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A three-dimensional interlayer composed of graphene and porous carbon for Long-life, High capacity Lithium-Iron Fluoride Battery
Electrochimica Acta 220 (2016) 75–82
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A three-dimensional interlayer composed of graphene and porous carbon for Long-life, High capacity Lithium-Iron Fluoride Battery Juan Yang, Zhanglin Xu, Hongxu Sun, Xiangyang Zhou* Central South University, China
A R T I C L E I N F O
Article history: Received 18 July 2016 Received in revised form 11 October 2016 Accepted 11 October 2016 Available online 15 October 2016 Keywords: iron fluoride interlayer porous carbon graphene sheets
A B S T R A C T
We design a macroscopic structure composing of porous carbon and graphene sheets, which are coated onto a cellulose paper as an interlayer inserted between electrode and separator. The interlayer mainly acts as a divertor to accommodate the discharge products breaking away from the electrode by mechanical degradation or cathode dissolution during cycling and keeps the close contact with current collector. Iron fluoride is a new-type lithium storage material developed in recent years, which can act as a cathode material candidate for the rechargeable lithium ion battery due to their large theoretical capacity and relatively high operating potential. Specifically, FeF30.33H2O, which possesses unusual tunnel structure, is attracting more and more attentions. However, FeF30.33H2O suffers from the poor electronic conductivity and volume effect during cycling, causing the large capacity fading. In this study, we design a macroscopic structure composing of porous carbon and graphene sheets, which are coated onto a cellulose paper as an interlayer inserted between electrode and separator. The interlayer can not only enhance the electronic conductivity, but also absorb the FeF30.33H2O nanoparticles breaking away from the Al foil due to the volume effect upon cycling. When the interlayer is applied in battery, discharge capacities of 600 and 460 mAh g 1 can be achieved at the rates of 100 and 600 mA g 1 after 60 cycles, respectively. Furthermore, the capacity of 435 mAh g 1 can be still retained at a high rate of 1000 mA g 1 after 250 cycles. The results demonstrate a potential feasibility for the porous carbon/graphene sheets to be applied to obtain a high-performance lithium-iron fluoride battery. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Energy crisis is becoming more and more severe since the growing consumption of fossil fuel in recent years. Although some renewable resources have been found, the utilization of these energy is difficult due to the instability and diffusion in space. Thus, the demand for an energy storage system with a high specific capacity is increasing [1]. Lithium ion battery is a promising candidate which has drawn immense attentions due to the high energy density and conversion efficiency [2–4]. However, the state-of-the-art cathode materials, such as LiCoO3 and LiFePO4, deliver only a actual specific capacity below 160 mAh g 1, which can not meet the demands of electric vehicles and energy storage systems [5–7]. As a conversion-type cathode material, transition metal fluorides can provide a large specific capacity and relatively high operation voltage [8]. Specifically, considerable attentions
* Corresponding author. E-mail address: [email protected] (X. Zhou). http://dx.doi.org/10.1016/j.electacta.2016.10.076 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
have recently been focused on iron fluoride due to the high theoretical specific capacity of 712 mAh g 1 in the range of 1.5– 4.5 V, electromotive force value of 2.7 V and low cost. Among the plentiful polymorphs of iron fluoride, FeF30.33H2O is prominent on account of the unusual tunnel structure, which is favorable for more Li-ion insertion [9]. Despite the theoretically attractive features of iron fluoride, the electronically insulating behavior introduced by the large bandgap affects the utilization of the active iron fluoride material [10]. Consequently, great efforts have been made to overcome the poor electrical conductivity by fabricating FeF3/C nanocomposites [11], which are based on the high electrochemical activity and conductivity of carbon [12–14], for example, the FeF3/graphite nanocomposites prepared by mechanical ball-milling of the asprepared FeF3 nanopowders with graphite [15], FeF3 nanoparticles deposited on carbon nanotube (CNT) surface [16] and FeF3graphene composite [17]. Besides the fabrication of FeF3/C nanocomposites, designing favorable morphologies is another approach to overcome the restrictions [18,19]. The poor electrochemical performance can be improved by fabricating the porous
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FeF3 nanospheres [20], FeF3 nanowires [21], and FeF30.33H2O with microporous morphology [22]. Enhancing the temperature of the battery operation is also an effective pathway to get a remarkable specific capacity, owing to the impact of temperature on the activity of iron fluoride [23]. A comparison of the electrochemical performance about above researches is shown in Table S1 (see the Supporting Information). In spite of some achievements attained by the preparation of FeF3/C nanocomposites or designing favorable morphologies, these modifications can not realize the desired specific capacity and cycling performance for practical applications. Indeed, the previous researches commonly exhibit a below-hundred cycle, and the capacity reveals a rapid degradation. When operated on the appropriate temperature, the battery can exhibit a higher specific capacity. However, the temperature spot can not be kept in the practical application. Another issue, which is not brought to enough attentions, is the large volume change during charge/ discharge, limiting the cycle performance of iron fluoride [24]. Moreover, a new finding that the substantial discharge product (such as LiF and Fe) shall dissolve into electrolyte in initial conversion reaction was recently reported [25]. To further improve the performance of iron fluoride cathode material, the volume expansion and cathode dissolution should be also considered besides enhancing the electronic conductivity of iron fluoride. Based on these issues, we designed a conductive interlayer consisting of porous carbon and graphene sheets (GS), which are coated on the cellulose paper as an interlayer inserted between FeF30.33H2O electrode and electrolyte. The interlayer mainly acts as a divertor to accommodate the discharge products breaking away from the electrode by mechanical degradation or cathode dissolution during cycling and keeps the close contact with current collector to retain the electrochemical activity of these portion of active material, which is expected to improve the cycle performance. With these considerations, the interlayer needs to provide high electronic conductivity, abundant pore volume and reasonable pore distribution. Hence, GS is chosen as conductive component, and porous carbon with rich pore structure acts as carrier. Fig. 1 shows the interlayer structure in battery. The porous carbon/GS (PGS) was fabricated by a hydrothermal treatment combining GO and hydrochars, followed by a heat treatment at 800 C. GS component which is dispersed throughout the interlayer, can greatly enhance the electron conductivity of electrode to improve the utilization of active material. Meanwhile, the porous carbon can not only adsorb the FeF30.33H2O stripping from the electrode due to volume expansion, but also capture the dissolved species, which greatly improves the storage capability
and cycle performance. Furthermore, it does not need to include complicated modification for FeF30.33H2O particles, but to insert a simply constructed porous interlayer, which is firstly applied to the lithium-iron fluoride battery. 2. Experimental 2.1. Preparation of the materials The FeF30.33H2O powder was prepared by a low-temperature method in which FeCl3 was added firstly into the excessive HF solution. Then the mixture was put in an ice-bath with a constant stirring for 24 h at 2 C. After that, the white powders could be obtained. To obtain the FeF30.33H2O powders, the white powders must be heated at 120 C for 24 h in air. When the colour turned to palegreen, the well-dispersed FeF30.33H2O grains could be obtained. Graphite oxide was prepared based on the modified Hummer's method [26]. The detailed steps could be seen in Supporting Information. The PGS was synthesized through a hydrothermal method, followed by a high temperature activation step at 800 C. In a typical synthesis procedure, we dissolved firstly 4.8 g of sucrose into the 77 ml of dispersed GO solution with a concentration of 2.4 mg mL 1, followed by a magnetical stir of4 h at room temperature. Then the aqueous mixture was poured into Tefon-lined stainless steel autoclave and maintained for 12 h at 180 C. The black powders could be obtain after the resulting precipitate was filtered off, washed thoroughly with deionized water and dried at 60 C. Then, it was activated using potassium hydroxide as activating agent at a ratio of 1:5. The mixture of KOH and the black powders was dissolved in a mix solution of ethanol and deionized water, which was stirred for 10 min before being dried at 120 C. The dried mixture was heated at a heating rate of 5 C min 1 and maintained at 800 C for 2 h in a muffle furnace filled with argon. The PGS could be obtained after washing the resulting product with diluted hydrochloric acid and deionized water. 2.2. Characterization The morphology and structure of the prepared samples were characterized using scanning electron microscope (SEM, Nova NanoSEM230) and transmission electron microscopy (TEM, JEOL JEM-2100F). The structure and composition of the samples were performed by X-ray diffraction (XRD, Rigaku-TTRIII) with Cu Ka radiation at a scanning rate of 8 min 1, and the Raman spectra was obtained using a LabRAM HR800 from HORIBA JOBIN YVON. The
Fig. 1. a) Schematic of a Li–FeF30.33H2O battery with PGS interlayer. b) The Schematic of four compositions in the battery.
