Unveiling Intrinsic Potassium Storage Behaviors of Hierarchical Nano Bi@N-Doped Carbon Nanocages Framework via In Situ Characterizations

Article abstract of DOI:10.1002/anie.202016082

Metallic bismuth has drawn attention as a promising alloying anode for advanced potassium ion batteries (PIBs). However, serious volume expansion/electrode pulverization and sluggish kinetics always lead to its inferior cycling and rate properties for practical applications. Therefore, advanced Bi-based anodes via structural/compositional optimization and sur-/interface design are needed. Herein, we develop a bottom-up avenue to fabricate nanoscale Bi encapsulated in a 3D N-doped carbon nanocages (Bi@N-CNCs) framework with a void space by using a novel Bi-based metal-organic framework as the precursor. With elaborate regulation in annealing temperatures, the optimized Bi@N-CNCs electrode exhibits large reversible capacities and long-duration cyclic stability at high rates when evaluated as competitive anodes for PIBs. Insights into the intrinsic K⁺-storage processes of the Bi@N-CNCs anode are put forward from comprehensive in situ characterizations.

Full text of DOI:10.1002/anie.202016082

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Research Articles
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How to cite: Angew. Chem. Int. Ed. 2021, 60, 7180–7187
International Edition:
German Edition:
doi.org/10.1002/anie.202016082
doi.org/10.1002/ange.202016082
Nanocages Hot Paper
Unveiling Intrinsic Potassium Storage Behaviors of Hierarchical Nano
Bi@N-Doped Carbon Nanocages Framework via In Situ
Characterizations
+
+
+
Zehang Sun , Yang Liu , Weibin Ye , Jinyang Zhang, Yuyan Wang, Yue Lin,* Linrui Hou, Ming-
Sheng Wang,* and Changzhou Yuan*
Abstract: Metallic bismuth has drawn attention as a promising
alloying anode for advanced potassium ion batteries (PIBs).
However, serious volume expansion/electrode pulverization
and sluggish kinetics always lead to its inferior cycling and rate
properties for practical applications. Therefore, advanced Bi-
based anodes via structural/compositional optimization and
sur-/interface design are needed. Herein, we develop a bottom-
up avenue to fabricate nanoscale Bi encapsulated in a 3D N-
doped carbon nanocages (Bi@N-CNCs) framework with
a void space by using a novel Bi-based metal-organic frame-
work as the precursor. With elaborate regulation in annealing
temperatures, the optimized Bi@N-CNCs electrode exhibits
large reversible capacities and long-duration cyclic stability at
high rates when evaluated as competitive anodes for PIBs.
the enhanced cost of conventional lithium ion batteries
[
1]
(LIBs). Accordingly, it seems more significant and mean-
ingful to explore other high-performance metal-ion batteries
as competitive alternatives of common LIBs.
Considering its appealing advantages including cost-
efficiency, earth-abundant resource of elemental potassium,
as well as similar chemical/electrochemical properties be-
tween Li and K, the emerging potassium ion batteries (PIBs),
as one of the most promising energy storage systems, have
[
2]
captured the worldwide research interests. Nevertheless, the
extensive development of PIBs is still struggled by its larger
atomic radius (0.138 nm), which induces more serious volume
change and sluggish kinetics of electrode materials during
[
3]
potassiation/depotassiation processes. Until now, tremen-
dous efforts have been devoted to pursuing appropriate
electrode materials for efficient storage of K ions towards
PIBs. Among the retrieved anodes, the typical carbon-based
materials always show more stable electrochemical behaviors,
but still suffer from lower capacities and potential safety
+
Insights into the intrinsic K -storage processes of the Bi@N-
CNCs anode are put forward from comprehensive in situ
characterizations.
Introduction
[
4]
issues.
Nowadays, with increasing requirements for electric
vehicles, hybrid electric vehicles, and portable electronic
devices, the demand for advanced energy storage devices with
high electrochemical performance turns out to be more
urgent. Unfortunately, the relative scarcity, increasing con-
sumption, and uneven geographic distribution of metal
lithium itself has become increasingly prominent, and induces
Metallic Bi, as an environment-friendly alloy anode
material, is considered to be an ideal electroactive material
candidate for high-performance PIBs, owing to its cost
efficiency, good electronic conductivity, high theoretical
À1
capacity (ca. 385 mAhg ), large interlayer spacing for the
+
easy accommodation of K ions, and similar standard hydro-
gen potential (À2.93 V) with the lithium (À3.04 V) towards
[2]
high energy density. While, the inherent volume expansion
(
ca. 411% for PIBs) and electrode pulverization commonly
[
+]
[+]
[
*] Dr. Z. Sun, Dr. Y. Liu, Dr. J. Zhang, Dr. Y. Wang, Prof. L. Hou,
Prof. C. Yuan
lead to short cycling life and inferior rate performance of Bi
anodes. One effective strategy to address these issues is to
hybridize active Bi with flexible carbonaceous materials.
