Tailoring Hollow Nanostructures by Catalytic Strategy for Superior

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Functional Nanostructured Materials (including low-D carbon)

Tailoring Hollow Nanostructures by catalytic strategy for Superior Lithium and Sodium Storage Kexuan Liao, Huanhuan Wei, Jinchen Fan, Qunjie Xu, and YuLin Min ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17541 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 20, 2018

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ACS Applied Materials & Interfaces

Tailoring Hollow Nanostructures by Catalytic Strategy for Superior Lithium and Sodium Storage Kexuan Liao a,b#, Huanhuan Weia,b#, Jinchen Fana,b*, Qunjie Xua,b*, Yulin Mina,b* a

Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power,

Shanghai Engineering Research Center of Energy-Saving in Heat Exchange Systems; Shanghai University of Electric Power, Shanghai 200090, P. R. China bShanghai #

Institute of Pollution Control and Ecological Security, Shanghai 200092, P.R. China

Kexuan Liao and Huanhuan Wei contributed equally to this work.

*E-mail:

[email protected]; [email protected]; [email protected];

ABSTRACT Nowadays, a novel catalyzed strategy for designing 3D carbon nanosheet frameworks is widely concerned in the field of energy storage. Herein, 3D hollow structure with nickel and nanographitic domains is presented to fabrication of functionalized with hollow microporous carbon embedded with expanded defective nanographitic domains or hollow nickel oxide composites followingly. The hollow microporous carbon coupling nanographitic domains exhibits excellent long-term cyclicity (4000 cycles for lithium storage, 2000 cycles for sodium storage), which is mainly due to the formation of defects in the nanographite for catalytic strategy. The hollow nickel oxide composites show the capacities of 1093 mA·hg−1 after 400 cycles with the high coulombic efficiency at a current density of 200 mA·g−1 for lithium storage and superior rate performance at different current densities for sodium storage. Stable and great energy storage features stem from the fact that the hollow structure can provide more active sites for ionic diffusion/storage and a free shuttle space for electrons.

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KEYWORDS: catalyzed strategy, nanographite, hollow structure, lithium ion batteries, sodium ion batteries

1. INTRODUCTION The increasing demand of rechargeable lithium and sodium ion batteries (LIBs/SIBs) with high capacities and low costs is balanced with electrode materials that can show longer and more stable cyclicity as well as higher storage energy.1-5 Considering the widespread availability, enhanced safety, and environmental benignity, hard carbons (HCs) and transitional metal oxides (TMOs) are promising anodes in this balance.6-10 The “house of cards” insertion and conversion reaction with ions are the basis for HCs and TMOs as anodes for lithium/sodium storage, respectively.8, 11-12 However, low conductivity and volume expansion block further increase in specific capacity and shorten the cycle.13 The employment of graphitizing or integrating conductive component and hollow structure will improve the stability and reaction kinetics of HCs or TMOs anodes.14-15 The electrons in the process of repeated charge and discharge rapidly pass through the interface between the electrode and the electrolyte to penetrate into the electrode to achieve rapid ion diffusion and shorten the transmission channel.15 This improved method curbs volumetric trends and enhances the cyclability of the electrode.8 On the other hand, the application of hierarchical porosity in 3D framework further increases the specific surface area (SSA) to provide more active sites and improve the wettability of the electrode.6, 8, 13, 16 A series of 3D hollow conductive structure engineering such as graphene aerogels, graphene foam


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composites confirm the feasibility of these schemes.14, 17-18 Accordingly, tailoring on the chemical composition and microstructure of active material are also critical for achieving high specific capacity targets.16, 19-20 Nowadays, the designed hollow structure basically has the advantages of huge SSA, high loading capacity, and thinner shell with internal free space that form a favorable ability to charge and discharge, but lacks optimization in conductivity.6, 8, 13, 21

Moreover, anode with high electron transport properties such as graphene-based

materials are costly and also scarce, which limits their large-scale practical application as electrode.22-23 Combining high conductivity and 3D hollow structure with hierarchical porous framework, a low-cost candidate will emerge to improve stability and enhance reaction kinetics that will be expected to achieve high capacity goals. Scheme 1. Schematic illustration of the catalytic fabrication of the hollow structural platform (Ni@G/C) and functionalized with microporous carbon embedded with expanded nanographite (NG/C) and metal oxide composites (NiO/C) through subsequent acidic etching and moderate oxidation, respectively.