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element analyses of the cycled electrodes were recorded using the X-ray photoelectron spectra (XPS, K-Alpha 1063) with a monochromatic Al Ka X-ray source. The elemental mapping analysis was obtained by an energy dispersive spectrometer (EDS). The specifc surface area was evaluated using the Brunauer–Emmett–Teller (BET). The micropore and mesoporous distribution were analyzed on the basis of Barrett–Joyner–Halenda (BJH) desorption pore and Horvath-Kawazoe (HK) respectively. 2.3. Electrochemical characterization For electrochemical characterization, the electrode was prepared by pasting a homogeneous slurry mixture of the assynthesized FeF30.33H2O powders, acetylene back and polyvinylidene fuoride (PVDF) binder at a weight ratio of 7:2:1 on the alumina foil with a blade. Then the coated alumina foil was dried overnight at 120 C in a vacuum chamber before assembling the test cell. The mass of active materials (FeF30.33H2O) loading each piece was 1.0 mg cm 2. The PGS and PVDF were coated onto the cellulose paper (Fosai New Material, TF4530) at a weight of ratio of 5:1, which was desiccated at 50 C for 12 h. The cellulose paper coated with the PGS and PVDF was tailored to a disk with a diameter of 2 cm as the PGS interlayer, which was the same as the size of separator. The weight of the blank cellulose paper was 4.2 mg, and the mass of PGS and PVDF was 2.2 mg on each blank cellulose paper. The test cells were assembled referring to Fig. 1a using CR2032 coin cells, polypropylene micromembranes as the separator, and nonaqueous solution of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1) in an Arflled universal glove box. Alternative coin cell with the blank cellulose paper as interlayer was assembled for comparison. Besides, another comparison cell (named ACP cell) employing the PGS and PVDF deposited on the cellulose paper with a weight ratio
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5:1 as cathode, which stays the same size and mass as the PGS interlayer, was prepared to identify the capacity contribution of PGS. Galvanostatic discharging/charging experiments were performed in the potential range of 1.5–4.5 V on a LAND CT-2001A. The specific capacity was calculated based on the mass of FeF30.33H2O active materials. Cyclic voltammetry (CV) experiment and electrochemical impedance spectroscopy (EIS) were carried on an electrochemical workstation (P4000, PARSTAT MC), The scanning speed of CV experiment was 0.2 mV s 1 in a voltage window of 1.5–4.5 V. The EIS data were obtained using an impedance analyser (Versa Studio) in the 100 kHz to 10 mHz frequency range in automatic sweep mode from high to low frequency. 3. Results and discussion To fully play the role of the PGS interlayer, it is extremely crucial to fabricate FeF30.33H2O particles of rational morphology and microstructure. The morphology of the prepared FeF30.33H2O powders is characterized by SEM. As shown in Fig. 2a, the FeF30.33H2O powder synthesized via a low-temperature method shows an average size of 300 nm. The nanoparticles can effectively minish the electronic transmission distance contributing to increasing the electron conductivity. Fig. 2b presents the SEM image of PGS, which exhibits the representative morphology of sucrose hydrochars. On the surface, we can observe some traces caused by potassium hydroxide at 800 C. As for the GS, we speculate it is mingled between the carbonaceous layers of hydrochars during hydrothermal treatment. The TEM image of the PGS sample as shown in Fig. 2c demonstrates the formation of graphene sheets with a highly folded morphology, which is typical for hydrothermally treated graphene-based sample. The Fig. 2d shows the high-resolution TEM (HRTEM) image, further indicating
Fig. 2. SEM images for (a) FeF30.33H2O powder and (b) PGS; (c) and (d) TEM images of PGS.
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the multi-sheet graphene, which contains approximately 3-5 layers graphene. Therefore, based on the results of SEM and TEM, it can be concluded that the PGS interlayer holds a 3-D structure stacked by GS and porous carbon. Moreover, the crystal structure of the fabricated powders and PGS is identified by XRD. The featured peaks of the fabricated powders before and after heat treatment are identified as those of FeF33H2O (Fig. S1) and FeF30.33H2O (Fig. 3a). The XRD patterns in Fig. 3b reveal two low-intensity and broad peaks centered at 24 and 43 respectively, which indicate the amorphous structure of the PGS similar to other carbon materials prepared via hydrothermal treatment as previously reported [27].
In order to investigate the porous structure of PGS, nitrogen adsorption/desorption measurements are carried out. As shown in Fig. 3c, the PGS exhibits a typical type IV isotherm. The steep initial region before relative P/P0 = 0.1 demonstrates the existence of micropores [28]. Meanwhile, the type II hysteresis loop (IUPAC classi-fcation) existing in a wide relative pressure (P/P0) range of 0.4 to 0.8 indicates the presence of abundant mesopores. The data illuminate that the BET specific surface area of PGS is 1948.8 m2 g 1, demonstrating that the porous structure comes into being due to the activation of KOH. Fig. 3d displays the micropore distribution of PGS on the basis of HK, which shows a sharp peak at around 0.65 nm. Additionally, the mesopore distribution (Fig. 3d) based of
Fig. 3. XRD patterns of FeF30.33H2O (a) and PGS (b); (c) Nitrogen adsorption/desorption isotherm; (d) Size distribution of PGS; Raman spectra of PGS before (e) and after (f) activating.
