Cu&Si Core–Shell Nanowire Thin Film As High-Performance Anode

applied sciences

Article Cu&Si Core–Shell Nanowire Thin Film as High-Performance Anode Materials for Lithium Ion Batteries

Lifeng Zhang 1, Linchao Zhang 1,*, Zhuoming Xie 1 and Junfeng Yang 1,2,*

1 Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei Institute of Physical Science, Chinese Academy of Sciences, Hefei 230032, China; [email protected] (L.Z.); [email protected] (Z.X.) 2 Lu’an Branch, Anhui Institute of Innovation for Industrial Technology, Lu’an 237100, China * Correspondence: [email protected] (L.Z.); [email protected] (J.Y.)

Abstract: Cu@Si core–shell nanowire thin ﬁlms with a Cu3Si interface between the Cu and Si were synthesized by slurry casting and subsequent magnetron sputtering and investigated as anode materials for lithium ion batteries. In this constructed core–shell architecture, the Cu nanowires were connected to each other or to the Cu foil, forming a three-dimensional electron-conductive network

and as mechanical support for the Si during cycling. Meanwhile, the Cu3Si layer can enhance the interface adhesion strength of the Cu core and Si shell; a large amount of void spaces between the Cu@Si nanowires could accommodate the lithiation-induced volume expansion and facilitate electrolyte impregnation. As a consequence, this electrode exhibits impressive electrochemical properties: the initial discharge capacity and initial coulombic efﬁciency is 3193 mAh/g and 87%, respectively. After 500 cycles, the discharge capacity is about 948 mAh/g, three times that of graphite, corresponding to an average capacity fading rate of 0.2% per cycle. Keywords: lithium ion battery; anode; silicon; core–shell structure; magnetron sputtering Citation: Zhang, L.; Zhang, L.; Xie, Z.; Yang, J. Cu&Si Core–Shell Nanowire Thin Film as High-Performance Anode Materials 1. Introduction for Lithium Ion Batteries. Appl. Sci. Silicon (Si) has the highest known theoretical capacity of 4200 mAh/g, almost ten 2021, 11, 4521. https://doi.org/ times that of the currently used graphite anode, and is therefore being considered as the 10.3390/app11104521 most promising anode material for high-energy-density lithium ion batteries (LIBs) [1,2]. Unfortunately, Si suffers from as high as 300% volume change upon full (de)lithiation, Academic Editor: Francesco Enrichi inducing pulverization of the silicon particles and therefore losing electrical connectivity from the current collector and the thickening of the solid electrolyte interface (SEI) layer Received: 28 April 2021 upon cycling; these eventually hinder the practical application of Si-based anode materials Accepted: 10 May 2021 Published: 15 May 2021 in LIBs [3,4]. To tackle these issues, enormous attention has been directed towards the develop-

Publisher’s Note: MDPI stays neutral ment of Si nanostructured materials [5], of which Si nanowires (SiNWs) have attracted with regard to jurisdictional claims in considerable interest owing to its small lithium diffusion length and facile strain relaxation published maps and institutional affil- during (de)lithiation [6,7]. However, the complete lithiation of Si nanowire impedes charge iations. transport in the longitudinal direction, limiting its rate performance [8]. Cui et al. successfully synthesized a core–shell structured Si nanowire with a crystallite Si (c-Si) core and amorphous Si (a-Si) shell, in which the a-Si shell can be cycled alone as lithium ion storage whereas the c-Si core remains intact as mechanical sturdy support and efficient electron transport pathways by setting an appropriate cut-off potential owing to the higher lithiation Copyright: © 2021 by the authors. ∼ ∼ Licensee MDPI, Basel, Switzerland. potential of a-Si than c-Si ( 220 mV vs. 120 mV, respectively), resulting in significant This article is an open access article improvement in electrochemical performance over traditional Si nanowires, such as a high distributed under the terms and charge capacity of 1000 mAh/g and a capacity retention of 90% over 100 cycles [9]. On conditions of the Creative Commons the heels of this work, a series of core–shell nanowires with Si as the core and electron Attribution (CC BY) license (https:// conductive material as shell [10,11], or with Si as the shell and other electron conductive creativecommons.org/licenses/by/ materials as the core [12,13], also have been fabricated, contributing to great achievement 4.0/). in Si nanowire-based anodes. However, they not only involved high temperature, complex

Appl. Sci. 2021, 11, 4521. https://doi.org/10.3390/app11104521 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 10

