Yolk-Shell Cu2O@CuO-decorated RGO for High-Performance Lithium-Ion Battery Anode

Fangzhao Pu,Yilu Bai,Jian Lv,Xintian Zhao,Guanchen Wu,Chuncai Kong*,Bosen Lei,Xiaojing Zhang,Hong Jin*,and Zhimao Yang*

Based on the great advantages of an inner hollow structure and excellent solid counterpart capacity,complex hierarchical structures have been widely used as electrodes for lithium-ion batteries.Herein,hierarchical yolk-shell Cu2O@CuO-decorated RGO(YSRs)was designed and synthesized via a multistep approach.Octahedron-like Cu2O-decorated RGO was firstly produced,in which GO was reduced slightly while cuprous oxide was synthesized.Subsequently,the controlled oxidation of Cu2O@RGO led to the synthesis of special YSRs,which were composed of a solid Cu2O core,spur-CuO,CuO shell,and RGO covered.As anode materials,YSRs could provide considerable capacity density.Meanwhile,the void existed between shells and solid active materials retaining the advantages of inner hollow structure.As a result,the unique architecture of the materials renders the composites with enhanced electronic and ionic diffusion kinetics,high specific capacity(～894 mAh g-1,0.1C),and an excellent rate capability.

Keywords

copper oxide,Cu2O,Li-ion battery,nanocomposites,yolk-shell

1.Introduction

With increasing requirements for portable electronics,electric vehicles,and consumer electronics,the development of high energy density,rechargeable batteries are expected.[1]In the meantime,the lithium-ion batteries(LIBs)have extensive applications in smart electrode grids and transportation facilities for decades due to their high energy density and long life span.[2-4]The need for superior LIB technology with high energy density and high power density is becoming essential.To date,the commercial anode used in LIBs is graphite,which has a low theoretical capacity of 372 mAh g-1and a low Li-ion transport rate.The development of electrode materials with high energy storage potential has attracted more attention.[5-7]

Since the key work of Tsrascon and colleagues,transition metal oxides(TMO)have attracted attention due to their high theoretical capabilities.The Li storage mechanism of based materials is “conversion mechanism.”[3]Among the TMOs,cuprous oxide(Cu2O)has attracted particular attention because of its nontoxic property,abundant in nature,and excellent electrochemical performance.However,compared with other TMOs electrode materials,the Cu2O electrode materials have a lower theoretical capacity(375 mAh g-1).To solve this problem,combining high electrochemical activity of Cu2O with high theoretical capacity materials has been noticed and studied for years.[8]For example,Sun et al.synthesized Cu2O/CuO/RGO composites using a single-step solvothermal method,and the anode exhibited remarkable cycling and high-rate performances.[9]Zeng et al.fabricated hierarchical CuO/MnO2composite hollow nanospheres via a water evaporation-induced method,and it exhibited high capacity and good rate performance.[10]However,similar to other TMO-based electrode materials,three key defects need to be solved to improve cycling capabilities are the poor electronic conductivity,limited anode conversion efficiency,and significant volume change during charge/discharge,respectively.[11]In particular,the volume variation causes the pulverization and aggregation of active materials and forms an unstable solid-electrolyte interphase(SEI)leading to fast capacity fading.[12,13]

To overcome these problems,incorporating TMOs into carbonaceous materials,including hollow/porous structures,has been reported and attracted considerable attentions.Because of the flexible and tough characteristics,carbon-containing materials can build an elastic buffer to avoid volume variation and keep the integrity of the electrode.At the same time,carbon-containing materials can provide a considerable electrical conductivity.[7,14,15]Moreover,the hollow-like structure would sustain a larger space for lithium insertion/extraction than solid materials.For instance,Hu et al.designed novel dendrite-shaped CuO hollow micro/nanostructures and used them to fabricate excellent hollow electrode materials for LIBs.[16]Zhou et al.synthesized a hollow CuOCu2O/graphene composite by microwave-assisted process and subsequent annealing to obtain excellent electrode materials with remarkable rate capability,reversible capacity,and cycling life.[17]However,the fabricated electrodes with hollow/porous structure are typically limited by relatively lower volumetric energy and power densities compared with parallel solid active materials.[18]Therefore,for practical applications,the structures of the electrode need to be properly designed.

