Towards excellent electrical conductivity and high-rate capability: A degenerate superlattice Ni3(S)1.1(S2)0.9 micropyramids electrode

Article abstract of DOI:10.1016/j.jallcom.2020.155590

Degenerate semiconductor is very highly desired in energy conversion and storage technologies due to its metal-like conduction behaviors. This is the first time the doping S₂ in Ni₃S₂ lattice into chemically homogeneous Ni₃(S)_1.1(S₂)_0.9 superlattice structure is proposed to induce a degenerate characteristic towards excellent electrical conductivity and high-rate capability. In this study, a series of the chemically homogeneous S₂-doped Ni₃(S)_1.8(S₂)_0.2, Ni₃(S)_1.6(S₂)_0.4, Ni₃(S)_1.3(S₂)_0.7, and Ni₃(S)_1.1(S₂)_0.9 micropyramid arrays on Ni foam were synthesized by reacting the Ni foam and alkaline sulfur aqueous solution in different S₂^2- concentrations. The perfect Ni₃(S)_1.1(S₂)_0.9 superlattice structure corresponds to the periodic S–Ni–S₂ atom arrangements in whole crystal lattice, which endows a degenerate characteristic of metal-like electrical conductivity to significantly improve the electrochemical performance. The bulk series resistance (R_s) value is only 0.62 Ω, while the charge-transfer resistance (R_ct) is nearly 0 Ω in the superlattice Ni₃(S)_1.1(S₂)_0.9 electrode. As a cathode material for application in lithium ion batteries (LIBs), a very high specific capacity of 874 mAh g^?1 is achieved at current density of 200 mA g^?1. Remarkably, it still holds a high capacity of 565 mAh g^?1 at current density of 500 mA g^?1, indicating its superior high-rate capability. This study reveals that the periodic S–Ni–S₂ atom arrangements in crystal lattice is a key factor in determining the superlattice structure, high specific capacity, and the dynamic behaviors of electron/ion transport.

Full text of DOI:10.1016/j.jallcom.2020.155590

Journal Pre-proof
Towards excellent electrical conductivity and high-rate capability: A degenerate
superlattice Ni (S) (S )
1.1 2 0.9
micropyramids electrode
3
Yuanhao Gao, Wensong Zhou, Wei Li, Tuo Cai, Yan Lei, Helin Niu, Pinjiang Li,
Hongwei Yue, Zhongjun Li
PII:
S0925-8388(20)31954-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.155590
JALCOM 155590
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 22 February 2020
Revised Date: 1 May 2020
Accepted Date: 9 May 2020
Please cite this article as: Y. Gao, W. Zhou, W. Li, T. Cai, Y. Lei, H. Niu, P. Li, H. Yue, Z. Li, Towards
excellent electrical conductivity and high-rate capability: A degenerate superlattice Ni (S) (S )
3
micropyramids electrode, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/
j.jallcom.2020.155590.
1.1 2 0.9
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Table of Contents graphic
Degenerate doping S2 in Ni3S2 lattice into chemically homogeneous superlattice Ni3(S)1.1(S2)0.9 multi-storey micropyramids
array on Ni foam towards excellent electrical conductivity and high-rate capability.
1

Towards excellent electrical conductivity and high-rate capability:
A degenerate superlattice Ni3(S)1.1(S2)0.9 micropyramids electrode
Yuanhao Gao*a, Wensong Zhoua, Wei Lia, Tuo Caia,Yan Leia, Helin Niub, Pinjiang Lia,
Hongwei Yue,*a Zhongjun Lic
a Henan Province Key Laboratory of Micro–Nano Materials for Energy Storage and
Conversion, College of Advanced Materials and Energy, Xuchang University, Xuchang
461000, P. R. China.
b Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybird
Functionalized Materials, College of Chemistry and Chemical Engineering, Anhui
University, Hefei 230039, P. R. China.
c College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou
450000, P. R. China.
* Corresponding author.
E-mail addresses: gyh–2007@sohu.com (Y. Gao), yuehw207@qq.com (H. Yue).
Abstract: Degenerate semiconductor is very highly desired in energy conversion and
storage technologies due to its metal-like conduction behaviors. This is the first time the
doping S2 in Ni3S2 lattice into chemically homogeneous Ni3(S)1.1(S2)0.9 superlattice
structure is proposed to induce a degenerate characteristic towards excellent electrical
conductivity and high-rate capability. In this study, a series of the chemically
homogeneous S2-doped Ni3(S)1.8(S2)0.2, Ni3(S)1.6(S2)0.4, Ni3(S)1.3(S2)0.7, and
Ni3(S)1.1(S2)0.9 micropyramid arrays on Ni foam were synthesized by reacting the Ni
foam and alkaline sulfur aqueous solution in different S22- concentrations. The perfect
1

Ni3(S)1.1(S2)0.9 superlattice structure corresponds to the periodic S–Ni–S2 atom
arrangements in whole crystal lattice, which endows a degenerate characteristic of
metal-like
electrical conductivity to significantly improve the electrochemical
performance. The bulk series resistance (Rs) value is only 0.62 Ω, while the charge-
transfer resistance (Rct) is nearly 0 Ω in the superlattice Ni3(S)1.1(S2)0.9 electrode. As a
cathode material for application in lithium ion batteries (LIBs), a very high specific
capacity of 874 mAh g-1 is achieved at current density of 200 mA g-1. Remarkably, it
still holds a high capacity of 565 mAh g-1 at current density of 500 mA g-1, indicating its
superior high-rate capability. This study reveals that the periodic S–Ni–S2 atom
arrangements in crystal lattice is a key factor in determining the superlattice structure,
high specific capacity, and the dynamic behaviors of electron/ion transport.
Keywords: degenerate semiconductor; electrical conductivity; S2-doping; superlattice;
lithium ion batteries.
2