Appl. Sci. 2021, 11, 4521 achievement in Si nanowire-based anodes. However, they not only involved high temper-2 of 9 ature, complex steps and the use of catalysts during preparation, but also have a low tap density when applied as an anode. Therefore, the Si core–shell nanowire-based anodes are relatively expensive to produce and hard to scale up. steps and the use of catalysts during preparation, but also have a low tap density when Alternatively, Si film anodes have also been studied since they are binder and con- applied as an anode. Therefore, the Si core–shell nanowire-based anodes are relatively ductive additive free. Ab-initio calculations suggested that the critical thickness of the sil- expensive to produce and hard to scale up. icon film to avoid crack is about 100 nm, beyond which the silicon film would fracture Alternatively, Si film anodes have also been studied since they are binder and conduc- and lead to rapid capacity fading [14]. Such a thin Si-film is not able to provide sufficient tive additive free. Ab-initio calculations suggested that the critical thickness of the silicon Si loading density for practical application [15]. By using a Si-based multilayer thin film, film to avoid crack is about 100 nm, beyond which the silicon film would fracture and with alternating layers of inactive materials as a buffer layer [16,17], its loading density lead to rapid capacity fading [14]. Such a thin Si-film is not able to provide sufficient Si can be increased but only to a very limited extent. loading density for practical application [15]. By using a Si-based multilayer thin film, with alternatingIn this study, layers to of combine inactive materialsthe advantage as a buffer of both layer a Si [ 16thin,17 film], its and loading core–shell density Si can nan- be owire,increased Cu@Si but core–shell only to a verynanowire limited thin extent. film (Cu&Si CSNWF) electrodes were fabricated throughIn thisslurry study, casting, to combine heat thetreatment, advantage and of magnetron both a Si thin sputtering. film and core–shell Subsequently, Si nanowire, their electrochemicalCu@Si core–shell properties nanowire were thin investigated film (Cu&Si in CSNWF) detail. In electrodes comparison were with fabricated other core through materials,slurry the casting, Cu nanowire heat treatment, core has and obvious magnetron advantage sputtering. such Subsequently,as (1) higher electrical their electrochem- conduc- tivityical properties and fracture were toughness investigated [18]; (2) in detail.it is inactive In comparison to lithium withand thus other experiences core materials, no vol- the umeCu nanowirechange during core hasthe lithiation/delithiation obvious advantage such of Si; as and (1) higher(3) the Cu electrical nanowires conductivity we used andare commerciallyfracture toughness available [18]; and (2) itvery is inactive cheap in to price, lithium and and easy thus to experiencesbe scale up in no practical volume changeappli- cation.during the lithiation/delithiation of Si; and (3) the Cu nanowires we used are commercially available and very cheap in price, and easy to be scale up in practical application. 2. Materials and Methods 2.1.2. MaterialsCu@Si Core–Shell and Methods Nanowire Thin Film (CSNWF) Preparation 2.1. Cu@Si Core–Shell Nanowire Thin Film (CSNWF) Preparation Cu nanowires were purchased from Hongwu Nanometer Material CO., LTD.,Xu- zhou China.Cu nanowires Cu&Si CSNWFs were purchased were prepared from Hongwu by three Nanometer steps as schematically Material CO., illustrated LTD.,Xuzhou in FigureChina. 1. Cu&SiFirstly, CSNWFs2 mg Cu werenanowires prepared (CuNWs), by three as stepsreceived as schematicallywithout any pretreatment, illustrated in wereFigure uniformly1 . Firstly, dispersed 2 mg Cu nanowiresinto 10 mL (CuNWs), isopropa asnol received (IPA), and without then anythe pretreatment,slurry was casted were ontouniformly 50 μm-thick dispersed Cu foil, into followed 10 mL isopropanol by heating at (IPA), 280 °C and for then 2 h under the slurry the reductive was casted atmos- onto ◦ phere50 µm-thick of 5 v% Cu H2 foil, + 95 followed v% Ar to by evaporate heating at IPA 280 andC forreduce 2 h under the oxide the reductiveon the surface atmosphere of the CuNWs.of 5 v% H2As +a 95result, v% Arthe to Cu evaporate foil was IPAcoated and with reduce a layer the oxide of cross-linked on the surface CuNW of the sediment. CuNWs. Finally,As a result, the Si the layer Cu foilwas was deposited coated withonto athem layer without of cross-linked intentional CuNW heating sediment. or cooling Finally, to obtainthe Si Cu@Si layer wascore–shell deposited nanowire onto thin them films without by direct intentional current heatingmagnetron or coolingsputtering to obtainof the SiCu@Si target. core–shell The sputtering nanowire power thin is about films by120 direct W, the current distance magnetron of the target sputtering to the substrate of the Si istarget. about The50 mm sputtering and the power work pressure is about 120is about W, the 0.4 distance Pa. For ofcomparison, the target topure the Si substrate film was is about 50 mm and the work pressure is about 0.4 Pa. For comparison, pure Si film was simultaneously deposited on Cu foil and glass substrate, respectively, under the same simultaneously deposited on Cu foil and glass substrate, respectively, under the same sputtering parameter. The thickness of the pure Si films is determined as 456 nm through sputtering parameter. The thickness of the pure Si films is determined as 456 nm through observing its cross-section thickness using SEM. The loading density of the as-deposited observing its cross-section thickness using SEM. The loading density of the as-deposited pure Si thin films was calculated under the assumption that the density of Si thin film is pure Si thin films was calculated under the assumption that the density of Si thin film is 2.33 g cm−3, as reported in literature [19], which is the highest density achievable for the 2.33 g cm−3, as reported in literature [19], which is the highest density achievable for the as-deposited films and will give the most conservative estimate for the specific capacity as-deposited films and will give the most conservative estimate for the specific capacity of of our electrode materials. our electrode materials.

