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carbon nanotube network as high volumetric performance cathode for lithium ion battery
Journal of Power Sources 447 (2020) 227303
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Packing FeF3⋅0.33H2O into porous graphene/carbon nanotube network as high volumetric performance cathode for lithium ion battery Qi Zhang a, Yu Zhang a, Yanyou Yin a, Lishuang Fan c, **, Naiqing Zhang b, c, * a
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China c Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China b
H I G H L I G H T S
� Porous graphene and CNTs are used as conductive carbon to form a 3D network. � A micro-nano structured FeF3∙0.33H2O possess high volumetric density. � Good Li-storage performance of GCFF can be obtained at high mass loading. A R T I C L E I N F O
A B S T R A C T
Keywords: Iron fluoride Prous graphene High density LIBs
Preparation of nanostructures and combining with carbon materials can solve the problem of poor conductivity of FeF3⋅0.33H2O cathode material. But the fatal drawback is that this will greatly reduce the volumetric energy density. To solve this problem, a multi-stage micro-nano structured porous graphene/carbon nanotube/ FeF3⋅0.33H2O possess high volumetric energy density is constructed in this work. The FeF3⋅0.33H2O nano particles generate by the solvothermal process are not only embedded on the surface of carbon nanotubes and graphene, but also partially agglomerated into micron-sized spheres embed in the graphene layers which can greatly increase the areal mass loading. And the addition of carbon nanotubes not only hinders the self-stacking of graphene layers, but also acts as highly conductive wires to enhance the conductivity between the graphene layers. Good rate capability with high mass loading can be achieved due to the effect of micro-nano-structured FeF3⋅0.33H2O particles packing into the three-dimensional conductive network constructed by carbon nanotubes and graphene. The composite exhibits excellent rate capability with discharge capacities of 148 mAh g 1at 1C (1C ¼ 200 mA g 1) rate under high mass loading of 5 mg cm 2.
1. Introduction In recent years, transition metal fluorides (MxFy, M ¼ Fe, Mn, Co, Ti, etc.) gradually attract particular attention, such as the iron fluoride (FeF3) [1–8]. Especially, the Hexagonal-bronze-tungsten type FeF3⋅0.33H2O possess advantages of high-capacity, high-voltage, and low-cost becomes a favorable cathode material. Theoretical capacity of FeF3⋅0.33H2O is 237 mAh g 1, and the reaction voltage is 2.74 V vs. Li/Liþ [9–12]. Moreover, the presence of a hexagonal channel in the FeF3⋅0.33H2O lattice facilitates the rapid intercalation/deintercalation of Liþ, making it great potential to become new generation of cathode material for lithium ion batteries.
However, the bottleneck which restricting the practical application of FeF3⋅0.33H2O is the low electron and ionic conductivity of the ma terial in nature. This is due to the strong ionic bond characteristics of the Fe–F bond [13–17]. The method of constructing nanostructures and composite materials can effectively alleviate this problem [18–22]. Combining with highly conductive material such as graphene and car bon nanotube can improves the overall conductivity and nanostructured FeF3⋅0.33H2O have large specific surface area would facilitate rapid reaction. However, nanostructures and carbon materials with high specific surface area would reduce the volumetric energy density of the electrode, which is more important than the gravimetric capacity for the battery in the fields of electronics and so on [23–29].
* Corresponding author. State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China., ** Corresponding author. E-mail addresses: [email protected] (L. Fan), [email protected] (N. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227303 Received 17 August 2019; Received in revised form 4 October 2019; Accepted 13 October 2019 Available online 8 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 447 (2020) 227303
In this work, one-dimensional carbon nanotubes (CNTs) and twodimensional porous graphene were used as conductive carbon matrix and micro-nano-structured FeF3⋅0.33H2O particles were prepared by solvothermal method. A multi-stage micro-nano structured porous gra phene/carbon nanotube/FeF3⋅0.33H2O (GCFF) possess high volumetric energy density was suceseeful constructed. The FeF3⋅0.33H2O nano particles generated by the solvothermal process are not only embedded on the surface of CNTs and graphene, but also partially agglomerated into micron-sized spheres embedded in the graphene layers which can greatly increase the areal mass loading. In addition, the pores on gra phene formed by H2O2 etching is more conducive to the mass transfer between PGO layers. And the addition of CNTs not only hinders the selfstacking of PGO layers, but also acts as highly conductive wires to enhance the conductivity between the PGO layers. It is expected that good rate capability with high mass loading can be achieved due to the effect of micro-nano-structured FeF3⋅0.33H2O particles packing into the 3D conductive network constructed by PGO and CNTs. The composite exhibits excellent rate capability with discharge capacities of 148 mAh g 1at 1C (1C ¼ 200 mA g 1) rate under high mass loading of 5 mg cm 2.
