Enhanced sodium storage kinetics by volume regulation and surface engineering via rationally designed hierarchical porous FeP@C/rGO

Ye Wang ab, Yew Von Lim b, Shaozhuan Huang b, Meng Ding b, Dezhi Kong ab, Yongyong Pei a, Tingting Xu a, Yumeng Shi c, Xinjian Li a and Hui Ying Yang *b
aKey Laboratory of Material Physics of Ministry of Education, School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450052, China
bPillar of Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, 487372, Singapore. E-mail: yanghuiying@sutd.edu.sg; Fax: +65 6779 5161; Tel: +65 6303 6663
cInternational Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, Engineering Technology Research Center for 2D Material Information Function Devices and Systems of Guangdong Province, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, China

Received 31st October 2019 , Accepted 28th December 2019

First published on 30th December 2019


Transition metal phosphides, such as iron phosphide (FeP), have been considered as promising anode candidates for high-performance sodium ion batteries (SIBs) owing to their high theoretical capacity. However, the development of FeP is limited by large volume change, low electrical conductivity and sluggish kinetics with sodium ions. Moreover, the sodium storage kinetics and dynamics behavior in FeP are still unclear. Herein, improved sodium storage ability of FeP is achieved by volume regulation and surface engineering via a rationally designed hierarchical porous FeP@C/rGO nanocomposite. This FeP@C/rGO nanocomposite exhibits excellent rate capability and long cycle life as the anode of SIBs. Specifically, the FeP@C/rGO nanocomposite delivers high specific capacities of 635.7 and 343.1 mA h g−1 at 20 and 2000 mA g−1, respectively, and stable cycling with 88.2% capacity retention after 1000 cycles. The kinetics and dynamics studies demonstrate that the superior performance is attributed to the rationally designed hierarchical porous FeP@C/rGO with a high capacitive contribution of 93.9% (at 2 mV s−1) and a small volume expansion of only 54.9% by in situ transmission electron microscopy (TEM) measurement. This work provides valuable insights into understanding the phase evolution of FeP during the sodiation/desodiation process for designing high-performance SIBs.


1. Introduction

Sodium ion batteries (SIBs) have received increasing attention owing to their low cost, high abundance of sodium and inherent safety in contrast to the conventional lithium ion batteries (LIBs).1–5 However, due to the large radius of sodium ions (0.102 nm), the electrode materials usually exhibit huge volume expansion/contraction of up to 525% and sluggish sodium storage kinetics during the sodiation/desodiation processes, leading to low rate capability and inferior cycle performance.6–12 These issues hamper the development of SIBs. Therefore, it is indispensable to understand the mechanism and develop strategies to relieve the volume change and enhance the sodium storage kinetics to obtain high performance SIBs based on rationally engineered nanomaterials.13–17

Recently, transition metal phosphides, such as iron phosphide (FeP), have been considered as one of the promising anode materials for high-performance SIBs owing to their higher theoretical specific capacity (∼924 mA h g−1 for FeP) than that of the insertion type anodes.18–22 Recent pioneering studies have reported micrometer sized FeP by the ball-milling method, 3D hollow FeP by multi-step processes and FeP/carbon derived from a Prussian blue analogue as the anode material for SIBs.23–28 The electrochemical performance is improved step by step. However, FeP based anode materials still suffered from low rate capability and/or fast capacity decay during cycling owing to the intrinsic physical properties including inferior reaction kinetics and large volume expansion (in the range of 241.5% (Fe2P)–428.2% (FeP2)).29 Moreover, there is still a lack of study of the kinetics and dynamics of sodium ions in FeP, which is critical for the design of high performance anodes. Therefore, it is highly interesting and demanding to design a rational FeP nanocomposite with a special functional hierarchical architecture to regulate the volume change and engineer the surface to achieve high sodium ion storage capacity. Meanwhile, it is essentially important to further investigate the kinetics and dynamics of sodium in FeP to understand the mechanism.