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BJH desorption pore exhibits an average pore diameter of 3.04 nm. The results indicate the majority of pores including micropores and mesopores are distributed within 0.5–5 nm, which are capable of absorbing the dissolved species (such as Fe). The characteristic D and G bands before and after the activation step are revealed in the Raman spectra (Fig. 3e and f). The distinct enhancement in the intensity of D band after the activation at 800 C demonstrates an increase in number of defects on PGS surface. Additionally, the intensityratios of D to G bands in PGS before and after activation are 1.75 and 1.04 respectively, further demonstrating the fact that the porous structure exists in PGS. Fig. 4a presents the typical discharge/charge profiles based on the coin cell employing the PGS interlayer. Somewhat similar to some other conversion materials, the first charge and discharge voltage profiles exhibit significant voltage hysteresis, which is the result of ohmic voltage drop, reaction overpotential and different spatial distributions of electrochemically active phases [29]. Moreover, a slant voltage plateau related to a Li-ion insertion reaction appears above 2 V according to the discharge curve. Another flat voltage plateau appearing at 1.5 V arises from the conversion reaction coupling with the formation of LiF and Fe0.33H2O. [30] However, the plateau gradually vanishes during cycling, indicating a portion of LiF and Fe0.33H2O may not return to FeF30.33H2O. An initial discharge capacity of 988.6 mAh g 1 is achieved at a current density of 100 mAh g 1 with a coulombic
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efficiency of 78.9%, which is beyond the theoretical capacity of FeF30.33H2O. The cycle performance of the cell with the PGS interlayer contrasting with that of the cell carrying the blank cellulose paper is displayed in Fig. 4b. The cell with the blank cellulose paper displays a initial discharge capacity of 145 mAh g 1, while it can reach up to 290 mAh g 1 after the activation of electrode material. Nevertheless, the capacity of 135 mAh g 1 can be remained after 60 cycles. As a contrast, the cell employing PGS interlayer delivers ultrahigh discharge and charge capacities of 988.6 and 1252.4 mAh g 1 respectively at a current density of 100 mAh g 1 in the first cycle. After 60th cycle, it can retain a large reversible capacity of 601 mAh g 1, and the coulombic efficiency stays above 95%. Besides, the cell with PGS interlayer also exhibits superior rate performance (Fig. 4c). The discharge capacity of 515 mAh g 1, 462 mAh g 1, 430 mAh g 1, and 375 mAh g 1, can be achieved when the current density varies from 300 mAh g 1, 600 mAh g 1, 1000 mAh g 1, 2000 mAh g 1, respectively. As the current density returns to 600 mAh g 1 and 100 mAh g 1, the corresponding discharge capacity of 460 and 590 can be retained. Moreover, the capacity of 435 mAh g 1 can be still sustained at a current density of 1000 mAh g 1 after 250 cycles (Fig. 4d), indicating the superior lithium storage capability at higher current rates. The better cyclability at higher rate can be attributed to the reason that Li-ion could slowly penetrate into FeF30.33H2O particles at a
Fig. 4. The results of charge-discharge tests for the cell with PGS interlayer. (a) Discharge-charge voltage profiles at a current density of 100 mAh g 1, (b) Cycling performance at a current density of 100 mAh g 1, (c) Rate performance at varying rates from 100 mA g 1 to 2 A g 1, (d) Cycling performance at a current density of 1000 mAh g 1 of the cell with PGS interlayer.
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lower current density, which permits more FeF30.33H2O to react with Li-ion during discharge and more serious degradation of recharge capabilities. In turn, the capacity retention is relatively low. However, at a higher rate, the FeF30.33H2O reacting with Liion decreases due to a shorter penetration time and transport path of the solvated electrolyte and lithium ions, thus leading to the better cyclability [31]. The results give the further evidence for the excellent power performance of the cell using PGS interlayer. Evidently, the excellent electrochemical performance is mainly ascribed to the PGS interlayer, as demonstrated above. Cyclic voltammetry (CV) is performed to investigate electrochemical reaction characteristics at a scanning rate of 0.2 mV s 1 in the potential window of 1.5 V to 4.5 V. Fig. S2a reveals the CV profiles of the cell with the blank cellulose paper. Two obvious reduction peaks are observed, one of which is reflected between 2 V and 3.5 V, corresponding to the Li+-insertion into FeF30.33H2O electrode without structural breakdown. The other one, a sharp peak, can be observed at 1.5 V, corresponding to the stage of conversion reaction. However, in reversed scan, there is only one oxidation peak at 3.2 V, implying that the larger polarization appears in conversion reaction, which is consistent with that in other reported FeF30.33H2O. The CV curves of the cell with the PGS interlayer are shown in Fig. S2b. Comparing with the profiles in Fig. S2a, there is no distinct change for the locations of cathodic peaks, but the width of the peaks increases, which probably accounts for excess Li+ inserting into the porous structure of PGS interlayer [32]. Thus, it seems that the PGS contained in interlayer also participates in the electrochemical reaction. To clarify the effects of PGS, the ACP cell employing the PGS interlayer as active
material is assembled to carry out CV experiment. As the CV curve shown in Fig. S2c, a wide reduction peak appears in the range of 1.5–4.5 V, illustrating that the behavior of Li+-insertion into the PGS exists in the voltage range. Accordingly, the PGS interlayer can contribute some capacity to the cell. To make certain how much capacity that PGS donates, the ACP cell is tested by galvanostatic discharging/charging technique. As shown in Fig. S3a and b, the specific capacity contributed by the PGS interlayer is calculated to be approximately 135 mAh g 1 after 60 cycles, and the discharge/charge profiles exhibit an oblique line corresponding to the Li+-insertion into the PGS. The real specific capacity of FeF30.33H2O is calculated to be about 300 mAh g 1 after deducting the effect of PSG interlayer. Therefore, the PGS interlayer can not only improve the cycle performance of FeF30.33H2O cathode, but also donate a portion of capacity. The electrochemical impedance spectroscopy (EIS) of the cell with PGS interlayer is carried out before cycling and after 60 cycles at a current density of 100 mAh g 1 to illustrate the superior electrochemical performance. The high–frequency semicircle and inclined line from each Nyquist plot in Fig. S3c can be observed. The depressed semicircle is attributed to the overlap between the SEI (solid electrolyte interphase) film and the interfacial charge transfer impedance, and the inclined line represents the lithium diffusion impedance. After 60 cycles, the semicircle of the cell exhibits a smaller diameter compared with that before cycling, suggesting that the charge transfer impedance minishes upon cycling. Additionally, the steeper inclined line manifests increase of lithium ion conductivity after the charge and discharge cycling. The relevant equivalent circuit model are shown in Fig. S3d. The results
Fig. 5. SEM images of FeF30.33H2O electrode before (a) and after (b) cycling; SEM images of PGS interlayer section before (c) and after (d) cycling; elemental maps of Fe (e) and F (f) corresponding to the area marked by rectangle part in Fig. 5d.
