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oxides nanosheets for high-performance sodium-ion battery anodes
Journal of Power Sources 414 (2019) 470–478
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Tailored synthesis of antimony-based alloy/oxides nanosheets for highperformance sodium-ion battery anodes
T
Tuan Loi Nguyena, Tejaswi Tanaji Salunkheb, Thuan Ngoc Vob, Hyung Wook Choic, Young-Chul Leed, Jin-Seok Choie, Jaehyun Hurb,∗∗, Il Tae Kimb,∗ a
Future Materials and Devices Laboratory, Institute of Fundamental and Applied Sciences, Duy Tan University, 03 Quang Trung, Da Nang, 550000, Viet Nam Department of Chemical and Biological Engineering, Gachon University, Seongnam-si, Gyeonggi-do, 13120, Republic of Korea c Department of Electrical Engineering, Gachon University, Seongnam-si, Gyeonggi-do, 13120, Republic of Korea d Department of Bionano Technology, Gachon University, Seongnam-si, Gyeonggi-do, 13120, Republic of Korea e Analysis Center for Research Advancement, Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon, 34141, Republic of Korea b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Ni Sb O nanosheet mixtures are syn• thesized via a galvanic replacement x
y
z
reaction.
Ni Sb O nanosheet mixtures contain • Sb, NiSb , and Sb O with sheet x
y
z
2
• • •
2
3
structure. Large interlayer spacing between sheets facilitate the movement of Na+ ions. NixSbyOz electrode exhibits high reversible cyclability and high-rate capability. NixSbyOz || NaxFeFe(CN)6 full cell demonstrates an energy density of 150 Wh kg−1.
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium-ion batteries Layered structure Antimony Antimony oxide NieSb alloy Nanosheet anodes
We report a facile approach to synthesize NixSbyOz nanosheet mixtures consisting of Sb, Sb2O3 and NiSb2 for application as potential anode materials in sodium-ion batteries. The reaction time affects the morphology, particle size, and phase components of the NixSbyOz nanosheet mixtures. The nanosheet mixture (specifically, NixSbyOz-6h) exhibits an impressive electrochemical performance by demonstrating a remarkable cycling stability (over 250 cycles) with high coulombic efficiency (∼99%), reversible sodium storage capacity (∼382 mAh g−1), and excellent rate capability (∼315 mAh g−1 at 10 A g−1). The notable electrochemical performance can be ascribed to the presence of layered structures and the Ni inactive phase functioning as buffer spaces for the significant volume expansion of Sb and preventing the collapse of the electrodes during the cycling process, as well as the shorter Na+ diffusion length, improving reaction kinetics. Furthermore, to realize a potential full-cell, the nanosheet anode is combined with a Prussian blue NaxFeFe(CN)6 as a cathode, which exhibits a notable electrochemical performance with stable and high rate capacity by achieving an energy density of ∼150 Wh kg−1.
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Hur), [email protected] (I.T. Kim).
∗∗
https://doi.org/10.1016/j.jpowsour.2019.01.033 Received 21 October 2018; Received in revised form 21 December 2018; Accepted 13 January 2019 Available online 19 January 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
Journal of Power Sources 414 (2019) 470–478
T.L. Nguyen et al.
1. Introduction
Prussian blue NaxFeFe(CN)6 cathode, which exhibited a working voltage of 2.5 V and generated an energy density of 150 Wh kg−1. The fullcell system had excellent electrochemical performance, exhibiting a stable cyclic and rate performance up to 50 cycles as well.
Lithium-ion batteries (LIBs), offering the largest energy density and output voltage compared to other existing energy storage systems, have become the prime choice as power sources for electric vehicles (EVs). Up to now, LIBs have attracted wide attention in energy storage; however, recently, sodium-ion batteries (SIBs) have been considered again due to the higher cost of Li resources [1,2]. The high cost remains a critical limiting factor for the widespread availability of battery energy storage. In the periodic table, Na is located below Li, and they share similar chemical properties in several aspects. However, Na is significantly cheaper than Li owing to the unlimited Na sources and its convenient recovery [3,4]. Therefore, SIBs can provide an alternative chemistry to LIBs [5], and possibly become competitive to LIBs in certain other fields of application including the energy storage industry. Although SIBs exhibit a lower energy density than LIBs owing to their large atomic weight of Na (∼23 g mol−1) compared with Li (∼7 g mol−1) as well as a reduction/oxidation potential ∼0.3 V above that of lithium, they can be applied in next generation stationary energy storage systems. The properties of Na ions required to be thoroughly studied to be able to introduce them in novel alternative energy storage systems. Recently, research interest in SIBs has been renewed, driven by new applications with requirements different from those of portable electronics [6]. A number of cathode materials have been studied and applied for SIBs [7–10], while, in comparison, research on anode materials is limited, and it is still more challenging to develop novel anodes with outstanding electrochemical performance and appropriately low redox potentials. Therefore, finding novel anode materials for SIBs has become extremely urgent and attracting significant interest. In recent years, two-dimensional (2D) materials have been popularly applied in a variety of applications, which could include energy storage [11–15], catalysts [16], and sensors [17] owing to their unique structures and properties [18,19]. Among the available 2D materials, 2D inorganic nanomaterials have received increasing attention for the application in SIBs, particularly mono- and multi-layered nanosheets [12,20]. 2D nanosheet materials can offer significant benefits when used as electrodes for SIBs as they exhibit the following beneficial properties owing to their structures. (1) The significant volume expansion during cycling, resulting in the collapse of the structures of electrodes, can be effectively reduced by exploiting the existence of 2D nanosheets with large interlayer spacing between the nanosheets. (2) Nanosheets with a large specific surface area can reduce the diffusion length of Na+ ions, resulting in a fast charge–discharge capability and an improvement of reaction kinetics. (3) The separate nature of the sheets allows the effective strain relaxation upon cycling and enables the integrity of the electrode as well. At the same time, Sb has been also known as a highcapacity anode material for SIBs owing to its theoretical capacity of 660 mAh g−1 when alloyed with a Na+ ion to form a Na3Sb alloy [21]. However, the large volume expansion of Sb (∼390%) during the sodiation process typically degrades the cycling stability [22]. To overcome these issues, Sb-based alloy materials, including MoSb [23], NiSb [24–26], ZnSb [27,28], CuSb [29,30], and SnSb [31,32] can be suitable choices as inactive metals in an Sb-based alloy, which is capable of functioning as a buffering matrix, reducing the drastic volume changes and maintaining the electrode structures, in addition to preventing the aggregation of active Sb, which results in a highly stable cycle life. In this work, we synthesized NixSbyOz nanosheet mixtures (NSMs) with a 2D structure consisting of Sb, Sb2O3 and NiSb2 by galvanic replacement reaction (GRR) for the first time. The NSMs were designed to exploit the benefits of the 2D structure and inactive phase (Ni), which reduces the volume expansion. The analysis of the as-prepared NixSbyOz NSMs as anodes in half-cells revealed an impressive electrochemical performance with remarkable cycling stability, high coulombic efficiency, reversible sodium storage capacity, and excellent rate capability. Furthermore, a full-cell was developed with an NSM anode and
