polyoxometalate hybrid as cathode for improved rechargeable lithium ion batteries

Materials Today Energy 6 (2017) 53e64

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Self-assembled three-dimensional graphene/polyaniline/ polyoxometalate hybrid as cathode for improved rechargeable lithium ion batteries Lubin Ni a, *, Guang Yang a, 1, Chunyu Sun a, 1, Guosheng Niu a, Zhen Wu a, Chong Chen b, Xiangxiang Gong b, Chuangqiang Zhou b, Gangjin Zhao a, Jie Gu a, Wei Ji a, Xin Huo a, Ming Chen a, Guowang Diao a, ** a b

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, Jiangsu, People's Republic of China Testing Center, Yangzhou University, Yangzhou, 225002, Jiangsu, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 May 2017 Received in revised form 23 July 2017 Accepted 21 August 2017

The energy crisis is currently a major concern worldwide due to the limited natural resources. Accordingly, lithium-ion batteries (LIBs) are in the focus of forefront energy storage investigations in our 21st century. Traditional lithium-insertion compounds for cathode materials, such as LiCoO2, LiMn2O4, LiNiO2 and LiFePO4, have been highly successful but they face serious limitations in energy storage density and production cost associated with their use. Therefore, the design of novel molecular cluster batteries (MCBs) as the next-generation energy storage device is an extremely important and hot topic of current research. Here, we first report preparation of zero-dimensional (OD) polyaniline/polyoxometalates [PW12O40]3
(PANI/PW12) nanospheres, and then have successfully embedded PANI/PW12 nanospheres into three-dimensional (3D) graphene sponge to construct a novel 3D graphene/polyaniline/polyoxometalates hybrid (rGO@PANI/PW12) as new cathode material in LIBs. The as-prepared rGO@PANI/ PW12 hybrid in half-cell exhibits extraordinary electrochemical performances with high specific capacity (around 285 mAh g
1 at 50 mA g
1), excellent rate capability (140 mAh g
1 at 2 A g
1), and outstanding cycling stability (capacity fade rate of 0.028% per cycle even after 1000 cycles at 2 A g
1), representing the best performance for long-cycle POMs-based cathode in LIBs to date. Furthermore, a rGO@PANI/PW12-C lithium ion full-cell is first fabricated with an initial discharge specific capacity of 145 mAh g
1 at 2 A g
1, and then shows excellent cycling stability with a capacity decay rate of 0.035% per cycle over 1000 cycles at 2 A g
1. Importantly, the discharge and degradation mechanisms of rGO@PANI/PW12 cathode in LIBs are further deeply investigated. The electron-transfer (ET) from reduced PANI polymer to PW12 polyanion as well as the “electron reservoir” model on PW12 molecule both contribute to the high electroactivity. This study sheds thus new lights to the design of new generation electrode materials for lithium-ion batteries. © 2017 Published by Elsevier Ltd.

Keywords: Molecular cluster batteries Polyoxometalate Graphene Polyaniline Electron transfer Discharge mechanism Lithium-ion batteries

1. Introduction With the increasing severity of the global energy crisis and environmental pollution, the development and applications of energy technologies have drawn extensive attention [1,2]. At

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Ni), [email protected] (G. Diao). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.mtener.2017.08.005 2468-6069/© 2017 Published by Elsevier Ltd.

present, rechargeable lithium-ion batteries (LIBs) have become one of the most important and promising renewable storage systems because of their high power density and energy density, which dominate as power sources for portable electronic devices, electric vehicles, and grid applications [3]. However, these widespread and long-term applications still pose many challenges in terms of high performance, good safety and low cost for LIBs [4e7]. For example, the high cost, toxic properties of cobalt and the low utilization ratio of theoretical capacity along with the safety problem in layered LiCoO2 material seriously inhibit its further practical applications in large-scale LIBs [8]. Spinel LiMn2O4 material suffers from the severe

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capacity fading upon extended electrochemical cycling, especially at elevated temperatures owing to the dissolution of the Mn2þ ions into the electrolyte, electrolyte decomposition at high voltages and the structural distortion caused by the Jahn-Teller Mn3þ ions [9]. Moreover, despite olivine LiFePO4 material recently has been used for electric vehicles or electric power storage devices, the poor intrinsic electronic conductivity (10
9 ~ 10
10 S cm
1), small lithium diffusion coefficient and low tap density of LiFePO4 led to the practical specific capacities below 200 mAh g
1 and poor rate capability that has limited its electrochemical performance and wide commercialization of LiFePO4 [10]. Thus, new cathode materials with the capability of rich reversible multi-electron redox reactions and rapid lithium ion diffusion are urgently needed, in order to satisfy both high battery capacity and fast charging/discharging [11,12]. Recently, molecular clusters materials (polyoxmetalates (POMs) [13e27], polynuclear metal complexes [28] and metal-organic frameworks (MOFs) [29]), have been considered as new cathode materials for high-performance rechargeable lithium batteries. POMs represent a large family of metal-oxygen clusters (generally Mo, W, V) in their highest oxidation states to form large, closed 3D frameworks [30]. The structural flexibility of POMs not only can generate a multitude of molecular architectures, but also bring forward their manifold potential applications, e.g. in the fields of catalysis, water splitting, medicine, electro-optics, multifunctional materials, and energy storage [31e40]. They are expected to achieve high capacity and broad working potential window (1.5e4.2 V) for energy storage applications owing to unique structural and electrochemical properties, namely electron and proton transfer/ storage abilities, high structural/thermal stability, and lability of their lattice oxygen [15]. POMs even undergo multi-electron reduction reaction during charging/discharging processes while they still can retain their structural integrity, compared to those lithium intercalation compounds. Thus far, tremendous efforts have been focused on the design and assembly of new POM-based materials in lithium batteries which exhibit unique structures and fascinating performances [13e27]. For example, Sonoyama and coworkers first reported Keggin-type POM, K3[PMo12O40] (KPM) exhibited a large capacity of over 200 mAh g
1 as the cathode electrode material for lithium battery in 2011 [13]. Later, bi-capped Keggin-type polyoxovanadate K5.72H3.28[PV14O42] (KPV) with higher energy density and cycle stability was also introduced by the same group in 2012 [14]. Meanwhile, Yoshikawa and Awaga further utilized In operando Mo K-edge XAFS measurements to reveal charging/discharging mechanism for TBA3[PMo12O40]3
molecular cluster battery: [PMo12O40]3
polyanion (PMo12) can be regarded as an “electron sponge”, which cycling reversibly by 24 electrons between initial state [PMo12O40]3
and super-reduced state [PMo12O40]27
during charging/discharging [15,16]. Afterwards, novel organically grafted POM organo-imido hexamolybdate (Mo6eSCN) was developed by Wei and Hou et al. as an interesting anode material for lithium ion batteries, showing initial discharge capacity of over 1600 mAh g
1 [17]. Vanadium-based POM [V10O28]6
as anode material in sodium-ion batteries (NIBs) is another representative example illustrated by Srinivasan et al. [18]. Recent progress on V-based POM {V15O36} as high performance cathode materials for rechargeable LIBs was achieved by Dong and Cronin et al. [19]. Moreover, POM-based nanocomposite cathodes in LIBs also have been explored to date, and outstanding examples include rGO/PMo12 and SWNT/PMo12 hybrids reported by Yoshikawa and Awaga et al. [20e22], electrochemically synthesized SiW12/rGO composite presented by Wu and co-workers [23], PANI/  mez-Romero [24] and Paik [25], rGO/ PMo12 hybrid designed by Go ethylenediamine(EDA)/Anderson-type Na3[AlMo6O24H6] (NAM) composite reported by Li [26], and rGO/POMOFs nanocomposite

