Design of Roughened Current Collector by Bottom- up Approach

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Design of Roughened Current Collector by Bottom-Up Approach Using Electroplating Technique: Charge–Discharge Performance of Sn Negative-Electrode for Na-Ion Batteries Masahiro Shimizu, Ryosuke Yatsuzuka, Masaomi Horita, Takahiro Yamamoto, and Susumu Arai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08512 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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The Journal of Physical Chemistry

Design of Roughened Current Collector by Bottomup

Approach

using

Electroplating

Technique:

Charge–Discharge Performance of Sn NegativeElectrode for Na-Ion Batteries Masahiro Shimizu,*,†,‡ Ryosuke Yatsuzuka,† Masaomi Horita§, Takahiro Yamamoto,† and Susumu Arai*,†,‡ †

Department of Materials Chemistry, Faculty of Engineering, Shinshu University 4-17-1 Wakasato, Nagano, 380-8553, Japan

‡

Institute of Carbon Science and Technology, Faculty of Engineering, Shinshu University 4-17-1 Wakasato, Nagano, 380-8553, Japan

§

Technical Division, Faculty of Engineering, Shinshu University 4-17-1 Wakasato, Nagano, 380-8553, Japan

*Corresponding author Assistant Prof. Masahiro Shimizu E-mail: [email protected] Tel: +81-26-269-5627; Fax: +81-26-269-5627 Prof. Susumu Arai E-mail: [email protected] Tel: +81-26-269-5413; Fax: +81-26-269-5432

ACS Paragon Plus Environment

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ABSTRACT The use of high capacity electrode materials based on alloying and dealloying reactions with Na is very effective for improving energy density of batteries. However, their application brings on electrical isolation such as detachment of the electrode mixture layer from a current collector, causing rapid capacity fading. We previously found that Cu electrochemically grows in sheet form by electroplating in a CuSO4-based aqueous solution with polyacrylic acid (PAA). In the present study, our goal was to elucidate the formation mechanism of Cu sheets by characterization using scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) analysis, and electron scatter diffraction patterns (EBSD) mapping. Then, the cycling performance of a Sn negative electrode for Na-ion batteries was significantly upgraded by the application of a roughened-Cu substrate with optimized sheet thickness. The STEM images and EBSD maps revealed that the Cu sheet was a single crystal, and the results obtained from XRD and the cathodic polarization behavior of Cu electrodeposition in PAA-containing solutions suggested that PAA molecules adsorbed onto Cu (100) to suppress the Cu growth on the plane, resulting in the formation of Cu sheets. Although the initial reversible capacities of flat-Cu/Sn and roughened-Cu/Sn electrodes were comparable, the developed Cu substrate (1.0×10−4 M PAA) delivered a noticeable increase in the reversible capacity by 210 mA h g−1 from the first to the second cycle, whereas the flat-Cu remained the increase by 100 mA h g−1. In addition, the roughened-Cu substrate suppressed the detachment of the active material layer to maintain a high capacity of 685 mA h g−1 with good capacity retention of more than 90% by the anchor effect. These results demonstrate that the roughened-Cu substrate prepared in the present work is a promising candidate as a current collector for rechargeable batteries.


