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Academic Editor: Shijun Liao
Received: 27 November 2024
Revised: 17 January 2025
Accepted: 26 January 2025
Published: 6 February 2025
Citation: Chu, H.; Li, Y.; Liu, Y.; Chai,
X.; Zhang, H.; Zhang, J. Synergistic
Effect of Anionic-Tuning and
Architecture Engineering in BiPO4@C
Anode for Durable and Fast Potassium
Storage. Molecules 2025,30, 729.
https://doi.org/10.3390/
molecules30030729
Copyright: © 2025 by the authors.
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Article
Synergistic Effect of Anionic-Tuning and Architecture
Engineering in BiPO4@C Anode for Durable and Fast
Potassium Storage
Heying Chu , Yong Li, Yuanjie Liu, Xueping Chai, Hongzhou Zhang * and Jingchuan Zhang *
College of Mechanical and Electronic Engineering, Tarim University, Alar 843300, China;
chuheying@taru.edu.cn (H.C.); deyuzhijia@163.com (Y.L.); 120050023@taru.edu.cn (Y.L.);
haixueping@taru.edu.cn (X.C.)
*Correspondence: shenzhouxing@taru.edu.cn (H.Z.); zhangjingchuan@taru.edu.cn (J.Z.)
Abstract: Bismuth-based materials that adhere to the alloy/dealloy reaction mechanism
are regarded as highly promising anode materials for potassium-ion batteries due to
their high volume-specific capacity and moderate reaction potentials. However, their
commercial viability has been limited by the effects of structural collapse due to volume
distortion and impeded electron conduction, resulting in rapid capacity decline. In this
work, a carbon-coated nanosized BiPO
4
rod (BiPO
4
@C) was designed and fabricated to
overcome the aforementioned challenges through the architecture engineering and anionic-
tuning strategy. In particular, the nanosized nanorods significantly reduce the volume
expansion; the incorporation of the bulk and open-skeleton anion PO
43−
serves to mitigate
the considerable volume distortion and generates the high ionic conductivity product
(K
3
PO
4
) to ameliorate the poor ionic transport due to the structural deformation. The
elaborated BiPO
4
rods exhibit high specific capacity (310.3 mAh g
−1
, at 500 mA g
−1
),
excellent cycling stability (over 700 cycles at 500 mA g
−1
) and superior rate performance
(137.8 mAh g
−1
, at 1000 mA g
−1
). Systematic ex-situ XRD and TEM, as well as kinetic
tests, have revealed the “conversion-multistep alloying” reaction process and the “battery-
capacitance dual-mode” potassium storage mechanism. Moreover, the thick electrodes
showed excellent specific capacity and rate performance, demonstrating their significant
application potential in the next generation of SIBs.
Keywords: bismuth phosphate; nanostructures; architecture engineering; anionic-tuning;
potassium-ion batteries
1. Introduction
The contradiction between the limited lithium resources (0.0017 wt% in the earth’s
crust) and the rapidly expanding energy storage and electric vehicle markets has consid-
erably constrained the further development of lithium-ion batteries (LIBs) [
1
]. Therefore,
the development of new energy storage devices with low-cost and high-specific energy is
an effective means to accelerate the development of the energy storage market. The abun-
dance of potassium resources (2.09% in the earth’s crust), the maturity of the processing
technology, the low electrochemical potential (K
+
/K:
−
2.93 V vs. E
o
), and the well-known
rocking chair charging/discharging mechanism have collectively positioned potassium-ion
batteries (PIBs) as a promising next-generation “beyond Li-ion” battery [
2
,
3
]. However, the
process of matching the large radius of K
+
(1.38 Å) with conventional electrode materials
leads to problems such as slow ion migration and high volume strain [
4
,
5
]. Consequently,
Molecules 2025,30, 729 https://doi.org/10.3390/molecules30030729
Molecules 2025,30, 729 2 of 14
there is an urgent requirement to develop anode materials with a robust structure and a
high ability to tolerate volume strain, as well as to obtain electrode materials with ultra-long
cycle stability and rapid potassiation/depotassiation capabilities.
In terms of anode materials, the potassium storage capacity and cycling stability of
graphite are much lower than that of commercial LIBs due to the drastic volume distor-
tion that occurs during K
+
insertion/extraction [
6
]. Metal-based materials (e.g., Sn [
7
,
8
],
Sb [
9
,
10
], Bi [
11
,
12
], and binary/ternary alloys [
13
,
14
]) capable of alloying with K
+
have
attracted considerable attention due to their high conductivity, considerable theoretical
specific capacity and safe redox potential. Among these materials, layered bismuth shows
promising prospects for K
+
-storage due to its advantageous characteristics, including a
high volumetric theoretical specific capacity (385 mAh g
−1
), non-toxicity, high electri-
cal conductivity, large lattice spacing, and moderate discharge voltage [
15
]. However,
the gradual alloying process with K
+
inevitably causes significant volume strain (390%),
which further leads to microstructural collapse and rapid deterioration of cycling perfor-
mance [
16
]. Many strategies have been attempted to overcome the above problems, such as
compositing with carbon media to mitigate the volume strain exerted on the active particles
and reduce particle fragmentation [
17
,
18
], Construction of special micro-nanostructures
to accommodate volume expansion and contraction through the incorporation of buffer
spaces to avoid rupture of the outer stable solid electrolyte interphase (SEI) films [
19
,
20
];
Anion auxiliary strategies (Bi
2
O
3
[
21
], Bi
2
S
3
[
22
], Bi
2
Se
3
[
23
], Bi
2
O
2
Se [
24
], Bi
2
Se
3−x
Se
x
[
25
])
to prevent agglomeration of metal elements and relieve structural stresses anchored by
bonding with anions. Undeniably, the aforementioned subtle enhancement strategies
have largely mitigated the damage to the electrode structure caused by volume distortion
and significantly improved the cycling stability of Bi-based electrodes. Nevertheless, the
scalability of this intricate process remains a significant challenge, and the difficulty of
disrupting the electron conduction path during volume expansion and contraction has yet
to be overcome. Furthermore, the poor conductivity of the discharge products of Bi-based
chalcogenides and the strong evolution and shuttle effects (K
2
S and K
2
Se) in the anion tun-
ing process are often overlooked, which are again the culprits for the capacity degradation
of Bi-based electrodes.
