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Three-dimensional carbon nanotubes-encapsulated Li2FeSiO4 microspheres as advanced positive materials for lithium energy storage
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Three-dimensional carbon nanotubes-encapsulated Li2FeSiO4 microspheres as advanced positive materials for lithium energy storage Haiyan Yan∗, Xiuxiu Xue, Yuqiao Fu, Xinming Wu, Jingwei Dong Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi'an Technological University, Xi'an, 710021, PR China
ARTICLE INFO
ABSTRACT
Keywords: Lithium energy storage 3D CNTs Li2FeSiO4 Microspheres Property
Polyanion-type Li2FeSiO4 has been considered as an advanced positive material for lithium energy storage owing to the low cost, good safety and high theoretical capacity. Unfortunately, the inferior electrical conductivity and bad diffusivity of Li+ inhibit its widespread applications for lithium energy storage. In the present research, we have used the simple spray-drying route to fabricate the three-dimensional carbon nanotubes (3D CNTs) encapsulated Li2FeSiO4 microspheres (3D-Li2FeSiO4/CNTs). Benefiting from the interconnected conducting networks, the as-fabricated 3D-Li2FeSiO4/CNTs cathode displays superior battery property with a specific capacity of 169.1 mAh g−1 at 0.1C. Besides, it also delivers a discharge capacity of 104.1 mAh g−1 at 5C over 500 cycles with a capacity retention ratio of around 91.2%. The above results suggest that the designed 3D-Li2FeSiO4/CNTs composite is an advanced positive material in lithium energy storage.
1. Introduction Lithium-ion batteries have received much attention as the main power sources for various applications because of their excellent lithium storage performances [1,2]. Actually, the development of novel electrodes with good rate property, superior cyclic-life and high reversible capacity, are crucial for next-generation lithium energy storage. Recently, many new cathodes such as LiCoMnO4 [3], Li3V2(PO4)3 [4], Li2MnSiO4 [5], LiVPO4F [6] as well as Li2FeSiO4 [7,8], have been widely fabricated and studied for lithium energy storage. Among these novel electrodes, polyanion-type Li2FeSiO4 is considered as a promising cathode because of its high stability and excellent safety [9–12]. Moreover, Li2FeSiO4 exhibits a theoretical capacity of 332 mAh g−1 by two-electron reactions [12]. Nevertheless, the deintercalation/intercalation of two Li+ in Li2FeSiO4 is hard to achieve owing to the inferior electrical conductivity (10−14 S cm−1) and bad diffusivity of Li+ (10−14 cm S−1) [13,14]. Therefore, these obstacles greatly present its practical applications for lithium energy storage. The effective strategies to solve these problems of Li2FeSiO4 have focused on doping with other metal ions [13,15–19], coating with conductive carbon materials [7–12,20–23], and decreasing the particle size [24,25]. Among these methods, carbon coating has been considered as the promising approach to enhance the conductivity of Li2FeSiO4 and inhibit the growth of Li2FeSiO4 crystals and oxidation of
∗
Fe2+ [26]. For instance, Zhang's group [22] used a sol-gel route to fabricate the graphene-decorated Li2FeSiO4@C nanocomposite and the obtained cathode displayed the specific capacity of 178 mAh g−1 at 0.1C over 40 cycles. Despite of the above mentioned methods, designing of different morphologies of Li2FeSiO4 has been proved to be of beneficial effect on its electrochemical performance. Particularly, constructing of 3D structure can endow the electrode with efficient electrolyte storage and decrease the pathway for Li+ transport [7,12,21,27]. Based on the advantages of 3D structure, Huang's group prepared the 3D hollow Li2FeSiO4/C microspheres, which displayed the discharge capacity of 151 mAh g−1 at 0.5C [27]. However, the effects of 3D CNTs on the battery performances of Li2FeSiO4 electrode have not been studied so far. In this work, the designed 3D CNTs-encapsulated Li2FeSiO4 microsphere has been synthesized through the spray-drying route. In the microspheres, the well-crystallized Li2FeSiO4 nanocrystals with monoclinic structure are dispersed in the 3D conducting CNTs. This constructed 3D CNTs networks can promote the electrical conductivity and diffusivity of Li+ during the charge/discharge procedure. Therefore, the fabricated 3D-Li2FeSiO4/CNTs electrode shows superior lithium storage properties including good rate performance and excellent cyclic-life. The results suggest that this designed 3D-Li2FeSiO4/CNTs microspheres can be utilized as the advanced positive material in lithium energy storage.