The complex hollow and hierarchical structures would take advantage of their interaction for better electrochemical performance and to make better utilization of inner hollow cavity.[18-20]The complex hollow structures,such as yolk-shell and multi-shell structure,have been prepared and demonstrated to shorten the Li+transfer length,provide more lithium/dilithium positions,have greater volume expansion space,and also overcome the poor low volumetric energy and power densities of hollow structures.[21,22]Nevertheless,the process for synthesizing complex hollow structures is usually complicated and challenging and should be designed appropriately with shell structure to obtain promising electrochemical performance.In particular,for Cu2O-based electrode materials,only a few hierarchical structures without any inner hollow structure have been designed and optimized for use in LIBs to date.Meanwhile,research that is based on Cu2O anode materials has only considered simple hybrid CuO/Cu2O nanoparticles with carbon,and there is almost no overall structure design of the composite material to maximize the material’s electrochemical characteristic superiority.[23,24]Under the circumstances,it is necessary to explore the effectiveness of the hierarchical structure Cu2O-based electrode materials with inner hollow structure on improving LIBs performance.This could be obtained with both the desired capacity of solid counterparts and structure advantages of inner hollow structure.

Herein,we developed a facile approach for fabricating hierarchical yolk-shell Cu2O@CuO-decorated RGO(YSRs).The hierarchical anode material was fabricated at room temperature,and the main structure was assembled as:Cu2O core-spur-CuO bridge-CuO shell-void-RGO.The novel synthetic 3D nanostructures possess several advantages as LIB anodes.The as-prepared hierarchical anode materials have all of the advantages of hollow sphere,including the cavity to accommodate the volume expansion during lithium insertion and shorten the Li+transfer length.At the same time,the structures inherit the high capacity of the solid counterpart,including the capacity of the Cu2O yolk,and have a reinforced capacity because of the spur-CuO bridge and CuO shell.Moreover,RGO flakes provide the excellent electrical conductivity,and the 3D hierarchical network insures a high specific surface area for excellent anode performance.Hence,the anode delivers a record capacity of 854 mAh g-1at 0.1C after 200 cycles,which shows excellent cycling and rate performances.

2.Results and Discussion

2.1.Structure and Morphological Features

The synthetic process is shown in Figure 1a.Copper chloride dehydrate and commercial GO were sonicated to produce a homogenous suspension in deionized water so that Cu2+can be uniformly absorbed on the surface of GO layers.Then,Cu2O@RGO was synthesized via a brief and environmentally friendly reduction route in which octahedron-like Cu2O was reduced by glucose,which is a mild reductant.Meanwhile,GO was reduced slightly,which means the RGO in Cu2O@RGO was in an incomplete reduction state.The incompletely reduced RGO is homogenously encapsulated on the surface of octahedron-like Cu2O.Using the as-prepared structure,Cu2O@RGO was then oxidized by H2O2,which is a mild oxidant;[25,26]the oxidant converted the octahedron-like Cu2O into an irregular core at the center of the entire composite.After mild oxidation,an ultrathin CuO shell formed,linking the irregular Cu2O core via spur-like CuO with abundant void.The formation of the CuO shell with voids is attributed to the Kirkendall effect,and it is associated with the different diffusion rates of atoms moving in and out of the sphere.[27,28]As a result,the slow oxidation process leads to the CuO shell and RGO flakes homogeneously encapsulating the Cu2O core with two inner hollow areas between the CuO shell and Cu2O yolk and between the CuO shell and RGO flakes.

Figure 1.a)Schematic illustration of the synthesis of Cu2O@CuO@RGO YSRs,inset small graph is the XRD pattern of Cu2O@CuO@RGO YSRs range from 32°-42°;b)XRD patterns of Cu2O@RGO and Cu2O@CuO@RGOYSRs;c)Raman spectra of Cu2O@RGO,Cu2O@CuO@RGO YSRs,and GO;d)FTIR spectra of Cu2O@CuO@RGO YSRs,Cu2O@RGO,and GO.