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2. RESULTS AND DISCUSSION Herein,

we

tailor

a

nanostructural

carbon

nanosheet

frameworks

with

nickel-nanographitic domains (denoted as Ni@G/C), whereby Ni2+ ions are absorbed absolutely with superabsorbent polymers (SAPs) to enable a catalyst confined graphitization process and then assure the formation of the fingerprint nanographite uniformly (Scheme 1). The chelation effect of carboxyl and Ni2+ in hydrogel ensures the homo-dispersion of Ni2+ in the SAP segments, prompting the effective transformation of carbon from sp3 to sp2 in the catalytic process of carbonization at high temperature. As expected, the internal nanographite of Ni@G/C with interconnected architecture is highly ordered in the local microscopic region; and nanographitic carbon shells or hollow nanostructure TMOs composites are converted perfectly in subsequent etching or oxidation process. On the one hand, Ni@G/C is converted into hollow carbon embedded with expanded nanographite domains (denoted as NG/C) through acidic etching serves as anode showing long-term cycles as well as high capacities in LIBs/SIBs. On the other hand, Ni@G/C is converted into hollow nickel oxide composites (denoted as NiO/C) through moderate oxidation act as anode displaying excellent capacities and better circulation ability for lithium and sodium storage.


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Figure 1. a-b) SEM (inset: AFM), and TEM images of 3DNF; c-f) SEM, TEM (inset: histogram of the sphere of Ni@G diameter), STEM, HRTEM images and corresponding elemental mappings of Ni@G/C; g-h) TEM (inset: histogram of the sphere of nanographite diameter) and HRTEM images of hollow NG/C; i-j) TEM (inset: histogram of the hollow NiO/C diameter) and corresponding mapping images of hollow NiO/C composites; k-1, k-2, k-3 and k-4) HRTEM and corresponding mapping images of hollow NiO/C composites.

The synthetic strategy of Ni@G/C is fabricated by hydrogel forming from SAPs with the adsorption of nickel acetate solution and carbonization of the precursors from dried hydrogel at high temperature, which involves the process of catalysis confined graphitization.6,

13, 18

Typically, SAPs are used as carbon precursor without nickel

acetate to obtain 3D carbon nanosheet frameworks (denoted as 3DNF). The scanning and electron transmission microscopic (SEM and TEM) images of the 3DNF (Figure 1a; Figure 1b) illustrate the interconnected carbon nanosheets and 3D framework with a hierarchical structure. The thickness walls of 3DNF that are composed of curved



and staggered carbon nanosheet like broken bubbles is 8 nm, which is presented on the atomic force microscope (AFM) inset image. Ulteriorly, SAPs are soaked with an aqueous solution containing Ni2+ ions to make sure the uniform distribution of Ni2+ ions in the hydrogel precursor, whereby the Ni2+ is adsorbed into SAPs to form green hydrogel (Figure S1) with its super absorbent property.24 To modify the microstructures of HCs from carbonized polymers, the adoption of catalysis of confined graphitization regularly stems from the aqueous solutions of transition metal salts such as Ni2+, Co2+, and Fe3+.6,

11, 13

Therefore, to yield more and better

nanographitic domains in carbonized SAPs, it is essential that the chelation effect between catalysts ions and SAPs, which can improve the overall degree of graphitization. The precursors of nickel-containing SAPs are translated to the molten fluid under a pyrolysis-carbonization process, which be accompanied by the emergence of the huge volume of the gas in the form of bubbles by released from the fast decomposition of nickel acetate above the glass transition temperature in the interior of decomposing SAPs. Meanwhile, the Ni nanoparticles are generated and embedded into carbon frameworks at high temperature under the reaction of in situ carbon thermal reductions from the decomposition of carbon component of precursors including acetate and SAPs, which is basis of the graphitization process that occurs concurrently.11 Simultaneously, the carbon shell is accompanied by the formation of nickel nuclei in the local region, which is a catalyst confined graphitization process. In most areas away from the nickel core, the remaining carbon area still exist as