Corresponding author. E-mail address: [email protected] (H. Yan).
https://doi.org/10.1016/j.ceramint.2019.12.241 Received 13 July 2019; Received in revised form 24 December 2019; Accepted 27 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Haiyan Yan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.241
Ceramics International xxx (xxxx) xxx–xxx
H. Yan, et al.
Fig. 1. (a–c) SEM and (d–f) TEM images for the resulted 3D-Li2FeSiO4/CNTs sample.
Fig. 2. (a) STEM and (b–e) EDS dot mapping images for the Fe, Si, O and C elements in the designed composite.
360 °C for 3.5 h and then at 700 °C for 9 h under the argon/hydrogen condition to form the 3D-Li2FeSiO4/CNTs composite. The phase structure of the fabricated 3D-Li2FeSiO4/CNTs composite was analyzed by XRD (D8, Bruker). Raman spectroscopy was tested through the RM2000 Raman spectrometer (514 nm laser). The morphology and microstructure for the obtained 3D-Li2FeSiO4/CNTs microspheres were observed by SEM and TEM. The residual CNTs content of the fabricated 3D-Li2FeSiO4/CNTs microspheres was studied by elemental analysis. The electronic conductivity of 3D-Li2FeSiO4/CNTs was measured with an RTS-8 linear four-point probe measurement
2. Experimental The designed 3D-Li2FeSiO4/CNTs microsphere was synthesized via the simple spray-drying route. In detail, the CNTs slurry was firstly dispersed into the deionized water with magnetic stirring for 15 min. Secondly, the stoichiometric amounts of lithium hydroxide (Aladdin, 99.9%), ferrous oxalate dehydrate (Aladdin, 99.5%) and silicon dioxide (Aladdin, 99.0%) were added into the above CNTs solution with stirring for 3 h. Thirdly, the mixture was spray-dried at 120 °C using a spraydryer to get the precursors. Finally, the precursors were annealed at
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According to Fig. 1a and b, it is found that the particle size of primary microsphere for the 3D-Li2FeSiO4/CNTs composite is around 1.7–5.3 μm. High-magnification SEM image (Fig. 1c) shows that the obtained 3D-Li2FeSiO4/CNTs microspheres are composed of many nanocrystals with size distribution of 150–400 nm. These small crystals are benefit to decrease the diffusion path for Li+ and thus improve the extraction/insertion of Li+ [14,25]. Moreover, the nanosized Li2FeSiO4 particles are embedded into the 3D conductive CNTs, which can significantly enhance the electronic conductivity [28,29]. As presented in Fig. 1d–f, the TEM images indicate that the designed 3D-Li2FeSiO4/ CNTs composite has a regular 3D morphology and the CNTs are anchored on the surface of main particles. To analyze the composition of 3D-Li2FeSiO4/CNTs microspheres, EDS mapping test was performed. Fig. 2 shows the STEM and the corresponding EDS images of iron, silicon, oxygen and carbon in the 3DLi2FeSiO4/CNTs composite. Clearly, the iron, silicon and oxygen elements are homogeneously distributed in the fabricated powder. In addition, the carbon element (Fig. 2e) is also dispersed in the 3DLi2FeSiO4/CNTs powder, which is benefit to provide a 3D conducting network for the transport of electrons and Li-ions during the charging/ discharging procedure [30]. XRD peaks for the resulted 3D-Li2FeSiO4/CNTs microspheres are displayed in Fig. 3. All the main diffraction peaks for 3D-Li2FeSiO4/ CNTs powder are well indexed to the monoclinic structure of Li2FeSiO4. No other impurities such as Li2SiO3 and Fe7(SiO4)O6 are detected