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1. INTRODUCTION To effectively utilize renewable energies such as natural sunlight, hydraulic power, and wind force, where the power generation amount changes significantly depending on the time of day, the development of stationary power source systems with a high performance is indispensable.1 In contrast to Li used in Li-ion batteries, Na resources are almost inexhaustible in sea water. The cost of renewable energy can thereby be dramatically reduced though the use of Na-ion batteries (NIBs), and their use is eagerly anticipated.2,3 To realize NIBs with high energy densities, the use of alloying/dealloying-type active materials such as Sn,4–6 Sb,7,8 P9–12 has attracted much attention. Sn can deliver a high theoretical capacity of 847 mA h g–1 by the formation of a Na-rich phase of Na15Sn4, and the capacity is much larger than that obtained from carbon-based materials.13,14 There is, however, a critical issue in that Sn undergoes a severe volume change during sodiation and desodiation reactions, and the volume expands and contracts by ~420%.4 This unfavorable change causes electrical isolation of the active material as a result of pulverization and detachment of the electrode mixture layer from the substrate, which is the main reasons for the poor cycle life.15,16 Typical approaches for improving the cycling performance of a Sn electrode are (i) utilization of Sn-based compounds17–20, (ii) optimization of a binder (polyimide, polyacrylate, etc.) and electrolyte additives21–23, (iii) morphological and structural modification of Sn active material.24–26 For instance, compounds such as SnO and SnO2 show stable cycling performances. The oxides undergo phase separation to form metallic Sn and Na2O, and the Na2O acts as a buffer matrix to reduce the stress induced by the large volume changes of Sn and to suppress the agglomeration of Sn.15) On the other hand, Mullins et al. synthesized Sn0.9Cu0.1 (Cu6Sn5/Sn) nanoparticles via a surfactant-assisted wet chemistry method, and they achieved a stable reversible capacity of greater than 420 mA h g–1, even after


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the 100th cycle at 0.2C.17 As for the optimization of binders, Komaba et al. reported that the reversibility of a Sn electrode is improved by using a polyacrylate binder. Kwon et al. successfully synthesized Sn nanofiber with a high aspect ratio using a simple electrodeposition process without the use of templates. The Sn nanofiber grown along one direction by the adsorption of Trion X-100 exhibited a high reversible capacity of 776 mA h g–1 even after the 100th cycle.24 As another strategy, there is the surface roughening of current collector. In the studies for electrodeposited Sn and vacuum deposited Si negative electrodes at the field of Li-ion batteries (LIBs), roughened substrates (i.e. pyramid and nodule-types) prepared by electrodeposition and electrolysis and subsequent heat treatment have been employed to enhance interface strength between the active material layer and the substrate.25–26 In addition, hydrogen bubble-templated Cu foams prepared by electrodeposition and porous Ni substrates have been also used to accommodate volumetric expansion of Sn and Si.27–28 In any of them, it has been demonstrated that peeling of the active material layer from the substrates during significant volume expansion and contraction is suppressed and it is effective for improving cyclability. However, in such the electrodes, a capacity fading should be inevitable when a loading mass increases due to their high denseness of film layers. We therefore believe that it is preferred to use a conventional slurry-type mixture electrode. Kataoka et al. roughened a high tensilestrength clad (Cu/Ni/Cu) foil by etching it in an acid bath and applied it as a current collector for a Si electrode for LIBs.31,32 Excellent performance with a reversible capacity of 1000 mA h g–1 was maintained for more than 1000 cycles in an electrolyte of 1 M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 50/50 with 10 vol.% fluoroethylene carbonate (FEC). They reported that the high tensile strength and roughness suppressed the generation of cracks and exfoliation of the electrode mixture layer


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from the current collector, respectively. Roué et al. prepared a roughened current collector by electrochemical etching using cyclic potential sweep in an aqueous solution of 1 M NaOH in the range from –1.6 V to 1.0 V versus a Hg/HgO reference electrode, followed by heat treatment at 300 oC in Ar/5%-H2 atmosphere (1 atom).33 Bundles of metallic Cu nanowires, which are tens of nanometers in diameter and approximately 5 µm in length, formed on the substrate surface. These Cu nanowires led to better adhesion of a Si-based coating on the modified current collector. The resulting anchor effect was pronounced in cycling tests in 1 M LiPF6/EC:DEC (50:50 vol.%) with 10 wt.% FEC and 2 wt.% vinylene carbonate (VC): the Si electrode using the modified Cu substrate exhibited a reversible capacity of 1200 mA h g–1 for 1000 cycles, whereas stable capacity retention in a Si electrode using flat-Cu was maintained only up to 600 cycles. The approach to achieving high electrode performance by modification of current collector in LIBs can also be applied to NIBs. This is because the major issues upon the use of metal-based negative electrode materials such as peeling off and electrical isolation are common in both the battery systems. In our previous electrodeposition studies, we found that Cu grows in sheet form on a substrate by electroplating in a copper (II) sulfate (CuSO4)-based aqueous solution containing polyacrylic acid (PAA).34–38 The results showed that the PAA molecules acted as a roughening agent in the preparation of a bottom-up type approach. Although an Al substrate can also be used as a current collector in view of the operating voltage of a Sn negative electrode for NIB21, it is expected that the application of a roughened-Cu substrate would improve the cycling performance. However, the mechanism of roughening by PAA remains unclear. Understanding the electrochemical Cu growth process would enable the development of design guidelines and allow free control of the roughness on a substrate surface. The goal of present study was to clarify the growth mechanism of Cu sheets on a substrate in the presence of PAA from the aspect


