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Rational Design of NiCoO2@SnO2 Heterostructure Attached on Amorphous Carbon Nanotubes with Improved Lithium Storage Properties Xin Xu, Sheng Chen, Chunhui Xiao, Kai Xi, Chaowei Guo, Shengwu Guo, Shujiang Ding, Demei Yu, and Ramachandran Vasant Kumar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11556 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 17, 2016

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ACS Applied Materials & Interfaces

Rational Design of NiCoO2@SnO2 Heterostructure Attached on Amorphous Carbon Nanotubes with Improved Lithium Storage Properties †

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Xin Xu, Sheng Chen, Chunhui Xiao, Kai Xi,* Chaowei Guo, Shengwu Guo, Shujiang Ding,* Demei Yu and R. ‡ Vasant Kumar †

Department of Applied Chemistry, School of Science, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China. ‡ Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom. Supporting Information Placeholderg ABSTRACT: It still remains very challenging to design proper heterostructures to enhance the electrochemical performance of transition metal oxides based anode materials for lithium-ion batteries. Here, we synthesized NiCoO2 nanosheets@SnO2 layer heterostructure supported by amorphous carbon nanotubes (ACNTs) which is derived from polymeric nanotubes (PNTs) by a step-wise method. The inner SnO2 layer not only provide a considerable capacity contribution, but also produce the extra Li2O to promote the charge process of NiCoO2 and thus result in a rising cycling performance. Combining with the contribution of ACNTs backbone and ultrathin NiCoO2 nanosheets, the specific capacities of these one-dimensional nanostructures show an interesting gradually increasing trend even after 100 cycles at 400 mA g-1 with a final result of 1166 mAh g-1. This approach can be an efficient general strategy for the preparation of mixedmetal-oxide one-dimensional nanostructures and this innovative design of hybrid electrode materials provides a promising approach for batteries with improved electrochemical performance. KEYWORDS: ternary nickel cobaltite; tin oxide; amorphous carbon nanotubes; synergistic effect; lithium ion batteries

INTRODUCTION Rechargeable lithium-ion batteries (LIBs) are key devices for mobile electronics and potentially power sources for electric vehicles (EVs), hybrid electric vehicles (HEVs) as well as 1-3 renewable energy storage. More recently, an intensified attention is being paid to design and fabricate advanced anode materials for LIBs with high energy density, stable 4,5 cycle life and good rate performance. In this field, selecting the right electrode material and then designing a proper microstructure are seen as promising way forward in 6,7 efficient electric energy storage . Transition metal oxides (TMOs) have been recently regarded as potentially as the predominant anodes for next-generation LIBs and supercapacitors arising from to their high specific/volumetric 8-15 capacity, safety and relatively low cost. However, the natural weaknesses such as poor electrical conductivity and mechanical stability which are common for most TMOs anode materials, leading to unsatisfactory lithium storage capability, still critically impede their practical applications. To date, numerous nanostructured TMOs have been prepared to mitigate the aforementioned problems. Despite good progress, the electrochemical performance of existing nanostructured TMOs are still not enough to meet the requirement for next-generation LIBs. Recently, an emerging strategy of designing and fabricating hybrid nanostructures has attracted much attention based upon “synergistic effect” from a combination

of materials, obtained from the individual constituents and 16-20 thereby enhancing the electrochemical performance. For example, Feng et al. fabricated graphene-based TiO2/SnO2 by 18 a step-wise approach. Fan et al. synthesized branched αFe2O3/SnO2 nano-heterostructures via a hydrothermal 19 Yang and co-workers also prepared γreaction. Fe2O3@SnO2@C porous core–shell nanorods through a layer20 by-layer deposition method for LIBs. These hybrid materials combine advantages of the individual constituents to offer enhanced lithium storage performance compared with the single components. But, the mechanism by which the so called synergy is achieved between the components is still not clear. In this study, we have chosen a combination of NiCoO2 and SnO2, which is already reported to be promising 21-24 anode materials for LIBs. The relevant redox reactions are st summarised in Scheme 1. In the 1 discharge cycle, both + NiCoO2 and SnO2 undergo reduction reactions with Li ions to form metallic elements Ni, Co and Sn. Further reductive discharge capacity is achieved from redox alloying of Sn with Li. During the charge process, Co metal produced after the discharge can be oxidized not only to CoO but also to Co3O4, as long as additional Li2O is available for the oxidation reaction. The situation is made worse, as a proportion of Li2O participate in the formation of solid electrolyte interphase (SEI) during the charge-discharge cycles. Interestingly, SnO2 can offer “synergy” by providing Li2O via what otherwise becomes an irreversible discharge process in st the 1 discharge cycle. In subsequent cycles the metallic Sn

