Yolk-shelled [email protected]@MnO hierarchical hybrid nanospheres for high performance lithium-ion batteries

Journal of Alloys and Compounds 829 (2020) 154579

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Yolk-shelled Sn@C@MnO hierarchical hybrid nanospheres for high performance lithium-ion batteries Fanchao Zhang , Yong Wang *, Wenbin Guo , Peiyuan Mao , Shun Rao , Pandeng Xiao Department of Chemistry, Capital Normal University, Beijing, 100048, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 December 2019 Received in revised form 7 February 2020 Accepted 27 February 2020 Available online 2 March 2020

Metallic tin exhibits great potential as an alternative to commercial graphite anode materials. However, its tremendous volume change during cycles leads to significant capacity fading, which hampers its practical application. In this work, Sn nanoparticles are confined to a carbon nanocage to form Sn@C@MnO yolk-shell hierarchical hybrid nanospheres as anode nanomaterials for Li-ion battery. In the novel nanostructures, the yolk-shell nanostructures can well buffer the volume contraction/expansion of Sn nanocores during cycles; the carbon shell can provide high conductivity and inhibit Sn and MnO nanocrystals from stacking; MnO nanocrystals on the C shell can not only offer sufficient active sites for Li-ion adsorption, but also validly consolidate the structural stability of the shell in the yolk-shell nanostructure. Benefiting from the unique nanostructures, Sn@C@MnO yolk-shell hierarchical hybrid nanospheres exhibit superior electrochemical lithium storage performances (822mAhg
1 after 250 cycles at 0.1Ag-1) and long-term cyclic stability (603mAhg
1 after 500 cycles at 0.5Ag-1). This work offers valuable insight into the synthetic strategy of Sn-based anode materials with novel hierarchical hybrid nanostructures. © 2020 Elsevier B.V. All rights reserved.

Keywords: Tin Manganese oxide Yolk-shell Hybrid nanomaterials Lithium-ion battery

1. Introduction Lithium-ion batteries (LIBs) have received extensive research interest and have been widely used as the power for various energy storage devices due to their high energy density [1e5]. Recently, metallic tin (Sn) has exhibited great potential as a new generation anode nanomaterial for LIBs [6e8], because of its abundant reserve, high theoretical capacity (994mAhg
1) and eco-friendliness [9e12]. However, the actual application of tin-based nanomaterials is hampered by the pulverization of the electrode and poor cyclic stability because of huge volume variation during the lithiation-delithiation process [13,14]. To address the aforementioned critical issues, lots of efforts have been devoted to tailoring the nanostructures, morphology and composition of tin-based materials for LIBs [7e13]. The first effective strategy is to reduce Sn partilces to the nanoscale. Nano-sized Sn can relieve the mechanical stress during the lithiationdelithiation process, thereby suppressing the tendency of Sn fracture [11,15,16]. Moreover, the nanostructure of Sn active materials

* Corresponding author. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.jallcom.2020.154579 0925-8388/© 2020 Elsevier B.V. All rights reserved.

can offer an excellent rate capability through shortening the path lengths for Liþ transport [11]. The second effective strategy is to disperse nanosized Sn in a carbon matrix, which can buffer the volumetric expansion of the electrode, leading to the outstanding cyclic stability [7e9]. For example, Xu et al. have dispersed uniform Sn nanoparticles in carbon matrix to form nanocomposites with outstanding properties (710 mAhg
1 after 130 cycles) as anodes for LIBs [17]. The third strategy is to introduce other new component (such Co, Ni and TiO2) to Sn to obtain nanocomposites [11,18e21]. For example, Zhang et al. have reported a 3D scaffold NieSn alloy anode nanocomposite with excellent electrochemical properties because their novel composite nanostructures well maintain electrical connectivity and accommodate the volume changes during the cycling process [19]. Although great advances have been achieved in the study on Sn-based anode nanomaterials, there are no reports on Sn@C@MnO yolk-shelled hierarchical hybrid nanospheres with desired nanoarchitectures for high-performance LIBs. Herein, we demonstrate a feasible synthetic strategy of yolkshelled Sn@C@MnO hierarchical hybrid nanospheres with novel nanoarchitectures for LIBs anode with superior electrochemical performance. In our design, metallic Sn nanoparticles are successfully encapsulated in spherical carbon nanocages with MnO nanoparticles. Notably, the yolk-shelled nanostructures can well

