A general strategy to construct yolk-shelled metal oxides inside carbon nanocages for high-stable lithium-ion battery anodes

Journal Pre-proof A general strategy to construct yolk-shelled metal oxides inside carbon nanocages for high-stable lithium-ion battery anodes Meng Liu, Hao Fan, Ou Zhuo, Junchao Chen, Qiang Wu, Lijun Yang, Luming Peng, Xizhang Wang, Renchao Che, Zheng Hu PII:

S2211-2855(19)31082-1

DOI:

https://doi.org/10.1016/j.nanoen.2019.104368

Reference:

NANOEN 104368

To appear in:

Nano Energy

Received Date: 10 October 2019 Revised Date:

28 November 2019

Accepted Date: 3 December 2019

Please cite this article as: M. Liu, H. Fan, O. Zhuo, J. Chen, Q. Wu, L. Yang, L. Peng, X. Wang, R. Che, Z. Hu, A general strategy to construct yolk-shelled metal oxides inside carbon nanocages for high-stable lithium-ion battery anodes, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104368. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

A general strategy to construct yolk-shelled metal oxides inside carbon nanocages for high-stable lithium-ion battery anodes Meng Liua,‡, Hao Fana,‡, Ou Zhuoa, Junchao Chena, Qiang Wua,*, Lijun Yanga, Luming Penga, Xizhang Wanga,*, Renchao Cheb, and Zheng Hua,* a

Key Laboratory of Mesoscopic Chemistry of MOE and Jiangsu Provincial Lab for Nanotechnology, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China.

b

Department of Materials Science and Collaborative Innovation Center of Chemistry for Energy Materials (iChem), Fudan University, Shanghai 200438, China.

*Corresponding authors E-mail: [email protected] (Q. Wu), [email protected] (X.Z. Wang), [email protected] (Z. Hu) ABSTRACT: Metal oxides (MOs) are attractive anode materials of Li-ion batteries for the large theoretical capacity, low cost and high density, but suffer from the poor cycling stability owing to the low conductivity, large volume variation, active component loss and instable solid electrolyte interface (SEI) during cycling. Herein, we develop a simple and general strategy to construct the yolk-shelled metal oxides inside 3D hierarchical carbon nanocages (hCNC). The obtained yolk-shelled MO@hCNC provides sufficient interior void to buffers the large volume variation, forms the stable SEI film, and greatly reduces the loss of active components during lithiation/delithiation, meanwhile ensures the fast electron transfer and ions/electrolyte transportation. Both SnO2@hCNC and Fe3O4@hCNC exhibit superior cycling stability and reversible capacity to the most corresponding SnO2- and Fe3O4-based anode materials reported to date. The simple and general approach to the yolk-shelled MO@hCNC is significant for exploring the high-performance energy-storage materials for Li-ion batteries or even beyond. Keywords: Lithium storage; Yolk-shelled; Hierarchical carbon nanocages; Tin dioxide; Ferroferric oxide

1. Introduction Performance improvement of rechargeable lithium-ion batteries (LIBs) is driven by the ever-increasing demand of portable electronics and electric vehicles[1]. The key challenge is to develop highperformance electrode materials with high energy density, high power density and excellent cycling 1