2. Experiment section 2.1. Synthesis of Ni nanosheets For the preparation of Ni nanosheets, 0.70 g of polyvinylpyrrolidone (MW = 360000, Sigma-Aldrich) and 0.15 g of poly-(2-ethyl-2-oxazoline) (MW = 50000, Sigma-Aldrich) were dissolved in 30 mL of triethylene glycol (TEG, 99%, Sigma-Aldrich) with vigorous stirring at 90 °C to form a homogeneous solution. After that, 30 mL of nickel chloride solution (0.14 M) in TEG was added to this solution. Next, the pH of the solution was increased to 12 using 2 M NaOH solution, followed by the dropwise addition of 40 mL of sodium borohydride solution (1.40 M) in TEG. After complete addition, the mixture was stirred at 90 °C under Ar atmosphere for 2 h. Then the Ni nanosheets were collected by centrifugation at 10000 rpm for 15 min and washed three times with ethanol solution. Finally, they were suspended again and sonicated in TEG to be used for the synthesis of the NixSbyOz NSMs at the next stage. 2.2. Synthesis of NixSbyOz nanosheet mixtures The NixSbyOz NSMs were prepared by refluxing the as-prepared Ni nanosheets and antimony chloride (SbCl3, 99%, Sigma-Aldrich) in 100 mL TEG at 180 °C under Ar atmosphere. First, the solution consisting of Ni nanosheets in 70 mL TEG was heated to 180 °C. Then, 30 mL of a solution consisting of 0.80 g SbCl3 in TEG was added dropwise to the solution. Following the complete addition of SbCl3, a constant temperature was maintained at 180 °C for 3 h and 6 h. The NixSbyOz NSMs were obtained by centrifugation at 10000 rpm for 15 min and washed with distilled water three times. Finally, the samples were collected after annealed at 300 °C for 4 h under Ar atmosphere. The final powders are referred to as NixSbyOz-3h and NixSbyOz6h. 2.3. Synthesis of NaxFeFe(CN)6 Prussian blue cubes To prepare Prussian blue cubes, 2.00 g of sodium ferrocyanide decahydrate (Na4Fe(CN)6.10H2O, 99%, Sigma-Aldrich) was dissolved in 100 mL of 1 M HCl solution at 80 °C by vigorous magnetic stirring under Ar gas, and the solution was then stirred for 2 h. At the end of the reaction, a blue solid powder was collected by centrifugation, washed with water several times, and then dried in oven at 80 °C for 24 h. A dark blue powder was obtained and identified as NaxFeFe(CN)6. 2.4. Materials characterization The samples were examined by X-ray powder diffraction (XRD) using an automated Rigaku D/max 2200 (Japan) X-ray diffractometer with monochromatic Cu Kα radiation. The morphology of each powder sample was investigated by scanning electron microscopy (SEM) using a Hitachi S-4700 field emission scanning electron microscope, while transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a Talos F200X (Thermo Fisher Scientific) instrument. The composition and surface characteristics of each sample were investigated by energy dispersive X-ray spectroscopy (EDS) using the same Talos F200X instrument and X-ray photoelectron spectroscopy (XPS, Kratos AXIS Nova). The specific surface areas of the materials were measured by the Brunauer–Emmett–Teller (BET) nitrogen adsorption/desorption method (ASAP 2020). The thickness of the materials was measured by atomic force microscopy (AFM) using a XE-100 (PSIA) instrument. 471
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2.5. Electrochemical measurements 2.5.1. Half-cell measurements The electrochemical performance was measured directly using coin cells (CR2032), which were assembled in an Ar filled glove box. First, NixSbyOz and NaxFeFe(CN)6 were individually evaluated in half-cells. The NixSbyOz anodes were prepared on Cu foil with slurries comprised of 70 wt% active material, 15 wt% poly (acrylic acid), 15 wt% Super P, and ethanol as solvent. To prepare the NaxFeFe(CN)6 cathode, NaxFeFe (CN)6 powder was blended with super P and polyvinylidene fluoride in a 70:15:15 wt ratio, and homogeneously dispersed in N-methylpyrrolidone, and the slurry was cast onto an Al foil. These electrodes were dried at 60 °C for 3 h in an oven, and at 70 °C overnight in a vacuum oven. The density of active material on the electrodes was about 1.4 ̴ 1.7 mg cm−2, and the capacities of the electrodes were calculated based on the total mass of active material. Sodium foil was used as the counter electrode, and a glass separator was utilized as the separator. A solution consisting of 1 M NaClO4 in a mixture of propylene carbonate and ethylene carbonate (1:1 by volume) with 2% fluoroethylene carbonate was used as the electrolyte. Cyclic voltammetry (CV) was performed using a ZIVE MP1 electrochemical workstation at a scan rate of 0.1 mV s−1 in the potential range of 0.00–2.00 V while the galvanostatic charge/discharge experiments were carried out at a constant current density of 100 mA g−1 in the range of 0.01 V–2.00 V (vs. Na/ Na+) for the NixSbyOz electrodes, and at a constant current density of 50 mA g−1 in the range of 2.00–4.00 V for the NaxFeFe(CN)6 electrode using a battery cycler (WBCS3000, WonAtech) system. The rate–cycling performance of the cells was tested at different charge current densities of 0.1, 0.5, 1.0, 3.0, 5.0, and 10.0 A g−1 for the NixSbyOz electrodes, and at 50, 100, 200, 400, and 1000 mA g−1 for the NaxFeFe(CN)6 electrode. Electrochemical impedance spectroscopy (EIS) was performed for the NixSbyOz electrodes using the ZIVE MP1 workstation at AC voltage with an amplitude of 10 mV in the frequency range of 100 kHz–0.1 Hz following the first 50 cycles. The CV tests were conducted for the NixSbyOz electrodes after two cycles using the ZIVE MP1 workstation in the potential range of 0.00–2.00 V at various scanning rates of 0.1, 0.2, 0.4, 0.6, and 1.0 mV s−1 to be applied in the calculation of the Na+ ion diffusion coefficients.
Fig. 1. Illustration for the formation of Ni nanosheets.
and reacted with Sb3+ to generate NixSbyOz NSMs in a TEG solvent by GRR. The difference in reduction potentials between Ni and Sb was the impetus of the electrochemical reaction. The potential of the Ni2+/Ni (−0.72 V vs. SHE) was lower than that of the Sb3+/Sb (−0.639 V vs. SHE) [35,36]. When the GRR was initiated, Ni atoms were transformed to Ni2+, and Sb3+ captured the electrons from the oxidation reaction of Ni to Ni2+ to generate Sb atoms on the surface of the Ni (eq. (1)). Subsequently, Sb atoms reacted with Ni atoms to form a NiSb2 alloy (eq. (2)). Then, Sb3+ formed a hydroxyl metal (eq. (3)) in TEG during the GRR. Finally, a Sb2O3 phase was formed by a reaction involving heat treatment at 300 °C for 4 h in Ar atmosphere (eq. (4)). 3 Ni + 2 Sb3+ → 3 Ni2+ + 2 Sb
(1)
Ni + 2 Sb → NiSb2
(2)
3+
Sb
+ 3 OH
−
→ Sb(OH)3
2 Sb(OH)3 → Sb2O3 + 3 H2O
(3) (4)
The phase components of Ni and NixSbyOz NSMs obtained by GRR were determined by XRD analysis as shown in Fig. 2. In the XRD pattern of Ni, shown in Fig. 2(a), the peaks at 39.1, 41.5, 44.5, 58.4, and 70.9° can be attributed to Ni (PDF 45–1027). The obtained Ni exhibited high purity and in good crystallinity. As can be seen in Fig. 2(b) and (c), the peaks of Ni disappear following 3 h and 6 h of GRR as they are dissolved by Sb3+ ions and generate alloys with Sb atoms during the reaction. The 2θ peaks at 28.5, 39.9, 41.7, 51.4, and 59.2° were observed, which could be indexed to Sb (PDF 71–1173), while the peaks at 25.8, 27.1, 31.5, 34.4, 45.4, and 56.6° can be attributed to Sb2O3 (PDF 71–0365). In addition, the 2θ peaks at 28.5, 31.9, 32.8, and 44.1° were related to NiSb2 (PDF 25–1083). According to powder XRD spectra, the NSMs are crystalline with the admixture of an amorphous part. The results were compatible with the EDS results shown in Table S1. The crystalline Ni structures were observed using HRTEM. The lattice fringes of Ni, shown in Fig. 3(a), correspond to the (011) crystallographic phase with an interlayer distance of ∼0.20 nm. The Ni phase with plate/spherical shapes was covered by carbon sources due
2.5.2. Full-cell measurements To create a full-cell, the NixSbyOz anode was assembled in a half-cell and discharged to 0.01 V to achieve a pre-sodiation state. Similarly, the NaxFeFe(CN)6 cathode was assembled in a half-cell as well and charged to 4.00 V to achieve a pre-desodiation state. This step is to introduced to prevent the large irreversible capacity loss associated with the NixSbyOz anode, and the low coulombic efficiency (CE) associated with the NaxFeFe(CN)6 cathode [33]. Subsequently, the NixSbyOz anode and the NaxFeFe(CN)6 cathode were removed from the corresponding half-cells, and paired together in a CR2032 cell. The mass balancing of anode and cathode were from 1:2 to 1:3. Finally, charge/discharge tests were carried out in the range of 0.50–4.00 V at a current density of 25 mA g−1 (based on the mass of the cathode). 3. Results and discussion The Ni nanosheets were prepared with NiCl2 solution in TEG, the support of NaOH and PVP as surface stabilizers, and NaBH4 as reduction agent. The formation of Ni nanosheets was related to the formation of Cl−⋅⋅⋅PVP in the medium, which tightly depends on the factors such as pH, temperature, concentration of halide ions (Cl−) in the solution [34], as shown in Fig. 1. During the nucleation step, Ni seeds (Ni2+ + nPVP + NaBH4 → Ni0) were generated, which consumes the amount of PVP. In the growth step, the Cl−⋅⋅⋅PVP interacted with Ni seeds to form Ni nanosheets. Therefore, it is noted that the concentration of Cl−⋅⋅⋅PVP could decide the yield of Ni nanosheets. The as-prepared Ni nanosheets were used as sacrificial templates
Fig. 2. XRD patterns of (a) Ni nanoparticles, (b) NixSbyOz-3h, (c) NixSbyOz-6h mixtures with the reference peaks of Ni, Sb, NiSb2, and Sb2O3 indicated. 472
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Fig. 3. TEM, HRTEM, and SAED images of (a) Ni phase, (b) NixSbyOz-3h, and (c) NixSbyOz-6h mixtures. STEM images with element mapping of (d) NixSbyOz-3h and (e) NixSbyOz-6h mixtures. (f) AFM images of NixSbyOz-6h mixture.
