Carbonized polydopamine coated single-crystalline NiFe2O4 nanooctahedrons with enhanced electrochemical performance as anode materials in a lithium ion battery

Accepted Manuscript Title: Carbonized polydopamine coated single-crystalline NiFe2 O4 nanooctahedrons with enhanced electrochemical performance as anode materials in a lithium ion battery Authors: Xinxin Liu, Tong Zhang, Yue Qu, Ge Tian, Huijuan Yue, Dong Zhang, Shouhua Feng PII: DOI: Reference:

S0013-4686(17)30270-0 http://dx.doi.org/doi:10.1016/j.electacta.2017.02.020 EA 28885

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

26-8-2016 15-1-2017 5-2-2017

Please cite this article as: Xinxin Liu, Tong Zhang, Yue Qu, Ge Tian, Huijuan Yue, Dong Zhang, Shouhua Feng, Carbonized polydopamine coated single-crystalline NiFe2O4 nanooctahedrons with enhanced electrochemical performance as anode materials in a lithium ion battery, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.02.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Carbonized polydopamine coated single-crystalline NiFe2O4 nanooctahedrons with enhanced electrochemical performance as anode materials in a lithium ion battery Xinxin Liua†, Tong Zhangb†, Yue Qua, Ge Tiana, Huijuan Yuea*, Dong Zhangb*, Shouhua Fenga a

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry, Jilin University, Changchun 130012, P. R. China. b

Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of

Education), College of Physics, Jilin University, Changchun 130012, P. R. China.

Corresponding author: [email protected] (Huijuan Yue) and [email protected] (Dong Zhang) †

These authors contributed equally.

1

Table of Contents

NiFe2O4@NC were successfully fabricated via a subsequent carbonization of polydopamine. A nanocomposite containing 20% mass fraction of dopamine exhibited enhanced lithium ion battery performance with high reversible cycle capacity and good rate retention performance.

2

Highlights

   

NiFe2O4 nanooctahedrons were synthesized by a facile hydrothermal process. A phase formation mechanism was studied by time-dependent experiments. NiFe2O4 with N-doped carbon shell was fabricated via carbonization of polydopamine. NiFe2O4@NC20 showed the best rate capability and cycle stability.

3

Abstract Combining nanostructure engineering with conductive carbonaceous material is a promising strategy to obtain high-performance lithium ion batteries (LIBs). In this work, spinel NiFe2O4 nanooctahedrons were initially synthesized at a low temperature without further annealing. We investigated the phase formation mechanism by timedependent experiments. Next, octahedral NiFe2O4 with a nitrogen-doped carbon shell (NiFe2O4@NC) were successfully fabricated via a subsequent carbonization of polydopamine (PDA). We systematically varied the dopamine content in the NiFe2O4/carbon nanocomposites and found that a nanocomposite containing 20% mass fraction of dopamine exhibited enhanced lithium ion battery performance with high reversible cycle capacity and good rate retention performance compared with the pure material. Remarkably, the hybrid nanocomposite delivered a high reversible capacity of 1297 mAh g-1 even after 50 cycles at a current density of 100 mA g-1. Additionally, a high capacity of 1204 mAh g-1 was retained at a high current density of 500 mA g-1 after 300 cycles. This improvement in electrochemical performance is attributed to the enhanced structural stability and electrical conductivity caused by the carbon layer, and is supported by TEM and EIS measurements.

Keywords: Nickel ferrite, Nanooctahedron, N-doped carbon layer, Phase formation mechanism, Electrochemical performance

1. Introduction The rapid development of electronic equipment increases the need for new generation of lithium ion batteries (LIBs) with long cyclic life, high rate 4