to irreversibly stack due to its strong π-π interaction, resulting in blocking of channels for electrolyte entry and rapid ion and electron transport. As a result, in GFF composite electrons can only be transferred quickly on the outer graphene surface during reaction. For GCFF com posite, one-dimensional (1D) CNTs and two-dimensional (2D) PGO are used as conductive carbon matrix to form a three-dimensional (3D) network structure. And micro-nano-structured FeF3⋅0.33H2O particles are packing into this 3D network. The one-dimensional CNTs added can incorporate into the PGO layers to prevent the irreversibly stack [30]. Moreover, although PGO has excellent planar electronic conductivity, but the electron conductivity between the layers is limited. The CNTs embedded in the PGO layers can be used as a wire to connect the layers of PGO in series to improve the overall conductivity [31,32]. Therefore, the incorporation of one-dimensional CNTs into a two-dimensional PGO interlayer can preserve the high specific surface area of PGO while forming a unique three-dimensional conductive network with extraor dinary planar and transplanar electron conductivity. As formed unique 3D conductive network can greatly improve the high loading perfor mance of FeF3⋅0.33H2O. Detailed preparation process can be found in the experimental part. Fig. 2 shows the SEM pictures of as prepared composite. The X-ray diffraction (XRD) patterns show that the synthetic materials are all HTBtype FeF3⋅0.33H2O (Fig. S1) [33–35]. Fig. 2a and b present the morphology of PGO/FeF3⋅0.33H2O (GFF). The SEM images show that the irregular spheres of FeF3⋅0.33H2O with a diameter about 500 nm in GFF are gathered together and completely wrapped by graphene. The magnified SEM image shows that the FeF3⋅0.33H2O irregular sphere is agglomerated by secondary nanoparticles. Fig. 2c and d present the morphology of CNTs/FeF3⋅0.33H2O (CFF). At low magnification, FeF3⋅0.33H2O spheres with a diameter of 1 μm and CNTs entangled can be seen. After magnification the SEM image shows that a large amount of secondary nanoparticles occupies the surface of the CNTs. The diameter of the CNTs used is 20 nm, and the size of these secondary nanoparticles are much smaller than the diameter of the CNTs. The reason for these secondary particles didn’t continue to grow is that the unreacted ionic liquid BmimBF4 in the solution adsorbs on the surface of the initially formed FeF3⋅0.33H2O crystal nucleus during the solvothermal process, preventing its growth. Fig. 2e and f show the SEM pictures of the GCFF composite. It can be seen that 1D CNTs doping can maintain a high
2. Results and discussion Fig. 1 shows the schematic illustration of the formation of GCFF material. Carbon nanotubes (CNTs) and graphene oxide (GO) are used as the conductive carbon matrix. Firstly, an acidification process is per formed on CNTs. After treated with HNO3, a large amount of oxygencontaining functional groups, such as –OH, –COOH, etc. are formed on the surface of the CNTs to enhance the hydrophilicity. The GO is also treated with H2O2 to form a porous GO (PGO). When CNT is used as the conductive matrix alone, the FeF3⋅0.33H2O nanoparticles generated during the solvothermal process will adhere to the surface of the CNTs. Moreover, the FeF3⋅0.33H2O nanoparticles will agglomerate into larger microspheres when all the surface of CNTs are occupied by the FeF3⋅0.33H2O nanoparticles. When PGO is used as the conductive car bon alone, the FeF3⋅0.33H2O nanoparticles will agglomerate into mi crospheres and be wrapped by PGO. The graphene nanosheets have excellent electronic conductivity and high mechanical strength, and the electrode can maintain structural stability even after under-going high pressure and long circulation. However, the graphene layer is very easy
Fig. 1. Schematic illustration of the formation of GCFF composite. 2
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Journal of Power Sources 447 (2020) 227303
Fig. 2. SEM pictures of as prepared composite. (a,b) GFF, (c,d) CFF, (e,f) GCFF.
specific surface area of PGO. There are a large number of FeF3⋅0.33H2O microspheres distributed between the PGO layers. These microspheres about 1 μm in diameter are connected in series by CNTs to form a continuous electron transfer highway. Moreover, a large number of small FeF3⋅0.33H2O nanoparticles are embedded on the surface of PGO, which further increases the proportion of FeF3⋅0.33H2O in the composite. The pores formed on PGO layer after etching were observed by transmission electron microscope (TEM). The TEM pictures show that pores appeared after the original GO layer has been etched (Fig. 3a and b). The presence of these pores allows the ions to pass through the GO layer smoothly, which facilitates the electrolyte to fully immerse into the interior of the electrode. Moreover, the H2O2 etching is not only to form holes, but also to further increase the degree of oxidation of GO. The Xray photoelectron spectroscopy (XPS) shows that more functional groups are formed on the surface of the GO layer after etching (Fig. S2). The intensity of C–O peak located at 286.5 eV becomes stronger and exceeds the C–C peak after etching. These functional groups are reduced by the ionic liquid during the solvothermal process, and the defect sites left could favor the nucleation and attachment of the FeF3⋅0.33H2O particles. The TEM pictures of the as-prepared GCFF shows that the large microspheres appearing in the SEM picture are agglomerated by smaller secondary particles (Fig. 3c and d). There are a large number of gaps between the secondary particles in the microspheres, which facilitates the entry of Liþ into the interior of the microspheres. The HRTEM pic ture of GCFF material contains a large number of regular lattice stripes and graphene folds appear at the edges of the particles. The lattice spacing of 0.37 nm corresponds to the (200) plane of FeF3⋅0.33H2O [36–38]. The lattice spacing of exposed (200) plane is large, which is
conducive to the intercalation of Liþ. The EDS mapping of Fe, F, and C elements in GCFF are shown in Fig. 3f. The distribution of Fe and F el ements is consistent with the shape of the particle in the Dark-Field TEM image. There is a distribution of C elements throughout the selected area, which should come from PGO coated on the surface of the particles and CNTs embedded in the particles. The TG test was used to detect the carbon content in the GCFF (Fig. S3). The result shows that carbon in GCFF is approximately 11 wt%, the content of the FeF3⋅0.33H2O component is 89 wt%. The chemical-bond information of each element in the GCFF com posite was analyzed using the XPS test. The results are shown in Fig. 4 below. The signals of C 1s, O 1s, Fe 2s, F 1s, and Fe 2p can be seen in the survey, which proves that there are only C, O, F, and Fe elements in the composite (Fig. 4a). Fitting results of the C 1s, F 1s, and Fe 2p peaks are shown in Fig. 4b–d. The C 1s peak can be divided into two peaks located at 284.6 eV and 285.9 eV, respectively. The peak at 284.6 eV corre sponds to the signal of C–C bond. The peak at 285.9 eV origins from the –O sp3 hybridization caused by defects in graphene. The C–O and C– peaks appeared in the previous GO and PGO disappeared in the C 1s peak of the composite, indicating that PGO has been reduced to gra phene during the solvothermal process [18]. Fig. 4c shows that the F 1s peak is located at 685.1 eV, which is consistent with the results of other iron fluorides [20]. The fitted Fe 2p3/2 peak can be divided into two peaks at 711.3 eV and 713.3 eV, respectively. The peak at 713.3 eV indicates that the Fe element is in the þ3 oxidation state, while the small peak at 711.3 eV indicates that trace amount of Fe is reduced to the þ2 state [39,40]. The ionic liquid is excessive in the solvothermal process, and it functions not only as a fluorine source but also as a soft template. The unreacted ionic liquid [Bmim][BF4] in the solution adsorbs on the 3
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Journal of Power Sources 447 (2020) 227303
Fig. 3. (a) TEM pictures of graphene oxide before etching, (b) After etching, (c–e) TEM and HRTEM pictures of GCFF, (f) Dark-Field TEM image of GCFF and the corresponding EDS mapping of Fe, F, and C elements.
