Tailoring hierarchical porous core–shell SnO2@Cu upon Cu–Sn alloys through oxygen binding energy difference for high energy density lithium-ion storage

Rational design and construction of self-supporting anodes with high energy density is an essential part of research in the field of lithium-ion batteries. Tin oxide (SnO2) is restricted in application as a prospective high energy density anode due to inherent low conductivity and huge volume expansion of the charge/discharge process. A new strategy that combines high energy ball milling and nonsolvent induced phase separation (NIPS) method was employed to synthesize self-supporting electrodes in which porous SnO2 was encapsulated in a three-dimensional hierarchical porous copper (Cu) shell structure (3DHPSnO2@Cu). This unique structure was constructed due to the different binding energy of the alloy with oxygen, which are −0.91 eV for Cu41Sn11 and −1.17 eV for Cu5.6Sn according to the density functional theory calculation. 3DHPSnO2@Cu electrodes exhibited excellent discharge capacity with an initial reversible capacity of 4.35 mAh cm−2 and a reversible capacity of 3.13 mAh cm−2 after 300 cycles at a current density of 1.4 mA cm−2. It is attributed that the porous Cu shell encapsulated with porous SnO2 provides buffer volume. Among them, the SnO2-Cu-SnO2 interface increases the electrical conductivity and the porous structure provides ion transport channels. This strategy opens a new pathway in the development of self-supporting electrode materials with high energy density.


Introduction
The traditional electrode manufacturing utilizes the slurry casting method, in which the active material, conductive agent and binder are uniformly dissolved in a solvent, after which the slurry is cast onto the collector fluid and formed into electrode sheets by drying and rolling [1][2][3].However, the problems of poor flexibility, low loading, poor conductivity, expensive additives and unfriendly process of traditional electrode sheet make another strategy to be proposed, which is self-supporting flexible electrode [4][5][6][7].To realize self-supported electrodes many strategies have been proposed: (1) carbon based materials are introduced as collectors due to their excellent electrical conductivity and easy realization of 'carbon paper' electrodes [8][9][10][11].(2) flexible polymers are used as substrate direct growth active materials due to their excellent flexibility and low price [12][13][14] (3) metal-based self-supporting electrodes have been widely studied due to their excellent electrical conductivity [15][16][17], unique porous structure [18][19][20], and structural stability [21][22][23].SnO 2 is considered to be the most promising next-generation lithium-ion anode material of particular interest due to its high energy density (1494 mAh g −1 ) [24], low discharge point and low price [25].However, problems such as the loss of activity and powdering off of SnO 2 due to volume expansion during charging and discharging [26], as well as poor electrical conductivity [27], make it an obstacle to practical applications.Consequently, researchers conducted a lot of research on SnO 2 based anode electrodes to address the above problems [28,29].Preparation of nanosized SnO 2 to increasing the specific surface area, thereby reducing the volume expansion stress, such as nanoflowers [30], nanospheres [31], nanoparticle [32], etc. Embedding SnO 2 in the carbon base material to buffer the volume expansion of SnO 2 by employing the pores, defects and hollow structure of the carbon base material [33,34].Compounding SnO 2 with a metallic material reduces poor conductivity by adopting the conductivity of the metal [35,36].A large number of works have combined the above strategies to address the application of SnO 2 [37][38][39].To the best of our knowledge, there are no reports to research the effect that the core-shell structure of metal base has on the lithium storage properties of SnO 2 , and the investigation of its formation mechanism.
Herein, We successfully prepared a self-supporting electrode with porous SnO 2 encapsulated in a three-dimensional hierarchical porous copper shell via high energy ball milling, NIPS and heat treatment.For the first time, we have adopted the principle that different Cu-Sn alloys have different binding energies with oxygen [40,41], as show in figure 1, which produces a clear sequential order during oxidation process to form the three-dimensional core-shell structure.Meanwhile, the DFT calculations show that Cu 5.6 Sn has a higher oxygen binding energy than Cu 41 Sn 11 .The 3DHPSnO 2 @Cu as a self-supporting electrode for Li-ion batteries demonstrates excellent storage performance with a reversible capacity of 1080 mAh g −1 and capacity retention at 849.5 mAh g −1 after 300 cycles at a current density of 100 mA g −1 .The design strategy provides a new perspective for the design of next-generation self-supporting electrode structures.