that the mixtures have type IV isotherms and type H3 hysteresis loops, indicating a mesoporous structure. The pore size distribution of the mixtures calculated by the Barrett–Joyner–Halenda method exhibited one dominant peak at 16 nm for the NixSbyOz-6h (and 30 nm for the NixSbyOz-3h) and another weak peak at 3.75 nm (Fig. S3(c)-(d)), suggesting that mesopores exist within the NSMs. The specific surface area of the NixSbyOz-6h is two times higher than that of the NixSbyOz-3h. The
to the PVP medium (Fig. 3(a)). The SEM images of the NixSbyOz NSMs also show large-sized plate-shaped and stacked structures (Fig. S1). By increasing the reaction time from 3 h to 6 h, NixSbyOz-6h NSM with larger lateral size and thinner thickness was gradually formed during the GRR, as shown in Fig. 3(f) and Fig. S2. Therefore, the specific surface areas of the samples are expected to increase with the increase of the reaction time as shown in Fig. S3(a)-(b) and Table S2. It is clear 473
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morphological features of NixSbyOz mixtures were further investigated by TEM, HRTEM, selected area electron diffraction (SAED), and AFM analyses, as shown in Fig. 3(b)–(f). It should be noted that black and white stripes can be seen in the HRTEM images, which correspond to the NixSbyOz lattice spaces between layers. This morphology is a direct evidence of the sheet structure of the NixSbyOz NSMs. The TEM and AFM images show a 2D thin sheet morphology (green circles) and stacks of thin sheets together (red circles). The distance between sheets is approximately 1 nm for both the NixSbyOz-3h and the NixSbyOz-6h, which is significantly greater than the radius of the Na+ ion (∼10 times); thus it is expected to enable convenient insertion and extraction of Na+ ions. When looking into the surface of the materials, HRTEM and SAED images (Fig. 3(b) and (c)) confirm the existence of crystalline Sb, Sb2O3, and NiSb2 phases in addition to the presence of amorphous regions in the mixtures, which is in good agreement with XRD results in Fig. 2. Moreover, the scanning TEM (STEM) images with element mapping shown in Fig. 3(d)-(e) illustrate the presence and distribution of Sb, Ni, and O elements in the NixSbyOz. It is noted that NixSbyOz-6h sample shows well-distributed Ni phase compared to NixSbyOz-3h sample, which could affect the electrochemical performance. In addition, the AFM analysis in Fig. 3(f) demonstrates the thickness of NixSbyOz-6h NSM. It clearly shows that the as-prepared NixSbyOz-6h has a 2D structure with several sheet layers exhibiting average thickness of ∼14 nm. The electrochemical properties of NixSbyOz anodes were analyzed for the application in SIBs. Fig. 4 shows the initial discharge and charge voltage profiles of the NixSbyOz electrodes at a current rate of 100 mA g−1 in the range of 0.01–2.00 V. When considering the weight percent of Sb in NSMs, which is more than 84% as seen in Table S1, the calculated capacity could be over 554 mAh g−1 (84% x 660 mAh g−1) for NSM electrodes. The NixSbyOz-3h electrode exhibited charge and discharge capacities of 349 and 564 mAh g−1, respectively, corresponding to an initial coulombic efficiency (ICE) of 62%. The NixSbyOz6h electrode demonstrated initial charge and discharge capacities of, respectively, 384 and 686 mAh g−1, exhibiting an ICE of 56%. The difference between the ICE values resulted from the excessive consumption of Na+ ions, which is mainly due to higher surface area of NixSbyOz-6h, to form a solid electrolyte interphase (SEI) layer during the first cycle. When the reaction time was increased from 3 h to 6 h, the charge/discharge capacities increased, which can be ascribed to an increase in the weight percent of the electrochemically active component of Sb and the specific surface areas, as shown in Table S1 and Table S2. Fig. 5(a) and (b) show CVs of the NixSbyOz electrodes in initial three cycles in the range of 0.00–2.00 V. The cathodic and anodic peaks for the NixSbyOz-3h and NixSbyOz-6h were the same, indicating that the same reaction occurred during the discharge/charge processes. In the first cycle, the CV contains a small peak at 1.00 V and a broad irreversible peak area at 0.82 V, which are related to the insertion of Na+
ions between the electrode sheets (1.00 V), the conversion reaction of Sb2O3 to form Sb and Na2O (eq. (5)), and the SEI layer formation (0.82 V). The broad peak at 0.34 V corresponds to the alloy reactions of NiSb2 and Sb to metallic Ni and alloy NaxSb, respectively (eq. (6) and eq. (7)). From the second cycle onward, no reaction corresponding to the conversion of Sb2O3 and alloy reaction of NiSb2 was observed, indicating that these reactions occurred only in the first cycle. We observed four cathodic peaks at 1.00, 0.67, 0.55, and 0.45 V for both the NixSbyOz-3h and NixSbyOz-6h electrodes in the subsequent cycles. The cathodic peak at 1.00 V corresponds to the insertion of Na+ ions between the sheets of mixture electrodes, while the cathodic peaks at 0.67, 0.55, and 0.45 V can mainly be attributed to the formation of the alloy NaxSb from Sb. In the first three cycles, three peaks at 0.77, 0.85, and 1.70 V were detected in the anodic processes. The peaks at 0.77 and 0.85 V are related to the phase transition from NaxSb to Sb (0 < x ≤ 3) [37–43]. A broad anodic peak at 1.70 V represents the removal of Na+ ions from the sheets [44–46]. After the first cycle, the shapes of the curves were nearly identical, indicating a stable electrochemical reaction in the NSMs. Sb2O3 + 6 Na+ + 6 e− → 2 Sb + 3 Na2O NiSb2 + 2x Na Sb + x Na
+
+
+ 2x e
+xe
−
−
→ Ni + 2 NaxSb
↔ NaxSb
(5) (6) (7)
The cycling performance of the NSM electrodes was also analyzed, as shown in Fig. 5(c). The NixSbyOz-3h electrode exhibited a charge capacity with high stability, demonstrating a reversible capacity of 311 mAh g−1 with a capacity retention of ∼89% after 120 cycles. At the same time, the NixSbyOz-6h electrode showed a high stability as well; after 250 cycles, the NixSbyOz-6h electrode delivered a charge capacity of 382 mAh g−1, corresponding to a capacity retention of 99.5%. It should be noted that NixSbyOz-6h NSM electrode demonstrated a superior cyclic performance compared with that of the NixSbyOz-3h NSM, which can be attributed to a reduction in the Na+ diffusion length of the NixSbyOz-6h NSM during the prolonged cycling, as well as the greater amount of Sb components. Furthermore, the CE of the electrodes increased gradually in the subsequent cycles and became greater than 98% after the first 10 cycles. The realized high CE of the electrodes is due to the presence of Ni, which functions as an electron conductor, resulting in stable structures in the charge/discharge processes. In addition, the cycling performance of NixSbyOz electrodes with different ratios of NixSbyOz material: PAA binder: super P was investigated as shown in Fig. S4. When looking into the cycling performances, there were no significant differences in cyclability up to 30 cycles. One thing is that the electrode with 15% of PAA binder shows slightly increased capacity than others, which could be due to the effective binding function from a number of functional groups in PAA binder, leading to the better cycling performance. The rate cyclic performance is also shown in Fig. 5(d). The NixSbyOz-6h electrode exhibited a significantly better rate capability than the NixSbyOz-3h electrode. The NixSbyOz-6h electrode delivered specific capacities of 350 mAh g−1 at 100 mA g−1, 348 mAh g−1 at 500 mA g−1, 341 mAh g−1 at 1 A g−1, 330 mAh g−1 at 3 A g−1, 328 mAh g−1 at 5 A g−1, and 315 mAh g−1 at 10 A g−1, corresponding to capacity retentions of, 100.00%, 99.53%, 97.41%, 94.35%, 93.73%, and 89.95%, respectively. When the current density returned to the value of 100 mA g−1, the charge capacity recovered to the value of 348 mAh g−1, demonstrating a capacity restoration of 99.5% and indicating high reversibility with structural stability. These indicate that the observed stable cycling and high rate capability for the NixSbyOz-6h electrode can be attributed to the formation of a thin and stable SEI layer together with an inactive phase (Ni), while the sheet structures that can offer a higher surface area and shorter path length for Na+ transport, maintain a stable contact with the current collector, and allow better reaction kinetics at the electrode surface. Furthermore, the Na2O and Ni phases, which are the products in eq. (5) and eq. (6),
Fig. 4. Initial voltage profiles of NixSbyOz-3h and NixSbyOz-6h electrodes. 474
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Fig. 5. CVs of (a) NixSbyOz-3h and (b) NixSbyOz-6h. (c) Cyclic performance and (d) rate cyclic performance of NixSbyOz-3h and NixSbyOz-6h electrodes.
electrochemical performance of NixSbyOz NSMs, EIS measurements were conducted. Typical Nyquist plots of the NixSbyOz electrodes after 50 cycles are shown in Fig. S6 and Table S4. In the equivalent circuit, the SEI layer impedance (Rs) is represented by the semicircle in the high frequency, the medium frequency semicircle corresponds to the charge transfer resistance (Rct), the inclined line represents the Na+ diffusion process (W), and Re is the ohmic resistance of the electrolyte and cell components. The Rs value for NixSbyOz-3h is larger than that of NixSbyOz-6h; for example, the Rs value of NixSbyOz-3h is 32.4 Ω while for the NixSbyOz-6h it is 13.7 Ω. The difference in Rs can be attributed to the thinner layered structures of the NixSbyOz-6h electrode, resulting in a stable cyclability. The lower Rct value indicates a faster the Na+ diffusion rate; thus, a better cycling performance can be obtained. The Rct value of NixSbyOz-3h, is 261.8 Ω while it is 169.5 Ω for the NixSbyOz6h. Both the Rs and Rct values for the NixSbyOz-6h electrode are lower than those of the NixSbyOz-3h electrode. The lowest resistance of the NixSbyOz-6h electrode can be attributed to the developed thin sheets, resulting in a shorter electron transport distance and the reduction of the diffusion length of the Na+ ions. In order to obtain further insight into the electrochemical reaction of sodiation and desodiation processes of the NixSbyOz electrodes, the diffusion coefficients (D) of the as-prepared electrodes were calculated by applying the classical Randles–Sevcik equation [48,49], as shown in Fig. 6(a) and Fig. S7(a)-(b). The D values of the electrodes were calculated as 1.10 × 10−7 cm2 s−1 for the NixSbyOz-3h and 3.16 × 10−7 cm2 s−1 for the NixSbyOz-6h electrodes, and they are in the order of approximately 10−7 cm2 s−1, where the NixSbyOz-6h electrode showed higher values than the NixSbyOz-3h electrode. Therefore, it can be concluded that the superior Na storage performance of the NixSnyOz-6h can also be attributed to its high Na+ diffusion ability. In order to further investigate on diffusion phenomena, the pseudo-capacitive and diffusion-controlled behaviors need to be considered and determined [15,50,51], as shown in Fig. 6(b). It can be clearly seen that the slopes (b value) of NixSbyOz-3h and NixSbyOz-6h are 0.64 and 0.60, respectively. These values are close to 0.5, indicating that the electrochemical reaction of NixSbyOz NSMs is dominated by a diffusion-controlled behavior. Moreover, the total capacitive contributions, including pseudo-capacitive and diffusion-controlled behaviors, were calculated [15,50,52], as shown in Fig. 6(c)−(d) and Fig. S7(c)−(d). As can be seen in Fig. 6(c) and (d), the quantities of the