1. Introduction
The increasing market demands of advanced energy storage devices have
promoted the development of advanced electrode materials.[1-5] In the past years,
carbon materials have been widely used in the lithium ion batteries (LIBs) and
supercapacitors because of
their special advantages in high electrical
conductivity, reusability, and environment benignity. However, the low specific
capacity of carbon materials is a serious defect, and hinders their large-scaled
application. The more effort is currently focused on various advanced electrode
materials, including carbon–based hybrid materials and inorganic nanostructured
materials for supercapacitors, LIBs and next-generation batteries, such as Li–S
batteries, Li–air batteries and aqueous batteries.[6-10] Among them, Ni3S2 has been
extensively explored as the most promising electrode materials for application in
supercapacitors, LIBs and next-generation batteries due to their good
electroactivity and low cost. In particular, the theoretical specific capacity of
Ni3S2 for LIBs is 472 mAh g-1, which is obvious higher than that of the mostly
used carbon materials. Up to now, various Ni3S2 nanostructures (e.g. nanosheets,
nanowires and nanotubes)[11-14] and their heterostructure nanomaterials (e.g.
Ni3S2@carbon, Ni3S2@MoS2, V2O5@Ni3S2, Ni3S2@NiMoO4)[15-18], have been
extensively studied. However, compared to the carbon materials, their electrical
conductivity are much lower, resulting in their damaged structure and poor rate-
capability, thus device performance is still unsatisfactory.
Towards excellent electrical conductivity and high-rate capability, the
degenerate semiconductor materials are very highly desired due to their metal-
like conduction behaviours. It is well known that, when a n-type semiconductor is
3

heavily doped with donors, it could be converted into degenerate semiconductor,
its Fermi level lies within its conduction band, and a metal-like conduction
occurs, namely doping-induced degenerate state.[19, 20] Similarly, with heavy
doping by acceptor in a p-type semiconductor, when Fermi level crosses its
valence band, the doping-induced degenerate state also arises.[19, 20] To date, some
efforts on the doping nanostructures have achieved many outstanding results,
such as V,[21] Zn,[22] Mo,[23] Sn,[24]or Fe[25] doped Ni3S2. In this case, the doping
metal atoms can form local impurity energy level within forbidden band to induce
the staggered alignment of band edges, and increase charge carrier concentration
correspondingly. [26, 27] However, it is difficult to obtain a periodic arrangement of
the doping atoms in whole crystal, thus the mobility of charge carrier is not high
enough and still hinder its electrical conductivity. Therefore, it is very important
to develop a new strategy toward the degenerately doping nanostructures with the
desired metal-like electrical conductivity.
In this work, we address this issue by doping S2 in Ni3S2 lattice to fabricate
the chemically homogeneous S2-doped Ni3Sx crystals on nickel foam. By reacting
nickel foam with appropriate amounts of sulfur powders in a NaOH aqueous
solution, a series of S2-doped Ni3Sx crystals were obtained. The key of this
reaction design is the use of the alkaline sulfur aqueous solution of NaOH. It is
well known that NaOH is very efficient to activate sulfur powders into high
2-
active polysulfide ions (Sn , n > 8), and finally generate S2- and additional
2-
2-
2-
species, such as S2 and S3
. Using homologous S2 species in appropriate
amounts to displace the bridging sulfur (S2-) in Ni3S2 units allows us to fabricate
the S2-doped Ni3Sx crystals, such as Ni3(S)1.8(S2)0.2, Ni3(S)1.6(S2)0.4,
4

2-
Ni3(S)1.3(S2)0.7, and Ni3(S)1.1(S2)0.9. It was found that the doping S2 species in
Ni3S2 lattice are favorable for significantly increasing electrical conductivity.
This is mainly due to the following natural characteristics:
2-
(i) The doping S2 species can enhance donor energy level and narrow's band
gap, and correspondingly increase charge carrier concentration, because the
energy level of the highest occupied molecular orbital (HOMO) of S2 is higher
than that of S due to orbital hybridization and electron delocalization in the sulfur
2-
dimer (S2 ).
(ii) The heterocontacts in the S2-doped Ni3S2 crystals can resist the charges
recombination, which hinder the neutralization of ions, resulting in high
electron mobility ratio.
Importantly, by tuning the S2/S atomic ratios to ~1:1, the S2-doped Ni3Sx
crystal structure could be converted to the chemically homogeneous
Ni3(S)1.1(S2)0.9 superlattice structure, consisting of the periodic alternate of Ni–S
and Ni–S2 atom layers. We found that the Ni3(S)1.1(S2)0.9 superlattice structure
with the periodic S–Ni–S2 atom arrangements has the degenerate doping induced
metal-like behaviors for excellent electrical conductivity and high-rate capability,
which were systematically investigated by electrochemical measurements and
electrochemical impedance spectroscopy (EIS) analyses. The bulk series
resistance (Rs) value is only 0.63 Ω, while the charge-transfer resistance (Rct) is
nearly 0 Ω in the superlattice Ni3(S)1.1(S2)0.9 electrode. To exploit its potential
application, LIBs were assembled with the superlattice Ni3(S)1.1(S2)0.9 electrode
as a working electrode. A very high specific capacity of 874 mAh g-1 has been
achieved at current density of 200 mA g-1. Remarkably, it still holds a capacity as
5