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amorphous. Interestingly, under the influence of fluid pressure, the bubble volume is continuously expanding and the thickness of the carbon shell is gradually shrinking until the bubble breaks, which generate thinner curved carbon nanosheet frameworks. Therefore, the formation of Ni@G/C with curved nanosheet can be considered as a thermal reduction process in which a nickel salt catalyzes the pyrolysis of SAPs to form the broken carbon bubble. It is displayed that there are similar stages in the carbonization process in three samples (SAPs, nickel acetate tetrahydrate, and Ni@G/C before carbonization) including the prophase of small molecule and residual water loss, the decomposition of the precursor in the high temperature as well as carbon graphitization under 650oC from thermogravimetric analysis (TGA; Figure S2). The SEM image and the corresponding inset elemental mapping images of the Ni@G/C (Figure 1c) illustrate the interconnected graphitic carbon nanoshell and the embedded Ni nanoparticles, which agree well with the initially envisaging of a catalytic strategy. The TEM image (Figure 1d) shows the random dispersion of Ni nanoparticles with an average size of 33.5 nm entirely embedding the curved carbon sheet. The elemental mapping images visualizes the homogenous distribution of Ni element in a carbon matrix, as shown in Figure 1e. From the conversation, Ni nanoparticles is formed and be accompanied by generating an ordered, fingerprint-like carbon layer, which are embedded in the curved nanosheets because of the accumulation of Ni species. High-resolution TEM (HRTEM; Figure 1f) image clearly reveals that the Ni nanoparticles are embedded in the center of the fingerprint-like graphitic carbon with multi thinner layers at ~0.4 nm



interval spacing, which is the effect of catalysis confined graphitization in the carbonization process.

Figure 2. a) XRD patterns of 3DNF, Ni@G/C, NG/C and NiO/C; b-c) Raman spectrum and nitrogen adsorption-desorption isotherms (inset: pore size distribution) of Ni@G/C; d) TGA curves of Ni@G/C and NiO/C; e-f) Ni2p XPS spectrum and nitrogen adsorption-desorption isotherms (inset: pore size distribution) of NiO/C.

As shown in Figure 2a, the sample of Ni@G/C contains metallic nickel in the X-ray diffraction (XRD) patterns, which exhibits a sharp and strong peak at 44° that can be assigned to the diffraction (111) plane of Ni (PDF04-0850).25 Consequently, the diameter of Ni nanoparticles is estimated to be 33.5 nm by TEM analysis. For comparison, a broad hump at ≈ 25.6° is attributed to the amorphous structure in XRD pattern of 3DNF corresponding without the introduction of the catalytic strategy. Accordingly, it can be found that a sharp peak at 26.2°, which indicate the plane of (002) of the nanographitic domains after etching the Ni@G/C sample and a series of standard cubic NiO phase (PDF47-1049) after oxidation the Ni@G/C sample,


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respectively.26 Figure 2b displays the representative Raman spectrum of Ni@G/C, which possesses the graphitic mode and defect-induced band (G-band, D-band) indicating the in-plane sp2 vibration and the structural defects in the graphitic structure at around 1585 cm−1and 1338 cm−1 respectively.13 It is illustrated that the structure of Ni@G/C is disordered from the ratio of peaks intensity of D-band and G-band (ID/IG=1.34) revealing the domination of disordered structure of carbon nanosheets.13 The peaks intensity ratio of the D and G-band for Ni@G/C have been heightened obviously comparing with 3DNF (ID/IG = 0.91; Figure S3), in which the ID/IG value can confirm an elevated degree of graphitization of carbon. Naturally, a two-phonon resonance peak (2D-band) of Ni@G/C reveals the extent of the stacked graphene layers of carbonized polymers under the influence of catalyst from Raman spectra results. The weak 2D-band intensity of Ni@G/C is ~2685 cm−1 furthervisualizes the formation of 2D nanosheet structure, being consistent with SEM and TEM observation and visualizing Ni@G/C with layer structure. The nanopores between the microcrystal lattice stripes in the graphitic domains is generated by the as-formed sodium species from SAPs in nickel-containing precursors when removal of impurities with acid washing. The XRD pattern of the carbonization product shows a series of diffraction peaks correspond to well crystalline Na2CO3 and Ni (Figure S4a).27 The inorganic sodium substances are removed completely from the carbonization product after washing, leaving lots of pores in the final carbon product. The N2 adsorption-desorption isotherms of Ni@G/C with features of type IV hint that micropores and mesopores coexist in the microstructure of the sample (Figure 2c).



