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Multidimensional Na4VMn0.9Cu0.1(PO4)3/C cottoncandy cathode materials for high energy Na-ion
Vaiyapuri Soundharrajan,a Muhammad H. Alfaruqi, ab Seulgi Lee,a
Balaji Sambandam,a Sungjin Kim,a Seokhun Kim,a Vinod Mathew,a
Duong Tung Pham,c Jang-Yeon Hwang,a Yang-Kook Sun d and Jaekook Kim
Sodium-ion batteries (SIBs) have attracted great attention for day-to-day applications as a replacement for
lithium-ion batteries (LIBs) that deliver high voltage and high energy because of the low battery-preparation
cost and vast availability of sodium resources. The recent exploration of Na+ superionic conductor or
NASICON-type Na4VMn(PO4)3 (NVMP) cathodes for SIBs is a pioneering approach because of the high
working voltage, high theoretical capacity, and stable three-dimensional framework of the NVMP
cathodes. However, the inherently low electronic conductivity results in mediocre rate outputs and poor
exploitation of the active material. Herein, we report, for the first time, the preparation of a cotton
candy-like carbon-coated Cu-doped NVMP or Na4VMn0.9Cu0.1(PO4)3 (NVMCP/C/CC) cathode by a facile
and ultrafast pyro-synthetic method. The robust structure of the NVMCP/C/CC and the highly reversible
two-phase reaction upon Na-ion insertion/extraction were systematically revealed by the in situ
synchrotron XRD and GITT studies, while the DFT calculations established the crucial reasons behind the
enhanced electronic conduction of the NVMCP/C/CC. The superior electrochemical properties of the
Received 6th April 2020
Accepted 24th May 2020
NVMCP/C/CC cathode at low (79 mA h g1 after 450 cycles at 1.5C) and high current rates (68 mA h g1
after 3000 cycles at 30C) demonstrate that the combination of a three-dimensional nanoarchitecture,
DOI: 10.1039/d0ta03767b
uniform carbon-coating, and Cu-doping is favorable for improving the electrochemical properties of the
NVMP cathodes.
1. Introduction
Among the many available energy storage devices, SIBs are
a favorable option in the electronics eld in the post-LIB era. At
present, LIBs appear to be inadequate in satisfying the massive
requirement of grid-scale energy storage devices for the fastgrowing electronic market; thus, SIBs have gained attention
because of their low cost and vast availability.1 Due to their
(EðNaþ =NaÞ ¼ 2:71 versus standard hydrogen electrode, similar to
Department of Materials Science and Engineering, Chonnam National University,
300Yongbong-dong, Bukgu, Gwangju 500-757, South Korea. E-mail: jaekook@
chonnam.ac.kr; Fax: +82-62-530-1699; Tel: +82-62-530-1703
Departemen Teknik Metalurgi, Universitas Teknologi Sumbawa, Jl. Raya Olat Maras,
Sumbawa, Nusa Tenggara Barat, 84371, Indonesia
Institute for Electrochemical Energy Storage, Helmholtz-Zentrum Berlin für
Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, German
Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea
† Electronic supplementary information (ESI) available: SEM and PXRD pattern
for MnS2 and NVMP samples, table with crystallographic information from
Rietveld renement, TGA, Raman spectra, XPS, BET, and ICP data of NVMCP
sample, CDC pattern at 30C rate, ex situ XRD and SEM, full-cell data. See DOI:
This journal is © The Royal Society of Chemistry 2020
that of lithium), and simple cell construction, the working
principle of SIBs is not just inspired by the chemistry of LIBs,
rather it is almost the same.2,3 For example, there are many
lithium-based intercalation compounds that are almost as good
as that in LIB cathodes and exhibit impressive electrochemical
performances as their sodium equivalents (Na1.1Li2V2(PO4)3,
Li2NaV2(PO4)3, Na2.4V2(PO4)3).4–6 Extensive investigations have
revealed that the major bottleneck in realizing high electrochemical performance in SIB cathodes is the sluggish diffusion
kinetics of the Na+ ions that results in poor rate performance
because of the larger ionic radius of Na+ (1.02 Å) than that of Li+
(0.76 Å).7–10
To overcome this setback, SIBs have been systematically
investigated in the current decade. Consequently, many cathodes with superior enhanced electrochemical properties have
been introduced for SIBs including layered oxides, polyanionic
compounds, and Prussian blue analogs.11–15 Among these,
NASICON-structured polyanionic compounds, with the general
formula NaxM2(PO4)3, are recognized as potential cathodes for
SIBs because of their unique 3D crystal framework owing to the
stable MO6 octahedral units (M ¼ transition metals) and XO4
tetrahedra (X ¼ P, S, Si, As), which ensure fast Na+ diffusion.16
More importantly, NASICON-based materials exhibit extreme
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Journal of Materials Chemistry A
safety, good thermal stability, and structural rmness.17 Among
the polyanionic groups, NVP-the most studied electrode material for SIBs-has a at working potential of 3.3–3.4 V operated by
a V4+/V3+ redox couple that causes an adequate reversible
capacity of 100 mA h g1 with a remarkable rate performance.18 Although, Na3V2(PO4)3 (NVP) is the most favored for
SIB cathode applications, the use of the expensive and noxious
vanadium component limits its large-scale applications.19 To
address this concern, Goodenough's group introduced a new
series of NASICON-structured NVMP cathodes for SIBs.20 The
introduction of inexpensive and eco-friendly Mn into the
NASICON framework has been advantageous in that the inexpensive Mn metal reduces the production cost of the cathode
material;21 the introduction of Mn into NVP results in additional Na+ sites in the framework, thereby leading to improved
capacity (110 mA h g1); also, the high-redox voltage of Mn2+/
Mn3+ (3.6 V) results in a higher working voltage for the NVMP
cathode.22 However, like most polyanionic materials, NVMP
exhibits poor electronic properties, which impedes the
complete utilization of the NVMP capacity. This problem can be
addressed by following the general strategies employed for
typical polyanion cathodes such as the development of
composites with carbon,23 swapping of transition-metal ions,24
particle downsizing,25,26 and controlling the morphology via
different preparation techniques.24,27,28 Most of the previous
documents have mainly focused on the NVMP/conducting
carbon composite aspect to enhance the electrochemical
Herein, to exploit the complete utilization and progress of
the potential of NMVP electrodes, an amalgamation approach
comprising of a crystal arrangement variation, tailored
morphology design with conned particle-sizes, and in situ
carbon coating has been proposed to develop a Cu-doped Na4VMn0.9Cu0.1(PO4)3/C (NVMCP/C/CC) cathode with a unique
cotton candy-like morphology through a modied pyrosynthetic reaction. In general for vanadium-based electrodes
via the pyro-synthetic process, we use vanadium acetylacetonate
directly, which is expensive, as a source for vanadium; also, it
resulted in an agglomerated nanoparticle morphology.4,18
However, in the proposed modied pyro-synthetic method, we
adopted a facile pre-reduction (vanadium) and successive pyrosynthetic method for the preparation of an NVMCP/C/CC
cathode by using a low-cost ammonium vanadate precursor
that cuts-down the nal product cost and also controls the
crystal growth to yield a specic morphology. In particular, the
achieved NVMCP/C/CC cathode with a unique cotton candy-like
nanoarchitecture exhibits superior electrochemical properties
compared to those of the NVMCP/C nanoparticle electrode
because of the enhanced electronic conductivity and facile Na+
ion diffusion in the unique cotton-candy skeleton, as revealed
from the experimental studies and the rst-principles calculations. Thus, this study demonstrates that polyanionic cathodes
can be prepared with a unique morphology with uniform
carbon coating within a short reaction time in an open-air
environment. We believe that the present modied pyrosynthetic approach will facilitate the development of potential
12056 | J. Mater. Chem. A, 2020, 8, 12055–12068
energy storage materials with distinctive morphology on
a mass-scale.
