carbon nanospheres as high-capacity cathode for lithium-ion batteries

Journal of Energy Chemistry 40 (2020) 1–6

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Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem

Design of pyrite/carbon nanospheres as high-capacity cathode for lithium-ion batteries Qinqin Xiong a, Xiaojing Teng a, Jingjing Lou a, Guoxiang Pan b, Xinhui Xia c,d,∗, Hongzhong Chi a, Xiaoxiao Lu a, Tao Yang a, Zhenguo Ji a,∗ a

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, Zhejiang, China Department of Materials Chemistry, Huzhou University, Huzhou 313000, Zhejiang, China State Key Laboratory of Silicon Materials, Department of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, China d Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China b c

a r t i c l e

i n f o

Article history: Received 31 January 2019 Revised 18 February 2019 Accepted 20 February 2019 Available online 26 February 2019 Keywords: Iron sulfide Carbon coating Cathode Nanosphere Lithium ion batteries

a b s t r a c t Transition metal sulfides are emerging as promising electrode materials for energy storage and conversion. In this work, hierarchical FeS2 /C nanospheres are synthesized through a controllable solvothermal method followed by the annealing process. Spherical FeS2 core is homogeneously coated by thin carbon shell. The hierarchical nanostructure and carbon coating can enhance electron transfer and accommodate the stress originated from the volume change as well as suppress the shuttle effect of polysulfide. Consequently, as the cathode material of lithium ion batteries (LIBs), the FeS2 /C nanospheres exhibit high reversible capacity of 676 mAh g−1 and excellent cycling life with the capacity retention of 97.1% after 100 cycles. In addition, even at the high current density of 1.8 C, a reversible capacity of 437 mAh g−1 is obtained for the FeS2 /C nanospheres, demonstrating its great prospect for practical applications in highperformance LIBs. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

1. Introduction In the wake of development in electric vehicles and portable electronic devices, the high energy-density lithium ion batteries (LIBs) with low cost have been the research focus and come into a period of rapid growth [1–5]. As we know, the capacity and the energy density of LIBs mainly depend on the cathode material. However, the conventional LIBs, such as LiMO2 (M=transition metal) and LiFeO4 cathode materials, exhibit specific capacities lower than 200 mAh g−1 , unable to meet the growing demand of electric vehicles field. Transition metal sulfides with higher capacity over traditional cathode have attracted great attention owing to their higher capacity, good electrochemical redox reactivity arising from electrochemical conversion reaction. Among the transition metal sulfide candidates, extensive efforts have been focused on pyrite (FeS2 ) because of its large energy density of 1273 Wh kg−1 , high theoretical capacity of 894 mAh g−1 , cost effectiveness, low toxicity and natural abundance [6,7]. However, the practical application of FeS2 in LIBs is still hampered. The capacity retention and rate capa-

∗

Corresponding authors. E-mail addresses: [email protected] (X. Xia), [email protected] (Z. Ji).

bility of FeS2 are not satisfactory due to its poor inherent conductivity and severe volume expansion via conversion reactions. Furthermore, polysulfide Li2 Sx (2
https://doi.org/10.1016/j.jechem.2019.02.005 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

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composite nanospheres. Additionally, the research on their lithium ion storage performance is also in the blank. In the present work, we report hierarchical FeS2 /C nanospheres composed of interconnected thin FeS2 /C nanosheets prepared by a facile solvothermal method. In the well-designed flower-like nanospheres, high surface and porous structure are combined. As a cathode of LIBs, the FeS2 /C nanospheres exhibit noticeable improvement in high rate performance and cycling stability, which can be ascribed to the flower-like nanospheres structure together with the uniform carbon layer. The composite can enhance electron/ion transfer, improve structure stability as well as the electrical conductivity, resulting in excellent rate capability. In addition, the proposed preparation method is also suited to the synthesis of other metal sulfides in the fields of electrochemical energy storage, catalysis and sensing.

