Constructing enhanced pseudocapacitive Li+ intercalation via multiple ionically bonded interfaces toward advanced lithium storage

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Constructing enhanced pseudocapacitive Liþ intercalation via multiple ionically bonded interfaces toward advanced lithium storage Zhikang Liu a, 1, Jing Huang b, 1, Bin Liu a, Dong Fang a, Tao Wang a, Quanling Yang a, Lijie Dong a, Guo-Hua Hu c, Chuanxi Xiong a, b, * a b c

State Key Laboratory of Silicate Materials for Architectures, School of Materials Science and Engineering, Wuhan University of Technology, 430070, China School of Materials Science and Engineering, Wuhan Textile University, Wuhan, 430070, China Laboratory of Reactions and Process Engineering (LRGP, CNRS UMR 7274), CNRS-University of Lorraine, 1 rue Grandville, BP 20451, 54001, Nancy, France

A R T I C L E I N F O

A B S T R A C T

Keywords: Fast charging Pseudocapacitance effect Fe3O4 Ionic bond Lithium-ion batteries

For lithium ion batteries, fast charging technology is one of the most important research topics in current society. Transition metal oxides show broad application prospects due to their obvious pseudocapacitive effect. We choose Fe3O4 as the template to design the enhanced pseudocapacitance effect considering of its high theoretical specific capacity and superior structural adjustability. The nanohybrid materials with multiple ionically bonded interfaces accumulated the giant pseudocapacitive effect to obtain superior specific capacity. As a result, the nanohybrid shows a ranking specific capacity over 800 mAh g1 at 1 A g1 for 1000 cycles as well as excellent rate capabilities. The existence of interfacial pseudocapacitive effect was confirmed by the kinetic analysis calculating from CV curves. Furthermore, the computation simulation based on the density functional theory (DFT) highly corresponded with our viewpoint about interfacial lithium storage. Our findings pave a novel path for designing novel electrode materials and could further advance the development of lithium-ion batteries with long-term stability and superhigh capacity.

1. Introduction With the growing markets for electric vehicles and hybrid electric vehicles due to the unavoidable environmental pollution from the conventional motor vehicles. The green energy is one of the predominant areas of research in current society [1,2]. Rechargeable lithium batteries, owning an unmatchable combination of high energy and power density, are considered as the most promising energy storage system, since the lithium has the lowest reduction potential and one of the smallest ionic radii of any single charged ion. Besides, they will significantly reduce greenhouse gas emissions [3–5]. Until now, fast charging technologies for lithium ion batteries has become one of the core technologies and draw extensive interests. As we know, the high migration rate of electronics and ions transfer within the battery (the electrode, electrolyte and their interfaces) is crucial for achieving the fast charge/discharge rate [6, 7]. However, both of the volume changes on the electrode and electrode-electrolyte chemical reactions can cause an irreversible capacity. Furthermore, the decomposition of electrolyte leads to the

formation of solid electrolyte interface (SEI) layer. The diffusion of lithium ions is limited due to the sluggish Liþ transfer through the SEI layer at high currents, which impedes the development of fast charging technology [8,9]. In view of the problems mentioned above, pseudocapacitive materials were considered as a promising candidate due to the potential capacitive behaviors. It is a complementary form of electric double layer capacitors and batteries, which are used to describe the properties of an electrode that behaves like a capacitor with fast charge/discharge feature in its electrochemical signature [10–13]. Pseudocapacitive materials depend predominantly on the surface faradaic electron transfer to metal centers, which is made possible by the intercalation or adsorption of charge-compensating lithium ions [14,15]. A faradaic charge-transfer with no crystallographic phase change can proceed when ions intercalate into the tunnels or layers of a redox-active material [16]. This is because the reactions that occur at the surface, without limitation from the surface or solid-state diffusion, lead to the release of greater energy density in combination with increased power density [17–19]. Therefore,

