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Sodium storage performance and mechanism of Ag2S nanospheres as electrode material for sodium-ion batteries
Solid State Ionics 343 (2019) 115071
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Sodium storage performance and mechanism of Ag2S nanospheres as electrode material for sodium-ion batteries Weijian Hao, Huinan Si, Wenting Li, Chao Zhang, Wentao Zhu, Xinping Qiu
T
⁎
Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China
A B S T R A C T
Metal sulfides have attracted tremendous attention as electrode materials for sodium-ion batteries due to their high theoretical capacity. In this article, Ag2S nanospheres with 70 nm diameter were prepared by a hydrothermal method. Compared with the bulk Ag2S, the as-prepared Ag2S nanospheres deliver the capacity of 198 mAh/g after 100 cycles at 1C, exhibiting excellent cycling and rate performance as the electrode material for sodium-ion batteries. The remarkable electrochemical performance can be attributed to the shorter Na+ diffusion path in Ag2S nanospheres. Structural characterization reveals that the sodiation processes of Ag2S include intercalation and conversion reactions, which were confirmed by potential relaxation technique (PRT) measurements.
1. Introduction In recent years, lithium-ion batteries (LIBs) are widely utilized in electric devices, electric vehicles and electricity storage systems due to their high energy densities and long cycle life [1–3]. However, extensive applications of LIBs are hindered by the shortage of lithium resources on earth [4]. Due to the low cost and abundant reserve of sodium resource, sodium-ion batteries (SIBs) have been considered as one of the potential candidates power sources for energy storage [5,6]. Owing to the similar chemistry between LIBs and SIBs, some electrode materials for LIBs, such as transition metal oxide, sulfide and fluoride, are also investigated in SIBs [7–9]. Among them, metal sulfides have attracted considerable attention in recent years because of their high theoretical capacity [10,11]. Two-dimensional sulfides, such as MoS2 [12], TiS2 [13] and WS2 [14], with layered structure of S-metal-S layers stacked through van der Waals interactions, allow Na+ to be reversibly intercalated into layers. During further Na+ insertion, these transition metal sulfides are reduced to metal nanoparticles which disperse in the Na2S matrix [15]. For non-layered sulfides, such as FeS [10] and CoS [16], the sodiation reactions involve intercalation and conversion reactions, while the sodiation of SnS [17], Sb2S3 [18] and Bi2S3 [19] involves sequential conversion and alloying reactions. α-Ag2S, known as argentite with a monoclinic structure, is widely applied as catalyst [20,21], electrochemical sensor [22,23] and also as the cathode material in lithium batteries [24] and magnesium batteries [25]. Our previous study found that lithiation of Ag2S is a kind of displacement reaction [24]. However, up to now, sodiation mechanism of Ag2S is still unclear. In this paper, we prepared Ag2S nanospheres via a hydrothermal
⁎
method. The as-prepared Ag2S nanospheres deliver the capacity of 198 mAh/g after 100 cycles at 1C rate. The structural characterization of Ag2S at different state of charge (SOC) indicates that the sodiation/ desodiation mechanism of Ag2S is a conversion reaction, which is different from the lithiation reaction. These differences may be caused by the sluggish Na+ diffusion in Ag2S. 2. Experimental section 2.1. Synthesis of Ag2S nanospheres We prepared Ag2S nanospheres via a hydrothermal method. In detail, 2 mmol of thiourea and 0.66 mol of AgNO3 were deionized in 15 mL distilled water. The obtained yellow solution was mixed with 15 mL of CTAB solution (10 mmol/L). After magnetic stirring for 10 min, the mixed solution was transferred into 50 mL Telfon-lined stainless autoclave and heated at 160 °C for 72 h. The resulting precipitate was centrifuged and washed with distilled water and ethanol for several times. Finally, the as-prepared product was dried under vacuum at 60 °C for 12 h. For comparison, bulk Ag2S was prepared via a simple solution deposition method. Briefly, 100 mL of 0.04 M AgNO3 solution was added into 300 mL of 0.05 M Na2S with magnetic stirring. The obtained black precipitate was collected and washed with deionized water for several times and dried at 80 °C overnight. 2.2. Materials characterization X-ray diffraction (XRD) patterns of samples were collected on a Rigaku D8 Advance X-ray diffractometer, with Cu Kα irradiation, and
Corresponding author. E-mail address: [email protected] (X. Qiu).
https://doi.org/10.1016/j.ssi.2019.115071 Received 2 August 2019; Received in revised form 1 September 2019; Accepted 18 September 2019 0167-2738/ © 2019 Published by Elsevier B.V.
