, and delithiated phase. Two well-defined peaks emerge at higher and lower
energy bands for pristine NaVO3 should be assigned to the V 2p1/2 and V 2p3/2
electrons, respectively [37, 38]. The atomic ratio of [Na]:[V] in NaVO3 calculated
from the measurement is approximately 1:1, which agrees well with the
stoichiometric ratio of raw materials (Na and V salts). After initial discharge and
charge, the shape and symmetry of these two peaks for the V 2p electrons are quite
different. The peak at higher-energy side for the lithiated phase (discharged to 1.5 V)
is composed of two splitting peaks located at 517.3 and 516.4 eV, which are assigned
to the 2p3/2 electrons of V5+ and V4+ ions, respectively [39]. Analogically, two peaks
at 524.8 and 523.6 eV corresponding to the 2p1/2 electrons of V5+ and V4+ ions can be
resolved from the peak at lower-energy side [40, 41]. After the first discharge, the
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content of V5+ decreases while that of V4+ increases, clearly indicating that NaVO3 is
reduced to V4+. Note that V5+ ions still exist after the initial discharge, probably
because of the high hygroscopicity of lithiated product. After the following charge,
the content of V4+ significantly decreases, and that of V5+ increases again, indicating
that V4+ ions are oxidized to V5+. The electrochemical reactions of the NaVO3 cathode
are thus summarized as follows:
Discharg e : NaVO3  Li  e  NaLiVO3
Charg e :
NaLiVO3  Li  e  NaVO3
In addition, the binding energies of Na 1s and Li 1s signals in the above three
samples were collected and presented in Figure S6 and S7. The molar ratios of
V5+/V4+, Na/V and Li/V for the three samples are summarized in Table 1. During the
initial discharge, the interlayer spacing of layered NaVO3 gradually widens with the
insertion of Li+, consequently the interaction between Na+ and O2- weakens and the
shielding effect of outer electron cloud lowers, finally leading to the increased binding
energy of inner electrons. On the contrary, the strengthened shielding effect results in
decreased binding energy of Na 1s electrons when Li+ is extracted from the interlayer.
Likewise, the Li 1s peak shifts towards lower binding energy when charged from
lithiated NaLiVO3 to delithiated NaLixVO3 (x≤1). These observations are in good
agreement with the ex situ XRD results in Figure 6.
Ex situ Raman spectroscopy is employed to further verify the phase changes of
NaVO3 electrode during discharge and charge. As shown in Figure 8, the Raman
spectrum of the pristine NaVO3 consists of a series of broad bands between 100 and
1100 cm−1, indicative of various internal and external vibration modes exist in the
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NaVO3 structure with a C2/c space group [42, 43]. Several small peaks emerge in the
low frequency region of 120-300 cm−1 correspond to the translational mode of the
3−
VO4 groups [44], which have been demonstrated as the featured reflections of
layered structure [45, 46]. Note that these peaks exist along the cycling, indicating
that the layered structure is well maintained upon lithium insertion and extraction.
The peaks located between 300–500 cm−1 and 500–700 cm−1 are attributed to the
bending vibrations of the triply coordinated oxygen (V3-O) bonds and the bridging
V-O-V bonds (doubly coordinated oxygen) [47, 48]. The strong peaks in the
high-frequency range of 800–1100 cm−1 are assigned to the V−O stretching vibrations
[44].
When the cell is successively discharged to 2.2 and 1.5 V, the characteristic bands
still preserve except that some of the main peaks vary in position. This variation is
relatively small for the translational mode, whereas is more evidenced for the
stretching mode. It can be clearly observed that the strongest peak at 955.5 cm−1 for
the pristine NaVO3 shifts gradually towards lower wavenumber positions as the depth
of discharge is increased. This movement of Raman reflections is related to the steric
V=O repulsion [44]. As lithium ions are inserted into layered structure of NaVO3, the
interlayer spacing widens, the steric repulsion of V=O therefore decreases, and
consequently the characteristic Raman peaks shift to lower frequency. Analogically,
these peaks gently move back to higher wavenumber positions as the potential
increases from 1.5 to 3.5 V, again indicating a high structural reversibility of NaVO3
towards lithium storage. The evolutions of Raman reflections are consistent with the
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XRD evolvement observed in Figure 6.