Fig. 1. The schematic illustration of the formation of FeS2 /C nanospheres.

2. Experimental 2.1. Material synthesis The FeS2 nanospheres were prepared by a solvothermal method. Typically, 1 g of FeSO4 ·7H2 O, 0.8 g of Na2 S2 O3 , 0.02 g polyvinyl pyrrolidone, 0.3 g urea and 0.1 g thioacetamide (CH3 CSNH2 ) were dissolved in 90 mL ethanol. Then, the above solution was sealed in Teflon-lined autoclave and heated at 150 °C for 24 h. After that, the sample was washed by DI water and then treated in furnace at 300 °C for 2 h in argon to obtain FeS2 nanospheres. Afterwards, the FeS2 nanospheres were dispersed in 0.05 M glucose solution overnight and finally calcined at 550 °C for 1.5 h in Ar atmosphere to obtain FeS2 /C nanospheres.

Fig. 2. SEM images of (a, b) FeS2 nanospheres and (c, d) FeS2 /C nanospheres.

2.2. Characterization The phase and purity of the as-obtained samples were characterized by X-ray diffractometry (XRD) (Miniflex600, Rigaku), using Cu Kα radiation at a scan rate of 0.02 °/s, and Raman spectroscopy under the excitation length of 532 nm (WITec-CRM200). The morphologies and microstructures of the as-obtained samples were characterized by a field emission scanning electron microscopy (SEM, ZEISS microscope), and high-resolution transmission electron microscopy (TEM, Tecnai G20). 2.3. Electrochemical measurements The FeS2 /C nanospheres as working electrodes were prepared in CR2025 coin-type cell as our previous work did [18]. The cells were assembled in an argon-filled glove box using 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1 in volume) as the electrolyte. The galvanostatic charge-discharge cycling test was conducted on battery test system (LAND, Wuhan, China) in the voltage range of 1.2–2.6 V at room temperature at different current densities. Cyclic voltammetry (CV) was tested at a scan rate of 0.1 mV s−1 on an electrochemical workstation (CHI660d). Electrochemical impedance spectroscopy (EIS) measurements were studied on the electrochemical workstation in the frequency range of 100 kHz to 10 mHz with the excitation voltage of 5 mV. The mass loading of the as-prepared composite was about 1.3 mg cm−2 . The capacity of electrodes was calculated based on the mass of FeS2 . For the as-obtained FeS2 /C nanospheres, the content of carbon was ∼3 wt%. 3. Results and discussion The synthesis process of the FeS2 /C nanospheres is schematically illustrated in Fig. 1. In the solvothermal step, the precursors

first form FeS2 nanosheets. In order to decrease the surface energy, the FeS2 nanosheets are assembled to nanospheres structures. Secondly, the FeS2 /C nanospheres are obtained by immersing the FeS2 nanospheres into the aqueous glucose and followed by an annealing process. The morphology of FeS2 nanospheres is shown in Fig. 2(a, b). The monodispersed hierarchical FeS2 nanospheres showdiameters of 500 to 600 nm, which are composed of ultrathin nanosheets as primary building blocks. In addition, there are lots of nanopores between interconnected nanosheets in this microstructure, which provides a high surface area. After coating carbon layer, the nanosphere morphology of FeS2 /C nanospheres is well preserved with the well-distributed size of about 550 nm (Fig. 2c, d). The detailed micro-morphology analysis of the two samples was monitored by TEM-HRTEM (Fig. 3), confirming that both FeS2 and FeS2 /C nanospheres have perfect spherical structure with the welldistributed diameter of 50 0–60 0 nm. Furthermore, HRTEM image in Fig. 3(d) clearly demonstrates a distinct composite structure of FeS2 /C nanospheres. The thickness of amorphous carbon layer is about 4–5 nm, uniformly coating on the FeS2 nanospheres. In addition, EDS elemental mapping analysis in Fig. 3(e) demonstrates the homogeneous distribution of Fe, S and C. The nanospheres composite structure and uniform carbon shell coating are confirmed in the FeS2 /C nanospheres. The conductive porous structure is favorable for fast lithium ion storage. The phase and purity of the as-prepared FeS2 and FeS2 /C nanospheres were confirmed by the XRD (Fig. 4a). All the characteristic peaks of both samples can be assigned to the cubic structure of FeS2 (JCPDS 42-1340). And there is no peak of carbon detected in the pattern of FeS2 /C nanospheres, which suggests the amorphous nature and low content of the C in the FeS2 /C nanospheres. To further confirm the conclusion from XRD patterns, Raman spectroscopy was conducted and the results are shown in Fig. 4(b). Both samples at low wave number have three