For instance, Su et al. reported the ultrathin carbon film@-
carbon nanorods@Bi nanoparticles (NPs) composite, which
[3]
School of Materials Science & Engineering, University of Jinan
Jinan, 250022 (P. R. China)
E-mail: mse_yuancz@ujn.edu.cn
[5–7]
ayuancz@163.com
À1
À1
[
+]
delivered an improved capacity (ca. 430 mAhg at 0.1 Ag )
Dr. W. Ye, Prof. M. Wang
À1
À1 [6]
State Key Lab of Physical Chemistry of Solid Surfaces,
College of Materials, Xiamen University
Xiamen, 361005 (China)
but poor rate behavior (140 mAhg at 1.0 Ag ). Cheng
and co-workers prepared three-dimensional (3D) porous
Bi@macroporous graphene framework composite electrode
that achieved a capacity of ca. 185 mAhg at 1.0 Ag along
with a modest cycling stability. Therefore, in spite of these
attractive advances, it is still a huge challenge to purposefully
design advanced Bi/C hybrids with exceptional electrochem-
ical behaviors in terms of high-rate cycling stability and
reversible capacities via smart optimization in microstruc-
tures and surfaces/interfaces towards PIBs. Especially note
that the underlying structure–performance relationship
E-mail: mswang@xmu.edu.cn
À1
À1
Dr. Y. Lin
[7]
Hefei National Laboratory for Physical Sciences at the Microscale,
University of Science and Technology of China
Hefei, 230026 (P. R. China)
E-mail: linyue@ustc.edu.cn
+
[
] Theses authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016082.
7
180
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should be further figured out for the Bi anodes as well with
a series of in situ techniques.
favor anchoring the formed bismuth atoms and prevent their
agglomeration over high-temperature calcination.
[
8]
With comprehensive considerations in mind, in the
contribution, we, to the best of our knowledge, first success-
fully fabricated Bi NPs encapsulated in a 3D foam-like N-
doped carbon nanocages (denoted as Bi@N-CNCs) frame-
work with a void space by precisely annealing a novel two-
dimensional (2D) Bi-based metal-organic framework (Bi-
MOF) precursor towards PIBs as an attractive anode. Thanks
to its unique architecture and composition, the optimized
Bi@N-CNCs anode was endowed with remarkable reversible
capacities and long-duration cyclic stability, especially at large
current densities. More significantly, the in-depth under-
standing of the intrinsic potassium storage behaviors of the
Bi@N-CNCs was reasonably proposed with various in situ
characterizations, which will guide the future design of high-
performance electrodes for next-generation PIBs.
Figure 1b demonstrates typical field-emission scanning
electron microscopy (FESEM) image of the 850-Bi@N-CNCs,
which is obtained by annealing the Bi-MOF at 8508C. It well
inherits the architecture of the Bi-MOF precursor without
noticeable calcination-induced structure collapse, which is
further evidenced by the low-magnification FESEM obser-
vation (Supporting Information, Figure S3a). It is worth
noting that the involved 2D NS subunits, like its precursor,
are evident with smooth surface (Supporting Information,
Figure S3b). Further transmission electron microscopy
(TEM) examinations visualize that the bismuth NPs of ca.
12 nm in size (Supporting Information, Figure S3c) are
uniformly and partially filled in the pore structure of the 2D
carbon skeleton constructed with the interconnected hollow
N-CNCs (Figure 1c,d; Supporting Information, Figure S3d).
Such characteristic hierarchical architecture can be vividly
supported by the reconstructed 3D tomograms (Figure 1e)
and corresponding rotation video of 3D reconstruction
Results and Discussion
(Supporting Information, Movie S1), obviously revealing the
In the work, the Bi@N-CNCs products were smartly
designed and fabricated via a two-step strategy including
solvothermal process and carbothermic reduction, as de-
scribed in Figure 1a. To the best of our knowledge, a novel
flower-shaped Bi-MOFassembled with 2D porous nanosheets
even distribution of these bismuth NPs in the pores of the
carbon framework, rather than simply attached on its surface.
Further high-magnification TEM inspection (Figure 1 f) also
visually confirms the location of ca. 13 nm Bi NPs in the
hollow N-CNCs with clear internal void space, that is, a typical
yolk–shell structure. Meanwhile, high-resolution TEM
(HRTEM) observation (Figure 1g), corresponding to the
magnified blue rectangular region in Figure 1 f, manifests the
well-defined lattice fringes with a spacing of about 0.39 nm,
larger than the (002) of classic graphite (0.335 nm), proving
(
NSs) with an average thickness of ca. 14 nm and a length of
up to tens of micrometers (Supporting Information, Figur-
es S1, S2) is first synthesized by using the glycol as the solvent,
along with the Bi(NO ) ·5H O and 1,3,5-Benzenetricarbox-
3
3
2
ylic acid (H BTC) ligand. Then, the Bi@N-CNCs is obtained
3
[9]
by calcinating the resultant Bi-MOF at a certain temperature
under Ar with the assistance of melamine as the nitrogen
source. The purposefully doped nitrogen into the CNCs will
that the elemental N is successfully doped in the CNCs.