FigureFigure 1. 1. SchematicSchematic diagram diagram of of the the preparation preparation procedur proceduree for for the the Cu@Si Cu@Si core–shell core–shell nanowire nanowire thin thin filmsﬁlms (CSNWFs). (CSNWFs).

2.2. Characterization and Electrochemical Measurement The crystal structure of the Cu nanowire and as-deposited pure Si and Cu&Si CSNWF were characterized, respectively, by X-ray diffraction (XRD, Rikagu Smart Lab) with Cu Kα radiation (λ = 1.541 Å). The surface morphology and structure of the ﬁlms were observed Appl. Sci. 2021, 11, 4521 3 of 9

by field-emission gun scanning electron microscopy (FEG-SEM, Zeiss Ultra-Plus) and scanning transmission electron microscopy (STEM, FEI Titan). The samples for STEM were made by the dual-beam focused ion beam (FIB) technique. A Swagelok-type two- electrode cell was constructed in an argon-filled glove box with a high-purity metallic lithium disk (Ø = 12 mm) as the counter electrode and reference electrode, Whatman filter paper (Ø = 13 mm) as the separator, and the Cu&Si core–shell nanowire thin film on copper foil (Ø = 10 mm) as the working electrode, without addition of any binder and conductive additive. The electrolyte was 1 mol L−1 LiPF6 in a 1:1 (volume ratio) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) tests were carried out on a PAR Versastat-2 electrochemistry system. EIS was measured over the frequency range from 0.01 to about 106 Hz with an AC amplitude of 10 mV. Cyclic voltammetry (CV) was recorded in the voltage range of 0.01–1 V (vs. Li/Li+) at a scan rate of 0.2 mV s−1. Discharge/charge capacities were tested in an Arbin BT 2000 multichannel battery tester by galvanostatic cycling the half cells over the potential range between 0.01 and 1.5 V vs. Li/Li+ at a 0.1 C rate. C rates were calculated on the basis of a theoretical capacity of 4200 mAh/g. After the cycling test, the working electrode was taken out through disassembling the Swagelok-type cell at first and then rinsing it in dimethyl carbonate (DMC) and acetone, finally dried, and subsequently put in a glove box for further examination. All the electrochemical properties were measured at room temperature.

3. Results and Discussion 3.1. Characterization of Cu@Si CSNWFs Figure2a shows the XRD pattern of the as-received Cu nanowires, Cu nanowire coated Cu foil after thermal treatment, and Cu&Si CSNWFs. The diffraction patterns of the as-received Cu nanowire can be well assigned to Cu and Cu oxides (CuO and Cu2O) [20], suggesting the oxidation of the Cu nanowire surface during storage. After heat treatment at 280 ◦C for 2 h in the reductive atmosphere of 5 v.% H2 + 95 v.% Ar, diffraction peaks corresponding to the Cu oxides disappear completely, indicating that this heat treatment is an effective way to remove Cu oxides. After heat treatment and Si deposition, four new peaks are observed; these peaks match well with Si and Cu3Si [21–24], respectively, demonstrating that the sputter-deposited Si is crystalline and there is formation of the crystalline Cu3Si phase. It is noteworthy that Chen et al. also found the formation of Cu3Si during sputtering of a Cu layer on the surface of Si nanowires [25]. Figure2b,c exhibit the SEM images of the Cu nanowire-coated Cu foil before and after Si deposition, respectively. It is clearly seen that the Cu nanowires connect either to each other or to the Cu foil substrate, forming a continuous electron-conductive pathway. After Si deposition, there is an apparent increase in diameter of the Cu nanowires, and meanwhile a large number of void spaces could be observed in between them (Figure2c). Based on Figure2b,c , the corresponding diameter distribution of the Cu nanowire before and after Si deposition was statistically analyzed and exhibited in Figure2d,e. The diameter of the Cu nanowire ranges from 50 to 300 nm, mainly between 150 and 200 nm. After Si deposition, the diameter is increased to 300–650 nm, and mainly lies in the range of 400–500 nm, suggesting the formation of a Cu@Si core–shell nanowire. The formation of voids was ascribed to the shadowing effect of the Cu nanowires during Si deposition. Figure2f exhibits the STEM image of an individual core–shell nanowire, in which the core and shell can be distinguished by the obvious contrast in the image. The brighter inner part is the Cu core, which is covered by a continuous uneven gray Si shell. In order to further conﬁrm the elemental composition, we have performed an EDS test in the mode of linear scanning along the straight line from A to B, as shown in Figure2g. It can be recognized that this single Cu@Si core–shell nanowire has a Cu core of ~200 nm in diameter and a Si shell of 125 nm in thickness, with only trace amounts of oxygen. Furthermore, an obvious mixed layer of Cu and Si is also observed, corresponding to a thin layer of Cu3Si, as conﬁrmed by the XRD in Figure2a, which is Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 10