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indicate the reversible reaction mainly takes place in the electrode during initial cycles, and the transfer of electron and ion suffers from the large resistance. After 60 cycles, a certain portion of FeF30.33H2O and discharge products have moved into the PGS interlayer. The synergistic effect of the porous carbon and GS with high conductivity improves the electron and ion conductivity of active material, which plays a significant role for the superior electrochemical performance. The change of electrode, PGS interlayer and Li electrode before and after cycling is characterized by scanning electron microscopy to investigate the function of PGS interlayer. Fig. 5a and b exhibit the SEM images of electrode before and after 60 cycles, respectively. It can be observed that a certain amount of FeF30.33H2O particles have dissolve away leaving many holes marked by the yellow rectangle in Fig. 5b after 60 cycles. In addition, the images of PGS interlayer before and after 60 cycles are shown in Fig. 5c and d. It can be confirmed that the PGS interlayer shows no distinct change after cycling, demonstrating the stability of PGS interlayer during cycling. EDS elemental maps (Fig. 5e and f) are carried out to analyze the elements of the cycled PGS interlayer (Fig. 5d). The maps analysis of interlayer section indicates that the Fe and F are distributed in the PGS interlayer, which demonstrates that a certain portion of unbonded FeF30.33H2O and discharge products caused by the volume expansion or chemical dissolution during charge/discharge enter into the porous structure of PGS
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interlayer. Hence, the PGS interlayer (Fig. S4) can not only increase the electron conductivity, but also act as an divertor trapping the dissolved discharge product and FeF30.33H2O particles breaking away from aluminum foil, which can improve the utilization of FeF30.33H2O. Moreover, EDS analysis (Fig. S5a and b) of the cycled Li electrode surface confirms the presence of a trace of Fe in the cell with blank cellulose paper, supporting the viewpoint on cathode dissolution. In contrast, the Li electrode of the cell with PGS interlayer does not contain detectable amount of Fe after 60 cycles, indicating the function of PGS interlayer for preventing cathode dissolution. The XPS is applied to analyze the elements of cycled PGS interlayer. The XPS survey spectrum (Fig. 6a) demonstrates the presence of Fe, C, F, and O in the porous structure of PGS, with most F and Fe in the form of discharge products [33,34]. The C 1 s spectra (Fig. 6b) reveals three major peaks, which are assigned to C-C bonds (284 eV), C-O bonds (286.7 eV), and the C-F bonds (290.9 eV) attributed to the PVDF binder. The F1 s spectra (Fig. 6c) exhibits LiF peak at 684.6 eV, indicating the some LiF particles formed during cycling are absorbed into the pores of PGS. Another major peak C-F bonds located at 687.4 eV is resolved into the component of PVDF binder. From the Fe2p spectra shown in Fig. 6d, two peaks at around 711 and 725 eV are corresponded to Fe2p3/2 and Fe2p1/2, suggesting the existence of Fe+3 in PGS interlayer [35]. The results correspond highly to that of EDS, further demonstrating that the
Fig. 6. XPS survey spectrum (a) of PGS interlayer after 60 cycles, XPS (b) C1s, (c) F1s, and (d) Fe2p spectra of the PGS interlayer.
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application of PGS interlayer improves the utilization of cathode material and stabilizes the structure of electrode. 4. Conclusions In summary, we develop a rational architecture that the composite of porous carbon and GS is coated onto a cellulose paper to form a conductive PGS interlayer. The PGS can be prepared by a simple hydrothermal treatment, followed an activation process, in which the GS can be mingled with porous carbon. The porous carbon in PGS interlayer can accommodate the dissolved species and FeF30.33H2O particles peeling off the Al foil caused by the volume expansion and cathode dissolution during the charge and discharge, which effectively increases the utilization of cathode material. Additionally, the GS can improve the poor electron conductivity of FeF30.33H2O, which is crucial for the electrochemical performance. When the PGS interlayer is applied in coin cell, the discharge capacities of 600, 460 and 370 mAh g 1 can be achieved at the rates of 100, 600 and 2000 mAh g 1 after 60 cycles respectively in the potential range of 1.5–4.5 V. Moreover, at a high rate of 1000 mAh g 1, it can retain a discharge capacity of 435 mAh g 1 after 250 cycles. The results further demonstrate the application of PGS interlayer can achieves an ultrahigh capacity, superior rate and cyclic performance. In addition, the proposed design of interlayer can be extended to other metal fluorides, which opens a new avenue for fabricating high performance Li-metal fluorides battery. Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant no. 51204209 and 51274240) and the Project of Innovation-driven Plan in Central South University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.10.076. References [1] X.P. Gao, H.X. Yang, Multi-electron reaction materials for high energy density batteries, Energy Environ. Sci. 3 (2010) 174. [2] Chao Chen, Xiaoping Xu, Shu Chen, Bin Zheng, Miao Shui, Lingxia Xu, Weidong Zheng, Jie Shu, Liangliang Cheng, Lin Feng, Yuanlong Ren, The preparation and characterization of iron fluorides polymorphs FeF30.33H2O and b-FeF33H2O as cathode materials for lithium-ion batteries, Materials Research Bulletin 64 (2015) 187. [3] K. Kang, Y.S. Meng, J. Bréger, C.P. Grey, G. Ceder, Electrodes with high power and high capacity for rechargeable lithium batteries, Science 311 (2006) 977. [4] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Highperformance lithium battery anodes using silicon nanowires, Nat. Nanotechnol. 3 (2008) 31. [5] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243. [6] F. Badway, N. Pereira, F. Cosandey, G.G. Amatucci, J. Electrochem. Soc. 150 (2003) A1209. [7] Feng Wang, Sung Wook Kim, Dong Hwa Seo, Kisuk Kang, Liping Wang, Dong Su, John J. Vajo, John Wang, Jason Graetz, Ternary metal fluorides as highenergy athodes, 2014 cathodes with low cycling hysteresis, Nature Communications 10 (2014) 7668. [8] X.P. Gao, H.X. Yang, Multi-electron reaction materials for high energy density batteries, Energy Environ. Sci. 3 (2010) 174. [9] Chilin Li, Lin Gu, Susumu Tsukimoto, Peter A. van Aken, Joachim Maier, LowTemperature Ionic-Liquid-Based Synthesis of Nanostructured Iron-Based Fluoride Cathodes for Lithium Batteries, J. Adv. Mater. 22 (2010) 36509. [10] R.E. Doe, K.A. Persson, Y.S. Meng, G. Ceder, First-principles investigation of the Li-Fe-F phase diagram and equilibrium and nonequilibrium conversion reactions of iron fluorides with lithium, Chem. Mater. 20 (2008) 5274.