Herein, a carbon-coated nanosized BiPO
4
rod (BiPO
4
@C) was synthesized by a simple
hydrothermal method to mitigate the drastic alloying reaction and improve ionic conductiv-
ity. Specifically, the small-sized nanorods reduce the transport distance of K
+
and electrons
within the solid phase, eliminate the size dependence, and mitigate the volume distortion.
The introduction of the large-sized PO
43−
with a highly thermally stable and open P-O
octahedral framework structure serves to buffer the significant volume distortion of the Bi
anode during the alloying/dealloying process. Crucially, the discharge product (K
3
PO
4
)
is generated in situ with high ionic conductivity, which boosts the electronic conductivity
in damaged structures. Furthermore, the carbon coating improves the conductivity of
the phosphate electrode and acts as an armor against structural degradation. As a conse-
quence of these properties, the BiPO
4
@C electrode exhibits high specific capacity, excellent
cycling stability and rate performance. Post-mortem analysis demonstrates the conversion-
multistep alloying reaction mechanism. The kinetic tests verify the “battery-capacitor”
dual-mode potassium storage behavior. Therefore, the synergistic strategy of anion tuning
and nanosize effect in this work provides a promising avenue for the development of rapid
and stable anodes for PIBs.
Molecules 2025,30, 729 3 of 14
2. Results and Discussion
2.1. Synthesis and Structural Characterization
Figure 1a illustrates a facile synthetic approach for BiPO
4
@C nanorod via a one-step
hydrothermal process, in which Bi(NO
3
)
3
and NH
4
H
2
PO
4
were employed as the sources
of Bi and P, respectively, while ethylene glycol (EG) was served as both the reaction
medium and the carbon source. According to the literature, Bi
3+
ions in solutions tend
to be hydrolyzed to produce various hydroxides, depending on the composition of the
solution [26]. Meanwhile, under mild heating, NH4H2PO4molecules can be decomposed
to form H
3
PO
4
. [
27
,
28
] Subsequent high temperature and pressure processes lead to the
formation of BiPO
4
nanorods. Based on the above analyses, we believe that the following
reactions occur in the water/ethylene glycol mixture:
Bi(NO3)3+ 3H2O→Bi(OH)3+ 3HNO3(1)
NH4H2PO4→H3PO4+ NH3↑(2)
Bi(OH)3+ H3PO4→BiPO4+ 3H2O (3)
Molecules 2025, 30, x FOR PEER REVIEW 4 of 14
Figure 1. (a) Schematic of the fabrication process of BiPO
4
@C composite. (b) SEM, (c,d) TEM, (e)
HRTEM images, and corresponding (f) HADDF and EDS mapping images of the obtained BiPO
4
@C.
X-ray diffraction (XRD), Raman spectra, Fourier transform infrared spectroscopy
(FTIR), and X-ray photoelectron spectroscopy (XPS) were used to determine the physical
phase and structural information of BiPO
4
@C. As shown in Figures 2a and S4 and Table
S1, the XRD results indicated that the diffraction peaks of BiPO
4
@C were consistent with
the pure phase of monoclinic BiPO
4
(PDF# 04-010-5606). Subsequently, Raman spectra
(Figure 2b) monitored a set of characteristic absorption peaks in the range of 300–1700
cm
−1
, wherein the peaks located at 395, 543, and 600 cm
−1
can be ascribable to PO
43−
, while
the peaks appearing at 972 and 1026 cm
−1
can ascribe to the symmetric (υ1) and asymmet-
ric (υ2) scaling modes of PO
43−
, respectively [29]. Two other characteristic peaks that ap-
pear at 1356 and 1587 cm
−1
can be attributed to disordered graphitic carbon (D-bond) and
sp
2
hybridized graphic carbon (G-bond) of the C component, respectively. Obviously, a
lower intensity ratio (I
D
/I
G
) value (0.37) for BiPO
4
@C indicates a regular graphitic structure
and high electrical conductivity, which can significantly improve the poor electrical con-
ductivity inherent in phosphate materials. Moreover, the FTIR spectra revealed the pres-
ence of chemical bonding information within the wavenumber range of 500 to 3700 cm
−1
for BiPO
4
@C composites (as shown in Figure 2c). The results indicated that the absorption
peaks at 530 and 601 cm
−1
were indicative of the vibrational absorption of δ(PO
43−
), while
the absorption peak at 1072 cm
−1
was attributed to the υ3 asymmetric stretching vibration
of the P-O bond [30,31]. The three absorption peaks situated between 2750 and 2950
−1
are
ascribed to the symmetric and antisymmetric stretching vibrations of the C−H bond in the
−CH
3
, while the vibrational absorption peaks of the benzenoid ring skeleton appear
Figure 1. (a) Schematic of the fabrication process of BiPO
4
@C composite. (b) SEM, (c,d) TEM,
(e) HRTEM
images, and corresponding (f) HADDF and EDS mapping images of the obtained
BiPO4@C.
Molecules 2025,30, 729 4 of 14
The microscopic morphology of BiPO
4
@C was investigated by scanning electron
microscopy (SEM) and transmission electron microscopy (TEM). First, the effects of raw
material concentration, reaction time and reaction temperature on the morphology of
BiPO
4
@C composites were systematically investigated. As shown in Figures S1–S3, the
diameter and length of BiPO
4
@C nanorods increase with increasing reaction concentration
and reaction temperature, while the reaction time significantly restricts the morphology
of the material. Taking all factors into consideration, we set the optimum raw material
concentration, reaction time and temperature to 0.5 mmol, 160
◦
C, and 12 h, respectively.