Hierarchically structure of reveals SSA of 307 m2 g-1, which is propitious to electrochemical reaction of electrodes due to the fast transfer speed. Similarly, 3DNF shows SSA of 276 m2 g-1 with micro/mesopores (Figure S5a). From the results of SSA analysis of 3DNF and Ni@G/C, it is known that the value of SSA is increased 3DNF to Ni@G/C mainly because of graphitic domains with micropores derived from nickel species occurring under a catalytic strategy.13 The surface chemical composition of the Ni@G/C is visualized by X-ray photoelectron spectrum (XPS), illuminating the carbothermal reduction of nickel. The XPS survey spectrum of Ni@G/C (Figure S6a) confirms the presence of C, O, locating at 284.7 and 532.5 eV of C and O peak, respectively. The poor nickel peaks intensity can be observed from XPS spectrum (Figure S6b) because the catalytic reaction of nickel occurs in the center of the pyrolytic carbon. As shown in Figure 2d, the weight loss of 54% of Ni@G/C demonstrate Reactions in the TGA test are the combustion of carbon species and the oxidation of Ni to NiO. Almost no sodium element on the surface shows the perfect process of washing and removal (Figure S6c). The hollow NG/C coupled with nanosized shells can be generated from Ni@G/C through etching the Ni nanoparticles embedded in fingerprint-like graphitic carbon layers. It is worth mentioning that the nickel nucleus completely leaves from the Ni@G/C center, as confirmed by TEM and XRD analysis. The hollow NG/C with well-crystalline structure is obtained and the aerogel-like morphology of NG/C is also observed from Figure 1g and Figure S7 after effective removal of Ni species by hydrofluoric acid. The average single shell size of NG/C is 11.0 nm, which agree with


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the XRD patterns. From the HRTEM image (Figure 1h), the d-spacing of hollow NG/C is 0.4 nm and highly ordered ring structure of NG/C is beneficial for the shuttle of ions easily. The HRTEM images also demonstrate the turbostratic carbon structure of hollow NG/C enjoys nanopores at the intersection of the parallel layers with fingerprint-like structure). Accordingly, the SSA of NG/C is increased to ~359 m2 g−1 drastically (Figure S5b). By virtue of porous properties of hollow structure, NG/C can absorb/desorb more lithium/sodium ions in the way of nano plating to achieve higher storage.28 In the nanographite region, the ring like crystal stripes are easy to cross and layers are interlaced, providing greater possibilities for the storage of ions.6 The composites of NiO/C (Figure 1i, Figure S8) can be generated from Ni@G/C through air oxidation. The formation of hollow NiO in the composite is generated from the nickel nucleus due to the nanoscale Kirkendall effect; and to a large extent, carbon matrix from Ni@G/C is not destroyed.13, 29 It is worth noting that the Ni@G/C is converted to hollow NiO/C by moderate oxidation in air at 400 °C, regulating the hollow structure of the metal oxide resorted to the as-formed nickel nucleus through the oxidation condition. As shown in Figure 1i, the NiO clusters with hollow structure are randomly dispersed on a carbon matrix and average diameter is 21.5 nm. It is demonstrated that the elements of Ni and O in hollow NiO clusters are uniformly distributed in Figure 1j. Furthermore, as shown in HRTEM images (Figure 1k), the distinct (111) lattice plane with 0.24 nm fringe is deemed to cubic NiO crystal. And the distinct (002) plane with 0.35 nm interlayer distance is designated as graphitic carbon. Evidently, the increment of the spacing between nano-size-layer graphene is



conducive to dredge the transmission channel and the mini-scaled cavity in the pyrolytic carbon could provide the space to hold the inward volume expansion.30 With increasing oxidation temperature to 500°C at a speed of 10 °C min−1 (Figure S9) or expanding longer heating time, the volume of nickel oxide will grow larger and conversely carbon matrix with nanographitic domains will gradually disappear under Kirkendall effect in the air.31 Bulk NiO without carbon is obtained when the oxidation temperature up to 800°C in the air because of the fusion of Ni@G/C and combustion of carbon (Figure S10). From TGA (Figure 2d) analysis results, the carbon content of the NiO/C composites is ~12 wt.%. The full scan further confirms the existence of these elements including Ni, C, and O in hollow NiO/C from XPS curve. Two distinct peaks centered at 873.4, 855.9 eV and other satellites peaks at 879.5, 861.7 eV can be placed in the Ni 2p1/2, Ni 2p3/2, Ni2p1/2, and Ni2p3/2 from the high-resolution XPS spectrum of Ni 2p (Figure 2e).30 As measured by N2 adsorption-desorption (Figure 2f), more and more mesopores are generated in hollow NiO/C being accompanied by destroying original micropores in Ni@G/C under oxidization conditions of in the air. From the results of SSA analysis of NiO/C and Ni@G/C, it is known that the value of SSA is sharply decreased to 166.8 m2 g−1.