Preparation of Cu-Sn alloy powders
Cu-Sn alloy powder is prepared by high-energy ball-milling.The Cu and Sn powders came from Shanghai Naiou Nano technology.The mass ratio of Cu-Sn powder was 6:4 denoted as Cu-6, which was ball milled in a high energy ball mill (Pulverisette 7 premium line) at 1000 rad min −1 for 10 h, employing ethanol as the dispersant.Other ratios of alloy powders were prepared by the same method, Cu-7 and Cu-5 denoted as 7:3 and 5:5 mass ratios of Cu and Sn powder, respectively.The lower ratio of Sn resulted in a lower loading of active material at the supporting electrode.And the electrode volume energy density is not dominant, so the preparation of the alloy with too low Sn content is not carried out.

Preparation of 3DHPSnO 2 @Cu composite electrode
The Cu-6 alloy powder and polyacrylonitrile (PAN, MACKIN) were uniformly dispersed into the N-Methyl-2-pyrrolidone (NMP, Kermel) at a mass ratio of 20:1, and the composite electrode precursors were prepared by NIPS method.The composite electrode precursors were oxidized at 600 • C and reduced at 250 • C under hydrogen atmosphere.To further enhance the atomic diffusion, the composite electrode was sintered at 300 • C, and the self-supported electrode obtained was named 3DHPSnO 2 @Cu-6.The preparation process of 3DHPSnO 2 @Cu is illustrated in figure S1.Self-supported electrodes prepared by the same method for Cu-5 and Cu-7 alloy powders, named 3DHPSnO 2 @Cu-5 and 3DHPSnO 2 @Cu-7, respectively.

Material characterization
The structure and morphology of the composite electrodes were tested by field emission scanning electron microscopy (SEM, Hitachi S-4800).The qualitative analysis of the composite electrode elements was carried out using an energy-dispersive x-ray spectrometer (EDX).X-ray diffraction (XRD) test was used to analyze the crystal structure of the composite and alloy powder.Transmission electron microscope (TEM) was used to further analyze the lattice structure of the composites.Thermogravimetric analysis (TGA) was performed on TG 209 F3 Tarsus instrument from ambient temperature to 800 • C with a heating rate of 5 • C min −1 .X-ray photoelectron spectroscopy (XPS) was applied to explore chemical composition and valence states of elements.The pore size distribution measurement was performed using a Autosorb-iQ-C analyzer at 140 K.To investigate the lattice vibration modes of the samples, a laser Raman spectrometer (LabRam HR800) with blue Ar laser (wavelength 1 /4 488 nm) and excitation power of 15 mW was employed.

Electrochemical measurements
The self-supporting electrode was cut into a square with a side length of 0.5 cm as the working electrode and a pure lithium sheet as the counter electrode.A CR2032 button cell was assembled in a argon-filled glove box.Dissolve 1 mol l −1 of LiPF 6 in an organic solvent with volume ratio dimethyl carbonate: ethylene carbonate: ethyl methyl carbonate (DMC) = 1:1:1 as the electrolyte.The dosage of electrolyte in each half-cell was 0.1 ml.The polypropylene microporous film (Cellgard 2325) was used as the separator.Each group of experiments was run 6 cells to obtain repeatable results.Constant current charge/discharge tests were performed on a battery test system (Land BT 2001 A) at different current densities in the voltage range of 0.01-3 V. Cyclic voltammetry (CV) was used to test the reaction peak levels of the cells at specific scan rates using an electrochemical workstation (versatility VMP-300, France).Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10 mHz-100 kHz, and electrochemistry was used for testing after five cycles of operation.After disassembly of the recovered cells, the electrode sheets were soaked in DMC for 24 h to remove the electrolyte residue, then rinsed several times with alcohol and dried in a vacuum drying oven at 60 • C for 2 h.

Computational details
All DFT calculations were performed using the Vienna ab initio simulation package.The exchangecorrelation functional was described using the Perdew-Burke-Ernzerhof generalized gradient approximation method [42,43].All self-consistent calculations were performed with a plane-wave cut-off of 450 eV.The convergence accuracy of the self-consistent process was set as 10 −5 eV, and the geometrical optimization was stopped when the Hellmann-Feynman forces on the atoms were smaller than 0.03 eV Å −1 .A vacuum space of 15 Å was constructed to eliminate the interactions between adjacent layers.The 2 × 2 Cu 41 Sn 11 (8 2 2) surface cell, 2 × 2 Cu 5.6 Sn (1 1 1) surface cell with 2 × 2 × 2 k-mesh were used for modeling the adsorption behavior of O 2 .The adsorption energy is calculated by where E surf+O is the energy of the absorbed system, E O is the energy of oxygen, and E surf is the energy of the optimized bare surface.