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buffer the volume contraction/expansion of nanosized Sn cores and maintain the structural integrity of the electrode nanomaterials, leading to excellent cyclic performance. In addition, MnO nanoparticles tightly anchored on the C shells can offer sufficient active sites for Li-ion adsorption and facilitate a fast electron transport. Meanwhile, it is important that the active MnO nanoparticles immobilized on the carbon shells can enhance the architecture stability of the yolk-shelled nanocomposites to obtain outstanding reversible capacity and cycling stability. More importantly, the nanocomposites with intriguing hierarchical nanostructures as anode materials for LIBs exhibit a high reversible capacity of 822mAhg
1 after 250 cycles at 0.1Ag-1 and 603mAhg
1 after 500 cycles at 0.5Ag-1. This investigation provides a rational strategy to prepare Sn-based anodes with novel architecture to enhance cyclic stability of LIBs. 2. Experimental 2.1. Fabrication of SnO2@C hollow nanospheres (SnO2CHNs) SiO2 nanospheres as template were synthesized by the previously reported method [22], and SnO2 coating on nanospheres was achieved as described in the previous work, leading to the formation of SiO2@SnO2 nanospheres [23]. Dopamine (0.12 g) was completely dissolved into a Tris-buffer aqueous solution (0.1 L, 0.01 M) containing SiO2@SnO2 nanospheres (0.1 g), and aged for 24 h. Then, the collected specimen was heated at 550  C under the argon protection. Finally, SnO2CHNs were successfully prepared by getting rid of the SiO2 template in 2 M NaOH solution according to the literature [24]. 2.2. Fabrication of Sn@C@MnO yolk-shelled hierarchical hybrid nanospheres (SnCMnOYNs) KMnO4 solution (0.09 L, 0.06 g) with well-dispersed SnO2CHNs (0.06 g) was heated in a 80  C water bath for 6 h. Then, the collected specimen was annealed at 700  C for 2 h under the protection of H2/ Ar (5:95 v/v) atmosphere to obtain the product of SnCMnOYNs. 2.3. Materials characterization The product was characterized by transmission electron microscopy (TEM, FEI TEcnai F20) with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD, Bruker D8 ADVANCE), thermogravimetric analysis (TG, TGA/Q5000IR), scanning electron microscopy (SEM, Hitachi SU8200) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250X). 2.4. Electrochemical measurements The anode slurry includes active material (SnCMnOYNs), acetylene black, PVDF (80, 10, and 10 wt%, respectively) and N-methyl2-pyrrolidone. The anode was prepared by coating the aforementioned slurry onto copper foil, then dried under vacuum at 80  C. The mass loading on the anode was carefully controlled at 1.0e1.5 mg cm
2. The anode was assembled into CR2032-type cells in an Ar-filled glovebox. The electrolyte is 1 M LiPF6 in a mixed solvent of dimethyl carbonate (DMC), ethylene carbonate (EC) and ethylene methyl carbonate (EMC) (1/1/1, vol%). The galvanostatic charge-discharge profiles were tested on LANHE CT2001A battery testers. The cyclic voltammetry (CV) curves and electrochemical impedance spectroscopy (EIS) were acquired from electrochemical workstation (Bio-Logic SP-200).

