N-Doped carbon encapsulating Bi nanoparticles derived from metal-organic frameworks for high-performance sodium-ion batteries

Article abstract of DOI:10.1039/d1ta06558k

Bismuth (Bi), as an alloy-based material, has been demonstrated as a promising anode for sodium-ion batteries (SIBs) due to its high theoretical capacity. However, the large volume change of the Bi anode during the sodiation/desodiation process results in poor cycling performance, which limits its practical application. In the present work, a simple one-step route was realized to fabricate Bi nanoparticles embedded into a N-doped carbon matrix (Bi@NC) by calcining Bi-containing metal-organic framework (Bi-MOF) precursors. Benefitting from the synergistic effect of Bi nanoparticles and the conductive N-doped carbon matrix, the composite can not only reduce the ion/electron diffusion pathways and enhance the reaction kinetics, but can also effectively alleviate the volume expansion during alloying/dealloying processes. As a result, the Bi@NC electrode displayed an excellent electrochemical performance with a superior rate capability of 86% capacity retention at 10 A g^?1and a high capacity of 326.9 mA h g^?1after 5000 cycles at 2 A g^?1. Furthermore, the assembled full cell with a Na₃V₂(PO₄)₃cathode and a Bi@NC anode also delivered an impressive electrochemical performance with a high energy density of 125 W h kg^?1(based on the total mass of cathode and anode materials). Furthermore, the sodium storage mechanism was also elucidated through in-depth fundamental investigation usingin situX-ray diffraction (XRD) and density functional theory (DFT) calculations.

Full text of DOI:10.1039/d1ta06558k

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ARTICLE
N-doped carbon encapsulating Bi nanoparticles derived from
metal-organic framework for high-performance sodium-ion
batteries
Received 00th January 20xx,
Accepted 00th January 20xx
Lin Chen, a Xiaojie He, b Huimin Chen, a Shuping Huang, * b Mingdeng Wei * a, b
DOI: 10.1039/x0xx00000x
Bismuth (Bi), as an alloy-based material, has been demonstrated as a promising anode for sodium-ion batteries (SIBs) due
to its high theoretical capacity. However, the large volume change of Bi anode during the sodiation/desodiation process
results in poor cycling performance, which limits its practical application. In the present work, a simple one-step route was
realized to fabricate Bi nanoparticles embedded into N-doped carbon matrix (Bi@NC) by calcining Bi-containing metal-
organic framework (Bi-MOF) precursors. Benefitting from the synergistic effect of Bi nanoparticles and conductive N-
doped carbon matrix, the composite can not only reduce the ion/electron diffusion pathways and enhance the reaction
kinetics, but also effectively alleviate the volume expansion during the alloying/dealloying process. As a result, the Bi@NC
electrode displayed an excellent electrochemical performance with a superior rate capability of 86% capacity retention at
10 A g-1 and a high capacity of 326.9 mA h g-1 after 5000 cycles at 2 A g-1. Furthermore, the assembled full cell with
Na3V2(PO4)3 cathode and Bi@NC anode also delivered an impressive electrochemical performance with a high energy
density of 125 W h kg-1 (Based on the total mass of cathode and anode materials). Furthermore, the sodium storage
mechanism was also achieved through in-depth fundamental investigated using in-situ X-ray diffraction (XRD) and density
functional theory (DFT) calculations.
mechanism can deliver fascinating sodium ion storage
capacity, but their realistic application is still limited due to the
high voltage platforms (about 1.5 V) and poor cycling
stability.11-15 Also, alloy-based materials (i.e., Sn, Bi, Sb and P)
can deliver a high sodium storage capacity, which display more
extensive development prospects.
1. Introduction
To date, lithium-ion batteries (LIBs) have been widely applied
in powering electric vehicles because of its high energy density
and long lifespan. However, the high cost and scarcity of
lithium resources on the earth restrict its further
applications.1-3 Sodium ion batteries (SIBs), as one kind of the
state-of-the-art energy storage devices, due to the similar
properties of sodium and lithium, low cost and abundant
natural resources, are regarded as a promising alternative to
LIBs, and have attracted extensive attention.4-7 At present, one
of the key challenges is to design anode materials with a
superior sodium storage property, which can play a significant
role in the output voltage, energy density and stability of SIBs.