Besides, the distinct lattice fringes with the spacing of ca.
0.33 nm are assigned to the interplanar distance of (012)
crystalline plane of the hexago-
nal Bi (JCPDS No.44-1246,
¯
space group R3m (166)). The
atomic resolution high-angle an-
nular dark-field scanning TEM
(
HAADF-STEM) image (the
inset in Figure 1h) of crystal face
012) demonstrates the arrange-
(
ment of bismuth atoms in
a unique rhombus as labeled
with the red circles, which is in
agreement with the crystallo-
graphic model of Bi (012) (Fig-
ure 1i). The STEM and corre-
sponding elemental mapping im-
ages (Figure 1j) confirm even
distributions of elemental N, Bi,
and C over the whole N-CNCs
network.
More interestingly, with the
annealing temperature dropping
to 5508C, although the NSs-con-
structed flower-like superstruc-
ture, similar to the 850-Bi@N-
CNCs, is retained well for the
Figure 1. a) Illustration for the synthesis processes of Bi@N-CNCs products. b) FESEM image;
c),d) TEM images; e) reconstructed 3D tomograms; f),g) high-magnification TEM images; h) HRTEM
image; i) crystallographic model of Bi (012); j) STEM and corresponding elemental mapping images of
the 850-Bi@N-CNCs. The insets in (f, h) correspond to the intensity profiles indicating the measured
interlayer spacing and atomic-resolution HAADF-STEM image of the red rectangular region, respectively.
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resultant 550-Bi@N-CNCs, sev-
eral insular bismuth particles of
hundreds nanometer in size are
attached to the surface of these
2
D NS building blocks (Support-
ing Information, Figure S4a,b).
Evidently, numerous Bi NPs with
an average size of ca. 18.2 nm are
inlaid in the carbon NSs (Sup-
porting Information, Figure S4c–
e), and especially, some isolated
Bi NPs are uniformly coated
with an amorphous N-doped car-
bon (or carbon nitride) shell
(
Supporting Information, Fig-
[
10]
ure S4d).
As for the case of
7
008C, no any obvious even
larger dissociated Bi can be
found, and just lots of Bi NPs
are anchored upon the surface of
Figure 2. a) XRD patterns and b) TG curves of 550-Bi@N-CNCs, 700-Bi@N-CNCs and 850-Bi@N-
CNCs; c) Raman spectrum of 850-Bi@N-CNCs; d) Pore size distribution plots of the three samples as
indicated. High-resolution XPS spectra of e) Bi 4f and f) N 1s of the 850-Bi@N-CNCs.
and/or in the obtained 700-
Bi@N-CNCs NSs (Supporting
[
11]
Information, Figure S5a-c), which is wholly distinguished
from the 550-Bi@N-CNCs and 850-Bi@N-CNCs. Addition-
ally, the size of these well-distributed Bi NPs located in the
carbon skeleton increases up to ca. 35.3 nm (Supporting
Information, Figure S6) when compare to that of 550-Bi@N-
CNCs. In sharp contrast, the as-fabricated 1000-Bi@N-CNCs
should be ascribed to its nanometer-scale size,
which is
consistent well with the aforementioned TEM analysis. The
specific contents of Bi in the 550-Bi@N-CNCs, 700-Bi@N-
CNCs and 850-Bi@N-CNCs are estimated as ca. 93.6, ca. 89.5,
and ca. 73.6 wt%, respectively, as calculated from TG plots
(Figure 2b; Supporting Information, Equation S1), consider-
ing the combustion of carbon materials and oxidation of pure
bismuth into Bi O3 at 350–6008C. The decrease in Bi
contents in Bi@N-CNCs with temperature increasing is
mainly attributed to the gradual evaporation of low-melting
Bi, similar to that of the antimony. It is for this reason that
the 850-Bi@N-CNCs is endowed with a specific surface area
of ca. 18.5 m g , higher than those of 550-Bi@N-CNCs (ca.
7.8 m g ) and 700-Bi@N-CNCs (ca. 9.7 m g ) (Supporting
Information, Figure S9), originating from its hierarchical
porosity with high-proportion tri-model micro-/meso-/mac-
ro-pores (Figure 2c), in good agreement with the TEM
observations (Figure 1c; Supporting Information, Fig-
ure S3d). Such unique pore feature effectively guarantees
convenient diffusion/transport of electrolyte ions to abundant
(
Supporting Information, Figure S7) shows typical 2D porous
[
7]
NSs structure (ca. 5 nm in thickness) with smooth surface and
interior nanocages, but contains a small amount of Bi species
maybe in the form of single atoms, which can be well
authenticated by its X-ray diffraction pattern (Supporting
Information, Figure S8). Accordingly, the evolution from the
Bi-MOF to Bi@N-CNCs samples coupled with the size
variation of metallic Bi in the temperature range from 550
to 10008C, as discussed above, rationally derives the forma-
tion mechanism of the Bi@N-CNCs. It probably comprises
the initial pyrolysis, Ostwald ripening, evaporation and
2
[12]
2
À1
2
À1
2
À1
[
8]
further atomization, similar to the report by Li et al.