along the straight line from A to B, as shown in Figure 2g. It can be recognized that this Appl. Sci. 2021, 11, 4521 single Cu@Si core–shell nanowire has a Cu core of ~200 nm in diameter and a Si shell4 of of 9 125 nm in thickness, with only trace amounts of oxygen. Furthermore, an obvious mixed layer of Cu and Si is also observed, corresponding to a thin layer of Cu3Si, as confirmed by the XRD in Figure 2a, which is beneficial for the enhancement of adhesion strength beneﬁcialbetween the for Cu the core enhancement and Si shell. of adhesion Based on strength the above, between it is concluded the Cu core that and the Si Cu@Si shell. BasedCSNWFs on thewere above, successfully it is concluded fabricated. that the Cu@Si CSNWFs were successfully fabricated.

FigureFigure 2.2.( (aa)) XRDXRD patternpattern of of the the as-received as-received Cu Cu nanowire nanowire on on a a Cu Cu foil foil substrate substrate before before and and after after heat treatmentheat treatment at 280 at◦ C280 for °C 2 hfor in 2 ah reductive in a reductive atmosphere atmosphere of 95 of v.% 95 Ar v.% + 5Ar v.% + 5 H2, v.% with H2, andwith withoutand with- Si deposition;out Si deposition; (b) SEM (b image) SEM of image the Cu of nanowirethe Cu nanowire (NW)-coated (NW)-c Cuoated foil substrate Cu foil substrate after heating after treatmentheating treatment and the corresponding size distribution of the diameter of the Cu nanowire before (b,d) and the corresponding size distribution of the diameter of the Cu nanowire before (b,d) and after and after (c,e) Si deposition; (f) STEM image of a single Cu@Si core–shell nanowire and (g) its (c,e) Si deposition; (f) STEM image of a single Cu@Si core–shell nanowire and (g) its composition composition analysis by line scanning EDS along the straight line. analysis by line scanning EDS along the straight line.

3.2.3.2. ElectrochemicalElectrochemical PerformancePerformance ofof Cu@SiCu@Si CSNWFsCSNWFs Figure3a exhibits the charge/discharge curves of the Cu@Si CSNWF electrode after the 1st, 2nd, 5th, and 10th cycle at a current rate of 0.1 C between 0.01 and 1.5 V. The potential window is 0.23–0.01 V during discharging and 0.23–0.55 V during charging, respectively. The voltage proﬁles exhibit a typical sloping and smooth curve without appearance of a distinct potential plateau. Figure3b demonstrates the speciﬁc discharge capacity and Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 10