The Cu2O@RGO composite structure was examined by X-ray diffraction.The observed peaks in the diffractogram(Figure 1b)correspond to the characteristic reflections of Cu2O and can be indexed to the cubic phase,according to JCPDS data(PDF#74-1230)for cuprite.The obvious sharp peaks indicate good crystallinity of the Cu2O core of Cu2O@RGO.The calculated lattice parameters are marched the(110),(111),(200),(211),(220),and(221)peaks of cuprite.Compared with Cu2O@RGO,the diffractogram for the CuO shell that formed in YSRs shows two peaks at 2-theta values of 35.4 °and 38.6 °,and these can be assigned to the(-111)and(111)planes of CuO(PDF#80-1916).However,the residual oxygen functional groups(carboxylic,hydroxyl,and epoxy groups)in the incompletely reduced RGO hindered crystallization and growth of the CuO shell,and this resulted in inferior crystallinity of the CuO shell.

Carbon-containing materials were generally characterized using Raman spectroscopy.Raman spectra of GO,Cu2O@RGO,and yolkshell Cu2O@CuO-decorated RGO(YSRs)are shown in Figure 1c,d.Two noticeable peaks located at 1324 cm-1and 1598 cm-1of Cu2O@RGO and YSRs are assigned to G-bond and D-bond,respectively.[29,30]The G-bond reflects the stretching of sp2-hybridized carbon atoms,which is corresponding with the symmetric characteristics and crystalline nature of RGO.The D-bond represents sp3vibrational defects and the randomness of graphitic carbon defects.[31]Therefore,the intensity ratio of D-to G-band(ID/IG)is a good measure for characterizing the defects and disorder in carbon-based materials.[32]Compared with the ID/IGvalue of 1.43 for GO,the Cu2O@RGO obviously had less defects and a higher amount of sp2domains,which means that there was a higher degree of graphitization,corresponding to the lower ID/IGof 1.12.This is a result of the reduction of GO and the defects reduced by intimate interaction between Cu2O and RGO.Moreover,the ID/IGof YSRs is a little bit higher than that of Cu2O@RGO,which indicates that the defects increased with the formation of the CuO shell and resulted in the destruction of the interaction between the Cu2O core and RGO.Meanwhile,the D-band and G-band shifted to a lower frequency region than those of GO(located at 1334 cm-1and 1607 cm-1,respectively).The trend is toward the graphitic characteristic,confirming the reduction of GO to RGO.Moreover,the Raman spectra range from 100-800 cm-1was detected to exclude the effect of RGO,as shown in Figure S2a.The peaks located at 149,199,and 220 cm-1are attributed to the Cu2O component in Cu2O@RGO,while the two peaks of 286 and 293 cm-1represent the typical CuO in YSRs.[33,34]

Figure 1d shows the FTIR spectra of YSRs,Cu2O@RGO,and GO.In GO,numerous oxygen-containing groups are present,such as the presence of a broad absorption peak centered at 3368 cm-1reflects the stretching vibration of hydroxyl groups.The typical peaks at 1728,1404,and 1050 cm-1are attributed to the C-O stretching vibration,OH bending mode,and C-O-C stretching vibration,respectively.The peak centered at 1624 cm-1is related to the vibration of water molecules adsorbed on GO.[35,36]After the formation of the Cu2O@RGO hybrid,the intensity of the absorption peaks at 3368,1728,1404,and 1050 cm-1decreased or disappeared,indicating that GO was reduced to RGO.However,the sharp peak at 631 cm-1can be assigned to the Cu-O vibration for Cu2O@RGO composites,suggesting Cu-O bonds are perturbed as RGO interacts with Cu2O.[37]The skeletal vibration of C=C of the RGO sheets,positioned at 1633 cm-1,[36]proves the efficiency of GO reduction.For the YSRs spectra,new bands appeared at around 1448 and 1379 cm-1in addition to the C=C vibration bond and can be attributed to O-H bending vibration,indicating the occurrence of H2O2oxidation process residual OH-.[38,39]Meanwhile,characteristic strong peaks at 504 cm-1are associated with the Cu-O vibrations of CuO while the 626 cm-1bond is associated with Cu-O vibrations of Cu2O.[40]Therefore,the FTIR studies clearly indicate the reduction of GO to RGO during the forming of YSR composites.