Herein, a hierarchical porous FeP nanocomposite is prepared through the decoration of reduced graphene oxide (rGO) and subsequent phosphorization of the r-MIL-88 Fe metal–organic framework (MOF). The FeP@C/rGO nanocomposite is composed of FeP@C nanorods wrapped with rGO nanosheets, where the FeP@C nanorods consist of abundant iron phosphide nanoparticles embedded in a porous carbon matrix, forming a hierarchical nanostructure. Porous carbon derived from the MOF provides a large surface area and serves as a conductive scaffold to support the iron phosphide nanoparticles, accommodate the volume expansion/contraction and prevent the pulverization/agglomeration of FeP nanoparticles during the sodiation/desodiation process. The rGO nanosheets can not only improve the electronic conductivity, but also effectively prevent the aggregation of FeP@C nanorods. Therefore, the designed FeP@C/rGO nanocomposites achieve a small volume expansion of only 54.9% and exhibit excellent sodium ion storage performance with excellent reversible specific capacity, high rate capability and long cycle life. Further ex situ X-ray powder diffraction (XRD), in situ transmission electron microscopy (TEM) and surface kinetic analysis study reveal that the sodiation/desodiation of FeP is based on a reversible conversion reaction and capacitive dominated process.

2 Experimental

2.1 Synthesis of r-MIL-88 MOF nanorods, MOF/GO and FeP@C/rGO nanocomposites

The synthesis process of r-MIL-88 Fe-MOF nanorods can be found elsewhere.30 In brief, 648.8 mg of FeCl3·6H2O and 310 mg of fumaric acid were dissolved in 20 ml of DI water and stirred for 10 min. Then, the mixture was transferred into a 50 ml Teflon-lined stainless steel autoclave and heated at 100 °C for 4 hours. The orange precipitate was collected by centrifugation, washed with DI water several times, and freeze dried.

The Fe-MOF/GO nanocomposite was prepared via a simple electrostatic assembly process, in which Fe-MOF is positively charged by poly(diallyldimethylammonium-chloride) (PDDA, a cationic polyelectrolyte) and GO is negatively charged in nature. In detail, 100 mg of Fe-MOF was added to 100 ml of DI water with 1% PDDA and 1 g of NaCl by stirring for 5 hours. After centrifugation and washing several times to remove the redundant PDDA and salt, the positively charged Fe-MOF was re-dispersed into 100 ml of DI water, followed by dropwise addition of 100 ml of 0.1 mg ml−1 GO solution under magnetic stirring overnight to drive an electrostatic attraction between the positively charged Fe-MOF and the negatively charged GO nanosheets. After that, Fe-MOF/GO was collected by centrifugation and freeze dried.

FeP@C/rGO was subsequently prepared by a simple phosphorization process via chemical vapor deposition in a normal tube furnace. In brief, 100 mg of Fe-MOF/GO nanorods and 300 mg of NaH2PO2 were placed at the two ends of a small ceramic boat which was placed in the center of a quartz tube, in which NaH2PO2 was at the upstream and FeP was at the downstream. The phosphorization was carried out at 400 °C for 5 hours at a slow ramping rate of 1 °C min−1 with Ar gas as the carrier gas. Finally, black powder was collected after cooling down to room temperature. FeP@C nanorods were prepared by phosphorizing Fe-MOF under the same conditions directly without the GO coating process.

2.2 Synthesis of Na3V2(PO4)3/C nanocomposites as cathode materials

The Na3V2(PO4)3/C nanocomposite preparation procedure is based on our previous work.31,32 In brief, 0.491 mg of vanadium trioxide, 0.971 mg of sodium dihydrogen phosphate, and 0.864 mg of citric acid were dissolved in distilled water with continuous stirring for 1 hour. Subsequently, the prepared solution was dried at 120 °C until all solvent evaporated. Then, the powder was annealed at 350 °C for 3 hours and at 800 °C for 8 hours under an Ar atmosphere. Finally, the Na3V2(PO4)3/C nanocomposite was collected after cooling down to room temperature.

2.3 Material characterization

The morphologies of the FeP nanorods were examined by field-emission scanning electron microscopy (FESEM, JSM-7600) and transmission electron microscopy (TEM, JEM-2100F). Energy-dispersive X-ray (EDX) analysis was carried out using a FESEM equipped with an EDX module. The crystal structure of the FeP nanorods was characterized by using an X-ray diffractometer (XRD, Bruker D8 Advances ECO) with Cu Kα (λ = 0.154 nm) radiation at an accelerating voltage of 40 kV. Raman spectra were recorded using a confocal Raman system with 532 nm laser excitation (WITec Instruments Corp, Germany). The content of FeP in the nanocomposites was measured by thermogravimetric analysis (TGA, Shimadzu, DTG-60, Japan). Specific surface area testing was carried out by N2 physisorption at 77 K by using the Brunauer–Emmett–Teller (BET, ASAP 2420, Micromeritics) method.