in the XRD curve. The above results are consistent with the formerly published papers [12,21,27,31]. It can be noted that no peaks originating from the added CNTs can be found, suggesting that the CNTs in the 3DLi2FeSiO4/CNTs sample are in amorphous state. As well known, it is hard to fabricate well-crystallized Li2FeSiO4 material with monoclinic structure. Therefore, this approach described in the present work demonstrates that this facile spray-drying method is a promising route to prepare the high-purity Li2FeSiO4 cathode for lithium energy storage. To further confirm the presence of CNTs in the obtained 3DLi2FeSiO4/CNTs microspheres, Raman spectrum was tested and the result is illustrated in Fig. 4. Two peaks centered at 1342.5 cm−1 and 1581.1 cm−1 are clearly detected in the curve, which are corresponded to the D-peak and G-peak respectively. Herein, the G-peak reveals the defect and disorder in the lattice of carbon, whereas the D-peak represents the existence of carbon material [32]. The graphitization degree of CNTs in the fabricated composite can be evaluated using the ID/ IG ratio [30,33]. Herein, the value of ID/IG for the designed 3DLi2FeSiO4/CNTs can be calculated to be around 1.05, indicating a high graphitization degree of CNTs. The charging/discharging curves of 3D-Li2FeSiO4/CNTs at 0.1C are presented in Fig. 5a. The 3D-Li2FeSiO4/CNTs electrode shows high discharge capacities of 169.1 and 168.5 mAh g−1 in the 1st and 2nd cycles. The charge curve of 1st cycle possesses two voltage plateaus, which correspond to the redox couples of Fe2+/Fe3+ and Fe3+/Fe4+ [34,35]. Subsequently, the charge plateau of 3D-Li2FeSiO4/CNTs shifts to a low level in the 2nd cycle, revealing a structural rearrangement. It can be found from Fig. 5b that the fabricated 3D-Li2FeSiO4/CNTs electrode displays a stable cycling property with the capacity retention ratio of 99.6% after 60 cycles. High-rate capabilities for the 3DLi2FeSiO4/CNTs electrode at various rates are presented in Fig. 5c. As expected, the 3D-Li2FeSiO4/CNTs delivers high discharge capacities of 144.3, 134.2, 124.5, 112.1 and 94.7 mAh g−1 at 0.5, 1, 2, 5 and 10C, respectively. After cycling for 20 cycles, this electrode still displays the high reversible capacity at each rate. For instance, it delivers the capacity of 121.2 mAh g−1 at 2C over 20 cycles with the low fading ratio of 0.13% per cycle. Fig. 5d presents the long cycle-life for the designed 3D-Li2FeSiO4/CNTs at 5C. Clearly, the Coulombic efficiency can
Fig. 3. XRD peaks for the 3D-Li2FeSiO4/CNTs powder.
Fig. 4. Raman spectrum for the 3D-Li2FeSiO4/CNTs sample.
system. The battery properties for the obtained 3D-Li2FeSiO4/CNTs electrode were tested with CR2025 cell. The working cathode was prepared by mixing the fabricated 3D-Li2FeSiO4/CNTs (80 wt%), PVDF (8 wt%) and carbon black (12 wt%) in the NMP solution. Then, the mixed slurry was casted on a clean aluminum film and heated at 100 °C for 16 h. The cells were assembled with Celgard 2300 as the separator, Li foil as the anode and 1 M LiPF6 in DMC/EC as the electrolyte. EIS and CV for the designed 3D-Li2FeSiO4/CNTs were measured using the CHI660e electrochemical workstation. Galvanostatic charging/discharging measurements were tested using the LAND CT2001 tester. 3. Results and discussion The morphology and nanostructure for the 3D-Li2FeSiO4/CNTs microsphere were studied by SEM and TEM as presented in Fig. 1.