Morphologies of Cu2O@RGO and YSRs were studied by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).As shown in Figure 2a-b,the Cu2O@RGO has an octahedronlike feature and is homogeneously embedded in a large sheet of RGO,whereas RGO uniformly enveloped a single Cu2O particle.Compared with Cu2O@RGO,the single YSRs particle has a hierarchical complex hollow structure,as shown in Figure 2d,and the irregular Cu2O yolk was linked to an ultrathin CuO shell via spur-like CuO,which can improve the specific capacity of the Cu2O yolk via mild oxidation to obtain slight CuO.The RGO cover enveloped all of the active electrochemical materials and built a void between CuO and RGO.Also,Figure 2c shows that some CuO fragments were dropped onto RGO flakes after oxidation because of the formation of voids.Figure 2e and f shows HRTEM images for a white array marked in Figure 2d for YSRs.Figure 2e shows the edge of the Cu2O yolk.The distinct lattice spacing of 0.24 between two adjacent planes matches well with the(111)lattice plane of Cu2O.Figure 2f shows the edge of the whole YSRs.The dspacing(0.25 nm)observed in the HRTEM images is consistent with the(-111)lattice plane of CuO,and the edge of the RGO layer can be clearly seen.And as shown in Figure S2b-c,the lattice spacing of 0.21 nm and FFT pattern all matched very well with the(111)plane of spur-like CuO for Cu2O@-CuO@RGO.The assignment is confirmed from the XRD pattern of YSRs.Figure 2g shows energy-dispersive spectrometry mapping images of a single YSR particle,which demonstrate the presence of C,Cu,and O elements.Also,CuO is observed to be uniformly located in the graphene layer.Meanwhile,both HAADF and the element images indicate the outline of the CuO shell,the void between the Cu2O yolk and CuO shell,and the void between the CuO shell and RGO cover.Linear scan analysis shows that the distribution of C is contrary to that of Cu and O(Figure S1),indicating RGO coating on the whole particle.Moreover,the atom ratio of Cu and O in the solid center is nearly 2:1,corresponding the Cu2O yolk located at the center of the particle.The linear correspondence to the shell nearest to the Cu2O yolk shows a Cu:O ratio that is nearly 1:1,indicating that the shell is formed by CuO.The observations demonstrate the formation of yolk-shell Cu2O@CuO-decorated RGO structure.

Figure 2.a,b)low-magnification FESEM images of Cu2O@RGO;c)low-magnification FESEM images of Cu2O@CuO@RGO YSRs;d)low-magnification TEM image of an individual Cu2O@CuO@RGO YSRs;e,f)HRTEM images of Cu2O@CuO@RGO YSRs;g)corresponding EDS mapping images of Cu2O@CuO@RGO YSRs.