Nevertheless, among many candidate materials, due to the
larger radius of Na+ (1.02 Å) as compared with Li+ (0.76 Å), it is
difficult for Na+ to be inserted into the interlayer of
commercial graphite, resulting in low theoretical capacity (35
mA h g-1) and energy density.8-10 On the other hand, transition
metal oxides/sulfides/selenides based on conversion-reaction
Bismuth (Bi) has been extensively studied due to its
excellent
intrinsic
advantages,
including
favorable
alloying/dealloying operating potential at about 0.6 V, large
lattice fringe along c-axis (d(003)= 3.95 Å) for sodium ion
insertion/extraction, and high theoretical capacity of 385 mA h
g-1.16-18 Regrettably, Bi still faces the problem of the huge
volume expansion (≈244%) during the charge-discharge
process, which leads to serious pulverization and capacity
degradation.19 According to previous investigations, designing
novel nanostructure, combining with conductive carbonaceous
and suitable electrolyte are considered as effective strategies
for improving its electrochemical properties.20-23 For instance,
Yu and co-workers24 synthesized Bi nanorods encapsulated in
N-doped carbon nanotubes, which displayed a high capacity of
302 mA h g-1 at 1 A g-1 after 1000 cycles. Xu group25 fabricated
the Bi@C composite that revealed an outstanding cycling
stability, with a large capacity of 265 mA h g-1 at 8 A g-1 after
30000 cycles. Also, Park et al.18 synthesized the Bi/C composite
and explored the mechanism of the difference of
electrochemical properties in ether and ester electrolytes.
Benefiting from the highly ionic-conducting and robust multi-
layered SEI, Bi/C electrode delivered a better sodium storage
a. Fujian Key Laboratory of Electrochemical Energy Storage Materials, Fuzhou
University, Fuzhou, 350002 Fujian, China.
b. State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou
University, Fuzhou, 350002 Fujian, China.
† E-mail: huangshp@gmail.com; wei-mingdeng@fzu.edu.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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capability in the ether-based electrolyte (270 mA h g-1 at 5 A g- 1). Then, the furnace was naturally cooleVdiew tAorticlerOonolimne
1 after 1000 cycles).
temperature, and the Bi@NC composite was obtained.
DOI: 10.1039/D1TA06558K
On the other hand, metal-organic frameworks (MOFs), as a 2.3 Preparation of pure Bi
kind of organic-inorganic compound that formed by Typically, Bi(NO3)3·5H2O (0.970g, 2 mmol) and NaBH4 (0.38g, 1
coordination bond between metal ions and organic ligands, mmol) were dissolved in 30 mL of deionized water,
owing to their high surface area, tunable pore size, respectively. Then, two solutions were mixed and stirred for 2
controllable structure and redox actively, have been widely h. The black precipitates were washed several times with
o
implemented
in
various
fields
(including
gas deionized water and dried at 70 C for 10 h in a vacuum, the
separation/storage, catalysis, sensors, drug delivery, energy pure Bi was achieved.
storage and so on).26-31 Meanwhile, MOFs have been 2.4 Structural characterization
employed as templates for manufacturing various functional The X-ray diffraction (XRD) of the samples were tested using a
composite materials, which can obtain homogeneously Rigaku Ultima IV using Cu Kα radiation. Thermogravimetric
dispersed nanoparticles embedded into carbon matrix and analysis (TGA) was recorded using a TG209F3 Tarsus® with a
improve the electrochemical performance.32-35 However, there rate of 10 oC min-1 in air atmosphere. The morphologies
are still few investigates on the preparation of Bi/carbon information was measured by scanning electron microscopy
composite by calcination of Bi-MOF materials and their (SEM, Hitachi 4800) and transmission electron microscopy
application in SIBs.
(TEM, TECNAI G2 F20). Raman spectroscopy was obtained
Herein, a facile method was developed to synthesize the using a Renishaw in Via Reflex Raman system with a 532 nm
composite of Bi nanoparticles/N-doped carbon (Bi@NC) Nd: YAG excitation source. FT-IR spectra were performed on a
through one-step calcination of Bi-containing metal-organic BRUKER-EQUINOX-55
IR
spectrophotometer.