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Fig. 5. (a) Charging/discharging profiles and (b) cyclic stabilities for the designed 3D-Li2FeSiO4/CNTs electrode at 0.1C; (c) Rate properties for 3D-Li2FeSiO4/CNTs at various rates; (d) Long cyclic-life for 3D-Li2FeSiO4/CNTs at 5C.
increase to nearly 99% after cycling for 3 cycles, demonstrating a good electrochemical reversibility. The 3D-Li2FeSiO4/CNTs shows the high capacities of 114.2 and 104.1 mAh g−1 in the 1st and 500th cycles respectively, suggesting a superior cyclic stability. The good lithium storage property for 3D-Li2FeSiO4/CNTs can be assigned to the nanosized Li2FeSiO4 crystals and 3D conductive CNTs networks, which can decrease the diffusion path for Li+ and improve the electrical conductivity. Fig. 6a shows the Nyquist plots of 3D-Li2FeSiO4/CNTs electrode material after 1st and 500th cycles at 5C. Obviously, both impedance spectra of 3D-Li2FeSiO4/CNTs exhibit similar characteristics: one depressed semicircle and one inclined line in the high- and low frequency regions respectively. The depressed semicircle represents the chargetransfer resistance (Rct) at the electrolyte/electrode interface, while the line represents the diffusion of Li+ in the electrode [36]. The 3DLi2FeSiO4/CNTs electrode shows a small value of Rct (106 Ω) after 1 cycle, revealing a high electrical conductivity [33]. Meanwhile, the value of Rct for 3D-Li2FeSiO4/CNTs is still as low as 133 Ω after 500 cycles. The electronic conductivity of the as-prepared 3D-Li2FeSiO4/ CNTs electrode material is about 4.5 × 10−4 S cm−1. The initial two CV profiles of 3D-Li2FeSiO4/CNTs cathode are displayed in Fig. 6b. In detail, the 3D-Li2FeSiO4/CNTs exhibits two peaks centered at 3.42 and 4.49 V in the charge process and one peak centered at 2.57 V in the discharge process for the 1st cycle, corresponding to the processes of Li+ extraction and insertion [20]. Differently, the anodic peak of 3DLi2FeSiO4/CNTs changes from 3.42 V to 3.14 V while the cathodic peak centered at 4.49 V disappears for the 2nd cycle, implying an evident structural transition [12].
4. Conclusions In a conclusion, the 3D-Li2FeSiO4/CNTs composite has been synthesized via the simple spray-drying approach. The phase structure, surface morphology, nanostructure and electrochemical performance of the obtained sample have been systemically studied in this research. The results demonstrate that the well-crystallized Li2FeSiO4 nanoparticles with monoclinic structure are anchored on the 3D conductive CNTs. Electrochemical tests show that the fabricated 3D-Li2FeSiO4/ CNTs electrode exhibits outstanding rate capability and cyclic stability with the capacity retention ratio of around 91.2% at 5C after 500 cycles. The excellent lithium storage performances for the 3D-Li2FeSiO4/ CNTs cathode material are attributed to the designed 3D CNTs networks, which can promote the electrical conductivity and diffusivity of Li+ during the charge/discharge process. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. Acknowledgements This work was financially supported by the Science and Technology Plan Project in Weiyang District of Xi'an City (201911) and Innovation and Entrepreneurship Training Program for College Students (201910702043, X201910702136).
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[10]
[11] [12] [13] [14] [15] [16] [17]
[18] [19]
[20] [21] [22]
[23] [24] [25]
Fig. 6. (a) Nyquist plots for 3D-Li2FeSiO4/CNTs electrode after 1st and 500th cycles at 5C and (b) Initial two CV profiles for the designed 3D-Li2FeSiO4/CNTs electrode.