Figure 3a exhibits the charge/discharge curves of the Cu@Si CSNWF electrode after the 1st, 2nd, 5th, and 10th cycle at a current rate of 0.1 C between 0.01 and 1.5 V. The Appl. Sci. 2021, 11, 4521 potential window is 0.23–0.01 V during discharging and 0.23–0.55 V during charging,5 ofre- 9 spectively. The voltage profiles exhibit a typical sloping and smooth curve without appearance of a distinct potential plateau. Figure 3b demonstrates the specific discharge ca- pacitycharge and efficiency charge of efficiency the Cu@Si of CSNWFs the Cu@Si electrode CSNWFs as aelectrode function as of a cycle function number. of cycle The initialnum- ber.discharge The initial capacity discharge is 3193 capacity mAh/g. is After 3193 500mA cycles,h/g. After the 500 discharge cycles, capacitythe discharge is still capacity as high isas still 948 mAh/g,as high as corresponding 948 mAh/g, tocorresponding an average capacity to an average fading rate capacity of only fading 0.2% perrate cycle. of only As 0.2%for the per charge cycle. efficiency,As for the thecharge initial efficiency, efficiency th ise initial close toefficiency 87%, and is thenclose quicklyto 87%, increasesand then quicklyand lies increases between 96%and andlies between 99.8% during 96% and subsequent 99.8% during cycling. subsequent These suggest cycling. a good These cycling sug- geststability a good of thecycling Cu@Si stability CSNWF of the electrodes. Cu@Si CSNWF In contrast, electrodes. the specificIn contrast, discharge the specific capacity dis- chargeof 456 nm-thickcapacity of pure 456 Sinm-thick films fades pure rapidly Si films with fades cycling, rapidly which with cycling, is typical which for ais thick typical Si forfilm aelectrode thick Si film [26]. electrode The higher [26]. initial The charge higher efficiency initial charge andbetter efficiency cycling and performance better cycling of performanceCu@Si CSNWF of Cu@Si electrodes CSNWF than electrodes pure Si film than could pure be Si attributed film could to be the attributed improved to electricthe im- provedconductivity electric and conductivity special core–shell and special structure. core–shell The structure. electrical conductivityThe electrical ofconductivity Cu3Si was − 4 -1 ofabout Cu3Si2 ×was10 about4 S cm 2 ×1, 10a little S cm lower, a little than lower the than Cu nanowirethe Cu nanowire but much but highermuch higher than purethan puresilicon silicon of about of about10–5 S10–5 cm− S1 cm[27-1].[27]. Indeed, Indeed, it has it beenhas been reported reported that increasingthat increasing the electronic the elec- tronicconductivity conductivity of Si films of Si could films improvecould improve the initial the coulombicinitial coulombic efficiency efficiency significantly significantly [28,29]. [28,29].Moreover, Moreover, the Cu 3theSi layerCu3Si betweenlayer between the Si the shell Si shell and Cuand core Cu core functions functions as an as adhesivean adhe- sivelayer, layer, and and could could enhance enhance the the adhesion adhesion strength strength between between the the Cu Cu nanowire nanowire core core andand Si shell and therefore improve the cycling stabilitystability of thethe Cu@SiCu@Si CSNWF electrodes in spite of a a little little sacrifice sacrifice in in capacity capacity due due to tothe the electrochemical electrochemical inactivity inactivity of Cu of3Si Cu to3Si Li to[24]. Li Kim [24]. etKim al. et[30] al. also [30] alsoattributed attributed the enhanced the enhanced cycling cycling stability stability of a of Si–Cu–graphite a Si–Cu–graphite composite composite to theto the formation formation of ofcopper copper silicide silicide in in the the interface interface between between Cu Cu and and Si. Si. Figure Figure 33cc exhibitsexhibits thethe rate capabilitycapability of of the the Cu@Si Cu@Si CSNWF CSNWF electrode. electrod Thee. dischargeThe discharge capacity capacity is about is 2060 about mAh/g 2060 mAh/gat 0.2 C at and 0.2 exhibits C and aexhibits decreasing a decreasing trend with trend the increase with the in increase current density.in current As density. the current As thedensity current is increased density is to increased 0.5 C, 1.5 to C, 0.5 3 C, 1.5 and C, 6 3 C, C, the and discharge 6 C, the discharge capacity droppedcapacity dropped down to ~1900down mAh/g,to ~1900 mAh/g, ~1600 mAh/g, ~1600 mAh/g, ~1200 mAh/g,~1200 mA and~700h/g, and~700 mAh/g, mAh/g, respectively. respectively. When When the thecurrent current density density was returnedwas returned to 0.2 to C, 0.2 the C, discharge the discharge capacity capacity was increased was increased to 2025 mAh/g,to 2025 mAh/g,almost thealmost same the value same as value that inas thethat beginning, in the beginning, demonstrating demonstrating a good a rate good capability rate capa- of bilityour Cu@Si of our CSNWF Cu@Si CSNWF electrodes. electrodes. In addition, In addition, the coulombic the coulombic efficiency efficiency at different at different current currentdensities densities is beyond is beyond 90%, with 90%, the with exception the except of theion firstof the cycle first at cycle 0.2 C,at 30.2 C, C, and 3 C, 6 and C rates, 6 C rates,further further confirming confirming the good the rategood performance rate performance of the of Cu@Si the Cu@Si CSNWF CSNWF electrodes. electrodes.