just developed by Lan [27]. Nevertheless, even after many years of detailed topical investigations, several remaining key issues are required to be urgently resolved for the practical applications of POMs-based materials in LIBs to date, such as incomplete POMs utilization, fast capacity degradation, and poor Coulombic efficiency of POMs-based Li-ion batteries. All these issues are mainly caused by (a) low electronic conductivity of POMs bulk crystals, (b) dissolution of POMs active materials into liquid electrolyte (c) irreversible Liþ insertion/extraction over POM clusters during charging/discharging. Furthermore, we still need deeper insight to understand the complex discharge-charge reaction mechanism of POMs-based electrodes on a molecular level in LIBs in order to fully tap their design options. In this work, we first report preparation of zero-dimensional (0D) polyaniline/polyoxometalate (PANI/PW12) nanosphere, and then utilized PANI/PW12 nanosphere as the precursor to successfully fabricate a novel three-dimensional (3D) nanostructure of graphene/polyaniline/polyoxometalate hybrid (rGO@PANI/PW12) as new cathode material in LIBs via a one-step hydrothermal process [41]. To the best of our knowledge, this 3D rGO@PANI/PW12 hybrid has rarely been documented for the POMs-based electrode materials in LIBs. Next, we further deeply investigated chargedischarge mechanism in LIBs by XPS spectra, and first proposed that electron-transfer (ET) mechanism from reduced PANI polymer to PW12 polyanion as well as the “electron reservoir” model on PW12 molecule. In comparison with bare PW12, PANI, rGO@PW12 and PANI/PW12 electrodes, the as-prepared 3D rGO@PANI/PW12 hybrid showed very remarkable electrochemical performance in terms of high specific capacity, excellent rate capability and outstanding cycling stability. The rGO@PANI/PW12 cathode material in half-cell delivers a reversible chargingedischarging capacity of 285 mAh g
1 at a current density of 50 mA g
1. In addition, the rGO@PANI/PW12 cathode in half cell and rGO@PANI/PW12-C full cell both demonstrate extremely low capacity-decay rates of 0.028% and 0.035% per cycle after 1000 cycles even at 2 A g
1, respectively, which represent the best performance for long-cycle POMs-based lithium-ion batteries to the best of our knowledge. These synergistic effects are achieved for the following reasons: (1) In PANI/ PW12 hybrid nanospheres, PANI as a conducting polymer can significantly increase the electrical conductivity of PW12 crystals, meanwhile PANI can enhance electron storage capability of PW12 polyanion through electron transfer mechanisms. (2) The 3D nanostructure of rGO@PANI/PW12 hybrid can maintain its structural integrity and efficiently prevent POMs dissolution into the liquid electrolyte. (3) The uniform 3D conductive network in selfassembled rGO@PANI/PW12 composite can supply highly-efficient 3D electron transfer pathways and ion diffusion channels. Therefore, we adopted a rational approach to design a novel 3D structure of POMs-based hybrid cathode and brought forward an accessible “hands-on” model for POM cluster to deeply understand the elusive discharge mechanisms, which have paved the way to explore the high-energy, long-life POMs-based lithium-ion batteries. 2. Experimental section 2.1. Synthesis of PANI/PW12 nanospheres First, 45 mL (45 mg) of freshly aniline and 0.3 g ammonium persulphate (APS) were dissolved in 200 mL of deionized water with stirring for 1 h 400 mg (0.14 mmol) of phosphotungstic acid H3PW12O40. xH2O (PW12) was slowly added to the reaction mixture, followed by addition of 2 mL of 0.5 M sulfuric acid. In the second step, the solution was left stirring for 90 min at room temperature, and then undisturbed in an ice bath for 24 h. The final products were collected by centrifuging and washed by water and ethanol

L. Ni et al. / Materials Today Energy 6 (2017) 53e64

for several times. Finally, PANI@PW12 nanosphere was collected after drying under vacuum condition, and the reaction yield for PANI@PW12 was about 73.2% (360 mg, calculated based on W). Elem anal. Found for PANI/PW12: C, 12.63%; H, 1.03%; N, 2.886%; P, 0.88%; Mo, 62.78%. Calcd for (C6H5N)6(PW12O40)$6H2O: C, 12.24%; H, 1.20%; N, 2.38%; P, 0.87%; W, 62.46%. 2.2. Synthesis of three-dimensional (3D) rGO@PANI/PW12 ternary hybrids 50 mL of the homogeneous GO suspension (2.4 mg mL
1) was sonicated for 1 h at room temperature. Then 360 mg of PANI/PW12 nanosphere was added into the above solution and stirred for 1 h. The obtained homogeneous rGO@PANI/PW12 colloidal suspension was sealed in a 50 mL Teflon-lined autoclave and hydrothermally treated at 180  C for 12 h. A black cylindrical graphene hybrid hydrogel was formed when the autoclave was cooled naturally. rGO@PANI/PW12 aerogel was obtained after the freezing-drying of graphene hydrogel. 2.3. Synthesis of three-dimensional (3D) rGO@PW12 hybrids The above synthetic procedure was only modified by using 1.5 g of H3PW12O40. xH2O instead of PANI/PW12 nanospheres. A black cylindrical graphene hybrid hydrogel was formed when the autoclave was cooled naturally. The rGO@PW12 aerogel was obtained after the freezing-drying of graphene hydrogel. 2.4. Materials and analytical characterization Natural graphite powder (99.99%) was obtained from Sigmae Aldrich (USA). Phosphotungstic acid, aniline and ammonium persulphate were purchased from Shanghai Chemical Reagents Company (Shanghai, China). Other chemicals and solvents were reagent grade and commercially available. Graphene oxide (GO) was prepared from natural graphite powder by improved Hummer's method [42]. TEM (Transmission Electron Microscopy) was conducted on a Philips TECNAI-12 instrument. SEM was carried out with Hitachi S-4800 (Japan). High-resolution TEM (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were conducted using a FEI Tecnai G2 F30 STWIN (USA) operating at 200 kV. X-ray powder diffraction (XRD) data were obtained with a graphite monochromator and Cu Ka radiation (l ¼ 0.1541 nm) on a D8 advance super speed powder diffractometer (Bruker). The thermogravimetric analyses (TG) were collected with a Netzsch TG209 F1 instrument with a 10  C min
1 from 30 to 800  C in flowing air atmosphere. Cyclic voltammetry (CV) measurements were performed using a electrochemical workstation (CHI660 E, Chenghua, China) at a scan rate of 0.1 mV s
1 between 1.5 and 4.2 V. Elemental analyses (C, H and N) were performed using an EA 1110 elemental analyzer. W and P were determined by an Optima 7300 V ICP-OES Spectrometer. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Optics Vertex 70 spectrometer. Raman spectroscopy was performed on a Renishaw Ramascope 1000 with a green Spectra Physics Argon laser with a wavelength of 524.5 nm and 50 mW capacity. The XPS experiments were carried out on a Thermo Escalab 250 system using Al Ka radiation (hn ¼ 1486.6 eV). The test chamber pressure was maintained below 2  10
9 Torr during spectral acquisition. Nitrogen adsorptionedesorption measurements were carried out on a Micrometrics ASAP 2020 Plus Surface Area & Porosity Analyzer at 77 K using the volumetric method and samples were degassed at 150  C for 6 h under vacuum before measurements. The desorption isotherm was used to determine the average pore size and distribution by the BarretteJoynereHalenda (BJH) method.