Experimental section
Synthesis of NVMP/C/NPs, NVMP/C/CC, and NVMCP/CC
As a control experiment, the NVMP/C/NPs were initially
prepared by the conventional pyro-synthetic process by dissolving (Na and Mn) metal acetate precursors (Sigma-Aldrich,
99%) and V (acetylacetonate) precursors (Sigma-Aldrich, 97%)
in tetraethylene glycol (TTEG) solvent before adding phosphoric
acid (Daejung, 85%) in a stoichiometric ratio, followed by the
ignition of the ammable reaction solution by a torch to obtain
the combustion deposits, which were collected and heated at
800 C for 12 h in an Ar atmosphere to achieve the agglomerated
NVMP/C nanoparticles. On the other hand, Na4VMn0.9Cu0.1(PO4)3/C Cotton candy-like material were synthesized by
a modied pyro-synthetic process. Firstly, (4 mmol) sodium
nitrate (Sigma-Aldrich, 99%) and (1 mmol) manganese nitrate
(Sigma-Aldrich, 97%) were dissolved in 100 mL of tetraethylene
glycol (TTEG) solvent to obtain solution A. Secondly, (1 mmol)
ammonium vanadate (JUNSEI, 99%) was added to 5 mL DI
water and (2 mmol) oxalic acid (DAEJUNG, 99.5%) was added as
the reducing agent and stirred well for 20 min to get solution B
(blue color). Aerwards, solution B was added to solution A,
followed by the addition of (3 mmol) phosphoric acid (Daejung,
85%), which was mixed well to get a homogeneous solution. The
resultant homogenous solution was poured into an aluminium
boat and kept on a hotplate maintained at 450 C. Then, the
polyol stock solution was ignited using a torch to activate the
ultrafast self-extinguishable combustion reaction. Later on, the
combustion deposits were harvested and annealed at 800 C in
the Ar atmosphere for 12 h to realize carbon-coated NVMP
cotton-candy-like micro-structures. In addition, Cu-doped Na4VMn0.9Cu0.1(PO4)3/C, Na4VMn0.85Cu0.15(PO4)3/C, and Na4VMn0.8Cu0.2(PO4)3/C Cotton candy-like products were also
synthesized under the same modied pyro-synthetic conditions, in which copper nitrate (Sigma-Aldrich, 99%) precursor is
used as the copper source. In all the above cases, 20 mL paint
thinner (KCC, commercial paint thinner) was added as an
igniter to trigger the pyro-reaction before transferring into an
aluminium boat.
Structure and morphology characterization
A Shimadzu X-ray diffractometer was used to obtain the powder
X-ray diffraction (PXRD, Cu Ka radiation, l ¼ 1.5406 Å) pattern
of the electrode materials. Thermogravimetric analysis (TGA)
was conducted using an SDT Q600 thermobalance in air with
a temperature gradient of 5 C min1. A 3D high-resolution Xray diffractometer (Empyrean, PANalytical, the Netherlands)
was used to achieve the high-resolution XRD patterns of the
samples. The surface morphology of the cathode materials was
characterized by eld-emission scanning electron microscopy
(FE-SEM) using a Hitachi S-4700 equipped with an energydispersive X-ray spectroscopic (EDS) detector and the lattice
fringes were calculated by eld-emission transmission electron
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microscopy (FE-TEM) using a Philips Tecnai F20 (200 kV; at
KBSI Chonnam National University) equipped with a selected
area electron diffraction system (SAED). The Raman spectra
were recorded using a JASCO Laser Raman Spectrometer NRS5100 series to conrm the presence of carbon in the samples.
The elemental compositons of the samples were characterized
via ICP-OES (inductively coupled plasma-optical emission
spectroscopy) analysis using a PerkinElmer 4300 DV analyzer.
The surface area of the samples was calculated using nitrogen
adsorption and desorption isotherms using a Brunauer–
Emmett–Teller surface analyzer (BET, Micromeritics ASAP
earlier by our group.29 The MnS2 anode (80%) was mixed with
super P carbon (10%) in the presence of 5 wt% aqueous solution
of sodium carboxymethyl cellulose (4%) and a 50 wt% aqueous
solution of styrene-butadiene rubber (6 wt%). The corresponding XRD pattern of the MnS2 anode (slurry-coated) along with
the SEM images are presented in the Supplementary
information† (Fig. S1†). The calculated and normalized weight
ratios of the two electrodes were 1 : 7 (anode:cathode). The
electrolyte was 1.0 M NaPF6 in DGM. The cathode and anode
materials were tested in half-cells in the same electrolyte and
the results are presented in the ESI.† The sodium full-cells were
tested in the potential window of 0–3.6 V at a current density of
40 mA g1.