3. Results and discussion The overall synthetic strategy for SnCMnOYNs is illustrated in Fig. 1. First, SiO2 nanospheres were facilely synthesized by the € ber method (Fig. 1a and Fig. S1a). Subsequently, a layer of SnO2 Sto nanocrystals with the thickness of about 17 nm grew on surface of SiO2 nanospheres to form core-shelled SiO2@SnO2 nanospheres (Fig. 1b and Fig. S1b). Afterward, polydopamine (PDA) deposited on the surface of SiO2@SnO2 nanospheres. The as-obtained SiO2@SnO2@PDA nanospheres were further annealed, and then SiO2 cores were removed by NaOH solution to form SnO2@C hollow nanospheres (SnO2CHNs, Fig. 1c and Figs. S1ced). Finally, the MnO2 could coat on the surface of SnO2CHNs by the redox reaction of the KMnO4 and C (Equation (1)) [25,26]. Notably, when treated at 700  C under H2/Ar atmosphere, MnO2 could be reduced to MnO (Equation (2)) while SnO2 could be reduced to metallic Sn (Equation (3)) [9,10,27]. It is noted that the metallic Sn has a low melting point (232  C) and tends to evaporate to form large beads at high temperature, and can be encapsulated by the appropriate carbon shell [10]. Therefore, the reduced Sn melts into the liquid state during the heating process due to its low melting point (232  C). After metallic Sn cools and solidifies, it pools in the bottom of the C nanocage to form the yolk-shelled nanostrucutures because of the gravity [10]. As a result, the reduced Sn nanoparticles were formed as the core in the carbon shell during the heating process, resulting in the generation of SnCMnOYNs (Fig. 1d). 2

4MnO
4 þ 3C þ H2 O/4MnO2 þ CO3 þ 2HCO3

(1)

MnO2 þ H2 /MnO þ H2 O

(2)

SnO2 þ 2H2 /Sn þ 2H2 O

(3)

The morphologies and microstructures of SnCMnOYNs were analyzed by SEM, TEM and high-resolution TEM (HRTEM). SnCMnOYNs (Fig. 2) inherit the spherical morphology of SnO2CHNs (Fig. S1c). SEM images reveal that MnO nanoparticles with the size of 4e13 nm are tightly anchored on the exterior surfaces of hollow nanospheres (Fig. 2a and b). Notably, the good combination of the C layer and MnO nanoparticles enhances the structural stability and reduces the content of carbon with relatively low theoretical capacity (372 mAhg
1) in SnCMnOYNs [28]. In TEM images of Fig. 2cee, the metal Sn nanoparticles are settled down to the inner surface of carbon nanocage, confirming the formation of the yolkshell nanostructure. The phenomenon of Sn attaching to the inner surface of C nanocage has also been reported in literatures [10]. As shown in the HRTEM image of SnCMnOYNs (Fig. 2f), the interlayer distance of lattice fringes is determined to be about 0.22 and 0.29 nm, corresponding to the MnO (200) plane and the Sn (200) plane, respectively [11,28,29]. EDS elemental mappings of Fig. 3 further reveal the elemental distribution of C, Sn, Mn and N. Fig. 3d shows that the distribution of Sn is concentrated in the inner nanoparticle of the SnCMnOYNs sample, implying that the existence of Sn cores in the carbon nanocages. Furthermore, the C and Mn elements are distributed over the entire surface of the sample, verifying that the nanosized MnO nanoparticles well deposit on the C shell. Notably, the minute amount of N element in SnCMnOYNs can be also observed from Fig. 3f, which derives from N-containing PDA [10,28]. The previous reports have confirmed that the N doping in the carbon shell can be utilized to validly enhance the electrical conductivity because the doping can increase the active site of the electrochemical reaction [30]. Moreover, the yolk-shelled nanostructure of the nanocomposites is also believed to be significant for their electrochemical performance because the void space of SnCMnOYNs is

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Fig. 1. Illustrated synthetic process of yolk-shelled Sn@C@MnO hierarchical hybrid nanospheres.

Fig. 2. (aeb) SEM and (cee) TEM images of SnCMnOYNs with different magnifications; (f) HRTEM images of SnCMnOYNs.