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D, respectively [21]. It reveals that the carbon layer originated from carbonization of glucose is amorphous. According to the BET test (Fig. 4c), the FeS2 /C nanospheres exhibit a specific surface area of ∼69 m2 g−1 , little higher than the FeS2 /C nanospheres (∼60 m2 g−1 ). Additionally, according to the TGA curve, we can deduce that the carbon content of the FeS2 /C nanospheres is about 3 wt%. The TGA test was conducted in air. The FeS2 /C would decompose and leave the final product: Fe2 O3 . As shown in Fig. 4(d), the weight loss is 35.4 wt%. By adverse inference, the FeS2 accounts for about 97 wt% in the FeS2 /C. Therefore, the carbon content is about 3 wt%. In view of the potential application of FeS2 /C nanospheres as a cathode in LIBs, we evaluated its reversible lithium storage properties. The CV curves of as-obtained electrodes with the potential window of 1.1–2.7 V (vs. Li/Li+ ) at a scan rate of 0.1 mV s−1 for the first two cycles are shown in Fig. 5(a, b). In the first cathodic scan, the curves of both electrodes (Fig. 5a) have only one sharp reduction peak, which is assigned to the conversion reaction [9,22,23]:

FeS2 + 4Li+ + 4e− → Fe + 2Li2 S

(1)

After the first lithiation process, the reduction products are Fe and Li2 S [24,25]. In the first anodic sweep, there are two welldefined peaks at 1.90 and 2.53 V observed due to the formation of FeS and sulfur, respectively [15,26], expressed as the following reactions [27,28]:

Fig. 3. TEM images of (a, b) FeS2 nanospheres and (c) FeS2 /C nanospheres; (d) HRTEM image of FeS2 /C nanospheres; (e) EDS mapping results of FeS2 /C nanospheres.

Fe + Li2 S − 2e− → FeS + 2Li+

(2)

Li2 S−2e− →1/8 S8 + 2Li+

(3)

For the second reduction-oxidation cycle, the reduction peak shifts notably to ∼1.29 V and a new reduction peak is observed at ∼1.97 V in Fig. 5(b). These two peaks are the reverse processes of Eqs. (2) and (3). Meanwhile, the current density of the FeS2 /C nanospheres is much larger than that of FeS2 nanospheres, indicating the sufficient reaction with Li+ after the carbon layer coating. Fig. 5(c,d) shows the galvanostatic discharge/charge profiles of

feature peaks at 338, 367, 420 cm−1 , which belong to the Eg , Ag and Tg modes of FeS2 , respectively [19,20]. Moreover, there are two strong peaks at high wave number of FeS2 /C nanospheres. The peaks located at about 1550 and 1355 cm−1 , which are assigned to graphitic G bands of carbon and typical disorder-induced

(b)

Ag Eg

Intensity (a.u.)

(311) (220) (023) (321)

(211)

(220)

(200) (111)

Intensity (a.u.)