Furthermore, it is the unique formation process that makes
the Bi NPs in the marginal location of the 850-Bi@N-CNCs
vanish more quickly than those in the central region (Fig-
ure 1c; Supporting Information, Figure S3b). With the syner-
gistic contributions from Bi evaporation/sublimation, sponta-
neous adsorption and N-involved anchoring effect, the unique
architecture, that is, Bi NPs encapsulated in the hollow N-
CNCs with a void space, is obtained for the case of 850-Bi@N-
CNCs.
+
active sites of the 850-Bi@N-CNCs for K storage. The
existence of carbon material in the composites can be well
supported by the Raman spectra (Figure 2d; Supporting
Information, Figure S10), where the characteristic peaks
centering at ca. 1235, ca. 1342, ca. 1490, and ca. 1592 cm
belong to the I, D, D’’, and G bands, respectively.
Particularly, besides the low-content D’’ band, the I /I value
À1
[9]
D
G
To further investigate the phase structure, compositions,
porosity and surface valence of the products, various charac-
terizations including XRD, thermal gravimetric (TG) analy-
(ca. 0.89) of the 850-Bi@N-CNCs, calculated with the
integrated areas of the D and G bands, is smaller than those
of the 550-Bi@N-CNCs (ca. 1.9), 700-Bi@N-CNCs (ca. 0.93),
and 1000-Bi@N-CNCs (ca. 1.2), which further highlights the
higher graphitization of the N-CNCs in the 850-Bi@N-CNCs,
favoring for the rapid transport of electrons in the conductive
carbon framework. Further XPS survey spectrum confirms
the co-existence of the C, Bi, and N species in the 850-Bi@N-
CNCs, along with a N/C atomic content of ca. 4.7%
(Supporting Information, Figure S11). The high-resolution
sis, N sorption, Raman, and X-ray photoelectron spectros-
2
copy (XPS) were conducted in detail. As illustrated in
Figure 2a, all the reflections in the XRD patterns can be
successfully indexed as the hexagonal Bi with a space group of
¯
R3m (166) for the three samples without any other discern-
able Bi-based impurities. Especially, the broadened and low-
intensity (012) peak (2q = 27.28) of the 850-Bi@N-CNCs
7
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[18]
spectrum of Bi 4f (Figure 2e) shows two characteristic peaks,
ly. The similar CV curves are clearly observed as well for
the 550-Bi@N-CNCs (Supporting Information, Figure S12a)
and 700-Bi@N-CNCs (Supporting Information, Figure S12b),
indicating the identical potassium-storage mechanism of the
three. Notably, the cathodic/anodic peaks of the 850-Bi@N-
CNCs after the first CV cycle well keep their shapes and
almost overlap, implying its high reversibility of the involved
reactions. Figure 3b profiles the selected discharge-charge
voltage plots of the 850-Bi@N-CNCs anodes. These discerned
discharge/charge platforms are in good agreement with the
CV observations (Figure 3a), and particularly show no
obvious difference in the voltage curves and capacity loss
with cycling from the 2nd to 300th cycles, which authenticates
the superior electrochemical reversibility of the 850-Bi@N-
CNCs electrode for efficient potassium storage. Furthermore,
that is, 4f7/2 (ca. 159.1 eV) and 4f (ca. 164.5 eV) orbitals of
5/2
[13]
metal-phase Bi. The N 1s spectrum (Figure 2 f) can be spilt
into three peaks at ca. 398.8, ca. 400.7, and ca. 401.7 eV, which
are related to the pyridinic-N, pyrrolic-N, and graphitic
[14]
nitrogen, respectively. The former two, with a total content
of ca. 2.9 at.%, favors for the enhancement of surface-
+
[15]
controlled capacitance, and rapid diffusion of K ions, and
the latter (graphitic nitrogen) is beneficial to electronic
[16]
transport in the hybrid anode.