2.4 Battery assembly and electrochemical measurements

The working electrodes were prepared by mixing FeP@C/rGO, conductive carbon black and carboxymethyl cellulose (CMC) binder in a weight ratio of 80[thin space (1/6-em)]:[thin space (1/6-em)]10[thin space (1/6-em)]:[thin space (1/6-em)]10 and adding several drops of DI water to form a slurry. Then the slurry was coated on copper foil and dried in a vacuum oven at 120 °C overnight. The loading mass of the anode materials is 2–3 mg cm−2. Then the anode was assembled into a CR2032 coin cell with the FeP@C/rGO active material as the working electrode and sodium foil as the counter electrode. 1 mol NaPF6 dissolved into ethylene-carbonate/diethyl-carbonate (EC/DEC) solution (1[thin space (1/6-em)]:[thin space (1/6-em)]1, v[thin space (1/6-em)]:[thin space (1/6-em)]v) with 2% fluoroethylene-carbonate (FEC, stabilizer) was used as the electrolyte. A glass Fiber Filter (GB-100R, Advantec, Japan) was used as the separator. All components listed above were assembled into a standard CR2032 coin cell in a glove box filled with argon gas with the oxygen and moisture level lower than 1 ppm. The full battery was assembled with Na3V2(PO4)3/C as the cathode and FeP@C/rGO as the anode. The cathode was prepared by coating Na3V2(PO4)3/C slurry on Al foil. The procedure is the same as that for the anode. In order to match the capacity, the mass loadings of the anode and cathode were approximately 1 to 7 mg cm−2. Before assembling the full battery, the anode was presodiated by contacting with Na foil for 6 hours to compensate for the initial irreversible capacity.

Electrochemical measurements were carried out 24 hours after battery assembly. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical workstation (VMP3, Bio-Logic, France). The galvanostatic charge/discharge test of the anode was carried out in the voltage range of 0.01–2.5 V at various current densities ranging from 20 to 2000 mA g−1 using a battery analyzer (Neware, Shenzhen, China). The full battery was measured in the voltage range of 1.5–3.8 V. All experiments were conducted at room temperature.

2.5 Ex situ XRD and in situ TEM characterization

In ex situ XRD measurements, the coin cells were discharged and charged to a fixed potential. After that, the cells were disassembled in the Ar filled glove box, washed with NMP several times, and dried and sealed with kapton tape.

The in situ TEM measurement was carried out using a JEOL JEM-2100F TEM equipped with a PicoFemto TEM specimen holder. A solid nano Li battery composed of FeP@C/rGO as the cathode, Na2O as the solid electrolyte, and Na metal as the anode was constructed to study the real-time morphology and phase evolution by TEM observation. The FeP@C/rGO nanostructure attached to a gold wire was used as the working electrode and metallic sodium coated to a sharp tungsten (W) wire was used as the counter electrode. The Na/Na2O-W probe was driven by a piezopositioner to approach the working electrode step by step. Once they came into contact, a bias of −0.1 V was applied to the FeP@C/rGO electrode to drive the flow of electrons and Na ions to initiate the sodiation process, and the bias was reversed to 1 V for the desodiation process.

3 Results and discussion

The synthesis process of the FeP@C/rGO nanocomposite is shown in Fig. 1a. First, rod-like Fe-MOF r-MIL-88 with a smooth surface was synthesized by a simple hydrothermal process.33 The diameter and length of the r-MIL-88 Fe-MOF are 200–400 nm and 3–10 μm, respectively (Fig. 1b). Then, the Fe-MOF was functionalized with the positively charged PDDA cationic polyelectrolyte molecule. When mixing the positively charged Fe-MOF with intrinsic negatively charged GO flakes, the Fe-MOF was immediately wrapped with GO nanosheets (indicated by the red arrow) through electrostatic adsorption (Fig. 1c), forming the Fe-MOF/GO nanocomposite.34 Elemental mapping images show that MOF/GO is composed of carbon and iron elements (Fig. S1). Then the FeP@C/rGO nanocomposite was obtained via the phosphorization of MOF/GO at 400 °C. During the phosphorization process, the Fe species were converted into iron phosphide nanoparticles and organic ligands were in situ transformed into the carbon scaffold, forming a composite with iron phosphide embedded in a carbon matrix.35,36 Meanwhile, graphene oxide (GO) was reduced into reduced-GO (rGO) by the thermal reduction process.37 The as-obtained FeP@C/rGO nanocomposite is composed of the FeP@C nanorod decorated on rGO nanosheets (Fig. 1d). It is also interesting to find that a single FeP@C nanorod is wrapped with rGO sheets (indicated by the red arrow). The EDX spectrum and related elemental mapping images indicate that the Fe and P elements are uniformly distributed over the nanorods (Fig. 1e–h). Without GO, the pristine Fe-MOF was converted into iron phosphide nanoparticles embedded in a carbon matrix with a rougher surface (Fig. S2).
image file: c9nr09278a-f1.tif
Fig. 1 (a) Schematic diagram of the synthesis process of FeP@C/rGO nanocomposites. SEM images of (b) r-MIL-88 Fe-MOF, (c) Fe-MOF@GO and (d) FeP@C/rGO. (e) EDX spectra and mapping of FeP@C/rGO nanocomposites with (f) carbon, (g) iron and (h) phosphorus element distributions of (d).

