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Nano-Sized Structurally Highly Disordered Metal Oxide Composite Aerogels as High-Power Anodes in Hybrid Supercapacitors Haijian Huang, Xing Wang, Elena Tervoort, Guobo Zeng, Tian Liu, Xi Chen, Alla Sologubenko, and Markus Niederberger ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b09062 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Nano-Sized Structurally Highly Disordered Metal Oxide Composite Aerogels as High-Power Anodes in Hybrid Supercapacitors Haijian Huang,† Xing Wang,‡¶ Elena Tervoort, † Guobo Zeng, † Tian Liu, † Xi Chen, † Alla Sologubenko, § and Markus Niederberger*,† †

Laboratory for Multifunctional Materials, Department of Materials, ETH Zurich, VladimirPrelog-Weg 5, 8093 Zurich, Switzerland.

‡

Institute for Chemistry and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland.

¶

Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland. §

Laboratory for Nanometallurgy, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland.

ABSTRACT

A general method for preparing nano-sized metal oxide nanoparticles with highly disordered crystal structure and their processing into stable aqueous dispersions is presented. With these


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nanoparticles as building blocks, a series of nanoparticles@reduced graphene oxide (rGO) composite aerogels are fabricated and directly used as high-power anodes for lithium-ion hybrid supercapacitors (Li-HSCs). To clarify the effect of the degree of disorder, control samples of crystalline nanoparticles with similar particle size are prepared. The results indicate that the structurally disordered samples show a significantly enhanced electrochemical performance compared to the crystalline counterparts. In particular, structurally disordered NixFeyOz@rGO delivers a capacity of 388 mAh g˗1 at 5 A g˗1, which is 6 times that of the crystalline sample. Disordered NixFeyOz@rGO is taken as an example to study the reasons for the enhanced performance. Compared with the crystalline sample, density functional theory (DFT) calculations reveal a smaller volume expansion during Li+ insertion for the structurally disordered NixFeyOz nanoparticles and they are found to exhibit larger pseudocapacitive effects. Combined with an activated carbon (AC) cathode, full-cell tests of the lithium-ion hybrid supercapacitors are performed, demonstrating that the structurally disordered metal oxide nanoparticles@rGO||AC hybrid systems deliver high energy and power densities within the voltage range of 1.0-4.0 V. These results indicate that structurally disordered nanomaterials might be interesting candidates for exploring high-power anodes for Li-HSCs.

KEYWORDS: metal oxides, crystallinity, power density, energy density, pseudocapacitance, aerogel, hybrid supercapacitors.

With the rechargeable battery technology moving to an era of medium/large-scale applications such as electric vehicles, hybrid electric vehicles and smart grid energy technology, the development of next-generation energy storage devices offering not only high energy density but


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also high power density becomes more and more imperative.1 Rechargeable lithium-ion batteries (LIBs), as a commercialized energy storage device, have attained great success owing to their competitive superiority in energy density.2-4 However, their energy storage mechanisms are based on Faradaic chemical reactions, which are highly sluggish, making the power density and cycle life of batteries far from satisfactory.5-7 Alternatively, supercapacitors exhibit rapid power delivery via fast non-Faradaic surface ion adsorption/desorption, but suffer from low energy density, because charges are only stored physically on the surface.8-10 As a result, bridging the complementary features of LIBs and supercapacitors is becoming an important issue to satisfy the demands from the emerging medium/large-scale applications in terms of both energy and power. To integrate high-energy output with high-power delivery, lithium-ion hybrid supercapacitors (Li-HSCs) have been explored in recent years.11-15 A typical Li-HSC consists of a supercapacitor-type cathode and a LIB-type anode in a Li salt containing electrolyte. The nonFaradaic capacitive cathode endures high-rate charge/discharge processes, while the Faradaic LIBs-type anode can effectively store more charges, providing an opportunity to efficiently combine high energy density with high power density within one device.16 Aside from the attractive advantages, however, one of the major obstacles for Li-HSCs is the imbalanced kinetics between the sluggish Faradaic anode and the capacitive cathode, which results in a high overpotential of the cathode and prevents full energy utilization of the anode.14 Accordingly, combining appropriate electrode materials, in particular developing high-power Faradaic anodes, to surmount the kinetic imbalance in the device remains challenging on the way to fabricating high performance Li-HSCs.6, 14, 17 To remedy the kinetics problem on the anode part in Li-HSCs, nanostructured materials show great promise.18-22 As the characteristic time constant for diffusion is proportional to the square


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of the diffusion length, lithium storage in nanosystems can be enormously fast.23 With decreasing particle size, larger surface areas of the nanomaterials enable pseudocapacitive behaviour that can be observed in many discharge curves close to 0 V (vs Li+/Li).6 This pseudocapacitive mechanism bridges the gap between a supercapacitor and a battery electrode.16, 23