Apart from the position shift of characteristic bands, peak splitting is also
observed for the discharged and charged electrode. As clearly displayed in the
enlarged XPS image shown in the right side, the single peak at 955.5 cm−1 for the
pristine NaVO3 is split into two separated peaks, suggesting the nonequivalent
character of the ladders in the lattice of NaLixVO3 [44]. This diversification is induced
by two different types of vanadium environments, of which the bands at lower and
higher wavenumber positions are associated with the V4+ and V5+ ions, respectively.
The changes in the relative intensities of these two peaks at different depths of
discharge and charge further demonstrate the electrochemical reactions of NaVO3
involving one electron transfer between V5+ and V4+ redox couple. Interestingly, it is
found that the intensity of the small peaks in the range of 120-300 cm−1 gradually
strengthens during discharge and returns to the original state after charge, probably
because of the intensified layered structure of lithiated product NaLixVO3.
4. Conclusions
In summary, NaVO3 nanorods are synthesized by a facile solid-state method and
evaluated as cathode materials for LRBs. The cathode delivers a high specific
capacity 224.8 mAh g−1 at the current density of 150 mA g−1, and a high rate
capability of 85 mAh g−1 even at a high current density of 3 A g−1. What is more, the
cathode also exhibits ultralong lifetime for 1000 cycles with high capacity retention of
77%. Ex situ XRD, Raman and XPS measurements suggest the remarkable cycling
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performance is ascribed to its considerable structure stability that can maintain
monoclinic structure upon repeated discharge and charge. In addition, a reversible
intercalation mechanism involving redox reaction between V5+ and V4+ couple is also
confirmed by ex situ characterizations. We believe that NaVO3 is a highly promising
cathode material for LRBs and our findings may help to pave the way for developing
vanadium-based layered structure materials for high-performance alkali and
alkaline-earth ion batteries.
Acknowledgements
This work was financially supported by the National Key Research Program of
China (No. 2016YFB0901500), the National Natural Science Foundation of China
(21875171), the Fundamental Research Funds for the Central Universities
(2042018kf0007) and the China Postdoctoral Science Foundation (2016M592383,
2017T100574).
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Figure captions:
Fig. 1. (a) Refined XRD patterns of NaVO3; (b) Schematic of layered NaVO3
structure.
Fig. 2. (a, b) SEM and (c, d) TEM images of NaVO3.
Fig. 3. (a) Cyclic voltammetry curves of NaVO3 at a sweep rate of 0.1 mV s−1
between 3.5 and 1.5 V; (b) typical galvanostatic charge-discharge curves of NaVO3 at
a current density of 300 mA g−1.
Fig. 4. (a) Cyclic voltammetry curves of NaVO3 at a sweep rate of 0.1 mV s−1
between 3.5 and 1.5 V; (b) typical galvanostatic charge-discharge curves of NaVO3 at
different current densities; (c) Cycling performance in terms of specific capacity at a
current density of 300 mA g−1.
Fig. 5. (a) CV curves of NaVO3 at different sweep rates (2, 5, and 10 mV s−1); (b)
relationship between anodic peak currents and the sweep rate, the point plots are
experimental values, and the dash lines are fitting results.
Fig. 6. Ex situ XRD patterns of NaVO3 electrode at different depths of discharge and
charge (discharge to 2.2, 1.5V; charge to 2.9, 3.5 V), left is the initial discharge/charge
curve, right is the magnified XRD patterns ranging from 17.0° to 18.4° and from 24°
to 25.4°.
Fig. 7. Ex situ XPS V 2p3/2 and V 2p1/2 core peaks of the pristine NaVO3 electrode,
lithiated phase (discharge to 1.5V), and delithiated phase (charge to 3.5 V).
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Fig. 8. Ex situ Raman spectra of NaVO3 electrode at different depths of discharge and
charge (discharge to 2.2, 1.5V; charge to 2.9, 3.5 V), right is the magnified Raman
reflections ranging from 928 to 970 cm−1.
Table 1. The molar ratios of V5+/V4+, Na/V and Li/V for the pristine NaVO3 electrode,
lithiated phase (discharge to 1.5V), and delithiated phase (charge to 3.5 V).
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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