surface of the FeF3⋅0.33H2O particles which can prevent its continue growth and a portion of Fe is reduced to þ2 valence in an ionic liquid atmosphere with graphene oxide [41,42]. The prepared composites were prepared into electrodes and assem bled into coin cells for electrochemical performance test. The charge/ discharge profiles for GCFF at different rates are shown in Fig. 5a. The discharge curve appears as a slowly decreasing slash without particu larly noticeable platform. Because the reaction that occurs during discharge is that Liþ enters into FeF3⋅0.33H2O lattice through a solid solution reaction instead of intercalation reaction [43,44]. Also the
reduction peak in the CV curves appears as a very wide peak (Fig. S4). When turns to charge, Liþ is precipitated from the FeF3⋅0.33H2O lattice and the charging curve also has no fixed platform. When the current density increases, the shape of the charge/discharge curve does not change greatly, but the voltage hysteresis between charge/discharge becomes larger due to the increase in polarization. The reaction that occurs during the charge/discharge process can be expressed as follows: FeF3⋅0.33H2O þ Liþ þe ⇌LiFeF3⋅0.33H2O (1). One FeF3⋅0.33H2O unit cell can dissolve up to one Liþ with one electron transfer, the theoretical capacity of FeF3⋅0.33H2O is 237 mAh
Fig. 4. XPS spectrum of GCFF composite. (a) Survey, (b) C 1s, (c) F 1s, (d) Fe 2p. 4
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Journal of Power Sources 447 (2020) 227303
Fig. 5. (a) Charge-discharge curves of GCFF material at different current rate, (b) Rate performance of CFF, GFF, and GCFF materials, (c) Cycle performance of CFF, GFF, and GCFF materials, (d) Nyquist plots of CFF, GFF, and GCFF electrode.
g
1
. In order to reflect the superiority of the GCFF structure to the GFF and CFF materials, three materials were tested for charge/discharge at different rates at a mass loading of 1 mg cm 2. The GCFF material dis plays a obviously better performance than GFF and CFF materials (Fig. 5b). The GCFF release a capacity of 162 mAh g 1, 132 mAh g 1, 100 mAh g 1, 83 mAh g 1, and 69 mAh g 1 at 1C, 2C, 5C, 10C, and 20C, respectively (1C ¼ 200 mA g 1). As a comparison, the CFF only delivers a capacity of 71 mAh g 1 at a high rate of 5C. When the rate rise to 20C, the capacity of CFF drops to 51 mAh g 1. And the high rate performance of GFF material is even worse than CFF. Only a capacity of 68 mAh g 1 and 48 mAh g 1 can be maintained at the rate of 5C and 20C. The excellent rate performance of GCFF can be attributed to the high-speed ion and electron transport network formed by PGO and highly conductive CNTs. Furthermore, the GCFF material also present a better cycling per formance than GFF and CFF materials at mass loading of 1 mg cm 2 (Fig. 5c). The capacity of GCFF electrode reaches 162 mAh g 1 and maintains at 120 mAh g 1 after 100 cycles at the current rate of 1C. While the GFF electrode delivers a capacity of 146 mAh g 1 for the first cycle and only 42% of the initial capacity can be retained after 100 cycles. The cycle performance of CFF electrode is a little better than GFF but worse than GCFF. The initial capacity is 138 mAh g 1, and there are still 97 mAh g 1 after 100 cycles charge/discharge at 1C. Long cycle test at 5C rate also shows that GCFF possess higher specific capacity than GFF and CFF electrode after 500 times charge/discharge (Fig. S5). The good cycling performance of GCFF origins from the structure con structed. Good conductivity of PGO and CNTs ensures the rapid trans port of electrons, while the pores in graphene and gaps in the microspheres ensure that Liþ can freely shuttle into the interior of the FeF3⋅0.33H2O lattice. Electrochemical impedance spectroscopy (EIS) shows that GCFF has a small Rct which present the small transfer impedance for electrode reaction (Fig. 5d). Although the CFF electrode possesses the smallest