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[24] W. Dreyer, J. Jamnik, C. Guhlke, R. Huth, J. Moskon, M. Gaberscek, The thermodynamic origin of hysteresis in insertion batteries, Nat. Mater. 9 (2010) 448. [25] Wentian Gu, Oleg Borodin, Bogdan Zdyrko, Huan-Ting Lin, Hyea Kim, Naoki Nitta, Jiaxin Huang, Alexandre Magasinski, Zoran Milicev, Gene Berdichevsky, Gleb Yushin, Lithium–Iron Fluoride Battery with In Situ Surface Protection, Adv. Funct. Mater 26 (2016) 150. [26] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [27] Nian Wu Li, Ya Xia Yin, Yu Guo Guo, Three-dimensional sandwich-type graphene@ microporous carbon architecture for lithium–sulfur batteries, RSC Adv. 6 (2016) 618. [28] A. Jahel, C.M. Ghimbeu, L. Moncondit, C. Vix-Guterl, Confined Ultrasmall SnO2 Particles in Micro/Mesoporous Carbon as an Extremely Long Cycle-Life Anode Material for Li-Ion Batteries, Adv. Energy Mater. 4 (2014) 1400025. [29] Linsen Li, Ryan Jacobs, Peng Gao, Liyang Gan, Feng Wang, Dane Morgan, Song Jin, Origins of Large Voltage Hysteresis in High-Energy-Density Metal Fluoride Lithium-Ion Battery Conversion Electrodes, J. Am. Chem. Soc. 138 (2016) 2839. [30] Li Liu, Haipeng Guo, Meng Zhou, Qiliang Wei, Zhenhua Yang, Hongbo Shu, Xiukang Yang, Jinli Tan, Zichao Yan, Xianyou Wang, A comparison among FeF33H2O, FeF30.33H2O and FeF3 cathode materials for lithium ion batteries: Structural, electrochemical, and mechanism studies, Journal of Power Sources 238 (2013) 505. [31] Guorong Hu, Manfang Zhang, Lili Wu, Zhongdong Peng, Ke Du, Yanbing Cao, Enhanced Electrochemical Performance of LiNi0.5Co0.2Mn0.3O2 Cathodes Produced via Nanoscale Coating of Li+-Conductive Li2SnO3, Electrochimica Acta 213 (2016) 547–556. [32] Yong Sheng Sheng, Philipp Adelhelm, Bernd M. 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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A three-dimensional interlayer composed of graphene and porous carbon for Long-life, High capacity Lithium-Iron Fluoride Battery Juan Yang, Zhanglin Xu, Hongxu Sun, Xiangyang Zhou* Central South University, China
A R T I C L E I N F O
Article history: Received 18 July 2016 Received in revised form 11 October 2016 Accepted 11 October 2016 Available online 15 October 2016 Keywords: iron fluoride interlayer porous carbon graphene sheets
A B S T R A C T
We design a macroscopic structure composing of porous carbon and graphene sheets, which are coated onto a cellulose paper as an interlayer inserted between electrode and separator. The interlayer mainly acts as a divertor to accommodate the discharge products breaking away from the electrode by mechanical degradation or cathode dissolution during cycling and keeps the close contact with current collector. Iron fluoride is a new-type lithium storage material developed in recent years, which can act as a cathode material candidate for the rechargeable lithium ion battery due to their large theoretical capacity and relatively high operating potential. Specifically, FeF30.33H2O, which possesses unusual tunnel structure, is attracting more and more attentions. However, FeF30.33H2O suffers from the poor electronic conductivity and volume effect during cycling, causing the large capacity fading. In this study, we design a macroscopic structure composing of porous carbon and graphene sheets, which are coated onto a cellulose paper as an interlayer inserted between electrode and separator. The interlayer can not only enhance the electronic conductivity, but also absorb the FeF30.33H2O nanoparticles breaking away from the Al foil due to the volume effect upon cycling. When the interlayer is applied in battery, discharge capacities of 600 and 460 mAh g 1 can be achieved at the rates of 100 and 600 mA g 1 after 60 cycles, respectively. Furthermore, the capacity of 435 mAh g 1 can be still retained at a high rate of 1000 mA g 1 after 250 cycles. The results demonstrate a potential feasibility for the porous carbon/graphene sheets to be applied to obtain a high-performance lithium-iron fluoride battery. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Energy crisis is becoming more and more severe since the growing consumption of fossil fuel in recent years. Although some renewable resources have been found, the utilization of these energy is difficult due to the instability and diffusion in space. Thus, the demand for an energy storage system with a high specific capacity is increasing [1]. Lithium ion battery is a promising candidate which has drawn immense attentions due to the high energy density and conversion efficiency [2–4]. However, the state-of-the-art cathode materials, such as LiCoO3 and LiFePO4, deliver only a actual specific capacity below 160 mAh g 1, which can not meet the demands of electric vehicles and energy storage systems [5–7]. As a conversion-type cathode material, transition metal fluorides can provide a large specific capacity and relatively high operation voltage [8]. Specifically, considerable attentions
* Corresponding author. E-mail address: [email protected] (X. Zhou). http://dx.doi.org/10.1016/j.electacta.2016.10.076 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
have recently been focused on iron fluoride due to the high theoretical specific capacity of 712 mAh g 1 in the range of 1.5– 4.5 V, electromotive force value of 2.7 V and low cost. Among the plentiful polymorphs of iron fluoride, FeF30.33H2O is prominent on account of the unusual tunnel structure, which is favorable for more Li-ion insertion [9]. Despite the theoretically attractive features of iron fluoride, the electronically insulating behavior introduced by the large bandgap affects the utilization of the active iron fluoride material [10]. Consequently, great efforts have been made to overcome the poor electrical conductivity by fabricating FeF3/C nanocomposites [11], which are based on the high electrochemical activity and conductivity of carbon [12–14], for example, the FeF3/graphite nanocomposites prepared by mechanical ball-milling of the asprepared FeF3 nanopowders with graphite [15], FeF3 nanoparticles deposited on carbon nanotube (CNT) surface [16] and FeF3graphene composite [17]. Besides the fabrication of FeF3/C nanocomposites, designing favorable morphologies is another approach to overcome the restrictions [18,19]. The poor electrochemical performance can be improved by fabricating the porous
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FeF3 nanospheres [20], FeF3 nanowires [21], and FeF30.33H2O with microporous morphology [22]. Enhancing the temperature of the battery operation is also an effective pathway to get a remarkable specific capacity, owing to the impact of temperature on the activity of iron fluoride [23]. A comparison of the electrochemical performance about above researches is shown in Table S1 (see the Supporting Information). In spite of some achievements attained by the preparation of FeF3/C nanocomposites or designing favorable morphologies, these modifications can not realize the desired specific capacity and cycling performance for practical applications. Indeed, the previous researches commonly exhibit a below-hundred cycle, and the capacity reveals a rapid degradation. When operated on the appropriate temperature, the battery can exhibit a higher specific capacity. However, the temperature spot can not be kept in the practical application. Another issue, which is not brought to enough attentions, is the large volume change during charge/ discharge, limiting the cycle performance of iron fluoride [24]. Moreover, a new finding that the substantial discharge product (such as LiF and Fe) shall dissolve into electrolyte in initial conversion reaction was recently reported [25]. To further improve the performance of iron fluoride cathode material, the volume expansion and cathode dissolution should be also considered besides enhancing the electronic conductivity of iron fluoride. Based on these issues, we designed a conductive interlayer consisting of porous carbon and graphene sheets (GS), which are coated on the cellulose paper as an interlayer inserted between FeF30.33H2O electrode and electrolyte. The interlayer mainly acts as a divertor to accommodate the discharge products breaking away from the electrode by mechanical degradation or cathode dissolution during cycling and keeps the close contact with current collector to retain the electrochemical activity of these portion of active material, which is expected to improve the cycle performance. With these considerations, the interlayer needs to provide high electronic conductivity, abundant pore volume and reasonable pore distribution. Hence, GS is chosen as conductive component, and porous carbon with rich pore structure acts as carrier. Fig. 1 shows the interlayer structure in battery. The porous carbon/GS (PGS) was fabricated by a hydrothermal treatment combining GO and hydrochars, followed by a heat treatment at 800 C. GS component which is dispersed throughout the interlayer, can greatly enhance the electron conductivity of electrode to improve the utilization of active material. Meanwhile, the porous carbon can not only adsorb the FeF30.33H2O stripping from the electrode due to volume expansion, but also capture the dissolved species, which greatly improves the storage capability