The XPS spectra are a significant method to investigate the chemical composition and electronic structure of prepared materials.[41,42]The survey spectra(Figure 3a)of YSRs confirm the presence of Cu and O peaks for Cu2O and CuO,and the C and O peaks are confirmed from RGO.Meanwhile,the spectrum of Cu2O@RGO is dominated by peaks associated with Cu and O from Cu2O.Further analysis of the high-resolution O 1s spectrum(Figure 3b)shows the corresponding peaks that arise from C=O,C-O,and O-C=O for RGO.[43]The characteristic peak that arises from Cu-O is for Cu2O decorated on RGO for Cu2O@RGO,whereas the Cu-O is for the CuO shell,and Cu2O yolk is for YSRs.As a result,the Cu-O peaks of YSRs are weaker than those of Cu2O@RGO,due to the void between RGO and the CuO shell,but Cu2O was decorated on the RGO surface for Cu2O@RGO.As shown in Figure 3c,the Cu2O@RGO and yolk-shell Cu2O@CuO-decorated RGO structure can obviously be distinguished by the high-resolution Cu 2p spectra,in which the spectra for YSRs display an obvious characteristic feature of Cu2+.One is spinorbital doublets that are located at 933 and 953 eV corresponding to Cu 2p3/2and Cu 2p1/2.The second is the two pronounced shake-up satellite(938-945 eV,960-965 eV)that is observed.Meanwhile,the characteristic Cu+peaks located at 934eV and 954 eV are detected in the Cu 2p spectra for both the Cu2O@RGO and YSRs composites,whereas the Cu 2p spectrum of Cu2O@RGO does not have the satellite.These results suggest that both Cu2O and CuO are present in the YSRs sample,which is consistent with the XRD pattern.[44]However,XPS is a surface-sensitive technique;CuO was observed to have a stronger signal in XPS than in XRD.Therefore,the presence of the CuO shell on the surface region was proven.As shown in Figure 3d,there are four different peaks that correspond to C=C,C-O,C=O,and C-C of RGO for both Cu2O@RGO and YSRs.[23]In contrast,after oxidization of Cu2O,the intensity of the C-O peak decreased dramatically,revealing that the CuO shell formation cut off the O of Cu2O from RGO.To quantify the pore size and surface area,BET analysis was performed,and the results are shown in Figure 3e.Typical nitrogen adsorptiondesorption isotherms for the Cu2O@RGO and YSRs powder samples were recorded.The observed hysteresis loop of YSRs in the relative pressure range of 0.4-1.0 represents a type IV isotherm,and this indicates a mesoporous structure,while the Cu2O@RGO shows a type Ⅱ isotherm.[45]The BET-specific surface area of YSRs was calculated to be 108 m2g-1,which is approximately 15 times greater than that of Cu2O@RGO(18 m2g-1).Obviously,the CuO shell and spur-like CuO formation increased the specific surface area.Pore-size-distribution histograms(Figure 3f)show that the YSRs samples had smaller pores(in the range of 2-4 nm)than the Cu2O@RGO samples(in the range of 10-12 nm).Thus,the 10-12 nm pores in Cu2O@RGO are attributed to the aggregation of nanoparticles.Also,the oxidation of Cu2O and formation of CuO decreased the aggregation of Cu2O particles,but the thin structure of CuO created small pores that were in the range 2-4 nm.

Figure 3.a)XPS survey spectra,b)O 1s,c)Cu 2p,and d)C 1s XPS spectra of Cu2O@RGO and Cu2O@CuO@RGO YSRs;e)BET surface area and f)corresponding pore-size distribution of Cu2O@RGO and Cu2O@CuO@RGO YSRs.

2.2.Electrochemical Performances

Figure 4 a-b shows the cyclic voltammetry(CV)of the Cu2O@RGO and YSRs composite electrodes in the voltage cutoff window of 0.01-3 V at a scanning rate of 0.1 mV s-1.For Cu2O@RGO,there are two reduction peaks that are observed at 0.5 and 0.82 V in the first cathodic scan.The peak at 0.82 V corresponds to Cu2O reduced to the intermediate phase of CuⅡ1-xCuⅠ1-x/2and the composition of Li2O.The peak at 0.5 V corresponds to the reduction of CuⅡ1-xCuⅠ1-x/2to metallic Cu.[9]Meanwhile,for the first anodic scan,the shoulder peak shown at the 2.24-2.8 V corresponds to re-oxidation of the Cu metal phase to Cu2O and decomposition of Li2O.The small peak located at 1.9 V is a result of the deformation of the SEI.[46]In subsequent scans,the cathode peaks are observed to shift to more positive potentials.This phenomenon is a common characteristic for the structure changing of metal oxide during the charge/discharge progress,and the decreasing of individual peak intensity and integral area indicating the losses of capacity and the formation of solid-electrolyte interphase(SEI)during the first cycle.The CV of YSRs is analogous to that of Cu2O@RGO.Three main cathode peaks are observed at 0.39,0.8,and 1.34 V.The peak at 1.34 V is correlated to Cu2O transfer to a solid-solution phase.The peak at 0.8 V is correlated to the Cu2O reduction to Cu,and the peak at 0.39 V corresponds to SEI formation.The anodic scans observed at 1.9 V result from decomposing of SEI,and 2.24-2.8 V indicates Cu returning to Cu2O,respectively.[47]In addition,the polarization during electrode reaction leads to the oxidation peaks shifts to high voltage significantly during the subsequent cycles.Furthermore,except for the first CV curve,the CV curves overlap well with each other,and this indicates that the YSR composites possess good reversible cycling performance.Figure 4c shows the CV of YSRs after 200 charge/discharge cycles at 0.1C.All of the peaks are corresponding with the CV curves of YSRs and overlapped with each other,which demonstrates the YSRs retains good cycle performance after even 200 cycles.