FeS2

(210)

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G D 1355 1550

Tg

FeS2/C FeS2

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20

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FeS2/C

(d) 100

Weight (%)

3

Quantity adsorbed (cm /g STP)

FeS2

1600

Raman shift (cm )

(c) 140

1200 -1

2 Theta (degree)

100 80 60

80

35.4%

60 40

40 20

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0.4

0.6

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Relative pressure (P/P0)

1.0

0

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o

Temperature ( C)

Fig. 4. (a) XRD patterns of FeS2 /C and FeS2 nanospheres; (b) Raman spectra of FeS2 /C and FeS2 nanospheres. (c) BET test: N2 adsorption–desorption isothermal curves of FeS2 and FeS2 /C nanospheres. (d) TGA curve of FeS2 /C nanospheres in air.

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(a)

1.91 V

2.53 V

0.0

-0.5

FeS2/C

-1.0

FeS2

1.18 V 1.22 V

1.0

1.5

2.0

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+

2 nd

-1

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0.5

-1.5

1.89 V

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1 st

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(b)

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0.5 0.0 1.97 V

-0.5 -1.0

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+

3.0

Potential (V vs. Li /Li)

Potential (V vs. Li /Li) (d)

(c) FeS2

FeS2/C

2.5

+

+

Potential (V vs. Li /Li)

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Potential (V vs. Li /Li)

FeS2

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0

200

400

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-1

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-1

Capacity (mAh g )

Capacity (mAh g )

Fig. 5. CV curves at a scan rate of 0.1 mV s−1 : (a) the first cycle; (b) the second cycle. Charge–discharge curves for the initial two cycles at rate of 0.3 C: (c) FeS2 nanospheres; (d) FeS2 /C nanospheres.

800

80 60

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0.3 C 200 0

FeS2/C Charge

FeS2/C Charge

FeS2 Discharge

FeS2 Discharge

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(b)

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FeS2 Charge

FeS2 /C Discharge

FeS2 Discharge

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(d) 300

Coulombic efficiency (%) -Z '' (Ω)

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-1

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10

FeS2 Charge

20

FeS2 Discharge

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(c) 700

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1.8 C

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80 0.9 C

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Coulombic efficiency (%) -1 Capacity (mAh g )

-1

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(a)

FeS2 /C

250

FeS2

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Simulated data Simulated data

150 100 50 0 0

50

100

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Z' (Ω)

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250

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Fig. 6. (a) Cycle stability of FeS2 /C and FeS2 nanospheres at the rate of 0.3 C; (b) Rate capability of FeS2 /C and FeS2 nanospheres at different current densities; (c) Cycle stability at 1.5 C. (d) Nyquist plots of FeS2 /C and FeS2 nanospheres after 10 cycles at the fully discharged state.

both electrodes at the rate of 0.3 C with the voltage window from 1.2 to 2.6 V. Both electrodes exhibit the similar curves, suggesting that the different surface modification does not change the electrochemical nature of FeS2 . In the first cycle, there is one discharge plateaus near 1.50 V, which corresponds to the reduction of FeS2 to Fe and Li2 S, corresponding to reaction (1). The second lithiation process is different from the first one, which is due to the different reaction kinetics and pathways. In the delithiation process, there are two plateaus shown at ∼1.75 and 2.5 V, which can be ascribed to the reaction (2) and reaction (3), respectively, in accordance

with the CV curves. Based on the mass of the FeS2 /C composite, the initial discharge capacities of FeS2 /C and FeS2 nanospheres are 808 and 681 mAh g−1 , respectively. Herein, the capacity contribution only comes from FeS2 in our voltage window between 1.2 to 2.6 V while the carbon layer will not contribute capacity. Note that the CE of FeS2 /C nanospheres observed in the first cycle is 88.2%, higher than that of FeS2 nanospheres (86.6%), mainly due to the improved reaction kinetics by introducing carbon layer. Fig. 6(a) exhibits the discharge capacities versus cycle number at current rate of 0.3 C. The FeS2 /C nanospheres shows much more

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Table 1. Performance comparison of FeS2 /C nanospheres with previous FeS2 /C-based materials. FeS2 /C-based matierlais

Potential cutoff (V)