capability, and high specific energy density [1-5]. Graphite is extensively used as an anode material for commercial batteries, but exhibits a low theoretical capacity of 372 mAh g-1 and poor safety stability, limiting its widespread applications [6-10]. Thus, there is a need to explore alternative materials with enhanced properties [11-12]. Recently, metal oxides as lithium electrode materials based on conversion reactions have been investigated in the past decade owing to various advantages such as high capacity, easy availability, well cyclability, and environmental benignity [13]. Among them, CuO [14], NiO [15,16], SnO2 [17], and Fe3O4 [18-20] merit attention. Studies have demonstrated that binary transition metal oxides can also be promising candidates for using as battery components [2129]. Hameed et al. investigated properties of pristine and (Mg, Cu) co-doped ZnFe2O4 nanoparticles [30]. The properties of (Ni1-xZnx)Fe2O4 samples have also been investigated by Reddy et al and gain reversible capacities of 819 mAh g-1 [31,32]. In addition, MgCo2O4 and CuCo2O4 can also deliver high capacities of 795 and 740 mAh g-1 [33,34]. Wei et al. successfully synthesized CuFe2O4 and MgFe2O4 materials and can maintain reversible capacity of 572.4 and 714 mAh g-1, respectively [35,36]. Qian et al. synthesized CuFe2O4 using a facile one-step solid state reaction route and gain a good performance of 950 mAh g-1 at 100 mA g-1 after 60 cycles [37]. Owing to its natural abundance and low cost, nickel ferrite NiFe2O4 (with theoretical capacity of 914 mAh g-1) offers high potential as a material for batteries. At present, NiFe2O4 has been successfully

synthesized

and

tested

for

anode

function

in

different

morphological forms, including nanospheres (709.0 mAh g-1 after 3 cycles at 200 mA g-1) [38,39], hollow spheres(850.4 mAh g-1 after 70 cycles at 200 mA 5

g-1) [40,41], nanofibers (1000 mAh g-1 after 100 cycles at 100 mA g-1, 514 mAh g-1 after 60 cycles at 50 mA g-1) [42,43] and nanorods (520 mAh g-1 after 300 cycles at 1000 mA g-1) [44]. However, NiFe2O4 has low columbic efficiency in the first cycle, poor cycling stability, and rapid capacity fading, due to intrinsically poor electrical conductivity and the drastic volume change that occurs during the charge/discharge process [30,45]. To address this problem and achieve enhanced electrochemical properties, extensive work has been performed to hybridize anodic systems into a carbon matrix that can not only function a buffer to relieve the stress arising from the volumetric expansion, but also improves electronic conductivity by providing a direct route for electron transport. For example, NiFe2O4/C nanoparticle composites [31], nanohybrids based on mixed oxides of NiFe2O4 and reduced graphene oxide [46], and core-shell structured NiFe2O4/onion-like carbon nanocapsules [47] exhibit improved strain accommodation and charge-transport capabilities and enhance cycling stability and rate capability. When nitrogen element is doped into a carbon matrix, there is increased charge efficiency and better cycle stability. For example, N-doped carbon has been used as an efficient conducting and buffering matrix for ZnFe2O4 [48], Fe3O4 [49], SnO2 [50], and NiO anodes [51] in LIBs owing to enhanced lithium-storage reaction kinetics. Therefore, it is reasonable to assess the electrochemical performance of N-doped carbon-coated NiFe2O4 electrode materials for the Li-ion battery. Polydopamine (PDA) can be easily converted to nitrogen-doped (N-doped) graphitized carbon. The graphitization of carbon promotes electrical conduction and nitrogen doping favors electron transport to further improve electrical conduction [52]. The high carbon yield of PDA 6

makes it well-suited for use in the preparation of carbon coating materials with controllable thickness [53,54] and it has been applied to some the reported systems such as GO-PDA and SiO2/PDA/Ag [55,56]. The thickness of the polydopamine layer can be controlled through varying the initial concentration of dopamine or the polymerization time [57]. In this work, we report preparation of N-doped carbon wrapped singlecrystalline NiFe2O4 nanooctahedrons (NiFe2O4/NC) by the hydrothermal synthesis of NiFe2O4 nanooctahedrons and in situ polymerization of dopamine on the surface of NiFe2O4, followed by carbonization of PDA at high temperature in inert atmosphere. The phase formation mechanism was studied in detail. For use as a potential anode for LIBs, NiFe2O4@NC shows improved specific capacity compared to unmodified NiFe2O4. We also compared the performances of the NiFe2O4@NC anode material containing different amounts of N-doped carbon coating by controlling the amount of dopamine. We characterized the carbon coating composition-dependent performance of these materials and found that NiFe2O4@NC20 shows the best electrochemical performance.

2. Experimental section 2.1 Synthesis of nano-octahedral NiFe2O4 and NiFe2O4/NC nanocomposites To prepare nano-spinel NiFe2O4, powders of Fe(NO3)3·9H2O (0.0076 mol) and Ni(NO3)2·6H2O (0.0038 mol), were dissolved in 30 mL of deionized water. Next, 30 mL portion of a 2 M KOH solution was added into the above solution. After magnetic stirring at room temperature for 2 h, the resulting mixture was transferred into a Teflon-lined stainless steel autoclave and maintained at 180 7

°C for 22 h. After cooling to room temperature, the brown precipitate was collected, washed (ethanol/water), and dried under vacuum at 80 °C for 5 h. The NiFe2O4/NC nanocomposite was synthesized through a simple polymerization reaction of dopamine. To do this, 0.4 g as-prepared NiFe2O4 was dispersed in 50 mL Tris (tris(hydroxymethyl) aminomethane)-buffer (TrisHCl, 10 mM, pH 8.5) by ultrasonication for 30 min to form a suspension. Next, 0.08 g dopamine (20% mass fraction of dopamine hydrochloride to NiFe2O4) was added to the mixture under stirring. The mixture was subjected to continuous magnetic stirring at room temperature for 24 h for the polymerization of dopamine. Afterwards, the product was centrifuged, washed 3 times with deionized water, and then dried overnight. Finally, the sample was calcined at 500 °C at a heating rate of 5 °C min-1 for 6 h under Ar atmosphere to obtain NiFe2O4/NC20 nanocomposite. NiFe2O4/NC10 and NiFe2O4/NC30 samples were similarly prepared, but with a mass fraction of dopamine hydrochloride to NiFe2O4 that was 10% and 30%, respectively.

2.2 Structural characterization The obtained products were characterized by X-ray diffraction (XRD; Rigaku D/MAX 2550 V/PC). The patterns were collected between 10 and 100° (2θ) at a scan rate of 10.0° 2θ/min. Raman spectra were recorded on a Micro-Raman (Renishaw) system with an Ar laser (514.5 nm). The X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG Scienta R3000 spectrometer with Al Ka (1486.6 eV) as the X-ray source. The thermal stability of the products was investigated using a TGA Q500 analyzer under an air-flow with a heating rate of 10 °C/min from room temperature to 800 °C. The morphology of the samples was investigated by 8

transmission electron microscopy (TEM; Tecnai G2 S-Twin F20), by scanning electron microscopy (SEM; Jeol JSM-6700F) and focused ion beam-scanning electron microscopy (FIB-SEM; Helios NanoLab 600i). The CHN elemental analysis was performed on an Elementar Vario MICRO CUBE elemental analyzer.

2.3 Electrochemical measurements Working electrodes were prepared by mixing a slurry containing 70% active material, 20% super P conductive additive, and 10% CMC-SBR (CMC:SBR =1:1 by weight ratio) binder on a copper current collector and then cut into square with the area of 0.64 cm2 and dried in a vacuum oven at 120 °C for 12 h. The loading mass of the active material was about 1-2 mg cm-2. The counter electrode and working electrode were separated by Celgard 2320 membrane. The electrolyte was a solution of 1 mol L-1 LiPF6 (lithium hexafluorophos-phate) in EC (ethylene carbonate) and DMC (dimethyl carbonate) at a 3:7 volume ratio. The batteries were assembled in an argonfilled

glove

box.

Galvonostatic

charge–discharge

measurements

were

performed on a LAND-2100 (Wuhan, China) battery tester for voltages between 0.01-3.0 V versus Li/Li+. The charge/discharge specific capacities were calculated for the total mass of NiFe2O4 and carbon. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed on a Bio-Logic VSP multichannel potentiostatic–galvanostatic system. The impedance spectra were recorded by applying an alternating-current voltage of 5 mV in the frequency range from 1 MHz to 5 mHz. All tests were carried out at room temperature.

9

Results and discussions 2.4 The phase and morphology evolution of nano-octahedral NiFe2O4 and NiFe2O4/NC nanocomposites. In the synthesis, nickel (II) nitrates and iron (Ш) nitrates were used as the metal particle precursors at a molar ratio of 1:2. The pH value was adjusted to 11 by addition of potassium hydroxide solution. Time-dependent experiments were performed to investigate the phase evolution of as-prepared NiFe2O4 nano-octahedrons. Fig. 1 shows the XRD patterns, SEM, and TEM images of the intermediate products obtained at different reaction times. In the first part (10 min) of the hydrothermal reaction, the sample was almost amorphous with irregular shapes and some needle products scattered around and on the top (Fig. 1b). The phases were identified by XRD as a mixture of poor crystallinity of goethite α-FeOOH (labelled as ▲, PDF No.29-0713) and Ni(OH)2 (labelled as ●, PDF No.38-0715). The formation of goethite α-FeOOH with its needle structure makes sense based on previous reports [58,59]. The non-distinct morphology of nickel hydroxide in the SEM images may be due to surface covering by α-FeOOH. Very little changes in phase components or morphology (Fig. 1c) occurred after 30 min reaction, except for enhanced diffraction peaks of α-FeOOH and Ni(OH)2. As the reaction time was extended to 60 min, more of the needle phase was present, as seen in Fig. 1d. The XRD pattern showed an obvious phase transformation from α-FeOOH and Ni(OH)2 to α-FeOOH and NiFe2O4 (labelled as □, PDF No. 54-0964) due to the reaction of local α-FeOOH with Ni(OH)2 to produce original nickel ferrite crystallites. The disappearance of the diffraction peaks corresponding to Ni(OH)2 in this step can be ascribed to the 10

overwhelming higher crystallinity of goethite and nickel ferrite. From the TEM imaging in Fig. 1e, three kinds of morphology are present: a majority needle phase, small cubic or spherical particles, and big irregular aggregations. The needle crystal lattice fringe showed a spacing of 0.230 nm in the highresolution (HRTEM) images, confirming that the structure is goethite (α-FeOOH), based on the (200) plane of orthorhombic α-FeOOH. The clear 0.253 nm interplane distance corresponding to the (311) crystal plane cubic NiFe2O4 suggests the formation of crystalline NiFe2O4 nanoparticles as well, consistent with the XRD result. The attempt to determine HRTEM structural information of the big aggregation failed due to severe conglomeration and bad crystallinity. Consistent with the gradual evolution of NiFe2O4 diffraction peaks in the XRD patterns as the reaction time extended to 5 h and 15 h, more visible nickel ferrites crystals and fewer needle α-FeOOH are evident in Fig. 1f and 1g, until pure nickel ferrites nano-octahedrons form after 22 h. Again, this is consistent with the XRD patterns. The hydrothermal reaction can be described as follows: Ni2+ + 2OH- → Ni(OH)2

(1)

Fe3+ + nOH- → Fe(OH)n3-n (n>3) → α-FeOOH

(2)

α-FeOOH + Ni(OH)2 → NiFe2O4

(3)

The shape evolution of nano-octahedral crystal with cubic structure in alkali media (pH 11) is governed by the preferential crystal growth which is favored along <100> rather than along <111> [60,61]. The preparation of the NiFe2O4/NC nanocomposite is achieved by in situ polymerization of dopamine on the surface of NiFe2O4 nanooctahedrons followed by carbonization of PDA at high temperature in

11

inert atmosphere. The overall synthesis procedure is depicted schematically in Scheme 1. 2.5 Characterization of NiFe2O4 and NiFe2O4/NC The X-ray diffraction (XRD) patterns of the obtained NiFe2O4 and NiFe2O4/NC20 are shown in Fig. 2. The major identified peaks at 2θ=18.39°, 30.42°, 35.77°, 37.37°, 44.22°, 51.47°, 57.32°, 63.07°, 71.52°, 74.42°, 79.72°, 87.67°, 90.47° and 95.23° correspond to the (111), (220), (311), (222), (400), (422), (511), (440), (620), (533), (444), (642), (731), and (800) planes of a cubic spinel NiFe2O4 structure, which can be perfectly indexed to PDF card no. 54-0964. No peaks of metal hydroxides and metal oxides were observed, insuring the phase purity of the nickel ferrites. The good crystallinity of the product was exhibited by the strong peak intensity and narrow peak width. From the spectrum, no diffraction peaks corresponding to graphitic carbon were observed, demonstrating the low degree of crystallinity of carbon, further confirmed by Raman spectroscopy [62,63]. Fig. S1 shows the XRD patterns of the NiFe2O4/NC10 and NiFe2O4/NC30 nanocomposites. All the diffraction peaks in NiFe2O4/NC10 are consistent with spinel NiFe2O4 without any secondary or parasitic phase peaks. However, the characteristic peaks of FeNi3 are observed in the XRD pattern of NiFe2O4/NC30, probably due to the reduction of the NiFe2O4 by the formed carbon materials. The Raman spectra of NiFe2O4/NC10, NiFe2O4/NC20, and NiFe2O4/NC30 are shown in Fig. S2, ESI†. The D band (defects/disorder-induced modes) at 1360 cm-1 and G band (in-plane stretching tangential modes) at 1587 cm-1 are observed, indicating the existence of carbon component in all the products. The X-ray photoelectron spectroscopy (XPS) spectra of NiFe2O4 and NiFe2O4/NC20 are shown in Fig. 3a. The XPS survey spectra of NiFe2O4 confirmed the presence of Fe and Ni in the nanocomposite. The peak at around 399 eV was 12

ascribed to N 1s in the NiFe2O4/NC20 survey spectra, suggesting the presence of N in the NiFe2O4/NC20 samples. The high-resolution N1s XPS spectrum of the NiFe2O4/NC20 (Fig. 3b) showed two component peaks at the Binding Energy (BE) of 400.2 and 398.5 eV, corresponding to C-N (pyrrolic nitrogen) and C=N (pyridinic nitrogen), respectively [64,65]. The XPS survey spectrum indicated that

polydopamine

was

transformed

into

nitrogen-doped

carbon

by

carbonization, which is beneficial for Li+ transport in the interface due to defects in the carbon caused by nitrogen doping [66-68]. Fig. S3 shows the TGA profiles of the as-prepared NiFe2O4 and NiFe2O4/NC samples under a flow of air. Compared with NiFe2O4, NiFe2O4/NC10 and NiFe2O4/NC20 showed two weight loss processes. The first step starts at 30 °C and goes to 300 °C, and is mainly due to the evaporation of surface absorbed water and/or surface hydroxyls in the samples. The TG curve also shows major weight loss over a temperature range of 300-800 °C, due to the combustion of the amorphous carbon coating. The results show that there is 4.03 wt% and 4.79 wt% of carbon in the NiFe2O4/NC10 and NiFe2O4/NC20 nanocomposites, respectively. Notably, there is an unsteady weight increase in the temperature range of 200-800 °C in the TG curve of NiFe2O4/NC30, possibly due to oxidation reactions of FeNi3 to Fe2O3 and NiO, along with the gradually combustion of carbon (6.80 wt% measured from CHN analysis), which can be further confirmed by powder XRD pattern of FeNi3/C calcined at 800 °C in air shown in Fig. S4. From TG test, 59.1 wt% of FeNi3 is calculated in NiFe2O4/NC30. The morphologies of the samples are examined by SEM and TEM. Fig. 4 indicates that the as-prepared NiFe2O4 possesses uniform octahedrons with 13

uneven sizes in the range of 100-300 nm. This relatively broad size distribution might be caused by overlap between the nucleation process and the growth process [69]. The SEM observations shown in Fig. 4b suggest that NiFe2O4/NC20 maintains its nanooctahedral configuration after N-doped carbon wrapping, indicating a homogenous distribution of the remaining carbon on the surface of the particles. For comparison, we also recorded the SEM images of NiFe2O4/NC10 and NiFe2O4/NC30 (Fig. S5, ESI†). The formation of a complete carbon coating was confirmed by TEM analysis of the sample, as illustrated in Fig. 5. The non-carbonaceous NiFe2O4 exhibited three kinds of lattice fringe spacing of 0.481 nm, 0.253 nm, and 0.295 nm, respectively, which are assigned to the (111), (311) and (220) planes of the cubic structural NiFe2O4, as shown in Fig.5 (b). The TEM images of NiFe2O4/NC20 (Fig. 5c and 5d) have bright and the dark areas, showing that the NiFe2O4 octahedrons are completely encapsulated by a carbon layer with an exterior shell thickness of about 10 nm. Fig. 5(d) shows the HRTEM image of NiFe2O4/NC20. Due to the effect of the surface carbon layer, the image of interior core part only reveals lattice fringe spacing of 0.481 nm of nickel ferrite. Additionally, we captured TEM images of NiFe2O4/NC10 and NiFe2O4/NC30 samples (Fig. S6, ESI†). Two TEM images (Fig. S6b and 6d) showed that a carbon layer 7 nm and 12.1 nm in thickness formed on the surface of the nanoparticles, due to the carbonization of polydopamine. In addition, the TEM images of NiFe2O4/NC30 show many nanoscaled FeNi alloy particles (dark dots) with an average size around 30 nm that are well-dispersed and confined within the carbon matrix, in agreement with the XRD analysis.

14

The cyclic voltammetry (CV) profiles of the NiFe2O4/NC material for the first three cycles for 0.01-3.0 V at a scan rate of 0.1 mV s-1 are shown in Fig.6 (a). The first cathodic scan differs from the subsequent scans. In the first cathodic sweep, the intensive peak at 0.5 V corresponds to the reduction of Ni 2+ and Fe3+ to Ni0 and Fe0 and an irreversible reaction of SEI that was related to the decomposition of the electrolyte [30,32]. The electrochemical reactions of the first discharge reaction can be expressed by equation (4): NiFe2O4 + 8e- + 8Li+ → Ni + 2Fe + 4Li2O

(4)

In the subsequent cycles, the cathodic peak down-shifted to ~0.9 V and the anodic peak was present at ~1.7 V, attributed to the oxidation of the Fe 0 and Ni0 into Fe3+ and Ni2+, respectively. The CV curves in the subsequent cycles almost completely overlapped, indicating excellent cyclic stability. The proposed reaction mechanism is as follows: Ni + Li2O ↔ NiO + 2Li+ + 2e-

(5)

2Fe + 3Li2O ↔ Fe2O3 + 6Li+ + 6e-

(6)

Galvanostatic charge-discharge cycling involves charging and discharging the cell at a constant current rate of 100 mA g-1 and a voltage range of 0.01-3.0 V to analyze the electrochemical properties of the cell. The voltage vs capacity profiles of both NiFe2O4 and NiFe2O4/NC20 are shown in Fig. 6(b) and 6(c). The charge-discharge voltage patterns correspond well to the CV curves. The NiFe2O4/NC20 material provides a much higher initial charge/discharge capacity of 1086/1383 mAh g-1 compared with untreated NiFe2O4 of 870/1105 mAh g-1. There are large irreversible capacity losses and higher initial discharge capacity for both electrodes in the first cycle. This can be attributed to (i) kinetic limitations of reactions 4-6 [23] and (ii) the formation of SEI during the 15

first discharge process, consuming extra Li-ions [23,29]. In addition, some of the coin cells evidently suffered from capacity fading in the first few cycles. This might be a result of the volume variations and crystal structure modifications during cycling, which is observed for many metal oxides undergoing conversion and alloying reactions [30,31]. Nevertheless, the higher initial capacities of the NiFe2O4/NC20 composite relative to the untreated NiFe2O4 may be due to the buffering and conducting effects of the carbon layer. Fig. 6(d) shows the cycling performance of NiFe2O4/NC20 and NiFe2O4 at current densities of 100 mAh g-1. The capacity of NiFe2O4/NC20 slightly increased, mainly due to the gradual activation of the carbon contained in the material. It eventually delivered a reversible capacity of 1297 mAh g-1 after 50 cycles, suggesting excellent cycle performance of the NiFe2O4/NC20 electrode. However, this capacity is much higher than the theoretical capacity of NiFe2O4, this is mainly resulting from 1) the interfacial storage of Li+ ions which profited by the carbon coating and can general found in metal oxide materials [32,33], 2) the reversible formation of a polymeric gel-like film that originated from kinetic activation in the electrode that provide extra capacity [12,13,70]. The NiFe2O4 electrode exhibited a rapid decrease in capacity, and showed a reversible capacity of only 472 mAh g-1 after 50 cycles. The rate performance was investigated to further evaluate the electrochemical performance of the NiFe2O4/NC20 composite electrode. As shown in Fig. 6(e), the cells were cycled at various current densities from 0.1 A g-1 to 2 A g-1 and the results indicate that the rate capability of NiFe2O4 was significantly improved by carbon coating. The NiFe2O4/NC20 composite can still deliver a reversible capacity of around 630 mAh g-1 at 2 A g-1, but the untreated NiFe2O4 at this current density has a 16

capacity of only 205 mAh g-1. The cycling performance of NC material has also been carried out and displayed in Fig. S7, ESI†. The high rate cycling performance of the NiFe2O4/NC20 composites was tested at a current density of 500 mA g-1. As shown in Fig. 7, the hybrid electrode showed an initial discharge specific capacity of 1250 mAh g-1. Notably, after a few cycles, the discharge capacity decreased over about 50 cycles. At that point, it exhibited a relatively stable state that was maintained at approximately 750 mAh g-1. Interestingly, the capacity showed a gradually rise again after cycling for about 100 cycles. This phenomenon is widely observed in TMO-based anodes and is generally ascribed to the reversible formation of a polymeric gel-like film that originated from kinetic activation in the electrode [12,13,70]. The specific discharge capacity of this composite remained at 1204 mAh g-1 over 300 cycles. The good cycling stability and rate performance of NiFe2O4/NC20 is likely due to the following factors: (1) The introduction of conductive carbon layer acts both as a skeleton to prevent the pulverization of the electrode and a buffer to accommodate the large volume changes that occur during the repeated charge-discharge cycles (2) The high electronic conductivity of the carbon layer allowed rapid and sufficient electrochemical reactions at high current densities. The structural stability of NiFe2O4/NC20 can also been cnfirmed by the TEM (Fig. S8, ESI†) images taken from the electrode cycled at 500 mA g-1 for 100 cycles. As we can see, there still some wellpreserved nanooctahedrons retain after cycled for 100 times at a high current rate, indicating its structural stability. The effects of different carbon content in NiFe2O4 material on electrochemical performance were also investigated, as shown in Fig. S9. The 17

specific capacity was not significantly different for the NiFe2O4/NC10 and NiFe2O4/NC20.

The

results

further

support

the

improvement

of

the

electrochemical performance of NiFe2O4 due to the incorporation of N-doped carbon. However, the capacity declined for the NiFe2O4/NC30 sample. This is likely related to the conversion of NiFe2O4 into FeNi alloy by reduction and the presence of the electrochemical-inactive phase FeNi alloy in the NiFe2O4/NC30. To test this, the electrochemical performance of FeNi3/C (prepared as described in the supporting information) was investigated. As shown in Fig. S10, both the cycling and rate performance of FeNi3/C were much lower than the NiFe2O4 samples. The FeNi3/C sample only maintained a capacity of about 130 mAh g-1 at a current density of 100 mA g-1 after 100 cycles. The rate performance is shown in Fig. S10(b), and the specific capacity decreased with current density and reached only about 90 mAh g-1 at a current of 1 A g-1. Therefore, this poor electrochemical performance of FeNi3/C may also explain the decline in capacity of NiFe2O4/NC30. To further investigate the effects of carbon coating on the NiFe 2O4 nanooctahedron material, EIS measurement was performed after the 4th charge to 3.0 V when cycled at 100 mA g-1, as seen in Fig. 8(a). The Nyquist plots were fitted by an equivalent circuit, as shown in the Fig. 8(c), where Re represents the electrolyte resistance, Rf denotes the SEI layer resistance, and Rct represents the charge transfer resistance. The fitting results are presented in Table 1. The NiFe2O4/NC20 shows a smaller Rct, which explains the better cycling stability and rate capability compared to untreated NiFe2O4. The EIS test of NiFe2O4/NC20 sample after 50th cycle at 100 mA g-1 at different voltages have also been carried out as shown in Fig. 8(b). The impedance parameters in the 18

charged to 3.0 V state of NiFe2O4/NC20 only slightly higher than that of the 4th cycle, which indicates its good Li-ion kinetics [31,71]. In addition, it is easy to find that the Rct resistance values during the discharge process show an increasing trend from 3.0 to 0.01 V and a decreasing trend during the charge process to 3.0 V, which is mainly caused by conversion reactions [72,73].

4 Conclusions NiFe2O4 and carbon-coated NiFe2O4 with octahedron morphology were successfully synthesized by a facile hydrothermal process and carbonization of polydopamine. The gradual evolution of needle-like goethite and irregular nickel hydroxide allows the ultimate formation of NiFe2O4 nano-octahedrons. As a potential anode material for LIBs, the NiFe2O4/NC composite exhibits improved cycling stability and rate capability compared with pure, untreated NiFe2O4. This improvement may be due to the strong framework, conductivity network, and increased electrical conductivity derived from the carbon layer. This improved electrochemical performance demonstrates that the NiFe2O4/NC nanohybrid is a promising anode material for LIBs.