high as 565 mAh g-1 at current density up to 500 mA g-1, indicating its superior
high-rate capability.
The article describes the formation, morphology, microstructure and
electrochemical properties of the degenerate Ni3(S)1.1(S2)0.9 superlattice electrode
material, which is illustrated in Scheme 1. This work will encourage more study
on the degenerately doped nanostructures for potential electrochemical,
photoelectric and photocatalytic applications.
2. Experimental
2.1 Materials and methods.
All reagents were of analytical grade and used without further purification. To
start the synthesis, 0.1 mol NaOH and appropriate amounts of S power (2.0, 3.0,
5.0, 7.0 mmol) were dissolved in 30 mL deionized water by magnetic stirring at
90°C for 3 h to make an alkaline sulfur aqueous solution. Different amounts of S
power were used to control the composition of the resulting alkaline sulfur
aqueous solution. Then, the 2mL of resulting alkaline sulfur aqueous solution
was transferred into a 15 mL Teflon-lined stainless steel autoclave. After that, the
1×2 cm2 piece of Ni foam with a thickness of 0.2 mm was enclosed in the
alkaline sulfur reaction solution and heated at 180°C for 12 h. After cooling to the
room temperature, the Ni foam was taken out and washed with deionized water
and then dried in a vacuum oven at 60°C for 6h. For comparison, the undoped
Ni3S2 nanowires array grown on Ni foam was also prepared by reacting Ni foam
with thioacetamide (TAA) in the aqueous solution of NaOH under the same
conditions (See the Supporting Information). The resulting Ni foam was weighed,
and the mass loading of Ni3Sx on Ni foam was calculated by the equation follow.
6

m
(Ni3Sx) = Δ
m × M (Ni3Sx) /x ÷ M (S)
Where Δ
m is the weight difference of the Ni foam before and after the
hydrothermal sulfidation,
respectively.
M (Ni3Sx) and M (S) is the molar mass of Ni3Sx and S,
2.2 Physical Characterizations.
X-ray diffraction (XRD) patterns were collected on a Philips MPD 18801
diffractometer with Cu- α radiation ( = 1.54178 Å). Scanning electron
K
λ
microscopy (SEM) micrographs were observed using field emission scanning
electron microscopy (FE-SEM, Nova NanoSEM 450) equipped with energy-
dispersive X-ray spectroscopy (EDS, OXFORD X-Max 50). The high resolution
TEM (HRTEM) images and the select-area electron diffraction (SAED) patterns
were obtained by a JEOL JEM 2100 high-resolution transmission electron
microscope operated at 200 kV. The Raman spectrum was recorded by a
LABRAM-1B confocal laser micro-Raman spectrometer with an excitation
wavelength of 532 nm. Ultraviolet visible (UV-vis) absorption spectrum was
obtained using a Shimadzu UV-3600 UV-vis-NIR spectrophotometer.
2.3 Electrochemical measurements.
The electrochemical measurements were conducted in 2M KOH electrolyte
on CHI760E electrochemical workstation with a three-electrode system, where
the Ni3Sx/Ni foam electrode was used as the working electrode, Pt foil and
standard Hg/HgO electrode as the counter electrode and reference electrode,
respectively. The electrochemical impedance spectroscopy (EIS) analyses were
performed in the frequency range of 100 kHz to 1 Hz with an AC oscillation of 5
mV.
7

The CR2025-type coin cells were assembled in a high-purity argon filled
glove box (H2O < 0.5 ppm, O2 < 0.5 ppm, MBraun, Unilab). The as-prepared
Ni3Sx on Ni foam were used as the working electrode, lithium metal foil as the
counter and reference electrode, and Celgard 2320 as the separator membrane.
The electrolyte was 1 M LiPF6 dissolved in EC:EMC:DMC = 1:1:1 in volume
ratio. Cyclic voltammogram (CV) tests were performed on a electrochemical
workstation (Autolab PGSTAT 302N) between 0.02 V and 3.00 V at a scanning
rate of 0.1 mV s-1. Charge–discharge measurements were performed with a
multichannel battery tester (Neware BTS-610) between 0.02 V and 3.00 V at
room temperature.
3. Results and discussion
3.1 Preparation and structure characterizations.
In this reaction design, sulfur powers were first activated in the aqueous
solution of NaOH at 90°C for 3 h to make an alkaline sulfur aqueous solution.
We observed that the solution color subsequently turns from yellow to orange and
then red within 1 h, and finally returns to pale yellow within 2 h. The color
2-
change originates from the formation of polysulfide ions (Sn ,
n > 8) by the
following well-known reaction (eqn (1)).
2-
2-
(2
n
+ 1)S + 6OH- → 2Sn + SO3 + 3H2O (1)
2-
With
n
value increasing, the polysulfide ions (Sn ) solution color changes from
2-
yellow to orange and then to red. Obviously, the polysulfide ions (Sn ) with long
S chains are more reactive than their yellow and orange counterparts, because the
S–S bond in long sulfur chains is much weaker due to electron delocalization
2-
2-
along the whole Sn chain. Thus, the red polysulfide (Sn ) precursors are highly
8