[26] [27]
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Three-dimensional carbon nanotubes-encapsulated Li2FeSiO4 microspheres as advanced positive materials for lithium energy storage Haiyan Yan∗, Xiuxiu Xue, Yuqiao Fu, Xinming Wu, Jingwei Dong Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi'an Technological University, Xi'an, 710021, PR China
ARTICLE INFO
ABSTRACT
Keywords: Lithium energy storage 3D CNTs Li2FeSiO4 Microspheres Property
Polyanion-type Li2FeSiO4 has been considered as an advanced positive material for lithium energy storage owing to the low cost, good safety and high theoretical capacity. Unfortunately, the inferior electrical conductivity and bad diffusivity of Li+ inhibit its widespread applications for lithium energy storage. In the present research, we have used the simple spray-drying route to fabricate the three-dimensional carbon nanotubes (3D CNTs) encapsulated Li2FeSiO4 microspheres (3D-Li2FeSiO4/CNTs). Benefiting from the interconnected conducting networks, the as-fabricated 3D-Li2FeSiO4/CNTs cathode displays superior battery property with a specific capacity of 169.1 mAh g−1 at 0.1C. Besides, it also delivers a discharge capacity of 104.1 mAh g−1 at 5C over 500 cycles with a capacity retention ratio of around 91.2%. The above results suggest that the designed 3D-Li2FeSiO4/CNTs composite is an advanced positive material in lithium energy storage.
1. Introduction Lithium-ion batteries have received much attention as the main power sources for various applications because of their excellent lithium storage performances [1,2]. Actually, the development of novel electrodes with good rate property, superior cyclic-life and high reversible capacity, are crucial for next-generation lithium energy storage. Recently, many new cathodes such as LiCoMnO4 [3], Li3V2(PO4)3 [4], Li2MnSiO4 [5], LiVPO4F [6] as well as Li2FeSiO4 [7,8], have been widely fabricated and studied for lithium energy storage. Among these novel electrodes, polyanion-type Li2FeSiO4 is considered as a promising cathode because of its high stability and excellent safety [9–12]. Moreover, Li2FeSiO4 exhibits a theoretical capacity of 332 mAh g−1 by two-electron reactions [12]. Nevertheless, the deintercalation/intercalation of two Li+ in Li2FeSiO4 is hard to achieve owing to the inferior electrical conductivity (10−14 S cm−1) and bad diffusivity of Li+ (10−14 cm S−1) [13,14]. Therefore, these obstacles greatly present its practical applications for lithium energy storage. The effective strategies to solve these problems of Li2FeSiO4 have focused on doping with other metal ions [13,15–19], coating with conductive carbon materials [7–12,20–23], and decreasing the particle size [24,25]. Among these methods, carbon coating has been considered as the promising approach to enhance the conductivity of Li2FeSiO4 and inhibit the growth of Li2FeSiO4 crystals and oxidation of
∗
Fe2+ [26]. For instance, Zhang's group [22] used a sol-gel route to fabricate the graphene-decorated Li2FeSiO4@C nanocomposite and the obtained cathode displayed the specific capacity of 178 mAh g−1 at 0.1C over 40 cycles. Despite of the above mentioned methods, designing of different morphologies of Li2FeSiO4 has been proved to be of beneficial effect on its electrochemical performance. Particularly, constructing of 3D structure can endow the electrode with efficient electrolyte storage and decrease the pathway for Li+ transport [7,12,21,27]. Based on the advantages of 3D structure, Huang's group prepared the 3D hollow Li2FeSiO4/C microspheres, which displayed the discharge capacity of 151 mAh g−1 at 0.5C [27]. However, the effects of 3D CNTs on the battery performances of Li2FeSiO4 electrode have not been studied so far. In this work, the designed 3D CNTs-encapsulated Li2FeSiO4 microsphere has been synthesized through the spray-drying route. In the microspheres, the well-crystallized Li2FeSiO4 nanocrystals with monoclinic structure are dispersed in the 3D conducting CNTs. This constructed 3D CNTs networks can promote the electrical conductivity and diffusivity of Li+ during the charge/discharge procedure. Therefore, the fabricated 3D-Li2FeSiO4/CNTs electrode shows superior lithium storage properties including good rate performance and excellent cyclic-life. The results suggest that this designed 3D-Li2FeSiO4/CNTs microspheres can be utilized as the advanced positive material in lithium energy storage.