Results and discussion
The physical phases of different alloy powders and 3DHPSnO 2 @Cu electrodes were analyzed by XRD.As show in figure 2(a), the Cu-5 diffraction peaks correspond to the standard Cu 41 Sn 11 (PDF#30-0510) phase.The Cu-7 diffraction peaks correspond to the standard Cu 5.6 Sn (PDF#30-0487) phase.The two main diffraction peaks of Cu-6 correspond to the diffraction peaks of Cu 41 Sn 11 and Cu 5.6 Sn, respectively, and the other peak positions also correspond, so Cu-6 is composed of two phase alloys.As shown in figures 2(b) and S2(a), the diffraction peaks of composite electrodes correspond to the standard Cu phase (PDF#04-0836) and the standard SnO 2 phase (PDF#41-1445).The diffraction peaks of electrode did not show any peak of Cu oxide, indicating that the in situ selective reduction process successfully reduced CuO alone, and finally the self-supporting 3DHPSnO 2 @Cu electrode was successfully fabricated.In order to investigate the changes in lattice structure of the electrode before and after cycling.The lattice parameter values (a and c), volume (V) and crystallite size of the 3DHPSnO 2 @Cu-6 were determined by Rietveld refining the diffraction pattern using an Fullprof software.The estimated lattice parameters are, a = b = 4.7381 Å, c = 3.1857 Å and the volume is 71.433 (Å) 3 .In conclusion, the active material loadings were calculated to be about 31% and 39%, corresponding to 3DHPSnO 2 @Cu-5 and 3DHPSnO 2 @Cu-7 composite electrodes, respectively.Meanwhile, as show in figure S2(b), the 3DHPSnO 2 @Cu-6 active material loading was calculated to be about 33% (8 mg cm −2 ) by TGA curve analysis.The element valence states of the 3DHPSnO 2 @Cu-6 composite were analyzed by XPS, as shown in figures 2(c)-(f).The results reveal that the 3DHPSnO 2 @Cu-6 composite is comprised of Sn, O, Cu.As show in figure 2(c), the high resolution of Sn 3d spectrum discloses that the relatively sharp peaks located at 486.4 and 494.8 eV are corresponded to Sn 3d 5/2 and Sn 3d 3/2 , respectively [44].The spin energy separation between Sn 3d 3/2 and Sn 3d 5/2 peaks is observed to be 8.4 eV, which is ascribed to the 3d binding energy of Sn 4+ in SnO 2 , without Sn 2+ metallic Sn.In addition, the high resolution of Cu 2p, as show in figure 2(e), there are fitting peaks located at 951.9 and 932.2 eV designate to the Cu 0 , which is in good agreement with the above XRD results.However, note that the conventional x-ray radiation sources (Al K α ) are not completely monochromatic but have small companion lines of slightly higher energy (K β , etc), thus resulting in the small companion peaks in the XPS spectrum in addition to the main peaks excited by K α .The O 1 s spectra show in figure 2(f), contain two peaks at 531.2 and 530.1 eV, which are correspond to the SnO 2 and surface adsorption of oxygen, respectively.
Raman scattering was conducted to investigate the lattice vibration modes in SnO 2 , as show in figure S2(c).Three Raman-active modes, A 1g mode at 605 cm −1 , B 2g at 712 cm −1 and E g at 477 cm −1 were detected.Besides, two IR-active modes, E u at 321 cm −1 and A 2u (LO) at 502 cm −1 , were also detected in the Raman spectrum [45].These Raman peaks were reported to be due to the nanoscale confinement phenomenon arisen from SnO 2 .The N 2 adsorption/desorption isotherms and the pore size distribution of 3DHPSnO 2 @Cu-6 are shown in figure S3.It is clear that 3DHPSnO 2 @Cu-6 exhibits isotherms of type IV according to the IUPAC classifications, with the pore size distribution dominates about 3.76 nm and the pore size range within 40 nm.
Further investigation of the morphological structure and phase distribution of alloy powder, precursors and composite electrodes by SEM, HRTEM and EDX.The Cu-6 alloy powder after high-energy ball milling is in the form of flakes as shown in figure S4(a).The composite electrode precursor is excellent in flexibility prepared by NIPS and can be bent 180 • , as shown in figure S4(b).From the cross section view, it is observed that the Cu-6 is uniformly distributed in the precursor as shown in figure S4(c).Self-supported electrodes were successfully fabricated by heat treatment with uniform thickness of about 50 µm, as shown in figure S4(d), can be bent over 90 • without cracking, as shown in figure 3(a).The 3DHPSnO 2 @Cu-6 electrode consists of three-dimensional hierarchical porous shell layer encapsulating three-dimensional porous core structure, and forms 3D continuous homogeneous porous skeleton, as shown in figures 3(a) and (b).Meanwhile, the characterization of the element distribution shows that the external shell layer is mainly Cu, and the internal core is uniformly distributed with SnO 2 and Cu, as shown in figure 3(c).The surface shell layer of the self-supported electrode is smooth and distributed with porous structures of uniform pore size (lower 50 nm), as show in figure S4(e).The internal core structure of the self-supported electrode is a homogeneous porous structure (lower 80 nm), as shown in figure S4(f).The structure of the internal core is further investigated, as shown in figures 3(d) and S5.Specifically, lattice fringe spacing is 0.33 nm, corresponding to the (110) crystal plane of SnO 2 .The lattice fringe spacing is 0.18 nm in the middle of the two SnO 2 , corresponding to the (200) crystal plane of Cu.It is obvious that Cu doping exists SnO 2 , forming a SnO 2 -Cu-SnO 2 highly conductive interface.Simultaneously, the internal pore size is about 3 nm, which corresponds to the results of the pore size distribution.To further understand the microstructure of the electrodes prepared from different alloy powders, as shown in figure S6, the 3D hierarchical porous shell structure was also formed from 3DHPSnO 2 @Cu-5 and 3DHPSnO 2 @Cu-7 electrodes, but the internal core with 3D porous structure was not formed.