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Figure 3. a-d) Galvanostatic charge and discharge profiles (at first 3 cycles), cycling performance (at a current density of 100 mA·g−1), rate performance (gradually increasing from 100 to 4000 mA·g−1 and then back to 100 mA·g−1), and long-term cycling performance at a high current density of 5000 mA·g−1 of the hollow NG/C in LIBs; e-h) Galvanostatic charge and discharge profiles (at first 3 cycles), cycling performance (at a current density of 100 mA·g−1), rate performance (gradually increasing from 100 to 4000 mA·g−1 and then back to 100 mA·g−1), and long-term cycling performance at a high current density of 5000 mA·g−1 of the hollow NG/C in SIBs.

The lithium storage behavior in hollow NG/C is visualized such as cyclic voltammetry (CV) curves and galvanostatic charge-discharge (GCD) profiles in the voltage range of 0.01–3.0 V versus Li+/Li (Figure S11, Figure 3a). The first four cycles of the CV curves are measured at a scanning rate of 0.1 mV s−1. The solid electrolyte interphase (SEI) formed at irreversible peaks of 0.69 V, being accompanied by the decomposition of the electrolyte in the first discharge round; and another irreversible process is the massive loss of surface sites at peaks of 1.31 and 1.6 V, directly leading to subsequent cycle capacity less than the first lap.32 It is concluded



that the as-formed SEI membrane and the irreversible reaction of the electrolyte and surface functional groups possesses incisive peak at ~0.01 V at the end of the first lithiation. It can be clearly seen from the CV curves that the electrochemical process of the electrode after the first lap is reversible. From Figure 3a, the decomposition reaction of electrolyte with NG/C and the shape of thicker SEI lead to the lower initial coulombic efficiency (CE). The electrode capacity of NG/C is located at 1105.8 mA·hg−1 in the third cycle at 100 mA·g−1 and the capacity of NG/C is attenuated to 853.2 mA·hg−1 after 150 cycles of circulation (Figure 3b). Most promisingly, the discharge specific capacity of the NG/C remains 399.2 mA·hg−1 even if the current density is increased from 100 to 4000 mA·g−1 (Figure 3c); the electrode capacity of NG/C rapidly rebound to and then gradually rise to 911 mA·hg−1 gradually at the current densities from 4000 to 100 mA·g−1. It demonstrated that the electrode of NG/C with hollow structure enjoys outstanding ability of charge transfer kinetics. In order to further test the long cycle capability of the NG/C electrode at high current density of 5000 mA·g−1, the electrode still exhibits a retention charge capacity of 197 mA·hg−1 after 4000 cycles and an initial capacity of 345.9 mA·hg−1, indicating an average charge capacity loss of 0.037 mA·hg−1 in each circle (Figure 3d). For sodium storage, HCs is currently one of the best candidates because the inherent “house of cards model” structure can be matched with the sodium ion size very well, especially HCs with graphitic domains and hollow structure. The reason why graphite is not suitable as the anode for SIBs lies in that its highly ordered layer spacing does not meet the requirements of storing sodium ion. Therefore, the HCs