The detailed microstructures of FeP@C and FeP@C/rGO nanocomposites are examined by TEM, as shown in Fig. 2. The FeP@C nanorod exhibits a highly porous configuration (Fig. 2a and b), where plenty of FeP nanoparticles (core) are coated with a uniform carbon thin layer (∼2 nm), forming a 3D interconnected porous structure (Fig. 2c). For the FeP@C/rGO sample, the porous FeP@C nanorods are decorated with a thin layer of rGO nanosheets (indicated by the red arrow in Fig. 2d and e). The FeP particles show an average size of approximately 10 nm. The high resolution TEM image of FeP@C/rGO nanocomposites shows that rGO is composed of 5–10 layers with a layer spacing of 0.34 nm, and a well-resolved lattice spacing of 0.251 nm corresponds to the (102) crystal lattice of FeP nanoparticles (Fig. 2f). It is worth mentioning that one of the merits of the porous nanomaterial is the large surface to volume ratio due to its nanostructure which can effectively shorten the sodium ion diffusion length and reduce the inner stress during the repeated sodiation/desodiation process.16 The surface areas characterized by BET analysis of FeP@C and FeP@C/rGO are 177.8 and 348.1 m2 g−1, respectively (Fig. S3).


image file: c9nr09278a-f2.tif
Fig. 2 (a and b) TEM and (c) HRTEM images of the FeP@C nanocomposites. The red lines in (c) indicate the amorphous carbon layer over FeP nanocomposites. The inset in (c) is the enlarged view of the square in white to show the lattice of FeP nanocrystals. (d and e) TEM and (f) HRTEM images of FeP@C/rGO nanocomposites. rGO nanosheets are indicted by red arrows in (d) and (e). The inset in (e) is the SAED pattern of FeP@C/rGO nanocomposites.

The crystal structure and composition of the FeP@C/rGO nanocomposite are determined by XRD, Raman, TGA and XPS analysis, as shown in Fig. 3. The diffraction peaks at 30.8°, 32.7°, 37.1°, 46.3°, 46.9°, 48.3°, 56.1° and 79.2° belong to the orthorhombic FeP (JPCDS card 65-2595, in red), and a small peak located at around 26° is indexed to carbon (amorphous carbon and rGO).38,39 In contrast, without rGO decoration, the phosphorized sample shows additional XRD peaks (Fig. S4a) at 40.4°, 44.6° and 47.3°, belonging to the hexagonal Fe2P (JPCDS card 85-1727, in blue). The semi-quantitative analysis indicates the ratio of FeP[thin space (1/6-em)]:[thin space (1/6-em)]Fe2P = 46%[thin space (1/6-em)]:[thin space (1/6-em)]56%. Therefore, the control sample without rGO incorporation is named FexP@C (1 < x < 2). For simplification, it is named FeP@C. The XRD result suggests that the GO coating can assist the Fe-MOF to be fully converted into FeP. The possible reasons are: (a) as a buffer layer, rGO prevents the aggregation of Fe-MOF and offers plenty of voids allowing the H3P gas to flow through the porous nanostructure, and (b) H3P gas is easily adsorbed by the rough GO functional groups and reacts with Fe-MOF. In the Raman spectra (Fig. 3b), there are two peaks located at around 1340 and 1585 cm−1, which are attributed to the D bands of carbon associated with the defects and the G bands of carbon related to the coplanar vibration of sp2 carbon atoms, respectively.40–45 The intensities of the D band and G band of FeP@C/rGO are much stronger than those of the D and G bands of FeP@C (Fig. S4b), suggesting that the rGO nanosheets are successfully introduced in the FeP@C/rGO sample. The contents of carbon and FeP in FeP@C/rGO nanocomposites were determined by TGA, as shown in Fig. 3c. A weight loss of 1.1% is attributed to the evaporation process of the moisture adsorbed on the surface of FeP@C/rGO.46,47 Afterwards, there are 5 stages including weight increment, decrement and stabilization. The reasons are ascribed to the stepwise combustion of carbon and oxidation of FeP. According to previous study, FeP is oxidized to FePO4 until 500–600 °C.48 This is the reason for the weight increment when the temperature range is 100–510 °C. The weight loss that occurs in the temperature range of 310–410 °C is due to the combustion of carbon, including the carbon shell and rGO. From the above analysis, the contents of FeP, carbon and rGO in FeP@C/rGO are found to be 58.8%, 32.4% and 8.8%, respectively. The contents of FeP and carbon in FeP@C are found to be 35.5% and 64.5%, respectively (Fig. S4c).