stability[2]. Metal oxides (MOs) with the large theoretical capacities, low cost and high density are promising candidates of anode materials to replace the commercial graphite[3]. However, their performances were usually limited owing to the poor intrinsic conductivity, large volume variation, easy loss of active components and instable solid electrolyte interface (SEI) formed on MO during cycling[3, 4]. Much efforts have been devoted to boosting their electrochemical performance typically via the morphology control[5-8], composition regulation[9-11], and hybridization with conductive sp2 carbons[12-18]. By supporting MO nanoparticles on sp2 carbons, the conductivity can be effectively improved, but the open structure cannot efficiently prevent the loss of active components and the collapse of electrode structures during repeated charge and discharge, resulting in the rapid capacity decay[12, 13]. By wrapping MO nanoparticles with sp2 carbons, the active components can be retained to certain content, but there is no enough cavity to accommodate the large volume variation during lithiation/delithiation, usually leading to the breaking of carbon coatings[14-18]. A recent study indicated the yolk-shelled Fe3O4@carbon could achieve an ultralong cycling life for lithium storage owing to the enough interior buffer space[19]. Usually, the yolk-shelled structures were obtained by Kirkendall effect[20], etching[19, 21], and very recently by in-situ pyrolysis of complex metal-organic framework hybrids[22, 23], which are rather complecated and, in most cases, unsuitable to get the conductive carbon shells. In recent years, we have developed the 3D hierarchical carbon-based nanocages (hCNC) featuring the large interior cavity, coexisting micro-meso-macropores and high conductivity, which are becoming a new platform for advanced energy storage and conversion[24, 25]. The sufficient interior void is particularly suitable for confining MO nanoparticles to alleviate the issues of volumetric expansion and active materials loss during the lithiation/delithiation. Herein, we report a general strategy to construct the yolk-shelled MO@hCNC by evacuation-filling the solutions of metal-containing precursors inside the nanocages through the micropores across the shells of hCNC, followed by post-processing. The so-obtained yolkshelled MO@hCNC (MO=SnO2, Fe3O4) exhibits the excellent long-term stability, accompanied by the high specific capacity and high-rate performance, far superior to the corresponding counterparts of MO nanoparticles supported outside hCNC (MO/hCNC). The improved performance is attributed to the 2

unique confinement structure with enough interior void, which efficiently buffers the volume variation, avoids the active material loss and stabilizes the SEI film during cycling. This study demonstrates a general strategy for improving the Li storage performance of MOs.

2. Experimental section 2.1 Sample preparation hCNC: hCNC was synthesized at 800 oC by in-situ MgO template method with benzene precursor, similar to our previous reports[26, 27]. The obtained hCNC presents the unique 3D hierarchical structure with coexisting micro-meso-macropores, high specific surface area (~1400 m2 g-1), well-graphitized shells and high conductivity (~300 S m-1). The inside cavities of the nanocages have the sizes of 10-30 nm (Fig. S1 and Table S1). MO@hCNC and MO/hCNC (MO=SnO2, Fe3O4): The MO@hCNC was prepared by a facile evacuation-filling plus post-processing (Route A in Fig. 1a). Taking SnO2 as an example, typically, a sealed vessel containing 100 mg hCNC was vacuumized to ca. 1.0 Pa, and then the ethanol solution (16 mL) containing 68 mg of SnCl4·5H2O and two drops (ca. 0.1 mL) of concentrated hydrochloric acid (HCl, ~37 wt.%) was quickly added into the vessel, followed by the magnetic stirring for 2.0 h. The suspension was filtrated and freeze-dried, and then the SnCl4 outside the nanocages was removed by rinsing with ethanol and water. The dried powder was heat-treated at 500 oC in argon flow for 4.0 h, leading to the SnO2@hCNC. By regulating the weight ratio of hCNC to SnCl4·5H2O and repeating evacuation-filling process, the SnO2 contents in SnO2@hCNC were be effectively adjusted in the range of 0-70.5 wt.%. For comparison, SnO2/hCNC was synthesized by a simple impregnation method (Route B in Fig. 1a)[28]. Specifically, 100 mg hCNC was first ultrasonically dispersed in 15 mL deionized water for 1.0 h, and then 15 mL SnCl4-ethanol solution containing 68 mg SnCl4·5H2O was dropwise added to the suspension at 90 oC and magnetically stirred for 2.0 h. After the solvent evaporated, the powder was heat-treated at