Acknowledgements This work was supported by grants from National Natural Science Foundation of China (No. 21401070, No.21201073), S&T Development Program of Jilin Province (No. 20140520078JH, No. 20140101115JC, 20150204030GX）

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Figure Captions Fig. 1. (a) XRD pattern of the growth process of NiFe2O4 at different reaction times. FIB-SEM images of samples after different hydrothermal times of (b) 10 min, (c) 30 min, (d) 60 min, (f) 5 h, and (g) 15 h. (e) TEM images of the precursors after 60 min hydrothermal treatment. Scheme 1. Schematic illustration of the possible formation mechanism of NiFe2O4. Fig. 2. XRD patterns of NiFe2O4 and NiFe2O4/NC20 nanoparticles. Fig. 3. (a) XPS survey spectra of NiFe2O4 and NiFe2O4/NC20 (b) the high-resolution N1s XPS spectrum of the NiFe2O4/NC20. Fig. 4. Scanning electron microscopy (SEM) images of (a) NiFe2O4, (b) NiFe2O4/NC20. Fig. 5. (a) TEM images of NiFe2O4; (b) HRTEM images of NiFe2O4; (c) TEM images of NiFe2O4/NC20; (d) HRTEM images of NiFe2O4/NC20.

25

Fig. 6. (a) CV curves of the NiFe2O4/NC20 electrode for 0.01–3.0 V at 0.1 mV s–1. Discharge–charge curves of the (b) NiFe2O4 and (c) NiFe2O4/NC20 electrodes at 100 mA g–1. Cycling and rate capabilities of the (d) NiFe2O4 and (e) NiFe2O4/NC20 electrodes. Fig. 7. Cycling performance of the NiFe2O4/NC20 composites measured at a current density of 500 mA g-1. Fig.8. Nyquist plots of the NiFe2O4 and NiFe2O4/NC20 samples after the 4th charge to 3.0 V (a), NiFe2O4/NC20 after 50th cycle at different voltage (b) and Equivalent circuit used for fitting the impedance spectra (c).

Table 1. Calculated impedance results of the NiFe2O4 and NiFe2O4/NC20 samples.

26

Fig. 1. (a) XRD pattern of the growth process of NiFe2O4 at different reaction times. FIB-SEM images of samples after different hydrothermal times of (b) 10 min, (c) 30 min, (d) 60 min, (f) 5 h, and (g) 15 h. (e) TEM images of the precursors after 60 min hydrothermal treatment.

Scheme 1. Schematic illustration of the possible formation mechanism of NiFe2O4.

27

Fig. 2. XRD patterns of NiFe2O4 and NiFe2O4/NC20 nanoparticles.

28

Fig. 3. (a) XPS survey spectra of NiFe2O4 and NiFe2O4/NC20 (b) the high-resolution N1s XPS spectrum of the NiFe2O4/NC20

29

Fig. 4. Scanning electron microscopy (SEM) images of (a) NiFe2O4, (b) NiFe2O4/NC20.

30

Fig. 5. (a) TEM images of NiFe2O4; (b) HRTEM images of NiFe2O4; (c) TEM images of NiFe2O4/NC20; (d) HRTEM images of NiFe2O4/NC20.

31

Fig. 6. (a) CV curves of the NiFe2O4/NC20 electrode for 0.01–3.0 V at 0.1 mV s–1. Discharge–charge curves of the (b) NiFe2O4 and (c) NiFe2O4/NC20 electrodes at 100 mA g–1. Cycling and rate capabilities of the (d) NiFe2O4 and (e) NiFe2O4/NC20 electrodes.

32

Fig. 7. Cycling performance of the NiFe2O4/NC20 composites measured at a current density of 500 mA g-1.

33

c Fig.8. Nyquist plots of the NiFe2O4 and NiFe2O4/NC20 samples after the 4th charge to 3.0 V (a), NiFe2O4/NC20 after 50th cycle at different voltage (b) and Equivalent circuit used for fitting the impedance spectra (c).

34

Table 1. Calculated impedance results of the NiFe2O4 and NiFe2O4/NC20 samples. Samples

Cycle times

Re (Ω)

Rf (Ω)

Rct (Ω)

NiFe2O4

4

15.02

8.49

6.45

NiFe2O4/NC20

4

9.48

5.21

3.93

NiFe2O4/NC20

50

2.782

9.965

18.89

35