Figure 3. (a)Voltage profileprofile ofof thethe Cu@SiCu@Si CSNWFs CSNWFs at at the the 1st, 1st, 2nd, 2nd, 5th, 5th, and and 10th 10th cycle cycle at at a currenta current rate rateof 0.1 of C 0.1 between C between 0.01 0.01 and and 1.5 V;1.5 (b V;) the(b) specificthe specific discharge discharge capacity capacity and and coulombic coulombic efficiency efficiency of the of Cu@Si CSNWFs and pure Si films as a function of cycle number at a rate of 0.16 C between 0.01 and 1.5 V; (c) the specific discharge capacity of Cu@Si CSNWFs as a function of the cycle number and current rate; (d) cyclic voltammetry curves of the Cu–Si core–shell nanowires embedded Si thin films (CSNEFs) in the potential range of 0.01–1.2 V at a scan rate of 0.2 mV s−1. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 10

Appl. Sci. 2021, 11, 4521 6 of 9 the Cu@Si CSNWFs and pure Si films as a function of cycle number at a rate of 0.16 C between 0.01 and 1.5 V; (c) the specific discharge capacity of Cu@Si CSNWFs as a function of the cycle number and current rate; (d) cyclic voltammetry curves of the Cu–Si core–shell nanowires embedded Si thin filmsThe (CSNEFs) good rate in the performance potential range can of be 0.01–1.2 understood V at a scan by rate the of fast 0.2 andmV s efficient-1. electron transfer of the Cu nanowire core and by the decrease in diffusion length of the lithium ions, as schematicallyThe good rate illustrated performance in Figure can be4 understood, in that the by diffusion the fast and length efficient of the electron lithium trans- ion in pure ferSi filmsof the isCu much nanowire longer core than and by in the our decrease Cu@Si CSNWF.in diffusion Figure length3 dof exhibits the lithium CV ions, curves as for the schematicallyCu@Si CSNWF illustrated electrode in Figure in the 4, potentialin that the rangediffusion of 0.01–1.2length of Vthe at lithium a scan ion rate in ofpure 0.2 mV s−1. Si films is much longer than in our Cu@Si CSNWF. Figure 3d exhibits CV curves for the It is clearly seen that the CV curve of the first cycle is distinct from those of subsequent Cu@Si CSNWF electrode in the potential range of 0.01–1.2 V at a scan rate of 0.2 mVs-1. It cycles, especially for the cathodic branch. In the first cathodic scan, the current increases is clearly seen that the CV curve of the first cycle is distinct from those of subsequent cycles,gradually especially as the for potential the cathodic is decreased branch. In the from first 0.76 cathodic V to aroundscan, the0.23 current V, whichincreases is ascribed graduallyto the formation as the potential of a solid is decreased electrolyte from interface 0.76 V to around (SEI) film 0.23 [V,31 ,which32]. Thereafter, is ascribed to it starts to theincrease formation rapidly, of a solid leading electrolyte eventually interface to the (SEI) formation film [31,32]. of a Thereafter, broad peak it betweenstarts to in- 0.05 V and crease0.20 V rapidly, and corresponding leading eventually to the to lithiationthe formation of Si.of a In broad the firstpeak anodic between scan, 0.05 aV distinctand peak 0.20at 0.31 V and V andcorresponding a weak and to the broad lithiation peak of at Si. 0.48 In the V were first anodic observed, scan, whicha distinct could peak be at assigned 0.31to the V and transformation a weak and broad between peak theat 0.48 Li–Si V were alloy observed, to Si. Since which the could second be assigned cycle, the to cathodic thecurrent transformation was kept between almost the unchanged Li–Si alloy in to the Si. Since potential the second range cycle, of 0.72 the tocathodic 0.23 V; cur- the current rentis substantially was kept almost lower unchanged than that in inthe the potential first cycle. range Theseof 0.72 datato 0.23 imply V; the that current the electrolyteis substantiallydecomposition lower during than that the in second the first cycle cycle. is These substantially data imply suppressed. that the electrolyte Instead, de- two new compositionpeaks located during at 0.15the second and 0.05 cycle V is appear, substantially which suppressed. could be Instead, assigned two to new the peaks formation of located at 0.15 and 0.05 V appear, which could be assigned to the formation of the Li–Si the Li–Si alloy [33]. As of the third cycle, the cathodic and anodic current peaks settled alloy [33]. As of the third cycle, the cathodic and anodic current peaks settled rapidly and maintainedrapidly and a maintainedstable pattern, a stableeventually pattern, resu eventuallylting in the resultingpeaks superimposing in the peaks on superimposing each other,on each suggesting other, suggesting a good cycling a good performance cycling performance in the following in thecycles. following In contrast, cycles. Cui Inet contrast, al.Cui reported et al. reported that the current that the peak current intensities peak intensitiesincreased gradually increased in gradually the first few in theto ten first few to cyclesten cycles for the for Si the nanowire Si nanowire electrodes electrodes becaus becausee the more the active more materials active materials are activated are activatedto to reactreact with with the the lithium lithium during during cycling cycling [7,34]. [7 ,Th34ese]. These results results indicate indicate that there that is no there gradual is no gradual activationactivation process process of ofthe the active active materials materials in ou inr electrode; our electrode; i.e., all i.e.,active all materials active materials can be can be fullyfully lithiated lithiated within within two two cycles, cycles, owing owing to the to superior the superior electrical electrical conductivity conductivity of the Cu of the Cu corecore in in the the core–shell core–shell structure structure and andthe void the voidspace space for electrolyte for electrolyte impregnation, impregnation, therefore therefore reducingreducing the the diffusion diffusion length length of the of lithium the lithium ion. ion.