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500 oC in argon flow for 4.0 h, leading to the SnO2/hCNC. The pristine SnO2 nanoparticles were synthesized by directly heat-treating SnCl4·5H2O at 500 oC in argon flow for 4.0 h. The Fe3O4@hCNC and Fe3O4/hCNC were prepared similarly with Fe(NO3)3·9H2O precursor, water solvent and heat temperature of 400 oC (see details in Supporting Information). The commercial Fe3O4 (Sinopharm Chemical Reagent, China) was used for comparison. 2.2 Sample characterizations The structure and composition of the samples were analyzed by high-resolution transmission electron microscopy (HRTEM, JEM-2100F operating at 200 kV) and X-ray diffraction (XRD, Bruker D8 Advance A25, Co Kα1 radiation of 1.78897 Å with Fe filter of 0.02 mm thickness). For electrochemical measurements, the working electrode contains the active materials, polyvinylidene difluoride (PVDF) and acetylene black with a weight ratio of 8:1:1. The coin-type cells were assembled with lithium foil as the counter electrode, a Celgard 2500 polypropylene membrane as the separator, and 1 mol L-1 LiPF6 in a 1:1 (V/V) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the electrolyte. The cells were charged and discharged on a Neware battery testing system in the potential range of 0.005~2.5 V (vs. Li/Li+), and the specific capacities were evaluated using the total mass of active materials. The cyclic voltammogram (CV) and electrochemical impedance spectra (EIS) measurements were performed on VMP3 electrochemical workstation (Bio-logic). The CV curves were recorded at a scan rate of 0.5 mV s-1 in the potential range of 0.005~2.5 V. The EIS measurements were performed at open circuit potential with a sinusoidal signal over the frequency range from 100 kHz to 0.01 Hz with an amplitude of 5 mV. Other characterizations are detailed in Supporting information, which include X-ray photoelectron spectroscopy (XPS),

119

Sn nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and

N2 adsorption/desorption isotherm.

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3. Results and discussion Fig. 1 shows the preparation scheme and structural characterizations of the optimal SnO2@hCNC and SnO2/hCNC with SnO2 contents of 65.5 and 68.5 wt.%, denoted as [email protected] and SnO2/hCNC68.5, respectively. Both samples keep the hierarchical structure and coexisting micro-meso-macropores of the hCNC support, and the SnO2 species have the tetragonal structure (Fig. 1e). For the SnO2@hCNC, the SnO2 nanoparticles are well encapsulated with the size of 6.1±1.0 nm, as confirmed by the tilting HRTEM observation, in agreement with the results of N2 adsorption/desorption isotherm, XPS, and 119Sn NMR (Figs. 1b,c, S1 and S2). For the SnO2/hCNC, the SnO2 nanoparticles are dispersed on the outer surface of hCNC with the larger size of 8.4±1.1 nm (Fig. 1d). The bulk conductivity of the two samples is three orders of magnitude higher than the pristine SnO2, implying the improved electron transfer kinetics (Table S1).

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Fig. 1. Preparation scheme and structural characterizations of [email protected] and SnO2/hCNC-68.5. (a) Schematic illustration of the preparation. Route A: the evacuation-filling process. Route B: the impregnating process. (b,c) TEM and tilting TEM images of SnO2@hCNC. The sample is tilted in the range of -5.2o~+10.2o around the x-axis or -10.2o~+15.2o around the y-axis. (d) TEM image of SnO2/hCNC. Insets in (b,d) are corresponding HRTEM images and particle size histograms. (e) XRD patterns.