semicircle diameter which representing the value of the Rct in the Nyquist plots, the rate and cycling performance of CFF materials are not the best. The reason for small Rct value is that FeF3⋅0.33H2O nano particles in the CFF that are directly attached to the surface of the CNTs. However, the large particles agglomerated in CFF exhibit poor perfor mance during high-rate charge/discharge and long cycle test. The semicircular diameter for GFF plot is almost twice of GCFF, indicating that reaction is difficult in GFF electrodes. It shows that the graphene coating has a limited effect on improving the conductivity of large FeF3⋅0.33H2O particles. The prepared three-dimensional conductive network of GCFF facili tates the rapid transmission of ions and electrons, so that the capacity of the electrode can be fully utilized even at a relatively high mass loading. Fig. 6 shows the cycling ability of as-prepared composites at different mass loadings (1 mg cm 2, 2 mg cm 2, and 5 mg cm 2). The initial spe cific capacity of GCFF composite at 1C rate are 162 mAh g 1, 154 mAh g 1,148 mAh g 1 at mass loading of 1 mg cm 2, 2 mg cm 2, and 5 mg cm 2, respectively (Fig. 6a). The retention of specific capacity is 92% when the areal mass loading rising from 1 mg cm 2 to 5 mg cm 2 (Fig. 6b). And the capacity retention rate after 100 cycles charge/ discharge didn’t decrease due to the increase in mass loading. Moreover, the GCFF composite can release a capacity of 100 mAh g 1, 91 mAh g 1, and 84 mAh g 1 at the mass loading of 1 mg cm 2, 2 mg cm 2, and 5 mg cm 2. The retention of specific capacity is 84% at 5C high rate (Fig. S5). As a comparison, the capacity retention of CFF electrodes is only 63%, the specific capacity drops to 87 mAh g 1 when mass loading rise to 5 mg cm 2 (Fig. 6c and d). For GFF composite, a high specific capacity of 146 mAh g 1 can be obtained at 1 mg cm 2 at 1C, but only half of the value can be reserved at mass loading of 5 mg cm 2 (Fig. 6e and f). The discharge capacity of GFF and CFF electrodes drop drasti cally when the mass loading increases. But the highway for electron transfer in the constructed conductive network and the micro-nano structure of the FeF3⋅0.33H2O ensure the high specific capacity of GCFF composite at high mass loading. 5
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Journal of Power Sources 447 (2020) 227303
Fig. 6. Cycle performance and capacity retention of as-prepared composites for different mass loading. (a, b) GCFF electrode, (c, d) CFF electrode, (e, f) GFF electrode.
As 5 mg cm 2 is the mass loading close to the actual application, which is beneficial to evaluate the practical application potential of the material. Therefore, cycle test under 5 mg cm 2 were conducted (Fig. 7). The GCFF composite still has a specific capacity of 51 mAh g 1 even after 100 times charge/discharge at high loading and 5C high rate, but
the specific capacity for CFF and GFF electrode drop to only 18 mAh g 1and 23 mAh g 1(Fig. 7a). In addition, the GCFF electrode possesses a specific capacity of 56 mAh g 1 at an ultra-high 20C rate (Fig. 7b). Packing the FeF3∙0.33H2O nanoparticles and microspheres into the 3D conductive network constructed by PGO and CNTs enable GCFF material
Fig. 7. (a) Cycle performance of as-prepared GCFF, CFF, and GFF at 5C for mass loading of 5 mg cm 6
2
, (b) GCFF at 20C.
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Journal of Power Sources 447 (2020) 227303
to have excellent performance even under high mass loading and high current density.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227303.
3. Conclusion
References
In summary, a three-dimensional conductive network was success fully constructed by porous graphene and CNTs, and FeF3∙0.33H2O nanoparticles and microspheres were loaded on the surface and between graphene layers by solvothermal method. The one-dimensional CNTs added can incorporate into the graphene layers to prevent the irre versibly stacking caused by strong π-π interaction. Porous graphene and CNTs can improve in-plane and portrait ion/electron conductivity which will benefit for mass migration at high mass loadings. A large amount of FeF3∙0.33H2O nanoparticles and microspheres can increase the volu metric energy density of composite. The GCFF composite exhibits excellent rate capability with discharge capacities of 162 mAh g 1 at 1 C, 132 mAh g 1 at 2 C, 100 mAh g 1 at 5 C, 83 mAh g 1 at 10 C, and 69 mAh g 1 at 20 C; The retention of specific capacity is 92% when the areal mass loading rising from 1 mg cm 2 to 5 mg cm 2 at 1C rate. The excellent performance of GCFF at high loading provide an strategy for the practical application of FeF3∙0.33H2O cathode for LIBs.