Current density

Cycles

Capacity (mAh g−1 )

Reference

FeS2 /C nanospheres FeS2 @C spheres FeS2 @C nanowires FeS2 @C spheres GF/FeS2 Al2 O3 -coated FeS2 @C fiber FeS2 @N-graphene n-FeS2 /rGO

1.2–2.6 0.01–3.0 1.0–3.0 1.0–3.0 1.1–2.6 1.0–3.0 1.0–3.0 0.8–3.0

0.3 C 0.1 A g−1 0.1 A g−1 0.3 A g−1 0.1 C 0.2 A g−1 0.5 A g−1 1 A g−1

100 100 100 100 100 100 400 200

676 560 570 614 555 530 401.7 435

Our work [16] [20] [23] [29] [30] [31] [32]

the FeS2 nanospheres. All these factors work together to contribute better rate performance of FeS2 /C nanospheres. 4. Conclusions

Fig. 7. SEM images of (a) FeS2 /C and (b) FeS2 nanospheres after 100 cycles at 0.3 C.

stable cycling performance compared with FeS2 nanospheres electrode. After 100 charge-discharge cycling, the discharge capacity of FeS2 /C nanospheres electrode achieves a capacity of 676 mAh g−1 with the capacity retention of 91.4% based on the second discharge capacity. However, the FeS2 nanospheres electrode suffered a serious capacity decay and the capacity retention is only 72.5%. Additionally, the lithium storage performance of FeS2 /C nanospheres is superior to the previous works on FeS2 /C-based composites, as shown in Table 1 [29–32]. Furthermore, the FeS2 /C nanospheres show better high-rate capability than that of the FeS2 nanospheres, as illustrated in Fig. 6(b). At different rates of 0.3, 0.6, 0.9, 1.2, 1.5, 1.8 C, high specific capacities of 814, 642, 598, 558, 504, 437 mAh g−1 are obtained, respectively. At the same rate, the discharge capacities of FeS2 /C nanospheres are always much higher than those of FeS2 nanospheres. With the increase of rates, both electrodes show the gradual capacity fading, indicating that the electrode reactions are mainly limited by the diffusion kinetics. In addition, the cycling performance at 1.5 C is presented in Fig. 6(c). Notice that the FeS2 /C electrode still shows much better stability than the unmodified FeS2 electrode. The enhanced cycling capability and high-rate performance of FeS2 /C nanospheres are mainly due to the following positive factors: (1) Improved electric conductivity and Li ion transfer during electrochemical process. EIS measurements of both electrodes at full-discharged state after 10 cycles were conducted to further study the kinetics of electrode reaction (Fig. 6d). The curves including three parts of both electrodes are almost the same. Firstly, the intersection point in the x-axis is related to the solution resistance. The depressed semicircles are assigned to the interfacial charge transfer resistance (Rct ) [33,34]. The inclined line in the low frequency is designated to Warburg impedance (Zw ), corresponding to ions diffusion into the bulk of the electrode [35,36]. The charge transfer resistance of FeS2 /C nanospheres is much lower compared to that of FeS2 nanospheres after 10 cycles. It suggests that carbon coating indeed provides more effective transfer of the Li+ and electrons, thus improving the electrochemical activity and reaction kinetics. That’s one of the key factors to enhance the electrochemical performance of FeS2 /C nanospheres [37–40]. (2) The carbon layer can stop the shuttle of polysulfides to achieve better cycles [41–43]. SEM images (Fig. 7) of FeS2 /C and FeS2 nanospheres after 100 cycles at 0.3 C are shown in the revised manuscript. It is seen that the structure of FeS2 /C nanospheres is better preserved than