As shown in Figure 1b,c, BiPO
4
@C exhibits a nanosized rod-like structure with a length of
approximately 50–80 nm and a diameter of 15–20 nm. Interestingly, as shown in Figure 1d,
each BiPO
4
nanorod exhibits a monocrystal structure with well-defined lattice fringes, and
the surface of the nanorods is encapsulated with an ultra-thin carbon layer. Furthermore,
the high-resolution TEM (HRTEM) image in Figure 1e clearly shows a set of periodic lattice
fringes with a spacing of 0.421 nm, corresponding to the (
−
111) plane of monoclinic BiPO
4
(PDF# 04-010-5606). Meanwhile, the energy dispersive spectroscopy (EDS) mapping results
indicate that the elements Bi, P and O are distributed uniformly in the BiPO
4
@C nanorod
composites (as shown in Figure 1f).
X-ray diffraction (XRD), Raman spectra, Fourier transform infrared spectroscopy
(FTIR), and X-ray photoelectron spectroscopy (XPS) were used to determine the physical
phase and structural information of BiPO
4
@C. As shown in Figures 2a and S4 and
Table S1,
the XRD results indicated that the diffraction peaks of BiPO
4
@C were consistent with
the pure phase of monoclinic BiPO
4
(PDF# 04-010-5606). Subsequently, Raman spectra
(Figure 2b) monitored a set of characteristic absorption peaks in the range of
300–1700 cm−1,
wherein the peaks located at 395, 543, and 600 cm
−1
can be ascribable to PO
43−
, while the
peaks appearing at 972 and 1026 cm
−1
can ascribe to the symmetric (
υ
1) and asymmetric
(
υ
2) scaling modes of PO
43−
, respectively [
29
]. Two other characteristic peaks that appear
at 1356 and 1587 cm
−1
can be attributed to disordered graphitic carbon (D-bond) and sp
2
hybridized graphic carbon (G-bond) of the C component, respectively. Obviously, a lower
intensity ratio (I
D
/I
G
) value (0.37) for BiPO
4
@C indicates a regular graphitic structure and
high electrical conductivity, which can significantly improve the poor electrical conductivity
inherent in phosphate materials. Moreover, the FTIR spectra revealed the presence of
chemical bonding information within the wavenumber range of 500 to 3700 cm
−1
for
BiPO
4
@C composites (as shown in Figure 2c). The results indicated that the absorption
peaks at 530 and 601 cm
−1
were indicative of the vibrational absorption of
δ
(PO
43−
), while
the absorption peak at 1072 cm
−1
was attributed to the
υ
3 asymmetric stretching vibration
of the P-O bond [
30
,
31
]. The three absorption peaks situated between 2750 and 2950
−1
are
ascribed to the symmetric and antisymmetric stretching vibrations of the C
−
H bond in
the
−
CH
3
, while the vibrational absorption peaks of the benzenoid ring skeleton appear
between 1459 and 1650 cm
−1
. Furthermore, the XPS survey spectrum clearly shows signals
corresponding to the Bi, P, O, and C elements in BiPO
4
@C composites, as illustrated in
Figure 2d. Specifically, the two peaks observed at 165.1 eV and 159.8 eV in the high-
resolution Bi 4f can correspond to Bi 4f
5/2
and Bi 4f
7/2
, respectively (Figure 2e) [
12
,
29
].
Additionally, the P 2p spectrum (Figure 2f) demonstrated the emergence of a prominent
peak associated with the P 2p
3/2
(at 133.7 eV) [
29
,
32
]. The high-resolution O 1s spectrum
is split into two peaks at 531.2 and 533.1 eV, which are attributed to P-O and C-O bonds,
respectively (Figure 2g) [
29
,
32
]. Additionally, we detected the characteristic peaks of the
C=O (288.7 eV), C-O (286.3 eV), and C-C (284.8 eV) bonds in the C 1s spectrum, respectively,
as shown in Figure 2h [
33
,
34
]. It is noteworthy that the presence of C-O bonding indicates
that the ultrafine BiPO
4
particles can be chemically bonded to form a linkage with the
carbon layer, which will undoubtedly facilitate electron transfer and enhance the rate
Molecules 2025,30, 729 5 of 14
performance. Subsequently, the content of the carbon-protective layer in BiPO
4
@C was
then determined by thermogravimetric analysis (TGA) test. As illustrated in Figure 2i,
the mass remained constant up to 450
◦
C, after which a rapid mass loss (reduction of
~6.48 wt%) occurred in the range from 450
◦
C to 600
◦
C. In light of the TGA result and
the deep oxidation products (BiPO
4
, as shown in Figure S5), the calculated contents of the
product BiPO
4
and carbon matrix in BiPO
4
@C composite are 93.58 and 6.42%, respectively.
A small amount of carbon layer with high graphitization can improve the conductivity of
BiPO4, alleviate the volume effect in the electrochemical process, and further improve the
potassium-ion storage performance of BiPO
4
@C electrode without excessively reducing
the specific capacity of the active component.
Molecules 2025, 30, x FOR PEER REVIEW 5 of 14
between 1459 and 1650 cm
−1
. Furthermore, the XPS survey spectrum clearly shows signals
corresponding to the Bi, P, O, and C elements in BiPO
4
@C composites, as illustrated in
Figure 2d. Specifically, the two peaks observed at 165.1 eV and 159.8 eV in the high-reso-
lution Bi 4f can correspond to Bi 4f
5/2
and Bi 4f
7/2
, respectively (Figure 2e) [12,29]. Addi-
tionally, the P 2p spectrum (Figure 2f) demonstrated the emergence of a prominent peak
associated with the P 2p
3/2
(at 133.7 eV) [29,32]. The high-resolution O 1s spectrum is split
into two peaks at 531.2 and 533.1 eV, which are attributed to P-O and C-O bonds, respec-
tively (Figure 2g) [29,32]. Additionally, we detected the characteristic peaks of the C=O
(288.7 eV), C-O (286.3 eV), and C-C (284.8 eV) bonds in the C 1s spectrum, respectively, as
shown in Figure 2h [33,34]. It is noteworthy that the presence of C-O bonding indicates
that the ultrafine BiPO
4
particles can be chemically bonded to form a linkage with the
carbon layer, which will undoubtedly facilitate electron transfer and enhance the rate per-
formance. Subsequently, the content of the carbon-protective layer in BiPO
4
@C was then
determined by thermogravimetric analysis (TGA) test. As illustrated in Figure 2i, the mass
remained constant up to 450 °C, after which a rapid mass loss (reduction of ~6.48 wt%)
occurred in the range from 450 °C to 600 °C. In light of the TGA result and the deep oxi-
dation products (BiPO
4
, as shown in Figure S5), the calculated contents of the product
BiPO
4
and carbon matrix in BiPO
4
@C composite are 93.58 and 6.42%, respectively. A small
amount of carbon layer with high graphitization can improve the conductivity of BiPO
4
,
alleviate the volume effect in the electrochemical process, and further improve the potas-
sium-ion storage performance of BiPO
4
@C electrode without excessively reducing the spe-
cific capacity of the active component.