In addition to the particle size, also their atomic structure plays an important role. Poorly crystalline or amorphous materials, composed of a large number of structural defects and a high degree of disorder, are capable of enhancing the alkali-ion diffusion coefficient.24,

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types of structurally disordered materials have been studied for application in high-power anodes.26-30 However, there are only very few reports on structurally disordered materials at the nanoscale. Xueliang Sun et al.30 reported amorphous tin oxide with particle size of 3-5 nm prepared by atomic layer deposition (ALD). In our work, we present a general method for synthesizing metal oxides (FexOy, CoxFeyOz, NixFeyOz) optimized for their use as high power anodes by combining these two attractive properties, namely small particle size and high structural disorder. Although the electrochemical performances of defective and crystalline materials were compared in some reports,29, 30 they typically differed in particle sizes, which made it difficult to correlate the electrochemical performance unambiguously to the degree of crystallinity. Thus, to study the effect of structural disorder on the electrochemical performance, structurally disordered and crystalline metal oxides with the same particle size are synthesized and compared in this study. Only in such a case, the influence of the particle size is excluded, providing the opportunity to study exclusively the impact of structural disorder on the electrochemical performance. In addition, density functional theory (DFT) calculations are


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performed to identify differences in the structural change during Li+ insertion in the crystalline and the disordered samples. To equip our electrodes with high electronic conductivity and flexible mechanical properties, reduced graphene oxide (rGO) is mixed with the metal oxide nanoparticles. For high-power anodes, rGO-based materials with a porous structure and high surface area benefit particularly well from the pseudocapacitive effect generated by the surface redox reactions.6, 14, 20, 31-33 In our study, the as-prepared nanoparticles are homogeneously embedded onto rGO nanosheets to form a composite aerogel. The obtained aerogel is directly used as a binder-free anode for Li-HSCs, which to the best of our knowledge has not been previously reported for lithium-ion hybrid devices. Combining the advantages of a small particle size and a highly disordered structure of the metal oxides with the hierarchical architecture of the composite aerogel, promising highpower anodes for lithium-ion hybrid supercapacitors can be expected.

RESULTS AND DISCUSSION

Scheme 1 illustrates the experimental procedure for the preparation of the materials. As control samples, a series of crystalline metal oxide (Fe3O4, CoFe2O4, NiFe2O4) nanoparticles (C-NPs) are synthesized through a one-pot microwave-assisted non-aqueous benzyl alcohol route developed before in our group.34, 35 To obtain structurally disordered metal oxide nanoparticles (D-NPs), 1,3-propanediol is employed during the synthesis (details in the methods part) to hinder the crystal growth, thus leading to the formation of a highly disordered structure. Powder X-ray diffraction (XRD) measurements are used to characterize the degree of crystallinity of the samples prepared by the two synthesis routes with and without 1,3-propanediol. As shown in Figure 1, broad, but clearly visible diffraction peaks are observed for the control samples, which


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are identified as Fe3O4 (ICSD PDF No. 01-089-0688), CoFe2O4 (ICSD PDF No. 00-022-1086) and NiFe2O4 (ICSD PDF No. 00-044-1485), respectively. By adding 1,3-propanediol during the synthesis, the crystallinity of the as-obtained powders significantly decreases, resulting in a highly disordered structure as confirmed by the diffraction patterns, in which the reflections nearly completely disappear. These results confirm that the growth of an ordered crystalline structure is successfully inhibited by the addition of 1,3-propanediol. We assume that the formation of the disordered structure is due to the coordination properties of 1,3-propanediol, suppressing, or at least strongly limiting, the crystallization of the nanoparticles.

Scheme 1. Schematic of the preparation of crystalline and structurally disordered composite aerogels. Photographs of the colloidal dispersions of the crystalline (a) and the structurally disordered metal oxide nanoparticles (b) together with a structural model of the crystalline and structurally disordered NixFeyOz as a representative example (color scheme: blue (Fe), yellow


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(Ni), red (O)). HRTEM images of a crystalline (c) and a structurally disordered NixFeyOz nanoparticle (d).

Figure 1. XRD patterns of the as-prepared crystalline and structurally disordered metal oxide nanoparticles together with D-NPs@rGO (vertical bars: reference patterns of Fe3O4 (ICSD PDF No. 01-089-0688), Fe2O3 (ICSD PDF No. 01-084-0308), CoFe2O4 (ICSD PDF No. 00-022-1086) and NiFe2O4 (ICSD PDF No. 00-044-1485)).

In the next step, the as-obtained structurally disordered nanoparticles are homogeneously anchored onto rGO sheets. To achieve high homogeneity of the nanoparticles in the final composite, colloidally stable nanoparticle dispersions need to be prepared beforehand. Previous study found that nanoparticles prepared by the benzyl alcohol route can be stabilized in polar


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solvents through the addition of 2-[2-(2-methoxyethoxy) ethoxy] acetic acid (MEEAA) as an efficient ligand.36, 37 Accordingly, MEEAA is also employed in this study to functionalize the structurally disordered nanoparticles to obtain stable aqueous dispersions as shown in Scheme 1(a,b). The as-obtained dispersions with a high mass concentration of 15 mg/ml are nearly transparent, indicating the good dispersibility of the nanoparticles. In fact, the aqueous dispersions are colloidally stable for months. The dispersions are then mixed with graphene oxide (GO), transferred onto the Ti current collector as mould and cured at 95°C. The GO sheets decorated with the structurally disordered nanoparticles start to cross-link and self-assemble into a macroscopic porous gel. After a supercritical drying step, the porous structure of the material is well maintained due to the absence of any capillary forces. The obtained composites are further annealed at 300°C for 2 h under continuous N2 flow to acquire the final D-NPs@rGO aerogels. According to thermogravimetric analyses the carbon content of the composites is around 20 wt% (Figure S8). The annealing temperature is selected such that the graphene oxide gets reduced, while preventing the crystallization of the metal oxide nanoparticles. To confirm the reduction of GO at such low temperature, X-ray photoelectron spectroscopy (XPS) measurements are carried out (Figure S1). As an example, the C 1s high resolution spectra of the annealed and the pristine NixFeyOz@GO samples (Figure S1e,f) are discussed in this paragraph, although the trend is the same for all samples (Figure S1a-d). The envelope of the C 1s peak can be deconvoluted into four components that are assigned to C-C/C=C (aromatic rings, 284.8 eV), C-O (286.5 eV), C=O (288.5 eV) and O-C=O (carboxyl groups, 290.6 eV). As expected, the signal corresponding to the C-O group is significantly decreased for the annealed sample, indicating that the oxygen-containing functional groups on the GO sheets are partially removed through the annealing at 300°C. In addition, Raman spectroscopy is performed on the


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annealed DPs@rGO and the data compared with that of the pristine GO. As shown in Figure S2, the prominent carbon peaks centered at around 1340 and 1590 cm-1 can be attributed to the Dand G-bands of graphene, respectively.38 The ID/IG ratios of all the annealed DPs@rGO samples are increased compared to that of the pristine GO, further confirming the reduction of GO in the annealed samples.33, 38-40 Such reduced GO sheets with the restoration of the sp2 hybridization imply a good electronic conductivity, enabling the rGO sheets to serve as the conductive network between the nanoparticles.41 The crystallinity of D-NPs@rGO is investigated by XRD measurements (Figure 1). Weak and broad peaks are characteristic for all the D-NPs@rGO samples, indicating the preservation of the disordered structure after annealing. Nevertheless, in comparison to the pristine nanoparticles, the reflections are more intensive due to the presence of a small number of nanoparticles with enhanced crystalline order as pointed out by the TEM results discussed below. Increase of crystallinity upon annealing is a widely observed phenomenon.42, 43 CoxFeyOz@rGO only shows a very broad peak with low intensity. In the case of the structurally disordered NixFeyOz@rGO, the diffraction peaks, also low in intensity, can be assigned to the cubic NiFe2O4 phase (ICSD PDF No. 00-044-1485). For the structurally disordered FexOy@rGO sample, the weak diffraction peaks are indexed according to α-Fe2O3 (ICSD PDF No. 01-084-0308). The disordered crystal structure of the annealed composite samples is further confirmed by a detailed transmission electron microscopy (TEM) study, including scanning TEM (STEM) imaging, high resolution TEM (HRTEM) imaging and energy dispersive spectroscopy (EDS) analyses in the STEM operation mode (Figure 2, Figure S4 and Figure S5). In the following, the results for NixFeyOz@rGO will be discussed in more depth (Figure 2). The element distribution maps are acquired in EDS-STEM using an atomic number sensitive high angle annular dark field


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detector (HAADF). Figure 2(c) reveals the co-existence of Ni and Fe inside the NixFeyOz nanoparticles. The STEM micrograph in Figure 2(d) displays the area of the TEM support foil covered with rGO nanosheets. The nanosheets are decorated with numerous tiny nanoparticles. The histogram and GaussAmp fitted curve of the particle size distribution in the inset of Figure 2(d) reveals that the average size of the structurally disordered NixFeyOz nanoparticles is about 4.5 nm, which is further confirmed by the HRTEM micrograph in Figure 2(e). Extensive Fast Fourier Transform (FFT) analyses of different areas in such HRTEM images (see insets in Figure 2(e-f)) does not show any well-defined reflections and thus confirm that the crystal structure of the nanoparticles is highly disordered. A representative HRTEM image of one NixFeyOz nanoparticle is shown in Figure 2(f), displaying a distinctly disordered structure over its whole volume. In fact, the same observation is made in most of the areas on the TEM grid, confirming the disordered structure of the vast majority of the nanoparticles. Nevertheless, there is also a small number of nanoparticles with a crystalline structure (Figure S3), which results in the development of weak XRD reflections in the corresponding XRD patterns. For comparison, a TEM overview image of crystalline NiFe2O4@rGO is presented in Figure 2(g). Although the average particle size of about 6 nm is slightly larger than the 4.5 nm of the disordered sample, the size distribution of both types of nanoparticles are well comparable. The crystalline nanoparticles are also homogeneously distributed on the rGO sheets (Figure 2(g)). Magnified images of the area in Figure 2(g) are presented in Figure 2(h-i). The images and the corresponding FFTs in the insets indicate a highly ordered atomic structure. The evaluation of the FFTs from the particles from different areas on the TEM grid confirms the high degree of structural order in the material. Figure 2(i) presents an example of a representative particle, where the FFT yields a spacing of 2.96 Å, corresponding to d220 of the NiFe2O4 (ICSD PDF No.


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00-044-1485) phase. The FFT indexing results in the 220, 331, and 400 reflections of the spineltype NiFe2O4. The results clearly confirm the high crystallinity of the nanoparticles. The same trend of TEM results is found for FexOy@rGO/Fe3O4/rGO (Figure S4) and for CoxFeyOz@rGO/CoFe2O4@rGO (Figure S5) and therefore the data is not further discussed anymore. To compare the disordered and the crystalline materials further, XPS is performed. The highresolution XPS spectra of Ni 2p, Fe 2p and O 1s of the structurally disordered NixFeyOz@rGO and of the corresponding crystalline NiFe2O4@rGO are shown in Figure S6. There is a pronounced chemical shift to higher binding energies for Ni 2p and Fe 2p and to lower binding energy for O 1s in the structurally disordered NixFeyOz@rGO. While the reversed chemical shift for O 1s excludes the possibility of miscalibration, the results indicate different chemical environments for the atoms in the disordered and the crystalline sample.


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Figure 2. (a) SEM overview image of the macroporous NixFeyOz@rGO aerogel. (b) HAADF image with (c) the corresponding element distribution maps. (d) STEM images of the structurally disordered NixFeyOz@rGO and (g) the crystalline NiFe2O4@rGO (insets: histograms and GaussAmp fitted curves of the corresponding particle size distributions). e, f) HRTEM images of the structurally disordered NixFeyOz@rGO and h, i) the crystalline NiFe2O4@rGO (insets: corresponding FFT patterns).


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For high-power anodes, surface reactions during charge/discharge play a critical role.18 With the traditional electrode preparation procedure, the active materials are grinded with binder and conductive additives, resulting in aggregation of the nanoparticles and thus leading to reduced surface active sites. In this study, after supercritical drying the aerogel on the current collector can directly be used as the anode. The nanoparticles are homogeneously embedded in the porous hierarchical structure, enabling the aerogel anodes to take full advantage of the small particle size of the nanoparticles. As shown in the SEM images in Figure 2(a), Figure S4(a) and Figure S5(a), disordered open macroporosity is clearly visible in all D-NPs@rGO samples. The pore size distribution obtained by N2 gas sorption measurements reveals the co-existence of mesoporosity and macroporosity with pore sizes between 5-70 nm (Figure S7). The observation that our method enables anchoring of the nanoparticles individually on the rGO sheets is a prerequisite to make a meaningful comparison between the electrochemical performance of the structurally disordered and the crystalline metal oxide nanoparticles under exclusion of any effects stemming from different agglomeration behavior. To study the electrochemical performance, half-cells are fabricated using the aerogel samples directly as one electrode and lithium foil as the counter and reference electrode. As a typical example, the results of NixFeyOz@rGO are discussed in more details. Figure 3(a) shows the first five galvanostatic discharge-charge curves of the structurally disordered and crystalline anodes at a current density of 0.1 A g˗1 within a voltage window of 0.001-3 V. In the initial cycle, the crystalline NiFe2O4@rGO anode delivers a discharge capacity of 1461 mAh g˗1 and a charge capacity of 706 mAh g˗1, corresponding to an irreversible capacity loss of 51.6%, which can be related to solid-electrolyte interphase (SEI) film formation.44-46 In comparison, the discharge and charge capacity of the structurally disordered NixFeyOz@rGO anode reaches 2534 mAh g˗1 and


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1156 mAh g˗1, respectively. The Coulombic efficiencies of 48.3% for the crystalline and 45.6% for the structurally disordered samples are comparable, but relatively low. The initial capacity loss of both samples is presumably a result of the high surface areas, which facilitate irreversible lithium loss due to the formation of SEI layer and the decomposition of electrolyte.47, 48 This problem, which is commonly observed for high-surface-area materials,49-53 can be solved to some extent by a prelithiation procedure.54 Our results also demonstrate that the initial capacity of the structurally disordered sample is much higher than that of the crystalline one. Further analysis reveals that during discharging within the voltage range of 0.8-3.0 V, there is a more pronounced drop in the capacity of the crystalline NiFe2O4@rGO compared to the structurally disordered NixFeyOz@rGO, indicating lower polarization of the disordered composite during cycling. This reduced polarization can probably be attributed to the structure without long-range order, providing percolation pathways for fast Li+ diffusion.24, 25 Both samples display a potential plateau at around 0.8 V, which corresponds to the reduction of Ni2+ to Ni0 and Fe3+ to Fe0 inside the Li2O matrix.55 A sloping curve observed from 0.8 V down to the cut-off voltage of 0.001 V for both samples can be attributed to the formation of SEI and double side adsorption of Li+ on the reduced graphene oxide layers.45, 46 From the second cycle on, the capacity within the voltage range of 0.001-0.8V is significantly reduced due to the irreversibility of SEI formation. Similar to the results obtained within the range of 0.8-3.0 V, the capacity of the crystalline NiFe2O4@rGO also decreases much faster within 0.8-0.001 V, underlining the reduced polarization of the structurally disordered sample. On the first charge, the peaks at 1.3 V and 1.7 V for both samples correspond to the oxidation of metallic iron and nickel and the partial decomposition of SEI.20, 55 More interestingly, it is observed that the profiles of the structurally disordered NixFeyOz@rGO in the following four cycles almost overlap with each other, while the


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capacity of the crystalline NiFe2O4@rGO anode keeps dropping with values much lower than those of the disordered composite, clearly indicating that the disordered NixFeyOz@rGO anode shows much better reversibility and capacity stability during cycling. Similar results are also found in the cases of FexOy and CoxFeyOz, which are shown in Figure S10. In this context it is important to remember that the Li+ storage mechanisms of the metal oxides involved in this study are all conversion-type.55-58


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Figure 3. (a) Galvanostatic charge-discharge profiles at 0.1 A g˗1 and (b) comparison of the rate performance of the structurally disordered NixFeyOz@rGO and the crystalline NiFe2O4@rGO


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composites. (c) CV curves of structurally disordered NixFeyOz@rGO at various sweep rates ranging from 0.2-1.0 mV s˗1 (inset: typical examples of the dependence of i/v1/2 on v1/2 for obtaining k1 values). (d) Voltammetric response of structurally disordered NixFeyOz@rGO at a scan rate of 1 mV s-1 (inset: comparison of the capacitive contributions of the structurally disordered NixFeyOz@rGO and the crystalline NiFe2O4@rGO at various scan rates (0.2, 0.3, 0.5, 0.7, 1.0 mV s-1)). (e) Long-term stability performance of the structurally disordered NixFeyOz@rGO and the crystalline NiFe2O4@rGO at 5 A g-1 and Coulombic efficiency of structurally disordered NixFeyOz@rGO.

Figure 3(b) compares the rate performance of the structurally disordered NixFeyOz@rGO with the crystalline NiFe2O4@rGO. With current density increasing from 0.1 A g˗1 to 5 A g˗1, much higher specific capacity can be retained in the disordered sample. In Figure S10(b) and Figure S10(d), similar results are obtained for disordered FexOy and CoxFeyOz. The rate capability results of the D-NPs@rGO composite aerogels confirm that the materials can be stably employed as anodes for Li-SHCs. To reveal further reasons for the electrochemical performance enhancement of the structurally disordered samples, DFT calculations and analysis of the kinetics are carried out (details in the methods section). As shown in Figure 4, the DFT results demonstrate that during the first cycle, the volume expansion of the crystalline structure after 1 Li+ insertion per unit cell is 8%, while it is only 6% for the disordered structure. The detailed values for the volume of the structural units are shown in Table S1. The smaller volume expansion of the disordered structure will efficiently suppress pulverization during cycling.28-30 The analysis of the kinetics is based on the cyclic voltammetry (CV) data of the structurally disordered NixFeyOz@rGO at sweep rates ranging


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from 0.2-1 mV/s as shown in Figure 3(c). Utilizing the method proposed by Dunn and coworkers, one can quantitatively separate the contributions from surface capacitive effects and diffusion-controlled insertion processes through analyzing the dependence of the CV curves on the sweep rates. According to this method, the current response at a fixed potential can be expressed as the combination of the current contributions from the surface capacitive effects and the diffusion-controlled insertion reactions:18

i = k1v + k2v1/2

Equation 1

where i is the current (A), v is the sweep rate (mV s-1), and k1v and k2v1/2 characterize the surface capacitive and the diffusion-limited contributions at a given potential. Equation 1 can be rearranged by dividing both sides by v1/ 2 , resulting in i / v1/2 = k1v1/2 + k2 . k1 and k 2 can be determined by plotting i / v1/2 versus v1/ 2 . Typical examples at specific potentials of 0.4 V and 1.8 V are shown in the inset of Figure 3(c). This analysis enables us to determine the relative contributions of the capacitive processes by comparing the shaded area with the total stored charge. As a result, Figure 3(d) shows that 64.7% of the total capacity of NixFeyOz@rGO at the scan rate of 1 mV s˗1 is attributable to the surface capacitive effect, indicating that capacitive charge storage is the dominant mechanism in the structurally disordered anode. In the inset of Figure 3(d), the contribution of the capacitive currents as obtained from the voltammetric response of structurally disordered NixFeyOz@rGO and the crystalline NiFe2O4@rGO aerogels at various scan rates (0.2, 0.3, 0.5, 0.7, 1.0 mV s˗1) is compared (the CV curves and detailed voltammetric response of the crystalline NiFe2O4@rGO are shown in Figure S9). The results illustrate that larger capacitive effects are observed for the structurally disordered sample. This can be due to the enhanced lithium diffusion kinetics in the structurally disordered NixFeyOz@rGO aerogel, leading to more pseudocapacitive reactions not only on the surface, but


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also within the near-surface region. As capacitive charge storage endows stable cycling performance, the enhanced surface pseudocapacitive effects in the structurally disordered sample result in higher capacity retention and better rate capability consistent with our observations (Figure 3(a) and Figure 3(b)). In addition, to corroborate the good electronic conductivity provided by the reduced graphene oxide in the NixFeyOz@rGO aerogel, electrochemical impedance spectroscopy (EIS) measurements are performed. Nyquist plots are recorded for NixFeyOz@rGO before and after 500 cycles at 5 A g˗1 (Figure S11). The plots consist of a semicircle in the high-frequency region, which is related to the charge transfer reaction and an incline in the low-frequency region, which is due to a diffusion-controlled process.59, 60 In Figure S11(a), the Nyquist plot of NixFeyOz@rGO (after annealing) is compared with pristine NixFeyOz@GO (without annealing) as a control sample. The diameter of the semi-circle in the high-frequency region is significantly reduced in the plot of NixFeyOz@rGO compared with that of the control sample, indicating that the reduced graphene oxide in NixFeyOz@rGO successfully decreases the charge-transfer resistance at the electrode/electrolyte interface. The enhanced conductivity enables higher transfer efficiency of e- and Li+. Interestingly, the results in Figure S11(b) show that the electrochemical impedance is greatly decreased for NixFeyOz@rGO after 500 cycles compared with that before cycling. This phenomenon has been reported in the literature and can be attributed to the activation process during charge/discharge, forming additional channels for ion transport.60, 61 To further elucidate the excellent electrochemical behavior of the structurally disordered NixFeyOz@rGO aerogel, long-term stability tests are performed and the result is shown in Figure 3(e). The NixFeyOz@rGO aerogel offers ultra-high stability, and a capacity of 388 mAh g˗1 is maintained after 500 cycles at 5 A g˗1 with an average Coulombic efficiency close to 100%. In


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contrast, the crystalline NiFe2O4@rGO aerogel can only discharge 59 mAh g˗1 after 500 cycles at the same current density of 5 A g˗1. Table S2 provides a comparison of the electrochemical performances between the structurally disordered NixFeyOz@rGO anode developed in this study and some other high power anodes recently reported in the literature, giving further evidence for the high potential of NixFeyOz@rGO aerogels as anodes in Li-HSCs.

Figure 4. Structural volume changes in crystalline and structurally disordered NiFe2O4 during the insertion of 1 Li+ per unit cell calculated with DFT (color scheme: red (O), blue (Fe), yellow (Ni) and green (Li)). Each structural model shows 16 unit cells.

Full cells of Li-HSCs are assembled using the composite aerogel as the anode and commercial activated carbon (denoted as AC, YP-50F from Kurary) as the cathode. Before fabricating the full cells, the as-prepared anode is pre-lithiated to reach the main insertion voltage (~0.7 V), thus keeping the anodic potential as low as the lithiation plateau. Hence, together with the AC


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cathode pair (3-4.5 V), a large operation voltage range (1-4 V) for the full cells can be achieved. Figure 5(a) displays the CV profiles of the structurally disordered NixFeyOz@rGO||AC hybrid supercapacitors. The CV curves display quasi-rectangular shapes. The deviation from the ideal rectangular shape at all scan rates implies the coexistence of insertion-type energy storage mechanism for the structurally disordered NixFeyOz nanoparticles. On the other hand, Figure 5(b) shows the nearly linear relationship between the redox peak current and the scan rate, indicating dominant pseudocapacitive behavior. Thus, the CV results imply a combination of fast intercalation reactions at the anode side and rapid capacitive behavior at the cathode side. As an instructive way to compare the performance of the Li-HSCs, the Ragone plots in Figure 5(c) represent the trade-off between the energy density and the power density. As expected, all the three D-NPs@rGO aerogel samples demonstrate much higher energy densities than the control samples of C-NPs@rGO aerogels at the same power densities. Maximum energy densities of 81, 120 and 85 Wh kg˗1 with a power density of 100 W kg˗1 are achieved for the structurally disordered FexOy@rGO||AC, CoxFeyOz@rGO||AC and NixFeyOz@rGO||AC, respectively. In contrast,

the

values

for

the

crystalline

Fe3O4@rGO||AC,

CoFe2O4@rGO||AC

and

NiFe2O4@rGO||AC are only 52, 63, 65 Wh kg˗1, respectively. The subsequent energy density degradation is observed for all samples, which is mainly attributable to the voltage drop at higher power densities. Notwithstanding, even at a high power density of 2500 W kg˗1, the Li-HSCs systems based on the structurally disordered FexOy@rGO, CoxFeyOz@rGO and NixFeyOz@rGO aerogel anodes still deliver energy densities of 28 Wh kg˗1, 43 Wh kg˗1 and 27 Wh kg˗1, respectively. In comparison to previously reported high performance Li-HSCs, most of our systems show a better performance as evidenced from Figure 5(c). In addition to specific energy and power density, also energy efficiency is a key parameter for practical applications.2, 62, 63 The


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energy efficiencies of the structurally disordered NixFeyOz@rGO||AC and the crystalline NiFe2O4@rGO||AC are calculated and compared as shown in Figure S12. The results show that the energy efficiency of the hybrid device based on the NixFeyOz@rGO anode is higher than that of NiFe2O4@rGO||AC. Specifically, at a current density of 0.1 A g˗1 an energy efficiency of 71.1% is achieved for disordered NixFeyOz@rGO||AC, while the value is only 66.3% for crystalline NiFe2O4@rGO||AC. Even at a high current density of 2.5 A g˗1, where the voltage hysteresis is much larger, the energy efficiency of NixFeyOz@rGO||AC still reaches 41.8%, while it is only 25.2% for NiFe2O4@rGO||AC. Furthermore, the long-term stability performance is tested for the structurally disordered NixFeyOz@rGO||AC hybrid supercapacitor. As shown in Figure 5(d), the energy density of this hybrid system is well maintained up to 1000 cycles with a capacity retention of 86% at the current rate of 2.5 A g˗1, again underlining the advantages of DNPs@rGO as high-power anodes in Li-HSCs.


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Figure 5. (a) CV curves of the structurally disordered NixFeyOz@rGO||AC hybrid supercapacitors at various scan rates ranging from 2-20 mV s-1. (b) Dependence of the redox peak current on the scan rate. (c) Ragone plots of D-NPs@rGO||AC (■:structurally disordered FexOy@rGO||AC, ●: structurally disordered CoxFeyOz@rGO||AC, ▲: structurally disordered NixFeyOz@rGO||AC) in comparison to C-NPs@rGO||AC (■: crystalline Fe3O4@rGO||AC, ●: crystalline CoFe2O4@rGO||AC, ▲: crystalline NiFe2O4@rGO||AC) and to other reported hybrid supercapacitors reported in literature (◇: TiO2 nanowires||carbon nanotubes (CNT) (ref. 11),

◇:

LiTi2(PO4)3||AC (ref. 12),

◇:

◇:

V2O5||CNT (ref. 13),

LiTi2(PO4)3@ carbon||AC (ref. 22),

◇:

◇:

V2O5@ CNT||AC (ref. 15),

Nb2O5@ CNT||AC (ref. 23)). (d) Long-term cycling


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stability performance of the structurally disordered NixFeyOz@rGO||AC at a current rate of 2.5 A g-1.

CONCLUSIONS

This study presents a general way to fabricate a series of stable aqueous dispersions of structurally highly disordered metal oxide nanoparticles. Using these nanoparticles as building blocks, hierarchical nanoparticles@rGO aerogels are prepared and directly used as high-power anodes. For comparison and as control samples, the crystalline metal oxide nanoparticles@rGO counterparts are also synthesized. The structurally disordered samples show much better electrochemical performance, which can be due to the following reasons: (1) the structurally disordered samples show lower polarization during cycling, which can be due to the isotropic structure of D-NPs providing random orientation of channels for faster lithium diffusion; (2) according to DFT calculations, the volume expansion during cycling is smaller for the structurally disordered nanoparticles than for the crystalline ones; (3) the higher capacitive contribution in the total capacity makes the D-NPs@rGO aerogel anodes more tolerant to faster charge/discharge rates. As anodes for full cells of lithium-ion hybrid supercapacitors, DNPs@rGO also shows good performance. In particular, 120 Wh kg-1 and 43 Wh kg˗1 are delivered for structurally disordered CoxFeyOz@rGO at power densities of 100 W kg˗1 and 2500 W kg˗1, respectively. These results demonstrate that the degree of disorder in the crystal structure plays a vital role for the electrochemical performance, offering an alternative strategy to improve the design of high-power anodes for lithium-ion hybrid supercapacitors.

METHODS


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Chemicals. Benzyl alcohol (anhydrous 99.8%, Aldrich), Fe(III) acetylacetonate (99.9%, Aldrich), Co(II) acetylacetonate (97%, Aldrich), Ni(II) acetate tetrahydrate (99.998%, Aldrich), 1,3-propanediol (99%, Alfa Aesar), 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (technical grade, Aldrich), graphene oxide dispersion (5 mg/ml in H2O, Royal Elite (Shanghai, China)), and activated carbon (YP-50F, Kurary) were all used without further purifications. Synthesis of D-NPs. Fe(III) acetylacetonate, Co(II) acetylactonate, Ni(II) acetate tetrahydrate were used as precursors. In a typical synthesis of FexOy, 4 mmol Fe(III) acetylacetonate were added to 18 ml anhydrous benzyl alcohol, stirred until completely dissolved, followed by adding 2 ml of 1,3-propanediol. The obtained solution was stirred for another 10 min before transferring it into four microwave glass tubes with inner volume of 10 ml. The reaction mixtures were heated to 160℃ for 30 minutes in a CEM Hybrid microwave reactor. The resulting product was centrifuged at 4000 rpm for 15 min. While the solid was discarded, the supernatant was combined with 40 ml diethyl ether. The obtained precipitate was then redispersed in Milli-Q water containing MEEAA (50 µl/ml) and ultrasonicated (Elmasonic P, frequency: 37 kHz, power: 100%) until a stable dispersion formed. The concentration of this dispersion was gravimetrically determined and further diluted with the same aqueous MEEAA solution to a final particle concentration of ~15 mg/ml. For the synthesis of CoxFeyOz and NixFeyOz, 2 mmol Fe(III) acetylacetonate with 1 mmol Co(II) acetylactonate or Ni(II) acetate tetrahydrate, respectively, were dissolved in 18.67 ml anhydrous benzyl alcohol followed by addition of 1.33 ml 1,3-propanediol. The microwave reaction temperature for CoxFeyOz and NixFeyOz was 165 °C and 150 °C, respectively. All the other experimental conditions were the same as for the FexOy synthesis.


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Synthesis of C-NPs. C-NPs (Fe3O4, CoFe2O4, NiFe2O4) were synthesized as the control samples using the methods reported before in our group.34, 35 All the synthesis procedures were the same as for the defective samples except that 1,3-propanediol was substituted by the same amount of benzyl alcohol and that the microwave reactions for all these three crystalline samples were carried out at 195 oC for 10 min. Synthesis of nanoparticles@rGO aerogels. The nanoparticle dispersion (~15 mg/ml) was homogeneously mixed with the GO dispersion (typically 5 mg/ml in water) through ultrasonication (Elmasonic P, frequency: 37 kHz, power: 100%). The gelling process was performed according to a procedure previously reported by our group.20, 64 Typically, the mixed dispersion was transferred onto a Petri-dish like current collector (made of Ti) and then sealed in a closed bottle, followed by curing under a saturated H2O vapor environment at 95oC for 6 h. A continuous gel film was obtained. This hydrogel film was then put into an excess of acetone for the solvent exchange. This solvent exchange procedure was repeated two times, until water as pore liquid was completely replaced by acetone. The gel was then dried in supercritical CO2 using a tousimis Autosamdri®-931. The aerogel was further annealed at 300oC for 2 h under N2 atmosphere with a ramping rate of 3 oC/min to obtain the final product. Characterization. Powder X-ray diffraction (XRD) measurements were taken on a PANalytical Empyrean equipped with a Cu Kα X-ray tube (45 kV, 40 mA) and a monochromator. For the morphology study, scanning electron microscopy (SEM) on a LEO 1530 Gemini and transmission electron microscopy (TEM) on an X-FEG FEI Talos were performed. X-ray photoelectron spectra (XPS) were measured with a Sigma 2 spectrometer (Thermo scientific) using a polychromatic Al Kα X-ray source, where binding energy was calibrated taking C 1s = 284.8 eV. Raman measurements were performed on a Raman


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spectrometer (NT-MDT, Russia, NTEGRA Spectra Upright) using 532 nm green laser as the excitation source. N2 gas sorption measurements were carried out on a Quantachrome Autosorb iQ at 77 K. Prior to the mearuement, the samples were outgassed at 100oC for at least 24 h. The surface area was determined by the Brunauer-Emmett-Teller (BET) method and the pore size distribution was obtained by a density functional theory (DFT) analysis using a Non Local DFT (NLDFT) calculation model for nitrogen at 77 K on cylindrical pores in silica. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA851e instrument. Electrochemical measurements. The as-obtained aerogel on the current collector was dried in the vacuum oven at 80 °C overnight and then directly used as the electrode without any other additives. The typical mass loading was 0.5-0.8 mg/cm2. In a typical AC electrode preparation, 70 wt% of AC, 20 wt% of carbon black and 10 wt% of polyvinylidene fluoride (PVDF) were homogeneously mixed in N-methyl-2-pyrrolidinone (NMP) under mechanical stirring. The typical mass ratio between the AC cathode and the aerogel anode is 3.5:1. The electrochemical performances were evaluated in Swagelok-type cells assembled in an argon-filled glove box (H2O, O2

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