In situ synchrotron XRD characterization
In situ synchrotron XRD measurements were performed at
beamline 1D KIST-PAL, Pohang Accelerator Laboratory, using
a MAR345 image plate sensor at 2.5 GeV with an assured storage
current of 200 mA. The X-ray beam was focused with a toroidal
mirror and monochromatized to 12.4016 keV (0.9997 Å) using
a double bounce Si (111) monochromator. An Si (111) monochromator and an Si (111) crystal detector were employed to
conrm high-resolution formation in the reciprocal space. The
patterns were established using a wavelength of 0.9997 Å. In
addition, the XRD peaks were plotted aer recalculating the 2q
values using Cu Ka radiation (l ¼ 1.5414 Å). For the construction of the in situ cell, 70% active material was mixed with 20%
carbon black (Lion Corporation, Japan) and 10% TAB binder
(Hohsen Corporation, Japan); then, the slurry was cast onto an
aluminium mesh and kept in a spectroelectrochemical cell. The
cell was cycled to a fully charged/discharged state using
a portable potentiostat at a xed current density of 20 mA g1.
The Kapton tape was glued on the apertures of the outer cases of
the test cell.
Journal of Materials Chemistry A
Electrochemical characterization
For electrochemical characterization, the cathodes were established from the active material (70%), Denta black conductive
carbon (15%), and polyacrylic acid binder (15%) in N-methyl-2pyrrolidone to create a homogeneous slurry. The slurry was
coated uniformly on the Al-foil current collector using the
doctor blade technique, dried at 80 C in a vacuum oven,
pressed between stainless steel twin rollers (maintained at 120
C), and tapped into circular discs. Sodium metal was employed
as the counter electrode. NaPF6 (1 M) in ethylene carbonate/
propylene carbonate (EC/PC) electrolyte with 2% uoroethylene carbonate (FEC) additive was assembled in an argonlled glovebox and aged overnight before carrying out electrochemical charge/discharge characterizations using a BTS2004H
(NAGANO KEIKI Co., LTD., Ohta-ku Tokyo, Japan) battery tester
at different current densities between 2.4 to 3.8 V vs. Na+/Na.
Cyclic voltammetry (CV) and galvanostatic intermittent titration
technique (GITT) measurements were conducted using BIOLogic Science Instruments. For the assembly of sodium fullcell batteries, MnS2 was used as the anode and NVMCP/C/CC
as the cathode in a CR2032-type coin-cell conguration. The
MnS2 anode material was prepared using a strategy reported
This journal is © The Royal Society of Chemistry 2020
Specic power and specic energy calculation
Specic energy was calculated as E (W h kg1) ¼ specic
capacity potential (average working potential).
Specific power was calculated as P (W kg1) ¼ I V/2m,
where I is the applied current (A), V is the average working
potential (V), and m is the active mass at the cathode side.30
First-principles calculations
First-principles calculations based on density functional theory
(DFT) were performed using Quantum-Espresso package with
projector augmented wave (PAW) pseudopotential and Perdew–
Burke–Ernzerhof (PBE) exchange-correlation functional.31,32 A
plane-wave basis set with a cutoff energy of 30 Ry (408 eV) was
used. The positions of the atoms in the primitive NVMP and
NVMCP structures were relaxed using Broyden–Fletcher–Goldfarb–Shanno (BFGS) and the Brillouin zones were sampled
using a k-point mesh of 2 2 2 and 2 2 1, respectively.
For the density of states (DOS) studies, the DFT + U method was
applied with an on-site potential U of 3.9 and 3.25 eV for
manganese and vanadium, respectively.33
Results and discussion
The realization of cathode materials using a fast and scalable
pyro-synthetic process with a short reaction time was demonstrated earlier by our group,34 followed by the fabrication of
polyanion-type cathodes with nanoparticle morphology for SIBs
in a short reaction time using a pyro-synthesis method.4,18,35 In
particular, the utilization of such methods to arrive at cathodes
with uniquely dened morphologies is of great necessity as it
will help to achieve mass-scale production in a short period.
With this concern in mind, we have demonstrated the synthesis
of carbon-coated NVMP with a cotton candy-like morphology
within a short reaction time. Scheme 1 compares the NVMP/C
preparation process using conventional pyro-synthesis and
modied pyro-synthetic methods. Conventional pyro-synthesis
involves the dissolution of acetate (Na, Mn) metal precursors
and V (acetylacetonate) precursors in tetraethylene glycol
(TTEG) solvent before the addition of phosphoric acid. TTEG
solvent with three glycol groups has higher boiling point and
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Journal of Materials Chemistry A
Scheme 1
Comparison of the conventional and modified pyro-synthetic processes.
viscosity compared to mono (di)-ethylene glycol solvents. These
advantages facilitate TTEG to strongly inuence the formation
of chelating complexes with transition metals. Further, the
higher boiling point of TTEG ensures sustained polyol
combustion for apparently longer durations that can facilitate
the formation of hierarchical or secondary particles. More
importantly, TTEG solvent with a long chain network serves as
a carbon-rich backbone for the active NVMP particles. This is
followed by the ignition of the ammable reaction mixture
using a torch to obtain the combustion deposits; these were
collected and heated at 800 C in an Ar atmosphere to produce
agglomerated NVMP/C nanoparticles (NVMP/NPs). On the other
hand, the modied pyro-synthetic process involves the dissolution of nitrate (Na, Mn) metal precursors in the TTEG solvent.
For the vanadium source, we initially dissolved ammonium
vanadate in deionized (DI) water, utilizing oxalic acid as the
reducing agent to induce the reduction of V(5+) to V(3+) and
added it to the metal stock solution. This was followed by the
addition of phosphoric acid. Later on, the homogenous solution was ignited using a torch to induce a self-extinguishable
combustion reaction to get cotton candy-like combustion
deposits, followed by thermal-treatment, similar to that of the
conventional pyro-synthetic method. The obtained cotton
candy-like NVMP/C with uniform carbon coatings was labelled
as NVMP/C/CC. By the inclusion of the Cu(NO3)2 precursor as
the copper source in the same modied pyro-synthetic reaction
conditions, the Cu-doped NVMCP/C cotton candy-like (NVMCP/
C/CC) cathode was also prepared. The doped NVMCP/C/CC
cathode with unique morphology demonstrates superior
12058 | J. Mater. Chem. A, 2020, 8, 12055–12068
electrochemical properties compared to those of the two
counterpart cathodes based on nanoparticles (NVMP/NP) and
undoped cotton candy-like structure (NVMP/C/CC); this will be
discussed in the following section. Moreover, to appreciate the
origin of the cotton candy-like morphology, the NVMCP/C/CC
combustion deposits were subjected to scanning electron
microscopy (SEM), demonstrating that even the as-prepared
sample itself consists of a cotton candy-like morphology (Fig.S2a†). The morphology retained its structure even aer
annealing; this will be further discussed in the following
section. On the other hand, the powder X-ray diffraction (PXRD)
pattern of the combustion deposit unveiled amorphous features
and their corresponding peaks were indexed to trigonal NVMP
(this will be discussed in detail in the following section)
(Fig. S2b†). However, several peaks were not completely grown
in the as-obtained sample, demonstrating that the combustion
deposits need heat treatment to yield the preferred product with
high crystallinity.