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2.5. Electrochemical measurements For the preparation of cathode, a mixture of 70 wt% of active material (pristine PANI, PANI/PW12, rGO@PW12 and rGO@PANI/ PW12), 20 wt% Super-P and 10 wt% Polyvinylidene fluoride (PVDF) was added in a N-methyl pyrrolidone (NMP) to form a homogeneous slurry. For pristine PW12 active material, the cathode was made by mixing PW12, Super-P, and PVDF at a weight ratio of 50:40:10. The mixtures were pasted on carbon paper (GDL 28 AA, SGL) current collectors. The electrode film was dried in an oven at 110  C overnight, and cut into round disks with a diameter of 15 mm. The 2032-type coin half-cells were assembled using Celgard 2300 membrane as separator and Li metal foil as anode. The electrolyte was composed of 1.0 M LiPF6 in a mixture of ethylene carbonate (EC)/diethyl carbonate (DEC; 1:1 v/v). Galvanostatic discharge-charge tests were performed on a Neware Battery Measurement System (Neware, China) with cutoff voltages of 4.2e1.5 V. To make a rGO@PANI/PW12-C full cell, rGO@PANI/PW12 and graphite were used as cathode and anode electrodes, respectively. For pristine graphite electrode, the anode was made by mixing graphite and PVDF at a weight ratio of 90:10. The mixtures were pasted on copper current-collector foils. Prior to full cells fabrication, both the graphite and rGO@PANI/PW12 electrodes were performed to carry out the second cycle activation in half-cell. The cycling test of the Cu-supported graphite electrode in half lithium cell was performed in the 0.001e2.0 V voltage range. The rGO@PANI/PW12-C full cell is investigated under galvanostatic cycling conditions in the voltage window of 0.5e4.2 V. The full cell balance was achieved by setting the electrode mass ratio of cathode/anode to ca. 2.0. The specific charge/discharge capacities of PW12, PANI/ PW12, rGO@PW12 and rGO@PANI/PW12 cathodes were calculated on the basis of the weight of POM with the mass loadings around 2.0 mg/cm2. All cell handling was performed in an argon-filled glove box. 3. Results and discussion 3.1. Synthesis and characterizations of 0D PANI/PW12 nanospheres and 3D rGO/PANI/PW12 nanocomposites The chemical structure and synthetic route for the preparation of 0D PANI/PW12 nanospheres and 3D rGO@PANI/PW12 nanocomposite assemblies are shown in Fig. 1. Firstly, the PANI/PW12 nanospheres were prepared by a facile polymerization and electrostatic self-assembly method in aqueous and acidic media. For a typical aniline polymerization, the aqueous solution of PW12 was first mixed with aniline in a 1:6 molar ratio at room temperature to create an aniline heteropolyacid salt (Aniline-PW12). The oxidant persulphate (APS) solution containing mineral acid H2SO4 was then added into the mixture of the aniline-PW12, and then the polymerization reaction took place in an ice bath undisturbed for 24 h. Meanwhile, the strong electrostatic interactive force fosters very close contact between the polyoxometalate PW12 and the PANI matrix, thus creating self-assembled 0D PANI@PW12 nanospheres. The scanning electron microscopy (SEM) images of PANI/PW12 hybrid polymers (Fig. 2a) exhibit uniform nanospheres with an average diameter of 1.1e1.3 mm (inset of Fig. 2a). Fig. 2b shows the magnified image of hybrid PANI/PW12 nanospheres. The transmission electron microscope (TEM, Fig. 3ced) and dark-field scanning transmission electron microscopy (STEM, Fig. 3f) images of PANI/PW12 also both confirm the formation of spherical aggregates. Energy dispersive X-ray (EDX) spectroscopy also demonstrates that the composite structure PANI/PW12 containing P, W, O, C, and N elements (Fig. 3l). Elemental mapping analysis (Fig. 3gek) shows that all the elements are homogeneously distributed in the

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Fig. 1. Schematic illustration of the assembly process of three-dimensional rGO@PANI/PW12 hybrid.

Fig. 2. SEM images of (aeb) PANI/PW12 nanospheres (inset: statistical analysis of the size of the spheres). (ced) SEM images of rGO@PANI/PW12 hybrids (inset: statistical analysis of the size of the spheres).

nanosphere. The high-resolution transmission electron microscopy (HRTEM) illustrates that the PANI/PW12 nanospheres are clearly composed of small dark spots with a diameter of ca. 1.5 nm, which correspond to those of the individual and aggregated PW12 clusters (Fig. 3e). Finally, the as-prepared PANI@PW12 nanospheres were further incorporated into three-dimensional rGO hydrogel via a one-step hydrothermal reaction of PANI@PW12 hybrid and GO aqueous solution, leading to the formation of 3D rGO@PANI/PW12 supramolecular assemblies. The macroporous structure of rGO hydrogel before and after incorporation of PANI@PW12 nanospheres were verified by SEM of the freeze-dried samples (Fig. S1 in

the supporting information). As it can be seen from Figs. S1ae1b, the bare rGO hydrogel or rGO@PANI/PW12 hybrid both has a welldefined and interlinked 3D porous network and the sizes of the pores are about 2e10 mm. A nitrogen adsorption/desorption measurement is an important analysis for calculating the pore size distribution in porous carbon materials. The pore distribution of 3D rGO hydrogel and rGO@PANI/PW12 composite are shown in Figs. S1ce1d. 3D rGO hydrogel has only a single mesopores pattern at ~3.7 nm, whereas rGO@PANI/PW12 composite exhibits both mesopores in ~2.3 and ~3.7 nm. These results further indicate the existence of macro@meso-porous structure in 3D rGO@PANI/PW12

L. Ni et al. / Materials Today Energy 6 (2017) 53e64

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Fig. 3. (aeb) TEM images of rGO@PANI/PW12 hybrids. (ced) TEM images of PANI/PW12 nanospheres. (e) HRTEM images of PANI/PW12 nanospheres. (fek) Dark field STEM image of PANI/PW12 nanosphere and EDX elemental mapping on C, N, O, P, and W. (i) EDX spectrum of PANI/PW12 nanosphere under SEM mode.

composite. PANI@PW12 nanospheres on the surface of 3D rGO hydrogel still can maintain their shape and structural integrity even after hydrothermal reaction, as confirmed by SEM and TEM in Fig. 2ced and Fig. 3aeb. The average size of nanospheres in rGO@PANI/PW12 composite is about 0.8e1.0 mm, smaller than that of pristine PANI@PW12, probably indicating the confinement effect of 3D rGO hydrogel or hydrothermal process on the spherical size. Therefore, all these visible evidences verify that PANI@PW12 spheres were successfully coated onto the surface of the 3D rGO sponge in a uniformly dispersed state. In addition, 3D rGO@PW12 composite was also prepared via hydrothermal reaction of PW12 clusters and GO aqueous solution in order to compare rGO@PANI/ PW12 ternary composite (See Exp. Part). The PANI/PW12, rGO@PW12 and rGO@PANI/PW12 composite were further characterized with a wide range of analytical methods