were contained inside the electrodes. They are both inactive components, functioning as effective buffers against volume changes for the following (de)alloying reaction, while Ni is an electron conductor; thus, an excellent stability of the electrodes can be achieved. Moreover, the significantly enhanced high-rate performance can also be closely related to the sheet structures of NixSbyOz NSMs. Specifically, the large interlayer spacing between the sheets has strong effects on the transport of Na+ at the high rate without instigating detrimental changes to the structure [47]. The NixSbyOz-6h electrode has the largest lateral size and the highest specific surface area; as a result, this electrode demonstrates the best performance. Based on the aforementioned discussion on the electrochemical performance, it is true that each component in NixSbyOz mixtures plays different roles on electrochemical performances. Two different samples with different compositions of Ni, Sb, and O were prepared by changing reaction time, and the weight percent of each component was calculated from EDS and XPS analyses, which are summarized in Table S1 and Table S3, respectively. As shown in Fig. S5, the spectra reveal the presence of Sb, O, and Ni in mixtures, and the peaks of Sb and O components are overlapped, which are deconvoluted (Fig. S5(a) and S5(c)), while the peak intensity of Ni is noisy due to the existence of very small amount of Ni in mixtures (Fig. S5(b) and Fig. S5(d)). It is noted that when increasing the reaction time, the amount of Ni and O components somewhat decreased while the amount of Sb increased even though the difference is not much significant. This tendency is well matched with that in EDS results. From this point of view, the existence of higher Sb and lower O components in NixSbyOz6h mixture could lead to higher capacity and less formation of SEI layer (related to Na2O phase during cycling), leading to better electrochemical performance. In specific, Sb component coming from Sb, Sb2O3, and NiSb2 phases acts as active material contributing to the capacity. The other components including Ni from NiSb2 and O (the actual phase: Na2O) from Sb2O3 phase could play roles, respectively, as conductive medium (conductivity of Ni: 1.43ⅹ105 S cm−1) and buffers to reduce large volume expansion of Sb during sodiation as discussed earlier. Even though the difference in the amount of components is small, finally, it can be summarized that the electrochemical performance is better for NixSbyOz-6h, which is mainly due to larger surface area, well-distributed Ni with high conductivity in the nanosheet mixture, and less formation of Na2O phase related to SEI layer. To further investigate the mechanisms contributing to the 475
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Fig. 6. (a) Relationships between Ip and ν1/2 for the NixSbyOz electrodes and (b) relationships between log i and log ν. Pseudo-capacitive contribution (yellow color) and diffusion-controlled contribution (olive color) at a scan rate of 0.4 mV s−1 for (c) NixSbyOz-3h and (d) NixSbyOz6h electrodes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
to achieve a pre-sodiation state in the half-cell and to form a stable SEI layer, while the cathode was pre-desodiated by applying a potential of 4.0 V. The electrochemical reaction of the full-cell can be described as follows:
pseudo-capacitive and ionic diffusion contributions are 21.9% and 78.1% for the NixSbyOz-3h, respectively, and 23.4% and 76.6% for the NixSbyOz-6h, respectively, at a scan rate of 0.4 mV s−1. The pseudocapacitive contributions of the NixSbyOz-6h are higher than that of the NixSbyOz-3h at the same scan rate, indicating that the kinetics can be expected to be transferred from diffusion-controlled to surface-controlled (pseudo-capacitive) reaction, resulting in a better electrochemical performance than that of the NixSbyOz-3h [51]. For the practical application of Na-ion cells, a full-cell system was developed with a NaxFeFe(CN)6 cathode. The crystalline structure and morphology of the NaxFeFe(CN)6 material were analyzed, as shown in Fig. S8 and Fig. S9. The XRD pattern of the NaxFeFe(CN)6 material has sharp and strong diffraction peaks, indicating a well-developed crystalline nature. The patterns of the NaxFeFe(CN)6 material are in good agreement with the standard Prussian blue Fe4 [Fe(CN)6]3 (PDF 73–0687), showing a typical face-centered cubic phase [53–56]. The SEM image shows that the NaxFeFe(CN)6 material has cubic shapes with a size of ∼800 nm, as can be seen in Fig. S9 [33]. The electrochemical properties of the NaxFeFe(CN)6 material were tested at a current density of 50 mA g−1 in the range of 2.00–4.00 V (Fig. 7). The different capacity plot (DCP) curves of the electrode showed two anode peaks at 3.00 V and 3.65 V and two cathode peaks at 2.88 V and 3.50 V, which are related to the redox potentials for the high-spin and low-spin Fe ions during the (de)sodiation process [54,57]. As can be seen in Fig. 7(a), the shape of these curves is nearly the same, indicating the stable electrochemical reaction of the NaxFeFe (CN)6 electrode. The voltage profiles shown in Fig. 7(b) exhibit a long sloping plateau in the range of 2.8–3.25 V with a working voltage of ∼3.0 V. Regarding the cycling performance of the NaxFeFe(CN)6 electrode, it exhibited a very stable cyclability, and demonstrated a specific capacity of 101 mAh g−1 with a high CE of ∼100% and capacity retention of 90% after 200 cycles (Fig. 7(c)). As can be seen in Fig. 7(d), the NaxFeFe(CN)6 electrode showed an excellent rate capability (83 mAh g−1 retained at 1 A g−1) for the rate cyclic performance. These results definitely indicate that NaxFeFe(CN)6 material can be used as a potential cathode material to pair with NixSbyOz anodes for Na-ion full batteries. A simple full-cell configuration with a NixSbyOz anode and a NaxFeFe(CN)6 cathode was established to test the electrochemical properties, as shown in Fig. 8(a). The anodes were discharged to 0.01 V
NaxFeFe(CN)6 + Sb ↔ NaxSb + FeFe(CN)6
(8)
The electrochemical performance of the full-cells with different mass ratios between anode and cathode was analyzed at a current density of 25 mA g−1 in the potential range of 0.50–4.00 V, as shown in Fig. 8(b)–(d) and Fig. S10, demonstrating that the 1:2 wt ratio between the anode and cathode exhibited a higher recharge capacity. Therefore, this mass ratio between the anode and cathode was employed in the full-cell study. Fig. 8(b) and (c) show typical discharge/charge curves of the NixSbyOz || NaxFeFe(CN)6 full-cell. From the voltage profiles, the NixSbyOz || NaxFeFe(CN)6 full-cell had a working voltage of ∼2.5 V. Fig. 8(d) and (e) show the cycling and rate performance of the NixSbyOz || NaxFeFe(CN)6 full-cell. It demonstrated an initial discharge capacity of ∼102 mAh g−1 and showed a stable cyclability; it exhibited a discharge capacity of 90 mAh g−1 after 50 cycles (a capacity retention of ∼88%) with a high CE of 100% during the entire cycling. Considering the rate cycling of the full-cell, it demonstrated a notable performance, as it exhibited a capacity retention of ∼72% at a high current rate of 1000 mA g−1. The details of the full-cell systems for SIBs are summarized in Table S5, and the capacities were calculated based on the total mass of the anode and cathode. As shown in Table S5 and Fig. S10(b), the capacity and energy density of the full-cells decreased with the decreasing mass ratio of the anode and cathode, which was possibly because the amount of Na+ ions were not enough from the anode to cathode. The full-cell with a mass ratio of 1:2 exhibited the highest energy density of ∼150 Wh kg−1. Although the working voltage and energy density (2.5 V and 150 Wh kg−1, respectively) are slightly lower than those of the LIBs, it is believed that the full-cell systems presented have the potential to be applied in high-performance sodium-ion batteries.
4. Conclusion Novel NixSbyOz nanosheet mixtures with a 2D structure were synthesized using a convenient GRR, and the as-prepared nanosheet 476
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Fig. 7. (a) Differential capacity plots, (b) voltage profiles, (c) cyclic performances, and (d) rate cyclic performance of the NaxFeFe(CN)6 cathode.
Fig. 8. (a) Schematic of the working principle and configuration of the NixSbyOz || NaxFeFe(CN)6 full-cell, (b) comparison of voltage profiles of the half-cell and fullcell, (c) voltage profile, (d) cyclic performances, (e) rate cyclic performance of the NixSbyOz || NaxFeFe(CN)6 full-cell. 477
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mixtures were employed as anode materials for sodium-ion cells. The NixSbyOz electrode prepared with a reaction time of 6 h (NixSbyOz-6h) exhibited a high reversible capacity of 382 mAh g−1 and a high-rate performance. The excellent electrochemical properties can be attributed to: (i) the existence of large interlayer spacing between sheets, leading to facile movement of Na+ ions, (ii) the formation of novel sheet structures providing large specific surface areas, resulting in effective electrolyte penetration and interface reactions, (iii) the generation of thin layers offering short diffusion paths of Na+ ions, (iv) the separate nature of the sheets allowing effective strain relaxation and the integrity of the electrode upon electrochemical cycling, and (v) the inactive phases (Na2O and Ni) functioning as effective buffers against volume changes during cycling, resulting in an enhanced performance for the Na storage. Furthermore, the anode was combined with a NaxFeFe(CN)6 cathode to form a sodium-ion full-cell, which exhibited a working voltage of 2.5 V and an energy density of ∼150 Wh kg−1. They showed a remarkable electrochemical performance with appropriate capacity and high rate capability as well. In conclusion, the nanosheet mixture electrodes (particularly, the NixSbyOz-6h electrode) presented in this study have great potential to be utilized as high-performance anode materials for sodium-ion batteries.