The comparative PXRD patterns of the prepared NVMP/C/
NP, NVMP/C/CC, and NVMCP/C/CC cathode samples revealed
a similarity and all the Bragg diffraction peaks could be wellindexed to the monoclinic NVMP sample achieved from X'Pert
Highscore Plus program (Fig. S3†).22 Moreover, to gain
a complete understanding of the crystal arrangements of the
NVMP/C/CC and NVMCP/C/CC materials, Rietveld renement
was conducted with the help of X'Pert Highscore Plus utility on
the obtained high-resolution XRD patterns of the NVMP/C/CC
and NVMCP/C/CC samples. The detailed patterns and renement data are shown in Fig. 1, Tables S1 and S2,† respectively.
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Journal of Materials Chemistry A
Fig. 1 XRD Rietveld refinement pattern for (a) Na4VMn(PO4)3 (inset: goodness-of-fit values), (b) Na4VMn0.9Cu0.1(PO4)3 (inset: goodness-of-fit
values) (c) structural model of NVMP/NVMCP, and (d) Facile Na-ion diffusion viewed from the expanded c-axis.
Both of the samples are well-indexed to the characteristic
c trigonal crystal system. The lattice paramefeatures of the R3
ters of the NVMP structure were calculated to be a ¼ b ¼ 8.9649
Å and c ¼ 21.47864 Å; noticeably, the “goodness-of-t” value for
both the samples (inset in Fig. 1a and b) further conrms the
renement process. The structural model of these samples was
constructed using the VESTA package36 and is depicted in
Fig. 1c. The NVMP/NVMCP structure is built from a cornershared M–O6 (M ¼ V, Mn, Cu) octahedron and PO4 tetrahedral units forming the [M2(PO4)3]4 anion framework. While
the M elements are located in a similar Wyckoff site, i.e., 12c
site, Na elements are situated at two different sites, i.e., 6b and
18e. Aer Cu-doping (NVMCP), the slightly different crystallographic parameters could be observed, i.e., a ¼ b ¼ 8.96072 Å
and c ¼ 21.48843 Å (Table S2†). The expansion of the c-axis is
also expected to provide facile Na-ion diffusion, as shown in
Fig. 1d.
Field emission-scanning electron microscopy (FE-SEM) was
conducted to observe the size and morphology of all the
prepared samples. The NVMP cathode prepared by the
conventional pyro-synthetic process showed a nanoparticle
morphology with an average particle size in the range of 50–
100 nm (Fig. S4a†), as witnessed in our previous reports.34,37 In
contrast, the NVMP/C/CC cathode achieved by the modied
pyro-synthetic process exhibited a unique cotton candy-like
This journal is © The Royal Society of Chemistry 2020
morphology composed of several nanoarrays with an average
particle size in the range of 1–2 mm (Fig. S4b†). Furthermore, the
Cu-doped NVMP or NVMCP/C/CC cathode maintained the
distinctive cotton candy-like morphology (composed of
multiple nanoarrays) and the same particle size (Fig. 2a).
As the NVMCP/C/CC cathode delivered remarkable electrochemical performance (described later in this section), further
investigations except for the galvanostatic measurements were
pursued only for this electrode sample. The transmission electron microscopy (TEM) studies to investigate the comprehensive crystallographic features conrmed that the NVMCP/C/CC
cathode consists of a cotton candy-like microstructure assembled from nanoarrays (Fig. 2b). The high-resolution transmission electron microscopy (HRTEM) targeted at the edge of
NVMCP/C/CC revealed a rich crystal lattice with an interlayer
spacing of 0.28 nm, attributable to the (116) plane of trigonal
NVMP (Fig. 2c). More importantly, it is clearly visible that the
NVMCP cotton candy-like cathode is wrapped with a smooth
amorphous carbon layer of 5–7 nm thickness. Further, the
energy dispersive X-ray (EDX) elemental mapping examination
was conducted to endorse the dissemination of all the elements
in the composite product. The resultant elemental mapping
images (Fig. 2d) illustrate the homogeneous distribution of
Na, V, Mn, P, O, and Cu elements in the NVMCP sample. The
additional TEM images (in Fig. S5a and b†) further endorse the
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Journal of Materials Chemistry A
(a) SEM image of NVMCP/C/CC. HRTEM images of NVMCP/C/CC at (b) low and (c) high magnifications, (d) elemental mapping in a bright
field image with corresponding elements (Na, Mn, V, P, O, Cu), XPS profile for NVMCP/C/CC (e), V 2p lines, and (f) Mn 2p lines after deconvolution
of the peaks.