The 65.5 wt.% loading for SnO2@hCNC ensures the high content of active species meanwhile the enough residual interior void to buffer the large volume expansion of SnO2 upon lithiation (Figs. 1b,c, and S3). Fig. 2 shows the electrochemical performance of SnO2@hCNC, SnO2/hCNC, SnO2 and hCNC. For pristine SnO2, the CV curve shows two anodic peaks at 0.63 and 1.31 V (vs. Li/Li+)[29]. Obviously, the two peaks downshift to 0.60 and 1.28 V for SnO2/hCNC, 0.52 and 1.25 V for SnO2@hCNC, respectively (Figs. 2a and S4). The lower polarization for SnO2@hCNC suggests the better reversible reaction between the confined SnO2 nanoparticles and Li+ ions in comparison with the cases for SnO2/hCNC and pristine SnO2[29]. Accordingly, SnO2@hCNC presents the excellent rate performance obviously superior to the counterparts of SnO2/hCNC, SnO2 and hCNC (Fig. 2b). Specifically, the SnO2@hCNC delivers the high specific capacities of 1129, 1044, 942, 836, 750 and 568 mAh g-1 at 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 A g-1, respectively, and then the capacity recovers to 1090 mAh g-1 at 0.1 A g-1 (Fig. 2b). The cycling stability was evaluated at different current densities (Fig. 2c,d). At 1.0 A g-1, SnO2@hCNC exhibits a high initial charge specific capacity of 1018 mAh g-1 and maintains at 792 mAh g-1 after 600 cycles with the capacity attenuation of only 0.037% per cycle, much lower than the corresponding ones of 0.094% for SnO2/hCNC (from 923 to 397 mAh g-1), 0.160% for SnO2 (from 642 to 39 mAh g-1), and 0.064% for hCNC (from 533 to 328 mAh g-1) (Figs. 2c and S5). At the high current density of 5.0 A g-1, the specific charge capacity still stabilizes at a high value of 477 mAh g-1 even after 1000 cycles, much larger than the corresponding 273, 44 and 194 mAh g-1 for SnO2/hCNC, SnO2 and pure hCNC, respectively (Fig. 2d). All the Coulombic efficiencies are higher than 98% after 50 cycles 6

(Fig. 2d). These results clearly indicate that the yolk-shelled SnO2@hCNC presents much better cycle stability and higher specific capacity than the supported SnO2/hCNC, SnO2 and hCNC. With increasing the SnO2 content in the range of 0~73.6 wt.%, the lithium storage capacities of SnO2@hCNC and SnO2/hCNC show the same trend of first increase and then decrease, reaching the highest capacity for SnO2 content of 65.5 and 68.5 wt.%, respectively (Figs. S3 and S6). For the samples with the comparable SnO2 loading, the yolk-shelled one presents the better performance than the supported one as expected due to the unique confinement for the former to hinder the loss of active species. For the SnO2@hCNC with high SnO2 content of 70.5 wt.%, the performance obviously deteriorates due to the insufficient interior space for accommodating the volume expansion of SnO2 upon lithiation (Figs. S3 and S6). As a result, the optimal SnO2@hCNC with SnO2 loading of 65.5 wt.% presents superior Li storage performance, which locates at the top level of the SnO2-based anodes to date (Figs. S7, S8 and Table S2).

Fig. 2. Electrochemical performances of [email protected] and SnO2/hCNC-68.5. (a) CV profiles at a scan rate of 0.5 mV s-1. (b) Rate capability. (c) Galvanostatic discharge/charge curves at 1.0 A g-1. (d) Cycling stability at 5.0 A g-1. The data for hCNC and SnO2 are plotted for comparison. 7

To get insight into the difference of lithium storage performances for SnO2@hCNC and SnO2/hCNC, their EIS evolutions during cycling were monitored and the Nyquist plots were simulated by the equivalent circuit[30], as shown in Fig. 3. The most prominent feature lies in the resistance of SEI film (Rf) which changes slightly for SnO2@hCNC (22.1~26.8 Ω) while increases significantly for SnO2/hCNC (21.7~56.4 Ω) (Fig. 3b and Table S3). This result indicates the better stability of the SEI film for SnO2@hCNC than SnO2/hCNC during cycling, which contributes to the better cycling stability for the former (Figs. 2c,d and S5). The similar cases were also observed for charge-transfer resistance (Rct) and intrinsic resistances (Rs). The drastic decrease of Rct after the first cycle is due to the formation of SEI (Fig. 3b and Table S3)[31].