Solid State Ionics 343 (2019) 115071
W. Hao, et al.
Fig. 1. Structural characterization of obtained Ag2S particles: (a) SEM image of Ag2S nanospheres, (b) SEM image of bulk Ag2S (c) TEM image of Ag2S nanospheres, (d) HR-TEM image with lattice fringes of Ag2S nanospheres, (e) TEM image with corresponding EDS mapping images of Ag2S nanospheres.
99%. Then, the cell was charged or discharged for 1 h at 0.1C. The current was cut off and the open circuit voltage (OCV) was recorded for 3 h. This procedure was repeated until the cut-off voltage was reached (0.1 V for discharging and 3 V for charging). The thickness of the electrode was 12.4 μm and loading mass of active material was 0.805 mg/cm2.
the data were collected between 20° to 60°. Morphology of samples were observed by a scanning electron microscopy (SEM, Zeiss Merlin 5 kV) and a transmission electron microscopy (TEM, JEOL JEM2100F). The chemical state of Ag in Ag2S at various state of charge was analyzed by a X-ray induced Auger electron spectroscopy (XAES, PHI Quantera). All samples were etched by Ar+ bombardment to the depth of 100 nm before XAES collection. For ex-situ XRD and XAES measurements, coin cells were disassembled in the glove box and the Ag2S electrode was collected and washed with DME solvent.
3. Results and discussion Ag2S nanospheres were prepared via a hydrothermal method with AgNO3 and thiourea as precursors. At elevated temperature, Agthiourea complex decomposes slowly to produce S2−, which reacts with the rest of Ag+ in thiourea complex to produce Ag2S nanospheres [26,27]. With the help of CTAB, the size and shape of Ag2S nanospheres were controlled. The morphologies of the Ag2S particles were observed by SEM and TEM. As shown in Fig. 1a, Ag2S nanospheres present the smooth and clean surface with an average particle size of 70 nm. For comparison, bulk Ag2S particles prepared by solution deposition method have the diameter over 10 μm (Fig. 1b). The spherical morphology of obtained Ag2S nanospheres was also confirmed by TEM image (Fig. 1c). HR-TEM image (Fig. 1d) shows obvious lattice fringes and d-space (0.26 nm) which can be attributed to the (220) plane, corresponding to the strongest peak in the XRD pattern. In addition,
2.3. Electrochemical characterization The electrode slurry was prepared by mixing active material (Ag2S nanospheres, 70 wt%), conductive additive (super P, 20 wt%), and binder (PVDF, 10 wt%) in NMP. The as-prepared slurry was coated onto an aluminum current collector foil and dried in a vacuum oven at 80 °C for 12 h. CR2032 coin cells were assembled in an Ar-filled glove box with water and oxygen contents below 0.1 ppm. Ag2S was employed as the working electrode, sodium foil was used as the counter electrode and 1 M NaPF6 in DME was the electrolyte. The galvanostatic charge and discharge tests were carried over the potential range from 0.1 to 3 V vs Na/Na+. For the PRT measurement, the cell was charged or discharged for several cycles until the Coulombic efficiency reached 2
Solid State Ionics 343 (2019) 115071
W. Hao, et al.
Fig. 2. XRD patterns and corresponding Rietveld refinement analysis of Ag2S nanospheres and bulk Ag2S.
For Ag, XAES measurement was used due to the difficulty in identifying the accurate chemical states of Ag from XPS [28]. As shown in Fig. 3b, the Ag MNN spectrum splits into two peaks at 356.8 eV and 351.2 eV, which can be assigned to Ag+ in Ag2S (Ag M4N5N5: 357.2 eV [29], Ag M5N5N5: 351.2 eV [30]). The S 2p spectrum splits into four peaks. The peaks at 160.7 eV and 161.9 eV are assigned to AgeS bond [31,32] and the peaks at 163.7 eV and 164.9 eV [33] can be assigned to elemental sulfur from the decomposition of absorbed thiourea in the surface. The electrochemical performance of Ag2S nanospheres is displayed in Fig. 4. Fig. 4a shows the galvanostatic charge/discharge profiles for different cycles. Three plateaus at 1.8 V, 1.5 V, 0.8 V in the discharge profile and three plateaus at 1.6 V, 2.2 V and 2.5 V in the charge profile can be observed, indicating multiple reaction mechanisms. The differential capacity curves of Ag2S nanospheres in different cycles are shown in Fig. 4b. With the increase in number of cycles, the height of peak at 2.5 V gradually decreases and that of peaks at 1.6 V and 2.2 V increase. The plateau migration can be attributed to dynamic cooperation between intercalation and conversion reaction or overpotential caused by resistance changes [34]. This phenomenon has been reported for Cubased sulfides such as Cu1.8S [34,35] and CuV2S4 [36]. The details of
Table 1 Structural parameters of as-prepared samples determined from XRD Rietveld refinement. Sample
a/Å
b/Å
c/Å
V/Å
Rp/%
Rwp/%
Nano-Ag2S Bulk-Ag2S
4.229 4.226
6.930 6.927
7.867 7.863
227.313 226.950
3.17 3.33
4.52 4.77
EDS element mapping indicates the uniform distribution of silver and sulfur elements (Fig. 1e). To investigate the crystal structure of the samples, XRD patterns and the corresponding Rietveld refinements of as-prepared Ag2S nanoparticles and bulk Ag2S are shown in Fig. 2. The calculated parameters of samples are shown in Table 1. All the diffraction peaks can be indexed to pure α-Ag2S phase (PDF#14–0072) with monoclinic P21/n space group. No additional peak is observed, indicating the high purity of the samples. XPS and XAES measurements were employed to investigate the chemical state of Ag2S nanospheres. The XPS spectrum, as shown in Fig. 3a, confirms the presence of Ag and S as well as C and O impurities.