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supported by SnO2 layer@amorphous carbon nanotubes (ACNT) core-shell nanocables (denoted as NiCoO2@SnO2@ACNT). The step-wise strategy is shown in Scheme 2. In step I, the sulfonated bamboo-like polymeric nanotubes (PNTs) are uniformly coated with a thin SnO2 30,31 layer through a precipitation method. Then the NiCoprecursor nanosheets are randomly assembled on the surface of SnO2@PNT nanocables through a solution method in step II. After a heating step under nitrogen gas (step III), the NiCo-precursor nanosheets are converted to a hierarchical and crystallized NiCoO2 shell, the PNTs are carbonized to an ACNT core in a simultaneous anneal/pyrolysis method. Scheme 1. The lithium insertion/extraction reactions for the NiCoO2@SnO2@ACNT electrode.

EXPERIMENTAL SECTION Material Synthesis

can continue to partake in dealloying during charging and 25,26 alloying during discharge. If the NiCoO2 and SnO2 form a heterostructure, the Li2O produced by SnO2 is directly available in the vicinity of Ni and Co for charging to NiO and all the way to Co3O4 thus offer the extra capacity. In the conventional SnO2 single oxide electrode, the 1st reduction of oxide is an irreversible process whereby the insulating Li2O formed is unused in subsequent cycles and accompanied by large volume change which detrimentally affects the carefully constructed morphology of the electrodes. If SnO2 can be suitably encapsulated in the interior of the hybrid nanostructure then volume change can be accommodated and the Li2O reversibly utilized by Ni and Co from the nickel cobaltite precursor. From a microstructural point of view, the higher capacity constituent should be exposed in the outer part. It is also worth reminding that some hybrid materials with amorphous carbon substrate fabricated through facile route under relatively low temperature conditions have shown to give excellent electrochemical performance as the anodes in LIBs. Compared to the common carbonaceous substrate such as CNTs and graphene, the looser structure possessed by amorphous carbon can allow it to buffer the 27-29 huge volume changes of TMOs more effectively. Based on the above considerations, we have designed a novel nanostructured hierarchical NiCoO2 nanosheets

Sulfonated PNTs: The bamboo-like PNTs were prepared through a modified method according to a literature 25 method. The obtained 3 g of PNTs were dispersed into concentrated sulfuric acid (PNTs: H2SO4 = 1: 30, w/w) and followed by a sonication for 10 min. The mixture was then stirred at 40 °C for 24 h, and then the resulting pink precipitate was collected by centrifugation, washed with 31,32 deionized water and ethanol for several times. SnO2@PNT: 0.1 g SPNTs was added to a mercaptoacetic acid solution (20 mM, 40 mL) in a 100 mL round bottom flask by sonication for 10 minutes. Then, 0.5 mL of HCl solution (37 wt%,), 0.1 g of SnCl2·2H2O and 0.5 g of urea were added into the above dispersion in order. The reaction solution was next stirred for 6 h under 60 °C. Aftre the reaction, the resulting grey precipitate was harvested by centrifugation, washed repeatedly by ethanol and dried at room temperature. NiCo-precursor@SnO2@PNT: 200 mL of trisodium citrate solution (0.7 mM) in a 500 mL round bottom flask was used to dissolve 375 mg of Ni(NO3)2·6H2O, 375 mg of Co(NO3)2·6H2O and 175 mg of hexamethylenetetramine (HMT). Then, 60 mg of as-prepared SnO2@PNT was added to the solution under sonication for 5 minutes. The above dispersion was next stirred for 6 h at 90 °C. Finally, the green precipitate was harvested by centrifugation, washed repeatedly by ethanol and dried at room temperature. NiCoO2@SnO2@ACNT: The NiCo-precursor@SnO2@PNT sample was further treated in nitrogen at 500 °C for 4 h with -1 a heating rate of 1 °C min , and then the final black products were obtained. NiO@SnO2@ACNT, Co3O4@SnO2@ACNT, NiCoO2@SnO2, NiCoO2@ACNT, SnO2@ACNT and ACNT are prepared via similar methods. Materials Characterization

Scheme 2. Schematic illustration of the synthetic process of NiCoO2@SnO2@ACNT hybrid nanostructures. I: Sulfonated PNTs are coated with uniform SnO2 layer through a simple precipitation method to form SnO2@PNT core-shell structures; II: the SnO2@PNT are covered by NiCo-precursor nanosheets via a further precipitation process; III: the NiCoprecursor@SnO2@PNT are annealed/pyrolysed in nitrogen to obtain the NiCoO2@SnO2@ACNT hybrid nanostructures.