Figure 4.a)CVs of Cu2O@RGO at a scan rate of 0.1 mV s-1in a voltage range of 0.01-3 V;b)CVs of Cu2O@CuO@RGO YSRs at a scan rate of 0.1 mV s-1 in a voltage range of 0.01-3V;c)CVs of Cu2O@CuO@RGO YSRs after 200 cycles;d)galvanostatic charge/discharge profiles of Cu2O@RGO;e)galvanostatic charge/discharge profiles of Cu2O@CuO@RGO YSRs.f)rate capability of Cu2O/CuO/RGO;g)cycling performances of Cu2O@RGO electrodes at 0.1 C;h)cycling performances of Cu2O@CuO@RGO YSRs electrodes at different current densities;i)AC impedance spectra of Cu2O@RGO and Cu2O@CuO@RGO YSRs.

The galvanostatic discharge-charge curves of the Cu2O@RGO composites between 0.01 and 3.0 V at 0.1C are shown in Figure 4d.There are two potential ranges at～0.88 V and～0.52 V in the first discharge curve,which is caused by the irreversible formation of SEI layer and Li+insertion reaction into Cu2O.In subsequent discharge curves,two short plateaus are observed at 1.5-1.6 V and 0.7-0.8 V corresponding to the insertion Li+reaction into Cu2O,which is consistent with the cathodic peaks in the CV curves.During the charge process,the main reaction in potential range at～1.8 V in all cycles is due to the recombination of Cu and Li2O turning into Cu2O and Li+.The galvanostatic of YSRs shows a similar trend,as shown in Figure 4e.The specific capacity of YSRs is larger than that of Cu2O/RGO composites in all cycles.For YSRs,at rates of 0.1C,0.3C,0.5C,1C,and 3C,the specific capacities are 516 mAh g-1,280 mAh g-1,189 mAh g-1,143 mAh g-1,and 136 mAh g-1,respectively.For Cu2O@RGO,the specific capacities are 282 mAh g-1,182 mAh g-1,145 mAh g-1,115 mAh g-1,and 106 mAh g-1,respectively.