Fig. 2
cotton candy-like architecture. The nite lattice fringes of Na4VMn0.9Cu0.10(PO4)3 material can be clearly visible from the high
magnication image with a calculated stripe space value of
0.61 nm, representing the (012) plane of the trigonal NVMP
structure (Fig. S5c†). To determine the exact carbon quantity,
thermogravimetric analysis was conducted and the precise
carbon content existing in the NVMCP/C/CC cathode was found
to be 3.5% (Fig. S6a†). Also, Raman spectroscopy was performed
to understand the nature of carbon in the NVMCP/C/CC material; the results are illustrated in Fig. S6b.† The resultant prole
presented two characteristic Raman footprints at 1346.3 cm1
(D-band) and 1587.4 cm1 (G-band). The ID/IG ratio (i.e., the
ratio of sp3 to sp2) was calculated to be 0.85, demonstrating the
amorphous nature of carbon.38 The specic surface area and
pore structure of all the three electrodes were furnished by
nitrogen adsorption–desorption investigation. The specic
surface area of the NVMCP/C/CC material was measured to be
141 m2 g1, which is higher than 38.2 m2 g1 for NVMCP/C/NPs
and near to 119.3 m2 g1 for NVMP/C/CC (Fig. S7†). It is to be
12060 | J. Mater. Chem. A, 2020, 8, 12055–12068
noted that a higher BET surface area could provide increased
electrolyte–electrode contact for boosted sodium storage
performance. To determine the inherent structural properties,
X-ray photoelectron spectroscopy (XPS) was carried out for the
NVMCP/C/CC material. Fig. 2e illustrates the V 2p spectrum,
which exhibits two predominant peaks at 517.1 eV and 522.2 eV,
corresponding to the 2p3/2 and 2p1/2 spin–orbit energy states,
respectively, and dening the trivalent nature of V in the
material.39 The Mn 2p prole in Fig. 2f shows two major peaks
with binding energies of 641.2 eV and 653.5 eV, which individually represent the 2p3/2 and 2p1/2 spin–orbit energy states
with an energy separation of 12.3 eV, respectively, indicating
that the energy state of Mn is bivalent.22,40 Furthermore, the
shake-up satellites existing at 645.3 eV and 658.1 eV were
attributed to the paramagnetic metal state.40,41 The highresolution XPS prole of C (1s) in Fig. S8a† demonstrates the
different forms of carbon at 282.3, 284.5, and 286.5 eV representing the (C–C), (C–O–C), and (COO) bonds, respectively, on
the surface.42 Similarly, the high-resolution XPS spectrum of O
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(1s) exhibits different peaks at 529.7, 530.6, and 533.2 originating from (M–O), (C–O–C), and (COO) bonds, respectively
(Fig. S8b†).42 Furthermore, the XPS peaks at 1070 and 132 eV
conrm the Na (1s) and P (2p) elements in the sample (Fig. S8c
and d†). More importantly, at 935 eV, a small peak can be seen
for Cu 2p, conrming that Cu2+ has been doped into the crystal
network of the NVMP/C product (Fig. S8e†). Overall, the XPS
survey results (Fig. S8f†) conrm the existence of multiple
elemental features of the NVMCP/C/CC material. To acquire the
exact elemental contribution information of the Na4VMn0.9Cu0.10(PO4)3 electrode material, inductively coupled plasmaoptical emission spectroscopy (ICP-OES) and elemental analysis were performed and the results are presented in Table S3 in
the ESI.† It is evident that the elemental ratio of the NVMCP
powder is in the order of (Na ¼ 0.556; V ¼ 0.16; Mn ¼ 0.14; Cu ¼
0.015), which is equivalent to the stoichiometric composition of
Galvanostatic cycling inspections were conducted to investigate the electrochemical behavior of the three electrodes,
prepared by the conventional and modied pyro-synthesis
methods. The comparative galvanostatic charge/discharge
pattern in the working voltage of 3.8–2.4 V at 0.25C is illustrated in Fig. 3a. All the electrodes exhibit two pairs of voltage
plateaus near 3.45/3.35 V and 3.63/3.51 V originating from the
V3+/V4+ and Mn2+/Mn3+ redox couples and signifying the
reversible Na+ (de)insertion from/into the NVMP NASICON
structure.20 The NVMCP/C/CC composite electrode delivered
a superior discharge capacity (117 mA h g1) in the initial cycle
compared to the NVMP/C/CC (112 mA h g1) and NVMP/C/NPs
(108 mA h g1) composite electrodes due to the combined
effects of copper doping and unique cotton candy-like
morphology. Moreover, amongst the three electrodes,
NVMCP/C/NPs exhibits the least electrochemical polarization,
signifying an enriched electronic conductivity due to the Cu
doping. To further clarify the morphological and doping effects
in the rate performance, we subjected all the cathodes to rate
testing at different current surges starting from 0.25C to 40C;
the resultant rate performances are illustrated comparatively in
Fig. 3b. Among the three cathodes, it is clear that the NVMP/C/
CC cathode has better rate capability than NVMP/C/CC and
NVMP/C/NP cathodes. When driven at low C-rates of 0.25C and
0.5C, the differences between the electrodes are less. However,
with the acceleration of C-rates, the reversible capacity and
cyclability of NVMCP/C/CC are improved more than those of
NVMP/C/CC and NVMP/C/NPs. For example, the reversible
capacity (68 mA h g1) realized for the NVMCP/C/CC cathode at
a very high rate (40C) is considerably higher than those for the
NVMP/C/CC and NVMP/C/NP (45 and 20 mA h g1, respectively)
at the same rate (40C). It is important to note that the appreciable rate performances in the present study are comparable to
the recent results in the literature.17,20,22,43 These outcomes
validate that the NVMP/C cathodes with a cotton candy-like
structure exhibit superior electrochemical performance
compared to that of NVMP/C/NPs due to the 3D electronic
passage system. On the other hand, the improved electrochemical stability realized in the Cu-doped NVMCP/C/CC with
cotton candy-like nano-architecture could originate from the
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Journal of Materials Chemistry A
combined electronic conductivity improvement due to the
presence of Cu in the crystal structure and 3D electronic
passage arrangements that arise from the unique 3D
morphology.24,44 In addition to the unique cotton-candy architecture, Cu doping was also found to enhance the electrochemical activity of the NVMP cathode. Hence, to understand
the effect of Cu content on the sodium storage properties, we
studied two NVMP samples, i.e., Na4VMn0.85Cu0.15(PO4)3 and
Na4VMn0.8Cu0.2(PO4)3 with 0.15 and 0.2 mol Cu content,
respectively, prepared by the same modied pyrosynthetic
technique. The obtained XRD patterns for the samples were
clearly indexed to the standard triagaonal NVMP phase estimated by the Xpert highscore plus program (Fig. S9a†). On
subjecting it to rate performance evaluation (Fig. S9b and c†),
the NVMP samples with high Cu concentration of 0.15 and
0.2 mol demonstrated low-rate capacities of 40 and 35 mA h g1
at 40C, respectively, compared to that of the NVMP sample with
low or 0.1 mol Cu concentration (68 mA h g1 at 40C). This
clearly manifests that the NVMCP cathode with optimized Cu
content (i.e., Na4VMn0.9Cu0.10(PO4)3), in addition, to the unique
morphology is essential to realize distinguished sodium storage
As NVMCP/C/CC exhibits the best rate performance, we
further observed the electrochemical properties of the NVMCP/
C/CC cathode by cycling at a 1.5C rate; the corresponding
cyclability results are illustrated in Fig. 3c. The NVMCP/C/CC
cathode delivers an initial discharge capacity of 87 mA h g1
and the cathode material undergoes a slight drop in capacity
(81.26 mA h g1, 25th cycle) in the initial cycles. Aerwards, the
cathode material underwent a slow activation process up to the
103rd cycle, which yielded an activation capacity of 84 mA h g1
due to the improved Na+ diffusion effects resulting from the
increased availability of the reversible sites along the periphery
of the electrode during the repeated cycling process.37,45 With
the increase in the number of cycles, the capacity is slowly
reduced; for example, aer the 200th cycle, the delivered specic
capacity was 82.87 mAhg1, aer which it was apparent that
the electrode exhibits adequate cycling stability; a stable
reversible capacity of 79 mA h g1 aer 450 cycles and capacity
retention of 90% were also observed. The superior cycling
stability at a low-current rate decides the stability of the electrode. Thus, excellent cycling stability at a low current rate
validates the long-term cycle life of the NVMCP/C/CC cathode.