and cycle performance. Furthermore, it does not need to include complicated modification for FeF30.33H2O particles, but to insert a simply constructed porous interlayer, which is firstly applied to the lithium-iron fluoride battery. 2. Experimental 2.1. Preparation of the materials The FeF30.33H2O powder was prepared by a low-temperature method in which FeCl3 was added firstly into the excessive HF solution. Then the mixture was put in an ice-bath with a constant stirring for 24 h at 2 C. After that, the white powders could be obtained. To obtain the FeF30.33H2O powders, the white powders must be heated at 120 C for 24 h in air. When the colour turned to palegreen, the well-dispersed FeF30.33H2O grains could be obtained. Graphite oxide was prepared based on the modified Hummer's method [26]. The detailed steps could be seen in Supporting Information. The PGS was synthesized through a hydrothermal method, followed by a high temperature activation step at 800 C. In a typical synthesis procedure, we dissolved firstly 4.8 g of sucrose into the 77 ml of dispersed GO solution with a concentration of 2.4 mg mL 1, followed by a magnetical stir of4 h at room temperature. Then the aqueous mixture was poured into Tefon-lined stainless steel autoclave and maintained for 12 h at 180 C. The black powders could be obtain after the resulting precipitate was filtered off, washed thoroughly with deionized water and dried at 60 C. Then, it was activated using potassium hydroxide as activating agent at a ratio of 1:5. The mixture of KOH and the black powders was dissolved in a mix solution of ethanol and deionized water, which was stirred for 10 min before being dried at 120 C. The dried mixture was heated at a heating rate of 5 C min 1 and maintained at 800 C for 2 h in a muffle furnace filled with argon. The PGS could be obtained after washing the resulting product with diluted hydrochloric acid and deionized water. 2.2. Characterization The morphology and structure of the prepared samples were characterized using scanning electron microscope (SEM, Nova NanoSEM230) and transmission electron microscopy (TEM, JEOL JEM-2100F). The structure and composition of the samples were performed by X-ray diffraction (XRD, Rigaku-TTRIII) with Cu Ka radiation at a scanning rate of 8 min 1, and the Raman spectra was obtained using a LabRAM HR800 from HORIBA JOBIN YVON. The
Fig. 1. a) Schematic of a Li–FeF30.33H2O battery with PGS interlayer. b) The Schematic of four compositions in the battery.
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element analyses of the cycled electrodes were recorded using the X-ray photoelectron spectra (XPS, K-Alpha 1063) with a monochromatic Al Ka X-ray source. The elemental mapping analysis was obtained by an energy dispersive spectrometer (EDS). The specifc surface area was evaluated using the Brunauer–Emmett–Teller (BET). The micropore and mesoporous distribution were analyzed on the basis of Barrett–Joyner–Halenda (BJH) desorption pore and Horvath-Kawazoe (HK) respectively. 2.3. Electrochemical characterization For electrochemical characterization, the electrode was prepared by pasting a homogeneous slurry mixture of the assynthesized FeF30.33H2O powders, acetylene back and polyvinylidene fuoride (PVDF) binder at a weight ratio of 7:2:1 on the alumina foil with a blade. Then the coated alumina foil was dried overnight at 120 C in a vacuum chamber before assembling the test cell. The mass of active materials (FeF30.33H2O) loading each piece was 1.0 mg cm 2. The PGS and PVDF were coated onto the cellulose paper (Fosai New Material, TF4530) at a weight of ratio of 5:1, which was desiccated at 50 C for 12 h. The cellulose paper coated with the PGS and PVDF was tailored to a disk with a diameter of 2 cm as the PGS interlayer, which was the same as the size of separator. The weight of the blank cellulose paper was 4.2 mg, and the mass of PGS and PVDF was 2.2 mg on each blank cellulose paper. The test cells were assembled referring to Fig. 1a using CR2032 coin cells, polypropylene micromembranes as the separator, and nonaqueous solution of 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1) in an Arflled universal glove box. Alternative coin cell with the blank cellulose paper as interlayer was assembled for comparison. Besides, another comparison cell (named ACP cell) employing the PGS and PVDF deposited on the cellulose paper with a weight ratio
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5:1 as cathode, which stays the same size and mass as the PGS interlayer, was prepared to identify the capacity contribution of PGS. Galvanostatic discharging/charging experiments were performed in the potential range of 1.5–4.5 V on a LAND CT-2001A. The specific capacity was calculated based on the mass of FeF30.33H2O active materials. Cyclic voltammetry (CV) experiment and electrochemical impedance spectroscopy (EIS) were carried on an electrochemical workstation (P4000, PARSTAT MC), The scanning speed of CV experiment was 0.2 mV s 1 in a voltage window of 1.5–4.5 V. The EIS data were obtained using an impedance analyser (Versa Studio) in the 100 kHz to 10 mHz frequency range in automatic sweep mode from high to low frequency. 3. Results and discussion To fully play the role of the PGS interlayer, it is extremely crucial to fabricate FeF30.33H2O particles of rational morphology and microstructure. The morphology of the prepared FeF30.33H2O powders is characterized by SEM. As shown in Fig. 2a, the FeF30.33H2O powder synthesized via a low-temperature method shows an average size of 300 nm. The nanoparticles can effectively minish the electronic transmission distance contributing to increasing the electron conductivity. Fig. 2b presents the SEM image of PGS, which exhibits the representative morphology of sucrose hydrochars. On the surface, we can observe some traces caused by potassium hydroxide at 800 C. As for the GS, we speculate it is mingled between the carbonaceous layers of hydrochars during hydrothermal treatment. The TEM image of the PGS sample as shown in Fig. 2c demonstrates the formation of graphene sheets with a highly folded morphology, which is typical for hydrothermally treated graphene-based sample. The Fig. 2d shows the high-resolution TEM (HRTEM) image, further indicating
Fig. 2. SEM images for (a) FeF30.33H2O powder and (b) PGS; (c) and (d) TEM images of PGS.
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the multi-sheet graphene, which contains approximately 3-5 layers graphene. Therefore, based on the results of SEM and TEM, it can be concluded that the PGS interlayer holds a 3-D structure stacked by GS and porous carbon. Moreover, the crystal structure of the fabricated powders and PGS is identified by XRD. The featured peaks of the fabricated powders before and after heat treatment are identified as those of FeF33H2O (Fig. S1) and FeF30.33H2O (Fig. 3a). The XRD patterns in Fig. 3b reveal two low-intensity and broad peaks centered at 24 and 43 respectively, which indicate the amorphous structure of the PGS similar to other carbon materials prepared via hydrothermal treatment as previously reported [27].