* Corresponding author. State Key Laboratory of Silicate Materials for Architectures, School of Materials Science and Engineering, Wuhan University of Technology, 430070, China. E-mail address: [email protected] (C. Xiong). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ensm.2019.08.026 Received 2 May 2019; Received in revised form 18 July 2019; Accepted 20 August 2019 Available online xxxx 2405-8297/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Z. Liu et al., Constructing enhanced pseudocapacitive Liþ intercalation via multiple ionically bonded interfaces toward advanced lithium storage, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.08.026

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the pseudocapacitive effects can enable electrode materials to exhibit excellent high-rate capability. Metal oxides, such as RuO2 [20], TiO2 [11], MoO3 [21], TiNb2O7 [22] and Fe3O4 [23], have shown promising properties with additional reversible capacity beyond their theoretical capacity as replacements for the currently employed graphite anodes, which greatly broadens the material selection for high-performance LIBs [24,25]. Graphite anodes with low Li-intercalation potential, is poorly matched with the lowest unoccupied molecular orbital (LUMO) of the organic liquid-carbonate electrolyte. Transition metal oxides have tunable reaction potentials relying on the strength of the ionic bond between the transition-metal cations and the anionic species, which can ensure the better safety of battery by avoiding the problem of lithium dendrite formation [8,26,27]. Meanwhile, the flexible structural adjustability of transition metal oxides will benefit for designing the controllable lithium storage interface. Recently, Fe3O4 is considered as a promising anode material owing to its low-cost, non-toxicity, high energy density and high theoretical energy capacity (926 mAh g1). Unfortunately, the large volume change of Fe3O4 during battery operation can cause the pulverization and exfoliation of active materials on the current collector, and the inherent lowconductivity of Fe3O4 also hampers the transports of Liþ and electrons in the battery, characters that both lead to the poor electrical performance [28–30]. To solve these issues of Fe3O4, the combination of nanoscale and carbon-encapsulated multi-layered structures remains the dominant approach. For example, Ding and co-works proposed rGO/Fe3O4/AC nanocomposite to embed Fe3O4 nanoparticles into the rGO substrate, which displayed a superior electrochemical performance with a considerable pseudocapacitive effect [31]. However, even though remarkable electrochemical performance has been achieved, this stream is difficult to overcome the intrinsic disadvantages, such as weak cycle stability or unsatisfactory diffusion kinetics of ions, severely hindering their practical applications in LIBs. Inspired by the pseudocapacitive effect of Fe3O4, we report a multiphase nanohybrid material of QFe3O4/SCNT/f-PANI based on the various ions. The obtained nanohybrids shown multi-layered ionic bonds interfaces, which significantly improve the ion diffusion behavior to maximize the pseudocapacitive effect of Fe3O4. Meanwhile, the incorporation of high viscosity flowable PANI ensure that the nanohybrids can be pasted on the surface of copper foil [32]. Then we demonstrated that a higher specific capacity and longer cycling life can be achieved in LIBs via the multiple ionically bonded interfaces, indicating that the method could be a promising enhancer of pseudocapacitive effect.

SCNT: 1.7 g p-aminobenzene sulfonic acid was dissolved into 10 mL sodium hydroxide solution (5 wt%), and 10 mL sodium nitrite solution (7 wt%) was added into the pre-mixed solution. Then 21 mL 1 M hydrochloric acid was added dropwise into the above solution at ice bath, till the color turned into creamy yellow, and kept stirring for 0.5 h on standby. Whereafter, 0.2 g commercialized CNT was added into the anterior p-diazobenzene sulfonic acid solution and stirred for 2 h. Finally, the product was collected by pumping filtration.

QFe3O4/SCNT nanohybrids: Both 0.5 g QFe3O4 and 0.15 g SCNT were dispersed into 100 mL deionized water and stirred for 5 h. Then the nanohybrids were obtained by filter. f-PANI: 14.55 g docosyl toluene sulfonic acid (DTSA) was dissolved into 100 mL xylene. In the case of ice bath, 5 mL aniline aqueous solution was injected into the mixture and stirred for 0.5 h. Naturally, preprepared 20 mL ammonium persulphate aqueous solution was dropped into the dispersion within 1 h and kept reacting for 8 h. After that, the resulting mixture was demulsified by additional dichloromethane. From now on, the solution was dialyzed for 72 h to remove the unreacted substance and the oligomer. In the end, the f-PANI was obtained by extraction with the dichloromethane and dried in the oven.

2. Experimental section 2.1. Sample preparation Materials: Ferric trichloride hexahydrate, ferrous chloride tetrahydrate, ammonium hydroxide, p-aminobenzene sulfonic acid, sodium hydroxide, sodium nitrite, hydrochloric acid, xylene, aniline and ammonium persulphate were purchased from Aladdin. The electrolyte (1.0 M LiPF6 in EC: DMC: EMC ¼ 1:1:1 vol%) was purchased from DoDoChem. Carbon nanotube was purchased from Chengdu Organic Chemicals Co. Ltd. (CH3O)3Si(CH2)3Nþ(CH3)2-(C18H37)Cl in methanol (40%) was purchased from Gelest. DTSA was purchased from Nanjing Shengxiong Chemical Co. Ltd. Dialysis tubing was purchased from USA Viskase with a molecular weight cut-off of 8000 Da. QFe3O4: 24.3 g ferric trichloride hexahydrate and 9.94 g ferrous chloride tetrahydrate were dissolved into 100 mL deionized water, then 30 mL ammonium hydroxide was added into the solution and kept at 60  C for 0.5 h under the nitrogen atmosphere to prepare the magnetite particles. 0.2 g Fe3O4 was dispersed into 100 mL deionized water with ultrasound for 0.5 h. Subsequently, 3.6 mL (CH3O)3Si(CH2)3Nþ(CH3)2(C18H37)Cl was injected into the dispersion and aged for 24 h in the table concentrator. At last, the precipitate was collected by magnetic washing.

PANI-eigenstate (PANI-EB): The solid PANI was doped with HCl based on the above method, then the PANI-HCl was de-doped with the excess ammonia for 24 h, which was washed with alcohol and deionized water. Finally, the PANI-EB can be obtained by vacuum drying at 60  C. 2.2. First principles calculations Density function theory calculation were performed by using the CP2K package [33]. PBE functional [34] with Grimme D3 correction [35] was used to describe the system. Unrestricted Kohn-Sham DFT has been used as the electronic structure method in the framework of the Gaussian and plane waves method [36,37]. The Goedecker-Teter-Hutter (GTH) pseudopotentials [38,39], DZVPMOLOPT-GTH basis sets [36] were utilized to describe the molecules. A plane-wave energy cut-off of 500 Ry has been employed. All the simulation has been carried out within a 17.06  17.06  503 2

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4000, Perkin Elmer) was used to study the degradation behaviors in nonisothermal conditions. The samples were heated up from RT to 800  C at a heating rate of 10  C min1 in air. Electrochemical testing: The as-prepared QFe3O4/SCNT nanohybrids and f-PANI were mixed with acetylene black at the weight ratio of 6:2:2. The active electrode was prepared by doctor-blading the slurry onto a Cu foil collector to assemble into half cells, and the lithium foil as counter electrode. Galvanostatic charge-discharge tests and rate performance were conducted at various current densities (LANHE China). The cyclic voltammograms (CV) curves was recorded in 0.01–3.00 V at a scanning rate of 0.1 mV s1. AC electrochemical impedance spectra was tested at the frequency ranging from 10 kHz to 10 mHz with an AC signal of 5 mV in amplitude as the perturbation.

box under periodic boundary condition. On the Z direction, there is about 30 Å vacuum to decouple the interaction between the images. For the SCNT, it is composed of 168 carbon atoms with about 8.2 Å diameter nanotube and on top, it was chemically modified by Benzenesulfonic group. For the QFe3O4, we use Fe3O4 (100) surface which composed of 96 Fe and 128 O atoms, on top a Quaternary Ammonium is bonded via Fe–O bonds. For the f-PANI, we build a periodic Aniline to mimic the fPANI. The interaction energy between the two component which composed the interface is calculated two subsystems as (r0) and the separated by certain distances r. Eint ¼ EAB(r)  EAB(r0)

(1)

Where EAB(r0) is the total energy of the optimized interface structure (as shown in the Fig. without Li ion), EAB (r) is the energy of the two subsystems has not interaction at distance of r. A represent SCNT, and B represent QFe3O4 or f-PANI. The adsorption energy of the Li ion is calculated by: Eads ¼ ELi þ EAB  ELi/AB

3. Results and discussion The Fe3O4 nanoparticles used in this work were obtained by the coprecipitation of ferric trichloride hexahydrate, ferrous chloride tetrahydrate and ammonium hydroxide (Fig. S1). We firstly quaternized Fe3O4 by grafting a quaternary ammonium salt, i.e. (CH3O)3Si(CH2)3Nþ(CH3)2(C18H37)Cl, through the dealcoholization and condensation of the salt with the hydroxyl units on the surface of Fe3O4 nanoparticle (QFe3O4, Fig. 1a). Upon employing the spontaneous transference of the delocalized electrons in CNT to the p-diazobenzene sulfonic acid, we sulfonated CNT by grafting benzene sulfonic acid (SCNT, Fig. 1b). QFe3O4 and SCNT were then dispersed in deionized water, and the quaternary ammonium salt on the QFe3O4 surface was exchanged with the sulfonic acid group on the SCNT surface to form ionically bonded QFe3O4/SCNT nanohybrids (Fig. 1c). This SCNT still can construct an extended electronically conductive network by entanglement (Fig. S2), stemming from our sulfonation strategy which doesn’t shorten CNT. To proof a favorable electrostatic assembly of QFe3O4 with SCNT, the surface charges were investigated by zeta potential measurements at aqueous solution (Fig. 2a–b). The results show that QFe3O4 remain positive and the SCNT are negative in contrast, which provides the driving force for the assembly of QFe3O4 with SCNT under neutral

(2)

Where Eads is the adsorption energy, ELi/AB is the energy for the whole system, and ELi is the energy of Lithium. The more positive adsorption energy suggests more stable adsorption. 2.3. Characterizations methods XRD patterns were acquired by a D/Max-IIIA (Rigaku) diffractometer using Cu Kα radiation (λ ¼ 1.54 Å). IR measurements were made on a FTIR spectrometer (Thermo Nicolet Nexus) using KBr powder. UV–vis spectrum was recorded using a Shimadzu UV-2250 Spectrophotometer. Morphologies of f-PANI and QFe3O4/SCNT were examined using the scanning electron microscope (SEM, HITACHI S-4800) and transmission electron microscope (TEM, JEOL JEM-2010), respectively. Rheological properties were studied using an ARES-RFS rheometer (TA) at an angular frequency of ω ¼ 1 s1 and with a strain amplitude of 10% in the temperature range from 20 to 80  C. A thermogravimetric analyzer (TGA

Fig. 1. (a) The quaternization of Fe3O4. (b) The sulfonation of CNT. (c) The electrostatic self-assembly of QFe3O4 and SCNT to form QFe3O4/SCNT hybrid. (d) The polymerization and doping of PANI. (e) The schematic of ionically bonded QFe3O4/SCNT/f-PANI nanohybrids. 3

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Fig. 2. Zeta potentials of (a) QFe3O4 and (b) SCNT. (c) The FTIR of SCNTs, QFe3O4 and QFe3O4/SCNT. (d) Nitrogen adsorption and desorption isotherm of QFe3O4/ SCNT nanohybrids. (e) The corresponding pore diameter distribution.

Fig. 3b), We validated that the NPs within the heterostructures can be assigned to the pure magnetite phase. High-resolution TEM shows that the QFe3O4 NPs locate on the outer surface of SCNT. The lattice fringes of 0.25 and 0.34 nm correspond to the (311) plane of QFe3O4 and the (002) plane of SCNT, respectively (Fig. 3c). The result can also be verified by XRay diffraction (XRD; Fig. S3). The X-ray diffraction peaks at 18, 30, 36, 43, 54, 57 and 63 are all ascribed to QFe3O4, with the peak at 26 is associated with the formation of graphene layer of adjacent SCNT [40]. Additionally, energy dispersive x-ray (EDX) elemental mapping (Fig. 3d–g) reveals that the Fe and O elements are uniformly distributed in the carbon element, suggesting that QFe3O4 NPs embedded in the carbon matrix originated from SCNT. The carbon, iron, sulfur and nitrogen elements can also be authorized by energy dispersive spectrometer (EDS) mapping (Fig. 3h), manifesting that a lot of QFe3O4 NPs are anchored onto SCNT. Polyvinylidene fluoride with the non-functionalized linear chain cannot sturdily adhere to the high-capacity electrode [41–43]. Therefore, a superior binder with intrinsic electrical conductivity and mechanical strength is promising to improve the electron and ion transport in batteries [44]. The polaron-bipolaron transition peaks at 450 and 725 nm appeared in ultraviolet–visible spectrophotometer (UV–Vis; Fig. 3i), suggesting that f-PANI is the protonation state. The XPS spectrum (Fig. S5) showed a quinoid-imine peak at 399.6 eV and a positively charged nitrogen peak at 402.3 eV, indicating that some nitrogen has been transmuted into protonated nitrogen species. Both of the UV–Vis and XPS are consistent with the FTIR (Fig. S4). The XRD (Fig. 3j) displayed the typical diffraction of smectic A phases [45] and the TEM (Fig. 3k) image showed lattice fringes of 0.35 nm. This analysis

conditions [23]. That indicated a strong interaction between SCNT and QFe3O4 or f-PANI due to the electrostatic interaction. Meanwhile, the Fourier transform infrared spectroscopy (FTIR; Fig. 2c) was further used to verify the interaction. As we can see, the Si–O–Fe stretching band located at 916 cm1 also confirmed the modification of Fe3O4 by (CH3O)3Si(CH2)3Nþ-(CH3)2(C18H37)Cl. Comparing the sulfonic acid group of SCNT and QFe3O4/SCNT nanohybrids, we can see that the peak position from –SO3H had a certain degree of redshift (1034 and 1008 to 975 and 950, respectively), which demonstrated that the energy of the system became smaller and more stable because of the formation of ionic bonds. The f-PANI was obtained by the inverse emulsion polymerization of aniline and doped with docosyl toluene sulfonic acid (DTSA) (Fig. 1d). Then the ionically bonded QFe3O4/SCNT/f-PANI nanohybrids (Fig. 1e) were prepared by mixing f-PANI with the QFe3O4/SCNT nanohybrid. The microstructure of QFe3O4/SCNT nanohybrid was characterized using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Fig. 3a demonstrates that QFe3O4 nanoparticles (QFe3O4 NPs) are bonded within SCNT network, and micropores are enclosed by SCNT in the nanohybrids. This 3D porous characteristic of the nanohybrids was determined by N2 sorption analysis. The BrunauerEmmett-Teller surface area of the nanohybrids was calculated to be ca. 87 m2 g1 (Fig. 2c). The pore size distribution obtained through the BJH method was given in Fig. 2d, confirming the hierarchical porosity in the range of 2–3, 5–7 and 10–20 nm respectively. This porous structure is favorable for the access of electrolyte, the rapid diffusion of lithium ions and the accommodation of volume change of Fe3O4 NPs, characters that will be beneficial for a high capacity and excellent cycling stability. Through the use of selected area electron diffraction (SAED, inset of 4

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Fig. 3. (a) TEM image of QFe3O4/SCNT nanohybrid. (b) TEM image and the selected area electron diffraction of QFe3O4/SCNT nanohybrid. (c) High resolution TEM image of QFe3O4/SCNT nanohybrid. (d–g) TEM image and elemental mapping of QFe3O4/SCNT nanohybrid. (h) SEM and EDX images of QFe3O4/SCNT nanohybrid. (i) UV–Vis image of f-PANI and PANI-Eigenstate. (j) XRD of f-PANI. (k) TEM image of f-PANI.

Fe3O4 as the anode. Notably, the voltage plateau is rarely observed in the following discharge/charge curves, indicating that the introduction of a highly entangled SCNT network can reduce the polarization and is favorable of improving the coulombic efficiency of cell [46]. These curves show good symmetry with representative feature of supercapacitor [47]. The cyclic behaviors of QFe3O4/SCNT/f-PANI nanohybrid at various current densities were shown in Fig. 3b. Five conditioning cycles at 0.1 A g1 current density was applied to activate all cells to reach a coulombic efficiency over 90% before the subsequent tests. The values of the specific capacity values were calculated based on the mass of QFe3O4/SCNT/f-PANI. The electrode delivers an initial discharge specific capacity close to 1200 mAh g1 at a current density of 0.1 A g1 in Fig. 4a–b, which is much higher than its theoretical specific capacity of 926 mAh g1, which we ascribe to the disintegration of electrolyte and the formation of a solid electrolyte interface with QFe3O4 NPs. Most surprisingly, the specific capacity kept increasing from the 20th cycle until 450 cycles, at which a maximum capacity about 830 mAh g1 was

corresponds to observations under a polarized light microscope (Fig. S6). The liquid-like behavior of f-PANI was affirmed by the loss modulus (G00 ) compared with storage modulus (G0 ) in the temperature-dependent (Figs. S7–8). These results illustrated that our f-PANI presented the molecular and morphological characteristics with high electrical conductivity as well as the rheological behavior with excellent adhesive performance. In fact, this f-PANI exhibit a high ionic conductivity which increases with temperature (Fig. S9). Therefore, our f-PANI can be used as the participator of the ionically interfaces to maximize the pseudocapacitive effect. Indeed, the QFe3O4/SCNT/f-PANI nanohybrid electrode remains a continuous network structure with multi-layered pores (Fig. S10). The charge and discharge curves of the QFe3O4/SCNT/f-PANI nanohybrids were performed at a current density of 0.1 Ag1 and 1A g1 from 0 to 3 V, as shown in Fig. 4a. The first discharge curve presents a voltage plateau at ~0.8 V corresponding to the reduction of Fe3þ and Fe2þ to Fe0 via the conversion reaction Fe3O4 þ 8Liþ þ 8e → 3Fe0 þ 4Li2O. The voltage trends agree well with the typical characteristic when using 5

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Fig. 4. (a) Voltage profiles plotted at a current density of 1 A g1. (b) Charge/discharge capacity curves at 1 A g1. (c) Charge/discharge capacity at various rates. (d) Charge/discharge capacity curves at 10 A g1.

reached at a current density of 1 A g1. What we emphasize here is the great reversibility and capacity retention. Even after 1000 cycles, a stable capacity around 800 mAh g1 is achieved for the electrode. Meanwhile, a high coulombic efficiency indicates a relatively stable solid electrolyte interface layer formed on the electrode due to the interpenetration of fPANI and interfacial ionic bonding in QFe3O4/SCNT/f-PANI nanohybrid, which reduces the number of irreversible lithium storage sites. This everincreasing trend can be attributed to the delayed wetting of electrolyte into the ionically bonded nanohybrid electrode, leading to the prolonged activation [48,49]. As expected, our QFe3O4/SCNT/f-PANI nanohybrid electrode exhibited an extremely durable high rate capability, as displayed in Fig. 4c. The specific capacities were measured under a number of rates increasing from 0.1 to 8 A g1, and the ionically bonded nanohybrid electrode has a discharge capacity value of ~907, 726, 692, 576, 426 and 268 mAh g1 at 0.1, 0.5, 1, 2, 4 and 8 A g1, respectively. Moreover, when the current rate was finally returned to 1 A g1, a capacity of ~800 mAh g1 was achieved and maintained up to the 100 cycles. To obtain the fast charge and discharge performance, a higher current density of 10 A g1 with deep cycling was carried out (Fig. 4d). What is encouraging is that a specific capacity of ~250 mAh g1 was still remained after 1000 cycles, indicating its superior and stable cycling performance. According to the voltammetric behavior, Dunn et al. pioneered a novel approach to distinguish the capacitive and diffusion-controlled contributions to the total measured capacity [17]. The relationship of the observed current i with the scan rate υ follows a power-law equation: i ¼ kυα, where α and k are two adjustable parameters. For a typical diffusion-controlled lithium storage process, α ¼ 0.5. For a pseudocapacitive-controlled behavior via a faradaic redox reaction, α ¼ 1 [21]. To identify the pseudocapacitive charge of the QFe3O4/SCNT/f-PANI nanohybrid electrode, we measured the cyclic voltammograms (CV) of the cell with the ionically bonded nanohybrid as the active material. Fig. 5a shows the voltammetric behaviors of postcycle cell (1000 cycles at 1A g1) from 0.05 to 0.2 mV s1. It is found that these curves

display analogous shapes with wide peaks when the sweep rates are risen from 0.05 to 0.2 mV s1, indicating the minimum polarization at high rates [50]. Fig. 5b exhibits the relationship between peak current ip and the sweep rate υ. The obtained α-values for peak 1 and peak 2 are 0.89 and 0.87 respectively, demonstrating that this electrochemical process prefers to pseudocapacitive-controlled behavior. This behavior was also can be confirmed by the electrochemical impedance spectra (Fig. S11). To further quantify the capacitive contribution to the total current response through the CV curves at various sweep rates, the power-law equation can be rewritten as follows: iv ¼ k1υþk2υ0.5 or iv/ υ0.5 ¼ k1υ0.5þk2, where k1 and k2 are two adjustable values, and iv, k1υ and k2υ0.5 represent the total current response at a given potential V, the current due to faradaic redox reactions and the current due to diffusioncontrolled Liþ insertion process respectively. By ploting iv/υ0.5 vs. υ0.5 at different potentials, we can calculate the values of k1 and k2 from the slope and intercept (Fig. 5c). It is universally accepted that the pseudocapacitive-controlled proportion in total currents increases gradually with the increasing sweep rate. We have choosed the CV curve at a scan rate of 0.2 mV s1 for the capactive currents (red area) as compared with the total current (Fig. 5d), and the calculated proportion of the capacitance via faradaic redox reactions is 77%. The high pseudocapacitive-contribution may be assigned to the existing of a vairety of ionic interfaces and bigger specific surfaces of the ionically bonded nanohybrid. Fig. 6a is a schematic of the ionic bonds in the QFe3O4/SCNT/f-PANI nanohybrids. It should be noted that there exist a large number of micropores enclosed by SCNT and f-PANI in the nanohybrid. Faraday redox reaction can operate on the surface of micropores during the charge and discharge processes because electrolyte and lithium ions can penetrate into these micropores. There is no doubt that a substantial number of the pseudocapacitance will be generated by faraday redox reaction on the surfaces of micropores. It should be emphasized that there exist four þ   kinds of, i.e. “SCNT and QFe3Oþ 4 ”, “QFe3O4 and DTSA ”, “DTSA and fPANIþ” as well as “f-PANIþ and SCNT” ionic bond interfaces in the nanohybrid. It is believed that electrolyte and lithium ions can penetrate

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Fig. 5. (a) CV curves at various scan rates, from 0.05 to 0.2 mV s1. (b) Determination of the k-value using the relationship between peak current ip to sweep rate ν. (c) Plots of ν0.5 vs. iv/ν0.5 used for calculating constants k1 at different potentials. (d) Capacitive charge storage contributions at a scan rate of 0.2 mV s1.

into these interfaces. Therefore, faraday redox reaction can occur in these interfaces during lithiation/delithiation processes. It is certain that a large number of pseudocapacitance can be produced by faraday redox reaction in the interfaces. The accessional pseudocapacitance is the reason why the ionically bonded nanohybrid exhibit a superhigh capacity, which is twice as high as the theoretical capacity of Fe3O4. To gain more insight into how the ionic bond interfaces effectively enhanced the capacity, the density functional theory (DFT) simulation was also conducted. The optimized two systems are shown in Fig. 7a–b. The lithium adsorption energy is about 4.35 eV in between SCNT/QFe3O4 interface and 2.86 eV for SCNT/f-PANI interface, with which a clear conclusion can be drawn that these interfaces could benefit for the storage of Liþ

[51]. This result highly matches our analysis of the pseudocapacitance. In addition to the beneficial effect of the ionic bond at interface on the specific capacitance, it is also believed to play a critical role in the cycling stability. Firstly, the interface cracks, caused by faraday redox reaction in the processes of charging and discharging, can be self-repaired by the rebonding of positive and negative ions at the interfaces, ensuring the long-term stability of the nanohybrid electrode. Secondly, it is widely agreed that the insertion of Liþ will cause Fe3O4 NPs to pulverize and the collector structure to collapse during the process of charging and discharging, thus reduce the specific capacity and the cycling stability of LIBs when Fe3O4 acts as the active substance. The pulverized QFe3O4 pieces in the nanohybrid, however, can rebond with other sulfonic

Fig. 6. (a–b) Schematics of ionic bonds in QFe3O4/SCNT/FPANI nanohybrids: the pulverized QFe3O4 could rebond and readhere with SCNT and DTSA via ion exchange. 7

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Fig. 7. (a) The Li ion adsorption on SCNT surfaces contacting with QFe3O4 and (b) f-PANI. Color code: white is hydrogen, pink is lithium, black is carbon, red is oxygen, blue is nitrogen, yellow is sulfur, orange is silicon, brown is iron. (c) The interface structure for SCNT surfaces with QFe3O4 and f-PANI (d). Color code: white is hydrogen, black is carbon, red is oxygen, blue is nitrogen, yellow is sulfur, orange is silicon, brown is iron. The distance between two components is separated by distance r, at r0 is the optimized structure. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

capacity and excellent cyclic stability is attributed to the huge pseudocapacitance produced at the ionically bonded interfaces and the inherent self-repair ability of the ionically bonded nanohybrid respectively. We believe that the unique ionically bonded nanohybrid electrode with an ever-increased pseudocapacitive effect can be extended to other electrode materials for electrochemical energy storage and conversion applications.

groups in the SCNT or with DTSA anion in f-PANI. Therefore, the pulverized Fe3O4 pieces will be trapped within the coating layer (Fig. 6b), leading to excellent rate capability and long cycle life at high rates. To gain more insight into how the ionic bond interfaces effectively assembled, the DFT simulation was conducted. Due to the complexity of our system, we build two models to understand the interfacial property by simulating SCNT/QFe3O4 and SCNT/f-PANI. The interaction energy between SCNT/QFe3O4 is about 4.3 eV, while SCNT/f-PANI is about 1.5 eV. That indicates a strong interaction between SCNT and QFe3O4 or f-PANI due to the electrostatic interaction. The optimized two systems are shown in Fig. 7c–d. At the sametime, the electrical properties of QFe3O4/SCNT/ PVDF, QFe3O4/f-PANI and unmodified Fe3O4 also indicate the importance of ionic bond in hybrid system (Fig. S12). Thirdly, both the porous SCNT network and f-PANI framework have empty space to tolerate the large volume expansion of QFe3O4 during lithiation/delithiation processes. Fourthly, f-PANI as a conductive coating provides good electrical channels for Liþ and electrons. Finally, f-PANI coating can benifit for a stable SEI on the surface of the nanohybrid, which is confirmed by SEM images (Fig. S13). TEM images shows that QFe3O4 NPs still adsorb on the SCNT surface without losing their electrical contacts (Fig. S14).

Conflicts of interest There are no conflicts to declare. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant no. 51673154, 51173139, 51503159). The authors thank Yi Guo, Juan Yi and Junming Kang for some instrumental support and helpful discussions on the manuscript. Appendix A. Supplementary data

4. Conclusions

Supplementary data to this article can be found online at https://do i.org/10.1016/j.ensm.2019.08.026.

In conclusion, the ionically bonded QFe3O4/SCNT/f-PANI nanohybrids with multi-level ionic bond interfaces, construct the enhanced pesudocapacitive effect through the Liþ insertion. In the meantime, the stronge interaction between the various ions trap the pulverized Fe3O4 pieces within the coating layer to maintain the integrity of electrodes. As a consequence, the nanohybrid is a promising anode material with superior specific capacities over 800 mAh g1 at 1 A g1 based on the total electrode mass as well as excellent rate capabilities. The superhigh

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