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of electrochemistry and inorganic chemistry and to analyze the microstructure. Furthermore, we attempted to prolong the cycle life of Sn negative electrode for NIBs.

2. EXPERIMENTAL SECTION 2.1 Fabrication of Roughened-Cu Substrate and Microstructural Characterization. Roughened-Cu current collectors were designed and produced by an electroplating technique using polyacrylic acid (PAA; average molecular weight: 5000). Cu foil was immersed in an electroplating bath consisting of an aqueous solution of 0.85 mol dm–3 (M) copper(II) sulfate (CuSO4; 99.5%), 0.55 M sulfuric acid (H2SO4), and PAA which acts as a surface roughening agent. The roughened-Cu surface was fabricated on the Cu foil by electrodeposition under galvanostatic conditions at 25 oC without stirring. The current density and charge amount were 5 mA cm–2 and 2.7 C cm–2, respectively. The PAA concentration which determines the surface roughness degree was set to 1.0 × 10–5 M, 5.0 × 10–5 M, and 1.0 × 10–4 M. The obtained substrate was washed with deionized water and dried under a vacuum for 24 h. The crystal structure and roughness degree for the surface of the current collectors were investigated by Xray diffraction (XRD; SmartLab, Rigaku), field-emission scanning electron microscopy (FESEM; SU-8000, Hitachi, Ltd.), and confocal laser scanning microscopy (CLSM, VK-8510; Keyence). The junction plane between the Cu sheet and the substrate was examined by scanning transmission electron microscope (STEM; HD2300A, Hitachi, Ltd.). Electron back-scatter diffraction (EBSD) analysis was applied to clarify the crystal orientation of the Cu sheets grown on the substrate. The mapping areas for the analysis from the vertical and horizontal directions against the structure were 10 × 5 µm2 and 4 × 6 µm2, respectively. The apparatus was operated at an acceleration voltage of 25 kV and an irradiation current of 0.5 nA.


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2.2 Electrode Preparation and Electrochemical Measurements. The working electrode was formed on the substrate by using a slurry composed of 70 wt.% Sn powder (diameter: 70–500 nm, EMJapan Co., Ltd.), 15 wt.% acetylene black (AB; Denka Co., Ltd.), 10 wt.% carboxymethyl cellulose (CMC, Mw = 90000; Sigma-Aldrich), and 5 wt.% styrene butadiene rubber (SBR; JSR Corporation). Methanol (150 µL) was used as a dispersant of the Sn powder. The electrode thickness was adjusted by a doctor blade to 10 µm in dry conditions, and the mass loading of the Sn active material powder was approximately 0.7−1.1 mg cm−2. To evaluate the adhesion strength between the Sn active material layer and the substrate, the force required to peel the electrode mixture layer from the substrate was measured by means of a tensile tester (Romulus; Quad Group Inc.). A stud pin with a diameter of 2.7 mm was bonded onto the surface of the Sn electrode using an epoxy resin. Then, the stud pin was pulled downward with a tensile force rate of 2 kg s−1. The influence of the surface roughness on the cycling performance of the Sn electrodes was investigated by charge–discharge tests using 2032-type coin cells that included Na metal foil (99.90%; Rare Metallic) as the counter electrode, a glass fiber filter (Whatman GF/A) as the separator, and an electrolyte of 1.0 M sodium hexafluorophosphate (NaPF6) dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 50/50 and with or without 5 vol.% fluoroethylene carbonate (FEC; Kanto Denka Kogyo Co., Ltd.). Sn electrodes were cycled under the constant current density of 85 mA g−1 (0.1C) in the voltage ranges of 0.005–2.000 and 0.005–0.650 V (vs. Na/Na+).