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Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (NRF2018R1A1A1A05018332). This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy of the Republic of Korea (MOTIE) (No. 20162010104190). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2019.01.033. References [1] B. Ellis, W. Makahnouk, Y. Makimura, K. Toghill, L. Nazar, Nat. Mater. 6 (2007) 749. [2] S. Komaba, W. Murata, T. Ishikawa, N. Yabuuchi, T. Ozeki, T. Nakayama, A. Ogata, K. Gotoh, K. Fujiwara, Adv. Funct. Mater. 21 (2011) 3859–3867. [3] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada, S. Komaba, Nat. Mater. 11 (2012) 512. [4] X. Lu, G. Xia, J.P. Lemmon, Z. Yang, J. Power Sources 195 (2010) 2431–2442. [5] S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, X. Ma, G. Ceder, Energy Environ. Sci. 4 (2011) 3680–3688. [6] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Adv. Energy Mater. 2 (2012) 710–721. [7] K.H. Ha, S.H. Woo, D. Mok, N.S. Choi, Y. Park, S.M. Oh, Y. Kim, J. Kim, J. Lee, L.F. Nazar, Adv. Energy Mater. 3 (2013) 770–776. [8] R. Berthelot, D. Carlier, C. Delmas, Nat. Mater. 10 (2011) 74. [9] Y. Lu, L. Wang, J. Cheng, J.B. Goodenough, Chem. Commun. 48 (2012) 6544–6546. [10] R. Tripathi, G.R. Gardiner, M.S. Islam, L.F. Nazar, Chem. Mater. 23 (2011) 2278–2284. [11] Q. Zhang, Y. Wang, Z.W. Seh, Z. Fu, R. Zhang, Y. Cui, Nano Lett. 15 (2015) 3780–3786. [12] F. Zhang, J. Zhu, D. Zhang, U. Schwingenschlögl, H.N. Alshareef, Nano Lett. 17 (2017) 1302–1311. [13] C. Liu, Z. Yu, D. Neff, A. Zhamu, B.Z. Jang, Nano Lett. 10 (2010) 4863–4868. [14] X. Wang, X. Shen, Z. Wang, R. Yu, L. Chen, ACS Nano 8 (2014) 11394–11400.
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Tailored synthesis of antimony-based alloy/oxides nanosheets for highperformance sodium-ion battery anodes
T
Tuan Loi Nguyena, Tejaswi Tanaji Salunkheb, Thuan Ngoc Vob, Hyung Wook Choic, Young-Chul Leed, Jin-Seok Choie, Jaehyun Hurb,∗∗, Il Tae Kimb,∗ a
Future Materials and Devices Laboratory, Institute of Fundamental and Applied Sciences, Duy Tan University, 03 Quang Trung, Da Nang, 550000, Viet Nam Department of Chemical and Biological Engineering, Gachon University, Seongnam-si, Gyeonggi-do, 13120, Republic of Korea c Department of Electrical Engineering, Gachon University, Seongnam-si, Gyeonggi-do, 13120, Republic of Korea d Department of Bionano Technology, Gachon University, Seongnam-si, Gyeonggi-do, 13120, Republic of Korea e Analysis Center for Research Advancement, Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon, 34141, Republic of Korea b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Ni Sb O nanosheet mixtures are syn• thesized via a galvanic replacement x
y
z
reaction.
Ni Sb O nanosheet mixtures contain • Sb, NiSb , and Sb O with sheet x
y
z
2
• • •
2
3
structure. Large interlayer spacing between sheets facilitate the movement of Na+ ions. NixSbyOz electrode exhibits high reversible cyclability and high-rate capability. NixSbyOz || NaxFeFe(CN)6 full cell demonstrates an energy density of 150 Wh kg−1.
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium-ion batteries Layered structure Antimony Antimony oxide NieSb alloy Nanosheet anodes
We report a facile approach to synthesize NixSbyOz nanosheet mixtures consisting of Sb, Sb2O3 and NiSb2 for application as potential anode materials in sodium-ion batteries. The reaction time affects the morphology, particle size, and phase components of the NixSbyOz nanosheet mixtures. The nanosheet mixture (specifically, NixSbyOz-6h) exhibits an impressive electrochemical performance by demonstrating a remarkable cycling stability (over 250 cycles) with high coulombic efficiency (∼99%), reversible sodium storage capacity (∼382 mAh g−1), and excellent rate capability (∼315 mAh g−1 at 10 A g−1). The notable electrochemical performance can be ascribed to the presence of layered structures and the Ni inactive phase functioning as buffer spaces for the significant volume expansion of Sb and preventing the collapse of the electrodes during the cycling process, as well as the shorter Na+ diffusion length, improving reaction kinetics. Furthermore, to realize a potential full-cell, the nanosheet anode is combined with a Prussian blue NaxFeFe(CN)6 as a cathode, which exhibits a notable electrochemical performance with stable and high rate capacity by achieving an energy density of ∼150 Wh kg−1.
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Hur), [email protected] (I.T. Kim).
∗∗
https://doi.org/10.1016/j.jpowsour.2019.01.033 Received 21 October 2018; Received in revised form 21 December 2018; Accepted 13 January 2019 Available online 19 January 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
Prussian blue NaxFeFe(CN)6 cathode, which exhibited a working voltage of 2.5 V and generated an energy density of 150 Wh kg−1. The fullcell system had excellent electrochemical performance, exhibiting a stable cyclic and rate performance up to 50 cycles as well.
Lithium-ion batteries (LIBs), offering the largest energy density and output voltage compared to other existing energy storage systems, have become the prime choice as power sources for electric vehicles (EVs). Up to now, LIBs have attracted wide attention in energy storage; however, recently, sodium-ion batteries (SIBs) have been considered again due to the higher cost of Li resources [1,2]. The high cost remains a critical limiting factor for the widespread availability of battery energy storage. In the periodic table, Na is located below Li, and they share similar chemical properties in several aspects. However, Na is significantly cheaper than Li owing to the unlimited Na sources and its convenient recovery [3,4]. Therefore, SIBs can provide an alternative chemistry to LIBs [5], and possibly become competitive to LIBs in certain other fields of application including the energy storage industry. Although SIBs exhibit a lower energy density than LIBs owing to their large atomic weight of Na (∼23 g mol−1) compared with Li (∼7 g mol−1) as well as a reduction/oxidation potential ∼0.3 V above that of lithium, they can be applied in next generation stationary energy storage systems. The properties of Na ions required to be thoroughly studied to be able to introduce them in novel alternative energy storage systems. Recently, research interest in SIBs has been renewed, driven by new applications with requirements different from those of portable electronics [6]. A number of cathode materials have been studied and applied for SIBs [7–10], while, in comparison, research on anode materials is limited, and it is still more challenging to develop novel anodes with outstanding electrochemical performance and appropriately low redox potentials. Therefore, finding novel anode materials for SIBs has become extremely urgent and attracting significant interest. In recent years, two-dimensional (2D) materials have been popularly applied in a variety of applications, which could include energy storage [11–15], catalysts [16], and sensors [17] owing to their unique structures and properties [18,19]. Among the available 2D materials, 2D inorganic nanomaterials have received increasing attention for the application in SIBs, particularly mono- and multi-layered nanosheets [12,20]. 2D nanosheet materials can offer significant benefits when used as electrodes for SIBs as they exhibit the following beneficial properties owing to their structures. (1) The significant volume expansion during cycling, resulting in the collapse of the structures of electrodes, can be effectively reduced by exploiting the existence of 2D nanosheets with large interlayer spacing between the nanosheets. (2) Nanosheets with a large specific surface area can reduce the diffusion length of Na+ ions, resulting in a fast charge–discharge capability and an improvement of reaction kinetics. (3) The separate nature of the sheets allows the effective strain relaxation upon cycling and enables the integrity of the electrode as well. At the same time, Sb has been also known as a highcapacity anode material for SIBs owing to its theoretical capacity of 660 mAh g−1 when alloyed with a Na+ ion to form a Na3Sb alloy [21]. However, the large volume expansion of Sb (∼390%) during the sodiation process typically degrades the cycling stability [22]. To overcome these issues, Sb-based alloy materials, including MoSb [23], NiSb [24–26], ZnSb [27,28], CuSb [29,30], and SnSb [31,32] can be suitable choices as inactive metals in an Sb-based alloy, which is capable of functioning as a buffering matrix, reducing the drastic volume changes and maintaining the electrode structures, in addition to preventing the aggregation of active Sb, which results in a highly stable cycle life. In this work, we synthesized NixSbyOz nanosheet mixtures (NSMs) with a 2D structure consisting of Sb, Sb2O3 and NiSb2 by galvanic replacement reaction (GRR) for the first time. The NSMs were designed to exploit the benefits of the 2D structure and inactive phase (Ni), which reduces the volume expansion. The analysis of the as-prepared NixSbyOz NSMs as anodes in half-cells revealed an impressive electrochemical performance with remarkable cycling stability, high coulombic efficiency, reversible sodium storage capacity, and excellent rate capability. Furthermore, a full-cell was developed with an NSM anode and