2-
2-
active to generate S2- and additional species, such as S2 and S3 (eqn (2)),
resulting in the final pale yellow solution.
2-
2-
2-
Sn → S2- + S2 + S3 + ... (2)
The final pale yellow sulfur solution was tested by a UV-vis
spectrophotometer. As shown in Fig. 1, the high absorbance in the 260–280 nm
regions is mainly derived from S2- species. The absorption band at 297 nm
2-
2-
stem from S2 , while the absorption about 370 nm is originated from S3 . This
assignment of spectral bands is consistent with previous reports.[28-30] From this
UV-vis absorption experiment, we observed that the increasing sulfur reactant
content leads to increased absorbance at all three wavelengths, and generates
considerable amounts of S2- and more S2 , and a few S3 species in the alkaline
2-
2-
2-
aqueous solution. No other Sn species are produced in the final pale yellow
2-
2-
sulfur solution, such as S4 (absorption of 420 nm), S5 (absorption of 438 nm),
and S62- (absorption of 450 nm).
The final pale yellow sulfur solution was used to react with Ni foam through
a hydrothermal process at 180°C (eqn (3)).
Ni + S2- + S22- → Ni3S2-y(S2)y
This simple method leads to the self-sulfidation of nickel foam, thus the S2
(3)
2-
species were combined into Ni3S2 lattice to produce the S2 -doped Ni3S2-y(S2)y
samples on the Ni foam. The S32- species cannot be combined into Ni3S2 lattice at
2-
180°C due to its steric bulkiness or decomposition to S2- and S2 . The
composition of the resulting S2 -doped Ni3S2-y(S2)y samples was controlled by
using the different amounts of S power reactants in the alkaline aqueous solution.
The Ni /S atomic ratio in the Ni3S2-y(S2)y series were validated by energy-
9

dispersive X-ray spectroscopy (EDS), and the obtained samples were marked as
Ni3S2.2, Ni3S2.4, Ni3S2.7, and Ni3S2.9, respectively (Figure S3 in the Supporting
Information). Just taking the obtained Ni3S2.9 sample as an example (Figure S3e),
the EDS data validate that the atomic ratio (at. %) of Ni and S in the Ni3S2.9
crystals is 50.81:49.19, which is very close to the marked 3.0:2.9 ratio. Thus,
according to the Ni3S2-y(S2)y form, the composition of Ni3S2.9 can be calculated
(by 2-y+2y=2.9, namely, y=0.9) and formally represented as Ni3(S)1.1(S2)0.9.
Similarly, the other samples can be formally represented as Ni3(S)1.8(S2)0.2,
Ni3(S)1.6(S2)0.4 and Ni3(S)1.3(S2)0.7. The increasing S2 content is due to a
2-
considerable number of S2 species substituting the S2- in the Ni3S2 units, which
was further validated by the below Raman investigation.
Fig. 2a shows the Raman spectra of all the obtained samples, for comparison.
For the undoped Ni3S2 sample (Pattern a in Fig. 2a), the observed Ni−S
vibration modes show six strong signal at 188, 202, 224, 304, 325 and 350 cm-1,
respectively, which can be readily indexed to six Raman-active modes (2A1 +
4E) of the heazlewoodite-phase Ni3S2, consistent with those reported earlier.[31,32]
2-
For all the S2 -doped samples, the Raman spectra exhibit distinct vibration
signals in two groups: S–S vibration mode at 478 cm-1,[33] and the six Ni–S
vibration modes in the broad range from 350 cm–1 to 188 cm–1. For the
Ni3(S)1.8(S2)0.2 sample (Pattern b in Fig. 2a), the weaker S–S stretching signal at
2-
478 cm–1 indicates only a few sulfur dimers (S2 ) in the crystal lattice, because
the S–S vibration mode typically produces a strong Raman signal.[33] With the
2-
increasing S2 doping ratio, for all the S2 -doped samples, the S–S stretching
signal at 478 cm-1 displays a systematic intensity increase, implying the
10

2-
2-
introduction of more sulfur dimers (S2 ) in the lattice. Moreover, for all the S2 -
doped samples, the Raman peaks from the Ni–S vibration modes obviously shift
downward in Raman frequency with the increasing S2 doping ratio (Fig. 2b). The
frequency shift is closely related to the induced strain/stress in crystal
structure.[34,35] If the Raman frequency is shifted downward, the induced-strain is
present in the crystal, the converse for induced-stress.[34,35] In our cases, the
Ni3(S)1.1(S2)0.9 sample shows the maximum downward shift in Raman frequency
(
Pattern e in Fig. 2b), indicating the greatest induced-strain in the Ni3(S)1.1(S2)0.9
crystals. We propose that the frequency shift is a result of the anion exchange
between S2- and S22- in the Ni3S2 units. Doping S22- in the Ni3S2 host lattice could
undoubtedly vary the force constants, because of the bond strength of S−Ni−S2
being different from the pure S−Ni−S bond. Thus, the varying bond strengths of
S−Ni−S2, depending on the S2 ratios in the lattice, induce the strain in the entire
2-
crystals. It is worth noting that, for all the S2 -doped samples, the Raman
intensity of the six Ni−S vibration modes is reduced, while the full width at half
maximum (fwhm) is increased (Fig. 2b), indicating the smaller polarizability of
2-
sulfur atoms in the these S2 -doped Ni3S2 samples. This result is originated from
the strong covalent nature of S-atom π-donation to the Ni metal center. In the
S−Ni−S2 case, the orbital hybridization and electron delocalization of sulfur
2-
dimer (S2 ) with its π-donation to neighboring Ni atoms could result in some
degree of electron delocalization over the S–Ni–S2 atom layer, increasing the
bond strength and force constant in the whole lattice, and thus decreasing the
polarizability of S atoms.
11