Fig. 3. XPS survey spectra (a) and the corresponding Ag MNN (b) XAES spectra and S 2p (c) XPS spectra of Ag2S nanospheres. 3
Solid State Ionics 343 (2019) 115071
W. Hao, et al.
Fig. 4. Electrochemical performances of Ag2S nanospheres: (a) Discharging and charging profiles of Ag2S electrode at 0.1C rate in sodium-ion batteries, (b) differential capacity of Ag2S nanospheres in different cycles, (c) Comparison of cycle performance between as-prepared Ag2S nanospheres and bulk Ag2S at 1C rate, (d) Comparison of rate performance of between as-prepared Ag2S nanospheres and bulk Ag2S at different current densities.
discharged to 0.1 V, only diffraction peaks of silver metal remain, which demonstrate the electrode is fully reduced to silver metal. Morphology of Ag2S nanospheres after being discharged to 0.1 V is shown in Fig. 5b. The Ag metal particle, confirmed by EDS (as shown in Fig. 5b), with size of ca. 30 nm, can be clearly observed. In order to verify the reversibility of sodiation process of Ag2S, XAES measurements were also performed for discharged and charged Ag2S. In the fully discharged state (0.1 V), the Ag (MNN) spectrum (as shown in Fig. 5 c) shows two peaks at 357.9 and 351.9 eV, corresponding to silver metal (Ag M4N5N5: 357.9 eV [29], Ag M5N5N5: 351.9 eV [30]), indicating that Ag2S is fully reduced to Ag. On the other hand, the Ag (MNN) spectrum, as shown in Fig. 5d, showing four peaks at 357.9, 356.8, 351.9, 351.2 eV in the fully charged state (3.0 V), where the peaks at 356.8 and 351.2 eV correspond to Ag+ in Ag2S. The area of Ag2S peaks is much higher than that of Ag metal peaks in the spectrum, indicating the oxidation of Ag metal to Ag2S. Combining with the large voltage hysteresis of (de)sodiation of Ag2S nanospheres (as shown in Fig. 4a) and results above, it can be inferred that the sodiation of Ag2S involves three steps of reactions: the first step of interaction reaction, second and third step of conversion reactions, corresponding to three potential plateaus in discharge curves. The detail of sodiation mechanism of Ag2S can be described as follows:
this mechanism still need further study. Cyclic performance of Ag2S nanospheres and bulk Ag2S at 1C rate are compared in Fig. 4c. For Ag2S nanospheres, the initial discharge capacity is 311 mAh/g, and charge capacity is 237 mAh/g, corresponding to the initial Coulombic efficiency of 76.2%, which may be attributed to the formation of SEI film. After 100 cycles, Ag2S nanospheres deliver the capacity of 198 mAh/g, much higher than that of bulk Ag2S (29 mAh/g). Fig. 2d exhibits the rate performance of Ag2S nanospheres and bulk Ag2S. Ag2S nanospheres deliver higher capacity than bulk Ag2S in each rate, especially at high current densities. When the current density was reset to 0.2C, the capacity of Ag2S nanospheres is fully recovered, indicating good rate performance. Sodiation mechanism for Ag2S nanospheres was investigated by exsitu XRD, XAES and TEM. Fig. 5a shows XRD patterns of Ag2S nanospheres after being discharged to different potential. For the pristine electrode, diffraction peaks of Ag2S nanospheres can be clearly observed. After being discharged to 1.75 V, corresponding to the first plateau in the discharge profile, the position of peaks is almost identical to the pristine of Ag2S nanospheres, indicating the crystal structure of Ag2S is unchanged during Na+ insertion. Therefore, the reaction mechanism for the first discharge plateau can be deduced as an intercalation mechanism [37]. When Ag2S is further discharged to 1.5 V, corresponding to the second plateau in the discharge profile, the diffraction peaks of Ag2S almost disappear. New peaks at 38.12° and 44.30° can be assigned to silver metal and peaks at 31.65°, 35.68°, 37.93° can be assigned to Na2Ag4S3 (PDF#43–0904). After being
Ag 2S + xNa+ → Na x Ag 2S (x < 0.33)