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4. Experimental section 4.1. Treatment of GO and CNTs Firstly, 0.2 g GO powder was dispersed in 50 ml of deionized water, then 5 ml H2O2 (30 wt%) was added and the mixture was heated and stirred at 95 � C for 30 min in a water bath. After cooling, the solution was centrifuged and washed 3 times with deionized water. PGO powder can be obtained after freeze-drying. Acid treatment of CNTs was per formed to enhance the hydrophilicity of CNTs. Typically, 1 g of CNTs (20 nm in diameter and 50 μm in length) was dispersed in 40 ml concentrated HNO3 and reflux heating at 80 � C for 1 h. Then the solution was centrifuged and washed with deionized water until neutral. Rich functional groups containing CNTs powder can be obtained after freeze drying. 4.2. Synthesis of GCFF In a typically synthetic process of GCFF, 20 mg treated CNTs and 20 mg PGO were dispersed in 50 ml of ethanol by ultrasonic. Then 0.5 g Fe(NO3)3⋅9H2O was added to the dispersed solution and 10 ml ionic liquid [Bmim][BF4] was slowly added after Fe(NO3)3⋅9H2O complete dissolution. The mixture was thoroughly stirred and transferred to a 100 ml Teflon reactor. The reactor was sealed and transferred to an oven at 120 � C for 12 h. After naturally cooling to room temperature, the reactor was opened and the product was washed with ethanol for at least 3 times. After vacuum dried at 80 � C a black powder can be obtained. As a comparison, CNT/FeF3⋅0.33H2O (CFF) and PGO/FeF3⋅0.33H2O (GFF) composites were also prepared by using 40 mg CNTs or PGO separately as conductive carbon and other conditions remains unchanged. Material characterization and electrochemical test part are described in supporting information. Declaration of competing interest There are no conflicts to declare. Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21646012). The Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 201836). 7
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Packing FeF3⋅0.33H2O into porous graphene/carbon nanotube network as high volumetric performance cathode for lithium ion battery Qi Zhang a, Yu Zhang a, Yanyou Yin a, Lishuang Fan c, **, Naiqing Zhang b, c, * a
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China c Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China b
H I G H L I G H T S
� Porous graphene and CNTs are used as conductive carbon to form a 3D network. � A micro-nano structured FeF3∙0.33H2O possess high volumetric density. � Good Li-storage performance of GCFF can be obtained at high mass loading. A R T I C L E I N F O
A B S T R A C T
Keywords: Iron fluoride Prous graphene High density LIBs
Preparation of nanostructures and combining with carbon materials can solve the problem of poor conductivity of FeF3⋅0.33H2O cathode material. But the fatal drawback is that this will greatly reduce the volumetric energy density. To solve this problem, a multi-stage micro-nano structured porous graphene/carbon nanotube/ FeF3⋅0.33H2O possess high volumetric energy density is constructed in this work. The FeF3⋅0.33H2O nano particles generate by the solvothermal process are not only embedded on the surface of carbon nanotubes and graphene, but also partially agglomerated into micron-sized spheres embed in the graphene layers which can greatly increase the areal mass loading. And the addition of carbon nanotubes not only hinders the self-stacking of graphene layers, but also acts as highly conductive wires to enhance the conductivity between the graphene layers. Good rate capability with high mass loading can be achieved due to the effect of micro-nano-structured FeF3⋅0.33H2O particles packing into the three-dimensional conductive network constructed by carbon nanotubes and graphene. The composite exhibits excellent rate capability with discharge capacities of 148 mAh g 1at 1C (1C ¼ 200 mA g 1) rate under high mass loading of 5 mg cm 2.
1. Introduction In recent years, transition metal fluorides (MxFy, M ¼ Fe, Mn, Co, Ti, etc.) gradually attract particular attention, such as the iron fluoride (FeF3) [1–8]. Especially, the Hexagonal-bronze-tungsten type FeF3⋅0.33H2O possess advantages of high-capacity, high-voltage, and low-cost becomes a favorable cathode material. Theoretical capacity of FeF3⋅0.33H2O is 237 mAh g 1, and the reaction voltage is 2.74 V vs. Li/Liþ [9–12]. Moreover, the presence of a hexagonal channel in the FeF3⋅0.33H2O lattice facilitates the rapid intercalation/deintercalation of Liþ, making it great potential to become new generation of cathode material for lithium ion batteries.
However, the bottleneck which restricting the practical application of FeF3⋅0.33H2O is the low electron and ionic conductivity of the ma terial in nature. This is due to the strong ionic bond characteristics of the Fe–F bond [13–17]. The method of constructing nanostructures and composite materials can effectively alleviate this problem [18–22]. Combining with highly conductive material such as graphene and car bon nanotube can improves the overall conductivity and nanostructured FeF3⋅0.33H2O have large specific surface area would facilitate rapid reaction. However, nanostructures and carbon materials with high specific surface area would reduce the volumetric energy density of the electrode, which is more important than the gravimetric capacity for the battery in the fields of electronics and so on [23–29].
* Corresponding author. State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China., ** Corresponding author. E-mail addresses: [email protected] (L. Fan), [email protected] (N. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227303 Received 17 August 2019; Received in revised form 4 October 2019; Accepted 13 October 2019 Available online 8 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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In this work, one-dimensional carbon nanotubes (CNTs) and twodimensional porous graphene were used as conductive carbon matrix and micro-nano-structured FeF3⋅0.33H2O particles were prepared by solvothermal method. A multi-stage micro-nano structured porous gra phene/carbon nanotube/FeF3⋅0.33H2O (GCFF) possess high volumetric energy density was suceseeful constructed. The FeF3⋅0.33H2O nano particles generated by the solvothermal process are not only embedded on the surface of CNTs and graphene, but also partially agglomerated into micron-sized spheres embedded in the graphene layers which can greatly increase the areal mass loading. In addition, the pores on gra phene formed by H2O2 etching is more conducive to the mass transfer between PGO layers. And the addition of CNTs not only hinders the selfstacking of PGO layers, but also acts as highly conductive wires to enhance the conductivity between the PGO layers. It is expected that good rate capability with high mass loading can be achieved due to the effect of micro-nano-structured FeF3⋅0.33H2O particles packing into the 3D conductive network constructed by PGO and CNTs. The composite exhibits excellent rate capability with discharge capacities of 148 mAh g 1at 1C (1C ¼ 200 mA g 1) rate under high mass loading of 5 mg cm 2.