Further to investigate the binding energy of different alloy phases with oxygen, the adsorption properties of Cu 41 Sn 11 and Cu 5.6 Sn were calculated using the DFT calculation.According to the calculated results, the adsorption energy of oxygen on the Cu 5.6 Sn (1 1 1) crystal plane is −1.17 eV, which is significantly higher than the adsorption energy of −0.91 eV on Cu 41 Sn 11 (8 2 2), as shown in figures 4(a) and S7.To further confirm the calculation results, we investigate the internal structure of 3DHPSnO 2 @Cu-6 before reduction, as shown in figure 4(b).It is observed that it has formed a core-shell structure with a three-dimensional porous core.The elemental distribution mapping shows that the interior is mainly SnO 2 and the exterior is mainly CuO.As we predicted, the three-dimensional porous core-shell structure is constructed due to the difference in the order of oxidation based on the principle of different binding energy of the alloy to oxygen.Further selective reduction, the CuO was reduced to Cu forming a porous structure on the surface in the shell layer, as shown in figure S4(e).Finally, a self-supported electrode was constructed with porous SnO 2 core encapsulated in three-dimensional hierarchical porous Cu shell layer.
To investigate the electrochemical performance of 3DHPSnO 2 @Cu, we assembled the electrodes into cells and tested them.The CV curves were recorded at a scan rate of 0.1 mV s −1 and at voltages from 0.01 V to 3 V (vs Li/Li + ), as show in figure 5(a).In the cathode scan, the first reduction peak observed near 1.28 V are attributed to the reaction of Sn 4+ to Sn 2+ (SnO 2 + 2Li + + 2e ↔ SnO + Li 2 O).The second reduction peak observed near 0.86 V are attributed to the reaction of Sn 2+ to Sn 0 (SnO + 2Li + + 2e ↔ Sn + Li 2 O).The distinct reduction peaks observed below 0.56 V are due to the alloying reaction of Sn to LixSn (Sn + xLi + + xe ↔ Li x Sn(0 < x < 4.4)).In the next anodic scan, the three reaction peaks correspond to the above reversible reactions described above, respectively.The first peak appearing near 0.6 V corresponds to the dealloying reaction.The subsequent peaks appeared near 1.33 and 1.9 V correspond to Sn and SnO oxidation reactions, respectively.Comparing the CV curves of 3DHPSnO 2 @Cu electrodes prepared from different alloys, it is observed that the 3DHPSnO 2 @Cu-6 electrode has a low oxidation potential and a maximum reduction potential, along with a large current response.Meanwhile, two alloying reactions were found near the low potentials 0.66 and 0.22 V.It is attributed to the fact that 3DHPSnO 2 @Cu-6 possesses a highly conductive SnO 2 -Cu-SnO 2 interface and a uniform internal and external porous structure, which reduced the redox energy barrier and promoted alloying.
Further investigation of the electrochemical properties reveals that the composite electrode produces a large irreversible capacity during the first discharge, while the reversible capacity reached 1080 mAh g −1 as shown in figure 5(b).It is attributed to the fact that the first discharge process produces a solid electrolyte interphase with a voltage plateau of around 0.75 V.The reaction platform at the same potential was not found during the subsequent cycling, and the SEI was seen to be stable.This is mainly attributed to the homogeneous three-dimensional hierarchical porous shell layer structure that can buffer the volume expansion of SnO 2 without exposing new SEI reaction sites.The charge/discharge response curves of the composite electrode overlap very well, exhibiting a very stable lithium embedding/delithiation process.As shown in figure 5(c), the electrochemical performance of 3DHPSnO 2 @Cu-6 was found to be the most outstanding at the same rates.The reversible discharge capacities were 998, 848, 799, 774, 634 and 625 mAh g −1 at rates of 100, 200, 400, 600, 800 and 1000 mA g −1 , respectively.The reversible capacity retention rate higher than 60% at high currents, and the reversible specific capacity can be maintained up to 845 mAh g −1 when the current density is restored to low currents density.At the same time, the Coulombic efficiency maintained 99.5% at different current densities.It is attributed to the three-dimensional hierarchical porous shell layer structure that provides an effective spatial domain limitation and buffering effect for the volume expansion of SnO 2 , as well as providing sufficient out Li + sites.The long-cycle performance of the composite electrode was further investigated as shown in figures 5(e) and S8.3DHPSnO 2 @Cu-6 exhibits excellent long cycle performance with reversible specific capacity up to 4.35 mAh cm −2 at current density