In order to investigate the porous structure of PGS, nitrogen adsorption/desorption measurements are carried out. As shown in Fig. 3c, the PGS exhibits a typical type IV isotherm. The steep initial region before relative P/P0 = 0.1 demonstrates the existence of micropores [28]. Meanwhile, the type II hysteresis loop (IUPAC classi-fcation) existing in a wide relative pressure (P/P0) range of 0.4 to 0.8 indicates the presence of abundant mesopores. The data illuminate that the BET specific surface area of PGS is 1948.8 m2 g 1, demonstrating that the porous structure comes into being due to the activation of KOH. Fig. 3d displays the micropore distribution of PGS on the basis of HK, which shows a sharp peak at around 0.65 nm. Additionally, the mesopore distribution (Fig. 3d) based of
Fig. 3. XRD patterns of FeF30.33H2O (a) and PGS (b); (c) Nitrogen adsorption/desorption isotherm; (d) Size distribution of PGS; Raman spectra of PGS before (e) and after (f) activating.
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BJH desorption pore exhibits an average pore diameter of 3.04 nm. The results indicate the majority of pores including micropores and mesopores are distributed within 0.5–5 nm, which are capable of absorbing the dissolved species (such as Fe). The characteristic D and G bands before and after the activation step are revealed in the Raman spectra (Fig. 3e and f). The distinct enhancement in the intensity of D band after the activation at 800 C demonstrates an increase in number of defects on PGS surface. Additionally, the intensityratios of D to G bands in PGS before and after activation are 1.75 and 1.04 respectively, further demonstrating the fact that the porous structure exists in PGS. Fig. 4a presents the typical discharge/charge profiles based on the coin cell employing the PGS interlayer. Somewhat similar to some other conversion materials, the first charge and discharge voltage profiles exhibit significant voltage hysteresis, which is the result of ohmic voltage drop, reaction overpotential and different spatial distributions of electrochemically active phases [29]. Moreover, a slant voltage plateau related to a Li-ion insertion reaction appears above 2 V according to the discharge curve. Another flat voltage plateau appearing at 1.5 V arises from the conversion reaction coupling with the formation of LiF and Fe0.33H2O. [30] However, the plateau gradually vanishes during cycling, indicating a portion of LiF and Fe0.33H2O may not return to FeF30.33H2O. An initial discharge capacity of 988.6 mAh g 1 is achieved at a current density of 100 mAh g 1 with a coulombic
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efficiency of 78.9%, which is beyond the theoretical capacity of FeF30.33H2O. The cycle performance of the cell with the PGS interlayer contrasting with that of the cell carrying the blank cellulose paper is displayed in Fig. 4b. The cell with the blank cellulose paper displays a initial discharge capacity of 145 mAh g 1, while it can reach up to 290 mAh g 1 after the activation of electrode material. Nevertheless, the capacity of 135 mAh g 1 can be remained after 60 cycles. As a contrast, the cell employing PGS interlayer delivers ultrahigh discharge and charge capacities of 988.6 and 1252.4 mAh g 1 respectively at a current density of 100 mAh g 1 in the first cycle. After 60th cycle, it can retain a large reversible capacity of 601 mAh g 1, and the coulombic efficiency stays above 95%. Besides, the cell with PGS interlayer also exhibits superior rate performance (Fig. 4c). The discharge capacity of 515 mAh g 1, 462 mAh g 1, 430 mAh g 1, and 375 mAh g 1, can be achieved when the current density varies from 300 mAh g 1, 600 mAh g 1, 1000 mAh g 1, 2000 mAh g 1, respectively. As the current density returns to 600 mAh g 1 and 100 mAh g 1, the corresponding discharge capacity of 460 and 590 can be retained. Moreover, the capacity of 435 mAh g 1 can be still sustained at a current density of 1000 mAh g 1 after 250 cycles (Fig. 4d), indicating the superior lithium storage capability at higher current rates. The better cyclability at higher rate can be attributed to the reason that Li-ion could slowly penetrate into FeF30.33H2O particles at a
Fig. 4. The results of charge-discharge tests for the cell with PGS interlayer. (a) Discharge-charge voltage profiles at a current density of 100 mAh g 1, (b) Cycling performance at a current density of 100 mAh g 1, (c) Rate performance at varying rates from 100 mA g 1 to 2 A g 1, (d) Cycling performance at a current density of 1000 mAh g 1 of the cell with PGS interlayer.
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lower current density, which permits more FeF30.33H2O to react with Li-ion during discharge and more serious degradation of recharge capabilities. In turn, the capacity retention is relatively low. However, at a higher rate, the FeF30.33H2O reacting with Liion decreases due to a shorter penetration time and transport path of the solvated electrolyte and lithium ions, thus leading to the better cyclability [31]. The results give the further evidence for the excellent power performance of the cell using PGS interlayer. Evidently, the excellent electrochemical performance is mainly ascribed to the PGS interlayer, as demonstrated above. Cyclic voltammetry (CV) is performed to investigate electrochemical reaction characteristics at a scanning rate of 0.2 mV s 1 in the potential window of 1.5 V to 4.5 V. Fig. S2a reveals the CV profiles of the cell with the blank cellulose paper. Two obvious reduction peaks are observed, one of which is reflected between 2 V and 3.5 V, corresponding to the Li+-insertion into FeF30.33H2O electrode without structural breakdown. The other one, a sharp peak, can be observed at 1.5 V, corresponding to the stage of conversion reaction. However, in reversed scan, there is only one oxidation peak at 3.2 V, implying that the larger polarization appears in conversion reaction, which is consistent with that in other reported FeF30.33H2O. The CV curves of the cell with the PGS interlayer are shown in Fig. S2b. Comparing with the profiles in Fig. S2a, there is no distinct change for the locations of cathodic peaks, but the width of the peaks increases, which probably accounts for excess Li+ inserting into the porous structure of PGS interlayer [32]. Thus, it seems that the PGS contained in interlayer also participates in the electrochemical reaction. To clarify the effects of PGS, the ACP cell employing the PGS interlayer as active
material is assembled to carry out CV experiment. As the CV curve shown in Fig. S2c, a wide reduction peak appears in the range of 1.5–4.5 V, illustrating that the behavior of Li+-insertion into the PGS exists in the voltage range. Accordingly, the PGS interlayer can contribute some capacity to the cell. To make certain how much capacity that PGS donates, the ACP cell is tested by galvanostatic discharging/charging technique. As shown in Fig. S3a and b, the specific capacity contributed by the PGS interlayer is calculated to be approximately 135 mAh g 1 after 60 cycles, and the discharge/charge profiles exhibit an oblique line corresponding to the Li+-insertion into the PGS. The real specific capacity of FeF30.33H2O is calculated to be about 300 mAh g 1 after deducting the effect of PSG interlayer. Therefore, the PGS interlayer can not only improve the cycle performance of FeF30.33H2O cathode, but also donate a portion of capacity. The electrochemical impedance spectroscopy (EIS) of the cell with PGS interlayer is carried out before cycling and after 60 cycles at a current density of 100 mAh g 1 to illustrate the superior electrochemical performance. The high–frequency semicircle and inclined line from each Nyquist plot in Fig. S3c can be observed. The depressed semicircle is attributed to the overlap between the SEI (solid electrolyte interphase) film and the interfacial charge transfer impedance, and the inclined line represents the lithium diffusion impedance. After 60 cycles, the semicircle of the cell exhibits a smaller diameter compared with that before cycling, suggesting that the charge transfer impedance minishes upon cycling. Additionally, the steeper inclined line manifests increase of lithium ion conductivity after the charge and discharge cycling. The relevant equivalent circuit model are shown in Fig. S3d. The results
Fig. 5. SEM images of FeF30.33H2O electrode before (a) and after (b) cycling; SEM images of PGS interlayer section before (c) and after (d) cycling; elemental maps of Fe (e) and F (f) corresponding to the area marked by rectangle part in Fig. 5d.