Fig. 3. EIS evolutions for [email protected] and SnO2/hCNC-68.5 during cycling at 1.0 A g-1. (a) Nyquist plots before cycling and after different cycles, respectively. Inset is the equivalent circuit model. (b) The variations of Rf , Rct and Rs versus the cycle number.

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The morphology and microstructure evolution of SEI film and Sn-based nanoparticles during lithiation/delithiation were further characterized by TEM, as shown in Fig. 4. For SnO2@hCNC, a thin and stable SEI film formed on the surface of the nanocages and nanoparticles during the cycling process (Fig. 4a-c), as schemed in Fig. 4g. In contrast, for SnO2/hCNC, the SEI film on the nanoparticles become thicker and thicker due to the repeated formation and fragmentation (Figs. 4d-f and S9), as schemed in Fig. 4h, similar to the case of Si-based anodes[32]. The thin and stable SEI film, together with the small and little-changed Rf of SnO2@hCNC (Fig. 3b), indicate that the confinement of SnO2 nanoparticles inside the nanocages can hinder the repeated formation and fragmentation of SEI films, leading to the improved recyclability. After the 1st discharge, the SnO2 nanoparticles of the two samples are transformed to the amorphous Li-Sn alloy particles and Li2O, accompanied by the large volume expansion (Fig. 4a,d). For SnO2@hCNC, after the 50th discharge, the Li-Sn alloy particles are pulverized (Fig. 4b). After the 600th charge, the pulverized Li-Sn alloy particles are transformed back to the crystalline SnO2 nanoparticles with the smaller size (2.5±0.8 nm) than the initial value (6.1±1.0 nm), which remain inside the nanocages without loss (Fig. 4c), as schemed in Fig. 4g. For SnO2/hCNC without the confinement, after the 50th discharge, the Li-Sn alloy particles are pulverized and part of them aggregated into larger size than the original one (Fig. 4e). After 600th charge, either the aggregated or pulverized particles are transformed back to the crystalline SnO2 nanoparticles. Since the pulverized particles are easy to get lost, the remaining aggregated SnO2 particles have the much larger average particle size (19.0±5.7 nm) than the initial value (8.9±1.1 nm) (Fig. 4f and S9), as schemed in Fig. 4h. The effective retention of electroactive species with small sizes and stable SEI film is the main reason for the excellent cyclic stability and rate performance of the SnO2@hCNC, in comparison with SnO2/hCNC.

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Fig. 4. TEM charactractions of the charge and discharge products for [email protected] and SnO2/hCNC-68.5 at different discharge/charge status. (a-f) TEM images for SnO2@hCNC (a-c) and SnO2/hCNC (d-f) after 1st, 50th, and 600th cycles, respectively. (g,h) Schematic evolution of SEI film and Sn-based nanoparticles for the two samples during cycling. In (c), the circled smaller-sized nanoparticles represent the pulverized particles, and the insert shows the interplanar spacing of 0.265 nm corresponding to d101 of tetragonal SnO2. In (a-f), the white and yellow arrows denote the SEI film and the fragment of SEI film, respectively. The insets on the right side of (c,f) are the corresponding size histograms of the pulverized SnO2 nanoparticle.

The advantage of hCNC confinement is also demonstrated by comparing the lithium storage performance of the yolk-shelled Fe3O4@hCNC and the supported Fe3O4/hCNC composites with Fe3O4 size of 14.2±3.5 and 24.5±6.3 nm, and comparable loading of 50.9 and 52.0 wt.%, respectively (Figs. 5, 10