3Na 0.33Ag2 S + Na+ → Na2Ag 4 S3 + 2Ag 4
Solid State Ionics 343 (2019) 115071
W. Hao, et al.
Fig. 5. (a) The ex-situ XRD patterns of Ag2S nanospheres at different state of charge. (b) TEM image of Ag2S nanospheres electrode in the fully discharged state (0.1 V) and EDS results of selected area. (c) XAES spectra of Ag2S nanospheres at fully discharged state (0.1 V) and (d) fully charged state (3 V).
Fig. 6. (a) PRT measurement profile for (de)sodiation process of Ag2S electrode. (b) Comparison of diffusion coefficient for Li+ and Na+ (discharge process).
Na2Ag 4 S3 + 4Na → 3Na2S + 4Ag
Normally, the reaction mechanism of sulfides with Li or Na is dependent on diffusion rate of insertion ions [41]. Here, we employed potential relaxation technique (PRT) [42,43] to measure the diffusion coefficient of Na+ (DNa), which can be obtained from following equation:
Similar sodiation mechanism was also observed for Cu1.8S [34]. After the electrode being further charged to 3.0 V, the diffraction peaks of Ag2S reappear, indicating the good reversibility of (de)sodiation of Ag2S. However, the diffraction peaks of silver metal can also be observed (Fig. 5a), meaning that Ag metal isn't fully oxidized to Ag2S due to the large polarization caused by low Na+ diffusion rate. The remaining silver metal can also enhance the conductivity of the electrode [38–40].
ln ⎡exp ⎛ ⎢ ⎝ ⎣
⎜
(Vm − Vt ) ⎞ π2 F − 1⎤ = −lnN − ⎛ 2 DNa ⎞ t ⎥ RT L ⎝ ⎠ ⎠ ⎦ ⎟
⎜
⎟
where Vt represents the potential when the relaxation time equals to t, 5
Solid State Ionics 343 (2019) 115071
W. Hao, et al.
Vm represents the stable potential after sufficient relaxation, and L refers to the thickness value of the Ag2S electrode. The charge and discharge profile of PRT measurement are shown in Fig. 6a. The diffusion coefficients of Na+ at different SOC are shown in Fig. 6b and the diffusion coefficients of Li+ are also presented for comparison [24]. It can be seen that DNa is about 10−11 cm/s2, three magnitudes lower than DLi, implying more sluggish kinetics in the sodiation process of Ag2S. The slow diffusion rate of Na+ in Ag2S may be attributed to the larger radius of Na+ (0.98 Å) compared to Li+ (0.69 Å) and cause the different reaction mechanisms between lithiation and sodiation.
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4. Conclusion In summary, we prepared Ag2S nanospheres via a hydrothermal method. As-prepared Ag2S nanospheres shows reversible sodiation capacity of 237 mAh/g. The capacity retention of Ag2S nanospheres after 100 cycles is 198 mAh/g, exhibiting excellent cycle performance. Exsitu XRD and XAES results show that Na2Ag4S3 intermediate and Ag metal formed during sodiation of Ag2S. Three step reactions for the sodiation of Ag2S is identified: the first step of interaction reaction, the second and the third step of conversion reactions, corresponding to three potential plateaus in discharge curves. The difference in reaction mechanisms between lithiation and sodiation can be attributed to sluggish diffusion kinetics of Na+ compared to Li+. Acknowledgements The authors appreciate financial supports from Project from National Key Research and Development Program (2016YFB0901700), Beijing Municipal Science and Technology Commission (Z181100004718006), Natural Science Foundation of China (U1664256, 51474133), China-US Electric Vehicle Project (S2016G9004) and National Key Project on Fundamental Research Program (2015CB251104). Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. References [1] J.B. Goodenough, Y. Kim, Chem. Mat. 22 (2010) 587–603. [2] Z.G. Yang, J.L. Zhang, M.C.W. Kintner-Meyer, X.C. Lu, D.W. Choi, J.P. Lemmon, J. Liu, Chem. Rev. 111 (2011) 3577–3613. [3] J.M. Tarascon, M. Armand, Nature 414 (2001) 359–367. [4] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947–958. [5] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez, T. Rojo, Energy Environ. Sci. 5 (2012) 5884–5901. [6] H.L. Pan, Y.S. Hu, L.Q. Chen, Energy Environ. Sci. 6 (2013) 2338–2360.