2-
The crystallographic structures of the S2 -doped Ni3Sx series are further
investigated by XRD analysis. Fig. 3 shows a representative XRD pattern of the
obtained Ni3(S)1.1(S2)0.9 sample scratched from Ni foam for eliminating the
backdrop of Ni foam. The diffraction pattern is in agreement with the feature of
the heazlewoodite Ni3S2 in hexagonal phase (JCPDS No. 44-1418). All of the
diffraction peaks can be readily indexed to the (101), (110), (003), (202), (113),
(211), (104), (303) and (214) planes of heazlewoodite hexagonal Ni3S2. No other
diffraction peaks were observed in the XRD pattern, which indicates that the
obtained Ni3(S)1.1(S2)0.9 crystal is chemically homogeneous solid solution.
Importantly, the diffraction angles of the all diffraction peaks are bigger than
those of normal Ni3S2 (JCPDS No. 44-1418), indicating a lattice contraction in
2-
the Ni3(S)1.1(S2)0.9 crystal. Considering the stronger electronic effects of S2 with
its π-donation to neighboring Ni atoms, we believe that the S2 species have
entered the Ni3S2 host lattice and further tuned the electronic structure of
chemical bonds, increasing bond strength and force constant, and thus resulting in
the lattice contraction. This XRD result is well consistent with the Raman results
2-
explained above. For all of the S2 -doped Ni3Sx series, there are just negligible
differences in their crystal structure of heazlewoodite hexagonal phase (FigureS4
in the Supporting Information).
Fig. 4 shows a representative SEM image of the obtained Ni3(S)1.1(S2)0.9
samples grown on Ni foam. As seen in Fig. 4a from a distant view, the SEM
image shows a net-like open 3D porous structure, namely, the skeleton structure
of Ni foam is completely retained after the hydrothermal process. From the close
views (Fig. 4b and c), it is clear that the Ni foam surface is uniformly covered
12

with the pyramid-like Ni3(S)1.1(S2)0.9 arrays. It can be seen that the
micropyramids are well-distributed aslant or perpendicular to the Ni foam support
with an average height of about 1.2 µm. In the high magnification SEM image
(
Fig. 4d), it appears that each of the micropyramids is a multi-storey schistous
structure along its entire height, consisting of many triangle nanosheets of about
30 nm in thickness.
To obtain more detailed structural information of the Ni3(S)1.1(S2)0.9
micropyramids, high-resolution TEM (HRTEM) and select area electron
diffraction (SAED) were measured. The SAED pattern in inset of Fig. 5a shows a
set of diffraction spots corresponding to the hexagonal Ni3S2 structure. The spots
indexed as spot (110) and spot (021) correspond to a plane distance of 2.81 Å and
2.29 Å, respectively, which is a slight deviation from the normal value (2.8730 Å
and 2.3498 Å, respectively) of hexagonal Ni3S2 structure (JCPDS No. 44-1418).
Interestingly, these diffraction spots are comprised of primary diffraction spots
with some subcell spots toward an almost continuous line, which is indeed a
characteristic of superlattice structures.[36-38] As shown in Fig. 5b, the HRTEM
image clearly reveals a novel superlattice structure. The superlattice structure has
a short alternate period of two atom layers interleaved by the alternate bright and
dark lattice fringes in the same spacing of 3.98 Å, which are highlighted by the
white lines and yellow dots in the HRTEM image. In addition, the distance for
two adjacent lattice fringes is about 1.99 Å, corresponding to the (202) plane,
smaller than that (2.04 Å) in the normal Ni3S2 structure (JCPDS No. 44-1418).
Typically, this is a chemically homogeneous superlattice structure, in which the
13

S2/S atomic ratio is near to 1:1 in lattice. So the bright and dark contrast fringes
should correspond to the Ni–S and Ni–S2 atom layers.
In our experiments, only 2 mL of the alkaline sulfur aqueous solution
2-
containing the S2- and S2 in different concentration were used to feed the
2-
nucleation and growth of S2 -doped Ni3Sx crystals on Ni foam at180°C with
enough time intervals of 12 h. In this case, the final structure strongly depends on
2-
the feeding amounts of S2- and S2 . Typically, the well-defined Ni3(S)1.1(S2)0.9
2-
superlattice structure were prepared in the high concentration of the S2- and S2
2-
resources. With the decreasing amounts of the S2- and S2 in the alkaline sulfur
aqueous solution, those comparative Ni3(S)1.8(S2)0.2, Ni3(S)1.6(S2)0.4 and
Ni3(S)1.3(S2)0.7 samples were produced, and the obtained SEM images (Figure S5)
and SAED patterns (Fig. 6) reveal the detailed evolution process of the
superlattice Ni3(S)1.1(S2)0.9 micropyramids. As seen in Figure S5, the different
SEM images shows the morphology evolution from the Ni3(S)1.8(S2)0.2 pyramid-
like bases to the final Ni3(S)1.1(S2)0.9 schistous multi-storey micropyramids,
implying an Ostwald ripening growth process. As to the multi-storey schistous
structure of the superlattice Ni3(S)1.1(S2)0.9 micropyramids, many forces could
cause its overgrowth, which follow Ostwald ripening kinetics, such as dipolar and
electrostatic fields associated with the crystal-face attraction, hydrophobic
interactions, aggregate, hydrogen bonds, and vander Waals forces.[39,40]
Significantly, the relevant SAED patterns explore the detailed structural
evolution of the Ni3(S)1.1(S2)0.9 superlattice. As shown in Fig. 6a-c, for all the
comparative samples, the corresponding SAED pattern is comprised of primary
diffraction spots with some subcell spots adjacent to primary spots, and the trend
14

is from separate spots toward a continuous line with the increasing S2 ratios. This
is a natural characteristic during the formation of a superlattice structure, tuned
by adjusting the atom ratio of S2 to S in lattice configuration. Hence, the
structural evolution of Ni3(S)1.1(S2)0.9 superlattice could be a result of the anion
2-
2-
exchange between S2- and S2 . Kinetically, the anion exchange between the S2
and S2- can easily occur because of the higher electronegativity of S2 (3s23p5)
than that of S atom (3s23p4) due to orbital hybridization and electron
delocalization in the S2 dimer. Crystallographically, the crystal phase systems of
the undoped Ni3S2 and S2-doped Ni3(S)1.8(S2)0.2, Ni3(S)1.6(S2)0.4 and
Ni3(S)1.3(S2)0.7 crystals all are the same heazlewoodite hexagonal phase, which
facilitate the bulky S2 to enter the host lattice at the hydrothermal condition. Also,
the Raman results above provide some support for the anion exchange
mechanism. For the well-defined superlattice Ni3(S)1.1(S2)0.9 structure, about 50%
S2 ratios in lattice, the Raman results demonstrate the greatest induced-strain in
its crystal structure. This means that the greatest induced-strain best cater to the
needs of the periodic S–Ni–S2 even arrangements in whole crystal lattice.
3.2 Electrochemical performance.
To investigate the electrochemical performance of the superlattice Ni3S2.9
micropyramids array grown on Ni foam, a series of electrochemical
measurements were carried out by directly using the architecture as the working
electrode in a three-electrode system with 2 M KOH as electrolyte.
Fig. 7a shows the cyclic voltammetry (CV) curves within a potential window
of 0−0.6 V (versus SCE) at scan rates from 5 to 20 mV s-1. A pair of battery-like
redox peaks is found in each curve with the cathodic peak around 0.25 V and the
15

anodic peak around 0.50 V, which can be attributed to the reversible
electrochemical reaction of Ni3S2.9/Ni3S2.9(OH)3 based on a battery-like diffusion-
controlled process.[41] Significantly, with the increasing scan rate, the peak
currents in both the cathode and anode are linearly increased with the square root
of the scan rate (Fig. 7b), indicating that the redox reactions of the superlattice
Ni3S2.9 electrode are perfectly reversible and ultra-fast. This means an excellent
electron/ion transport behavior in the superlattice Ni3S2.9 electrode. Fig. 7c
displays its galvanostatic charge-discharge (GCD) curves at various current
densities. The observed nonlinear curves confirm its typical battery-like behavior,
while the stable charge/discharge platform match well with the potential positions
of redox peaks in the CV curves. Based on the GCD curves, the rate
performances from 5 mA cm-2 to 100 mA cm-2 were illustrated in FigureS6. As
shown in FigureS6, the specific capacity is up to 1.02C cm-2 at current density of
5 mA cm-2. Increasing the current density to 100 mA cm-2, a high capacity of 0.63
C cm-2 (~ 61.8 % of the maximum value) still retains, demonstrating its superior
high-rate capability.
Such superior electrochemical performance of the superlattice Ni3S2.9
electrode is expected to be a result of the degenerate doping S2 in Ni3S2 lattice. In
order to explore the influence, the electrochemical impedance spectroscopy (EIS)
measurements were carried out, and several control experiments reveal some
interesting impedance changes. Fig. 8 shows the Nyquist plot of all the obtained
Ni3Sx series electrode after 10th cycle in the frequency range from 100 kHz to 1
Hz, where Z' and Z" are the real and imaginary parts of the impedance,
respectively. Usually, the Nyquist impedance plot display a small semicircle in
16

the high-frequency region and a straight line in the low-frequency region. In the
high frequency region, the first intercept at the real axis (Z’) represents a bulk
series resistance (Rs), including the inherent resistance of electroactive material,
the bulk resistance of electrolyte, and the contact resistance between the active
material and the current collector.[42, 43] The small semicircle corresponds to the
charge-transfer resistance (Rct), which is associated with the electron/ion transport
behaviors in the electrochemical redox process of the electroactive materials. For
the undoped Ni3S2 nanowires (Fig. 8a), the Rs value is 0.87Ω, while the Rct value
is 2.12 Ω, which is consistent with the result of early reported 3D Ni3S2
nanosheet arrays on Ni foam.[13] Comparatively, the measured Rs value for the
2-
four S2 -doped Ni3S2.2, Ni3S2.4, Ni3S2.7 and Ni3S2.9 samples was 0.70, 0.55, 0.64
and 0.62 Ω, respectively (Fig. 8b-e). The general trend is that the Rs value is
decreased with the increasing S2 ratios. Obviously, this is a typical doping-
induced degenerate process, and the Ni3S2.9 superlattice structure has the
degenerate-induced metal-like characteristic for excellent electrical conductivity
at Rs = 0.62 Ω. While the radius of the high frequency semicircles is decreased
systematically with the increasing S2 ratios, until disappeared in the superlattice
Ni3S2.9 electrode (inset in Fig. 8), which demonstrate the improving charge-
transfer kinetics with the increasing S2 ratios. For the superlattice Ni3S2.9
electrode, the charge-transfer resistance (Rct) is nearly 0 Ω (Fig. 8e). Such Rs and
Rct value in the superlattice Ni3S2.9 electrode is much smaller than most of
recently reported values, such as hierarchical V2O5@Ni3S2 hybrid arrays (Rs
=1.25Ω, Rct =0.92 Ω),[17] Ni3S2 wrapped by rGO on carbon cloth (Rs =1.1Ω),[44]
hierarchical urchin-like Ni3S2 (Rs =1.15Ω, Rct =1.02Ω),[45] honeycomb-like Ni3S2
17

nanosheet arrays
(
Rs =0.92Ω, Rct =4.5 Ω),[46] and Hierarchical
carbon@Ni3S2@MoS2 double core–shell nanorods (Rs=0.68Ω, Rct = 4.8Ω).[47]
Besides, the line in low-frequency region is described as Warburg resistance
(Zw), demonstrating the impeded diffusion/transport process of electrolyte inside
the electrode. Cdl and Cl are the double layer capacitance and the limit
capacitance, respectively.[22, 48, 49] For all the Ni3Sx series electrode (Fig. 8a-e),
the impedance plot tends to become vertical line with the increasing S2 ratios,
where the largest slope degree is achieved in the Ni3S2.9 superlattice electrode,
indicating its excellent ion diffusion activity in the electrochemical process.[44]
The excellent electron/ion transport behavior of the Ni3S2.9 superlattice could be
attributed to its unique structures. First, First-principle calculations indicate that
the doping S2 species could naturally tune the whole electronic structure by
orbital hybridization and electron delocalization over its adjacent S atoms, thus its
Fermi level lies within its valence band, and a metal-like electrical conductivity
occurs (See First-principle calculations in the Supporting Information). Secondly,
the alternate S−Ni−S2 atomic arrangement could act as the periodic
heterojunction to resist the charges recombination, which hinder the
neutralization of ions, resulting in high electron mobility ratio and high-rate
capability. Moreover, the multi-storey schistous structure can provide efficient
pathways for ions transport and electrolyte diffusion along its whole surface, and
also provide some flexible space to accommodate the volume change in the
ion/electrolyte diffusion processes.
To demonstrate further applications, the superlattice Ni3S2.9 architecture as
the working electrode was assembled into a CR2025 coin cell with lithium metal
18

foil as the counter electrode. Also, we assembled the undoped Ni3S2 nanowirws
architecture into a CR2025 coin cell under the same conditions, for comparison.
The mass loading of the superlattice Ni3S2.9 micropyramids and the undoped
Ni3S2 nanowirws on Ni foam was calculated to be 4.28 mg.cm-2 and 5.70 mg.cm-
2, respectively.
Cyclic voltammetry (CV) curves for the superlattice Ni3S2.9 sample are shown
in Fig. 9a, which are recorded in the potential range of 0.02–3.0 V (vs. Li+/Li)
with a scan rate of 0.1 mV s-1. At the first scan, two cathodic peaks around 0.90
and 0.62 V is evident, which can attribute to the insertion of Li+ in the Ni3S2.9
active materials{Ni3S1.1(S2)0.9 + 4Li+ + 4e → 3Ni + 1.1Li2S + 0.9Li2(S2) } and the
formation of a solid electrolyte interface (SEI) film on the active material surface
[22, 49]. In contrast, there are four anodic peaks at about 1.05, 1.71, 1.94 and 2.26 V
due to the extraction of Li+ from the Ni3S2.9 active materials and other side
reactions.[22, 49] There are several obvious differences in subsequent cycles
compared to the first scan. The reduction peak shifts to 1.28 V, then to 1.33 V,
while three anodic peaks around 2.26, 1.71 and 1.05 V are loosening from the
second cycle due to the activation of the Ni3S2.9 materials.[22, 49] The CV curves
well overlap each other in the subsequent cycles, demonstrating the good
reversibility of the Ni3S2.9 electrode. However, it is noticeable that the current
intensity in Li+ insertion process is continually decreasing with the increased
cycling number in the CV curves. This means the diffusion-controlled Li+
insertion process becomes to be easier with the increasing cycles. Fig. 9b
presents the galvanostatic charge−discharge profiles recorded for different cycles
at a current density of 500 mA g-1. It is evident that the discharge and charge
19

plateaus match well with the respective peaks on the CV curves. Remarkably, the
discharge curves display a constant discharge plateaus at ~1.94 V with the
increasing cycle number, revealing the excellent discharge stability at ~1.94 V.
Fig. 10 shows the discharge–charge cycling performance between 0.02 and
3.0 V. For the undoped Ni3S2 sample, at the current density of 200 mA g-1, the
discharge–charge cycles show a continuous capacity decay process, which is a
common phenomenon for pure Ni3S2 nanocrystals due to its structural
degradation in the lithium insertion/extraction reactions. Comparatively, for the
superlattice Ni3S2.9 sample, at the current density of 200 mA g-1, the first
discharge–charge cycle has a discharge capacity of 944 mAh g-1 and a charge
capacity of 669 mAh g-1. The irreversible capacity in the initial cycle is well
consistent with the initial CV scan, which has been widely referred to the
formation of SEI film and other side reactions.[49, 50] The reversible capacity are
observed at the 3th cycle for 646 mAh g-1, and then gradually increase until the
80th cycle up to 874 mAh g-1. Subsequently, a capacity decay phenomenon
appears similar to that of the undoped Ni3S2 sample, however, a high capacity of
824 mAh g-1 still holds at the 100th cycle. Importantly, the superlattice Ni3S2.9
sample has the superior high-rate capability. As seen in Fig. 10, at the current
density of 500 mA g-1, the reversible capacity continuously increase up to 565
mAh g-1 at the 100th cycle, which is consistent with the level of discharge–charge
profile measurements above. Such a capacity increase phenomenon could be
attributed to the presence of sulfur dimer (S2) in Ni3S2.9 structure, in which the S2
species could provide higher electronegativity (3s23p5) and additional sulfur
active sites to store more Li+ ions. At the same time, the morphology of Ni3S2.9
20

2-
micropyramids could be changed during the cycling reaction of 2Li++ S2
↔
Li2(S2). As seen in Figure S10, after 100 cycles at current density of 500 mA g-1,
the micropyramid-like Ni3S2.9 structure is transformed into graphene-like
irregular nanosheets with the unchanged crystallization phase . This means more
2-
S2 active sites with higher electronegativity could be gradually exposed during
2-
the cycling reactions, convenient for the Li+ storage. Also, the exposed S2
species would finally dissolve to generate the S2- species in the tricky
electrochemical processes. Therefore, the reversible discharge–charge capacity is
gradually increased to a maximum value, then gradually decayed accompanied
with the structural degradation in the cycling tests. On the other hand, the higher
electronegativity of sulfur dimer (S2) has great effect on the extraction behavior
of Li+ ions, so that the rate-capacity at the higher current density of 500 mA g-1 is
reduced, not well match with the excellent electrical conductivity of Ni3S2.9.
However, the actual rate-capacity up to 565 mAh g-1 is really better than the
previously reported nickel sulfide [11-14, 50] , also almost matched to the carbon-
based Ni3S2 hybrid materials.[51-53] Moreover, It is worth noting that the
Coulombic efficiency for the superlattice Ni3S2.9 has been maintained above 97%
with the prolonged cycles except the initial two cycles for 71% and 94%,
respectively. These results indicate that the Ni3S2.9 superlattice is indeed a
promising candidate for application in LIBs.
4. Conclusions
In this paper, we demonstrate a simple wet-chemical S2 doping method to
construct
a
novel S2-tuned chemically homogeneous Ni3(S)1.1(S2)0.9
superlattice multi-storey micropyramid arrays on Ni foam as a battery-type
21

electrode material. The homogeneous superlattice Ni3(S)1.1(S2)0.9 structure
is composed of the alternate S–Ni–S2 atom arrangements in whole crystal lattice,
which is different from the chemically heterogeneous and nonhomogeneous
superlattice, and has the doping induced metal-like electrical conductivity. The Rs
value is only 0.62 Ω, while the Rct is nearly 0 Ω, indicating an excellent
electron/ion transport behavior in the superlattice Ni3(S)1.1(S2)0.9 electrode. As a
cathode material for application in LIBs, a very high specific capacity of 874
mAh g-1 has been achieved at current density of 200 mA g-1. Remarkably, it still
holds a high capacity of 565 mAh g-1 at current density of 500 mA g-1, indicating
its superior high-rate capability. The superior specific capacity and high-rate
capability benefit from the periodic alternate arrangements of S2 species in
Ni3(S)1.1(S2)0.9 superlattice, which could naturally improve band structure and
spatial field effect over the whole crystal to optimize the charge transfer kinetics
for much lower resistance, also provide higher electronegativity and additional
sulfur active sites to store more Li+ ions. Moreover, the schistous multi-storey
micropyramid structure could facilitate the electrolyte diffusion and ion transport
along its entire surface, and also accommodate the volume change in ion
diffusion processes. The convenient and successful degenerate doping S2 strategy
could be large-scale applied to other metal sulfides with novel superstructures for
material chemists and physicists to further study on electrochemical,
photoelectric and photocatalytic applications.
Acknowledgements
22

This work was financially supported by the National Natural Science Foundation of
China (Grant No. 21271152, 11804289). We thank Dr. Xuanjun Zhang, Prof. Zhonglin
Lu for helpful discussions.
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