(FT-IR, Raman and XRD spectroscopic techniques, TGA, and elemental analysis). FTIR spectra of PANI, PW12, PANI@PW12 polymers hybrid, 3D rGO@PW12 and rGO@PANI/PW12 composites were collected and presented in Fig. 4a. The characteristic peaks in the FT-IR spectra of PANI@PW12 nanosphere, rGO@PW12 and rGO@PANI/PW12 composite at 1083, 989, 892, and 810 cm
1 are assigned to nas(PeOa), terminal nas(WeOt), corner-sharing nas(WeObeW), and edge-sharing nas(W-Oc-W) asymmetrical vibration peaks of Keggin [PW12O40]3
[43]. Raman spectra of PW12 and rGO@PANI/ PW12 in the solid state are closely related to FT-IR spectra, and the characteristic peaks are assigned to PeO bonds of PO4 tetrahedra and WeO vibrations of PW12 unit (Fig. 4b) [43]. Moreover, a broad D band at 1319 cm
1 and G band at 1582 cm
1 of graphene oxide were also observed in the Raman spectrum of rGO@PANI/PW12, suggesting that PANI/PW12 nanospheres are embedded into 3D rGO

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Fig. 4. (a) FT-IR spectra of PANI, PW12, PANI/PW12, rGO@PW12 and rGO@PANI/PW12. (b) Raman spectra of GO, PW12, PANI, rGO@PANI/PW12. (c) XRD patterns of PW12, PANI, PANI/ PW12, rGO@PW12 and rGO@PANI/PW12. (d) TGA curves of PW12, PANI/PW12, rGO@PW12 and rGO@PANI/PW12.

architectures (Fig. 4b) [44]. Therefore, the FTIR and Raman spectra both give direct evidences that the primary Keggin-type [PW12O40]3
polyanions serve as counterions in the positively charged PANI matrix of PANI/PW12 nanosphere and 3D rGO@PANI/ PW12 composite. In addition, the XRD patterns of hybrids PANI/ PW12 and rGO@PANI/PW12 are in good agreement with primary PW12, revealing that the Keggin structure of PW12 remains intact in the hybrids (Fig. 4c). The XRD pattern of rGO@ PW12 hybrid indicates the absence of crystalline peaks of PW12, suggesting that the Keggin PW12 polyanion units are dispersed at the molecular level [25]. Elemental analyses (EA, See Exp. Part) in combination with thermogravimetric (TGA) measurements (Fig. 4d) are usually performed to determine the composition of hybrids PANI/PW12, rGO@PW12 and rGO@PANI/PW12. As shown in Fig. 4d, the TGA curve of PW12 exhibits one weight loss of 11.7% below 200  C via a two-step process, which is corresponding to the release of crystal water molecules and adsorbed water molecules (~24.0 H2O). In the case of hybrid PANI/PW12, the observed weight losses of 2.8% below 215  C (calcd. 3.0%) and 6.3% between 215 and 365  C correspond to the removal of physisorbed water molecules (~6.0 H2O) and the dopant/smaller chain oligomers, respectively [45,46]. A rapid weight loss step of 22.9% in the temperature range of 365  C to 610  C is due to thermal decomposition of the main molecular chain of the polymer [45]. The EA and TGA results also reflect that the chemical structure for the hybrid PANI/PW12 should be {[(C6H5N)6(PW12O40)]. 6H2O}n, viz, each [PW12O40]3
polyoxoanion electrostatically combined with six aromatic rings from the polaronic form of polyaniline-emeraldine salt (PANI-ES) to stabilize three negative charge as shown in Fig. 1 [47,48]. For rGO@PANI/ PW12 composite, one weight loss step of 48.1% in the temperature range from 30  C to 650  C can be assigned to the loss of water molecules, and the decomposition of PANI polymer and 3D rGO

network. Moreover, the observed remaining weights 77% in PANI/ PW12, 56% in rGO@PW12 and 52% in rGO@PANI/PW12 all can correspond to their PW12 content of the hybrids. 3.2. Electrochemical properties for Li-ion battery and structural stability of rGO@PANI/PW12 cathode Next, we evaluated the electrochemical performance of 3D rGO@PANI/PW12 composite as cathode materials in applications of lithium-ion batteries. The prepared POM-based cathode materials were fabricated in 2032-type coin half-cells with Li metal as reference anode, which exhibit a high open-circuit voltage (OCV) of 3.2 V (for details, See Exp. Part). The galvanostatic discharge/charge curves of the rGO@PANI/PW12 nanocomposite exhibit four characteristic discharge plateaus at ~2.7, 2.4, 2.1 and 1.7 V during chargeedischarge process in the potential range of 4.2e1.5 V, corresponding well to the four redox waves of CV (Fig. 6a). The rGO@PANI/PW12 composite delivers an initial discharging capacity of 205 mAh g
1 at a current density of 50 mA g
1. When charging back to 4.2 V versus Liþ/Li, a reversible chargingedischarging capacity of 285 mAh g
1 is obtained. After that, the charge-discharge curve shows identical plateaus, indicating the excellent reversible capability of rGO@PANI/PW12 hybrid (Fig. 5a). As current densities increases, the specific capacity is reduced gradually to 271, 235, 190, 160, 140 mAh g
1 at 100 mA g
1, 200 mA g
1, 500 mA g
1, 1 A g
1 and 2 A g
1, respectively, confirming the excellent electronic/ionic transport properties and reaction kinetics of the three-dimensional conductive network (Fig. 5b). Fig. 5c demonstrates the consecutive cycling performance of rGO@PANI/PW12, rGO@PW12 and PANI/ PW12 nanocomposites, bare PANI and PW12 materials at different current densities, measured for 6 cycles at each current density in ascending steps from 50 mA g
1 to 2 A g
1, followed by a return to

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Fig. 5. (a) First and second Charge-discharge curves of rGO@PANI/PW12 as cathode at a current density of 50 mA g
1. (b) The second galvanostatic charge-discharge profiles for rGO@PANI/PW12 over the voltage range of 1.5e4.2 V versus Liþ/Li at various current densities, collected from different cells. (c) Rate performance of PW12, PANI, PANI/PW12, rGO@PW12 and rGO@PANI/PW12 cathodes at different current densities, ranging from 50 mA g
1 to 2 A g
1. (d) Cycling performance of PW12, PANI, PANI/PW12, rGO@PW12 and rGO@PANI/PW12 cathodes at a current density of 200 mA g
1 over 50 cycles. (e) Long-term cycling performance of rGO@PANI/PW12 cathode at 100 mA g
1 for initial 9 cycles and then 2 A g
1 for subsequent 1000 cycles (red), or at 2 A g
1 for 1000 cycles (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

50 mA g
1. The discharge capacities for three dimensional rGO@PANI/PW12 composites steadily changed along with the increase of the current density from 50 mA g
1 to 2 A g
1. Noticeably, higher specific capacities can be obtained for all applied current densities in the case of rGO@PANI/PW12, rGO@PW12 and PANI/PW12 nanocomposite electrodes, whereas bare PANI and PW12 electrodes revealed very lower capacities. The rGO@PANI/PW12 composite still can exhibit a higher average reversible capacity of 140 mAh g
1 even after cycling at high current rate 2 A g
1. Compared to rGO@PW12 and PANI/PW12 nanocomposite cathodes, the original

capacity of rGO@PANI/PW12 composite can be fully recovered when the current density was abruptly switched back to 50 mA g
1 after a gradual increase of the current density from 50 mA g
1 to 2 A g
1, indicating that the high stability of the electrode and structural reliability of the rGO@PANI/PW12 composite (Fig. 5c). As shown in Fig. 5d, the rGO@PANI/PW12 composite electrode also exhibit best cycling stability at 200 mA g
1 over 50 cycles among them. After 50 cycles, the rGO@PANI/PW12 composite still can achieve discharge specific capacity of 205 mAh g
1 with the capacity retention of 87%. However, rGO@PW12 and PANI/PW12 composites only can deliver