6
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Sodium storage performance and mechanism of Ag2S nanospheres as electrode material for sodium-ion batteries Weijian Hao, Huinan Si, Wenting Li, Chao Zhang, Wentao Zhu, Xinping Qiu
T
⁎
Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China
A B S T R A C T
Metal sulfides have attracted tremendous attention as electrode materials for sodium-ion batteries due to their high theoretical capacity. In this article, Ag2S nanospheres with 70 nm diameter were prepared by a hydrothermal method. Compared with the bulk Ag2S, the as-prepared Ag2S nanospheres deliver the capacity of 198 mAh/g after 100 cycles at 1C, exhibiting excellent cycling and rate performance as the electrode material for sodium-ion batteries. The remarkable electrochemical performance can be attributed to the shorter Na+ diffusion path in Ag2S nanospheres. Structural characterization reveals that the sodiation processes of Ag2S include intercalation and conversion reactions, which were confirmed by potential relaxation technique (PRT) measurements.
1. Introduction In recent years, lithium-ion batteries (LIBs) are widely utilized in electric devices, electric vehicles and electricity storage systems due to their high energy densities and long cycle life [1–3]. However, extensive applications of LIBs are hindered by the shortage of lithium resources on earth [4]. Due to the low cost and abundant reserve of sodium resource, sodium-ion batteries (SIBs) have been considered as one of the potential candidates power sources for energy storage [5,6]. Owing to the similar chemistry between LIBs and SIBs, some electrode materials for LIBs, such as transition metal oxide, sulfide and fluoride, are also investigated in SIBs [7–9]. Among them, metal sulfides have attracted considerable attention in recent years because of their high theoretical capacity [10,11]. Two-dimensional sulfides, such as MoS2 [12], TiS2 [13] and WS2 [14], with layered structure of S-metal-S layers stacked through van der Waals interactions, allow Na+ to be reversibly intercalated into layers. During further Na+ insertion, these transition metal sulfides are reduced to metal nanoparticles which disperse in the Na2S matrix [15]. For non-layered sulfides, such as FeS [10] and CoS [16], the sodiation reactions involve intercalation and conversion reactions, while the sodiation of SnS [17], Sb2S3 [18] and Bi2S3 [19] involves sequential conversion and alloying reactions. α-Ag2S, known as argentite with a monoclinic structure, is widely applied as catalyst [20,21], electrochemical sensor [22,23] and also as the cathode material in lithium batteries [24] and magnesium batteries [25]. Our previous study found that lithiation of Ag2S is a kind of displacement reaction [24]. However, up to now, sodiation mechanism of Ag2S is still unclear. In this paper, we prepared Ag2S nanospheres via a hydrothermal
⁎
method. The as-prepared Ag2S nanospheres deliver the capacity of 198 mAh/g after 100 cycles at 1C rate. The structural characterization of Ag2S at different state of charge (SOC) indicates that the sodiation/ desodiation mechanism of Ag2S is a conversion reaction, which is different from the lithiation reaction. These differences may be caused by the sluggish Na+ diffusion in Ag2S. 2. Experimental section 2.1. Synthesis of Ag2S nanospheres We prepared Ag2S nanospheres via a hydrothermal method. In detail, 2 mmol of thiourea and 0.66 mol of AgNO3 were deionized in 15 mL distilled water. The obtained yellow solution was mixed with 15 mL of CTAB solution (10 mmol/L). After magnetic stirring for 10 min, the mixed solution was transferred into 50 mL Telfon-lined stainless autoclave and heated at 160 °C for 72 h. The resulting precipitate was centrifuged and washed with distilled water and ethanol for several times. Finally, the as-prepared product was dried under vacuum at 60 °C for 12 h. For comparison, bulk Ag2S was prepared via a simple solution deposition method. Briefly, 100 mL of 0.04 M AgNO3 solution was added into 300 mL of 0.05 M Na2S with magnetic stirring. The obtained black precipitate was collected and washed with deionized water for several times and dried at 80 °C overnight. 2.2. Materials characterization X-ray diffraction (XRD) patterns of samples were collected on a Rigaku D8 Advance X-ray diffractometer, with Cu Kα irradiation, and
Corresponding author. E-mail address: [email protected] (X. Qiu).
https://doi.org/10.1016/j.ssi.2019.115071 Received 2 August 2019; Received in revised form 1 September 2019; Accepted 18 September 2019 0167-2738/ © 2019 Published by Elsevier B.V.