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coupled with expanded nano graphite is a key step in achieving efficient sodium storage. It not only solves the inherent low conductivity problem of HCs to a certain extent but also compensates for the poor sodium storage performance of graphitization. Promisingly, NG/C displays distinguish specific discharge capacities of 197 mA·hg−1 at 100 mA·g−1 after 150 cycles in SIBs (Figure 3e, f). From rate test, with the gradual increase of current density from 100 to 4000 mA·g−1 (Figure 3g, Figure S12), the GCD curves of NG/C are denser and shows a great capacity of 104.2 mA·hg−1 at 4000 mA·g−1. In subsequent rate cycling tests from 100 to 350 laps (Figure S13), there is a slight rise to 159.7 mA·hg−1in discharge specific capacity at 100 mA·g−1 current density. In order to extend the number of cycles of the charge and discharge test, NG/C preserves an excellent capacity of 69.7 mA·hg−1 after 2000 steady cycles at 5000 mA·g−1 (Figure 3h). Furthermore, compared with previously reported carbon anode (Table S1, S2), NG/C electrode exhibits the excellent performance for lithium/sodium storage. It is found that a phenomenon of curves fluctuation is distinct in the cycle performance of the NG/C electrode for LIBs (Figure 3b) but not obvious for SIBs (Figure 3f). It is generally believed that the intercalation and deintercalation of lithium ions leads to structure change of electrode materials is accompanied by the creation of a new electrode-electrolyte interface, resulting in more active sites for LIBs.11 The process of generating new SEI film with continuously consuming electrolyte. However, the storage of sodium ions is considered to be a adsorption and desorption process, which is relatively stable and gentle, and the effect of this process on the structure of the electrode material is



relatively weak. This phenomenon is also in line with the literature report.6-7, 11, 33 It is concluded that the as-prepared NG/C electrode exhibits superior properities such as cycle and rate performance and is crucial that the as-formed NG/C structure possesses hollow nanoshells and fingerprint-like nanographite. The NG/C anode displays clearly superiority in enhancing the capability of energy storage for LIBs/SIBs as shown in Scheme 2a. The interconnected graphitic nanoshells with thinner walls provides a continuous transmission channel for electrons in NG/C, shortens the diffusion path, improves electron conductivity, and alleviates volume expansion. The nanopores produced by the formed nanographite provide more storage sites. Thence, hollow structure NG/C with nanopores exhibits a high specific capacity, great rate performance and excellent cycle performance. Scheme 2. a-b) Li+/Na+ storage mechanism for the hollow NG/C and hollow NiO/C. c) Simulation of formation process of NG/C with the NiCx modification.

Based on the above analysis of the structures and performances, the importance of formation mechanism of nanographitic domains in Ni@G/C is stressed as well as


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carbon micropores by removal of nickel and sodium species. The Ni@G/C is obtained from carbonizing the precursors of SAPs containing nickel acetate and the component of packed nanosheets from carbonized polymers with “house of card model” structure is made of internal sp2 carbons and external sp3 carbons. These sp3 carbons bridge hydrogen atom or the other nanosheets.6 These sp3 carbons comes from methylene of SAPs (a sodium salt of polyacrylic acid), showing the rough process of graphitization even under high temperature. During the formation of green hydrogels, Ni2+ can chelate with carboxyl groups of SAPs and the retained nickel species can push the external sp3 carbon to internal sp2. As shown in Scheme 2c, the retained nickel species can break the bonding of carbon nanosheets, and react with carbon to generate NiCx. The Nickel carbide NiCx also migrates and accumulates while reacting with sp3 carbon to form new species of sp2 carbons and nickel. The carbonized polymers realize localized graphitization in the form of snowballing under the action of nickel species. And that little nickel species is the beginning of the small snowball. To visualize this catalytic process in a snowballing manner, the simulation process presents the gradual conversion of disordered carbon to nano-graphite where nickel species migrate under the action of nickel species as outlined in Scheme 2c. The illustration displays the nickel species dispersed in the amorphous carbon matrix pass through the disordered carbon to produce ordered nanographite microdomains. It is generally believed that the nickel species are dissolved in relatively disordered carbons and then precipitated in the graphitized region. The negative change of free energy makes the process of



dissolution and precipitation easier. And the as-formed nickel catalyst is not molten state and nor need a stoichiometric compound with carbon at a carbonization temperature of 650 oC. The scale of as-formed nickel species with hexagonal lattice (Cell=6.171*6.171*16.84 ) is larger than the interplanar spacing of (002) plane of graphite (d-spacing of ~0.34 nm), which leads to catalytic production of expanded nanographite (d-spacing of 0.36~0.4 nm) in the process of nickel species migration and accumulation.6, 11, 13 In a word, the introduction of nickel species has a good modification of the inherent structure of HCs, including as-generated hollow structure, graphitized microcrystalline regions with expanded d-spacing and nanopores. It should be pointed that it is difficult to convert all amorphous regions inside the pyrolytic carbon material into ordered graphitic carbon. Conversely, the retained edge sp3 carbons can bridge the catalytically formed graphite carbon regions, is beneficial to the overall structural stability of HCs.