to irreversibly stack due to its strong π-π interaction, resulting in blocking of channels for electrolyte entry and rapid ion and electron transport. As a result, in GFF composite electrons can only be transferred quickly on the outer graphene surface during reaction. For GCFF com posite, one-dimensional (1D) CNTs and two-dimensional (2D) PGO are used as conductive carbon matrix to form a three-dimensional (3D) network structure. And micro-nano-structured FeF3⋅0.33H2O particles are packing into this 3D network. The one-dimensional CNTs added can incorporate into the PGO layers to prevent the irreversibly stack [30]. Moreover, although PGO has excellent planar electronic conductivity, but the electron conductivity between the layers is limited. The CNTs embedded in the PGO layers can be used as a wire to connect the layers of PGO in series to improve the overall conductivity [31,32]. Therefore, the incorporation of one-dimensional CNTs into a two-dimensional PGO interlayer can preserve the high specific surface area of PGO while forming a unique three-dimensional conductive network with extraor dinary planar and transplanar electron conductivity. As formed unique 3D conductive network can greatly improve the high loading perfor mance of FeF3⋅0.33H2O. Detailed preparation process can be found in the experimental part. Fig. 2 shows the SEM pictures of as prepared composite. The X-ray diffraction (XRD) patterns show that the synthetic materials are all HTBtype FeF3⋅0.33H2O (Fig. S1) [33–35]. Fig. 2a and b present the morphology of PGO/FeF3⋅0.33H2O (GFF). The SEM images show that the irregular spheres of FeF3⋅0.33H2O with a diameter about 500 nm in GFF are gathered together and completely wrapped by graphene. The magnified SEM image shows that the FeF3⋅0.33H2O irregular sphere is agglomerated by secondary nanoparticles. Fig. 2c and d present the morphology of CNTs/FeF3⋅0.33H2O (CFF). At low magnification, FeF3⋅0.33H2O spheres with a diameter of 1 μm and CNTs entangled can be seen. After magnification the SEM image shows that a large amount of secondary nanoparticles occupies the surface of the CNTs. The diameter of the CNTs used is 20 nm, and the size of these secondary nanoparticles are much smaller than the diameter of the CNTs. The reason for these secondary particles didn’t continue to grow is that the unreacted ionic liquid BmimBF4 in the solution adsorbs on the surface of the initially formed FeF3⋅0.33H2O crystal nucleus during the solvothermal process, preventing its growth. Fig. 2e and f show the SEM pictures of the GCFF composite. It can be seen that 1D CNTs doping can maintain a high
2. Results and discussion Fig. 1 shows the schematic illustration of the formation of GCFF material. Carbon nanotubes (CNTs) and graphene oxide (GO) are used as the conductive carbon matrix. Firstly, an acidification process is per formed on CNTs. After treated with HNO3, a large amount of oxygencontaining functional groups, such as –OH, –COOH, etc. are formed on the surface of the CNTs to enhance the hydrophilicity. The GO is also treated with H2O2 to form a porous GO (PGO). When CNT is used as the conductive matrix alone, the FeF3⋅0.33H2O nanoparticles generated during the solvothermal process will adhere to the surface of the CNTs. Moreover, the FeF3⋅0.33H2O nanoparticles will agglomerate into larger microspheres when all the surface of CNTs are occupied by the FeF3⋅0.33H2O nanoparticles. When PGO is used as the conductive car bon alone, the FeF3⋅0.33H2O nanoparticles will agglomerate into mi crospheres and be wrapped by PGO. The graphene nanosheets have excellent electronic conductivity and high mechanical strength, and the electrode can maintain structural stability even after under-going high pressure and long circulation. However, the graphene layer is very easy
Fig. 1. Schematic illustration of the formation of GCFF composite. 2
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Fig. 2. SEM pictures of as prepared composite. (a,b) GFF, (c,d) CFF, (e,f) GCFF.
specific surface area of PGO. There are a large number of FeF3⋅0.33H2O microspheres distributed between the PGO layers. These microspheres about 1 μm in diameter are connected in series by CNTs to form a continuous electron transfer highway. Moreover, a large number of small FeF3⋅0.33H2O nanoparticles are embedded on the surface of PGO, which further increases the proportion of FeF3⋅0.33H2O in the composite. The pores formed on PGO layer after etching were observed by transmission electron microscope (TEM). The TEM pictures show that pores appeared after the original GO layer has been etched (Fig. 3a and b). The presence of these pores allows the ions to pass through the GO layer smoothly, which facilitates the electrolyte to fully immerse into the interior of the electrode. Moreover, the H2O2 etching is not only to form holes, but also to further increase the degree of oxidation of GO. The Xray photoelectron spectroscopy (XPS) shows that more functional groups are formed on the surface of the GO layer after etching (Fig. S2). The intensity of C–O peak located at 286.5 eV becomes stronger and exceeds the C–C peak after etching. These functional groups are reduced by the ionic liquid during the solvothermal process, and the defect sites left could favor the nucleation and attachment of the FeF3⋅0.33H2O particles. The TEM pictures of the as-prepared GCFF shows that the large microspheres appearing in the SEM picture are agglomerated by smaller secondary particles (Fig. 3c and d). There are a large number of gaps between the secondary particles in the microspheres, which facilitates the entry of Liþ into the interior of the microspheres. The HRTEM pic ture of GCFF material contains a large number of regular lattice stripes and graphene folds appear at the edges of the particles. The lattice spacing of 0.37 nm corresponds to the (200) plane of FeF3⋅0.33H2O [36–38]. The lattice spacing of exposed (200) plane is large, which is
conducive to the intercalation of Liþ. The EDS mapping of Fe, F, and C elements in GCFF are shown in Fig. 3f. The distribution of Fe and F el ements is consistent with the shape of the particle in the Dark-Field TEM image. There is a distribution of C elements throughout the selected area, which should come from PGO coated on the surface of the particles and CNTs embedded in the particles. The TG test was used to detect the carbon content in the GCFF (Fig. S3). The result shows that carbon in GCFF is approximately 11 wt%, the content of the FeF3⋅0.33H2O component is 89 wt%. The chemical-bond information of each element in the GCFF com posite was analyzed using the XPS test. The results are shown in Fig. 4 below. The signals of C 1s, O 1s, Fe 2s, F 1s, and Fe 2p can be seen in the survey, which proves that there are only C, O, F, and Fe elements in the composite (Fig. 4a). Fitting results of the C 1s, F 1s, and Fe 2p peaks are shown in Fig. 4b–d. The C 1s peak can be divided into two peaks located at 284.6 eV and 285.9 eV, respectively. The peak at 284.6 eV corre sponds to the signal of C–C bond. The peak at 285.9 eV origins from the –O sp3 hybridization caused by defects in graphene. The C–O and C– peaks appeared in the previous GO and PGO disappeared in the C 1s peak of the composite, indicating that PGO has been reduced to gra phene during the solvothermal process [18]. Fig. 4c shows that the F 1s peak is located at 685.1 eV, which is consistent with the results of other iron fluorides [20]. The fitted Fe 2p3/2 peak can be divided into two peaks at 711.3 eV and 713.3 eV, respectively. The peak at 713.3 eV indicates that the Fe element is in the þ3 oxidation state, while the small peak at 711.3 eV indicates that trace amount of Fe is reduced to the þ2 state [39,40]. The ionic liquid is excessive in the solvothermal process, and it functions not only as a fluorine source but also as a soft template. The unreacted ionic liquid [Bmim][BF4] in the solution adsorbs on the 3
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Fig. 3. (a) TEM pictures of graphene oxide before etching, (b) After etching, (c–e) TEM and HRTEM pictures of GCFF, (f) Dark-Field TEM image of GCFF and the corresponding EDS mapping of Fe, F, and C elements.