Figure 2. (a) XRD pattern, (b) Raman spectra, (c) FTIR spectra, (d) XPS survey spectra, high-resolu-
tion XPS spectra of (e) Bi 4f, (f) P 2p, (g) O 1s, (h) C 1s, and (i) TGA curve of BiPO
4
@C composite.
2.2. Electrochemical Properties
The potassium ion storage performance of BiPO
4
@C ultrafine nanorods and reference
sample (commercial micro-sized BiPO
4
, morphology and crystalline phase are shown in
Figure 2. (a) XRD pattern, (b) Raman spectra, (c) FTIR spectra, (d) XPS survey spectra, high-resolution
XPS spectra of (e) Bi 4f, (f) P 2p, (g) O 1s, (h) C 1s, and (i) TGA curve of BiPO4@C composite.
2.2. Electrochemical Properties
The potassium ion storage performance of BiPO
4
@C ultrafine nanorods and reference
sample (commercial micro-sized BiPO
4
, morphology and crystalline phase are shown in
Figure S6) was investigated in button-type half-cells and coupled with potassium metal as
the counter electrode. Figure 3a depicts the cyclic voltammetry (CV) curves of the BiPO
4
@C
electrode within the voltage range of 0.01 to 1.6 V at a scan rate of
0.1 mV s−1.
A reduction
peak at 0.96 V was observed in the initial cathodic scan, corresponding to the conversion
reaction to produce Bi element and K
3
PO
4
(BiPO
4
+3K
+
+3e
−→
K
3
PO
4
+ Bi), as well
as the formation of SEI layer [
31
]. Furthermore, a subsequent peak located at 0.09 V is
associated with the alloying reaction to produce K
3
Bi (3 Bi + K
+
+ e
–→
K
3
Bi). Subsequently,
Molecules 2025,30, 729 6 of 14
three anodic
peaks located at 0.60, 0.79, and 1.12 V were detected due to the gradual deal-
loying reaction (K
3
Bi
→
K
x
Bi
→
Bi) [
29
]. During successive scans, the reduction peak at
0.96 V
disappeared, suggesting an irreversible transformation process. Undoubtedly, the
high ionic conductivity of K
3
PO
4
is always maintained around the Bi particles, thereby ac-
celerating the K
+
conduction kinetics by repairing the ion transport channels. In subsequent
cycles, the electroreception process of the Bi-metal gradually evolved into a unique, multi-
step alloy/dealloy process, exhibiting three pairs of oxidation/reduction peaks at 1.12/0.84,
0.79/0.35 and 0.6/0.14 V, respectively. [
29
,
31
]. Comfortingly, the alloying/dealloying
reaction is highly reversible in terms of the positions and shapes of the CV curves, indi-
cating the strong structural stability of BiPO
4
@C. Furthermore, the voltage platforms in
the galvanostatic charge-discharge (GCD) curve (Figure 3b) are well consistent with the
redox peaks observed in the CV curves, confirming the proposed mechanism of initial
“conversion-alloying/dealloying” and the subsequent “alloying–dealloying” reactions.
Molecules 2025, 30, x FOR PEER REVIEW 6 of 14
Figure S6) was investigated in button-type half-cells and coupled with potassium metal
as the counter electrode. Figure 3a depicts the cyclic voltammetry (CV) curves of the
BiPO
4
@C electrode within the voltage range of 0.01 to 1.6 V at a scan rate of 0.1 mV s
−1
. A
reduction peak at 0.96 V was observed in the initial cathodic scan, corresponding to the
conversion reaction to produce Bi element and K
3
PO
4
(BiPO
4
+ 3 K
+
+ 3 e
−
→ K
3
PO
4
+ Bi),
as well as the formation of SEI layer [31]. Furthermore, a subsequent peak located at 0.09
V is associated with the alloying reaction to produce K
3
Bi (3 Bi + K
+
+ e
–
→ K
3
Bi). Subse-
quently, three anodic peaks located at 0.60, 0.79, and 1.12 V were detected due to the grad-
ual dealloying reaction (K
3
Bi → K
x
Bi → Bi) [29]. During successive scans, the reduction
peak at 0.96 V disappeared, suggesting an irreversible transformation process. Undoubt-
edly, the high ionic conductivity of K
3
PO
4
is always maintained around the Bi particles,
thereby accelerating the K
+
conduction kinetics by repairing the ion transport channels. In
subsequent cycles, the electroreception process of the Bi-metal gradually evolved into a
unique, multistep alloy/dealloy process, exhibiting three pairs of oxidation/reduction
peaks at 1.12/0.84, 0.79/0.35 and 0.6/0.14 V, respectively. [29,31]. Comfortingly, the alloy-
ing/dealloying reaction is highly reversible in terms of the positions and shapes of the CV
curves, indicating the strong structural stability of BiPO
4
@C. Furthermore, the voltage
platforms in the galvanostatic charge-discharge (GCD) curve (Figure 3b) are well con-
sistent with the redox peaks observed in the CV curves, confirming the proposed mecha-
nism of initial “conversion-alloying/dealloying” and the subsequent “alloying–dealloy-
ing” reactions.
Figure 3. (a) CV curves at 0.1 mV s
−1
and (b) galvanostatic charge/discharge curves at 0.2 A g
−1
of the
BiPO
4
@C electrode. (c) Cycling performances at 50 mA/g. (d) Rate performances and (e) correspond-
ing discharge/charge profiles at various current densities of BiPO
4
@C electrode. Long cycling per-
formances at (f) 200 mA/g and (g) 500 mA/g.