Thanks to the remarkable structural/compositional ad-
vantages mentioned above, the as-fabricated 850-Bi@N-
CNCs can be highly anticipated with competitive electro-
chemical potassium storage properties for PIBs. Figure 3a
shows the initial cyclic voltammetry (CV) curves of the 850-
À1
À1
Bi@N-CNCs at a scanning rate of 0.1 mVs . During the first
a large initial charge capacity of ca. 321.7 mAhg can be
cathodic (potassiation) process, the strong peak located at
delivered by the anode along with an initial Coulombic
efficiency (CE) of ca. 78.0%, corresponding to a low irrever-
sible capacity of ca. 90.8 mAhg due to the formation of SEI
+
0
.33 V (vs. K /K) is assigned to the formation of the solid
À1
electrolyte interface (SEI) layer, and the small shoulder peak
+
located at 0.28 V is related to the alloy reaction between K
film. Noticeably, the first CE value of the 850-Bi@N-CNCs is
somewhat higher than other reported Bi-based anodes
[
5]
and metallic bismuth. Upon the first depotassiation process,
three anodic peaks appearing at approximately 0.47, 0.52, and
[3,5–7,17,19–24]
(Supporting Information, Table S1).
Upon the
1
.13 V should be attributed to typical three-step dealloying
second discharge-charge cycle, the CE is increased rapidly
up to ca. 95.9%, coupled with a large charge capacity of ca.
[17]
reactions (namely, K Bi!K Bi !KBi !Bi). In the sub-
3
3
2
2
À1
sequent 3rd and 5th discharge processes, three stable cathodic
peaks centering at 0.93, 0.44, and 0.34 V are related to the
alloying process of Bi to KBi , K Bi , and K Bi, respective-
328.7 mAhg , which is even higher than that of the 1000-
À1
Bi@N-CNCs anode (ca. 104.6 mAhg ) (Supporting Infor-
mation, Figure S13a), revealing that metallic Bi is the real
electroactive phase for the hy-
brid anode.
2
3
2
3
Figure 3c collects the rate
behaviors of the three anodes
as indicated within the wide
current density range from 0.5
À1
to 10 Ag . Competitively, the
8
50-Bi@N-CNCs electrode ex-
hibits reversible capacities of
ca. 334.3, ca. 321.8, ca. 312.7, ca.
À1
2
59.7, and ca. 235.5 mAhg at
current densities of 0.5, 1.0, 2.0,
À1
5
.0, and 10.0 Ag , correspond-
ing to a capacity retention of ca.
0.4%. While, with a 20-time
7
increase in current rate from 0.5
À1
to 10 Ag , the 550-Bi@N-
CNCs, 700-Bi@N-CNCs, and
1
000-Bi@N-CNCs (Supporting
Information, Figure S13b) just
obtain ca. 284.5/ca. 170.1, ca.
3
9
18.4/ca. 201.1, and ca. 104.2/ca.
2.8 mAhg at 0.5/10 Ag , re-
À1
À1
spectively. The serious capacity
degradations are evident for the
5
50-Bi@N-CNCs (ca. 40.2%)
and 700-Bi@N-CNCs (ca.
6.9%). Particularly, the largest
À1
Figure 3. a) Initial three CV profiles (0.1 mVs ) and b) selected galvanostatic charge-discharge voltage
3
À1
plots (1.0 Ag ) of the 850-Bi@N-CNCs. c) Rate behaviors of the three Bi@N-CNCs anodes.
capacities of the 850-Bi@N-
CNCs with the lowest Bi content
at the same rates reveal the
highest electrochemical utiliza-
d) Comparison in rate capabilities of 850-Bi@N-CNCs with other reported alloy anodes for KIBs in the
À1
literature. e) Cycling behaviors (1.0 Ag ) of the three Bi@N-CNCs anodes as indicated. f) Long-term
À1
À1
cycling performance and CE values of the 850-Bi@N-CNCs at 5.0 Ag after activation at 1.0 Ag for
0 cycles.
2
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+
tion of active Bi for high-efficiency K storage. Additionally,
porting Information, Figure S20) are both only ca.
À1
À1
À1
with the current rate switched back to 1.0 Ag , a reversible
capacity of ca. 332.2 mAhg still can be achieved by the 850-
Bi@N-CNCs, and further remains stable for a long cycle
period, which is apparently superior to those of the 550-
Bi@N-CNCs (ca. 241.5 mAhg ), 700-Bi@N-CNCs (ca.
03.6 mAhg ), and 1000-Bi@N-CNCs (ca. 96.4 mAhg )
Supporting Information, Figure S13b). It is particularly
worth mentioning that the rate behaviors of the 850-Bi@N-
CNCs are comparable to, and even better than other reported
183 mAhg over 500 cycles at 5.0 Ag , corresponding to
capacity degradations of ca. 0.044% and ca. 0.041% per cycle
for the two, respectively. Furthermore, the 1000-Bi@N-CNCs
À1
electrode only has
a
low reversible capacity of ca.
À1
À1
À1
79.1 mAhg
after 1000 cycles at 5.0 Ag
(Supporting
À1
À1
3
Information, Figure S21). One especially notes that the
high-rate cycling properties of the 850-Bi@N-CNCs anode
evidently surpass other reported Bi-based anodes (Support-
(
[3,5–7,17,19–24]
ing Information, Table S1).