of 1.4 mA cm −2 , area specific capacity up to 3.13 mAh cm −2 after 300 cycles.Meanwhile, initial reversible specific capacity up to 1080 mAh g −1 with the initial Coulombic efficiency of 64.3%, retention specific capacity up to 849.5 mAh g −1 with the Coulombic efficiency of 99.5% at current density of 0.1 A g −1 after 300 cycles.3DHPSnO 2 @Cu-6 has a relatively high performance compared to recent research related to lithium storage in SnO 2 anode.It is worth emphasizing that its loading can be 8 mg cm −2 while maintaining a high stability of cyclability, as show in table S1.The high energy density of the electrodes is attributed to the three-dimensional porous core structure that provides an abundance of reactive sites for the active material.The long cycle life is attributed to the three-dimensional hierarchical porous shell and core porous structure that provides a two-way mechanical strain buffer for the volume expansion of SnO 2 .The rapid capacity decay of 3DHPSnO 2 @Cu-5 and 3DHPSnO 2 @Cu-7 after several cycles, attributed to the absence of the porous core structure.The porous Cu shell alone could not provide sufficient buffer space for SnO 2 , leading to the rapid capacity decay.
The excellent electrochemical properties of 3DHPSnO 2 @Cu-6 are further interpreted by AC impedance.Nyquist plots show semicircles in the high and mid-frequency regions and long straight lines in the low-frequency region, as shown in figure 5(d), corresponding to the charge transfer and diffusion processes.Detailed analysis of the EIS data using an equivalent circuit, where the parameter R s represents the resistance of the electrolyte and electrical components.R SEI indicates the resistance of Na + through the SEI layer, and R ct indicates the charge transfer resistance.C PE1 and C PE2 are the constant phase elements corresponding to the surface film and capacitance.It is obvious that the 3DHPSnO 2 @Cu electrode R s resistance is extremely small compared to the previous self-supporting electrode.3DHPSnO 2 @Cu-6 electrode has a minimum resistance of 8.2 Ω.It is attributed to the highly conductive interface SnO 2 -Cu-SnO 2 and the highly conductive Cu shell layer, which establishes a complete continuous three-dimensional conductive network.The R ct semicircle diameter of 3DHPSnO 2 @Cu-6 is much smaller than the other two electrodes, and the measured resistance is 32.4 Ω. Indicates that its charge transfer resistance is low, which is attributed to the high specific surface area provided by the three-dimensional porous core for the electrode greatly enhances the reactive sites and improves the charge transfer kinetics.The slope in the low frequency region is the Warburg resistance (Z w ), which is inversely proportional to the diffusion coefficient [46].As show in figure S9, the linear slope of Z w for 3DHPSnO 2 @Cu-6 is significantly lower than the other two electrodes, indicating that its Li + diffusion resistance is the smallest.It is attributed to the synergistic effect of the 3D hierarchical porous shell structure and the porous core structure, which provides an effective path for Li + diffusion.
To further investigate the failure mode of the composite electrodes, the 3DHPSnO 2 @Cu-6 lattice parameter values (a) and (c), volume (V) and crystallite size of the 3DHPSnO 2 @Cu-6 after cycling were determined by Rietveld refining the diffraction pattern using an Fullprof software, as show in figure 6  of the XRD patterns before cycling shows that the lattice structure of SnO 2 is basically unchanged, indicating that the conversion reaction from SnO 2 to Sn is highly reversible during the lithiation/delithiation process.The element valence states of the 3DHPSnO 2 @Cu-6 after cycling were analyzed by XPS, as shown in figure 6(b).The high resolution of Sn 3d spectrum discloses that the relatively sharp peaks located at 486.6 and 495.8 eV are corresponded to Sn 3d 5/2 and Sn 3d 3/2 , respectively.The spin energy separation between Sn 3d 3/2 and Sn 3d 5/2 peaks is observed to be 8.4 eV, which is ascribed to the 3d binding energy of Sn 4+ in SnO 2 .Meanwhile, two small peaks at 483.4 and 492.3 eV correspond to Sn 3d5 /2 and Sn 3d 3/2 , attributed to rarely SnO failing to reduce completely to Sn.The self-supporting electrode structure was found to be stable without structural slumping or pulverization, but a uniform and consistent crack appeared at the edge of the shell structure, after 100 cycles, as show in figures 6(c)-(e).It is possible that the alloy powder is flake by high energy ball milling, and its morphology was not changed in the subsequent process, and the electrode eventually formed an elliptical shell structure along such morphology.As we all know, the elliptical edges are prone to stress concentration and cracking when the volume of SnO 2 expands.In detail, as shown in figure 6(f), the elliptical porous shell layer edge first appeared cracks due to SnO 2 volume expansion in the discharge process.However, due to the excellent flexibility and buffering ability of Cu, the crack only becomes slightly larger and tends to stabilize after multiple charging and discharging, and the electrode still maintains its original structure to achieve a stable cycle of the self-supporting electrode.