Additionally, the BiPO
4
@C electrode displays a low polarization potential (Figure 3c)
and excellent rate performance (Figure 3d). Accordingly, the specific charging capacities
at 50, 100, 200, 500 and 1000 mA g
−1
current density were 307.2, 254.9, 229.5, 187.9 and
137.8 mAh g
−1
, respectively, which are significantly higher than those of commercial BiPO
4
electrodes and other reported Bi-based compound anode materials (Figure S7) [35–39].
Figure 3. (a) CV curves at 0.1 mV s
−1
and (b) galvanostatic charge/discharge curves at
0.2 A g−1
of the BiPO
4
@C electrode. (c) Cycling performances at 50 mA/g. (d) Rate performances and
(e) corresponding
discharge/charge profiles at various current densities of BiPO
4
@C electrode. Long
cycling performances at (f) 200 mA/g and (g) 500 mA/g.
Additionally, the BiPO
4
@C electrode displays a low polarization potential (Figure 3c)
and excellent rate performance (Figure 3d). Accordingly, the specific charging capacities
at 50, 100, 200, 500 and 1000 mA g
−1
current density were 307.2, 254.9, 229.5, 187.9 and
137.8 mAh g
−1
, respectively, which are significantly higher than those of commercial BiPO
4
electrodes and other reported Bi-based compound anode materials (Figure S7) [
35
–
39
]. The
superior rate performance can be attributed to the short and efficient ion transport path
provided by the nanostructuring effect and the accelerated ion transport kinetics provided
Molecules 2025,30, 729 7 of 14
by the fast ion conductor intermediate (K
3
PO
4
). It is encouraging to note that following
multiple current shocks, the BiPO
4
@C electrode still exhibited a reversible capacity of
278.2 mAh g−1
when the current density was returned to 50 mA g
−1
, substantiating the fast
electron/ion transport capacity and robust structure of the BiPO
4
@C electrode. Subsequent
cycling performance tests highlighted the high specific charge capacity and exceptional
electrochemical stability of the BiPO
4
@C electrode. As illustrated in Figure 3e, the BiPO
4
@C
electrode exhibits an initial charge capacity of 310.3 mAh g
−1
and reaches a specific capacity
of 216 mAh g
−1
after 300 cycles at a current density of 50 mA g
−1
with a slower average
decay rate of 0.101% per cycle. Not surprisingly, the reversible capacities of the BiPO
4
@C
electrode after 600 cycles at 200 and 500 mA g
−1
were an astonishing 161 mAh g
−1
and
142 mAh g−1,
respectively (Figure 3f,g). Nevertheless, the specific capacitance of the micro-
sized BiPO
4
electrode rapidly decreases due to the fact that the unprotected large-area
structure cannot accommodate the large volume distortion that occurs during repeated
alloying and dealloying. In addition, even at a current density of 1000 mA/g, the BiPO
4
@C
electrode demonstrated stable operation over 150 cycles, maintaining a specific capacity of
171 mAh g
−1
(Figure S8). These superior reversibility rate properties far exceed those of
some previously reported phosphate anode electrodes [32,40,41].
To elucidate the robust cycle stability of the BiPO
4
@C electrode, we added the struc-
tural tests of the electrode surface and cross-section with multiple sets and performed
a statistical analysis of the volume expansion. As expected, the BiPO
4
@C electrode sur-
face retained an intact structure even after 100 cycles (Figure S9a,b), whereas commercial
micro-sized BiPO
4
electrodes exhibited obvious structural fragmentation (Figure S10a,b).
Figure S9c,d
shows the cross-sectional changes of the BiPO
4
@C electrodes before and after
cycling. Specifically, the thickness of the BiPO4@C electrode is 20.31 µm before potassium
insertion. After repeated potassiation/depotassiation, the thickness increases to 29.74
µ
m,
and the corresponding volume expansion rate is 46.4%. In contrast, the commercial micro-
sized BiPO
4
electrode showed a volume expansion of 95.5%, and the electrode showed
obvious signs of loosening and cracking (Figure S10c,d). It is clear that the BiPO
4
@C
electrode, which combines the nanoscale effect and carbon-constrained structure, has sig-
nificant resistance to volume expansion, which is the key to achieving stable potassium-ion
storage. In addition, Electrochemical Impedance Spectroscopy (EIS) tests were carried out
under different cycle conditions (Figure S11). Clearly, at high frequencies, the BiPO
4
@C
electrode has a smaller semicircle, which means a lower impedance. Especially after the
formation of the SEI film during the first cycle, the impedance decreases significantly
and gradually stabilizes even after 50 to 100 cycles (Figure S11a). In contrast, the BiPO
4
electrode consistently showed a large semicircle (Figure S11b), which can be attributed
to the large interfacial impedance due to continuous volume expansion and electrode
cracking. Overall, further testing confirms that the BiPO
4
@C electrode combines both
nanoscale dimensions and carbon layer properties, is highly resistant to the volume strain
inherent in alloy–dealloy reactions, achieves a robust structure and significantly improves
cycle stability.
2.3. Kinetic Analysis
To ascertain the origin of the excellent rate performance of the BiPO
4
@C electrode,
further tests were carried out on the kinetic and pseudocapacitive behavior. In detail, as
depicted in Figure 4a, the five CV curves obtained at different sweep rates (0.1, 0.2, 0.6, 0.8,
and 1.0 mV/s) show similar profiles and regular change trends of the redox peaks. The ca-
pacitive effect is calculated from the relationship between the measured peak current (i) and
the sweep rate (v) (i= av
b
), where bis the slope of the plot of log iversus log v. The atypical
diffusion-controlled process corresponds to b= 0.5, while b= 1.0 indicates surface induced
Molecules 2025,30, 729 8 of 14
capacitive behavior. Furthermore, Figure 4b shows that the bvalues of the several peaks are
distributed between 0.61 and 0.75, suggesting that the BiPO
4
@C electrode follows a “battery-
capacitance dual-mode” potassium storage mechanism dominated by surface capacitive
behavior. Moreover, the proportion of capacitance-controlled and diffusion-controlled
processes contributing to the total specific capacity can also be quantified according to the
equation of i= k
1
v+ k
2
v
1/2
, where k
1
vand k
2
v
1/2
are to be assigned as the relative contribu-
tions of the capacitive and intercalation processes, respectively. As expected, the capacitive
percentage was observed to be 31, 31, 43, 49 and 60% at scanning rates of 0.1, 0.2, 0.6, 0.8, and
1.0 mV s
−1
, respectively (Figure 4c,d). The high capacitive behavior is primarily attributable
to the abundant surface reaction sites afforded by the ultrafine structure of BiPO
4
@C and
the adsorption/desorption capacity of K
+
by the ultrathin carbon layer, undoubtedly result-
ing in fast K-ion storage kinetics and ultralong cycling life. Meanwhile, the galvanostatic
intermittent titration technique (GITT) measurements were carried out to identify the
underlying causes of the superior K-ion storage kinetics obtained in the BiPO
4
@Celectrode.