Fig. 3. (a) TEM and (b) STEM images of SnCMnOYNs. The corresponding EDS elemental mapping images of SnCMnOYNs: (c) C, (d) Sn, (e) Mn and (f) N.

sufficient to buffer the grievous volume change of Sn nanocores, resulting in their enhanced cyclic stability [10,13,14]. In the XRD pattern of SnCMnOYNs (Fig. 4a), all the XRD peaks

with signal * are indexed to Sn (JCPDS No. 04e0673) [7,10], while all the other peaks with signal # match well MnO (JCPDS No. 07e0230) [28,29]. Meanwhile, no reflection can be observed for

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Fig. 4. (a) XRD pattern and (b) TG profile of SnCMnOYNs. High-resolution XPS spectra of (c) Sn3d, (d) Mn2p, (e) C1s and (f) N1s.

crystalline graphite in SnCMnOYNs, primarily due to the existence of the amorphous C in SnCMnOYNs [7,24]. The weight ration of carbon in SnCMnOYNs was confirmed by TG curves (Fig. 4b). Accordingly, the small weight loss (3.3 wt%) at below 210  C may stem from the desorption of absorbed moisture in SnCMnOYNs [8e10], while the main weight loss at 210e800  C (13.1 wt%) can be related to the complete combustion of C and the gradual oxidation of MnO and Sn. It is reported that Sn and MnO can be oxidized to SnO2 and Mn3O4 after being heated at 800  C in the previous works, respectively [31e33]. According to Inductively coupled plasma mass spectrometry (ICP-MS), the Sn:Mn weight ratio in SnCMnOYNs is 1.43:1. Based on the results of ICP-MS and TG, the content of Sn, C and MnO in SnCMnOYNs is ca. 38.7, 26.4 and 34.9 wt% (Detailed calculation method can be seen in Supplementary Material). Fig. S2a shows the full survey XPS spectrum of SnCMnOYNs, revealing the coexistence of Sn, Mn, C, O and N in SnCMnOYNs. The Sn3d spectrum of Fig. 4c shows that two peaks at 495.3 and 486.7 eV arise from Sn3d3/2 and Sn3d5/2, respectively, which agrees well with those of the reported SnOx [34], indicating the generation of tin oxide on the surface of Sn nanoparticles [35]. Meanwhile, the peaks at 485.5 and 493.9 eV correspond to spin orbit peaks of metallic Sn (Fig. 4c) [11,33]. As for the Mn2p spectrum (Fig. 4d), two peaks at 641.6 and 653.2 eV are attributed to Mn2p3/2 and Mn2p1/2, respectively, confirming the presence of MnO [36]. As seen from the C1s spectrum of Fig. 4e, the bonds CeOH, C]O, C]C, CeC and CeN in SnCMnOYNs can be found at 286.7, 289.3, 284.4, 284.9 and 285.7 eV, respectively [28,36]. Interestingly, the N 1s spectrum of SnCMnOYNs (Fig. 4f) exhibits three distinct peaks at 402.8, 398.2 and 400.8 eV, corresponding to graphitic, pyridinic and pyrrolic N, respectively [36,37]. Notably, the N incorporation in the C layer can effectively provide more active sites for the fast Liþ intercalation, leading to excellent lithium storage performance [10,33]. Moreover, the O1s spectrum of SnCMnOYNs (Fig. S2b) shows four distinct peaks at 533.9, 532.4, 529.9 and 531.0 eV, corresponding to OeCeO, chemisorbed oxygen-related species (OC), lattice oxygen (OL, Mn] O) and oxygen vacancies (OV), respectively [27,38].

To investigate MnO nanoparticles on the effect of nanostructures and properties of SnCMnOYNs, Sn@C yolk-shelled nanospheres (SnCYNs) have been tailored (Fig. S3). Furthermore, the XPS spectra of SnCYNs are also shown in Fig. S4, confirming the coexistence of the Sn, C, O and N elements. The N2 ab-/de-sorption analysis was performed to investigate the surface area and porous characteristic of SnCMnOYNs (Fig. S5). As exhibited in Fig. S5, the specific area of SnCMnOYNs is 71.5 cm2g-1, and the pore volume is 0.08 cm3g-1. Such high surface area and porous nanostructures can offer sufficient electrochemical active sites, facilitating the fast diffusion of Li-ion during cycles [28,36]. The electrochemical performances of SnCMnOYNs as a LIBs anode are evaluated. Fig. 5a displays the CV curves of SnCMnOYNs at 0.2 mVs
1. The electrochemical reaction mechanisms of Sn and MnO can be described as Equations [10,27,36,39]:

MnO þ 2Liþ þ 2e
4Mn þ Li2 O

(4)

Sn þ 4:4Liþ þ 4:4e
4Li4:4 Sn

(5)

During the initial cycle, the reduction peak at 0.25 V is accredited with the reduction process of Mn2þ to Mn0 (Equation (4)), and the peak at 0.35e0.60V is related to the gradually alloying reaction process of Sn to LixSn alloy (Equation (5)) [7,9,10,28,36]. The weak broad peak at about 0.8 V in the initial cathodic scanning arises from the formation of solid-electrolyte interphase (SEI) film [7,11,12]. Meanwhile, the distinct oxidation peaks at 0.70e0.85V in the anodic scanning are ascribed to the multi-step dealloying reactions of LixSn (Equation (5)) [9e11], and the broad oxidation peak occurring at 1.25 V is related to the reaction of Mn0 to Mn2þ (Equation (4)) [28,36]. Notably, one distinct oxidation peak can be observed at ca. 2.1 V in the CV curves of the SnCMnOYNs electrode (Fig. 5b). Moreover, with cycles going on, this oxidation peak of 2.1 V appears and gradually enhances, deriving from the reoxidation of Mn2þ to higher oxidation state manganese, which can introduce extra specific capacity in the subsequent cycles [36,40,41]. According to the literature, the gradual emergence of

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Fig. 5. (aeb) CV curves of SnCMnOYNs and (c) SnCYNs. Charge-discharge curves of (d) SnCMnOYNs and (e) SnCYNs at 0.1 Ag-1. (f) Cyclic performance at 0.1 Ag-1 and (g) rate performance of SnCMnOYNs and SnCYNs. (h) Comparison of rate capability of SnCMnOYNs with previously reported Sn-based and MnO-based anode materials. (i) Prolonged cycling performance of SnCMnOYNs at 0.5 A g
1 for 500 cycles. CE at 0.1 Ag-1 and 0.5Ag-1 for SnCMnOYNs is presented in (f) and (i), respectively.

higher oxidation state manganese may be related to the structural change of electrode materials during the cyclic process [42e44]. For example, Guo et al. have reported that higher Mn (II) oxidized to a higher oxidation state can be related to the synergistic effects of C and MnOx as well as the amorphous structure of MnOx/C nanocomposites [45]. In the subsequent scans, the CV curves strongly overlap each other, confirming the high reversibility and outstanding cyclic stability of SnCMnOYNs. Fig. 5c displays the initial six CV curves of SnCYNs, manifesting the similar characteristics with SnCMnOYNs apart from the peaks of manganese oxide. Fig. 5d shows discharge/charge profiles of SnCMnOYNs. SnCMnOYNs exhibit an excellent 1st discharge/charge capacity of 1281/898mAhg
1 at 0.1 Ag-1, respectively, resulting in an initial Coulombic efficiency (CE) of 70.1%. The irreversible capacity loss is mainly resulted from the pre-activation of the electrode and the formation of SEI film [10,13]. Meanwhile, SnCYNs exhibit the initial discharge/charge capacity of 1211/748 mAhg
1, corresponding to the initial CE of 61.8% (Fig. 5e). Obviously, the initial CE of SnCMnOYNs is superior to that of SnCYNs, resulting from the MnO nanoparticles anchored on SnCYNs. The cycling performances of SnCMnOYNs and SnCYNs are shown in Fig. 5f. After 250 cycles, SnCMnOYNs still preserve a high capacity of 822mAhg
1, and their capacity retention (versus the second discharge capacity) is 93.4%, definitively proving that