The corresponding selected charge–discharge patterns for the
50th, 100th, 200th, and 300th cycle, respectively, are presented in
Fig. 3d. The shape of the charge–discharge pattern and position
of the redox pairs for both V3+/V4+ and Mn2+/Mn3+ are retained
over the entire cycling process period again, indicating that the
cathode maintains a stable Na+ extraction/insertion mechanism
due to the robust NASICON framework structure; this is further
enhanced by the unique cotton candy-like morphology and
electronic conductivity support from the Cu-doping.24,44
Also, fast Na+ intercalation and de-intercalation properties of
NVMCP/C/CC were also assessed at a high current rate (30C)
and the resultant long-term cycling stability curve is given in
Fig. 3e. The cathode exhibits extraordinary cycle stability and
stable reversible capacity of 68 mA h g1 even aer 3000
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Fig. 3 (a) Comparison of the charge/discharge profiles for NVMP/C/NPs, NVMP/C/CC, and NVMCP/C/CC cathodes; (b) rate performance
comparison for NVMP/C/NPs, NVMP/C/CC, and NVMCP/C/CC cathodes; (c) cyclability pattern for the NVMCP/C/CC cathode obtained at 1.5C
rate, (d) charge/discharge profiles for the NVMCP/C/CC cathode at 1.5C rate, (e) the cyclability curve for the NVMCP/C/CC cathode at 30C rate,
(f) Ragone-plot of the NVMCP/C/CC cathode along with that of different layered electrodes available in the literature.
repeated cycles. During cyclability analysis, the coulombic efficiency is nearly entirely preserved. On the whole, aer delivering a high initial discharge capacity (79 mA h g1), the
cathode showed a small drop in the capacity (70 mA h g1) up to
the 40th cycle. Aer that, the electrode achieved its stabilization
process and underwent a stable cycling passage. Thus, almost
constant cycling performance and delivery of steady discharge
capacity of 68 mA h g1 aer 3000 cycles were demonstrated,
realizing an 86% capacity retention. This indicates the fast
sodium ion reversibility of the present NVMCP/C/CC cathode.
The corresponding selected discharge patterns for the 100th,
1000th, 1500th, 2000th, and 3000th cycles at the applied current
rate are seen in Fig. S10a.† An analogous discharge shape was
realized throughout the long run of the cycle, indicating the
extremely stable framework of the NVMCP/C/CC cathode. Also,
the trigonal Na4VMn0.9Cu0.1(PO4)3/C structure was retained
even aer the deep cycling process (ex situ XRD in Fig. S10b†),
yet again indicating that the lattice structure is effortlessly
12062 | J. Mater. Chem. A, 2020, 8, 12055–12068
preserved. More importantly, we investigated the morphological changes in the NVMCP cathode aer 3000 cycles at 30C
using ex situ SEM analysis and the corresponding SEM image is
presented in Fig. S10c.† It is evident that the NVMCP/CC/C
cathode retained most of its cotton-candy architecture even
aer harsh cycling conditions. This clearly conrms that the
NVMCP/C/CC cathode with a unique cotton candy microstructure supports the overall electrochemical properties in addition
to the Cu doping into the structure. Thus, employing dual
strategies, i.e., engineering Cu-doped NVMP cathode with
a distinctive architecture is highly benecial for achieving
superior electrochemical properties, as revealed by the NVMCP/
C/CC composite cathode. Hence, the ex situ XRD and SEM
studies signify the synergetic importance of the 3D cotton
candy-like morphology for facile Na+ ion diffusion property and
the Cu dopant that assists the enriched electronic conductivity.
More remarkable features of our NVMCP/C/CC composite
cathode were appreciated by comparing the Ragone plot of our
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Journal of Materials Chemistry A
recommended cathode with several other recently recorded
polyanion-type cathodes (Fig. 3f). The specic energy and power
values were calculated based on the entire active mass of the
cathode attained from the charge/discharge plots at different
current surges starting from 0.25C to 40C. The plot indicates
that the NVMCP/C/CC cathode retains high a specic energy of
398 W h kg1 at a specic power of 90 W h kg1 and shows
a distinct position amongst polyanion-based cathodes, formerly
documented for SIBs. Thus, the NVMCP/C/CC composite
cathode outperforms other cathodes such as NMVP/C/rGO,17
NMVP/C/GA,22 NMVP/C/CNTs,19 Na3MnTi(PO4)3/C,46 Na2FeP2O7/C/rGO,47 NaVTi(PO4)3/C,48 Na3.32Fe2.34(P2O7)2/C,49 Na7V4(P2O7)4(PO4)/C/graphene,50 and NaVPO4/C51 documented so far. It
is notable to compare the electrochemical outputs of the
present NVMCP/C/CC cathode with available registered reports
on Na4MnV(PO4)3. A statistical correlation between different
Na4MnV(PO4)3 materials is provided in Table S4† to underline
the sodium storage properties of the present NVMCP/C/CC
cathode synthesized through the ultrafast pyro-synthetic technique. To the best of the author's knowledge, the documented
sodium-storage properties of the NVMCP/C/CC cathode realized
via the super-fast pyro-synthestic process is comparable to the
NVMP cathodes prepared by the time-consuming sol–gel and
spray-drying methods.