As depicted in Figure 4e, the GITT curve of the BiPO
4
@C electrode exhibits a high spe-
cific capacity, low electrochemical polarization and standard redox potential. Moreover,
from the pulse current time (s), potential difference (∆ES) and electrode parameters in the
collected GITT profiles, the potassium-ion diffusion coefficient (D
K+
) is calculated. The
calculated D
K+
values during discharging
(4.20 ×10−13~3.95 ×10−12 cm2s−1)
and charging
(1.2 ×10−12~1.65 ×10−11 cm2s−1)
are significantly higher than micro-sized BiPO
4
electrode
(discharging:
7.8 ×10−13~1.82 ×10−14 cm2s−1,
charging:
1.2 ×10−13~3.5 ×10−11 cm2s−1)
(Figures 4f and S12), also significantly exceeds some of the reported phosphate
anodes [42–44].
The high K
+
values indicate fast potassiation and depotassiation kinetics and excellent rate
performance. The rapid potassium storage capacity of well-designed ultra-fine BiPO
4
@C
electrodes can be attributed to their ingenious structural configuration, which provides an
abundance of K
+
insertion sites and a short K
+
migration path, thereby accelerating the K
+
reaction kinetics (as summarized in Figure S13).
Molecules 2025, 30, x FOR PEER REVIEW 8 of 14
atypical diffusion-controlled process corresponds to b = 0.5, while b = 1.0 indicates surface
induced capacitive behavior. Furthermore, Figure 4b shows that the b values of the several
peaks are distributed between 0.61 and 0.75, suggesting that the BiPO
4
@C electrode fol-
lows a “battery-capacitance dual-mode” potassium storage mechanism dominated by
surface capacitive behavior. Moreover, the proportion of capacitance-controlled and dif-
fusion-controlled processes contributing to the total specific capacity can also be quanti-
fied according to the equation of i = k
1
v+ k
2
v
1/2
, where k
1
v and k
2
v
1/2
are to be assigned as
the relative contributions of the capacitive and intercalation processes, respectively. As
expected, the capacitive percentage was observed to be 31, 31, 43, 49 and 60% at scanning
rates of 0.1, 0.2, 0.6, 0.8, and 1.0 mV s
−1
, respectively (Figure 4c,d). The high capacitive
behavior is primarily attributable to the abundant surface reaction sites afforded by the
ultrafine structure of BiPO
4
@C and the adsorption/desorption capacity of K
+
by the ul-
trathin carbon layer, undoubtedly resulting in fast K-ion storage kinetics and ultralong
cycling life. Meanwhile, the galvanostatic intermittent titration technique (GITT) meas-
urements were carried out to identify the underlying causes of the superior K-ion storage
kinetics obtained in the BiPO
4
@Celectrode. As depicted in Figure 4e, the GITT curve of the
BiPO
4
@C electrode exhibits a high specific capacity, low electrochemical polarization and
standard redox potential. Moreover, from the pulse current time (s), potential difference
(ΔE
S
) and electrode parameters in the collected GITT profiles, the potassium-ion diffusion
coefficient (D
K+
) is calculated. The calculated D
K+
values during discharging (4.20 ×
10
−13
~3.95 × 10
−12
cm
2
s
−1
) and charging (1.2 × 10
−12
~1.65 × 10
−11
cm
2
s
−1
) are significantly
higher than micro-sized BiPO
4
electrode (discharging: 7.8 × 10
−13
~1.82 × 10
−14
cm
2
s
−1
, charg-
ing: 1.2 × 10
−13
~3.5 × 10
−11
cm
2
s
−1
) (Figures 4f and S12), also significantly exceeds some of
the reported phosphate anodes [42–44]. The high K
+
values indicate fast potassiation and
depotassiation kinetics and excellent rate performance. The rapid potassium storage ca-
pacity of well-designed ultra-fine BiPO
4
@C electrodes can be attributed to their ingenious
structural configuration, which provides an abundance of K
+
insertion sites and a short K
+
migration path, thereby accelerating the K
+
reaction kinetics (as summarized in Figure
S13).
Figure 4. (a) CV curves at various scan rates, (b) the relationship between log (i) and log (v), and
(c,d) contribution ratios of the capacitive-controlled capacity of the BiPO
4
@C electrode. (e) GITT
curves and (f) the calculated potassium-ion diffusion coefficients of the BiPO
4
@C.
Figure 4. (a) CV curves at various scan rates, (b) the relationship between log (i) and log (v), and
(c,d) contribution
ratios of the capacitive-controlled capacity of the BiPO
4
@C electrode. (e) GITT
curves and (f) the calculated potassium-ion diffusion coefficients of the BiPO4@C.