Figure 4g shows the cycling performances of the Cu2O@RGO composites at 0.1C.The sample delivers a high initial lithium storage capacity of about 622 mAh g-1.After a trend of capacity loss of about 40 cycles,the capacity increases to 400 mAh g-1at 200 cycles and remains at a high capacity for the next 100 cycles.The mild irreversible capacity loss is ascribed to the incomplete formation of an SEI film on the surface of the electrode,and this is confirmed by the CV curves.The cycling performance of YSRs is shown in Figure 4h.Compared with Cu2O@RGO,Cu2O@CuO@RGO YSRs electrode delivers an initial discharge capacity of 892 mAh g-1,and the discharge capacity is maintained at 849 mAh g-1after 200 cycles with a high coulombic efficiency of about 100% .The high capacity retention benefits from the unique structure of YSRs.On the one hand,the hierarchical two voids of YSRs can buffer the volume expansion during cycling.As shown in Figure S3a-b,the TEM images of the YSRs electrode confirm that the unique structure is highly stable even after 200 charge/discharge cycles.On the other hand,the CuO shell and spur-like CuO with high theoretical capacity can promote the specific capacity of YSRs and provide more lithiation sites.In addition,the RGO can further enhance the electrical conductivity for charge/discharge progress.However,it shows a relatively higher capacity fading than Cu2O@RGO during the first few cycles.Then,it increases to an excellent high value with a good capacity retention after a drop progress.This phenomenon is partially attributed to the formation of the SEI film in the initial few cycles,and the main reason is the unique structure of YSRs,which results in the electrolyte slowly acted the role after transporting through the two voids of Cu2O core-void-CuO shell-void-RGO structure.To investigate the rate performance and cycling stability of the YSRs electrode,the battery was tested at 0.1C,0.3C,0.5C,and 1C.At a current density of 0.1C,a high initial capacity of 892 mAh g-1is obtained.The capacity drops to 400 mAh g-1for a few cycles because of the formation of the SEI,and then,it gently increased to 849 mAh g-1after 200 cycles.The results show that the electrode presented a high initial capacity of 1098 mAh g-1even at the current density of 0.3C,and it maintained the high capacity(808 mAh g-1)after 200 cycles.Similar electrochemical performance results were verified at 0.5C and 1C,and the capacity remained 576 mAh g-1at a high current density of 0.5C.Furthermore,we synthesized CuO@RGO for comparison,and the TEM image and cycling performance at 0.1c are shown in Figure S4d.The capacity falls rapidly and shows very low cyclic capacities.The reason may be the poor conductivity because of the degradation of RGO,[48]and also too much CuO covering the RGO leads the as-prepared CuO@RGO not having space structure to buffer the volume change of CuO during charge and discharge.Meanwhile,the agglomeration of CuO particles results in a poor specific capacity of CuO@RGO.Therefore,the combination of reasonable structures and good inherent capacity materials can achieve ideal battery performance.

Electrochemical impedance spectroscopy(EIS)was used to study the electrochemical performance of the battery.As shown in Figure 4i,the EIS diagrams of Cu2O@RGO and YSRs batteries are both highfrequency semicircles and low-frequency straight line,corresponding to charge transfer resistance(Rct)and lithium-ion diffusion process(Wo).[49-51]Both the Rct values before and after 200 cycles for the Cu2O@RGO battery have values of 21 Ω.Compared with Cu2O@RGO,the Rct value of the YSRs battery is 19 Ω,which is higher than the Rct value after 200 cycles(12 Ω).The results show that the unique structure of YSRs needs an electrolyte infiltration progress to obtain the best electrical conductivity,and this results in decreased cycling capacity.

The YSR structure anode materials show the obvious increase in capacity.Generally,the reversible formation of the SEI layers and the electrode materials activation process during charge/discharge are the two main reasons for capacity increase of metal oxide anode materials.DFT simulation has been used to analyze the reason of specific capacity.As shown in Figure S5,the conductivity between CuO layer and RGO layer is higher than that of Cu2O and RGO.Particle charge density simulation shows clearly that more active electrons surround the interfaces between CuO and RGO than the interface between Cu2O and RGO.The DOS calculations indicate that the CuO-RGO displays higher state of density than Cu2O-RGO.It demonstrates that the CuO shell construction between Cu2O core and RGO flakes plays a vital role to enhance the conductivity for YSRs structure and get remarkable capacity performance.Thus,the DFT proves that CuO covered Cu2O complex metal oxide anode material shows vital beneficial at preferable conductivity,so the proper structure design is still a promising method to get good anode materials by ameliorating the conductivity and volume change.