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indicate the reversible reaction mainly takes place in the electrode during initial cycles, and the transfer of electron and ion suffers from the large resistance. After 60 cycles, a certain portion of FeF30.33H2O and discharge products have moved into the PGS interlayer. The synergistic effect of the porous carbon and GS with high conductivity improves the electron and ion conductivity of active material, which plays a significant role for the superior electrochemical performance. The change of electrode, PGS interlayer and Li electrode before and after cycling is characterized by scanning electron microscopy to investigate the function of PGS interlayer. Fig. 5a and b exhibit the SEM images of electrode before and after 60 cycles, respectively. It can be observed that a certain amount of FeF30.33H2O particles have dissolve away leaving many holes marked by the yellow rectangle in Fig. 5b after 60 cycles. In addition, the images of PGS interlayer before and after 60 cycles are shown in Fig. 5c and d. It can be confirmed that the PGS interlayer shows no distinct change after cycling, demonstrating the stability of PGS interlayer during cycling. EDS elemental maps (Fig. 5e and f) are carried out to analyze the elements of the cycled PGS interlayer (Fig. 5d). The maps analysis of interlayer section indicates that the Fe and F are distributed in the PGS interlayer, which demonstrates that a certain portion of unbonded FeF30.33H2O and discharge products caused by the volume expansion or chemical dissolution during charge/discharge enter into the porous structure of PGS
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interlayer. Hence, the PGS interlayer (Fig. S4) can not only increase the electron conductivity, but also act as an divertor trapping the dissolved discharge product and FeF30.33H2O particles breaking away from aluminum foil, which can improve the utilization of FeF30.33H2O. Moreover, EDS analysis (Fig. S5a and b) of the cycled Li electrode surface confirms the presence of a trace of Fe in the cell with blank cellulose paper, supporting the viewpoint on cathode dissolution. In contrast, the Li electrode of the cell with PGS interlayer does not contain detectable amount of Fe after 60 cycles, indicating the function of PGS interlayer for preventing cathode dissolution. The XPS is applied to analyze the elements of cycled PGS interlayer. The XPS survey spectrum (Fig. 6a) demonstrates the presence of Fe, C, F, and O in the porous structure of PGS, with most F and Fe in the form of discharge products [33,34]. The C 1 s spectra (Fig. 6b) reveals three major peaks, which are assigned to C-C bonds (284 eV), C-O bonds (286.7 eV), and the C-F bonds (290.9 eV) attributed to the PVDF binder. The F1 s spectra (Fig. 6c) exhibits LiF peak at 684.6 eV, indicating the some LiF particles formed during cycling are absorbed into the pores of PGS. Another major peak C-F bonds located at 687.4 eV is resolved into the component of PVDF binder. From the Fe2p spectra shown in Fig. 6d, two peaks at around 711 and 725 eV are corresponded to Fe2p3/2 and Fe2p1/2, suggesting the existence of Fe+3 in PGS interlayer [35]. The results correspond highly to that of EDS, further demonstrating that the
Fig. 6. XPS survey spectrum (a) of PGS interlayer after 60 cycles, XPS (b) C1s, (c) F1s, and (d) Fe2p spectra of the PGS interlayer.
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application of PGS interlayer improves the utilization of cathode material and stabilizes the structure of electrode. 4. Conclusions In summary, we develop a rational architecture that the composite of porous carbon and GS is coated onto a cellulose paper to form a conductive PGS interlayer. The PGS can be prepared by a simple hydrothermal treatment, followed an activation process, in which the GS can be mingled with porous carbon. The porous carbon in PGS interlayer can accommodate the dissolved species and FeF30.33H2O particles peeling off the Al foil caused by the volume expansion and cathode dissolution during the charge and discharge, which effectively increases the utilization of cathode material. Additionally, the GS can improve the poor electron conductivity of FeF30.33H2O, which is crucial for the electrochemical performance. When the PGS interlayer is applied in coin cell, the discharge capacities of 600, 460 and 370 mAh g 1 can be achieved at the rates of 100, 600 and 2000 mAh g 1 after 60 cycles respectively in the potential range of 1.5–4.5 V. Moreover, at a high rate of 1000 mAh g 1, it can retain a discharge capacity of 435 mAh g 1 after 250 cycles. The results further demonstrate the application of PGS interlayer can achieves an ultrahigh capacity, superior rate and cyclic performance. In addition, the proposed design of interlayer can be extended to other metal fluorides, which opens a new avenue for fabricating high performance Li-metal fluorides battery. Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant no. 51204209 and 51274240) and the Project of Innovation-driven Plan in Central South University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.10.076. References [1] X.P. Gao, H.X. Yang, Multi-electron reaction materials for high energy density batteries, Energy Environ. Sci. 3 (2010) 174. [2] Chao Chen, Xiaoping Xu, Shu Chen, Bin Zheng, Miao Shui, Lingxia Xu, Weidong Zheng, Jie Shu, Liangliang Cheng, Lin Feng, Yuanlong Ren, The preparation and characterization of iron fluorides polymorphs FeF30.33H2O and b-FeF33H2O as cathode materials for lithium-ion batteries, Materials Research Bulletin 64 (2015) 187. [3] K. Kang, Y.S. Meng, J. Bréger, C.P. Grey, G. Ceder, Electrodes with high power and high capacity for rechargeable lithium batteries, Science 311 (2006) 977. [4] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Highperformance lithium battery anodes using silicon nanowires, Nat. Nanotechnol. 3 (2008) 31. [5] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243. [6] F. Badway, N. Pereira, F. Cosandey, G.G. Amatucci, J. Electrochem. Soc. 150 (2003) A1209. [7] Feng Wang, Sung Wook Kim, Dong Hwa Seo, Kisuk Kang, Liping Wang, Dong Su, John J. Vajo, John Wang, Jason Graetz, Ternary metal fluorides as highenergy athodes, 2014 cathodes with low cycling hysteresis, Nature Communications 10 (2014) 7668. [8] X.P. Gao, H.X. Yang, Multi-electron reaction materials for high energy density batteries, Energy Environ. Sci. 3 (2010) 174. [9] Chilin Li, Lin Gu, Susumu Tsukimoto, Peter A. van Aken, Joachim Maier, LowTemperature Ionic-Liquid-Based Synthesis of Nanostructured Iron-Based Fluoride Cathodes for Lithium Batteries, J. Adv. Mater. 22 (2010) 36509. [10] R.E. Doe, K.A. Persson, Y.S. Meng, G. Ceder, First-principles investigation of the Li-Fe-F phase diagram and equilibrium and nonequilibrium conversion reactions of iron fluorides with lithium, Chem. Mater. 20 (2008) 5274.
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