Molecules 2025,30, 729 9 of 14
2.4. Electrochemical Mechanism
Systematic ex-situ XRD and TEM testing were further employed to reveal the potas-
sium storage mechanism of the BiPO
4
@C electrode. As depicted in Figure 5a, the BiPO
4
@C
electrode exhibits a series of diffraction peaks located at 21.3
◦
, 25.5
◦
, 27.1
◦
, 28.3
◦
, 29.1
◦
,
31.2
◦
, 34.4
◦
, 36.8
◦
, and 36.8
◦
at open circuit voltage (OCV), which are attributed to the
(
−
111), (020), (200), (002), (120), (
−
112), (
−
202), and (112) crystal planes of monoclinic
BiPO
4
. As the discharge progresses, the diffraction peak of BiPO
4
gradually diminishes,
and three characteristic peaks (located at 27.3
◦
, 38.1
◦
, and 39.8
◦
) corresponding to the
metal Bi emerges at 0.7 V, indicating a conversion reaction of BiPO
4
(BiPO
4→
Bi). Subse-
quently, the metal Bi is gradually alloyed into KBi
2
(31.1
◦
and 32.6
◦
) at 0.4 V and further
converted to K
3
Bi (28.6
◦
) at 0.01 V (Bi
→
KBi
2→
K
3
Bi). In the reverse charge process,
the derivative peak of KBi
3
gradually disappears and converts into KBi
2
at 0.5 V before
ultimately converting back to metallic Bi at 1.5 V, suggesting a gradual dealloying process
(K3Bi →KBi2→Bi).
Subsequently, the ex-situ TEM test results provided further confirmed
the experimental findings. Figure 5b,c clearly shows that the BiPO
4
@C electrode produced
uniform nanoparticles when discharged to 0.01 V, with crystal plane spacings of 0.20 and
0.311 nm, corresponding to the (400) crystal plane of K
3
PO
4
and the (220) crystal phase
of K
3
Bi, respectively. Meanwhile, the (202) and (104) crystal planes of K
3
Bi and the (200)
and (422) crystal planes of K
3
PO
4
were identified by selected area electron diffraction
(SAED) (Figure 5d). Upon reverse charging to 1.6 V, ultrafine metallic Bi particles belonging
to the (012) and (014) crystal planes were successively detected (Figure 5e–g), thereby
demonstrating the excellent reversibility of the stepwise alloying-dealloying reaction of the
BiPO
4
@C electrode. To sum up, the in-situ/ex-situ test results combined with the CV curves
reveal the reversible conversion and stepwise alloying/dealloying reaction mechanism of
the BiPO
4
@C electrode. The detailed evolution process of the mechanism is illustrated in
Figure 5h and described as follows:
Stage I: (OCV–0.8 V, Conversion reaction and the formation of SEI layer):
BiPO4+3K++3e−→K3PO4+ Bi
Stage II: (0.8–0.01 V, Step-alloying reaction):
2Bi+K++ e–→KBi2
KBi2+5K++5e−→2 K3Bi
Stage III: (0.01–0.5 V, Dealloying reaction):
2 K3Bi −5 K+−5 e−→KBi2
Stage IV: (0.5–1.6 V, Dealloying reaction):
KBi2−K+−e–→2 Bi
Molecules 2025,30, 729 10 of 14
Molecules 2025, 30, x FOR PEER REVIEW 10 of 14
Figure 5. Investigation of the reaction mechanism of the BiPO
4
@C anode: (a) In-situ XRD and corre-
sponding discharge-charge curve. (b,c) HRTEM images and (d) SAED pattern after discharge to 0.01
V. (e,f) HRTEM images and (g) SAED pattern after charge to 1.6 V. (h) Illustration of the reaction
mechanism of the BiPO
4
@C anode.
2.5. Application Potential
To fully demonstrate the application potential of BiPO
4
@C, we have re-prepared
thick electrodes (about 4.0 mg/cm
2
) and investigated their potassium storage performance.
As shown in Figure S14, the BiPO
4
@C thick electrode showed high specific capacity and
superior cycling stability, maintaining 218.8 mAh g
–1
after 80 cycles at 50 mA g
–1
(Figure
S14a), 180.9 mAh g
–1
after 200 cycles at 200 mA g
–1
(Figure S14b), and 130.3 mAh g
–1
after
80 cycles at 1000 mA g
–1
(Figure S14c). Impressively, the thick electrode demonstrated ex-
cellent rate performance, with capacities of 280.1, 219.9, 194.8, 152.4, and 102.6 mAh g
–1
at
50–1000 mA g
–1
, respectively (Figure S14d). Compared to the commercial BiPO
4
thick elec-
trode, the BiPO
4
@C thick electrode has significantly better structural advantages, as evi-
denced by its specific capacity, capacity retention rate and rate performance at various
current densities (Figure S15). Overall, the BiPO
4
@C electrodes maintained excellent cy-
cling stability and rate performance even at the increased loading mass of 4.0 mg cm
–2
,
which also confirms its potential for commercialization. The excellent performance is at-
tributed to the nanosized BiPO
4
significantly reducing the volume strain and increasing
the embedded potassium sites; the incorporation of the bulk and open-skeleton anion
PO
43−
serves to mitigate the considerable volume distortion and generates the high ionic
conductivity product (K
3
PO
4
) to ameliorate the poor ionic transport due to the structural
deformation.
Figure 5. Investigation of the reaction mechanism of the BiPO
4
@C anode: (a) In-situ XRD and
corresponding discharge-charge curve. (b,c) HRTEM images and (d) SAED pattern after discharge to
0.01 V. (e,f) HRTEM images and (g) SAED pattern after charge to 1.6 V. (h) Illustration of the reaction
mechanism of the BiPO4@C anode.
2.5. Application Potential
To fully demonstrate the application potential of BiPO
4
@C, we have re-prepared thick
electrodes (about 4.0 mg/cm
2
) and investigated their potassium storage performance. As
shown in Figure S14, the BiPO
4
@C thick electrode showed high specific capacity and supe-
rior cycling stability, maintaining 218.8 mAh g
–1
after
80 cycles
at 50 mA g
–1 (Figure S14a),
180.9 mAh g
–1
after 200 cycles at 200 mA g
–1
(Figure S14b), and 130.3 mAh g
–1
after
80 cycles
at 1000 mA g
–1
(Figure S14c). Impressively, the thick electrode demonstrated
excellent rate performance, with capacities of 280.1, 219.9, 194.8, 152.4, and 102.6 mAh g–1
at
50–1000 mA g–1,
respectively (Figure S14d). Compared to the commercial BiPO
4
thick
electrode, the BiPO
4
@C thick electrode has significantly better structural advantages, as
evidenced by its specific capacity, capacity retention rate and rate performance at various
current densities (Figure S15). Overall, the BiPO
4
@C electrodes maintained excellent cycling
stability and rate performance even at the increased loading mass of 4.0 mg cm
–2
, which
also confirms its potential for commercialization. The excellent performance is attributed to
the nanosized BiPO
4
significantly reducing the volume strain and increasing the embedded
Molecules 2025,30, 729 11 of 14
potassium sites; the incorporation of the bulk and open-skeleton anion PO
43−
serves to
mitigate the considerable volume distortion and generates the high ionic conductivity
product (K3PO4) to ameliorate the poor ionic transport due to the structural deformation.