[
5,17,21,25–30]
alloy-type anodes,
as compared in Figure 3d, for
) of ca.
To further figure out the superior structural stability
enabled by sufficient void design in the case of 850-Bi@N-
CNCs, in situ TEM investigations into the microstructure/
morphology evolution over the electrochemical potassiation/
depotassiation processes were performed. Figure 4a shows
the in situ TEM nano battery set-up, which is composed of the
electroactive Bi@N-CNCs and potassium metal counter
+
which its exceptional K ion diffusion coefficients (D
þ
K
À7
À2 À1
1
0
cm
s
(Supporting Information, Figure S14) and su-
perb electronic conductivity (Figure 2d) should account.
Furthermore, the 850-Bi@N-CNCs anode displays re-
markable cycling performance. As illustrated in Figure 5e, ca.
9.6% of the second capacity, that is, ca. 327.5 mAg , still
À1
9
can be retained after 300 consecutive cycles at a current
density of 1.0 Ag . In contrast, the faster capacity decay can
electrode, coupled with the K O layer grown on the surface
of metallic potassium as solid electrolyte in the cell. Once
2
À1
[29]
be observed for both the 550-Bi@N-CNCs and 700-Bi@N-
CNCs, along with capacity degradation of ca. 67.8% and ca.
the contact between two electrodes was built, an appropriate
bias of 2.0 V was applied to initiate the involved electro-
chemical reactions. Figure 4b–d present the time-resolved
TEM images of the 850-Bi@N-CNCs during consecutive
potassiation process, which can be further visualized clearly
from the video (Supporting Information, Movie S2). Clearly,
9
0.2%, respectively. Although relatively stable capacities can
be obtained by the 1000-Bi@N-CNCs, a low capacity of ca.
À1
9
0.4 mAhg is just maintained (Supporting Information,
Figure S15). The rigid structure integrity of the 850-Bi@N-
CNCs over the uninterrupted cycle is reasonably responsible
for its superior cyclic behaviors, benefiting from the collab-
orative contributions from nano-dimensional Bi and inherent
void space in the rigid N-CNCs skeleton with exceptional
electronic conductivity (Supporting Information, Fig-
ure S16a). Obviously, after repeated alloying/dealloying pro-
cesses, the ultra-small Bi NPs are still perfectly wrapped with
the N-CNCs (Supporting Information, Figure S16b–d), and
the intact 3D hybrid architecture with accordant elemental
distributions can be well retained (Supporting Information,
Figure S17). The expanded lattice space of ca. 0.40 nm for the
N-CNCs is conducive to the convenient transport of potas-
sium ions (Supporting Information, Figure S16d). While, as
for the 550-Bi@N-CNCs (Supporting Information, Fig-
ure S18a–c) and 700-Bi@N-CNCs (Supporting Information,
Figure S18d–f) electrodes, owing to the serious volume
change of electrodes in each cycle, the contact form of the
Bi NPs will inevitably change, resulting in the ongoing
formation of an unstable SEI layer on its surface. As a result,
the significant capacity decay is observed for the two during
+
the K is first driven to penetrate the N-CNCs framework,
and then, the alloying process of Bi with K occurs along with
the noticeable volume expansion. With the continuous
+
insertion of K , the Bi NPs are gradually potassiated
accompanied with size expansion from ca. 26 to ca. 39 nm
after potassiation for 240 s, as indicated by red circles. The
interior void of hollow N-CNCs was fully filled by the
expanded Bi NPs. If the NPs were assumed as the regular
spheres with isotropic volume expansion, the 3D volume
expansion is estimated to be ca. 338% for the case, while the
2D projection width of the hierarchical carbon framework
expands from ca. 195 to ca. 201 nm, confirming the rigid
structure stability of the conductive carbon skeleton. During
the following depotassiation process driven by employing
a reversed bias of À2.0 V, the whole Bi-K alloy is gradually
de-alloyed and finally recovers to Bi NPs, as presented in
Figure 4e–g. Apparently, the elastic shrinkage of the N-CNCs
on the 2D projection to its original width of ca. 191 nm is
observed, along with the discerned Bi NPs of ca. 22 nm in size
(Figure 4g). After the first charge-discharge cycle, the 850-
Bi@N-CNCs almost restores their original state (Supporting
Information, Movie S2). Moreover, after 20 potassiation/
depotassiation cycles, the well-defined 850-Bi@N-CNCs still
can maintains its structural integrity without noticeable
structure rupture (Supporting Information, Movie S3), which
authenticates that the internal void space and elastic shell of
N-CNCs can effectively relieve the volume expansion of Bi
NPs. By contrast, the bismuth NPs in the 550-Bi@N-CNCs
(Supporting Information, Figure S22; Movie S4) and 700-
Bi@N-CNCs (Supporting Information, Figure S23; Movie S5)
are both broken and seriously agglomerated after the depot-
assiation process. The comparative real-time observations
here visualize the superior structural reversibility of the 850-
Bi@N-CNCs, which ensures the formation of a stable yet thin
3
00 cycles. The 1000-Bi@N-CNCs anode well maintains its
unique 3D porous architecture (Supporting Information,
Figure S18g–i) owing to the absence of alloying/dealloying
reactions, which is reasonably responsible for its appealing
electrochemical stability. More impressively, the 850-Bi@N-
CNCs hybrid anode shows long-duration cycle stability
especially at high rates, as illustrated in Figure 3 f. Remark-
À1
ably, after 20-cycle activation at 1.0 Ag , the capacity of the
À1
8
50-Bi@N-CNCs electrode remains as ca. 224 mAhg over
further 1200 consecutive cycles at a high current density of
À1
5
.0 Ag , indicating a superb capacity retention of ca. 95.3%,
along with an average reversible capacity loss of 0.004% per
cycle. The specific capacities of the 550-Bi@N-CNCs (Sup-
porting Information, Figure S19) and 700-Bi@N-CNCs (Sup-
7
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as shown in Figure 5b. After full
potassiation, all the diffraction
signals are well indexed into the
hexagonal K Bi with a space
3
group of P63/mmc (194) (Fig-
ure 5c). Therefore, the phase
transition of electroactive bis-
muth in the first potassium pro-
cess of the 850-Bi@N-CNCs is as
follows: Bi-KBi -K Bi, indicat-
2
3
ing the two-step alloying reac-
tion for our case here. Corre-
sponding ex situ TEM and SA-
ED observations (Supporting In-
formation, Figure S24) further
corroborate that the final potas-
siation product of metal Bi is the
K Bi phase indeed. Correspond-
3
ing crystal lattice structure evo-
lution of the 850-Bi@N-CNSs is
schematically shown in Fig-
ure 5d for more intuitively un-
derstanding of the two-step al-
loying potassiation during the
first discharge process.
To further authenticate the
two-step alloying reactions in-
+
volved K -storage mechanism of
our 850-Bi@N-CNCs electrode,
in situ XRD was conducted as
well. As manifested in Figure 5e,
the initial discharge profile of
the 850-Bi@N-CNCs shows
a long slope. In the initial dis-
charge stage, the intensities of
typical reflections (2q = 27.2,
Figure 4. a) Illustration of the in situ TEM nanobattery device. b)–g) Time-resolved TEM images of the
8
50-Bi@N-CNCs during (b–d) potassiation and (e–g) depotassiation. h) Description of the potassia-
tion/depotassiation processes of the Bi@N-CNCs electrodes with Bi particles of different size confined
in the carbon nanocages. The size changes of the N-CNCs and Bi particles were measured and
indicated by red arrows.
3
7.9 and 39.68) for the metallic
Bi gradually decrease. Then,
with the voltage dropping down
[31–34]
SEI layer on the outer robust N-CNCs,
and a close
to ca. 0.4 V, two new peaks arise at 31.1 and 32.58, which can
electronic contact between the continuous carbon network
be indexed to cubic KBi (JCPDS No. 03-0698). When the
discharge voltage further decreases to ca. 0.2 V, the peaks of
2
[35–37]
and Bi NPs meantime.
While for the 550-Bi@N-CNCs or
7
00-Bi@N-CNCs, the repetitive rupturing and reformation of
Bi and KBi still exist. Meanwhile, two new diffraction peaks
2
the passivating SEI film occur, and always convert the
cyclable K species in electrode sur-/interfaces to dead K in
the SEI, eventually leading to the death of the batteries due to
the K exhaustion.
Coupled with the in situ TEM observations, in situ
selected area electron diffraction (SAED) is further em-
ployed for in-depth understanding of the phase evolution over
the alloying process of the 850-Bi@N-CNCs during the first
discharge process (Supporting Information, Movie S6). Typ-
ical SAED pattern of the fresh 850-Bi@N-CNCs show a strong
diffraction ring, corresponding to the (012) plane of the well-
crystallized Bi NPs (Figure 5a). In the early stage of
potassiation reaction, the diffraction rings related to the
related to the hexagonal K Bi (JCPDS No. 04-0642) emerge
at 2q = 28.9 and 29.58, accompanied by the complete disap-
3
pearance of characteristic signals from Bi and KBi . There-
2
fore, the two-step alloying reactions involved in the 850-
Bi@N-CNCs can be well supported by both in situ SAED and
XRD observations. One should particularly note that the
observation here is somewhat distinguished from other
previous reports about Bi-based anodes in the same electro-
lyte systems, where the three-step or one-step alloy reactions
[5,18]
take place.
Such unique difference is probably ascribed to
specific microstructures, that is, nanoscale Bi NPs (< 20 nm)
confined in the 3D porous carbon framework, which shows
enormous influence on the surfaces/interfaces and phase
transition of active Bi species, and verifies that the potassia-
tion process of Bi anodes is mainly controlled by kinetics
¯
(
(
400)/(511) planes of cubic KBi with a space group of Fd3m
2
227) appear, which evidences that bismuth NPs are partially
[
31]
[5]
alloyed to form KBi due to insufficient potassium supply,
rather than the thermodynamics system. The in situ XRD
2
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cathode/anode is designed as
around 1:1.2 to guarantee that
the cathode is the capacity-limit-
ing electrode. Corresponding dif-
ferential capacity vs. voltage pro-
À1
files (dQ dV ) of the two electro-
des reveal an average operating
voltage of ca. 3.01 V for the
unique
850-Bi@N-CNSs//KFe-
[
Fe(CN) ] device (Supporting In-
6
formation, Figure S26c). The
average operating voltage of the
potassium ion full battery is ca.
2
.2 V, with an initial discharge
À1
capacity of ca. 78.7 mAhg at
0
À1
.1 Ag
(Supporting Informa-
tion, Figure S26d). It should be
noted that the device displays
large energy densities of about
À1
1
2
70–105 Whkg (from the 1st to
10th cycle) calculated based on
the total mass of cathode and
anode materials, along with high
CE values from ca. 94.1 to ca.
9
8.7% (Supporting Information,
Figure S26d). This corroborates
that the 850-Bi@N-CNSs hold
an enormous promise towards
[38–40]
next-generation PIBs
attractive anode.
as an
Conclusion
Figure 5. Time-lapse diffraction patterns revealing the phase evolution of the 850-Bi@N-CNCs in
reaction with K: a) pristine Bi NPs in the N-CNCs; b) Bi alloyed with K in the early stage to form KBi2;
In the work, we first devised
a simple yet efficient two-step
method including solvothermal
process and subsequent calcina-
tion to controllably synthesize
c) formation of K Bi upon further alloyed with K. d) In situ XRD analysis of the 850-Bi@N-CNCs upon
3
the first galvanostatic potassiation/depotassiation process: contour plot (the right) and line plot (the
middle), and corresponding discharge/charge curves (the left). e) Illustration of crystalline structure
evolution during the first potassiation process.
examination further demonstrates that the well-defined K Bi
nano Bi encapsulated in 3D porous N-CNCs framework with
an appropriate void space by using a novel Bi-based MOF as
the precursor. With attentive regulation of calcination
temperatures, the underlying formation processes of the
Bi@N-CNCs products were proposed, and the optimized 850-
Bi@N-CNCs exhibited exceptional electrochemical potassi-
um-storage properties both in half-cell and full-cell systems,
thanks to its unique structure/composition merits. With the
synergistic contributions from 3D conductive N-CNCs net-
work and nanoscale effect of active Bi NPs, typically, the 850-
Bi@N-CNCs was optimized and endowed with large rever-
3
phase can be transformed to the pure metal bismuth over the
first charge process via a typical three-step way, correspond-
ing to the three identifiable platforms, which is slightly distinct
+
from the initial discharge counterpart. Typically, the K
extraction process of K Bi first happens at ca. 0.55 V, and
3
[19]
results in the formation of K Bi .
Then, further depot-
3
2
assiation process of K Bi produces the KBi phase at ca.
3
2
2
0
.7 V. Finally, pure bismuth recovers once again with the
disappearance of KBi at ca. 1.10 V.
2
To explore potential applications of the 850-Bi@N-CNSs
towards commercial PIBs, a full PIB device was assembled by
À1
sible capacities of ca. 334.3 and ca. 221.3 mAhg at current
[32]
À1
using the Prussian blue analogue of KFe[Fe(CN)6] (Sup-
porting Information, Figure S25) as cathode material and pre-
potassiated 850-Bi@N-CNSs as the anode (Supporting In-
densities of 0.5 and 10.0 Ag , respectively, along with a long-
term cycling stability of ca. 0.004% capacity degradation per
À1
cycle over 1200 cycles at 5.0 Ag . Besides, the assembled
formation, Figure S26a). The obtained KFe[Fe(CN) ] can
provide a stable reversible capacity of ca. 70 mAhg along
850-Bi@N-CNCs//KFe[Fe(CN) ] full cell displayed a large
energy densities of ca. 170 Whkg . More importantly, a series
6
6
À1
À1
with a high CE value of ca. 98.8% within the potential range
of in situ characterizations including TEM, SAED and XRD
were comprehensively conducted to shed light on in-depth
understanding of the intrinsic electrochemical K -storage
+
À1
from 2.0 to 4.5 V (vs. K/K ) at 0.1 Ag
(Supporting
+
Information, Figure S25b, S26b). The capacity ratio of
7
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behaviors of the 850-Bi@N-CNCs anode. More significantly,
we strongly believe that our methodology here is highly
universal to versatile fabrication of other Bi-based alloys
towards high-performance secondary batteries, and other
energy-related application.
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