Cyclic voltammetry (CV) was performed at 0.1 mV s1 for the
NVMCP/C/CC cathode in the potential window from 3.9 to 2.4 V
vs. Na+/Na, as shown in Fig. 4a. The CV pattern comprises of two
pairs of predominate redox peaks at 3.48/3.28 and 3.66/3.45 V
resulting from the reversible transformation of V3+/V4+ and
Mn2+/Mn3+, respectively, during Na+ (de)insertion from/into the
NVMCP/C/CC framework. To determine the capacitive impact
effects in the NVMCP/C/CC cathode, CV analyses at varying scan
speeds (0.1–1 mV s1, Fig. 4b) were performed. At a denite
potential, the current was revealed to be comprised of two
inuences, surface (k1v) and diffusion (k2v) limited processes,
which can be noted by the following equation:52
i ¼ k1v + k2v1/2
where i and v are the peak current and scan rate of the CV
results, respectively, and k1v and k2v1/2 are the surface- and
diffusion-controlled redox reactions, respectively.45 From this
equation, the proportion of these two contributions at each
sweep rate can be determined. The diagnostic outcome at
0.2 mV s1 (Fig. 4c) discloses a 17.12% ratio of the surfaceoriented contribution. Furthermore, the histogram in Fig. 4d
illustrates the two different contributions at multiple scan rates.
The proportion of the surface-driven reaction is increased with
increasing scan rates. In particular, the contribution of the
surface-oriented reaction to the specic capacity increased from
17.12% (0.2 mV s1) to 72% (1 mV s1), thereby representing the
overriding role of the surface reaction at high sweep rates.
The reaction mechanism behind the NVMCP/C/CC cathode
material is analyzed using an in situ synchrotron XRD technique. Fig. 5 elucidates the in situ XRD patterns recorded under
different charge/discharge states in the rst cycle, in which the
constructed cell is initially charged to 3.8 V and then discharged
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to 2.4 V at a current density of 20 mA g1. The resultant charge/
discharge prole along with the selected in situ scan numbers
are provided in Fig. 5a and the selected XRD 2q regions are
presented in the remaining panels from b to d. Aer the initiation of Na+ extraction, a clear shi is perceived for the (211),
(116), (030), (223), (226), (140), and (146) reections. During the
charging process, all the planes exhibited changes in the peak
position towards a higher 2q angle (aer the 220th scan number)
and returned progressively to their original spot aer the
discharge process (scan no. 450, indicating the completion of
the cycle), thus lightening the lattice breathing in the course of
Na+ (de)insertion in the controlled electrochemical working
agenda. On the other hand, the (104) and (113) reections split
into two peaks from scan no. 120–220 (i.e., from 3.5 to 3.8 V)
during the charge response. This clearly suggests that the initial
Na+ extraction was a coupled V4+/V3+ redox induced singlephase reaction and the remaining Na+ extraction allied with
the bi-phasic reaction originated from the Mn3+/Mn2+ redox
couple.22 More importantly, aer the end of the discharge
process, two peaks were fused into a single peak, as perceived in
the open-circuit voltage (OCV), at scan no. 450 (end of
discharge, 2.4 V). This conrms that the original structure is
recovered at the end of the discharge process. The NVMCP/C/CC
cathode experienced sequential two-phase transitions during
the reversible Na+ (de)intercalation process that preserved the
complete framework,43 indicating the two well-established
charge/discharge voltage plateaus.
Besides, to completely understand the biphasic reaction
mechanism and the diffusion rate of Na+ ions in the Cu-doped
and un-doped NASICON framework, galvanostatic intermittent
titration technique (GITT) analysis was performed on the
NVMCP/C/CC and NVMP/C/CC cathodes at the rate of
20 mA g1 in the specied working voltage region (OCV – 3.8 V)
for the rst cycle (Fig. 6a and b), with an equal interval of pulses
(10 min); the rest time was xed as 1 h to reach a quasielectrochemical equilibrium voltage. This was preserved
throughout the operating potential space. The resultant single
titration output during the cycling process for the NVMCP/C/CC
and NVMP/C/CC cathodes are illustrated in (Fig. 6c and d),
respectively. Signicant variation in the equilibrium voltage
shape for both the cathodes (Fig. 6a and b) indicated the presence of bi-phasic response during the Na+ de-intercalation
between 3.5 and 3.8 V. The transient potential responses evidenced from the GITT results can be used to calculate the
sodium ion diffusion coefficient according to Fick's second
DNa ¼ 4/Ps(mBVM//MwA)2(DEs/DEs)2
where DNa (cm2 s1) indicates the diffusion coefficient, MB (g) is
the total mass loading of the active material, VM (cm3 mol1) is
the molar volume, MW (g mol1) is the molecular weight, A
(cm2) is the sum of the surface area of the cathode, and s (s) is
the current pulse time; DEs and DEs are the variations in the
steady-state voltage and total variation in the cell voltage taking
place during the constant pulse of a single-stage GITT analysis.54 The chemical diffusivity plots for the NVMCP/C/CC and
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Fig. 4 (a) CV profile for NVMCP/C/CC at 0.1 mV s1, (b) multi-scan CV profile at various scan rates, (c) the contribution ratio of the diffusionlimited capacities and capacitive capacities at 0.2 mV s1, (d) bar chart for capacity contribution ratios at different scan rates.
NVMP/C/CC cathodes for charge and discharge reactions are
provided in Fig. 6e, f and g, h, respectively. The GITT results
clearly demonstrate that the NVMCP/C/CC cathode exhibited
enhanced sodium diffusion attainment than the NVMCP/C/CC
cathode. For example, the calculated diffusion coefficient
values for the NVMCP/C/CC sample (2.46 108 to 8.6 109
cm2 s1) is higher than that of the NVMP/C/CC sample (6.83 109 to 9.14 1010 cm2 s1) for the Na+ de-insertion process,
respectively, which are comparable to those of NASICON based
cathodes materials,22,55,56 thereby depicting the superior
mobility of Na+ ions in the NVMCP/C/CC NASICON framework.
This clearly portrays that Cu doping signicantly encourages
the mobility of Na+ ions, which helps in the rapid sodium
storage properties of the NVMCP/C/CC cathodes. Thus, it is
clear that the Cu-doped sample exhibits exceptional rate
performances and remarkably fast current testing capabilities
when compared to the un-doped sample due to the superior Na+
diffusion properties.
First-principles calculations based on density functional
theory were also performed to gain more insight into the
evolution of the electronic structures of pure (NVMP/C/CC) and
Cu-doped (NVMCP/C/CC) samples. The total density of states
12064 | J. Mater. Chem. A, 2020, 8, 12055–12068
(DOS) of the NVMP and NVMCP samples are displayed in Fig. 7a
and b, respectively. It can be seen that the NVMP sample
exhibited a semiconductor feature with a bandgap of 0.83 eV.