image file: c9nr09278a-f3.tif
Fig. 3 (a) XRD pattern, (b) Raman spectra, (c) TGA, (d) XPS profiles and (e) Fe 2p and (f) P 2p spectra of FeP@C/rGO nanocomposites.

The chemical states of the FeP@C/rGO nanocomposite were investigated by X-ray photoelectron spectroscopy (XPS) measurement. Fig. 3d shows the XPS survey profile of the FeP@C/rGO nanocomposite which is composed of C, O, Fe and P elements. The carbon peak of FeP@C/rGO is much more obvious than that of FeP@C (Fig. S5), indicating that more carbon exists in FeP@C/rGO due to the wrapping by rGO. In the high resolution XPS spectrum of Fe 2p, there are two main peaks located at 710.9 and 723.8 eV (Fig. 3e), corresponding to the doublet peaks of Fe–O 2P3/2 and Fe–O 2P1/2, respectively.48 Besides, a pair of weak peaks located at 707.1 and 720 eV should be assigned to Fe–P 2p3/2 and Fe–P 2p1/2, respectively.28,49,50 In the XPS spectrum of P 2p (Fig. 3f), there are three peaks located at 129.3, 130.1 and 133.4 eV, corresponding to the binding energies of P 2p3/2 and 2p1/2, and P–O, respectively.49,51 These results agree with the XRD results and previous reports.

The sodium storage performance of FeP@C/rGO and FeP@C nanocomposites was evaluated by using a standard half-cell battery configuration with sodium foil as the counter/reference electrode. Fig. 4a shows the CV tests of the FeP@C/rGO electrode for the first 5 cycles at a scan rate of 0.1 mV s−1 in the voltage range of 0.01–2.50 V (vs. Na/Na+). During the 1st cathodic process, the peak at 0.77 V corresponds to the reduction of FeP to Fe nanoparticles embedded in a Na3P matrix and the formation of a solid electrolyte interface (SEI) layer.52–57 During the 1st anodic process, the peak located at 1.42 V is attributed to the reversible oxidation of Fe to FeP.58 The related sodiation/desodiation processes are described by the following equations:

 
Sodiation: FeP + Na+ + 3e → Fe + Na3P(1)
 
Desodiation: Fe + Na3P → FeP + Na+ + 3e(2)


image file: c9nr09278a-f4.tif
Fig. 4 Electrochemical performance of FeP@C/rGO nanocomposites as the anode material for SIBs. (a) CV curves of the FeP@C/rGO electrode of the first 5 cycles at a scan rate of 0.1 mV s−1 in the potential range of 0.01–2.5 V vs. Na/Na+. (b) Galvanostatic discharge/charge curves of the FeP@C/rGO electrode of the first 5 cycles at a current density of 20 mA g−1. (c) Rate capability of FeP@C/rGO and FeP@C nanocomposite electrodes at current densities ranging from 20 to 2000 mA g−1. (d) A comparison of the electrochemical performance of FeP based anode of SIBs. (e) Long-term cycling performance and related coulombic efficiency of FeP@C/rGO and FeP@C nanocomposite electrodes at a current density of 200 mA g−1 for 1000 cycles.