SnCMnOYNs owns excellent cycling stability and capacity retention. Interestingly, the CE of SnCMnOYNs maintains near 99% in the subsequent cycles. For comparison, after 250 cycles, SnCYNs deliver a capacity of 574 mAhg
1, which is lower than that of SnCMnOYNs. Fig. 5g reveals the rate performance of SnCMnOYNs with various rate currents of 0.2, 0.5, 1, 2 and 5 Ag-1, corresponding to the reversible capacities of 708, 620, 560, 430 and 265 mAhg
1, which are superior to those of SnCYNs (485, 350, 280, 190 and 130 mAhg
1). Notably, the reversible capacity is back to 810mAhg
1 as the testing current is back to 0.1Ag-1, confirming the good reversibility of SnCMnOYNs. Compared with previously reported Snbased and MnO-based nanocomposites, SnCMnOYNs exhibit superior rate performances (Fig. 5h) [8,11,29,40,41,46e52]. Moreover, Fig. 5i shows the long-life cyclic performances of SnCMnOYNs at 0.5Ag-1. After 500 cycles, SnCMnOYNs preserve a high capacity of 603mAhg
1 with a superior CE of above 99%. The as-presented electrochemical performance clearly suggests that synergetic effects of yolk-shelled nanostructures, nano-sized Sn cores, N-doping C intermediate layer and MnO nanoparticles external layer lead to the significantly improved electrochemical performances. EIS is also performed on SnCMnOYNs after the 10th, 50th and 100th cycles (Fig. 6a). In the equivalent circuit, Rsf and Rct can be ascribed to surface film resistance and charge transfer resistance, respectively, while Re is electrolyte resistance [28,53]. In addition,

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Fig. 6. (a) EIS of SnCMnOYNs after the 10th, 50th and 100th cycles. The equivalent circuit is shown in the insert in (a). (b) The linear relationship between u
1/2 and Z0 in the lowfrequency region. (c) Schematic Liþ and e
conduction/diffusion within SnCMnOYNs during intensive cycles.

Zw and CPEi denote Warburg impedance and constant-phase element. In general, R(sfþct) of the high-frequency semicircle in EIS can be applied to assessing the electrode conductivity [54,55]. In Table S1, the R(sfþct) value of SnCMnOYNs electrode after 100 cycles (46.7 U) is much less than that after 10 cycles (84.8 U) and 50 cycles (71.3 U), implying the enhanced activation and the improved reaction kinetics as cycles go on [43,56]. The Liþ diffusion coefficient (DLiþ) is evaluated based on the following Equations (6) and (7) [57,58]:

. DLiþ ¼ R2 T2 2A2 n4 F4 C2 s2

(6)

Z’ ¼ Re þ Rct þ Rsf þ su
1=2

(7)

Where s denotes the Warburg factor, which is related to Z0 , as described in Equation (7) and can be acquired from Fig. 6b. If other parameters are identical, the electrode with the lowest s value should have the highest DLiþ value (Table S1). Obviously, compared with the 10th and 50th cycles, the SnCMnOYNs electrode after the 100th cycle possesses the lowest s value, so the SnCMnOYNs electrode after the 100th cycle has the higher DLiþ value and the faster Liþ diffusion process (Fig. 6c), resulting in the gradual enhanced reversible capacity and superior rate performances. Notably, the DLiþ value of SnCMnOYNs after the 10th, 50th and 100th cycle can be calculated to 1.95  10
14, 2.65  10
14 and 5.16  10
14 cm2 s
1, respectively, confirming the aforementioned conclusion. To deeply understand the influence of MnO nanoparticles on the performances of SnCMnOYNs, the morphology of SnCMnOYNs and SnCYNs after 200 cycles have been investigated (Fig. 7 and Fig. S6). As shown in Fig. 7a, the interior void space of SnCMnOYNs is filled with Sn nanoparticles after the lithiation process. The STEM image