Field-emission scanning electron microscopy (FESEM; HITACHI, su-8010) and transmission electron microscopy (TEM; JEOL, JEM-2100) were employed to investigate the morphology of products. The crystal structure of the sample was acquired by powder X-ray diffraction (XRD; SHIMADZU, Lab X XRD-6000). Specific surface area and pore size distribution of the sample were measured by a Brunauer– Emmett–Teller instrument (BET; ASAP 2020M) at 77 K. The chemical information of the different samples was investigated by the X-ray photoelectron spectroscopy (XPS)

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equipment, performed on an Axis Ultra, Kratos (UK) at monochromatic Al Ka radiation (150 W, 15 kV and 1486.6 eV). Electrochemical measurements Two-electrode coin-type cells (CR2016) were assembled for the electrochemical tests. The working electrode was prepared by spreading a mixture of NiCoO2@SnO2@ACNT composites, conductive agent (carbon black, C-NERGY™ Super C65) and polymer binder (poly(vinylidenedifluoride), PVDF, Aldrich) in a weight ratio of 70: 20: 10 on a copper foil. The electrolyte used in the cells was 1.0 M LiPF6 in a 50: 50 (w/w) mixture of ethylene carbonate and diethyl carbonate. Both of the counter and reference electrode are served by lithium only. Every cell was assembled in argon-filled dry glovebox. The galvanostatic charge-discharge tests were performed on a NEWARE battery tester. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out using an electrochemical workstation (CHI 660D). Results and discussion With the adsorption ability of functional groups (–SO3), the sulfonated PNTs can serve as the template on which the SnO2 layer deposits. The morphology of the as-synthetized SnO2@PNT nanocables is shown in Figure 1a and b. In Figure 1a, it is can be seen that plenty of SnO2@PNT nanocables with uniform exterior are obtained after the SnO2 coating reaction. From the TEM image, a homogeneous SnO2 layer can be directly observed, and its thickness is about several nanometres (Figure 1b). Figure 1c–f show the as-prepared NiCo-precursor@SnO2@PNT with different resolution. After a solution process, the previous SnO2@PNT nanocables are uniformly coated with hierarchical NiCo-precursor shell (Figure 1c and d). Moreover, TEM image further

Figure 1. (a) SEM image and (b) TEM image of the asprepared SnO2@PNT; (c, d) SEM image and (e, f) TEM image of NiCo-precursor@SnO2@PNT.

demonstrates that the NiCo-precursor nanosheets are totally surrounding the SnO2@PNT nanocables (Figure 1e). In addition, a bit of bare SnO2@PNT (or sulfonated PNT) directly shows the core-shell structure.It can be distinctly identified from Figure 1f that the hierarchical NiCo-precursor shell is constituted by free-standing nanosheets with a height of 35-60 nm. To acquire highly crystalline NiCoO2 nanosheets and ACNT core, the NiCo-precursor@SnO2@PNT composite is calcinated at 500 °C for 4 h. As can be seen in Figure 2a, the NiCoO2@SnO2@ACNT composites still maintain the hierarchical and tubular structure, which illustrates their good thermal stability. In Figure 2b, it indicates that the constituent NiCoO2 nanosheets are distinctly apparent and randomly standing on the surface. From the corresponding TEM image (Figure 2c), we can see that the shell of the composite is breach in some positions. This phenomenon can be attributed to the slight shrink of inside SnO2 layer 31 during the annealing procedure. Figure 2d shows the detailed structural information of the NiCoO2@SnO2@ACNT. Owing to the existence of ultrathin SnO2 layer, the outline of ACNT backbone can be distinctly found out. Figure 2e reveals that the thickness of NiCoO2 nanosheets are approximately 3-5 nm and SnO2 layer is about 8 nm. Energydispersive X-ray spectroscopy (EDX) elemental mapping of a single NiCoO2@SnO2@ACNT nanotube is also employed to study its elemental spatial distribution (Figure 2f). The result demonstrates that the C and Sn elements form the core of the nanocable as well as the outside nanosheets are mainly comprised by Ni and Co, which proves that the NiCoO2

Figure 2. SEM images (a and b), TEM images (c and d) and HRTEM image (e) of the NiCoO2@SnO2@ACNT. (f) TEM image of NiCoO2@SnO2@ACNT composite with the corresponding elemental mapping images of carbon, tin, nickel, cobalt and oxygen.