In the 2nd cathodic process, the reduction peak shifts to 0.86 V, which is related to the conversion reaction into Fe and Na3P.49 The overlapping curves of the following cycles indicate the good cycling stability of the synthesized FeP@C/rGO nanostructure. The shape of the CV curve of the FeP@C electrode is almost the same as that of the FeP@C/rGO electrode but with a slight peak shift (Fig. S6a). The cathodic peak for the FeP@C electrode in the 1st cycle is located at 0.75 V, which is slightly lower than that of the FeP@C/rGO electrode. The anodic peak for the FeP@C electrode in the 1st cycle becomes broad. The galvanostatic discharge/charge curves of the FeP@C/rGO and FeP@C electrodes are shown in Fig. 4b and S6b, respectively. The initial discharge/charge capacities of the FeP@C/rGO electrode are 1264.5 and 675.7 mA h g−1, with an initial coulombic efficiency (CE) of 53.4%. The long plateau located at 1.1–0.8 V in the 1st discharge process is due to the conversion of FeP into Fe nanoparticles and Na3P and the formation of a SEI film.25 The plateau at 1.2–1.9 V in the 1st charge process is related to the oxidation of Fe to FeP due to the desodiation process.49 These results agree well with the CV results. The initial discharge/charge capacities of FeP@C are 1163.6 and 346.7 mA h g−1, respectively (Fig. S6b), with a much lower initial CE of 29.8%. The higher initial CE of FeP@C/rGO suggests that the protection from the rGO layer can avoid severe SEI layer formation and effectively stabilize the electrode surface.

The detailed sodiation/desodiation processes were further investigated by the ex situ XRD study (Fig. S7). Ex situ XRD measurements at different sodiation/desodiation stages were carried out to further investigate the crystal structure evolution of FeP@C/rGO in the 1st cycle. In the fresh electrode, two diffraction peaks located at approximately 46.5° and 48.4° belong to the (112/202) and (211) planes of orthorhombic FeP. When the electrode is discharged to 1.0 V, the (112/202) and (211) peaks of FeP become weak and two tiny peaks located at 36° and 37° appear, which can be indexed to the (110) and (103) planes of Na3P (JCPDS card 74-1164), respectively.48 Further discharging the battery to 0.01 V, the peaks belonging to Na3P become more obvious, while the peaks of FeP completely disappear. This result perfectly confirms the reaction described by eqn (1). When the battery is recharged to 1.0 V, the peaks of FeP (∼46.5° and 48.4°) appear again. Further recharging to 2.5 V results in an increased peak intensity of FeP. The disappearance/reappearance of FeP and generation/decomposition of Na3P demonstrate that the sodiation and desodiation reactions are based on a reversible conversion reaction.

The rate performance of the FeP@C/rGO and FeP@C electrodes at various current densities from 20 to 2000 mA g−1 is shown in Fig. 4c. The FeP@C/rGO electrode delivers reversible specific capacities of 635.7, 597.1, 573.7, 523.2, 448.9, 398.7 and 343.1 mA h g−1 at current densities of 20, 50, 100, 200, 500, 1000 and 2000 mA g−1, respectively. When the current density returns to 20 mA g−1, the capacity quickly recovers to a high value of 636.3 mA h g−1. In contrast, FeP@C delivers much lower specific capacities of 279.1, 239.8, 203.4, 184.6, 143.5, 93.3, and 51.7 mA h g−1 at current densities of 20, 50, 100, 200, 500, 1000 and 2000 mA g−1, respectively. The improved electrochemical performance of FeP@C/rGO is due to the introduction of rGO nanosheets which not only improve the electrical conductivity of the active material, but also prevent the electrode from aggregation. The improved reaction kinetics can be confirmed by the fitted Nyquist plots of all evaluated electrodes (ESI, Fig. S8, and Table S1), in which the FeP@C/rGO electrode has a much smaller charge transfer resistance (30.7 Ω) than the FeP@C electrode (258.3 Ω). We summarized and compared the rate capability of various FeP based anode electrodes in SIBs (Fig. 4d). The rate capability of FeP@C/rGO nanoarchitectures is one of the best values reported recently for iron phosphide anodes.25–28,51,59,60 The long-term cycling performance of the FeP@C/rGO and FeP@C electrodes was further evaluated at a current density of 200 mA g−1 (Fig. 4e). The FeP@C/rGO electrode delivers a high specific capacity of 450.5 mA h g−1 after 1000 cycles, with a high capacity retention of 88.2% (versus 2nd discharge capacity, 0.011% capacity decay per cycle). In contrast, the FeP@C electrode exhibits a much lower capacity of 23.6 mA h g−1 after 1000 cycles, with a poor capacity retention of 9.6% (versus 2nd discharge capacity). The initial coulombic efficiency of FeP@C/rGO is 51.6%, and increases to 91.2% in the 2nd cycle and then remains higher than 99% after 10 cycles, indicating the excellent cycling reversibility.

To further reveal the sodium storage behavior of the FeP@C/rGO nanocomposite, pseudocapacitance contribution to the total capacity was analyzed by the CV curves at various scan rates, as shown in Fig. 5a. According to the power-law equation, the relationship between the current and the sweep rate is shown as below:61–65

 
i(V) = b(3)
where a and b are adjustable values. According to the previous studies, a b-value of 0.5 represents a diffusion-controlled behavior and a b-value of 1 indicates a surface capacitive behavior.61,66,67 In the current work, the b-values of the FeP@C/rGO electrode based on cathodic and anodic peaks are 0.832 and 0.824, respectively, indicating that the charge storage in FeP@C/rGO is capacitive dominated (Fig. 5b).65,68,69 However, the b-values of the FeP@C electrode for cathodic and anodic peaks are only 0.569 and 0.542, respectively, indicating a slow kinetics due to the diffusion-controlled electrochemical behavior (Fig. S9).


image file: c9nr09278a-f5.tif
Fig. 5 Kinetic analysis of the FeP@C/rGO nanocomposites as the anode material for SIBs. (a) CV curves at various scan rates; (b) b value fitting; (c) capacitive contribution (red part) at a scan rate of 1 mV s−1; (d) normalized capacitive contribution (red part) ratio at various scan rates.

Furthermore, the capacitive contribution of the FeP@C/rGO electrode is analyzed in more detail, as shown in Fig. 5c and d. The current response to a fixed potential can be distinguished into capacitive contribution (proportional to the scan rate ν) and diffusion-controlled contribution (proportional to ν1/2) according to the following equation:70–74

 
i(V) = k1ν + k2ν1/2(4)
using which the capacitive contribution of the FeP@C/rGO electrode can be analyzed. The capacitive contributions are 58.6%, 65.8%, 71.5%, 73.9%, 77.3%, 85.3% and 93.9% at scan rates of 0.1, 0.2, 0.4, 0.6, 0.8, 1 and 2 mV s−1, respectively (Fig. 5d). These results indicate that the capacitive charge-storage occupies a large portion of the entire capacity.75,76

In order to further investigate the structure evolution and reveal the dynamic processes of sodiation/desodiation in FeP@C/rGO, the in situ TEM measurement was performed, as shown in Fig. 6. A schematic diagram of the in situ TEM setup is shown in Fig. 6a. The FeP@C/rGO nanostructure attached to a gold wire was used as the working electrode, and metallic sodium attached to the tungsten wire was employed as the counter electrode, and the naturally formed Na2O in air on the sodium surface acts as the solid electrolyte.77–79 A 1.0 V bias voltage was applied with a duration of 120 s when the FeP@C/rGO working electrode came into contact with Na/Na2O during the sodiation/desodiation processes. The microstructure evolution of FeP@C/rGO at various sodiation/desodiation stages is shown in Fig. 6b–g. The corresponding videos have been taken to show the dynamic structure evolution during the sodiation (Video S1) and desodiation (Video S2) processes. The diameter of the FeP@C nanorod increases from 73.2 to 84.7 nm after 120 s of sodiation, leading to a small volume expansion of approximately 54.9% (Fig. 6b–e), which is much smaller than that for other phosphides.80,81 An enlarged TEM image of FeP@C/rGO after the sodiation process indicates that the FeP nanoparticles are pulverized into smaller nanocrystals, but still confined inside the carbon shell (Fig. S10b). The SAED pattern indicates the formation of metallic Fe and Na3P (Fig. 6i). When the voltage bias is switched off, the diameter of the FeP@C nanorod is reduced to 82.7 nm. The volume of the FeP@C nanorod continues to decrease to 75.4 nm until the end of the desodiation process, with a volume contraction of 41.8% compared with the sodiation process (Fig. 6f and g). The pulverized nanoparticles still maintain 3–5 nm inside the carbon matrix after the desodiation process (Fig. S10c). The SAED pattern (Fig. 6j) indicates the formation of FeP after the desodiation process. These results agree well with the ex situ XRD results.


image file: c9nr09278a-f6.tif
Fig. 6 (a) Schematic diagram of the experimental setup of in situ TEM analysis. Micro-structure evolution of FeP@C/rGO nanocomposites during the 1st sodiation (b–e) and 1st desodiation (f and g) processes by in situ TEM observation. Related SAED patterns of (h) before sodiation, (i) end of 1st sodiation and (j) end of the 1st desodiation process.