X-ray
framework (Bi-MOF) precursor. It was found that the Bi@NC photoelectron spectroscopy (XPS) was measured on a Thermo
composite showed a microrod-like morphology, in which Bi ESCALAB 250XI to analyze the elements states. N2
nanoparticles were uniformly embedded into the N-doped adsorption/desorption analysis were collected on
a
carbon matrix. The electrode composed of Bi nanoparticles Micromeritics ASAP 2020 instrument. The pore size
and conductive N-doped carbon matrix can shorten the distribution of samples was calculated by a Barrett-Joyner-
diffusion distance of Na-ion/electron, ensuring a fast kinetics Halenda (BJH) analyses based on the adsorption branch.
and inhibiting the volume expansion during the charge- 2.5 Electrochemical measurement
discharge process. As a result, when employed as an anode for The CR2025 type coin cells were assembled in a highly pure Ar-
SIBs, the Bi@NC electrode displayed a high specific capacity of filled glove box (O2 and H2O levels < 0.1 ppm) to assess the
386.2 mA h g-1 at 0.2 A g-1, a superior rate capability of 86% electrochemical performance of half and full cell. Typically, the
capacity retention at 10 A g-1, and a long-term cycling stability anode slurry was made by mixing 80 wt% of Bi@NC, 10 wt% of
(326.9 mA h g-1 over 5000 cycles at 2 A g-1). The full cell Super P and 10 wt% of polyvinylidene difluoride (PVDF) with N-
constructed by combining Na3V2(PO4)3 cathode with Bi@NC methyl-2-pyrrolidone (NMP). The Na3V2(PO4)3 (NVP) cathode
anode delivered impressive electrochemical performance was purchased from Hubei Energy Technology Co., Ltd. The
(223.3 mA h g-1 at 1 A g-1 after 200 cycles). Furthermore, the cathode slurry was made by a mixture including NVP powder,
kinetics-controlled phase transitions of alloying reaction were Super P and PVDF (weight ratio 7:2:1), and then they were
o
also revealed combining in-situ X-ray diffraction and density coated on the Cu foil and Al foil followed drying at 110 C in
functional theory (DFT) calculations.
vacuum for 12 h, respectively. The average active material
mass loading was around 1.0 and 3.5 mg cm-2 for anode and
cathode, respectively. 1 M NaPF6 in dimethoxyethane (DME)
solution and glass fiber (Whatman) served as the electrolyte
and separator in coin cell. Also, the metallic sodium was used
as the counter electrode for half cells. Galvanostatic charge-
discharges and cyclic voltammograms (CV) curves were
measured within a voltage range of 0.01-1.5 V for Bi@NC
anode and 2.0-3.8 V for NVP cathode (vs. Na+/Na) at room
temperature. To evaluate the NVP // Bi@NC full cell
properties, the Bi@NC anode and NVP cathode were paired up
in a weight ratio of 1: 3.5. The Bi@NC anodes were pre-treated
by charging/discharging 5 cycles at current density of 0.1 A g-1
and finally charged to 1.5 V. Galvanostatic charge/discharge
experiments were carried out between 1.6 and 3.2 V.
Electrochemical impedance spectroscopy (EIS) measurements
were carried out on an electrochemical workstation (Chenhua
CHI660c, Shanghai, China).
2. Experimental section
2.1 Preparation of Bi-MOF precursor
All chemicals were purchased commercial and used are
received without further purification. Typically, Bi(NO3)3·5H2O
(0.485g, 1 mmol), 1, 3, 5-benzenetricarboxylic acid (0.420g, 2
mmol), 4, 4-bipyridine (0.312g, 2 mmol) and 2.0 g of polyvinyl
pyrrolidone (PVP) were dissolved in 35 mL of methanol. After
stirring of 20 min, the resulting solution was transferred into a
Teflon-coated stainless-steel autoclave (50 mL) and kept 140
oC for 12 h. The product was naturally cooled to room
temperature, the wash by ethanol and collected after dried at
60 oC for 12 h under air.