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nanosheets and SnO2 layer are uniformly coated on the surface of ACNT skeleton. We also removed the ACNT core of NiCoO2@SnO2@ACNT by calcining them in air as revealed in Figure S1, the hollow interior further confirmed the coreshell structure.

Co oxides back to corresponding metallic Ni and Co, Thus the starting NiCoO2 is not expected to reform again. There is no obvious peak associated with the electrochemical transformation of SnO2, which illustrate the main capacity contribution is provided by the NiCoO2.

To investigate the detailed information about NiCoO2@SnO2@ACNT composite, X-ray diffraction (XRD) was carried out with the results shown in Figure 3a. Due to the relatively small content of SnO2, the peaks of annealed sample are totally belong to the cubic NiCoO2 phase (JCPDS card no. 10-0188). The resultant NiCoO2@SnO2@ACNT is also examined by X-ray photoelectron spectroscopy (XPS) to study the SnO2 constituent (Figure 3b). The binding energy of Sn 3d 5/2 is 486.2 eV, which is between 484.7 eV (Sn) and 487.2 eV (SnO2), suggesting that SnO2 is partially reduced by ACNT during the calcination process. As determined by N2 sorption measurements (Figure 3c), the Brunauer-EmmettTeller (BET) specific surface area of NiCoO2@SnO2@ACNT 2 -1 composite is 264.3 m g , and its pore size is ~4 nm (Figure + 3d), which offers more active sites to accommodate Li ion and electrolyte. In addition, the mass fraction of amorphous carbon in the NiCoO2@SnO2@ACNT composite is about 41.8% as confirmed by TGA curve (Figure S2).

The charge–discharge curves of NiCoO2@SnO2@ACNT at -1 400 mA g are revealed in Figure 4b. As can be seen, the NiCoO2@SnO2@ACNT electrode can deliver large discharge -1 -1 and charge capacity of 1347 mA h g and 908 mA h g during st the 1 cycle, showing a moderate irreversible loss of about 33%. Formation of the solid–electrolyte interface (SEI) and/or decomposition of the electrolyte are the main reasons 23,33 for the large capacity loss. Figure 4c shows the discharge capacities of -1 NiCoO2@SnO2@ACNT electrode cycling at 400 mA g , and the Coulombic efficiency during the whole cycles. After the slight decrease, the discharge capacity is gradually increased -1 up to 1166 mAh g at the end of the tests. As a comparison, the cycling performance of NiCoO2@SnO2@ACNT electrode is also better than Co3O4@SnO2@ACNT, NiO@SnO2@ACNT and NiCoO2@SnO2 (Figure S3). In addition, to prove the role of SnO2, we also test the cycling performance of NiCoO2@SnO2@ACNT, NiCoO2@ACNT, SnO2@ACNT and -1 ACNT at the same current density of 400 mA g (Figure 27 S4). As can be seen, the discharge capacities of are lower than that of NiCoO2@SnO2@ACNT th NiCoO2@ACNT before the 43 cycle. However, the capacities th of NiCoO2@ACNT are decreased after the 32 cycle as well as the capacities of NiCoO2@SnO2@ACNT are still increasing th even after the 100 cycle. The role of SnO2 thus can be

Figure 3. XRD patterns (a) and Sn 3d XPS spectra (b) of NiCoO2@SnO2@ACNT composite. (c) N2 adsorption– desorption isotherms of the NiCoO2@SnO2@ACNT. (d) The pore-size distribution calculated from desorption branch. Then, the electrochemical properties of NiCoO2@SnO2@ACNT anode materials are investigated. Figure 4a displays the typical cyclic voltammograms (CVs) of NiCoO2@SnO2@ACNT electrode. The appearance of CV profiles is generally in accordance with previously reported 22-24,32-34 For the first nickel cobaltite based nanostructures. cathodic scan, an intense peak located at ca. 0.39 V 2+ 2+ originates from the reduction of Ni and Co in the oxide, respectively. It is noteworthy that the weak peak located around 1.15 V in the first cathodic sweep probably corresponds to lithium intercalation in Ni2SnO4 and Co2SnO4 35 formed during the annealing process. Two peaks located at the following anodic scan around 1.47 V and 2.21 V could be ascribed to the oxidation of metallic Ni and Co. During the succedent sweeps, the cathodic peaks shift to ca. 0.85 V and 1.50 V arising from reversible reduction of individual Ni and

Figure 4. Electrochemical characterization of the NiCoO2@SnO2@ACNT : (a) CV profiles at a scan rate of 0.5 -1 mV s between 0.01 and 3 V; (b) charge–discharge voltage -1 profiles at a current density of 400 mA g ; (c) cycling performance together with the Coulombic efficiency at a -1 current density of 400 mA g ; (d) rate capability at different current densities indicated.