surface of the FeF3⋅0.33H2O particles which can prevent its continue growth and a portion of Fe is reduced to þ2 valence in an ionic liquid atmosphere with graphene oxide [41,42]. The prepared composites were prepared into electrodes and assem bled into coin cells for electrochemical performance test. The charge/ discharge profiles for GCFF at different rates are shown in Fig. 5a. The discharge curve appears as a slowly decreasing slash without particu larly noticeable platform. Because the reaction that occurs during discharge is that Liþ enters into FeF3⋅0.33H2O lattice through a solid solution reaction instead of intercalation reaction [43,44]. Also the
reduction peak in the CV curves appears as a very wide peak (Fig. S4). When turns to charge, Liþ is precipitated from the FeF3⋅0.33H2O lattice and the charging curve also has no fixed platform. When the current density increases, the shape of the charge/discharge curve does not change greatly, but the voltage hysteresis between charge/discharge becomes larger due to the increase in polarization. The reaction that occurs during the charge/discharge process can be expressed as follows: FeF3⋅0.33H2O þ Liþ þe ⇌LiFeF3⋅0.33H2O (1). One FeF3⋅0.33H2O unit cell can dissolve up to one Liþ with one electron transfer, the theoretical capacity of FeF3⋅0.33H2O is 237 mAh
Fig. 4. XPS spectrum of GCFF composite. (a) Survey, (b) C 1s, (c) F 1s, (d) Fe 2p. 4
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Fig. 5. (a) Charge-discharge curves of GCFF material at different current rate, (b) Rate performance of CFF, GFF, and GCFF materials, (c) Cycle performance of CFF, GFF, and GCFF materials, (d) Nyquist plots of CFF, GFF, and GCFF electrode.
g
1
. In order to reflect the superiority of the GCFF structure to the GFF and CFF materials, three materials were tested for charge/discharge at different rates at a mass loading of 1 mg cm 2. The GCFF material dis plays a obviously better performance than GFF and CFF materials (Fig. 5b). The GCFF release a capacity of 162 mAh g 1, 132 mAh g 1, 100 mAh g 1, 83 mAh g 1, and 69 mAh g 1 at 1C, 2C, 5C, 10C, and 20C, respectively (1C ¼ 200 mA g 1). As a comparison, the CFF only delivers a capacity of 71 mAh g 1 at a high rate of 5C. When the rate rise to 20C, the capacity of CFF drops to 51 mAh g 1. And the high rate performance of GFF material is even worse than CFF. Only a capacity of 68 mAh g 1 and 48 mAh g 1 can be maintained at the rate of 5C and 20C. The excellent rate performance of GCFF can be attributed to the high-speed ion and electron transport network formed by PGO and highly conductive CNTs. Furthermore, the GCFF material also present a better cycling per formance than GFF and CFF materials at mass loading of 1 mg cm 2 (Fig. 5c). The capacity of GCFF electrode reaches 162 mAh g 1 and maintains at 120 mAh g 1 after 100 cycles at the current rate of 1C. While the GFF electrode delivers a capacity of 146 mAh g 1 for the first cycle and only 42% of the initial capacity can be retained after 100 cycles. The cycle performance of CFF electrode is a little better than GFF but worse than GCFF. The initial capacity is 138 mAh g 1, and there are still 97 mAh g 1 after 100 cycles charge/discharge at 1C. Long cycle test at 5C rate also shows that GCFF possess higher specific capacity than GFF and CFF electrode after 500 times charge/discharge (Fig. S5). The good cycling performance of GCFF origins from the structure con structed. Good conductivity of PGO and CNTs ensures the rapid trans port of electrons, while the pores in graphene and gaps in the microspheres ensure that Liþ can freely shuttle into the interior of the FeF3⋅0.33H2O lattice. Electrochemical impedance spectroscopy (EIS) shows that GCFF has a small Rct which present the small transfer impedance for electrode reaction (Fig. 5d). Although the CFF electrode possesses the smallest
semicircle diameter which representing the value of the Rct in the Nyquist plots, the rate and cycling performance of CFF materials are not the best. The reason for small Rct value is that FeF3⋅0.33H2O nano particles in the CFF that are directly attached to the surface of the CNTs. However, the large particles agglomerated in CFF exhibit poor perfor mance during high-rate charge/discharge and long cycle test. The semicircular diameter for GFF plot is almost twice of GCFF, indicating that reaction is difficult in GFF electrodes. It shows that the graphene coating has a limited effect on improving the conductivity of large FeF3⋅0.33H2O particles. The prepared three-dimensional conductive network of GCFF facili tates the rapid transmission of ions and electrons, so that the capacity of the electrode can be fully utilized even at a relatively high mass loading. Fig. 6 shows the cycling ability of as-prepared composites at different mass loadings (1 mg cm 2, 2 mg cm 2, and 5 mg cm 2). The initial spe cific capacity of GCFF composite at 1C rate are 162 mAh g 1, 154 mAh g 1,148 mAh g 1 at mass loading of 1 mg cm 2, 2 mg cm 2, and 5 mg cm 2, respectively (Fig. 6a). The retention of specific capacity is 92% when the areal mass loading rising from 1 mg cm 2 to 5 mg cm 2 (Fig. 6b). And the capacity retention rate after 100 cycles charge/ discharge didn’t