2.2 Preparation of Bi@NC composite
The Bi-MOF precursors and 500 mg of dicyandiamide were
placed at two separate positions in a tube furnace and heated
at 800 oC for 2 h under the Ar gas flow (heating ramp: 5 oC min-
2.6 Computational Method
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Computational calculations were performed by using the reaction (4, 4-bipyridine and 1, 3, 5-benzenetricarboxylic acid
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DOI: 10.1039/D1TA06558K
Vienna Ab Initio Simulations Package (VASP) based on density as organic ligands). Then, the Bi@NC composite composed of
functional theory (DFT).36 Generalized gradient approximation N-doped carbon-encapsulated Bi nanoparticles was obtained
(GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange- via annealing the Bi- MOF in an Ar atmosphere at 800 ℃. The
correlation functional was applied.37 In the calculations, the morphology of Bi-MOF precursor was observed by SEM
plane wave cut-off energy was set at 500 eV. For geometry measurements. As depicted in Fig. S1a, the Bi-MOF shows a
optimization, the energy and force on the atoms were uniform structure of microrod with an average diameter of
converged to less than 10-5 eV and 0.02 eV Å-1. A Γ-centered about 500 nm. The XRD pattern and IR spectrum are shown in
grid with 13 × 13 × 13 mesh size was adopted to sample in Fig. (b, c). The well-defined diffraction peaks are detected in
Brillouin zones for Bi-bulk phase. The calculated lattice the former, presenting a high degree of crystallinity. The IR
parameters of bulk Bi are a = b = 4.562 Å and c = 12.185 Å, spectrum delivers that the characteristic peaks of vas(-COO-)
which are in good agreement with experimental ones (a = b = and vs(-COO-) stretching vibrations are located at 1548 and
4.546 Å, c = 11.862 Å).38 Then, we constructed the 2 × 3 sized 1360 cm-1. The peaks at 1429 and 1613 cm-1 can be attributed
periodic slabs for Bi (110) and Bi (104) surfaces with 4 layers to the stretching vibrations of the pyridyl rings of 4,4’-
(Fig. S9). To simulate the surface state, the top two layers of bipyridine ligands, and the band at 729 cm-1 can be ascribed to
atoms were allowed to relax while the bottom two layers were plane vibration of the benzene.40-42
held at the bulk position. A vacuum layer of 15 Å was used to
The crystal structure of the Bi@NC composite and pure Bi
avoid the interaction between repeated slabs. Besides, dipole were performed by XRD measurement. As displayed in Fig. 1b,
moment correction was also considered in our calculations for all the diffraction peaks can be consistent with a standard
that asymmetric slabs were employed. For Bi (110) and Bi phase of rhombohedral Bi (JCPDS No. 85-1329), and no
(104) surfaces, a 2 × 2 × 1 k-point mesh was adopted to sample impurity peaks are observed, indicating that the Bi-MOF
in Brillouin zones. In addition, sodium migration paths and precursor has been transferred into the Bi@NC composite
barriers are determined using the climbing-image nudge after calcination. The N2 adsorption-desorption isotherms
elastic band (CI-NEB) method.39 All the transition states were were tested to analyze the specific surface area (Fig. 1c). The
confirmed by frequency calculations and only one imaginary Bi@NC composite shows a typical IV type isotherm, and the
frequency was found for each transition state.
specific surface area is calculated to be about 73.99 m2 g-1,
while that of pure Bi is only 3.84 m2 g-1 (Fig. S2a). In the inset
of Fig. 1c, the pore size distribution demonstrates that the
Bi@NC composite exhibits micro/mesoporous structure with
the pores centered at about 1-15 nm. Also, the Raman spectra
are shown in Fig. 1d, two characteristic peaks at 1341 and
1590 cm-1 can be corresponded to the D- and G- band of
3. Results and discussion
The schematic illustration for the preparation procedure of
Bi@NC composite is shown in Fig. 1a. Firstly, the Bi-MOF
precursor was synthesized through a simple solvothermal
Fig. 1 (a) Schematic drawing of the formation process to obtain the Bi@NC
composite, (b) XRD patterns, (c) N2 adsorption-desorption isotherms, (d) Raman
spectra, and (e) TGA curve of the Bi@NC composite.
Fig. 2 (a-b) SEM images, (c-d) TEM images, and (e) STEM image as well as (f-h)
elemental mappings of the Bi@NC, respectively.
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carbon, respectively. The value of IG/ID is 1.03, demonstrating
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DOI: 10.1039/D1TA06558K
that the N-doped carbon is partially graphitized and has a high
44
conductively.43,
Moreover, the content of Bi in Bi@NC
composite was conducted by the thermogravimetric analysis
(TGA). As shown in Fig. 1e, the weight change process can be
attributed to the loss of adsorbed water, the combustion of C
and N, and the oxidation of Bi. According to the amount of
final residual product Bi2O3 is 88.2%, the Bi content in the
Bi@NC composite is calculated to be about 78.9%.