To sum up, a facile solvothermal synthesis followed by the annealing process is proposed to prepare hierarchical FeS2 /C nanospheres composed of interconnected thin FeS2 nanosheets with carbon layer coating. The FeS2 /C nanospheres have the advantages of hierarchical porous nanostructure and conductive uniform amorphous carbon layer. Due to the reduced Li+ diffusion distance, enhanced electrical conductivity and good accommodation of volume stress, the FeS2 /C nanospheres show high specific capacity, excellent long-term cycling performance and superior rate capability, which is promising for the application of LIBs with high-energy and high-power densities. Acknowledgments The work was funded by the Natural Science Foundation of Zhejiang Provincial (No. LQ17E020 0 03) and the National Natural Science Foundation of China (No. 51804092). Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.02.005. References [1] W.M. Seong, K. Yoon, M.H. Lee, S.K. Jung, K. Kang, Nano Lett. 19 (2019) 29–37. [2] J.B. Zhao, Y.Y. Zhang, Y. H.Wang, H. Li, Y.Y. Peng, J. Energy Chem. 27 (2018) 1536–1554. [3] Y.F. Yuan, F. Chen, G.C. Cai, S.M. Yin, M. Zhu, L.N. Wang, J.L. Yang, S.Y. Guo, Electrochim. Acta 296 (2019) 669–675. [4] G. Wu, Y.K. Zhou, J. Energy Chem. 28 (2019) 151–159. [5] H. Su, Y.F. Xu, S.Y. Shen, J.O. Wang, J.T. Li, L. Huang, S.G. Sun, J. Energy Chem. 27 (2018) 1637–1643. [6] V. Sridhar, H. Park, J. Alloys Compd. 732 (2018) 799–805. [7] S.A. Khateeb, T.D. Sparks, J. Mater. Sci. 54 (2019) 4089–4104. [8] D.T. Pham, J.P. Baboo, J. Song, S. Kim, J. Jo, V. Mathew, M.H. Alfaruqi, B. Sambandam, J. Kim, Nanoscale 10 (2018) 5938–5949. [9] F. Cao, G.X. Pan, J. Chen, Y.J. Zhang, X.H. Xia, J. Power Sources 303 (2016) 35–40. [10] M. Caban-Acevedo, D. Liang, K.S. Chew, J.P. Degrave, N.S. Kaiser, S. Jin, ACS Nano 7 (2013) 1731–1739. [11] J.Z. Chen, X.Y. Zhou, C.T. Mei, J.L. Xu, S. Zhou, C.P. Wong, Electrochim. Acta 222 (2016) 172–176. [12] L.S. Li, M. Caban-Acevedo, S.N. Girard, S. Jin, Nanoscale 6 (2014) 2112–2118. [13] X. Wen, X.L. Wei, L.W. Yang, P.K. Shen, J. Mater. Chem. A 3 (2015) 2090–2096. [14] Y.N. Chen, S.M. Xu, Y.C. Li, R.J. Jacob, Y.D. Kuang, B.Y. Liu, Y.L. Wang, G. Pastel, L.G. Salamanca-Riba, M.R. Zachariah, Adv. Energy Mater. 7 (2017) 1700482. [15] H.H. Fan, H.H. Li, K.C. Huang, C.Y. Fan, X.Y. Zhang, X.L. Wu, J.P. Zhang, ACS Appl. Mater. Interfaces 9 (2017) 10709–10716. [16] Y.J. Liu, W.Q. Wang, Q.D. Chen, C. Xu, D.P. Cai, H.B. Zhan, Inorg. Chem. 58 (2019) 1330–1338. [17] Q.F. Su, Y.H. Lu, S.H. Liu, X.C. Zhang, Y.H. Lin, R.W. Fu, D.C. Wu, Carbon 140 (2018) 433–440. [18] Q.Q. Xiong, J.J. Lou, X.J. Teng, X.X. Lu, S.Y. Liu, H.Z. Chi, Z.G. Ji, J. Alloys Compd. 743 (2018) 377–382. [19] Y. Li, Z. Han, L. Jiang, Z. Su, F. Liu, Y. Lai, Y. Liu, J. Sol–Gel Sci. Technol. 72 (2014) 100–105.

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