2. Experiment section 2.1. Synthesis of Ni nanosheets For the preparation of Ni nanosheets, 0.70 g of polyvinylpyrrolidone (MW = 360000, Sigma-Aldrich) and 0.15 g of poly-(2-ethyl-2-oxazoline) (MW = 50000, Sigma-Aldrich) were dissolved in 30 mL of triethylene glycol (TEG, 99%, Sigma-Aldrich) with vigorous stirring at 90 °C to form a homogeneous solution. After that, 30 mL of nickel chloride solution (0.14 M) in TEG was added to this solution. Next, the pH of the solution was increased to 12 using 2 M NaOH solution, followed by the dropwise addition of 40 mL of sodium borohydride solution (1.40 M) in TEG. After complete addition, the mixture was stirred at 90 °C under Ar atmosphere for 2 h. Then the Ni nanosheets were collected by centrifugation at 10000 rpm for 15 min and washed three times with ethanol solution. Finally, they were suspended again and sonicated in TEG to be used for the synthesis of the NixSbyOz NSMs at the next stage. 2.2. Synthesis of NixSbyOz nanosheet mixtures The NixSbyOz NSMs were prepared by refluxing the as-prepared Ni nanosheets and antimony chloride (SbCl3, 99%, Sigma-Aldrich) in 100 mL TEG at 180 °C under Ar atmosphere. First, the solution consisting of Ni nanosheets in 70 mL TEG was heated to 180 °C. Then, 30 mL of a solution consisting of 0.80 g SbCl3 in TEG was added dropwise to the solution. Following the complete addition of SbCl3, a constant temperature was maintained at 180 °C for 3 h and 6 h. The NixSbyOz NSMs were obtained by centrifugation at 10000 rpm for 15 min and washed with distilled water three times. Finally, the samples were collected after annealed at 300 °C for 4 h under Ar atmosphere. The final powders are referred to as NixSbyOz-3h and NixSbyOz6h. 2.3. Synthesis of NaxFeFe(CN)6 Prussian blue cubes To prepare Prussian blue cubes, 2.00 g of sodium ferrocyanide decahydrate (Na4Fe(CN)6.10H2O, 99%, Sigma-Aldrich) was dissolved in 100 mL of 1 M HCl solution at 80 °C by vigorous magnetic stirring under Ar gas, and the solution was then stirred for 2 h. At the end of the reaction, a blue solid powder was collected by centrifugation, washed with water several times, and then dried in oven at 80 °C for 24 h. A dark blue powder was obtained and identified as NaxFeFe(CN)6. 2.4. Materials characterization The samples were examined by X-ray powder diffraction (XRD) using an automated Rigaku D/max 2200 (Japan) X-ray diffractometer with monochromatic Cu Kα radiation. The morphology of each powder sample was investigated by scanning electron microscopy (SEM) using a Hitachi S-4700 field emission scanning electron microscope, while transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a Talos F200X (Thermo Fisher Scientific) instrument. The composition and surface characteristics of each sample were investigated by energy dispersive X-ray spectroscopy (EDS) using the same Talos F200X instrument and X-ray photoelectron spectroscopy (XPS, Kratos AXIS Nova). The specific surface areas of the materials were measured by the Brunauer–Emmett–Teller (BET) nitrogen adsorption/desorption method (ASAP 2020). The thickness of the materials was measured by atomic force microscopy (AFM) using a XE-100 (PSIA) instrument. 471
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2.5. Electrochemical measurements 2.5.1. Half-cell measurements The electrochemical performance was measured directly using coin cells (CR2032), which were assembled in an Ar filled glove box. First, NixSbyOz and NaxFeFe(CN)6 were individually evaluated in half-cells. The NixSbyOz anodes were prepared on Cu foil with slurries comprised of 70 wt% active material, 15 wt% poly (acrylic acid), 15 wt% Super P, and ethanol as solvent. To prepare the NaxFeFe(CN)6 cathode, NaxFeFe (CN)6 powder was blended with super P and polyvinylidene fluoride in a 70:15:15 wt ratio, and homogeneously dispersed in N-methylpyrrolidone, and the slurry was cast onto an Al foil. These electrodes were dried at 60 °C for 3 h in an oven, and at 70 °C overnight in a vacuum oven. The density of active material on the electrodes was about 1.4 ̴ 1.7 mg cm−2, and the capacities of the electrodes were calculated based on the total mass of active material. Sodium foil was used as the counter electrode, and a glass separator was utilized as the separator. A solution consisting of 1 M NaClO4 in a mixture of propylene carbonate and ethylene carbonate (1:1 by volume) with 2% fluoroethylene carbonate was used as the electrolyte. Cyclic voltammetry (CV) was performed using a ZIVE MP1 electrochemical workstation at a scan rate of 0.1 mV s−1 in the potential range of 0.00–2.00 V while the galvanostatic charge/discharge experiments were carried out at a constant current density of 100 mA g−1 in the range of 0.01 V–2.00 V (vs. Na/ Na+) for the NixSbyOz electrodes, and at a constant current density of 50 mA g−1 in the range of 2.00–4.00 V for the NaxFeFe(CN)6 electrode using a battery cycler (WBCS3000, WonAtech) system. The rate–cycling performance of the cells was tested at different charge current densities of 0.1, 0.5, 1.0, 3.0, 5.0, and 10.0 A g−1 for the NixSbyOz electrodes, and at 50, 100, 200, 400, and 1000 mA g−1 for the NaxFeFe(CN)6 electrode. Electrochemical impedance spectroscopy (EIS) was performed for the NixSbyOz electrodes using the ZIVE MP1 workstation at AC voltage with an amplitude of 10 mV in the frequency range of 100 kHz–0.1 Hz following the first 50 cycles. The CV tests were conducted for the NixSbyOz electrodes after two cycles using the ZIVE MP1 workstation in the potential range of 0.00–2.00 V at various scanning rates of 0.1, 0.2, 0.4, 0.6, and 1.0 mV s−1 to be applied in the calculation of the Na+ ion diffusion coefficients.
Fig. 1. Illustration for the formation of Ni nanosheets.
and reacted with Sb3+ to generate NixSbyOz NSMs in a TEG solvent by GRR. The difference in reduction potentials between Ni and Sb was the impetus of the electrochemical reaction. The potential of the Ni2+/Ni (−0.72 V vs. SHE) was lower than that of the Sb3+/Sb (−0.639 V vs. SHE) [35,36]. When the GRR was initiated, Ni atoms were transformed to Ni2+, and Sb3+ captured the electrons from the oxidation reaction of Ni to Ni2+ to generate Sb atoms on the surface of the Ni (eq. (1)). Subsequently, Sb atoms reacted with Ni atoms to form a NiSb2 alloy (eq. (2)). Then, Sb3+ formed a hydroxyl metal (eq. (3)) in TEG during the GRR. Finally, a Sb2O3 phase was formed by a reaction involving heat treatment at 300 °C for 4 h in Ar atmosphere (eq. (4)). 3 Ni + 2 Sb3+ → 3 Ni2+ + 2 Sb
(1)
Ni + 2 Sb → NiSb2
(2)
3+
Sb
+ 3 OH
−
→ Sb(OH)3
2 Sb(OH)3 → Sb2O3 + 3 H2O
(3) (4)
The phase components of Ni and NixSbyOz NSMs obtained by GRR were determined by XRD analysis as shown in Fig. 2. In the XRD pattern of Ni, shown in Fig. 2(a), the peaks at 39.1, 41.5, 44.5, 58.4, and 70.9° can be attributed to Ni (PDF 45–1027). The obtained Ni exhibited high purity and in good crystallinity. As can be seen in Fig. 2(b) and (c), the peaks of Ni disappear following 3 h and 6 h of GRR as they are dissolved by Sb3+ ions and generate alloys with Sb atoms during the reaction. The 2θ peaks at 28.5, 39.9, 41.7, 51.4, and 59.2° were observed, which could be indexed to Sb (PDF 71–1173), while the peaks at 25.8, 27.1, 31.5, 34.4, 45.4, and 56.6° can be attributed to Sb2O3 (PDF 71–0365). In addition, the 2θ peaks at 28.5, 31.9, 32.8, and 44.1° were related to NiSb2 (PDF 25–1083). According to powder XRD spectra, the NSMs are crystalline with the admixture of an amorphous part. The results were compatible with the EDS results shown in Table S1. The crystalline Ni structures were observed using HRTEM. The lattice fringes of Ni, shown in Fig. 3(a), correspond to the (011) crystallographic phase with an interlayer distance of ∼0.20 nm. The Ni phase with plate/spherical shapes was covered by carbon sources due
2.5.2. Full-cell measurements To create a full-cell, the NixSbyOz anode was assembled in a half-cell and discharged to 0.01 V to achieve a pre-sodiation state. Similarly, the NaxFeFe(CN)6 cathode was assembled in a half-cell as well and charged to 4.00 V to achieve a pre-desodiation state. This step is to introduced to prevent the large irreversible capacity loss associated with the NixSbyOz anode, and the low coulombic efficiency (CE) associated with the NaxFeFe(CN)6 cathode [33]. Subsequently, the NixSbyOz anode and the NaxFeFe(CN)6 cathode were removed from the corresponding half-cells, and paired together in a CR2032 cell. The mass balancing of anode and cathode were from 1:2 to 1:3. Finally, charge/discharge tests were carried out in the range of 0.50–4.00 V at a current density of 25 mA g−1 (based on the mass of the cathode). 3. Results and discussion The Ni nanosheets were prepared with NiCl2 solution in TEG, the support of NaOH and PVP as surface stabilizers, and NaBH4 as reduction agent. The formation of Ni nanosheets was related to the formation of Cl−⋅⋅⋅PVP in the medium, which tightly depends on the factors such as pH, temperature, concentration of halide ions (Cl−) in the solution [34], as shown in Fig. 1. During the nucleation step, Ni seeds (Ni2+ + nPVP + NaBH4 → Ni0) were generated, which consumes the amount of PVP. In the growth step, the Cl−⋅⋅⋅PVP interacted with Ni seeds to form Ni nanosheets. Therefore, it is noted that the concentration of Cl−⋅⋅⋅PVP could decide the yield of Ni nanosheets. The as-prepared Ni nanosheets were used as sacrificial templates
Fig. 2. XRD patterns of (a) Ni nanoparticles, (b) NixSbyOz-3h, (c) NixSbyOz-6h mixtures with the reference peaks of Ni, Sb, NiSb2, and Sb2O3 indicated. 472
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Fig. 3. TEM, HRTEM, and SAED images of (a) Ni phase, (b) NixSbyOz-3h, and (c) NixSbyOz-6h mixtures. STEM images with element mapping of (d) NixSbyOz-3h and (e) NixSbyOz-6h mixtures. (f) AFM images of NixSbyOz-6h mixture.