decrease due to the increase in mass loading. Moreover, the GCFF composite can release a capacity of 100 mAh g 1, 91 mAh g 1, and 84 mAh g 1 at the mass loading of 1 mg cm 2, 2 mg cm 2, and 5 mg cm 2. The retention of specific capacity is 84% at 5C high rate (Fig. S5). As a comparison, the capacity retention of CFF electrodes is only 63%, the specific capacity drops to 87 mAh g 1 when mass loading rise to 5 mg cm 2 (Fig. 6c and d). For GFF composite, a high specific capacity of 146 mAh g 1 can be obtained at 1 mg cm 2 at 1C, but only half of the value can be reserved at mass loading of 5 mg cm 2 (Fig. 6e and f). The discharge capacity of GFF and CFF electrodes drop drasti cally when the mass loading increases. But the highway for electron transfer in the constructed conductive network and the micro-nano structure of the FeF3⋅0.33H2O ensure the high specific capacity of GCFF composite at high mass loading. 5
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Journal of Power Sources 447 (2020) 227303
Fig. 6. Cycle performance and capacity retention of as-prepared composites for different mass loading. (a, b) GCFF electrode, (c, d) CFF electrode, (e, f) GFF electrode.
As 5 mg cm 2 is the mass loading close to the actual application, which is beneficial to evaluate the practical application potential of the material. Therefore, cycle test under 5 mg cm 2 were conducted (Fig. 7). The GCFF composite still has a specific capacity of 51 mAh g 1 even after 100 times charge/discharge at high loading and 5C high rate, but
the specific capacity for CFF and GFF electrode drop to only 18 mAh g 1and 23 mAh g 1(Fig. 7a). In addition, the GCFF electrode possesses a specific capacity of 56 mAh g 1 at an ultra-high 20C rate (Fig. 7b). Packing the FeF3∙0.33H2O nanoparticles and microspheres into the 3D conductive network constructed by PGO and CNTs enable GCFF material
Fig. 7. (a) Cycle performance of as-prepared GCFF, CFF, and GFF at 5C for mass loading of 5 mg cm 6
2
, (b) GCFF at 20C.
Q. Zhang et al.
Journal of Power Sources 447 (2020) 227303
to have excellent performance even under high mass loading and high current density.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227303.
3. Conclusion
References
In summary, a three-dimensional conductive network was success fully constructed by porous graphene and CNTs, and FeF3∙0.33H2O nanoparticles and microspheres were loaded on the surface and between graphene layers by solvothermal method. The one-dimensional CNTs added can incorporate into the graphene layers to prevent the irre versibly stacking caused by strong π-π interaction. Porous graphene and CNTs can improve in-plane and portrait ion/electron conductivity which will benefit for mass migration at high mass loadings. A large amount of FeF3∙0.33H2O nanoparticles and microspheres can increase the volu metric energy density of composite. The GCFF composite exhibits excellent rate capability with discharge capacities of 162 mAh g 1 at 1 C, 132 mAh g 1 at 2 C, 100 mAh g 1 at 5 C, 83 mAh g 1 at 10 C, and 69 mAh g 1 at 20 C; The retention of specific capacity is 92% when the areal mass loading rising from 1 mg cm 2 to 5 mg cm 2 at 1C rate. The excellent performance of GCFF at high loading provide an strategy for the practical application of FeF3∙0.33H2O cathode for LIBs.
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4. Experimental section 4.1. Treatment of GO and CNTs Firstly, 0.2 g GO powder was dispersed in 50 ml of deionized water, then 5 ml H2O2 (30 wt%) was added and the mixture was heated and stirred at 95 � C for 30 min in a water bath. After cooling, the solution was centrifuged and washed 3 times with deionized water. PGO powder can be obtained after freeze-drying. Acid treatment of CNTs was per formed to enhance the hydrophilicity of CNTs. Typically, 1 g of CNTs (20 nm in diameter and 50 μm in length) was dispersed in 40 ml concentrated HNO3 and reflux heating at 80 � C for 1 h. Then the solution was centrifuged and washed with deionized water until neutral. Rich functional groups containing CNTs powder can be obtained after freeze drying. 4.2. Synthesis of GCFF In a typically synthetic process of GCFF, 20 mg treated CNTs and 20 mg PGO were dispersed in 50 ml of ethanol by ultrasonic. Then 0.5 g Fe(NO3)3⋅9H2O was added to the dispersed solution and 10 ml ionic liquid [Bmim][BF4] was slowly added after Fe(NO3)3⋅9H2O complete dissolution. The mixture was thoroughly stirred and transferred to a 100 ml Teflon reactor. The reactor was sealed and transferred to an oven at 120 � C for 12 h. After naturally cooling to room temperature, the reactor was opened and the product was washed with ethanol for at least 3 times. After vacuum dried at 80 � C a black powder can be obtained. As a comparison, CNT/FeF3⋅0.33H2O (CFF) and PGO/FeF3⋅0.33H2O (GFF) composites were also prepared by using 40 mg CNTs or PGO separately as conductive carbon and other conditions remains unchanged. Material characterization and electrochemical test part are described in supporting information. Declaration of competing interest There are no conflicts to declare. Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21646012). The Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 201836). 7