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Fig. 6. (a) Cyclic voltammetry curves of rGO@PANI/PW12 electrode swept in the voltage range of 1.5e4.2 V at a sweep rate of 0.1 mV s
1. (b) Nyquist plots for PW12, PANI, PANI/PW12, and rGO@PANI/PW12 cathodes for lithium ion batteries after second charge/discharge cycle. (c) The equivalent circuit model of R(Q(RW)). (d) Light-emitting diode (LED) demonstration: one coin-type Li-ion battery based on rGO@PANI/PW12 electrode can light up two LEDs.

discharge specific capacities of 125 and 110 mAh g
1 with the capacity retention of 76% and 80%, respectively. In contrast, bare PW12 and PANI cathodes have worse electron conductivity and ion/ electron transport pathways, giving rise to lower capacity of 37 and 27 mAh g
1 after 50 cycles (Fig. 5d). Therefore, the rGO@PANI/PW12 sample displays a superior rate capability and better cycle performance than rGO@PW12 hybrid, PANI/PW12 nanosphere, bare PW12 and PANI materials. Furthermore, long-term cycling stability and Coulombic efficiency of the Li-ion cells based on the rGO@PANI/ PW12 nanocomposite were also performed at practical level current density of 2 A g
1 (Fig. 5e). With an initial reversible capacity of 135 mAh g
1, the rGO@PANI/PW12 cathode still can maintains 80 mAh g
1 of capacity at the 1000th cycle, representing only 0.040% loss per cycle (Fig. 5e). Moreover, when the current density was switched to 2 A g
1 after the first nine cycles at the low current density of 100 mAh g
1, an initial discharge capacity of 140 mAh g
1 was achieved at 2 A g
1 (Fig. 5e). Then the capacity gradually decreased and stabilized at around 100 mA g
1 even after prolonged cycling over 1000 cycles. Most importantly, the capacity retention was found to be 72%, which corresponds to an extremely low capacity decay of 0.028% per cycle, representing the best performance for long-cycle POMs-based lithium-ion batteries to the best of our knowledge (Fig. 5e). Meanwhile, the average Coulombic efficiency is above 98% throughout. Cyclic voltammograms (CVs) of rGO@PANI/PW12 composite electrode for the first two cycles at a scan rate of 0.1 mV s
1 in the potential range of 1.5e4.2 V are shown in Fig. 6a. The several couples of reversible redox peaks clearly elucidate the reversible electrochemical reactions between the lithium electrode and the rGO@PANI/PW12 nanocomposite in the battery [23]. Electrochemical impedance spectroscopy (EIS) measurements are widely used to investigate the conductivity and electron transfer/Liþ ion

diffusion for the Li-ion batteries. Nyquist plots of PW12, PANI, PANI/ PW12, and rGO@PANI/PW12 electrodes after ten galvanostatic charge/discharge cycles at 1 A g
1 are shown in Fig. 6b. Each Nyquist plot is composed of an intercept at the real axis (Z0 ) in the high frequency region, a depressed semicircle in the middle frequency region and a sloping line in the low frequency region. The EIS data can be fitted by an equivalent circuit modeling of R(Q(RW)) as shown in Fig. 6c, and the impedance parameters are listed in Table 1. The equivalent circuit model consists of the solution resistance Rs, a constant phase element (CPE), the charge-transfer resistance Rct, and the Warburg impedance W. In the high frequency region, Rs is the real axis intercept (also known as equivalent series resistance (ESR)), which represents the total ionic resistance of the electrolyte, the intrinsic resistance of electrode materials and contact resistance at various phase interfaces. The depressed semicircle in the middle frequency region is assigned to the charge-transfer resistance Rct, which is correlated with Liþ migration at the electrode materialeelectrolyte interface. The constant phase element (CPE or Q, 0 < n < 1) was largely used to describe the double layer capacitance and passivation film capacitance in real electrochemical systems due to non-homogeneity of the conductance [49] and the electrode [50]. The linear portion is designated to Warburg impedance (W), which expresses the diffusion of Liþ into the bulk of the electrode materials. As shown in Table 1, the values of Rs are all around 3 U owing to the fact that the same electrolyte was applied in the Li-ion cells. Moreover, the values of Rct are 109.8 U and 170.6 U for rGO@PANI/PW12 and PANI/ PW12 cells, which are obviously lower than those of the PANI and pure PW12 electrodes (202.6 U and 376.9 U, respectively). This result also indicates that three dimensional conductive structure of rGO@PANI/PW12 composite can not only greatly enhance the conductivity of the PANI/PW12 electrode to accelerate electron transfer,

L. Ni et al. / Materials Today Energy 6 (2017) 53e64

61

Table 1 Fitted equivalent circuit elements of PW12, PANI, PANI/PW12, and rGO@PANI/PW12 electrodes. Sample

c2

rGO@PANI/PW12 PANI/PW12 PANI PW12

9.16 3.73 1.89 4.74

   

10
4 10
3 10
3 10
3

Rs (U)

Rct (U)

Q (CPE) (10
5) (S secn)

W/Y0 (S sec5)

n

3.13 3.27 3.08 3.49

109.8 170.6 202.6 376.9

4.30 2.30 2.16 1.66

0.0265 0.0132 0.0175 0.0054

0.68 0.76 0.77 0.76

but also significantly improve the electrochemical activity of PW12 electrode during the chargeedischarge process. Moreover, we only used one coin cell based on rGO@PANI/PW12 electrode with high OCV to power two LEDs easily at the same time in order to demonstrate practical applicability of this POM-based Li-ion cell (Fig. 6d). We further investigate the morphology and structural changes of the rGO@PANI/PW12 cathode after cycling, to demonstrate the structural stability of the 3D rGO@PANI/PW12 electrode. The rGO@PANI/PW12 cathode in Li-ion coin was disassembled and characterized by SEM, TEM, EDX, FT-IR, and XRD (Figs. S2eS4). The SEM and TEM images together with EDX elemental mapping confirm that the morphology of the 3D rGO@PANI/PW12 electrode is still maintained after running 100 cycles at 1 A g
1 in coin cell (Fig. S2), indicating the structural integrity and improved cycle performance of rGO@PANI/PW12 electrode. In addition, FTIR spectrum and XRD pattern of rGO@PANI/PW12 cathode after cycling both agree well with original rGO@PANI/PW12 cathode before cycling (Figs. S3eS4), which underlines relative good structural stability during cycling. Moreover, we also performed dissolution experiments to investigate the stability of the rGO@PANI/PW12 hybrid cathode in Li-ion battery electrolyte solutions. As indicated by Fig. 7aeb and Fig. S5, pristine Keggin-type POMs (e.g. PW12, PMo12, SiW12 and TBA3[PMo12]) and PANI materials are all fully dissolved into the liquid electrolyte, which might lead to their fast capacity fading during charge/discharge cycling. The problem of the dissolution of the POM-based cathode achieved an obvious improvement by constructing insoluble POM-based nanocomposites. The partial dissolution of PANI/PW12 nanosphere into the electrolyte solution still existed (Fig. 7c), whereas a 3D nanostructure of rGO@PANI/PW12 hybrid was almost insoluble in electrolyte solutions (Fig. 7d). Thus, it can effectively protect active materials against dissolution in electrolyte solutions when PANI/ PW12 hybrid nanospheres were successfully embedded into the 3D graphene sponge to form rGO@PANI/PW12 hybrid. This might be an important reason that rGO@PANI/PW12 cathode presents the highlevel performance with the large discharge capacity, high-rate capability and long cyclability. In short, rGO@PANI/PW12 cathode exhibits superior electrochemical performance than known POMsbased cathodes in LIBs [13e16,19e25].