that the mixtures have type IV isotherms and type H3 hysteresis loops, indicating a mesoporous structure. The pore size distribution of the mixtures calculated by the Barrett–Joyner–Halenda method exhibited one dominant peak at 16 nm for the NixSbyOz-6h (and 30 nm for the NixSbyOz-3h) and another weak peak at 3.75 nm (Fig. S3(c)-(d)), suggesting that mesopores exist within the NSMs. The specific surface area of the NixSbyOz-6h is two times higher than that of the NixSbyOz-3h. The
to the PVP medium (Fig. 3(a)). The SEM images of the NixSbyOz NSMs also show large-sized plate-shaped and stacked structures (Fig. S1). By increasing the reaction time from 3 h to 6 h, NixSbyOz-6h NSM with larger lateral size and thinner thickness was gradually formed during the GRR, as shown in Fig. 3(f) and Fig. S2. Therefore, the specific surface areas of the samples are expected to increase with the increase of the reaction time as shown in Fig. S3(a)-(b) and Table S2. It is clear 473
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morphological features of NixSbyOz mixtures were further investigated by TEM, HRTEM, selected area electron diffraction (SAED), and AFM analyses, as shown in Fig. 3(b)–(f). It should be noted that black and white stripes can be seen in the HRTEM images, which correspond to the NixSbyOz lattice spaces between layers. This morphology is a direct evidence of the sheet structure of the NixSbyOz NSMs. The TEM and AFM images show a 2D thin sheet morphology (green circles) and stacks of thin sheets together (red circles). The distance between sheets is approximately 1 nm for both the NixSbyOz-3h and the NixSbyOz-6h, which is significantly greater than the radius of the Na+ ion (∼10 times); thus it is expected to enable convenient insertion and extraction of Na+ ions. When looking into the surface of the materials, HRTEM and SAED images (Fig. 3(b) and (c)) confirm the existence of crystalline Sb, Sb2O3, and NiSb2 phases in addition to the presence of amorphous regions in the mixtures, which is in good agreement with XRD results in Fig. 2. Moreover, the scanning TEM (STEM) images with element mapping shown in Fig. 3(d)-(e) illustrate the presence and distribution of Sb, Ni, and O elements in the NixSbyOz. It is noted that NixSbyOz-6h sample shows well-distributed Ni phase compared to NixSbyOz-3h sample, which could affect the electrochemical performance. In addition, the AFM analysis in Fig. 3(f) demonstrates the thickness of NixSbyOz-6h NSM. It clearly shows that the as-prepared NixSbyOz-6h has a 2D structure with several sheet layers exhibiting average thickness of ∼14 nm. The electrochemical properties of NixSbyOz anodes were analyzed for the application in SIBs. Fig. 4 shows the initial discharge and charge voltage profiles of the NixSbyOz electrodes at a current rate of 100 mA g−1 in the range of 0.01–2.00 V. When considering the weight percent of Sb in NSMs, which is more than 84% as seen in Table S1, the calculated capacity could be over 554 mAh g−1 (84% x 660 mAh g−1) for NSM electrodes. The NixSbyOz-3h electrode exhibited charge and discharge capacities of 349 and 564 mAh g−1, respectively, corresponding to an initial coulombic efficiency (ICE) of 62%. The NixSbyOz6h electrode demonstrated initial charge and discharge capacities of, respectively, 384 and 686 mAh g−1, exhibiting an ICE of 56%. The difference between the ICE values resulted from the excessive consumption of Na+ ions, which is mainly due to higher surface area of NixSbyOz-6h, to form a solid electrolyte interphase (SEI) layer during the first cycle. When the reaction time was increased from 3 h to 6 h, the charge/discharge capacities increased, which can be ascribed to an increase in the weight percent of the electrochemically active component of Sb and the specific surface areas, as shown in Table S1 and Table S2. Fig. 5(a) and (b) show CVs of the NixSbyOz electrodes in initial three cycles in the range of 0.00–2.00 V. The cathodic and anodic peaks for the NixSbyOz-3h and NixSbyOz-6h were the same, indicating that the same reaction occurred during the discharge/charge processes. In the first cycle, the CV contains a small peak at 1.00 V and a broad irreversible peak area at 0.82 V, which are related to the insertion of Na+
ions between the electrode sheets (1.00 V), the conversion reaction of Sb2O3 to form Sb and Na2O (eq. (5)), and the SEI layer formation (0.82 V). The broad peak at 0.34 V corresponds to the alloy reactions of NiSb2 and Sb to metallic Ni and alloy NaxSb, respectively (eq. (6) and eq. (7)). From the second cycle onward, no reaction corresponding to the conversion of Sb2O3 and alloy reaction of NiSb2 was observed, indicating that these reactions occurred only in the first cycle. We observed four cathodic peaks at 1.00, 0.67, 0.55, and 0.45 V for both the NixSbyOz-3h and NixSbyOz-6h electrodes in the subsequent cycles. The cathodic peak at 1.00 V corresponds to the insertion of Na+ ions between the sheets of mixture electrodes, while the cathodic peaks at 0.67, 0.55, and 0.45 V can mainly be attributed to the formation of the alloy NaxSb from Sb. In the first three cycles, three peaks at 0.77, 0.85, and 1.70 V were detected in the anodic processes. The peaks at 0.77 and 0.85 V are related to the phase transition from NaxSb to Sb (0 < x ≤ 3) [37–43]. A broad anodic peak at 1.70 V represents the removal of Na+ ions from the sheets [44–46]. After the first cycle, the shapes of the curves were nearly identical, indicating a stable electrochemical reaction in the NSMs. Sb2O3 + 6 Na+ + 6 e− → 2 Sb + 3 Na2O NiSb2 + 2x Na Sb + x Na
+
+
+ 2x e
+xe
−
−
→ Ni + 2 NaxSb
↔ NaxSb
(5) (6) (7)
The cycling performance of the NSM electrodes was also analyzed, as shown in Fig. 5(c). The NixSbyOz-3h electrode exhibited a charge capacity with high stability, demonstrating a reversible capacity of 311 mAh g−1 with a capacity retention of ∼89% after 120 cycles. At the same time, the NixSbyOz-6h electrode showed a high stability as well; after 250 cycles, the NixSbyOz-6h electrode delivered a charge capacity of 382 mAh g−1, corresponding to a capacity retention of 99.5%. It should be noted that NixSbyOz-6h NSM electrode demonstrated a superior cyclic performance compared with that of the NixSbyOz-3h NSM, which can be attributed to a reduction in the Na+ diffusion length of the NixSbyOz-6h NSM during the prolonged cycling, as well as the greater amount of Sb components. Furthermore, the CE of the electrodes increased gradually in the subsequent cycles and became greater than 98% after the first 10 cycles. The realized high CE of the electrodes is due to the presence of Ni, which functions as an electron conductor, resulting in stable structures in the charge/discharge processes. In addition, the cycling performance of NixSbyOz electrodes with different ratios of NixSbyOz material: PAA binder: super P was investigated as shown in Fig. S4. When looking into the cycling performances, there were no significant differences in cyclability up to 30 cycles. One thing is that the electrode with 15% of PAA binder shows slightly increased capacity than others, which could be due to the effective binding function from a number of functional groups in PAA binder, leading to the better cycling performance. The rate cyclic performance is also shown in Fig. 5(d). The NixSbyOz-6h electrode exhibited a significantly better rate capability than the NixSbyOz-3h electrode. The NixSbyOz-6h electrode delivered specific capacities of 350 mAh g−1 at 100 mA g−1, 348 mAh g−1 at 500 mA g−1, 341 mAh g−1 at 1 A g−1, 330 mAh g−1 at 3 A g−1, 328 mAh g−1 at 5 A g−1, and 315 mAh g−1 at 10 A g−1, corresponding to capacity retentions of, 100.00%, 99.53%, 97.41%, 94.35%, 93.73%, and 89.95%, respectively. When the current density returned to the value of 100 mA g−1, the charge capacity recovered to the value of 348 mAh g−1, demonstrating a capacity restoration of 99.5% and indicating high reversibility with structural stability. These indicate that the observed stable cycling and high rate capability for the NixSbyOz-6h electrode can be attributed to the formation of a thin and stable SEI layer together with an inactive phase (Ni), while the sheet structures that can offer a higher surface area and shorter path length for Na+ transport, maintain a stable contact with the current collector, and allow better reaction kinetics at the electrode surface. Furthermore, the Na2O and Ni phases, which are the products in eq. (5) and eq. (6),
Fig. 4. Initial voltage profiles of NixSbyOz-3h and NixSbyOz-6h electrodes. 474
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Fig. 5. CVs of (a) NixSbyOz-3h and (b) NixSbyOz-6h. (c) Cyclic performance and (d) rate cyclic performance of NixSbyOz-3h and NixSbyOz-6h electrodes.