Corresponding author. E-mail address: [email protected] (H. Yan).
https://doi.org/10.1016/j.ceramint.2019.12.241 Received 13 July 2019; Received in revised form 24 December 2019; Accepted 27 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Haiyan Yan, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.241
Ceramics International xxx (xxxx) xxx–xxx
H. Yan, et al.
Fig. 1. (a–c) SEM and (d–f) TEM images for the resulted 3D-Li2FeSiO4/CNTs sample.
Fig. 2. (a) STEM and (b–e) EDS dot mapping images for the Fe, Si, O and C elements in the designed composite.
360 °C for 3.5 h and then at 700 °C for 9 h under the argon/hydrogen condition to form the 3D-Li2FeSiO4/CNTs composite. The phase structure of the fabricated 3D-Li2FeSiO4/CNTs composite was analyzed by XRD (D8, Bruker). Raman spectroscopy was tested through the RM2000 Raman spectrometer (514 nm laser). The morphology and microstructure for the obtained 3D-Li2FeSiO4/CNTs microspheres were observed by SEM and TEM. The residual CNTs content of the fabricated 3D-Li2FeSiO4/CNTs microspheres was studied by elemental analysis. The electronic conductivity of 3D-Li2FeSiO4/CNTs was measured with an RTS-8 linear four-point probe measurement
2. Experimental The designed 3D-Li2FeSiO4/CNTs microsphere was synthesized via the simple spray-drying route. In detail, the CNTs slurry was firstly dispersed into the deionized water with magnetic stirring for 15 min. Secondly, the stoichiometric amounts of lithium hydroxide (Aladdin, 99.9%), ferrous oxalate dehydrate (Aladdin, 99.5%) and silicon dioxide (Aladdin, 99.0%) were added into the above CNTs solution with stirring for 3 h. Thirdly, the mixture was spray-dried at 120 °C using a spraydryer to get the precursors. Finally, the precursors were annealed at
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According to Fig. 1a and b, it is found that the particle size of primary microsphere for the 3D-Li2FeSiO4/CNTs composite is around 1.7–5.3 μm. High-magnification SEM image (Fig. 1c) shows that the obtained 3D-Li2FeSiO4/CNTs microspheres are composed of many nanocrystals with size distribution of 150–400 nm. These small crystals are benefit to decrease the diffusion path for Li+ and thus improve the extraction/insertion of Li+ [14,25]. Moreover, the nanosized Li2FeSiO4 particles are embedded into the 3D conductive CNTs, which can significantly enhance the electronic conductivity [28,29]. As presented in Fig. 1d–f, the TEM images indicate that the designed 3D-Li2FeSiO4/ CNTs composite has a regular 3D morphology and the CNTs are anchored on the surface of main particles. To analyze the composition of 3D-Li2FeSiO4/CNTs microspheres, EDS mapping test was performed. Fig. 2 shows the STEM and the corresponding EDS images of iron, silicon, oxygen and carbon in the 3DLi2FeSiO4/CNTs composite. Clearly, the iron, silicon and oxygen elements are homogeneously distributed in the fabricated powder. In addition, the carbon element (Fig. 2e) is also dispersed in the 3DLi2FeSiO4/CNTs powder, which is benefit to provide a 3D conducting network for the transport of electrons and Li-ions during the charging/ discharging procedure [30]. XRD peaks for the resulted 3D-Li2FeSiO4/CNTs microspheres are displayed in Fig. 3. All the main diffraction peaks for 3D-Li2FeSiO4/ CNTs powder are well indexed to the monoclinic structure of Li2FeSiO4. No other impurities such as Li2SiO3 and Fe7(SiO4)O6 are detected