S10 and S11). For Fe3O4@hCNC, a reversible specific capacity of 466 mAh g-1 after 1000 cycles at 5.0 A g-1 is achieved, with the capacity attenuation of 0.056% per cycle, much lower than the corresponding 0.067% for Fe3O4/hCNC (255 mAh g-1@1000th cycle) and 0.089% for Fe3O4 (79 mAh g-1@1000th cycle) (Fig. 5a). The Fe3O4@hCNC also shows an excellent rate performance with the reversible capacities of 873, 793, 733 and 264 mAh g-1 at the specific current densities of 0.1, 0.5, 1.0 and 25 A g-1, respectively, much superior to the corresponding ones of Fe3O4/hCNC and Fe3O4 (Fig. 5b). The enhanced performance for Fe3O4@hCNC is ascribed to the confinement and buffering effects of nanocages, as supported by the similar evolutions of SEI film and Fe-based nanoparticles to the case of SnO2@hCNC (Figs. 4 and S12).[19] Actually, the yolk-shelled Fe3O4@hCNC presents the superior performance to the most Fe3O4based anodes to date (Fig. S13 and Table S4). In principle, the strategy to construct the yolk-shelled MO@hCNC should be also applicable to exploring advanced electrode materials of some other energy storage devices, e.g., Na-ion, K-ion, Mg-ion batteries or even beyond.

Fig. 5. Electrochemical performances of Fe3O4@hCNC and Fe3O4/hCNC. (a) Cycling stability at 5.0 A g1

. (b) Rate capability.

4. Conclusions In summary, we have demonstrated a general strategy to construct the yolk-shelled metal oxides inside carbon nanocages, through a simple evacuation-filling plus post-processing, by making use of the large cavity and subnanometer microchannels across the shells of hCNC. The so-obtained yolk-shelled MO@hCNC provides sufficient interior void to buffer the large volume variation, forms the stable SEI 11

film, and greatly reduces the loss of active components during lithiation/delithiation. In addition, the 3D conductive carbon network and coexisting micro-meso-macropores ensures the fast electron transfer and ions/electrolyte transportation. All these features favor the long-term cycling stabilities and high reversible specific capacities. As a result, the optimized SnO2@hCNC (SnO2 loading: 65.5 wt.%) presents a superior cycling stability with a reversible capacity of 792 mAh g-1 at the current density of 1.0 A g-1 after 600 cycles, and a remarkable rate capability of 568 mAh g-1 at 5.0 A g-1, which locates at the top level of the SnO2-based anode materials to date. The Fe3O4@hCNC (un-optimized Fe3O4 loading: 50.9 wt.%) also presents a high cycling stability with a reversible capacity of 466 mAh g-1 at 5.0 A g-1 after 1000 cycles, and an excellent rate capability of 264 mAh g-1 at 25 A g-1, exceeding the most reported Fe3O4-based anode materials. The correlation between the improved electrochemical performance and the yolk-shelled structure are corroborated by the EIS and HRTEM characterizations which present the much smaller increase of the resistance of SEI film (Rf) and much less loss of the electroactive species for MO@hCNC than MO/hCNC. In comparison with the complicated construction of yolk-shelled structures by the previous methods, this study provides a simple and general approach to the yolk-shelled metal oxides inside carbon nanocages, which is of great significance for exploring the high-performance anode materials for LIBs or even beyond.

Acknowledgments We appreciate the jointly financial support from the National Key Research and Development Program of China (2018YFA0209100, 2017YFA0206500) and the National Natural Science Foundation of China (21832003, 21773111, 21972061, 21573107 and 51571110).

Declaration of interests ‡

M. Liu and H. Fan contributed equally to this work. The authors declare no competing financial interest.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at 12