and EDX mappings (Fig. 7beg) also confirm that the flexible carbon shell tightly wraps the expanding active nanocores in SnCMnOYNs. In Fig. 7h-n, it can be observed that the Sn nanocores in the SnCMnOYNs undergo obvious volume shrinkage during delithiation, and SnCMnOYNs well maintain the structural integrity after 200 cycles. However, many broken C shells of SnCYNs after 200 cycles can be observed (Fig. S6), verifying the effective strengthening of MnO nanoparticles for the C shell of SnCMnOYNs due to the nanosized MnO layer coated on the surface of the carbon intermediate layer. Moreover, the C, Sn, Mn, N and F elements are still uniformly distributed over SnCMnOYNs (Fig. 7ceg and Fig. 7jen), which confirms their prominent structural integrity during the lithiation/delithiation process, leading to the excellent cycling stability. Notably, owing to the repeated volume expansion/shrinkage of Sn nanocores in the SnCYNs, the C shells without MnO nanoparticles gradually collapse after intense cycles (Fig. 7o). However, owing to the effective strengthening of MnO nanoparticles for the C shell of SnCMnOYNs, the C shells with MnO nanoparticles well maintain after intense cycles (Fig. 7p). The electrochemical performance of SnCMnOYNs is superior to that of Sn-based composite materials in the literatures (Table S2). The excellent properties of SnCMnOYNs can be explained as follows: (1) The yolk-shell nanostructure of the sample is believed to be important for the excellent cyclic stability, conducive to accommodating the volume variation of active Sn nanocores during cycling [9e14,59e61]; (2) The Sn nanocores can shorten Li-ion transfer path, favorable to enhancing the capacity [13,14,17]. (3) The conductive and flexible C intermediate layer can not only provide high conductivity and inhibit the Sn and MnO nanocrystals from stacking, but also be conducive to accommodating the volume variation of active Sn nanocores during cycling, and thus maintain structural stability of the yolk-shelled nanostructures [13,14,62]. (4) MnO

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Fig. 7. (a) TEM, (b) STEM and (ceg) EDS elemental mapping images of SnCMnOYNs after lithiation: (c) C, (d) Sn, (e) Mn, (f) N and (g) F; (h) TEM, (i) STEM and (jen) EDS elemental mapping images of SnCMnOYNs after delithiation: (c) C, (d) Sn, (e) Mn, (f) N and (g) F. Illustrated cycling model of (o) SnCYNs and (p) SnCMnOYNs.

nanoparticles tightly anchored and well-dispersed on the C layer can not only offer sufficient active sites for Li-ion adsorption, but also consolidate the structural stability of the shell in the yolkshell nanostructure [25,63]. (5) The capacity of SnCMnOYNs decreases initially, and then increases gradually after subsequent cycles (Fig. 5f.), which can be ascribed to the following two reasons: (a) Because of the gradual activation of Sn nanocores and MnO nanoparticles, the interface between anode and electrolyte gradually increases as cycles go on. The increased interface is advantageous to e
/Liþ transfer and the improvement of the reaction kinetics (Fig. 6) [10,36,64,65], resulting in the increased capacity. (b) The reversible generation of a polymeric gel-like film during cycles is also one of reasons on the gradual increase of the capacity [28,66].

4. Conclusions In summary, we have successfully designed novel Sn@C@MnO yolk-shelled hierarchical hybrid nanospheres. In the novel nanostructures, the yolk-shelled nanostructures can well buffer the volume contraction/expansion of nanosized Sn cores during cycles; the carbon shell can provide high conductivity and inhibit the Sn and MnO nanocrystals from stacking; MnO nanocrystals on the C shell can not only offer sufficient active sites of Li-ion adsorption, but also validly consolidate the structural stability of the shell in the yolk-shell nanostructure. Benefiting from the unique nanostructures, SnCMnOYNs exhibit improved Li storage properties. Importantly, our investigation will contribute to the rational design of Sn-based anode nanomaterials with novel yolk-shelled,

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hierarchical and multi-shelled nanostructures to improve Li ion storage properties. [14]

Declaration of competing interest 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.

[15]

[16]

CRediT authorship contribution statement Fanchao Zhang: Investigation, Validation, Formal analysis, Data curation, Writing - original draft. Yong Wang: Conceptualization, Methodology, Data curation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Wenbin Guo: Formal analysis, Investigation. Peiyuan Mao: Formal analysis, Data curation. Shun Rao: Formal analysis, Data curation. Pandeng Xiao: Formal analysis, Data curation.

[17]

[18]

[19]

[20]

Acknowledgements This work was supported by the Open Research Fund of State Key Laboratory of Multiphase Complex Systems (No. MPCS-2019-D02) and Beijing Municipal Natural Science Foundation (2152010). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154579.

[21]

[22]

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