in the XRD curve. The above results are consistent with the formerly published papers [12,21,27,31]. It can be noted that no peaks originating from the added CNTs can be found, suggesting that the CNTs in the 3DLi2FeSiO4/CNTs sample are in amorphous state. As well known, it is hard to fabricate well-crystallized Li2FeSiO4 material with monoclinic structure. Therefore, this approach described in the present work demonstrates that this facile spray-drying method is a promising route to prepare the high-purity Li2FeSiO4 cathode for lithium energy storage. To further confirm the presence of CNTs in the obtained 3DLi2FeSiO4/CNTs microspheres, Raman spectrum was tested and the result is illustrated in Fig. 4. Two peaks centered at 1342.5 cm−1 and 1581.1 cm−1 are clearly detected in the curve, which are corresponded to the D-peak and G-peak respectively. Herein, the G-peak reveals the defect and disorder in the lattice of carbon, whereas the D-peak represents the existence of carbon material [32]. The graphitization degree of CNTs in the fabricated composite can be evaluated using the ID/ IG ratio [30,33]. Herein, the value of ID/IG for the designed 3DLi2FeSiO4/CNTs can be calculated to be around 1.05, indicating a high graphitization degree of CNTs. The charging/discharging curves of 3D-Li2FeSiO4/CNTs at 0.1C are presented in Fig. 5a. The 3D-Li2FeSiO4/CNTs electrode shows high discharge capacities of 169.1 and 168.5 mAh g−1 in the 1st and 2nd cycles. The charge curve of 1st cycle possesses two voltage plateaus, which correspond to the redox couples of Fe2+/Fe3+ and Fe3+/Fe4+ [34,35]. Subsequently, the charge plateau of 3D-Li2FeSiO4/CNTs shifts to a low level in the 2nd cycle, revealing a structural rearrangement. It can be found from Fig. 5b that the fabricated 3D-Li2FeSiO4/CNTs electrode displays a stable cycling property with the capacity retention ratio of 99.6% after 60 cycles. High-rate capabilities for the 3DLi2FeSiO4/CNTs electrode at various rates are presented in Fig. 5c. As expected, the 3D-Li2FeSiO4/CNTs delivers high discharge capacities of 144.3, 134.2, 124.5, 112.1 and 94.7 mAh g−1 at 0.5, 1, 2, 5 and 10C, respectively. After cycling for 20 cycles, this electrode still displays the high reversible capacity at each rate. For instance, it delivers the capacity of 121.2 mAh g−1 at 2C over 20 cycles with the low fading ratio of 0.13% per cycle. Fig. 5d presents the long cycle-life for the designed 3D-Li2FeSiO4/CNTs at 5C. Clearly, the Coulombic efficiency can
Fig. 3. XRD peaks for the 3D-Li2FeSiO4/CNTs powder.
Fig. 4. Raman spectrum for the 3D-Li2FeSiO4/CNTs sample.
system. The battery properties for the obtained 3D-Li2FeSiO4/CNTs electrode were tested with CR2025 cell. The working cathode was prepared by mixing the fabricated 3D-Li2FeSiO4/CNTs (80 wt%), PVDF (8 wt%) and carbon black (12 wt%) in the NMP solution. Then, the mixed slurry was casted on a clean aluminum film and heated at 100 °C for 16 h. The cells were assembled with Celgard 2300 as the separator, Li foil as the anode and 1 M LiPF6 in DMC/EC as the electrolyte. EIS and CV for the designed 3D-Li2FeSiO4/CNTs were measured using the CHI660e electrochemical workstation. Galvanostatic charging/discharging measurements were tested using the LAND CT2001 tester. 3. Results and discussion The morphology and nanostructure for the 3D-Li2FeSiO4/CNTs microsphere were studied by SEM and TEM as presented in Fig. 1.
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Fig. 5. (a) Charging/discharging profiles and (b) cyclic stabilities for the designed 3D-Li2FeSiO4/CNTs electrode at 0.1C; (c) Rate properties for 3D-Li2FeSiO4/CNTs at various rates; (d) Long cyclic-life for 3D-Li2FeSiO4/CNTs at 5C.
increase to nearly 99% after cycling for 3 cycles, demonstrating a good electrochemical reversibility. The 3D-Li2FeSiO4/CNTs shows the high capacities of 114.2 and 104.1 mAh g−1 in the 1st and 500th cycles respectively, suggesting a superior cyclic stability. The good lithium storage property for 3D-Li2FeSiO4/CNTs can be assigned to the nanosized Li2FeSiO4 crystals and 3D conductive CNTs networks, which can decrease the diffusion path for Li+ and improve the electrical conductivity. Fig. 6a shows the Nyquist plots of 3D-Li2FeSiO4/CNTs electrode material after 1st and 500th cycles at 5C. Obviously, both impedance spectra of 3D-Li2FeSiO4/CNTs exhibit similar characteristics: one depressed semicircle and one inclined line in the high- and low frequency regions respectively. The depressed semicircle represents the chargetransfer resistance (Rct) at the electrolyte/electrode interface, while the line represents the diffusion of Li+ in the electrode [36]. The 3DLi2FeSiO4/CNTs electrode shows a small value of Rct (106 Ω) after 1 cycle, revealing a high electrical conductivity [33]. Meanwhile, the value of Rct for 3D-Li2FeSiO4/CNTs is still as low as 133 Ω after 500 cycles. The electronic conductivity of the as-prepared 3D-Li2FeSiO4/ CNTs electrode material is about 4.5 × 10−4 S cm−1. The initial two CV profiles of 3D-Li2FeSiO4/CNTs cathode are displayed in Fig. 6b. In detail, the 3D-Li2FeSiO4/CNTs exhibits two peaks centered at 3.42 and 4.49 V in the charge process and one peak centered at 2.57 V in the discharge process for the 1st cycle, corresponding to the processes of Li+ extraction and insertion [20]. Differently, the anodic peak of 3DLi2FeSiO4/CNTs changes from 3.42 V to 3.14 V while the cathodic peak centered at 4.49 V disappears for the 2nd cycle, implying an evident structural transition [12].
4. Conclusions In a conclusion, the 3D-Li2FeSiO4/CNTs composite has been synthesized via the simple spray-drying approach. The phase structure, surface morphology, nanostructure and electrochemical performance of the obtained sample have been systemically studied in this research. The results demonstrate that the well-crystallized Li2FeSiO4 nanoparticles with monoclinic structure are anchored on the 3D conductive CNTs. Electrochemical tests show that the fabricated 3D-Li2FeSiO4/ CNTs electrode exhibits outstanding rate capability and cyclic stability with the capacity retention ratio of around 91.2% at 5C after 500 cycles. The excellent lithium storage performances for the 3D-Li2FeSiO4/ CNTs cathode material are attributed to the designed 3D CNTs networks, which can promote the electrical conductivity and diffusivity of Li+ during the charge/discharge process. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. Acknowledgements This work was financially supported by the Science and Technology Plan Project in Weiyang District of Xi'an City (201911) and Innovation and Entrepreneurship Training Program for College Students (201910702043, X201910702136).
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[11] [12] [13] [14] [15] [16] [17]
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Fig. 6. (a) Nyquist plots for 3D-Li2FeSiO4/CNTs electrode after 1st and 500th cycles at 5C and (b) Initial two CV profiles for the designed 3D-Li2FeSiO4/CNTs electrode.
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