3. Experimental
3.1. Sample Preparation
Synthesis of BiPO
4
@C nanorods. All chemicals were analytical reagent grade and
used as received without further treatment. BiPO4@C nanorods were synthesized using a
typical solvothermal method. First, 0.5 mmol Bi(NO
3
)
3
(99%, Aladdin Reagent Co., Ltd.,
Shanghai, China) and 0.5 mmol NH
4
H
2
PO
4
(99%, Aladdin Reagent Co., Ltd., China) were
dissolved in 30 mL of ethylene glycol (99%, Aladdin Reagent Co., Ltd., China) with stirring
for 6 h. Then, 1 mL of deionized (DI) water was added and stirred rapidly for 5 min. The
mixture was transferred to a 50 mL autoclave and heated at 160
◦
C for 12 h. The BiPO
4
nanorods were washed six times by centrifugation with ethanol and DI water. Finally, they
were dried overnight at 80 ◦C.
3.2. Electrochemical Measurements
The working electrodes were prepared by mixing active materials, Super P, and
polyvinylidene fluoride (PVDF) in a weight ratio of 7:2:1 with N-methyl-2-pyrrolidone
(NMP) as solvent. The CR2032 half-cells were assembled with K-metal, glass fiber (What-
man), and 3 M potassium bis(fluorosulfonyl)imide (KFSI) in dimethyl ether (DME) as the
counter electrodes, the separators, and the electrolyte, respectively. Cyclic voltammetry
(CV) and electrochemical impedance spectroscopy (EIS, frequency range from 1.0
×
10
5
to 0.1 Hz) tests were performed on an Autolab instrument (PGSTAT 302). Galvanostatic
charge-discharge tests were performed on a Neware battery tester (Neware CT-4008). The
mass loading of the active materials (BiPO
4
@C) was about 1.2 mg cm
−2
. And the specific
capacity was based on the mass of BiPO4@C.
4. Conclusions
In summary, we have integrated the dual strategies of architecture engineering and
anion tuning to construct an ultrathin carbon-coated BiPO
4
nanorods (BiPO
4
@C) electrode
to suppress volume distortion and unclog electron transport pathways. The ultrathin
nanorods provided a substantial number of electrochemical reaction sites, negated the
size-related volume strain effect, and also shortened the ion transport distance. Meanwhile,
the incorporation of an ultrathin carbon layer and bulk PO
43−
served to defend and buffer
the volume expansion associated with the alloying-dealloying process, thereby markedly
enhancing the cycling stability of the BiPO
4
@C anode. Significantly, the in-situ generated
K
3
PO
4
with high ionic conductivity acts as a bridge to unblock the electron/ion transport
pathway and significantly improves the rate performance. Moreover, the preparation and
testing of thick electrodes confirmed the excellent electrochemical performance and appli-
cation potential of BiPO
4
@C electrodes. Thus, this work highlights the significance of archi-
tecture engineering and anion-tuning strategy in rationally designing high-performance
electrodes for K-ion batteries.
Supplementary Materials: The following supporting information can be downloaded at:
https://www.mdpi.com/article/10.3390/molecules30030729/s1, Figure S1: SEM images of BiPO
4
@C
composites with different raw material concentrations; Figure S2: SEM images of BiPO
4
@C compos-
ites at different reaction temperatures; Figure S3: SEM images of BiPO
4
@C composites at different
reaction times; Figure S4: XRD patterns and Rietveld refinement plots of BiPO
4
@C; Figure S5: XRD
pattern of BiPO
4
@C residue after the TGA test; Figure S6: SEM image and XRD patterns of commer-
cial micro-sized BiPO
4
; Figure S7: Comparison of rate performance between BiPO
4
@C anode and
Molecules 2025,30, 729 12 of 14
the previously reported Bi-based compound anode in PIBs.; Figure S8: Cycling performances of (a)
BiPO
4
@C and (b) commercial micro-sized BiPO
4
anodes at a current density of 1.0 A g
−1
; Figure S9:
Surface and cross-sectional SEM images of the BiPO
4
@C electrode after 100 cycles; Figure S10: Surface
and cross-sectional SEM images of the BiPO
4
electrode after 100 cycles; Figure S11: EIS results of the
(a) BiPO
4
@C electrode and (b) commercial micro-sized BiPO
4
electrode before and after different
cycles at 500 mA g
−1
; Figure S12: (a) GITT curves, (b) the corresponding detailed voltage response
in a single current pulse, and the K
+
diffusion coefficient values (D
K+
) during the (c) discharge and
(d) charge process; Figure S13: Schematic illustration of potassium-ion diffusion in the nanosize
structure; Table S1: Crystallographic parameters from Rietveld refinement for the XRD pattern of
BiPO4@C.
Author Contributions: Conceptualization, H.C. and H.Z.; methodology, H.C.; investigation,
Y.L. (Yong Li) and Y.L. (Yuanjie Liu); resources, X.C.; writing—original draft preparation, H.C.;
writing—review
and editing, H.Z. and J.Z.; All authors have read and agreed to the published
version of the manuscript.
Funding: This research was funded by the Science and Technology Program of XPCC (NO.2023AB031;
NO.2023AA007) and the Xinjiang Production and construction corps new energy industry innovation
research institute construction project (NO.2023-02-20240106).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The dataset is available upon request from the authors.
Acknowledgments: The authors thank all the help from their laboratory companions.
Conflicts of Interest: The authors declare no conflicts of interest.
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