Remarkably, aer the inclusion of Cu within the structure, the
impurity levels appeared within the Fermi energy, indicating an
increase in the conductivity of the NVMCP sample. In addition,
the partial DOS of the NVMP sample showed that the conduction and valence bands were dominated by transition metal-3d
electrons with some portions of P and O 2p electrons (Fig. 7c).
Aer the addition of Cu, a similar trend can still be observed,
however, with some impurity levels (Fig. 7d). Based on the DOS
studies, it is worth noting that the Cu-doped sample essentially
showed a metallic behavior, which indeed has a much higher
conductivity compared to the pure sample, thus facilitating fast
electron transport, which is also in agreement with the experimental results.
To evaluate the diverse application of the NVMCP/C/CC
cathode, the full cell is assembled with the MnS2 anode. Since
the MnS2 anode exhibits stable electrochemical properties in
the diglyme-based electrolyte (1 M NaPF6 in DGM) before the
full cell examination, both the anode and cathode were optimized in the sodium half-cell setup using the diglyme-based
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(a) Electrochemical charge/discharge profile of the constructed NVMCP/C/CC spectro-electrochemical cell tested within 3.8–2.4 V at
20 mA g1. The resultant profile kickoff from scan no. 1 (OCV) to scan no. 450 (2.4 V, end of first discharge). In situ XRD profiles within the selected
scanning angle (2) regions of (b) 18–26 , (c) 27–36 , and (d) 41–66 .
Fig. 5
electrolyte and the comprehensive electrochemical outputs are
illustrated in the ESI (Fig. S11a and d†). This gives an opportunity to test the endurance of the NVMCP/C/CC cathode in
highly viscous diglyme-based electrolytes as well and the stable
electrochemical properties validate the permanence of the
cathode even in a high-viscosity electrolyte. More importantly,
the mass ratio of 1 : 7 for MnS2 : NVMCP/C/CC was employed to
gain the desired yield. The corresponding galvanostatic charge/
Fig. 6 Voltage profile during GITT at 20 mA g1 current density for (a) the NVMCP/C/CC and (b) NVMP/C/CC cathodes. The expansion of single
titration curve during the charging process for (c) NVMCP/C/CC and (d) NVMP/C/CC cathodes. Sodium ion chemical diffusion coefficient
calculated from GITT curves from the charge/discharge process during the electrochemical reaction of (e and f) NVMCP/C/CC and (g and h)
NVMP/C/CC cathodes.
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Fig. 7 The total DOS of the (a) NVMP and (b) NVMCP samples and the projected DOS of the (c) NVMP and (d) NVMCP samples.
discharge prole along with the cyclability pattern of the
MnS2 : NVMCP/C/CC full-cell is presented in Fig. S11e and f†
and the cycling pattern clearly illustrates that the full-cell
exhibits a stable cycle life aer 50 cycles with 73% capacity
retention of its initial value. It is worth noting that the high
cathode mass ratio and slow-diffusion kinetics of Na+ ion in
highly viscous diglyme electrolyte denitely affected the nal
productivities of the full cell. However, the reasonably good
performance of the NVMCP/C/CC cathode along with the MnS2
anode in full-cell formations even in the high-viscosity electrolyte demonstrates their high suitability for real-world applications. It is to be noted that the explored research outputs for the
NVMP cathode mainly uses hard carbon anode for full-cell
construction. However, the present study demonstrates the
versatility of the NVCMP/C/CC cathode in the sulde-based
The present NVMCP/C/CC cathode demonstrated superior
sodium storage ability because of the following features. It is
rational to propose that the high electrochemical performance
can be attributed to the distinctive cotton candy-like
morphology with the thin uniform carbon coating, which
improves the interfacial redox reactions due to the decreased
diffusion pathways for Na+ ions originating from the electrochemically active spots owing to the 3D cotton candy-like
morphology and good conductive assistance owing to the
uniform carbon coating. Furthermore, for the present
composite cathode, Cu doping played an important role in
providing conductive channels that enable electronic conductivity. Thus, we have demonstrated the successful synthesis of
Cu-doped carbon-coated NASICON-type NVMCP/C cathode with
12066 | J. Mater. Chem. A, 2020, 8, 12055–12068
a unique cotton candy-like morphology in a short time by an
open-air ultrafast modied pyro-synthetic method for highperformance SIBs.
4. Conclusions
In this study, a Cu-doped NVMCP cotton candy-like cathode
with uniform carbon coating was produced by a facile and
ultrafast modied pyro-synthetic method. The distinctive
structural assemblies, such as stable and open NASICON
framework, nanoake-constructed cotton candy-like architecture, optimized Cu-doping, and uniform carbon coating provide
the NVMCP/C/CC cathode with a superior rate capability
(68 mA h g1 at 40C) and outstanding cycling stability both at
low (79 mA h g1 aer 450 cycles at 1.5C rate with 90% capacity
retention) and high current rates (68 mA h g1 aer 3000 cycles
at 30C rate with 86% capacity retention). More importantly,
a two-phase reversible electrochemical reaction mechanism in
the NVMCP/C/CC cathode was revealed using real-time in situ
XRD and GITT studies. First-principles calculations based on
the DOS studies authenticated the enhanced conductivity
originating from the Cu-doping compared to the pure sample
that encourages fast electron transport, leading to enhanced
rate capability. More importantly, the achieved cathode was
subjected to full-cell fabrication using an MnS2 anode and
demonstrated the possibilities of using this cathode diversely.
Thus, we believe that this cotton candy-like architecture model
and the modied pyro-synthetic protocol can be expanded to
develop high-performance cathodes with unique nanoarchitecture and enhanced electrochemical consistency and can
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accelerate the development of stable electrochemical energy
storage devices.
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Author contribution
Vaiyapuri Soundharrajan Conceptualization, Investigation,
Methodology, Formal analysis, Writing-Original Dra,
Muhammad H. Alfaruqi performed Rietveld renement and
DFT computations. Seulgi Lee performed in situ XRD analysis.
Balaji Sambandam and Sungjin Kim performed GITT and
reviewed the manuscript; Seokhun Kim and Vinod Mathew
Writing, Reviewing, and Editing. Duong Tung Pham directed
the full-cell fabrication. Jang-Yeon Hwang, Yang-Kook Sun, and
Jaekook Kim Conceptualization, Supervision, and Funding
Conflicts of interest
There are no conicts to declare.
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP)
(2017R1A2A1A17069397). This work was also supported by the
National Research Foundation of Korea (NRF) grant funded by
the Korea government (MSIT) (2018R1A5A 1025224).
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