Conclusion
In summary, based on the principle that different Cu-Sn alloys have different binding energies with oxygen, we propose a new strategy combining high-energy ball milling, NIPS and heat treatment processes to successfully fabricate self-supporting 3DHPSnO 2 @Cu electrodes with porous SnO 2 encapsulated in a three-dimensional continuous hierarchical porous copper shell.This unique structure was constructed due to the different binding energy of the alloy with oxygen, which are −0.91 eV for Cu 41 Sn 11 and −1.17 eV for Cu 5.6 Sn according to DFT calculation.Furthermore, the reversible area specific capacity reaches 4.35 mAh cm −2 at a current density of 1.4 mA cm −2 , with a capacity retention rate of nearly 75% after 300 cycles.The specific capacity retention reaches more than 60% when the current density is slowly expanded from 0.1 to 1 A g −1 .The excellent structure exhibited is largely attributed to the special core-shell structure.The high conductivity interface of the core SnO 2 -Cu-SnO 2 reduces the problem of poor SnO 2 conductivity, meanwhile, the three-dimensional hierarchical porous core-shell structure not only provides an effective channel for Li + ion diffusion, but also provides a buffer space and spatial restriction for the volume expansion and powdering of SnO 2 .Therefore, this article suggests a strategy combining energy directed control of alloy distribution, which provides a new theory for the design of next generation self-supporting electrodes.

Figure 1 .
Figure 1.Schematic diagram of oxidation process of different Cu-Sn alloys, where Cu-Sn-S and Cu-Sn-W represent Cu-Sn alloys with strong and weak oxygen binding energies, respectively.

Figure 4 .
Figure 4. (a) Adsorption energy of different alloys with oxygen, (c) SEM and EDX elemental mapping of 3DHPSnO2@Cu-6 before reduction.

Figure 5 .
Figure 5. Electrochemical performance curves of composite electrodes at different alloys.(a) CV curves, (c) cyclic charge/ discharge curve of 3DHPSnO2@Cu-6, (b) rate curve of self-supporting electrodes, (d) Nyquist plots of self-supporting electrodes, (e) long cycle curve of self-supporting electrodes.