SEM and TEM measurement were applied to further
investigate the morphology and structure information of the
Bi@NC composite. Fig. 2(a, b) show that the Bi@NC composite
keeps a microrod-like structure with uniform size, implying
that original shape of the Bi-MOF precursor can be perfectly
inherited after calcination. Furthermore, the high-resolution
TEM (HRTEM) images in Fig. 2(c, d) show that the Bi
nanoparticles are uniformly embedded into the N-doped
carbon matrix. In the inset of Fig. 2d, a well-defined lattice
fringe with a spacing of 0.322 nm can be detected, which can
be ascribed to the interplanar of the (012) plane for Bi. The
high-angle annular dark-field scanning TEM (HAADF-STEM) and
corresponding elemental mappings are shown in Fig. 2 (e-h),
which indicate the uniformly distribution of Bi, C and N
elements in the Bi@NC composite. By contrast, pure Bi
obtained by NaBH4 reduction route presents an agglomeration
and irregular morphology (Fig. S2b).
Fig. 4 Sodium storage performance of the Bi@NC electrode: (a) The CV curves at
0.2 mV s-1, (b) the charge-discharge curves at 0.2
A
g-1, (c) the cycling
performance at 0.2
A
g-1, (d) rate capability, and (e) long-term cycling
performance at 2.0 A g-1.
XPS measurement was performed to determine the surface
composition and chemical state of Bi@NC composite. As
shown in Fig. 3a, the full survey spectrum presents that the
elements of Bi, N and C can be observed. The high-resolution
spectrum of Bi 4f is displayed in Fig. 3b, the main two peaks
locate at 159.0 and 164.3 eV, which can be assigned to Bi 4f7/2
and Bi 4f5/2, respectively.24 The three binding energies at
284.7, 286.2 and 288.3 eV can be observed in the C1s core-
level spectrum (Fig. 3c), corresponding to the C-C, C-N and C=O
bonds, respectively.45 Also, as presented in Fig. 3d, the N 1s
spectrum can be separated into four characteristic peaks at
398.3, 399.5, 401.0 and 403.5 eV, which can be ascribed to
pyridinic N, pyrrolic N, graphitic N and quaternary N,
respectively, demonstrating that the N was successfully doped
47
into carbon.46,
After quantitative analysis, the atomic
content of N element in Bi@NC was about 3.08%. According to
previously experimental results, the N-doped carbon can not
only enhance the electronic conductivity but also provide
more sodium storage active sites, which will greatly improve
the electrochemical performance of the composite.48
The sodium storage performance of Bi@NC composite and
pure Bi were investigated by the assembled half-cell (2025-
type) in a voltage range of 0.01-1.5 V (vs. Na+/Na). The typical
three CV curves of Bi@NC composite were tested at a scan
rate of 0.2 mV s-1 and the results are shown in Fig. 4a. In the
first cycle, a broad cathodic peak at 0.6-0.75 V is observed,
which can be corresponded to the formation of the SEI layer
and alloy reaction process from Bi to NaBi. Also, a distinct peak
at 0.47 V can be attribute to the NaBi to produce Na3Bi phase.
In the anodic process, two peaks at 0.62 and 0.77 V are
attributed to the desodiation reactions of Na3Bi → NaBi →
Bi.24, 49 Compared with the CV curves of pure Bi (Fig. S3a), in
the subsequent second and fifth cycle, the CV curves of Bi@NC
composite almost overlap, indicating a high electrochemical
reversibility of the Bi@NC composite. The corresponding three
galvanostatic charge-discharge profiles at 0.2 A g-1 are shown
in Fig. 4b, the clear plateaus are well matching with the redox
peaks of the CV curves. The first charge and discharge
capacities were 383.7 and 595 mA h g-1, respectively, leading
to an initial Coulombic efficiency of 64.5%. The irreversibility of
initial capacity loss could be imputed to the decomposition of
Fig. 3 (a) Typical XPS survey spectrum and the corresponding (b) Bi 4f, (c) C1s,
and (d) N 1s of the Bi@NC.
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electrolyte to generate SEI layer and side reaction between the based anodes.
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51
carbon matrix and the electrolyte.50,
In the subsequent
To better understand the excellent Na-ion
DOI: 10.1039/D1TsAt0o6r5a5g8eK
cycles, the Coulombic efficiency was increased rapidly to performance of Bi@NC, the CV curves at different scan rates
nearly 100%, and obvious change in voltage curves was not from 0.2 to 4.0 mV s-1 were tested. As shown in Fig. 5a, the CV
detected, indicating that the irreversible reaction was only curves of Bi@NC possess a similar profile of redox peaks and
occurred in initial period. As depicted in Fig. 4c, the Bi@NC position, indicating an excellent reversibility. On the basis of
composite exhibited a high capacity of 386.2 mA h g-1 after 400 previous literatures, the possible mechanism of Na-ion
cycles at a current density of 0.2 A g-1.
reaction for the Bi@NC electrode can be qualitatively analyzed
The rate capabilities of the Bi@NC electrode were by following formula (1-2):52-54
measurement at different current densities (0.1 to 10 A g-1)
and the results are shown in Fig. 4d and Fig. S4. The average
i = a vb
(1)
(2)
capacities of 392.9, 376.7, 363.9, 359.5, 347.5, and 344.1 mA h
g-1 can be obtained at current densities of 0.1, 0.2, 0.5, 1.0,
2.0, and 5.0 A g-1, respectively. Even at 10 A g-1, the Bi@NC
electrode still delivers a high capacity of 338.2 mA h g-1 (86%
retention of the capacity at 0.1 A g-1), while the pure Bi only
retains a capacity of 307.2 mA h g-1. When the current density
was returned to 0.1 A g-1, the capacity can be restored to 392.8
mA h g-1. The results present that the capacities hardly
dropped with the increase of current density, reflecting a
robust rate capability of the Bi@NC anode. Also, the long
cycling stability of pure Bi and Bi@NC composite was
examined at 2 A g-1. As presented in Fig. 4e and Fig. S5, the
latter retained a large capacity of 326.9 mA h g-1 after 5000
cycles, while the capacity of pure Bi is only 286.6 mA h g-1 after
1600 cycles, indicating that the Bi@NC composite has a
superior structural stability and a long-term cycling
performance. Such an excellent long-term cycling performance
could be ascribed to the structure of Bi nanoparticles
immobilized into the N-doped carbon, which not only has a
fast kinetics, but also retards the volume expansion during the
charge-discharge process. Also, as shown in Fig. S6, when the
mass loading was arrived to 3.5 mg cm-2, the Bi@NC composite
can still deliver a high reversible capacity of 300.2 mA h g-1
after 700 cycles at a current density of 2 A g-1. To the best of
our knowledge, as listed in Table S1, such an impressive
sodium storage performance is one of the best results for Bi-
log i =b log v + log a
where the value of variable b can be calculated through the
slop of log i - log v plots. When the value of b is close to 0.5, it
represents a diffusion-dominated process, while b reaches 1
for a capacitive behavior. As presented in Fig. 5b, the slop of
four peaks is 0.576, 0.551, 0.554 and 0.581, respectively,
presenting that the sodium storage reaction is mixture of the
diffusion-dominated process and capacitive behavior, which is
conducive to improving the sodium storage performance.
Electrochemical impedance spectroscopy (EIS) and
galvanostatic intermittent titration technique (GITT) were
tested to further explore the kinetic process of the Bi@NC
composite. As shown in Fig. 5c, the Nyquist plots of two
samples have similar shapes, consisting of a semicircle in the
high-frequency and a straight line in low-frequency region.
Obviously, the Bi@NC composite has a smaller charge-transfer
resistance (Rct) value, which is less than that of the pure Bi,
confirming that the incorporation of Bi nanoparticles into N-
doped carbon matrix are beneficial to improve the rapid
electron transfer during the charge-discharge process. In
addition, the Na+ diffusion coefficients (DNa+
) can be
quantitatively analyzed according to the following equations
(3-4):55-57
Z' = Re + Rct + σω-1/2
(3)
(4)
D = R2T2/2A2n4F4C2σ2
where ω, R, T, A, F, n, C are the angular frequencies, gas
constant (8.314 J K-1 mol-1), absolute temperature (298 K),
surface area of the electrode, Faraday constant (96500 C mol-
1), electrons number and concentration of sodium ions,
respectively. The DNa+ is inversely proportional to σ, where the
value of σ is determined by the slop of the line Z' - ω-1/2. Fig. 5d
shows the linear fitting of Z' - ω-1/2 in the low-frequency region
of Bi@NC composite and pure Bi. It can be clearly found that
the σ value of the Bi@NC electrode is significantly smaller than
that of pure Bi electrode, and the DNa+ values of the Bi@NC
and pure Bi electrode are calculated to be 7.64 × 10-13 and
1.03×10-13 cm2 S-1, respectively, demonstrating faster kinetics
of the Bi@NC electrode. Also, the GITT data (Fig. S7) was
collected at a current pulse of 0.2 A g-1 for 15 min followed by
an open-circuit stand for 60 min to allow fully relaxation to a
quasi-equilibrium potential. The overpotentials for the
sodiation and desodiation reactions are very small, the
calculated sodium ion diffusion coefficients range from 10-15 to
Fig. 5 (a) CV curves at different scan, (b) determination of the b-value using the
relationship between peak current and scan rate of the Bi@NC, (c) EIS plots after
5th cycles and (d) the relationship plots of Z′ versus ω-1/2 at low-frequency
region.
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observed again. The XRD pattern and TEM image of Bi/NC after
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the first cycle were analyed to determine itDs OcoI:m10p.1o0s3it9i/oDn1T(AF0ig6.5S588)K.
As shown in Fig. S8a, the characteristic peaks of Bi can be observed
clearly after the first cycle, and the HRTEM image (Fig. S8b) shows a
lattice fringe with lattice spacing of 0.32 nm, which can be
corresponded to the interplanar of the (012) for Bi. The results
mentioned above have proven that the Bi@NC electrode has
undergone the reversible alloying reaction process of
Bi↔NaBi↔Na3Bi.
The DFT calculations were further performed to understand
the Na+ diffusion properties in bismuth. Two main surfaces
(104) and (110) were chosen to estimate the diffusion process
and get the minimum energy path (MEP). Fig. S10 shows the
most favorable insertion sites of Na on Bi (104) and Bi (110)
surfaces, in which A1 and A2 are the 4-fold locations on Bi
(104), B1 and B2 are the 4-fold locations on Bi (110). For Bi
(104) surface, we studied three diffusion paths (Fig.6 c): (i)
from A1 to the neighboring A1; (ii) from A1 to the neighboring
A2; (iii) from A2 to the neighboring A2. The NEB result (Fig. 6d)
shows that the energy barrier of Na+ diffusing via A1→A1
pathway is about 0.40 eV, while the one for A1→A2 pathway
and for A2→A2 pathway is 0.23 eV and 0.41 eV, respectively.
So, the most favorable diffusion pathway of Na+ on Bi (104)
surface is the A1→A2 pathway with a barrier of 0.23 eV.
Similarly, for Bi (110), we considered three Na+ diffusion paths
(Fig. 6e): (i) B1→B1 pathway, (ii) B1→B2 pathway, and (iii)
B2→B2 pathway. The corresponding energy barriers (Fig. 6f)
are 0.35 eV, 0.18 eV and 0.40 eV, respectively. Thus, the
diffusion barrier of the most favorable pathway (0.18 eV) of
Na+ on (110) surface is lower than that on (104) surface,
confirming that Na+ is more inclined to diffuse on (110)
surface. Moreover, these barriers are much lower than the
ones of sodium diffusion in Bi bulk (1.28 eV and 1.92 eV) in
previous study, which suggests that sodium diffusion on
surface is much more favorable.50
Fig. 6 (a) In situ XRD patterns recorded, (b) charge-discharge profiles of the
Bi@NC electrode, (c) sodium diffusion paths on Bi (104) surface, (d)
corresponding energy barrier profile, (e) sodium diffusion paths on Bi (110)
surface, and (f) corresponding energy barrier profile.
10-10 cm2 s-1, presenting a fast sodium ion diffusion in Bi@NC
electrode. Based on the above analysis, the Bi@NC composite
has faster charge transfer and sodium ion diffusion coefficient,
thus exhibiting excellent electrochemical properties.
To further investigated the sodiation/desodiuation mechanism
of the Bi@NC composite, the uninterrupted in situ XRD test
was carried out. The battery mold for in situ XRD testing was
equipped with a metal Be window to allow X-ray passage. Fig.
6b displays the four characteristic stages (I to IV) of the charge-
discharge curves, which correspond to the phase transition of
the Bi@NC electrode at different (de)sodiation states in Fig.
6a. During the sodiation process, in the I stage of discharge to
0.5 V, the characteristic peak of Bi located at 27.1° gradually
attenuated and fade away, while a new characteristic peak at
31.8° is formed simultaneously and enhanced, which is
attributed to the generation of NaBi. Next, in second voltage
window of 0.50-0.01 V (II stage), the new peaks at 32.8° and
33.6° of Na3Bi are evolving and accompanied by the
disappearance of the peak of NaBi, indicating that the NaBi
Based on the excellent electrochemical performance of the
Bi@NC composite, a sodium-ion full cell was assembled to
further evaluate its potential application. As shown in Fig. 7a,
the full cells constructed with Bi@NC composite anode and
Na3V2(PO4)3 (NVP) cathode are represented as NVP//Bi@NC.
phase has been transformed to Na3Bi and the alloying reaction
58
has been closed to completion.22,
Upon the desodiation
process, the Na3Bi is decomposed in two steps. In the voltage
window of 0.01-0.57 V (III stage), the characteristic peaks of
Na3Bi gradually disappear, accompanied by the regeneration
of NaBi. Then, in the process of charging to 1.5 V (IV stage), the
NaBi decomposes gradually, while the peak of Bi can be
Fig. 7 Electrochemical performance of NVP // Bi@NC full cells: (a) schematic
working mechanism, (b) typical charge/discharge curves of the NVP cathode, the
Bi@NC anode and NVP // Bi@NC full cells, (c) CV curves at 0.2 mV s-1, (d)
galvanostatic charge/discharge curves, e) cycling performance at 1 A g-1, and f)
rate performance.
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The SEM images and XRD pattern of NVP are shown in Fig. S11. The
XRD peaks are consistent with a standard phase of Na3V2(PO4)3
(JCPDS No. 62-0345).59-61 Also, the NVP//Na half-cell with 1 M
NaPF6/DME as an electrolyte was assembled to investigate the
electrochemical performance of NVP. As presented in Fig. S12, a
high voltage plateau (3.4 V) and stable cycling performance for 150
cycles, indicating that NVP is a reliable cathode material for SIBs.
According to the voltage differences between Bi@NC composite
anode and NVP cathode, the NVP//Bi@NC full cell was explored
within a potential window of 1.6-3.2 V, and showed two flat voltage
plateaus at 2.60 and 2.75 V (Fig. 7b). Also, as shown in Fig. 7c, the
full cell presents two pairs of redox peaks at 2.60/2.70 and
2.75/2.90 V in CV curves, respectively, which are well consistent
with the distinct plateaus of the discharge-charge profiles (Fig. 7e).
The CV curves of different cycles almost overlap, and the charge-
discharge profiles are the same, which demonstrates that the
NVP//Bi@NC full cell has excellent electrochemical reversibility and
cycling stability. Meanwhile, Fig. 7d shows the cycling performance
of NVP//Bi@NC full cell at 1 A g-1 (the capacity calculated by the
mass of the Bi/NC composite), it delivers a high capacity of 223.3
mA h g-1 after 200 cycles. Based on the total mass of the active
materials, conductive agents and binders of cathode and anode, the
energy density of the full cell can be calculated to be about 125 W h
kg-1. The result of rate capability is also satisfactory and is shown in
Fig. 7f and Fig. S13, the average capacities of 246.7, 231.0, 223.4,
210.4, 198.2 and 180.2 mA h g-1 can be maintained at current
densities of 0.2, 0.5, 0.8, 1.0, 2.0 and 5.0 A g-1, respectively. Such an
impressive electrochemical performance of NVP//Bi@NC full cell
definitely indicate that the Bi@NC composite can act as a reliable
anode material for future commercial SIBs.
Acknowledgements
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This work was financially supported bDyOfI:ro10m.103t9h/eD1TNAa0t6io55n8aKl
Natural Science Foundation of China (21703036) and Fujian
provincial project (2019H6005) as well as National Key
Laboratory (6142808190203).
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Conclusions
In summary, the Bi@NC composite composed of Bi
nanoparticles embedded into the N-doped carbon matrix was
synthesized successfully by using Bi-MOF as a precursor.
Employed as an anode material for SIBs, the Bi@NC electrode
presented excellent performance, an exceptional rate
capability of 86% capacity retention at a current density of 10
A g -1 and a long-term cycling life of 5000 cycles at 2 A g-1. The
results of in-situ XRD and DFT calculations have proved that
benefitting from the synergistic effect of the Bi nanoparticles
and N-doped carbon matrix, this composite can not only
decrease the ion/electron diffusion pathways and ensure a fast
kinetics, but also efficient alleviation of stress/strain caused by
the volume expansion during the sodiation/desodiation
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and displayed a high capacity of 223.3 mA h g-1 after 200 cycles
and an energy density of 125 W h kg-1 (Based on the total mass
of cathode and anode materials). These promising results will
provide a significant guidance for the development of alloy
anode materials and its potential alkaline ion batteries.
Conflicts of interest
There are no conflicts to declare.
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