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confirmed. In addition, the cycling performance of NiCoO2@SnO2@ACNT is much better than that of SnO2@ACNT and ACNT, as well as some previous reported 36-38 NiCo2O4 and SnO2 anode materials. The origin of this unusual result can be ascribed to the synergistic effect between SnO2 and NiCoO2 enhanced by ACNT as discussed above. The continuous growth of gel-like polymeric layer and possible activation of anode materials also make a 39-48 contribution to the increased capacity. In addition, the Coulombic efficiency of NiCoO2@SnO2@ACNT anode is almost 100% during the whole cycles. Besides the cycling behavior, rate capability is also very important for LIBs. As shown in Figure 4d, the discharge capacities of NiCoO2@SnO2@ACNT anode are 988, 987, 953, -1 881, 781, 623, and 475 mA h g at the current densities of 0.1, -1 0.2, 0.4, 0.8, 1.6, 3.2 and 6.4 A g , respectively. Finally, the -1 discharge capacity can still return to 1094 mA h g when the -1 current density switches back to 0.1 A g again. To prove that the oxidation of CoO to Co3O4 takes place as suggested, we also performed a ex situ XPS analysis of the NiCoO2@SnO2@ACNT electrode in Figure 5. With regards to the original anode, there are two characteristic peaks at 780.1

investigate the resistances at different stages of charge– -1 discharge cycling at a current density of 400 mA g . As shown in Figure 6, the diameter of the semicircle increases th th until the 5 cycle, and then decreases after the 10 cycle, indicating an enhanced charge-transfer. This result is also consistent with the cycling performance of NiCoO2@SnO2@ACNT electrode These outstanding lithium storage properties can be ascribed to the smart architecture and synergistic effect of NiCoO2@SnO2@ACNT composite in Scheme 3. Firstly, the ACNT core can enhance the electronic conductivity of the electrode by forming a cross-linked conducting networks and serve as the conducting skeleton. The looser structure also allow it to more effectively buffer the large volume change and relieve the aggregation of NiCoO2 and SnO2 during the charge-discharge processes. Secondly, the inner SnO2 can not only provide a considerable capacity contribution from the alloying/dealloying ability of metallic Sn, but also can help to produce the extra Li2O to promote the oxidation during charge process of CoO to Co3O4 and thus contributing to additional reversible capacity and better cycling performance. In addition, the metallic Sn emerging from the first discharge

Figure 5. Co 2p XPS spectra of NiCoO2@SnO2@ACNT composite in the initial cycle (a) and the 10th cycle (b). and 796.0 eV, corresponding to the 2p3/2 and 2p1/2 spin–orbit 2+ peaks of Co . After 10 cycles, these two peaks are shifted to 3+ 49 778.3 and 793.1 eV, representing the existence of Co . For better understanding the gradually increasing capacity of NiCoO2@SnO2@ACNT electrode, electrochemical impedance spectroscopy (EIS) analysis was employed to

Scheme 3. Illustration of synergistic NiCoO2@SnO2@ACNT composite.

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process can also enhance the electrical conductivity of the + hybrid system. Thirdly, the Li diffusion pathways can be effectively shorted by the ultrathin NiCoO2 nanosheets, and the hierarchical structure is also useful to buffer the volume variation during the charge-discharge cycles. CONCLUSION

Figure 6. Nyquist plots of NiCoO2@SnO2@ACNT electrodes in different cycles.

In conclusion, we have rationally designed a step-wise templating method for the fabrication of NiCoO2 nanosheets@SnO2 layer@amorphous carbon nanotubes coreshell nanocables as an enhanced anode material for lithiumion batteries. The designed NiCoO2@SnO2@ACNT composite displays an unusual increase in capacity up to 100 charge-discharge cycles as well as an outstanding rate performance. The improved lithium storage performance can be attributed to the appropriate structural design of the material and the beneficial synergistic effect between NiCoO2, SnO2 and ACNT. In addition, our system presented in this work offers significant implications on material selection and structural design for improving performances of electrodes in LIBs.

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Corresponding Author *E-mail: [email protected]; [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported partially by the National Natural Science Foundation of China (No. 51273158, 21303131); We thank Penghui Guo for the help of materials characterization. REFERENCES (1)

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Rational Design of NiCoO2@SnO2 Heterostructure Attached on

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