Finally, we successfully assembled a rGO@PANI/PW12-C LIB full cell by employing the graphite anode and the rGO@PANI/PW12 cathode (See Exp. Part). In order to reduce the polarization and irreversible capacity loss of the electrode materials in the first discharge process (both for the anode and cathode sides), the graphite anode and rGO@PANI/PW12 cathode are electrochemically activated for two charge/discharge cycles at the low current density of 100 mA g
1 in half-cells before the full cell is assembled (Figs. S6ae6b). The full cell is investigated under galvanostatic cycling conditions at room temperature in the voltage window of 0.5e4.2 V. The initial charge/discharge capacities are ~242 and 239 mAh g
1 at 100 mA g
1, respectively (Fig. S6c). After electrochemical activation of full cell for two cycles at 100 mA g
1, an initial discharge capacity of about 145 mAh g
1 was achieved at 2 A g
1 (Fig. S6c). The reversible discharge capacity still remains at ~93 mAh g
1 even after 1000 cycles at 2 A g
1, with an extremely low capacity decay of 0.035% per cycle and Coulombic efficiency of ca. 98% (Fig. S6d), thus suggesting excellent cycling performance and reversible capacity for rGO@PANI/PW12-C full cells. 3.3. The charge-discharge mechanism in Li-ion batteries In order to gain further insight into discharge mechanism of POMs-based Li-ion battery cathode, X-ray photoelectron spectroscopy (XPS) of rGO@PANI/PW12 electrode before and after discharging were employed (Fig. 8). For the original rGO@PANI/PW12 electrode before discharging, the W4f spectra display a characteristic doublet centered at 37.9 and 35.9 eV corresponding to the spineorbit split peaks of W6þ4f5/2 and W6þ4f7/2 (Fig. 8a). The spineorbit splitting of the doublet is 2.0 eV, and W4f5/2: W4f7/2 peak ratio of 0.77: 1 [51,52]. The position and shape of these doublet peaks confirm only W(VI) valence state existed in the initial rGO@PANI/PW12 electrode before discharging, as expected for Keggin-type structure [PWVI 12 O40]3
(PW12) in the highest oxidation state [30]. After 20 full discharge cycles at 1 A g
1, W4f XPS spectra of the rGO@PANI/PW12 electrode were also investigated (Fig. 8bec). As shown in Fig. 8b, the W6þ 4f doublet peaks shift toward lower binding energies of 35.9 eV and 32.9 eV, and 34.0 eV and 31.8 eV, corresponding to the W5þ 4f and Wxþ 4f (x ¼ 3, 2, or 1) doublet peaks, respectively. This reveals that the W6þ valence state

Fig. 7. Digital pictures of (a) PW12, (b) PANI, (c) PANI/PW12, and (d) rGO@PANI/PW12 in LiPF6/EC/DEC electrolyte.

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L. Ni et al. / Materials Today Energy 6 (2017) 53e64

Fig. 8. W(4f) XPS spectra of the rGO@PANI/PW12 electrode: (a) before discharge; (b) after 20 full discharge cycles at 1 A g
1 (without ethanol treatment); (c) after 20 full discharge cycles at 1 A g
1 (with ethanol treatment).

of Keggin-type POM (PW12) in the composite electrode is partially reduced to W4þ and Wxþ (x ¼ 3, 2, or 1) after full discharge [51]. The Wxþ state should be an intermediate state between W4þ and metallic W0 (such as W3þ, W2þ or W1þ), which might originate from the formation of WeW bonds [51]. Moreover, the peak at 30.0 eV is assigned to the F 2s line from LiPF6/EC/DEC electrolyte in the electrode (Fig. S7), and then the F 2s line disappeared when rGO@PANI/PW12 electrode was treated with the excess of ethanol in order to remove the residual electrolyte (Fig. 8c). To further prove valence change mechanism for the Wnþ ions in Li-ion cell during the discharging process, W4f XPS analysis of the composite samples retrieved after first and second full discharge cycle was also carried out (Fig. S8). Compared to 100% (W6þ) in PW12 structure of rGO@PANI/PW12 cathode before discharge (Fig. 8a), the reduction of 23% W6þ to Wnþ (n ¼ 1e4) lead to the relative intensities of W4þ and Wxþ (x ¼ 3, 2, or 1) increase to 13% and 10% in the W4f XPS spectrum after initial full discharge to 1.5 V, respectively (Fig. S8a). After second full discharge to 1.5 V, the relative intensities of W6þ obviously decrease to 55% whereas the relative amounts of W4þ and Wxþ continuously increase to 20% and 25% in the W 4f XPS spectrum (Fig. S8b). This result also prove that the tungsten atoms in POMs are partially reduced as a result of accepting and storing multiple electrons together with the intercalation of Liþ ions in POMs as counter cations during the discharging process. However, the previous studies [15,16,23] demonstrate all 12 highest-valent transition metal ions M6þ (M ¼ Mo and W) in Keggin-type POMs [PMo12O40]3
(PMo12) or [SiW12O40]4
(SiW12) are reduced to tetravalent metal ions M4þ in the discharging process, which were identified by In operando Mo K-edge XAFS and W4f XPS measurements, respectively. This also means that Keggin-type POMs [PMo12O40]3
and [SiW12O40]4
are both converted toward their super-reduced [PMo12O40]27
and [SiW12O40]28
species by storing 24 electrons in the discharging

process. Hence different trend in W valence change was observed for the rGO@PANI/PW12 electrode during discharge compared to PMo12 and SiW12 polyanaions, and is summarized as follows: First, not each W6þ ion in the rGO@PANI/PW12 electrode is involved in electron transfer processes during the lithiation. Second, W6þ ions in the electrode can be partially reduced to Wnþ (n ¼ 1e4) ions in low oxidation states during discharge. Such phenomenon might be due to the redox reaction between PANI polycation and PW12 polyanion during discharging process, in which electrons move from reduced type PANI polycation to PW12 polyanion. Based on the above results, the charge-discharge mechanism of rGO@PANI/PW12 cathode in LIBs can be proposed as indicated in Scheme 1: Initially, PANI emeraldine salt (ES) can turn into reduced type PANI leuco-emeraldine base (LEB) polycation through gaining two electrons during discharge (Scheme 1) [53e55]. Because Keggin-type PW12 polyanion has been viewed as one of the most classical POMs with strong oxidative ability [30], it could undergo reduction reaction to generate its reductive species heteropoly blue (HPB) by accepting electrons from PANI-LB polycation, and simultaneously oxidize PANI-LEB back to PANI-ES (Scheme 1). Afterwards, HPB cluster can continue to be reduced by storing excess electrons from the Li atoms, finally resulting in the formation of a super-reduced state of the PW12 cluster (Scheme 1) [15]. PW12 cluster in rGO@PANI/PW12 or PANI/PW12 composite can store excess electrons both from PANI polymers and Li atoms, thus exceeding the 24-electron super-reduction of pristine PW12 cluster. This is the most probable cause that Wnþ (n ¼ 1e4) ions in low oxidation states were observed during reduction. Furthermore, it is well known that this electron number also correspond to the large capacity of the POM-based lithium ion batteries. Therefore, the resulting rGO@PANI/PW12 and PANI/PW12 electrodes exhibit much higher specific capacities than bare PW12 and PANI electrodes.

L. Ni et al. / Materials Today Energy 6 (2017) 53e64

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Scheme 1. Schematic illustration of the proposed chargeedischarge mechanism of rGO@PANI/PW12-based cathode materials in Li-ion battery.

4. Conclusions In summary, PNAI/PW12 hybrid nanospheres are first synthesized via a facile polymerization reaction, and then they were successfully self-assembled into a 3D nanostructure of rGO@PANI/ PW12 hybrid through hydrothermal method. Next, we studied the electrochemical properties of the rGO@PANI/PW12 hybrid as cathode material for lithium ion batteries. Compared with bare Keggin POMs-based cathode materials, this as-prepared 3D self-assembled rGO@PANI/PW12 hybrids exhibit significantly improved electrochemical performances in terms of high specific capacity, remarkable rate capability, and excellent cycling stability. More importantly, the discharge mechanisms of the PANI/PW12 or rGO@PANI/PW12 hybrids were deeply investigated with XPS spectra, and then we first have proposed that the potential electrons transfer from reduced PANI polymer to PW12 polyanion can play an important role in the process of discharging. Therefore, the benefits of this the rGO@PANI/PW12 composite are mainly attributed to three following factors: (1) Polyaniline (PANI) as a conducting polymer not only can substantially improve the electrical conductivity of [PW12O40]3
polyoxometalate particle, but also can enhance electron storage capability of PW12 polyanion through electron transfer mechanisms. (2) PANI/PW12 hybrid nanospheres were embedded in a 3D nanostructure of rGO@PANI/PW12 hybrid, which can maintain its structural integrity and avoid dissolution into the liquid electrolyte. (3) Moreover, the 3D self-assembled rGO@PANI/PW12 composite supply a fully conductive network with efficient 3D electron transfer pathways and ion diffusion channels. In short, our present work not only brought forward a novel 3D structure of to POMs-based hybrid cathode to deeply

understand the elusive discharge mechanisms of molecular cluster batteries but also opened up new ways to explore the high-energy, long-life POMs-based lithium-ion batteries. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant nos. 21401162, 21773203), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant no. 14KJB430024), the Jiangsu Provincial Postdoctoral Sustentation Fund (Grant no. 1402015B), High-Level Entrepreneurial and Innovative Talents Program of Jiangsu, and Lvyangjinfeng Talent Program of Yangzhou. Financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Natural Science Foundation of Education Committee of Jiangsu Province (no. 12KJB150023) is gratefully acknowledged. We also thank the testing center of Yangzhou University for TGA, SEM, TEM and XPS measurements. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.mtener.2017.08.005. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652e657. [2] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359e367.

64

L. Ni et al. / Materials Today Energy 6 (2017) 53e64

[3] M.S. Whittingham, Lithium batteries and cathode materials, Chem. Rev. 104 (2004) 4271e4302. [4] D. Linden, T.B. Reddy, Handbook of Batteries, third ed., McGraw-Hill, New York, 2002. [5] K.T. Lee, S. Jeong, J. Cho, Roles of surface chemistry on safety and electrochemistry in lithium ion batteries, Acc. Chem. Res. 46 (2013) 1161e1170. [6] O.K. Park, Y. Cho, S. Lee, H.C. Yoo, H.K. Song, J. Cho, Who will drive electric vehicles, olivine or spinel? Energy Environ. Sci. 4 (2011) 1621e1633. [7] K.H. Seng, M.H. Park, Z.P. Guo, H.K. Liu, J. Cho, Self-assembled germanium/ carbon nanostructures as high-power anode material for the lithium-ion battery, Angew. Chem. Int. Ed. 51 (2012) 5657e5661. [8] X. Zuo, C. Fan, X. Xiao, J. Liu, J. Nana, Methylene methanedisulfonate as an electrolyte additive for improving the cycling performance of LiNi0.5Co0.2Mn0.3O2/graphite batteries at 4.4 V charge cutoff voltage, ECS Electrochem. Lett. 1 (2012) A50eA53. [9] M. Jeong, M.-J. Lee, J. Cho, S. Lee, Surface Mn oxidation state controlled spinel LiMn2O4 as a cathode material for high-energy Li-ion batteries, Adv. Energy Mater. 5 (2015) 1500440. [10] S. Deng, H. Wang, H. Liu, J. Liu, H. Yan, Research progress in improving the rate performance of LiFePO4 cathode materials, Nano-Micro Lett. 6 (2014) 209e226. [11] X.P. Gao, H.X. Yang, Multi-electron reaction materials for high energy density batteries, Energy Environ. Sci. 3 (2010) 174e189. [12] M.S. Islam, C.A.J. Fisher, Lithium and sodium battery cathode materials: computational insights into voltage, diffusion and nanostructural properties, Chem. Soc. Rev. 43 (2014) 185e204. [13] N. Sonoyama, Y. Suganuma, T. Kume, Z. Quan, Lithium intercalation reaction into the keggin type polyoxomolybdates, J. Power Sources 196 (2011) 6822e6827. [14] S. Uematsu, Z. Quan, Y. Suganuma, N. Sonoyama, Reversible lithium chargeedischarge property of Bi-dapped Keggin-type polyoxovanadates, J. Power Sources 217 (2012) 13e20. [15] H. Wang, S. Hamanaka, Y. Nishimoto, S. Irle, T. Yokoyama, H. Yoshikawa, K. Awaga, In operando x-ray absorption fine structure studies of polyoxometalate molecular cluster batteries: polyoxometalates as electron sponges, J. Am. Chem. Soc. 134 (2012) 4918e4924. [16] Y. Nishimoto, D. Yokogawa, H. Yoshikawa, K. Awaga, S. Irle, Super-reduced polyoxometalates: excellent molecular cluster battery components and semipermeable molecular capacitors, J. Am. Chem. Soc. 136 (2014) 9042e9052. [17] R. Naumaan, N. Khan, N. Mahmood, C. Lv, G. Simaa, J. Zhanga, J. Haoa, Y. Hou, Y. Wei, Pristine organo-imido polyoxometalates as an anode for lithium ion batteries, RSC Adv. 4 (2014) 7374e7379. [18] S. Hartung, N. Bucher, H.Y. Chen, R. Al-Oweini, S. Sreejith, P. Borah, Z. Yanli, U. Kortz, U. Stimming, H.E. Hoster, Vanadium-based polyoxometalate as new material for sodium-ion battery anodes, J. Power Sources 288 (2015) 270e277. [19] J.J. Chen, M.D. Symes, S.C. Fan, M.S. Zheng, H.N. Miras, Q.F. Dong, L. Cronin, High-performance polyoxometalate-based cathode materials for rechargeable lithium-ion batteries, Adv. Mater. 27 (2015) 4649e4654. [20] N. Kawasaki, H. Wang, R. Nakanishi, S. Hamanaka, R. Kitaura, H. Shinohara, T. Yokoyama, H. Yoshikawa, K. Awaga, Nanohybridization of polyoxometalate clusters and single-wall carbon nanotubes: applications in molecular cluster batteries, Angew. Chem. Int. Ed. 50 (2011) 3471e3474. [21] H. Wang, N. Kawasaki, T. Yokoyama, H. Yoshikawa, K. Awaga, Molecular cluster batteries of nano-hybrid materials between Keggin POMs and SWNTs, Dalton Trans. 41 (2012) 9863e9866. [22] K. Kume, N. Kawasaki, H. Wang, T. Yamada, H. Yoshikawa, K. Awaga, Enhanced capacitor effects in polyoxometalate/graphene nanohybrid materials: a synergetic approach to high performance energy storage, J. Mater. Chem. A 2 (2014) 3801e3807. [23] S. Wang, H. Li, S. Li, F. Liu, D. Wu, X. Feng, L. Wu, Electrochemical-reductionassisted assembly of a polyoxometalate/graphene nanocomposite and its enhanced lithium-storage performance, Chem. Eur. J. 19 (2013) 10895e10902. mez-Romero, Electrochemical and chemical syntheses of [24] M. Lira-Cantu, P. Go the hybrid organic-inorganic electroactive material formed by phosphomolybdate and polyaniline. application as cation-insertion electrodes, Chem. Mater. 10 (1998) 698e704. [25] H.X. Yang, T. Song, L. Liu, A. Devadoss, F. Xia, H. Han, H. Park, W. Sigmund, K. Kwon, U. Paik, Polyaniline/polyoxometalate hybrid nanofibers as cathode for lithium ion batteries with improved lithium storage capacity, J. Phys. Chem. C 117 (2013) 17376e17381. [26] J.J. Xie, Y. Zhang, Y.L. Han, C.L. Li, High-capacity molecular scale conversion anode enabled by hybridizing cluster-type framework of high loading with amino functionalized graphene, ACS Nano 10 (2016) 5304e5313. [27] T. Wei, M. Zhang, P. Wu, Y.J. Tang, S.L. Li, F.C. Shen, X.L. Wang, X.P. Zhou, Y.Q. Lan, POM-based metal-organic framework/reduced graphene oxide

[28] [29]

[30] [31] [32] [33]

[34]

[35] [36] [37]

[38]

[39]

[40]

[41] [42]

[43]

[44] [45] [46] [47]

[48]

[49]

[50]

[51]

[52]

[53] [54] [55]

nanocomposites with hybrid behavior of battery-supercapacitor for superior lithium storage, Nano Energy 34 (2017) 205e214. H. Yoshikawa, C. Kazama, K. Awaga, M. Satoh, J. Wada, Rechargeable molecular cluster batteries, Chem. Commun. 30 (2007) 3169e3170. rey, F. Millange, M. Morcrette, C. Serre, M.-L. Doublet, J.-M. Grene che, J.G. Fe M. Tarascon, Mixed-valence Li/Fe-based metal-organic frameworks with both reversible redox and sorption properties, Angew. Chem. Int. Ed. 46 (2007) 3259e3263. M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer, Berlin, 1983. M.T. Pope, A. Muller, Polyoxometalate chemistry: an old field with new dimensions in several disciplines, Angew. Chem. Int. Ed. Engl. 30 (1991) 34e48. B. Ni, H. Liu, P.P. Wang, J. He, X. Wang, General synthesis of inorganic singlewalled nanotubes, Nat. Commun. 6 (2015) 8756. N. Mizuno, K. Yamaguchi, K. Kamata, Epoxidation of olefins with hydrogen peroxide catalyzed by polyoxometalates, Coord. Chem. Rev. 249 (2005) 1944e1956. Z. Kang, E. Wang, B. Mao, Z. Su, L. Gao, S. Lian, L. Xu, Controllable fabrication of carbon nanotube and nanobelt with a polyoxometalate-assisted mild hydrothermal process, J. Am. Chem. Soc. 127 (2005) 6534e6535. D.L. Long, R. Tsunashima, L. Cronin, Polyoxometalates: building blocks for functional nanoscale systems, Angew. Chem. Int. Ed. 49 (2010) 1736e1758. D. Zhou, B.H. Han, Graphene-based nanoporous materials assembled by mediation of polyoxometalate nanoparticles, Adv. Funct. Mater. 20 (2010) 2717e2722. P.E. Car, M. Guttentag, K.K. Baldridge, R. Alberto, G.R. Patzke, Synthesis and characterization of open and sandwich-type polyoxometalates reveals visiblelight-driven water oxidation via POM-photosensitizer complexes, Green Chem. 14 (2012) 1680e1688. A. Rubinstein, P. Jimenez-Lozanao, J.J. Carbo, J.M. Poblet, R. Neumann, Aerobic carbonecarbon bond cleavage of alkenes to aldehydes catalyzed by first-row transition-metal-substituted polyoxometalates in the presence of nitrogen dioxide, J. Am. Chem. Soc. 136 (2014) 10941e10948. Y. Ji, L. Huang, J. Hu, C. Streb, Y.F. Song, Polyoxometalate-functionalized nanocarbon materials for energy conversion, energy storage and sensor systems, Energy Environ. Sci. 8 (2015) 776e789. S.M. Lauinger, J.M. Sumliner, Q. Yin, Z. Xu, G. Liang, E.N. Glass, T. Lian, C.L. Hill, High stability of immobilized polyoxometalates on TiO2 nanoparticles and nanoporous films for robust, light-induced water oxidation, Chem. Mater. 27 (2015) 5886e5891. Y.X. Xu, K.X. Sheng, C. Li, G.Q. Shi, Self-assembled graphene hydrogel via a one-step hydrothermal process, ACS Nano 4 (2010) 4324e4330. D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, Acs Nano 4 (2010) 4806e4814. K. Li, J. Hu, W. Li, F. Ma, L. Xu, Y. Guo, Design of mesostructured H3PW12O40silica materials with controllable ordered and disordered pore geometries and their application for the synthesis of diphenolic acid, J. Mater. Chem. 19 (2009) 8628e8638. S. Eigler, C. Dotzer, A. Hirsch, Visualization of defect densities in reduced graphene oxide, Carbon 50 (2012) 3666e3673. X.R. Zeng, T.M. Ko, Structures and properties of chemically reduced polyanilines, Polymer 39 (1998) 1187e1195. E. Erdem, M. Karakısla, M. Saçak, The chemical synthesis of conductive polyaniline doped with dicarboxylic acids, Eur. Polym. J. 40 (2004) 785e791. E.C. Gomes, M.A.S. ́Oliveira, Chemical polymerization of aniline in hydrochloric acid (HCl) and formic acid (HCOOH) media. differences between the two synthesized polyanilines, Am. J. Polym. Sci. 2 (2012) 5e13. M. Canales, J. Torras, G. Fabregat, A. Meneguzzi, C. Aleman, Polyaniline emeraldine salt in the amorphous solid state: polaron versus bipolaron, J. Phys. Chem. B 118 (2014) 11552e11562. D. Giray, T. Balkan, B. Dietzel, A.S. Sarac, Electrochemical impedance study on nanofibers of poly (m-anthranilic acid)/polyacrylonitrile blends, Eur. Polym. J. 49 (2013) 2645e2653. N. Zehani, S.V. Dzyadevych, R. Kherrat, N.J. JaffrezicRenault, Sensitive impedimetric biosensor for direct detection of diazinon based on lipases, Front. Chem. 2 (2014) 44. F.Y. Xie, L. Gong, X. Liu, Y.T. Tao, W.H. Zhang, S.H. Chen, H. Meng, J. Chen, XPS studies on surface reduction of tungsten oxide nanowire film by Arþ bombardment, J. Electron. Spectrosc. Relat. Phenom. 185 (2012) 112e118. M. Vasilopoulou, A. Soultati, D.G. Georgiadou, T. Stergiopoulos, L.C. Palilis, S. Kennou, N.A. Stathopoulos, D. Davazogloua, P. Argitisa, Hydrogenated under-stoichiometric tungsten oxide anode interlayers for efficient and stable organic photovoltaics, J. Mater. Chem. A 2 (2014) 1738e1749. T. Matsunaga, H. Daifuku, T. Nakajima, T. Kawagoe, Development of polyanilineelithium secondary battery, Polym. Adv. Technol. 1 (1990) 33e39. F. Goto, K. Abe, K. Ikabayashi, T. Yoshida, H. Morimoto, The polyaniline/lithium battery, J. Power Sources 20 (1987) 243e248. L. Yang, W. Qiu, Q. Liu, Polyaniline cathode material for lithium batteries, Solid State Ion. 86‒88 (1996) 819e824.