FigureFigure 4. 4. (a(,ab,)b Schematic) Schematic diagram diagram of the of thelithium lithium ion diffusion ion diffusion in pure in pureSi film Si and film CSNEFs, and CSNEFs, respec- respectively. tively. In order to clarify the mechanism of the enhanced electrochemical performance of the Cu@SiIn order CSNWF to clarify in comparison the mechanism with pureof the Si enhanced films, EIS, electrochemical SEM, and STEM performance measurements of were theperformed Cu@Si CSNWF after the in comparison cycling test. with Figure pure5 Sia,b films, exhibit EIS, the SEM, EIS and spectra STEMof measurements the Cu@Si CSNWFs wereand pureperformed Si films, after respectively, the cycling aftertest. Figure the cycling 5a,b exhibit test in the the EIS state spectra of full of de-lithiation. the Cu@Si As can CSNWFs and pure Si films, respectively, after the cycling test in the state of full de-lithia- be seen, for both electrodes the obtained Nyquist plots consisted of a high-frequency (HF) tion. As can be seen, for both electrodes the obtained Nyquist plots consisted of a high- frequencydepressed (HF) semicircle depressed and semicircle a low-frequency and a low-frequency (LF) inclined (LF) line. inclined It is line. accepted It is accepted that in the fully thatdelithiated in the fully state, delithiated the diameter state, the of diamet the HF-depresseder of the HF-depressed semicircle semicircle is mainly is mainly ascribed to the ascribedintrinsic to electronic the intrinsic resistance electronic and resistance the contact and the resistance contact resistance (materia–material (materia–material and material– current collector) while the LF inclined line is attributed to the lithium ion diffusion within the electrode [35,36]. The diameter of the HF semicircle for the pure Si film electrode is about 310 Ω after the first cycle, larger than that of the Cu@Si CSNWF (about 100 Ω), indicating that the electronic conductivity of the Cu@Si CSNWFs is much better than pure Si films. As the cycle proceeds, the HF semicircle diameter for the Cu@Si CSNWF increase to about 110 Ω after 30 cycles whereas that for pure Si films is increased up to about 530 Ω after 20 cycles, demonstrating the better microstructure stability of the Cu@Si CSNWFs than pure Si films. Figure6a exhibit the surface morphology of the Cu@Si Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 10

and material–current collector) while the LF inclined line is attributed to the lithium ion diffusion within the electrode [35,36]. The diameter of the HF semicircle for the pure Si film electrode is about 310 Ω after the first cycle, larger than that of the Cu@Si CSNWF (about 100 Ω), indicating that the electronic conductivity of the Cu@Si CSNWFs is much Appl. Sci. 2021, 11, 4521 7 of 9 better than pure Si films. As the cycle proceeds, the HF semicircle diameter for the Cu@Si CSNWF increase to about 110 Ω after 30 cycles whereas that for pure Si films is increased up to about 530 Ω after 20 cycles, demonstrating the better microstructure stability of the CSNWFCu@Si CSNWFs after 30 than cycles; pure Si the films. core–shell Figure 6a nanowires exhibit the stillsurface preserve morphology their of shape the Cu@Si and integrity CSNWF after 30 cycles; the core–shell nanowires still preserve their shape and integrity with a diameter distribution ranging between 400 and 850 nm (Figure6b) larger overall with a diameter distribution ranging between 400 and 850 nm (Figure 6b) larger overall than those before cycling, as a result of the lithiation-induced volume expansion upon the than those before cycling, as a result of the lithiation-induced volume expansion upon the cycled discharge–charge discharge–charge processes. processes. Furthermor Furthermore,e, after 500 after cycles, 500 cycles, the core–shell the core–shell nanowire nanowire shape is is hard hard to to discern discern because because the theCu@Si Cu@Si core–shell core–shell nanowires nanowires expand expand enough enoughto make to make contact with with each each other other and and form form a continuous a continuous film (Figure film (Figure 6c). A6 morec). A detailed more detailed analysis analysis by cross-sectional cross-sectional STEM STEM (Figure (Figure 6d) illustrates6d) illustrates that the that Si shell the still Si shell adheres still well adheres to the Cu- well to the Cu-corecore and andthe Cu the nanowires Cu nanowires remain remain as effici asent efficient electron electron transport transport pathways, pathways, securing a securing agood good rate rate capability capability of the of Cu the@Si Cu@Si CSNWF CSNWF anode. In anode. contrast, In contrast,a large amount a large of cracks amount are of cracks areobserved observed on the on surface the surface of the pure of the Si purefilm af Siter film 20 cycles, after 20separating cycles, separatingthe Si film into the 1–5 Si film into 1–5μm islandsµm islands (Figure (Figure 6e) and6e) resulting and resulting in poor incycling poor performance. cycling performance.