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Author's Vitae Meng Liu received her M.S. degree from Zhengzhou University of Light Industry in 2015. She is currently a Ph.D. candidate under the supervision of Prof. Xizhang Wang and Prof. Zheng Hu in School of Chemistry and Chemical Engineering, Nanjing University. Her research interests mainly focus on the development of new energy storage systems especially for Li-ion and Na-ion batteries, addressing the electrochemical mechanism and materials design. Hao Fan received his Ph.D. degree in Chemistry from Nanjing University in 2018. He is currently a postdoctoral research fellow in School of Materials Science and Engineering in Xi’an Jiaotong University. His research interests include the design and synthesis of advanced functional materials and their applications in energy conversion and storage devices. Currently his research mainly focuses on the development of high-performance water-soluble organic electrolytes and metallic zinc-based anode in low-cost redox flow battery systems for large-scale energy storage. Ou Zhuo received his M.E. degree from Guilin University of Technology in 2012, and obtained his Ph.D. from Nanjing University in 2019. He was appointed a lecturer in chemical engineering of Jishou University in 2019. His research focuses on Fischer-Tropsch synthesis. Junchao·Chen received his M.S. (2015) and Ph.D. (2019) degrees in chemistry from Nanjing University. His principal research experience and interests lie in applying various state-of-the-art solid-state NMR and XRD techniques to solve scientific problems in the fields of catalysis and energy storage. The ultimate goal of his work is to have a deeper comprehension on the relationship of accurate structures to applied performances, which can be used to rationally design nano/mesostructures with desired properties. Qiang Wu obtained his Ph.D. from Nanjing University in 2004. He was appointed an associate professor in 2006, and a professor of Nanjing University in 2015. As a Hua-Ying Scholar, he visited Stanford University for one year. His scientific interests focus on the rational design of nano/mesostructured materials and their applications in energy storage and conversion.

Lijun Yang received his Ph.D. in solid mechanics from Harbin Institute of Technology in 2006, and gradually converged to chemistry after two post-doc periods in IMEC Belgium and Nanjing University. Now he is an associate professor in Nanjing University and mainly focuses on the theoretical understanding of the mechanisms in energy conversion and storage systems, such as fuel cells, supercapacitors and lithium batteries. Luming Peng is a Professor at the School of Chemistry and Chemical Engineering, Nanjing University (NJU), Nanjing, China. He received his B.S. from NJU in 2001, and Ph.D. from State University of New York at Stony Brook in 2006 under the supervision of Prof. Clare P. Grey. Prior to his appointment at NJU, he was a postdoctoral associate at Stanford University, working with Prof. Jonathan F. Stebbins. His research interests lie in applying various state-of-the-art solid-state NMR techniques to solve tough scientific problems where other techniques have failed.

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Xizhang Wang received his Ph.D. in chemistry from Nanjing University in 2001. He was appointed an associate professor of Nanjing University in 2003 and full professor in 2011. He was a JSPS Fellow in Tokyo University (2003-2005). His scientific interests mainly focus on nanomaterial chemistry, sustainable energy, and heterogeneous catalysis.

Renchao Che received his Ph.D. degree in 2003 from the institute of physics, Chinese Academy of Sciences. Dr. Che was a research fellowship in National Institute for Materials Sciences (NIMS), Japan (2003–2006) and has been appointed as a Professor and PhD tutor at the Laboratory of Advanced Materials at Fudan University since 2008. He was the winner of National Science Fund for Distinguished Young Scholars in 2017. He mainly contributes in the field of microstructure research of nano-functional materials and microwave absorption materials by advanced transmission electron microscopy techniques and focuses on the structure-activity relationship for complex nano-composite systems. Zheng Hu received his BS (1985) and Ph.D. (1991) degrees in physics from Nanjing University. After two-year’s postdoctoral research in Department of Chemistry, he became an associate professor in 1993, and subsequently acquired the professor position in 1999, and Cheung Kong Scholar professor in 2007. He is the owner of the NSFC fund for outstanding young scientists of China (2005). Hu is engaged in the research field of physical chemistry and materials chemistry addressing the growth mechanism, materials design and energy applications of a range of nano-/mesostructured materials, especially the carbon-based materials, group III nitrides and transition metal oxides.

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Highlights 1. A simple and general strategy to construct yolk-shelled MO@hCNC (MO=SnO2, Fe3O4). 2. The yolk-shelled MO@hCNC samples are excellent anode materials for LIBs. 3.

The performance~structure correlation is well demonstrated.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: