Carbon‐anchored Sb nanoparticles as high‐capacity and stable anode for aqueous alkaline batteries

Antimony (Sb) holds a high theoretic capacity and suitable redox potential as a promising anode for aqueous alkaline batteries (AABs). However, the uncontrollable nucleation for SbO2− and promiscuous water‐induced side reactions severely degrade the reversibility of Sb anode. Herein, the carbon‐anchored Sb nanoparticles are constructed to induce uniform Sb plating/stripping for high‐performance AABs. The experimental results reveal that the enhanced interaction between carbon and antimony as well as defective carbon can significantly improve the electrical conductivity and decrease the Sb nucleation overpotential. Accordingly, the as‐prepared Sb anode enables preferential plating of Sb rather than parasitic side reactions. As a result, the cycle life of A‐Sb/CF is sustained over 500 cycles at 10 mA cm−2/2 mAh cm−2. Even at the high capacity of 4 mAh cm−2, this anode can cycle stably for 225 cycles, which is significantly better than the Sb/CF counterpart. Furthermore, the assembled Ni3S2@Ni(OH)2//A‐Sb/CF full battery demonstrates a high capacity of 2.17 mAh cm−2 and a stable cycle life of over 500 cycles.


| INTRODUCTION
As the demand for green and sustainable energy storage increases, advanced energy storage technologies like lithium-ion batteries (LIBs) have attracted wide attention. 1,2owever, the large-scale application of LIBs is further limited by the scarcity of Li resources, high production cost, and flammable organic electrolytes. 3As a promising complement, aqueous alkaline batteries (AABs) have attracted a lot of attention due to their abundant resources, ease of handling and high security. 4,5Antimony (Sb) metal as an anode material for AABs has the following advantages: (1) high theoretical capacity of 660 mAh g −1 , (2) low redox potential of −0.66 V (vs.7][8] Moreover, Sb anode in alkaline electrolytes relies on the transformation of SbO 2 − /Sb for energy storage, 9,10 ensuring fast reaction kinetics.As opposite to Zn anode, [11][12][13][14][15] Sb exhibits dendrite-free deposition behavior.Nevertheless, Sb anode suffers from limited reversibility issues, such as low coulombic efficiency and poor rechargeable life, which are all related to the fundamental mechanisms of Sb plating reactions.Specifically, during the plating process, the charge carrier (SbO 2

−
) and the electrode surface have the same charge, making it difficult for SbO 2 − to approach the electrode surface for depositing into Sb due to electrostatic repulsion. 16Unfortunately, such a process might trigger water-induced side reactions (H 2 evolution), further leading to batteries failure.To solve the above issues, an effective strategy is to construct the functional group on the surface of electrode to facilitate the diffusion and deposition behaviors of SbO 2 − ions.For example, Zhang et al. 16 prepared an oxygen-rich interface on the carbon cloth, which firstly achieved the reversible plating/stripping of Sb over 30 h at a low capacity of 0.47 mAh cm −2 .As another effective method, the design of nitrogen-doped carbon frameworks could also improve the charge transport and induce uniformly deposited Sb. 17 For example, Zhang et al. 18 fabricated the N-doped carbon nanocages as the substrate of Sb anode, which improved the [Sb(OH) 4 ] − deposition reaction kinetics, and remarkably alleviating parasitic side-reaction.In addition, Wang et al. found that the carbon with defects could effectively decrease the surface charge density and provide a large number of nucleation sites, which is beneficial to inducing uniform deposition of Sb. 19 However, the insufficient lifespan of Sb anodes or the laborious material preparation process still greatly hinders their further large-scale application.Therefore, it is of great significance to develop simple and effective approaches for constructing stable Sb anodes at high current density/capacity.
Herein, we develop an effective KCl-assisted strategy to construct active carbon-anchored Sb nanoparticles (A-Sb/CF) as high-performance Sb anode for AABs.Different from the Sb/CF obtained by direct calcination of C 8 H 10 O 15 Sb 2 K 2 , there is a strong interaction between carbon and Sb as well as defective carbon in A-Sb/CF sample, which accelerate electron transfer and decrease the Sb nucleation overpotential.Benefiting from these merits, the A-Sb/CF electrode shows stable cycles over 500 cycles at a capacity of 2 mAh cm −2 and the average coulombic efficiency is maintained at 93%.When the capacity increases to 4 mAh cm −2 , this as-prepared Sb anode still works stably for about 225 cycles, which outperforms the Sb/CF electrode (150 cycles).In addition, the full battery coupled with Ni 3 S 2 @Ni(OH) 2 exhibits a high areal capacity of 2.17 mAh cm −2 and a stable cycle life of 500 cycles with 87.6% retention, surpassing the most recently reported AABs.

| EXPERIMENTAL SECTION 2.1 | Preparation of A-Sb/CF and Sb/CF composites
First, 1.5 g C 8 H 10 O 15 Sb 2 K 2 and 12 g KCl were ground in a ball mill for 30 min.Then the mixture was transferred into a tube furnace and annealed at 600°C for 2 h under a nitrogen atmosphere.After cooling to room temperature, the as-prepared powder was washed with distilled water and anhydrous ethanol, followed by drying at 60°C for 12 h.For comparison, the Sb/CF were synthesized under the same conditions without the addition of KCl before annealing.

| Preparation of Co x Ni 3−x S 2 @NiCo-OH (CNSOH)
The CNSOH was prepared directly on the clean nickel foam (NF) through a solvothermal reaction according to the previously reported work. 20NF was cleaned in absolute ethanol, distilled water, 1 M HCl aqueous solution, and distilled water in an ultrasonic cleaner for 15 min, respectively.Specifically, 0.73 g Ni(NO 3 ) 2 •6H 2 O, 0.73 g Co(NO 3 ) 2 •6H 2 O, and 1.40 g hexamethylenetetramine were first dissolved in 30 mL of deionized water and stirred for 1 h to form a homogeneous solution.Then, the above solution was transferred into a 50 mL Teflon-lined stainless steel autoclave with two clean NF substrates placed inside the solution.The autoclave was put in the oven at 100°C for 8 h.After cooling to room temperature, the obtained samples were washed several times with distilled water and anhydrous ethanol and dried at 60°C for 12 h to obtain Co-Ni precursor samples.Subsequently, the as-prepared CoNi precursor samples, along with 30 ml sodium sulfide (0.08 M) solution were put into a Teflon−lined stainless steel autoclave, and then placed in an oven at 120°C for 8 h.After cooling to room temperature, the samples were washed several times with distilled water and anhydrous ethanol, and dried at 60°C for 12 h to obtain Co x Ni 3−x S 2 sample.Finally, the obtained Co x Ni 3−x S 2 sample along with 30 mL deionized water was put into autoclaves and heated to 150°C for 2 h.After that, the sample was washed with distilled water, followed by drying at 60°C for 12 h in air to obtain the CNSOH.

| Material characterization
The microstructures and compositions of the materials were characterized using field-emission SEM (JSM-6330F), TEM (FEI Tecnai G2 F30), XPS (ESCALab 250, Thermo VG), Raman spectroscopy (Renishaw inVia), XRD (D8 ADVANCE), and FT-IR (NICOLET 6700, Thermo).The contact angles of the as-prepared film electrodes were measured at 25°C using a contact angle meter (SL150, Kino Industrial Co., Ltd.).Themogravimetric analysis (TGA) was carried out to measure the Sb content by using a thermogravimetric analyzer (TG209F1 libra) with a heating rate of 10°C min −1 in the air for Sb/CF and A-Sb/ CF composite.

| Electrochemical measurement
The electrodes were prepared by casting the slurry containing 90% of active materials and 10% carboxymethylcellulose sodium binder onto carbon paper.After that, the coated carbon paper was dried at 60°C for 12 h in the vacuum, and the mass loading of the active material was about 3.0-3.5 mg cm −2 .The size of the working electrode is 0.5 × 1 cm 2 .The electrochemical performance of the electrodes was tested in a threeelectrode system containing a working electrode (Sb/CF, A-Sb/CF), a graphite rod counter electrode and a Hg/ HgO reference electrode in 3 M KOH and 0.1 M C 8 H 4 K 2 O 12 Sb 2 aqueous electrolyte.The A-Sb/CF and Sb/CF symmetric cells were assembled into coin-type batteries (CR2032).Cyclic voltammetry and electrochemical impedance spectroscopy measurements were carried out on a CHI 760E electrochemical workstation.The galvanostatic charge/discharge measurements were conducted on a Neware battery testing system.EIS was measured from 100 kHz to 0.01 Hz with a deposition voltage of −0.90 V.The EIS spectra were fitted according to the classic Arrhenius law (Equation ( 1)), and the activation energy for Sb plating are obtained.

| RESULTS AND DISCUSSION
The preparation process of A-Sb/CF is illustrated in Figure 1A.Initially, the C 8 H 10 O 15 Sb 2 K 2 precursor was homogeneously mixed with KCl by ball milling, and then annealing at 600°C for 2 h under an N 2 atmosphere.Lastly, the A-Sb/CF was obtained by washing away soluble impurities, such as inorganic salts.During the carbonization process, KCl was used as an activation agent.As a comparison, pristine Sb/CF was synthesized by directly heating antimony tartrate precursor in an N 2 atmosphere at 600°C for 2 h.Scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology of Sb/CF and A-Sb/CF.As displayed in Figure S1, Sb/CF is composed of micron-sized lumpy particles, where the surface of the particles is uneven.Compared to Sb/CF, the A-Sb/CF shows distinct morphology (Figure 1B-C), in which small-sized Sb particles are encapsulated by amorphous carbon.The elemental mapping images (Figure 1D) clearly exhibit the uniform distribution of Sb, C, and O in the A-Sb/CF sample.In the high-resolution TEM (HRTEM) image (Figure 1F) of A-Sb/CF, the lattice spacings of 0.208, 0.220, and 0.321 nm can be discerned, well corresponding to the (110), (104), and (012) planes of metal Sb (JCPDS card No. 35-0732), respectively.In addition, the presence of amorphous carbon was also observed around the Sb nanoparticles (Figure 1E).The selected-area electron diffraction (SAED) pattern shows a series of diffraction spots (Figure 1G), assigned to the diffractions of ( 014), ( 024), ( 116), (110), and (015) planes of the hexagonal Sb, further demonstrating the highly crystalline nature of the Sb nanoparticles. of redox peaks were observed, which belonged to the plating/stripping behavior of Sb.The galvanostatic cycling stability of the two electrodes at different capacities was displayed in Figure 3A,B.At the current density/capacity of 10 mA cm −2 /2 mAh cm −2 , the charging voltage plateau of the A-Sb/CF electrode does not change significantly around 0.92 V for 200 h.By contrast, the charging profiles of Sb/CF electrode gradually become unstable after 140 h charge/discharge cycles and sharply drop to −1.60 V, resulting in a large number of bubbles.As the capacity increases to 4 mAh cm −2 , no significant change in the charging voltage plateau of A-Sb/CF electrode after 175 h of charge/discharge cycle, indicating its excellent stability.Conversely, the charging voltage of Sb/CF electrode becomes much larger after 110 h.More importantly, the A-Sb/CF electrode can be cycled steadily for 110 h at a high capacity of 8 mAh cm −2 without any significant change in the charging voltage plateau (Figure S7).
To further investigate the reversibility of Sb plating/ stripping process on the Sb/CF and A-Sb/CF electrodes, we compared the Coulomb efficiency (CE) (ratio of the stripping capacity to the plating capacity) of the two electrodes at different capacities.As illustrated in Figure 3C, the CE of A-Sb/CF electrode was close to 93% during the 500 cycles at 2 mAh cm −2 .In sharp contrast, the CE of Sb/CF experienced a sharp drop after the 350 charge/discharge cycles.A similar phenomenon is observed when the areal capacity was increased to 4 mAh cm −2 (Figure 3D).The average CE of A-Sb/CF cycled at 4 mAh cm −2 is about 92%.Such disappointed performance is mainly attributed to the occurrence of severe side effects.The charge-discharge curves of Sb/CF and A-Sb/CF electrodes at different cycles are displayed in Figure 3E.The Sb/CF electrode exhibits a capacity of 3.3 mAh cm −2 in the first cycle, but drops to 1.1 mAh cm −2 after 150 cycles, demonstrating the presence of irreversible side reactions.Remarkably, the capacity of A-Sb/CF did not change significantly during the cycle.The cycling performance of Sb/CF and A-Sb/CF symmetric cells is shown in Figure S8.As expected, the A-Sb/ CF symmetric cell exhibits a long lifespan of over 690 h, which is far more stable than Sb/CF symmetric cell.
To gain more insight into the improved performance of A-Sb/CF electrode, the water contact angle and temperature-dependent EIS were performed.As shown in Figure 4A, the contact angle of Sb/CF electrode is around 10°, while the electrolyte is completely wetted on the A-Sb/CF electrode, indicating that the A-Sb/CF electrode is more conducive than the uniform deposition of Sb according to the capillary effect of homogeneous nucleation. 21In temperature-dependent EIS (Figure S9), the semicircle in the Nyquist plot represents the activation process of Sb plating and the activation energy can be calculated according to the classic Arrhenius law. 22As depicted in Figure 4B, the activation energy for Sb plating on A-Sb/CF electrode (11.9 kJ mol −1 ) is much lower than that on Sb/CF electrode (12.8 kJ mol −1 ).Additionally, the nucleation overpotential is also an important parameter to evaluate the uniform Sb plating and cycling stability.The difference between the sharp tip voltage and the following stable voltage is known as the nucleation overpotential in which a smaller nucleation overpotential means more nucleation sites and more uniform nucleation.As displayed in Figure 4C, the initial Sb nucleation overpotential on A-Sb/CF electrode was 77 mV, much lower than that on Sb/CF electrode (112 mV), demonstrating the uniform Sb nucleation on A-Sb/CF electrode.
The morphologies evolution of Sb deposits on Sb/CF and A-Sb/CF electrodes during electrocrystallization process were observed by ex-situ SEM.The Sb deposits obtained on the Sb/CF electrode were predominantly particulate and did not cover the electrode surface (Figure 4D).It is found that the newly deposited Sb is preferentially deposited on some of the already formed Sb particles to form larger particles rather than on the blank substrate as the capacity increases from 2 to 8 mAh cm −2 .This is due to the fact that these electrons tend to accumulate on the protuberant Sb particles, which is more favorable for Sb deposition and leads to further partial accumulation of Sb, thus forming "dead Sb." 23 By comparison, the Sb deposits on the A-Sb/CF electrode exhibit different morphologies.As shown in Figure 4E, dense and compact Sb grains are stacked in order without evident separation and the size of the grains grows globally as the amount of Sb deposition increases.Even when the capacity increased to 8 mAh cm −2 , no aggregation of Sb deposits was observed on the surface of the A-Sb/CF electrode, demonstrating that the presence of Sb-O-C plays an important role in regulating the homogeneous distribution of the SbO 2 − .To further demonstrate the potential of the A-Sb/CF electrode in the field of energy storage, we assembled a full Ni//Sb battery by employing the as-prepared A-Sb/CF as anode and Co x Ni 3−x S 2 @NiCo-OH (CNSOH) as cathode.Note that the CNSOH was synthesized according to the previously reported hydrothermal method (details are given in the experimental section). 15The SEM characterization of CNSOH is shown in Figure S10. in Figure 5B.The polarization of the redox peak gradually increases as the scan rate increases.The charge/discharge profiles of Ni//Sb battery in Figure 5C exhibit flat discharge plateaus at approximately 1.0 V.More importantly, an impressive areal capacity of 2.17 mAh cm −2 was obtained at 4 mA cm −2 .In addition, with the current densities increased to 40 mA cm −2 , a 93% retention of discharge voltage plateaus is still obtained, further indicating the rapid electrolysis kinetics of the electrodes. 24To fully evaluate the electrochemical performance of the Ni//Sb battery, the comparison of the area capacity between our as-prepared battery and other reported alkaline aqueous batteries is plotted in Figure 5D.The areal capacity of Ni//Sb battery in this work remarkably outstrip most aqueous batteries, such as P-NiCo 2 O //Zn battery (0.19 mAh cm −2 , 4 mAcm −2 ), 25 FCO//Zn battery (0.24 mAh cm −2 , 4 mAcm −2 ), 26 Zn-MnO 2 Battery (0.28 mAh cm −2 , 4 mAcm −2 ), 27 AGrE//Sn full battery (0.45 mAh cm −2 , 5mAcm −2 ), 28 NCM@CC-3// Zn@CC battery (1.78 mAh cm −2 , 5 mAcm −2 ) 29 and Ni//Bi battery (1.79 mAh cm −2 , 4 mAcm −2 ), 30 and so on.The cycling stability performance of the Ni//Sb battery was also explored.The Ni//Sb battery based on A-Sb/CF anode delivers a capacity retention of 87.6% after 500 cycles (Figure 5E).In stark contrast, the battery based on Sb/CF anode retained 16.7% of the capacity after 400 cycles, which should be caused by the unstable Sb/CF anode.

| CONCLUSION
In summary, we have successfully developed an effective KCl-assisted strategy to fabricate carbonanchored Sb nanoparticles (A-Sb/CF) as the anode for AABs.The introduction of KCl as an activator and template makes the calcined product into a carbonmodified structure that is a protective shell covered with a carbon layer on Sb nanoparticles, thus providing electron paths to enhance the conductivity of the A-Sb/CF.Moreover, the formation of defective carbon in the obtained sample can significantly decrease the Sb nucleation overpotential.Owing to the above advantages, the as-prepared A-Sb/CF electrode exhibited improved stability with a high CE of about 93% over 500 cycles at the current density of 10 mA cm −2 with 2 mAh cm −2 and high reversibility with an average CE of 92% over 225 cycles even at the high capacity of 4 mAh cm −2 .Moreover, a Ni//Sb battery based on A-Sb/CF anode delivered a high areal capacity of 2.17 mAh cm −2 , as well as a stable cycle life of 87.6% after 500 cycles.This work provides a simple design strategy to prepare the high-performance Sb anode, which shows great potential in the applications of AABs.

AUTHOR CONTRIBUTIONS
Figure 2A displays the X-ray diffraction (XRD) patterns of Sb/CF and A-Sb/CF samples.Most of the diffraction peaks of the samples are well consistent with Sb (JCPDS card No. 35-0732), except the five peaks diffraction peaks located at 27.5°, 31.9°,45.8°, and 54.3°t hat can be attributed to the (222), (400), (440), and (622) of the Sb 2 O 3 , respectively.This result indicates that the antimony in C 8 H 10 O 15 Sb 2 K 2 is completely converted to Sb and Sb 2 O 3 .In addition, the presence of carbon is further confirmed by Raman spectroscopy.The distinct D and G bands of carbon at 1332.4 cm −1 and 1592.5 cm −1 are clearly observed in both samples (Figure2B).The intensity ratio of the D band and G band (I D /I G ) is 0.91 for A-Sb/CF and 0.85 for Sb/CF, indicating the increased disorder degree of carbon after KCl activation.Thermogravimetric analysis (TGA) was collected to quantify the amount of amorphous carbon and Sb in Sb/CF and A-Sb/ CF samples.As shown in FigureS2, A significant weight loss can be seen from 350°C to 600°C, which is assigned to a combination of oxidation of Sb to Sb 2 O 3 and the consumption of carbon in the air.The final weight percentages of Sb 2 O 3 are about 72.0% and 87.1% for A-Sb/ CF and Sb/CF, respectively.Accordingly, the calculated carbon in A-Sb/CF and Sb/CF is 39.9% and 27.2%, respectively.Apparently, the increase of carbon content is beneficial to improve the electronic conductivity and stability of the composite, which is further verified by the electrochemical impedance spectroscopy (EIS) results.As shown in Figure2C, the A-Sb/CF exhibits a much smaller diameter of the semicircle (related to the charge-transfer resistance) than that of Sb/CF, indicating a faster charge transfer for A-Sb/CF.The chemical composition of Sb/CF and A-Sb/CF samples was investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR).The XPS survey spectrum of A-Sb/CF confirms the chemical composition of Sb, O and C without other impurities (FigureS3).The high-resolution Sb 3d XPS spectrum of Sb/CF sample displays two peaks at 531.2 eV and 540.5 eV (Figure2D), assigned to the presence of Sb-O bonds, suggesting that the surface of Sb nanoparticles is completely oxidized.Compared with Sb/CF sample, two peaks for the Sb-O bonds in the A-Sb/CF sample shifted toward the lower binding energy, indicating more electron transfer from carbon to antimony, which results in an enhanced interaction between carbon and Sb.The C 1s spectrum of the A-Sb/CF exhibits a broader shoulder peak than Sb/CF, indicating the slightly increased functional groups on carbon (FigureS4).The FT-IR results of all samples are collected in FigureS5.The absorption band at 1690 cm −1 is assigned to ν (C=C).Yet, for A-Sb/CF sample, an extra medium intensity band at ≈699 cm −1 generated by the stretching vibration of Sb-O-C, indicating that Sb and carbon are linked by a strong oxygen bond.Moreover, a weaker Sb-O-C vibration peak was also observed in Sb/CF.The electrochemical measurements of Sb/CF and A-Sb/CF electrodes were conducted in three-electrode cells with 3 M KOH and 0.1 M C 8 H 4 K 2 O 12 Sb 2 as the electrolyte.The plating/stripping electrochemistry of Sb on Sb/CF and A-Sb/CF was firstly investigated by cyclic voltammograms (CV).As shown in Figure S6, A couple F I G U R E 1 (A) Scheme diagram of the synthesis procedure of the A-Sb/CF sample.(B) Large-scale and (C) enlarged SEM images of A-Sb/CF.(D) Elemental mapping of C, O, and Sb distributions in the A-Sb/CF sample.(E) TEM image, (F) HRTEM image, and (G) SADE pattern of A-Sb/CF sample.HRTEM, high-resolution TEM; SEM, scanning electron microscope; TEM, transmission electron microscopy.

F
I G U R E 2 (A) XRD patterns, (B) Raman spectra, (C) Nyquist plots, and (D) high-resolution Sb 3d XPS spectra of Sb/CF and A-Sb/CF samples.XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction.
Figure 5A shows the energy storage mechanism of the Ni//Sb battery.During the charge/discharge process, the A-Sb/CF anode experiences a reversible plating/stripping of Sb/SbO 2 − redox reaction, while the CNSOH undergoes a Faradaic redox reaction.CV curves of Ni//Sb battery at various scan rates are displayed F I G U R E 3 Voltage profiles of Sb/CF and A-Sb/CF electrodes at 10 mA cm −2 with a fixed charging capacity of (A) 2 mAh cm −2 and (B) 4 mAh cm −2 .(C and D) Corresponding CE of the Sb plating/stripping on Sb/CF and A-Sb/CF electrodes.(E) The discharging curves of Sb plating/stripping at different cycles on Sb/CF and A-Sb/CF electrodes.

F
I G U R E 4 (A) Contact angle of Sb/CF and A-Sb/CF with electrolyte.(B) Arrhenius behavior of the resistance corresponding to Sb 3+ deposition.(C) The first Sb deposition voltage curves at 10 mA cm −2 and 1 mAh cm −2 on Sb/CF and A-Sb/CF electrodes.SEM images for Sb deposits with different deposition capacities from 2 to 8 mAh cm −2 on (D) Sb/CF and (E) A-Sb/CF electrodes.SEM, scanning electron microscope.