Appl. Sci. 2021, 11, x FOR PEER REVIEWFigure 5. Electrochemical impedance spectroscopy spectra of (a) Cu@Si CSNWFs after 1 and 308 of 10 Figure 5. Electrochemical impedance spectroscopy spectra of (a) Cu@Si CSNWFs after 1 and 30 cycles; cycles; and (b) pure Si films after 1 and 20 cycles. and (b) pure Si ﬁlms after 1 and 20 cycles.

FigureFigure 6. 6. (a(a,b,b) SEM) SEM image image and and corresponding corresponding diameter diameter distribution distribution of the Cu@Si of the CSNWFs Cu@Si after CSNWFs 30 after cycles; (c,d) SEM image and cross-section STEM image of the Cu@Si CSNWFs after 500 cycles; and 30 cycles; (c,d) SEM image and cross-section STEM image of the Cu@Si CSNWFs after 500 cycles; (e) SEM image of the pure Si film after 20 cycles at a current rate of 0.1 C between 0.01 and 1.5 V. and (e) SEM image of the pure Si ﬁlm after 20 cycles at a current rate of 0.1 C between 0.01 and 1.5 V. 4. Conclusions Cu@Si CSNWFs were fabricated through slurry casting and subsequent magnetron sputtering and investigated as anode materials for lithium ion batteries. Both the silicon shell and copper nanowire core are in the crystal state and they are adhered by an interface layer of Cu3Si, which could enhance the Cu/Si interface strength; the Cu nanowire network functions well as an electron conductive path. These endow Cu@Si CSNWFs with a superior electrochemical performance with a high discharge capacity of about 948 mAh/g after 500 cycles, corresponding to a capacity fading rate of 0.2% per cycle. This work demonstrates that Cu@Si CSNWF is a promising anode material for high-energy-density lithium ion batteries.

Author Contributions: Formal analysis, Z.X.; methodology, J.Y.; writing—original draft, L.Z. (Lifeng Zhang); writing—review and editing, L.Z. (Linchao Zhang) All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by National Natural Science Foundation of China (Grant No. 51502300), The fund of Science and Technology on Reactor Fuel and Materials Laboratory (Grant No. 6142A06200310), Anhui Provincial Natural Science Foundation (Grant No. 1608085QE88) and Hefei Center of Materials Science and Technology (2014FXZY006). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable Appl. Sci. 2021, 11, 4521 8 of 9

4. Conclusions Cu@Si CSNWFs were fabricated through slurry casting and subsequent magnetron sputtering and investigated as anode materials for lithium ion batteries. Both the silicon shell and copper nanowire core are in the crystal state and they are adhered by an interface layer of Cu3Si, which could enhance the Cu/Si interface strength; the Cu nanowire network functions well as an electron conductive path. These endow Cu@Si CSNWFs with a superior electrochemical performance with a high discharge capacity of about 948 mAh/g after 500 cycles, corresponding to a capacity fading rate of 0.2% per cycle. This work demonstrates that Cu@Si CSNWF is a promising anode material for high-energy-density lithium ion batteries.

Author Contributions: Formal analysis, Z.X.; methodology,J.Y.; writing—original draft, L.Z. (Lifeng Zhang); writing—review and editing, L.Z. (Linchao Zhang). All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by National Natural Science Foundation of China (Grant No. 51502300), The fund of Science and Technology on Reactor Fuel and Materials Laboratory (Grant No. 6142A06200310), Anhui Provincial Natural Science Foundation (Grant No. 1608085QE88) and Hefei Center of Materials Science and Technology (2014FXZY006). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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