Solid State Ionics 343 (2019) 115071
W. Hao, et al.
Fig. 1. Structural characterization of obtained Ag2S particles: (a) SEM image of Ag2S nanospheres, (b) SEM image of bulk Ag2S (c) TEM image of Ag2S nanospheres, (d) HR-TEM image with lattice fringes of Ag2S nanospheres, (e) TEM image with corresponding EDS mapping images of Ag2S nanospheres.
99%. Then, the cell was charged or discharged for 1 h at 0.1C. The current was cut off and the open circuit voltage (OCV) was recorded for 3 h. This procedure was repeated until the cut-off voltage was reached (0.1 V for discharging and 3 V for charging). The thickness of the electrode was 12.4 μm and loading mass of active material was 0.805 mg/cm2.
the data were collected between 20° to 60°. Morphology of samples were observed by a scanning electron microscopy (SEM, Zeiss Merlin 5 kV) and a transmission electron microscopy (TEM, JEOL JEM2100F). The chemical state of Ag in Ag2S at various state of charge was analyzed by a X-ray induced Auger electron spectroscopy (XAES, PHI Quantera). All samples were etched by Ar+ bombardment to the depth of 100 nm before XAES collection. For ex-situ XRD and XAES measurements, coin cells were disassembled in the glove box and the Ag2S electrode was collected and washed with DME solvent.
3. Results and discussion Ag2S nanospheres were prepared via a hydrothermal method with AgNO3 and thiourea as precursors. At elevated temperature, Agthiourea complex decomposes slowly to produce S2−, which reacts with the rest of Ag+ in thiourea complex to produce Ag2S nanospheres [26,27]. With the help of CTAB, the size and shape of Ag2S nanospheres were controlled. The morphologies of the Ag2S particles were observed by SEM and TEM. As shown in Fig. 1a, Ag2S nanospheres present the smooth and clean surface with an average particle size of 70 nm. For comparison, bulk Ag2S particles prepared by solution deposition method have the diameter over 10 μm (Fig. 1b). The spherical morphology of obtained Ag2S nanospheres was also confirmed by TEM image (Fig. 1c). HR-TEM image (Fig. 1d) shows obvious lattice fringes and d-space (0.26 nm) which can be attributed to the (220) plane, corresponding to the strongest peak in the XRD pattern. In addition,
2.3. Electrochemical characterization The electrode slurry was prepared by mixing active material (Ag2S nanospheres, 70 wt%), conductive additive (super P, 20 wt%), and binder (PVDF, 10 wt%) in NMP. The as-prepared slurry was coated onto an aluminum current collector foil and dried in a vacuum oven at 80 °C for 12 h. CR2032 coin cells were assembled in an Ar-filled glove box with water and oxygen contents below 0.1 ppm. Ag2S was employed as the working electrode, sodium foil was used as the counter electrode and 1 M NaPF6 in DME was the electrolyte. The galvanostatic charge and discharge tests were carried over the potential range from 0.1 to 3 V vs Na/Na+. For the PRT measurement, the cell was charged or discharged for several cycles until the Coulombic efficiency reached 2
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Fig. 2. XRD patterns and corresponding Rietveld refinement analysis of Ag2S nanospheres and bulk Ag2S.
For Ag, XAES measurement was used due to the difficulty in identifying the accurate chemical states of Ag from XPS [28]. As shown in Fig. 3b, the Ag MNN spectrum splits into two peaks at 356.8 eV and 351.2 eV, which can be assigned to Ag+ in Ag2S (Ag M4N5N5: 357.2 eV [29], Ag M5N5N5: 351.2 eV [30]). The S 2p spectrum splits into four peaks. The peaks at 160.7 eV and 161.9 eV are assigned to AgeS bond [31,32] and the peaks at 163.7 eV and 164.9 eV [33] can be assigned to elemental sulfur from the decomposition of absorbed thiourea in the surface. The electrochemical performance of Ag2S nanospheres is displayed in Fig. 4. Fig. 4a shows the galvanostatic charge/discharge profiles for different cycles. Three plateaus at 1.8 V, 1.5 V, 0.8 V in the discharge profile and three plateaus at 1.6 V, 2.2 V and 2.5 V in the charge profile can be observed, indicating multiple reaction mechanisms. The differential capacity curves of Ag2S nanospheres in different cycles are shown in Fig. 4b. With the increase in number of cycles, the height of peak at 2.5 V gradually decreases and that of peaks at 1.6 V and 2.2 V increase. The plateau migration can be attributed to dynamic cooperation between intercalation and conversion reaction or overpotential caused by resistance changes [34]. This phenomenon has been reported for Cubased sulfides such as Cu1.8S [34,35] and CuV2S4 [36]. The details of
Table 1 Structural parameters of as-prepared samples determined from XRD Rietveld refinement. Sample
a/Å
b/Å
c/Å
V/Å
Rp/%
Rwp/%
Nano-Ag2S Bulk-Ag2S
4.229 4.226
6.930 6.927
7.867 7.863
227.313 226.950
3.17 3.33
4.52 4.77
EDS element mapping indicates the uniform distribution of silver and sulfur elements (Fig. 1e). To investigate the crystal structure of the samples, XRD patterns and the corresponding Rietveld refinements of as-prepared Ag2S nanoparticles and bulk Ag2S are shown in Fig. 2. The calculated parameters of samples are shown in Table 1. All the diffraction peaks can be indexed to pure α-Ag2S phase (PDF#14–0072) with monoclinic P21/n space group. No additional peak is observed, indicating the high purity of the samples. XPS and XAES measurements were employed to investigate the chemical state of Ag2S nanospheres. The XPS spectrum, as shown in Fig. 3a, confirms the presence of Ag and S as well as C and O impurities.