electrochemical performance of NixSbyOz NSMs, EIS measurements were conducted. Typical Nyquist plots of the NixSbyOz electrodes after 50 cycles are shown in Fig. S6 and Table S4. In the equivalent circuit, the SEI layer impedance (Rs) is represented by the semicircle in the high frequency, the medium frequency semicircle corresponds to the charge transfer resistance (Rct), the inclined line represents the Na+ diffusion process (W), and Re is the ohmic resistance of the electrolyte and cell components. The Rs value for NixSbyOz-3h is larger than that of NixSbyOz-6h; for example, the Rs value of NixSbyOz-3h is 32.4 Ω while for the NixSbyOz-6h it is 13.7 Ω. The difference in Rs can be attributed to the thinner layered structures of the NixSbyOz-6h electrode, resulting in a stable cyclability. The lower Rct value indicates a faster the Na+ diffusion rate; thus, a better cycling performance can be obtained. The Rct value of NixSbyOz-3h, is 261.8 Ω while it is 169.5 Ω for the NixSbyOz6h. Both the Rs and Rct values for the NixSbyOz-6h electrode are lower than those of the NixSbyOz-3h electrode. The lowest resistance of the NixSbyOz-6h electrode can be attributed to the developed thin sheets, resulting in a shorter electron transport distance and the reduction of the diffusion length of the Na+ ions. In order to obtain further insight into the electrochemical reaction of sodiation and desodiation processes of the NixSbyOz electrodes, the diffusion coefficients (D) of the as-prepared electrodes were calculated by applying the classical Randles–Sevcik equation [48,49], as shown in Fig. 6(a) and Fig. S7(a)-(b). The D values of the electrodes were calculated as 1.10 × 10−7 cm2 s−1 for the NixSbyOz-3h and 3.16 × 10−7 cm2 s−1 for the NixSbyOz-6h electrodes, and they are in the order of approximately 10−7 cm2 s−1, where the NixSbyOz-6h electrode showed higher values than the NixSbyOz-3h electrode. Therefore, it can be concluded that the superior Na storage performance of the NixSnyOz-6h can also be attributed to its high Na+ diffusion ability. In order to further investigate on diffusion phenomena, the pseudo-capacitive and diffusion-controlled behaviors need to be considered and determined [15,50,51], as shown in Fig. 6(b). It can be clearly seen that the slopes (b value) of NixSbyOz-3h and NixSbyOz-6h are 0.64 and 0.60, respectively. These values are close to 0.5, indicating that the electrochemical reaction of NixSbyOz NSMs is dominated by a diffusion-controlled behavior. Moreover, the total capacitive contributions, including pseudo-capacitive and diffusion-controlled behaviors, were calculated [15,50,52], as shown in Fig. 6(c)−(d) and Fig. S7(c)−(d). As can be seen in Fig. 6(c) and (d), the quantities of the
were contained inside the electrodes. They are both inactive components, functioning as effective buffers against volume changes for the following (de)alloying reaction, while Ni is an electron conductor; thus, an excellent stability of the electrodes can be achieved. Moreover, the significantly enhanced high-rate performance can also be closely related to the sheet structures of NixSbyOz NSMs. Specifically, the large interlayer spacing between the sheets has strong effects on the transport of Na+ at the high rate without instigating detrimental changes to the structure [47]. The NixSbyOz-6h electrode has the largest lateral size and the highest specific surface area; as a result, this electrode demonstrates the best performance. Based on the aforementioned discussion on the electrochemical performance, it is true that each component in NixSbyOz mixtures plays different roles on electrochemical performances. Two different samples with different compositions of Ni, Sb, and O were prepared by changing reaction time, and the weight percent of each component was calculated from EDS and XPS analyses, which are summarized in Table S1 and Table S3, respectively. As shown in Fig. S5, the spectra reveal the presence of Sb, O, and Ni in mixtures, and the peaks of Sb and O components are overlapped, which are deconvoluted (Fig. S5(a) and S5(c)), while the peak intensity of Ni is noisy due to the existence of very small amount of Ni in mixtures (Fig. S5(b) and Fig. S5(d)). It is noted that when increasing the reaction time, the amount of Ni and O components somewhat decreased while the amount of Sb increased even though the difference is not much significant. This tendency is well matched with that in EDS results. From this point of view, the existence of higher Sb and lower O components in NixSbyOz6h mixture could lead to higher capacity and less formation of SEI layer (related to Na2O phase during cycling), leading to better electrochemical performance. In specific, Sb component coming from Sb, Sb2O3, and NiSb2 phases acts as active material contributing to the capacity. The other components including Ni from NiSb2 and O (the actual phase: Na2O) from Sb2O3 phase could play roles, respectively, as conductive medium (conductivity of Ni: 1.43ⅹ105 S cm−1) and buffers to reduce large volume expansion of Sb during sodiation as discussed earlier. Even though the difference in the amount of components is small, finally, it can be summarized that the electrochemical performance is better for NixSbyOz-6h, which is mainly due to larger surface area, well-distributed Ni with high conductivity in the nanosheet mixture, and less formation of Na2O phase related to SEI layer. To further investigate the mechanisms contributing to the 475
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Fig. 6. (a) Relationships between Ip and ν1/2 for the NixSbyOz electrodes and (b) relationships between log i and log ν. Pseudo-capacitive contribution (yellow color) and diffusion-controlled contribution (olive color) at a scan rate of 0.4 mV s−1 for (c) NixSbyOz-3h and (d) NixSbyOz6h electrodes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
to achieve a pre-sodiation state in the half-cell and to form a stable SEI layer, while the cathode was pre-desodiated by applying a potential of 4.0 V. The electrochemical reaction of the full-cell can be described as follows:
pseudo-capacitive and ionic diffusion contributions are 21.9% and 78.1% for the NixSbyOz-3h, respectively, and 23.4% and 76.6% for the NixSbyOz-6h, respectively, at a scan rate of 0.4 mV s−1. The pseudocapacitive contributions of the NixSbyOz-6h are higher than that of the NixSbyOz-3h at the same scan rate, indicating that the kinetics can be expected to be transferred from diffusion-controlled to surface-controlled (pseudo-capacitive) reaction, resulting in a better electrochemical performance than that of the NixSbyOz-3h [51]. For the practical application of Na-ion cells, a full-cell system was developed with a NaxFeFe(CN)6 cathode. The crystalline structure and morphology of the NaxFeFe(CN)6 material were analyzed, as shown in Fig. S8 and Fig. S9. The XRD pattern of the NaxFeFe(CN)6 material has sharp and strong diffraction peaks, indicating a well-developed crystalline nature. The patterns of the NaxFeFe(CN)6 material are in good agreement with the standard Prussian blue Fe4 [Fe(CN)6]3 (PDF 73–0687), showing a typical face-centered cubic phase [53–56]. The SEM image shows that the NaxFeFe(CN)6 material has cubic shapes with a size of ∼800 nm, as can be seen in Fig. S9 [33]. The electrochemical properties of the NaxFeFe(CN)6 material were tested at a current density of 50 mA g−1 in the range of 2.00–4.00 V (Fig. 7). The different capacity plot (DCP) curves of the electrode showed two anode peaks at 3.00 V and 3.65 V and two cathode peaks at 2.88 V and 3.50 V, which are related to the redox potentials for the high-spin and low-spin Fe ions during the (de)sodiation process [54,57]. As can be seen in Fig. 7(a), the shape of these curves is nearly the same, indicating the stable electrochemical reaction of the NaxFeFe (CN)6 electrode. The voltage profiles shown in Fig. 7(b) exhibit a long sloping plateau in the range of 2.8–3.25 V with a working voltage of ∼3.0 V. Regarding the cycling performance of the NaxFeFe(CN)6 electrode, it exhibited a very stable cyclability, and demonstrated a specific capacity of 101 mAh g−1 with a high CE of ∼100% and capacity retention of 90% after 200 cycles (Fig. 7(c)). As can be seen in Fig. 7(d), the NaxFeFe(CN)6 electrode showed an excellent rate capability (83 mAh g−1 retained at 1 A g−1) for the rate cyclic performance. These results definitely indicate that NaxFeFe(CN)6 material can be used as a potential cathode material to pair with NixSbyOz anodes for Na-ion full batteries. A simple full-cell configuration with a NixSbyOz anode and a NaxFeFe(CN)6 cathode was established to test the electrochemical properties, as shown in Fig. 8(a). The anodes were discharged to 0.01 V
NaxFeFe(CN)6 + Sb ↔ NaxSb + FeFe(CN)6
(8)
The electrochemical performance of the full-cells with different mass ratios between anode and cathode was analyzed at a current density of 25 mA g−1 in the potential range of 0.50–4.00 V, as shown in Fig. 8(b)–(d) and Fig. S10, demonstrating that the 1:2 wt ratio between the anode and cathode exhibited a higher recharge capacity. Therefore, this mass ratio between the anode and cathode was employed in the full-cell study. Fig. 8(b) and (c) show typical discharge/charge curves of the NixSbyOz || NaxFeFe(CN)6 full-cell. From the voltage profiles, the NixSbyOz || NaxFeFe(CN)6 full-cell had a working voltage of ∼2.5 V. Fig. 8(d) and (e) show the cycling and rate performance of the NixSbyOz || NaxFeFe(CN)6 full-cell. It demonstrated an initial discharge capacity of ∼102 mAh g−1 and showed a stable cyclability; it exhibited a discharge capacity of 90 mAh g−1 after 50 cycles (a capacity retention of ∼88%) with a high CE of 100% during the entire cycling. Considering the rate cycling of the full-cell, it demonstrated a notable performance, as it exhibited a capacity retention of ∼72% at a high current rate of 1000 mA g−1. The details of the full-cell systems for SIBs are summarized in Table S5, and the capacities were calculated based on the total mass of the anode and cathode. As shown in Table S5 and Fig. S10(b), the capacity and energy density of the full-cells decreased with the decreasing mass ratio of the anode and cathode, which was possibly because the amount of Na+ ions were not enough from the anode to cathode. The full-cell with a mass ratio of 1:2 exhibited the highest energy density of ∼150 Wh kg−1. Although the working voltage and energy density (2.5 V and 150 Wh kg−1, respectively) are slightly lower than those of the LIBs, it is believed that the full-cell systems presented have the potential to be applied in high-performance sodium-ion batteries.
4. Conclusion Novel NixSbyOz nanosheet mixtures with a 2D structure were synthesized using a convenient GRR, and the as-prepared nanosheet 476
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Fig. 7. (a) Differential capacity plots, (b) voltage profiles, (c) cyclic performances, and (d) rate cyclic performance of the NaxFeFe(CN)6 cathode.
Fig. 8. (a) Schematic of the working principle and configuration of the NixSbyOz || NaxFeFe(CN)6 full-cell, (b) comparison of voltage profiles of the half-cell and fullcell, (c) voltage profile, (d) cyclic performances, (e) rate cyclic performance of the NixSbyOz || NaxFeFe(CN)6 full-cell. 477
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mixtures were employed as anode materials for sodium-ion cells. The NixSbyOz electrode prepared with a reaction time of 6 h (NixSbyOz-6h) exhibited a high reversible capacity of 382 mAh g−1 and a high-rate performance. The excellent electrochemical properties can be attributed to: (i) the existence of large interlayer spacing between sheets, leading to facile movement of Na+ ions, (ii) the formation of novel sheet structures providing large specific surface areas, resulting in effective electrolyte penetration and interface reactions, (iii) the generation of thin layers offering short diffusion paths of Na+ ions, (iv) the separate nature of the sheets allowing effective strain relaxation and the integrity of the electrode upon electrochemical cycling, and (v) the inactive phases (Na2O and Ni) functioning as effective buffers against volume changes during cycling, resulting in an enhanced performance for the Na storage. Furthermore, the anode was combined with a NaxFeFe(CN)6 cathode to form a sodium-ion full-cell, which exhibited a working voltage of 2.5 V and an energy density of ∼150 Wh kg−1. They showed a remarkable electrochemical performance with appropriate capacity and high rate capability as well. In conclusion, the nanosheet mixture electrodes (particularly, the NixSbyOz-6h electrode) presented in this study have great potential to be utilized as high-performance anode materials for sodium-ion batteries.
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