To further analyze the phenomenon of YSRs,we selected several constant current charge/discharge curves of Cu2O@CuO@RGO and calculated differential curves of capacity versus potential.As shown in Figure S6a,from 3rd to 10th,the curves show the same shape.While from the 10th to 100th,the discharge platform at 1.0-0.01 V keeps increasing,which is corresponding to the formation of SEI layer.Meantime,the increase charge platform at 2.5-3.0 V indicates electrode materials activation process.As shown in Figure S6b,the dQ/dV curves show the connection between the changed platform and increased reversible capacity of Cu2O@CuO@RGO.In the first 10th cycles,the peak at 1.42 V(Trend 2)is the reduction of CuI to Cu0.The increased reduction peak(Trend 3)at 0.8 V is ascribed to the continuous formation of the SEI film.During the charging process,the oxidation peaks at 1.18 V(Trend 4)represent the decomposition of SEI membrane.Notably,by comparing the increased intensity of trend 3 with unchanged intensity of trend 4,it can be concluded that the formation quantity of the SEI film is more than that of decomposition during the all discharge/charge process.In other words,the SEI film tended to be thicken constantly in this stage.Besides,the next oxidation peak at 2.5 V(Trend 5)can be assigned to the generation of Cu2O by the reaction of copper metal with Li2O.Compared trend 2 with trend 5,it is noted that Cu2O as an active anode material keeps increasing from 3rd to 100th cycles.In addition,a new gradually increasing reduction peak appears at 2.3 V(Trend 1),which is the typical reduction peak of CuO to Cu2O.It is easy to observe that Cu2O is the main lithium storage material,and new CuO is formed during the charge/discharge process,which is corresponding with the atom radio(Cu:O=12:11)measured by EDS(Figure S3c).Combined with the DFT calculation,the more CuO generated between RGO and Cu2O core,the more active electron created in anode materials and the increasing in capacity.It is worth noting that the trend 2 is increasing from 3rd to 30th cycle while decreasing from 30th to 100th cycle,which indicates that Cu2O creates new Cu content during cycles and part of them is transformed into new active Cu2O anode materials,thereby improving conductivity and capacity during the charge/discharge process.Therefore,the capacity trend of Cu2O@CuO@RGO increases with the increasing cycles.Based on this,it is shown that complex metal oxide anode material shows a synergistic effect on the excellent anode performance.

3.Conclusions

In summary,hierarchical yolk-shell Cu2O@CuO-decorated RGO(YSRs)electrode materials were fabricated using a simple chemical method.At 0.1C and 0.3C conditions,the materials show a high specific capacity of 892 mAh g-1and 1098 mAh g-1,and capacities of 849 mAh g-1and 808 mAh g-1were maintained after 200 cycles.However,the unique structure of the Cu2O yolk with spur-like CuO-void-CuO shell-void-RGO requires an electrolyte infiltration process during the cycling capacity test.Therefore,we used a different current density to optimize the parameter of cycling capacity test,and the results show that 0.1C can be stable and gently obtain a steady specific capacity while 0.3C can faster get the steady specific capacity.The present work demonstrates an economic and environmental way to build hierarchical Cu2O@CuO@RGO(YSRs)nanostructured electrodes for high-performance lithium-ion batteries.In the analysis of the rising trend of capacity,the active substance is transformed from Cu2O to Cu and then to CuO,which plays a key role in the latter charge/discharge process.On the other hand,the DFT results point that complex metal oxides provide an opportunity to enhance the conductivity of metal oxide electrode materials.It provides a new perspective for the study of complex metal oxides and the electrode materials structure design.For future work,the problem of electrolyte infiltration rate should be solved using a more suitable plan.

4.Experimental Section

Chemical used in this study,the preparation method,the characterizations of YSRs,and electrochemical characterization are mentioned in supplementary data.

Acknowledgements

This work was supported by National Natural Science Foundation of China(U1866203,11674263,21805221),Postdoctoral Research Foundation of China(2019M663690,2020M671606),National Natural Science Foundation of Shaanxi Province(2020JZ-03),Key Scientific and Technological Innovation Team of Shaanxi Province(2020TD-001),the Fundamental Research Funds for the Central Universities and the World-Class Universities(Disciplines)and the Characteristic Development Guidance Funds for the Central Universities,and the Instrument Analysis Center of Xi’an Jiaotong University.

Conflict of Interest

The authors declare no conflict of interest.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.