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Figure 4. a-e) Cyclic voltammogram, cycling performance (current density: 200 mA·g−1), galvanostatic charge and discharge profiles (at first 3 cycles), rate performance (current densities: from 200 to 4000 mA·g−1), and long-term cycling performance (current density: 2000 mA·g−1) of the hollow NiO/C composites in LIBs. f-j) Cyclic voltammogram (scan rate: 0.1 mV s−1), cycling performance (current density: 100 mA·g−1), galvanostatic charge and discharge profiles (at first 3 cycles), rate performance (current densities: from 100 to 2000 mA·g−1), and long-term cycling performance (current density: 2000 mA·g−1) of the hollow NiO/C composites in SIBs.

For another, the NiO/C electrode with 3D hollow structure also shows excellent lithium/sodium storage performance. These excellent features of NiO/C composites are mainly reflected in the volume change resistance and the shorter lithium/sodium ion diffusion path during the charge and discharge process. As shown in Figure 4a, the CV profiles of the NiO/C electrodes is from the first three laps of the scan. A peak at 0.44 V is obtained from the first cathodic scan, vanishing in subsequent scans due to the as-generated SEI membrane and nickel clusters.8,

25, 30

The two broad peaks

located at about 1.4 and 2.2V in the anodic scan curves can be attributed to the decomposition of the SEI layer and Li2O decomposition (Ni + Li2 O → NiO + 2Li+ + 2e_ ), respectively. From the subsequent scans results, it is indicated that the reaction of NiO and Ni enjoys great reversibility.

26, 34

The current peaks corresponding to the

redox reactions of the NiO/C electrodes can basically overlap in each scan of the circle. The electrodes reaction of NiO/C involves two reversible processes, including the reaction for conversion and insertion/desertion for lithium storage: NiO + 2Li 



Ni + Li2O, C + xLi++xe−  LixC.

30

It is obviously found that the hollow NiO/C

electrodes involved reactions are like other NiO composites electrodes in LIBs. Figure 4b displays a GCD curves of NiO/C, showing an excellent specific discharge capacity of 1428 mA·hg−1 at 200 mA·g−1 current density in the first lap. The as-formed SEI film and the conversion between lithium ions and NiO lead to loss of specific capacity, which is a normal phenomenon that often occurs in anode materials. The discharge capacity is 1093 mA·hg−1 with higher CE after 400 cycles (Figure 4c). With the increase of the number of cycles, the rising capacity is also a universal phenomenon for metal oxide anode. And the underlying reason for this intuitive phenomenon is the activation of electrode materials penetration and the pseudo-capacitance effect caused by decomposition of electrolyte at lower voltage.26, 30, 35 As shown in Figure 4d, the higher specific capacities can remain from 926 to 536 mA·hg−1 with current densities from 200 to 4000 mA·g−1. When the working current density is reduced to 200 mA·g−1, the specific capacity of the electrode is also recovered to the original higher capacities, which reflects the superior reversibility of the NiO/C electrode and excellent rate performance. Note that from Figure 4e, the NiO/C electrodes possesses the superior ability of long-term cycling with higher CE at current density of 2000 mA·g−1. After 1000 cycles, the reserved capacity can still be higher than 600 mAh·g−1. Moreover, the fresh electrode of hollow NiO/C indicates lower charge transfer resistance (Rct; Rct=40 Ω) and the fresh electrode of bulk NiO exhibits a resistance value of 1000 Ω in electrochemical impedance spectroscopy (EIS) plots (Figure S14).25, 30 It is concluded that the electrode of hollow NiO/C enjoys conductive shells,