The excellent electrochemical performance of the designed FeP@C/rGO hybrid nanoarchitecture is attributed to the rationally designed hierarchical porous nanostructure. First, FeP nanoparticles with an average size of 10 nm can effectively shorten the sodium diffusion length and release the local strain, and improve the utilization of electrode materials. Second, organic ligands are carbonized to produce a porous carbon structure. The porous structure can provide large surface areas and pore volumes to facilitate the contact of active materials and the electrolyte. The porous carbon matrix can also improve the electron transportation and avoid the aggregation of FeP nanoparticles, and facilitate the accommodation of volume change during the sodiation and desodiation processes. Moreover, plenty of pores and voids can further alleviate the volume expansion/contraction. A volume expansion of only 54.9% is observed in such a porous FeP@C/rGO nanocomposite, which is 4.4 to 7.9 times smaller than the theoretical expansion of metal phosphides and most conversion reaction anodes.29,82 Third, the in situ carbon coating on the surface of FeP nanoparticles can effectively confine pulverized FeP nanoparticles, reducing the formation of a SEI layer on the surface newly exposed to the electrolyte as the SEI film is only formed on the surface of the carbon layer, leading to long-term cycling stability and high CE. Fourth, rGO can increase the conductivity of the overall electrode and effectively prevent the aggregation of FeP@C nanorods during cycling. Last but not least, the rationally designed FeP@C/rGO composite provides fast surface kinetics and the synergistic effect between FeP@C nanorods and rGO nanosheets, leading to high rate capability and stable, long-term cycling performance.

A full SIB was assembled by using FeP@C/rGO and Na3V2(PO4)3/C as the anode and cathode, respectively. As shown in Fig. 7a, the Na3V2(PO4)3/C electrode can deliver a discharge capacity of 88.2 mA h g−1 in the voltage range of 2.5–3.8 V and at a current density of 100 mA g−1, and the FeP@C/rGO electrode can deliver a capacity of 574.8 mA h g−1 at the same current density. Then the full battery was assembled and measured in the voltage window of 1.0–3.5 V (Fig. 7b). At a current density of 100 mA g−1, the full battery exhibits an initial charge and discharge capacity of 78.7 and 73.3 mA h g−1 (based on the total mass of the anode and cathode), respectively. The full battery exhibits an excellent rate capacity. It can deliver reversible capacities of 72.4, 66.7, 60.4, 53.2 and 35.2 mA h g−1 at current densities of 100, 200, 500, 1000 and 2000 mA g−1, respectively (Fig. 7c). Moreover, the full battery can retain a capacity of 61.7 mA h g−1 at a current density of 200 mA g−1 after 100 cycles (Fig. 7d). The excellent rate capability and long-term cycling stability of the full battery demonstrate that our designed FeP@C/rGO anode is a promising anode material for SIBs.


image file: c9nr09278a-f7.tif
Fig. 7 Electrochemical performance of the full battery with FeP@C/rGO and Na3V2(PO4)3/C as the anode and cathode, respectively. (a) Charge and discharge profiles of FeP@C/rGO (dark lines) and Na3V2(PO4)3/C (blue lines). (b) Galvanostatic charge/discharge curves of the full battery at a current density of 100 mA g−1. (c) Rate capability at various current densities. (d) Cycling stability measurement at a current density of 200 mA g−1.

4 Conclusions

In summary, we have synthesized the FeP@C/rGO nanocomposite via a hydrothermal process, rGO modification and subsequent phosphorization process. The resultant FeP@C/rGO nanocomposite exhibits high specific capacity, excellent rate capability and long-term cycling stability. Furthermore, ex situ XRD and in situ TEM studies reveal that the sodiation/desodiation of FeP is based on a reversible conversion reaction. This rationally designed FeP@C/rGO hierarchical porous nanostructure achieves a very small volume expansion of only 54.9%, which is much smaller than most of the conversion type anodes. The kinetics and dynamics analysis reveals that the excellent electrochemical performance is attributed to the large capacitive contribution behavior and small volume expansion owing to the unique nanostructure design and the synergistic effect between FeP, porous in situ carbon coating and the conductive buffer rGO. The rational nanostructure architecture and effective ex situ/in situ characterization provide a possible route to design high performance SIBs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21603192 and U1804132), Academic Improvement Program of Physics, Zhengzhou University, Zhengzhou University Youth Talent Start-up Grant and the Outstanding Young Talent Research Fund of Zhengzhou University (Grant No. 1521317005), Education Commission of Guangdong Province (Grant No. 2016KZDXM008), and SUTD Digital Manufacturing and Design (DManD) Centre.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr09278a

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