Fig. 3. XPS survey spectra (a) and the corresponding Ag MNN (b) XAES spectra and S 2p (c) XPS spectra of Ag2S nanospheres. 3
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Fig. 4. Electrochemical performances of Ag2S nanospheres: (a) Discharging and charging profiles of Ag2S electrode at 0.1C rate in sodium-ion batteries, (b) differential capacity of Ag2S nanospheres in different cycles, (c) Comparison of cycle performance between as-prepared Ag2S nanospheres and bulk Ag2S at 1C rate, (d) Comparison of rate performance of between as-prepared Ag2S nanospheres and bulk Ag2S at different current densities.
discharged to 0.1 V, only diffraction peaks of silver metal remain, which demonstrate the electrode is fully reduced to silver metal. Morphology of Ag2S nanospheres after being discharged to 0.1 V is shown in Fig. 5b. The Ag metal particle, confirmed by EDS (as shown in Fig. 5b), with size of ca. 30 nm, can be clearly observed. In order to verify the reversibility of sodiation process of Ag2S, XAES measurements were also performed for discharged and charged Ag2S. In the fully discharged state (0.1 V), the Ag (MNN) spectrum (as shown in Fig. 5 c) shows two peaks at 357.9 and 351.9 eV, corresponding to silver metal (Ag M4N5N5: 357.9 eV [29], Ag M5N5N5: 351.9 eV [30]), indicating that Ag2S is fully reduced to Ag. On the other hand, the Ag (MNN) spectrum, as shown in Fig. 5d, showing four peaks at 357.9, 356.8, 351.9, 351.2 eV in the fully charged state (3.0 V), where the peaks at 356.8 and 351.2 eV correspond to Ag+ in Ag2S. The area of Ag2S peaks is much higher than that of Ag metal peaks in the spectrum, indicating the oxidation of Ag metal to Ag2S. Combining with the large voltage hysteresis of (de)sodiation of Ag2S nanospheres (as shown in Fig. 4a) and results above, it can be inferred that the sodiation of Ag2S involves three steps of reactions: the first step of interaction reaction, second and third step of conversion reactions, corresponding to three potential plateaus in discharge curves. The detail of sodiation mechanism of Ag2S can be described as follows:
this mechanism still need further study. Cyclic performance of Ag2S nanospheres and bulk Ag2S at 1C rate are compared in Fig. 4c. For Ag2S nanospheres, the initial discharge capacity is 311 mAh/g, and charge capacity is 237 mAh/g, corresponding to the initial Coulombic efficiency of 76.2%, which may be attributed to the formation of SEI film. After 100 cycles, Ag2S nanospheres deliver the capacity of 198 mAh/g, much higher than that of bulk Ag2S (29 mAh/g). Fig. 2d exhibits the rate performance of Ag2S nanospheres and bulk Ag2S. Ag2S nanospheres deliver higher capacity than bulk Ag2S in each rate, especially at high current densities. When the current density was reset to 0.2C, the capacity of Ag2S nanospheres is fully recovered, indicating good rate performance. Sodiation mechanism for Ag2S nanospheres was investigated by exsitu XRD, XAES and TEM. Fig. 5a shows XRD patterns of Ag2S nanospheres after being discharged to different potential. For the pristine electrode, diffraction peaks of Ag2S nanospheres can be clearly observed. After being discharged to 1.75 V, corresponding to the first plateau in the discharge profile, the position of peaks is almost identical to the pristine of Ag2S nanospheres, indicating the crystal structure of Ag2S is unchanged during Na+ insertion. Therefore, the reaction mechanism for the first discharge plateau can be deduced as an intercalation mechanism [37]. When Ag2S is further discharged to 1.5 V, corresponding to the second plateau in the discharge profile, the diffraction peaks of Ag2S almost disappear. New peaks at 38.12° and 44.30° can be assigned to silver metal and peaks at 31.65°, 35.68°, 37.93° can be assigned to Na2Ag4S3 (PDF#43–0904). After being
Ag 2S + xNa+ → Na x Ag 2S (x < 0.33)
3Na 0.33Ag2 S + Na+ → Na2Ag 4 S3 + 2Ag 4
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Fig. 5. (a) The ex-situ XRD patterns of Ag2S nanospheres at different state of charge. (b) TEM image of Ag2S nanospheres electrode in the fully discharged state (0.1 V) and EDS results of selected area. (c) XAES spectra of Ag2S nanospheres at fully discharged state (0.1 V) and (d) fully charged state (3 V).
Fig. 6. (a) PRT measurement profile for (de)sodiation process of Ag2S electrode. (b) Comparison of diffusion coefficient for Li+ and Na+ (discharge process).
Na2Ag 4 S3 + 4Na → 3Na2S + 4Ag
Normally, the reaction mechanism of sulfides with Li or Na is dependent on diffusion rate of insertion ions [41]. Here, we employed potential relaxation technique (PRT) [42,43] to measure the diffusion coefficient of Na+ (DNa), which can be obtained from following equation:
Similar sodiation mechanism was also observed for Cu1.8S [34]. After the electrode being further charged to 3.0 V, the diffraction peaks of Ag2S reappear, indicating the good reversibility of (de)sodiation of Ag2S. However, the diffraction peaks of silver metal can also be observed (Fig. 5a), meaning that Ag metal isn't fully oxidized to Ag2S due to the large polarization caused by low Na+ diffusion rate. The remaining silver metal can also enhance the conductivity of the electrode [38–40].
ln ⎡exp ⎛ ⎢ ⎝ ⎣
⎜
(Vm − Vt ) ⎞ π2 F − 1⎤ = −lnN − ⎛ 2 DNa ⎞ t ⎥ RT L ⎝ ⎠ ⎠ ⎦ ⎟
⎜
⎟
where Vt represents the potential when the relaxation time equals to t, 5
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Vm represents the stable potential after sufficient relaxation, and L refers to the thickness value of the Ag2S electrode. The charge and discharge profile of PRT measurement are shown in Fig. 6a. The diffusion coefficients of Na+ at different SOC are shown in Fig. 6b and the diffusion coefficients of Li+ are also presented for comparison [24]. It can be seen that DNa is about 10−11 cm/s2, three magnitudes lower than DLi, implying more sluggish kinetics in the sodiation process of Ag2S. The slow diffusion rate of Na+ in Ag2S may be attributed to the larger radius of Na+ (0.98 Å) compared to Li+ (0.69 Å) and cause the different reaction mechanisms between lithiation and sodiation.
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4. Conclusion In summary, we prepared Ag2S nanospheres via a hydrothermal method. As-prepared Ag2S nanospheres shows reversible sodiation capacity of 237 mAh/g. The capacity retention of Ag2S nanospheres after 100 cycles is 198 mAh/g, exhibiting excellent cycle performance. Exsitu XRD and XAES results show that Na2Ag4S3 intermediate and Ag metal formed during sodiation of Ag2S. Three step reactions for the sodiation of Ag2S is identified: the first step of interaction reaction, the second and the third step of conversion reactions, corresponding to three potential plateaus in discharge curves. The difference in reaction mechanisms between lithiation and sodiation can be attributed to sluggish diffusion kinetics of Na+ compared to Li+. Acknowledgements The authors appreciate financial supports from Project from National Key Research and Development Program (2016YFB0901700), Beijing Municipal Science and Technology Commission (Z181100004718006), Natural Science Foundation of China (U1664256, 51474133), China-US Electric Vehicle Project (S2016G9004) and National Key Project on Fundamental Research Program (2015CB251104). Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. References [1] J.B. Goodenough, Y. Kim, Chem. Mat. 22 (2010) 587–603. [2] Z.G. Yang, J.L. Zhang, M.C.W. Kintner-Meyer, X.C. Lu, D.W. Choi, J.P. Lemmon, J. Liu, Chem. Rev. 111 (2011) 3577–3613. [3] J.M. Tarascon, M. Armand, Nature 414 (2001) 359–367. [4] M.D. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Funct. Mater. 23 (2013) 947–958. [5] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez, T. Rojo, Energy Environ. Sci. 5 (2012) 5884–5901. [6] H.L. Pan, Y.S. Hu, L.Q. Chen, Energy Environ. Sci. 6 (2013) 2338–2360.
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