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leading to a severe reduction in the radius of the high frequency region in EIS plots. With the increase of charge and discharge cycles of the battery, the impedance value of NiO/C electrode decreases as a whole, which fully indicates that the impedance value is related to the conductivity of the electrode. For sodium storage, the electrode of NiO/C composites is also measured. As shown in Figure 4f, two peaks around at 0.52 V involves the formation of SEI film and reduction reaction of nickel oxide in the first cathodic scan. The process of re-oxidation of metallic Ni occurs at 1.49 V from the anodic scan.8 Moreover, it is observed that the decomposition of SEI film like lithium storage corresponds to the peak of 0.5 V. Based on this result, the overall reaction of NiO for SIB is concluded as NiO + 2Na+ + 2e-  Ni + Na2O.8 As shown in Figure 4g, NiO/C shows a discharge capacity of 921.2 mA·g−1. The initial CE (∼61%) is lower due to the as-formed SEI film and the inherent poor theoretical capacity from Figure 4h, displaying the cycle performance. It is demonstrated that the great rate performance of the NiO/C electrode is visualized in Figure 4i, displaying the capacities of 461, 333.7, and 259.3 mAh·g−1 at the current densities of 200, 1000, and 2000 mA·g−1. Promisingly, the specific capacity reaches to 440 mAh·g−1 with higher and stable CE at the lower current density of 100 mA·g−1(Figure S15) and CE is increased to ~98.5% even at 2000 mA·g−1 (from the fifth cycle). Therefore, the electrode of hollow NiO/C composite indicates excellent cycle persistence, which is comparable with reported works in LIBs and SIBs (Table S3, S4). Based on the above analysis of the NiO/C electrode, the superior lithium and



sodium storage properties is obtained. There are several reasons as shown in Scheme 2b, including that the electrode with hollow structure possesses huge free spacing for releasing mechanical strain caused by cubic expansion and with conductive carbon matrix improves electronic conductivity, and with nanopores provides more active site for ions.

3. CONCLUSION In summary, a simple method for adsorbing metal salts by superabsorbent polymers for tailoring hollow structure is proposed. It is expounded that the nickel species introduced with a chelation effect undergo a catalytic reaction to form a core-shell (Ni@G) structure under the action of the confined graphitization, forming the hollow structure by etching and oxidation. At the same time, the hollow structure involves NG/C with localized graphitization region and NiO/C with clustered framework, exhibiting long term and stable cycling properties as well as excellent rate capability as high-performance anodes for lithium and sodium storage.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: [Experimental section, digital photograph about the synthesis of precursor process of SAP, TGA curves of nickel acetate tetrahydrate, SAPs, and Ni@G/C before


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carbonization, Raman spectra of 3DNF, XRD patterns of the cooled carbonization product (Ni@G/C) exposed to air before washing and XRD patterns of bulk NiO, nitrogen adsorption-desorption isotherms and (inset: pore size distribution) of 3DNF, nitrogen adsorption-desorption isotherms and (inset: pore size distribution) of NG/C, XPS spectrum of Ni@G/C, SEM image of NG/C, SEM image of hollow NiO/C nanoparticles, TEM and HRTEM images of hollow NiO/C composites, TEM image of bulk NiO (800 ℃ ), cyclic voltammogram of the hollow NG/C in LIBs, charge/discharge curves of the NG/C electrode at the current densities from 100 to 4000 mA•g-1 in SIBs, Rate performances of the NG/C with the corresponding coulombic efficiency being displayed on the right axis at different densities of 100 to 4000 mA•g-1 after 350 cycles within 0.01-3.0 V vs. Na+/ Na, EIS curves of the hollow NiO/C and bulk NiO, Charge/discharge curves of the NiO/C electrode at the current densities from 200 to 2000 mA•g-1 in SIBs, cyclic voltammogram of the Ni@G/C and b) Cyclic voltammogram of the bulk NiO in LIBs, Simulations showing the NiCx modification to generate locally graphitized carbon. Simulation experiments of NiCx by “SIESTA” procedure, a-b) Cyclic voltammogram, cycling performance. c) Capacitive (red) and diffusion-controlled (blue) contribution to charge storage of Hollow NiO/C electrudes at 0.8 mVs-1. d) Normalized contribution ration of capacitive (red) and diffusion-controlled (blue) capacities at different scan rate, Electrodes after cycle 400th of Hollow NG/C. b) Electrodes after cycle 400th of Hollow NiO/C. c and d) sectional view of electrodes, comparison of cycle performance of carbon-based anodes for LIBs with previous reports, comparison of



cycle performance of carbon-based anodes for SIBs with previous reports, comparison of cycle performance of metal-oxide-based anodes for LIBs with previous reports, comparison of cycle performance of metal-oxide-based anodes for SIBs with previous reports.]

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC) (Grants no. 21671133; 21271010, 21604051, 21507081); the Shanghai Municipal Education Commission (No.15ZZ088; No.15SG49); Technology Commission of Shanghai Municipality (18020500800) and International Joint Laboratory on Resource Chemistry.

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Tailoring Hollow Nanostructures by Catalytic Strategy for Superior

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