3. RESULTS AND DISCUSSION


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3.1 Characterization of Microstructure and Crystal Orientation of Cu Sheets. In our previous studies for preparing co-deposited electroplating films consisting of metal and multiwalled carbon nanotubes (MWCNTs), we clarified that PAA delivers good dispersibility of MWCNTs, which affects the MWCNTs content inside the composite film. Furthermore, it was found that Cu electrochemically grew in sheet form on a substrate in the absence of MWCNTs. We assumed that the sheet formation proceeded as follows (Figure 1a): (1) adsorption of PAA molecules onto a Cu substrate; (2) Cu nucleus formation in regions where PAA molecules do not exist; (3) re-adsorption of PAA on specific planes of electrochemically deposited Cu; (4) development of Cu deposition in the direction where PAA does not adsorb (formation of Cu sheet form). In the STEM image of the Cu sheets, grain boundaries are not observed (Figure 1b), which suggests that the formation of the sheet proceeds according to the mechanism mentioned above. In the junction plane between the deposited Cu and the substrate, it was clearly observed that there were no voids, suggesting highly uniform electrodeposition.


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Figure 1. (a) Schematic illustration of proposed growth process for Cu sheets by electroplating using polyacrylic acid (PAA; average molecular weight: 5000) as a threedimensional structure former. (b) STEM images of Cu sheets and an interface between the substrate and the desposited Cu.

Figure 2 displays cross-sectional FE-SEM images of roughened-Cu obtained from an electroplating bath with 3.0×10−4 M PAA at a current density of 20 mA cm−2. With increasing charge amounts, the Cu sheets grew more densely on the substrate in the height direction. To investigate the growth orientation of the Cu sheet, we tried to analyze the XRD analysis. However, because the underlayer used in the present study was also Cu, it was difficult to distinguish the diffraction peak of the sheets from those of the underlayer. We therefore applied an amorphous Ni−P coating on a Cu substrate by electroplating using a bath (an aqueous solution consisted of 0.2 M C6H5Na3O7, 0.5 M (NH4)2SO4, 0.1 M NiSO4･6H2O, and 0.2 M NaH2PO2･ H2O) at a charge amount of 450 C cm−2 (equal to the film thickness of about 40 µm)39. As shown in Figure 2b, obvious diffraction peaks derived from the Cu substrate were not detected, which


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enabled us to explore the crystal orientation. When the Cu electroplating treatment by 3.8 C cm−2 was conducted, diffraction peaks located at 43.5o, 50.5o, and 74.2o were detected, and the peaks originated from Cu planes of (111), (100), and (110), respectively. The important point to emphasize is that only the peak intensities of the Cu (111) and (110) planes became strong with increasing electrodeposition amounts, whereas the change in intensity of the diffraction peak derived from (100) plane is little. Figure 2c shows cathodic linear sweep voltammograms corresponding to Cu electrodeposition in aqueous solution with various PAA concentrations versus a saturated calomel electrode (SCE reference electrode: +0.244 V vs. SHE). In the absence of PAA, the electrodeposition of Cu proceeded at a potential of 0.06 V vs. SCE. On the other hand, the cathodic reaction (Cu2+ + 2e− → Cu) occurred at the more negative side with increasing PAA concentration. The shift of electrodeposition potential to the negative side (generation of overpotential) means that PAA molecules suppressed the Cu electrodeposition reaction.35 We evaluated iconicity of PAA (0.3 mM) in the electroplating bath using zeta potential as the indicator (Figure S1, Supporting Information). It was found that the PAA molecules ionize (proton release) to behave like an anion (−8.457 mV). It is therefore thought that the adsorption mechanism of PAA molecules is not electrostatic attraction but is specific adsorption. We will investigate the adsorption characteristic such as crystal surface and potential selectivity by electrochemical quartz crystal microbalance (EQCM) measurements and the use of single crystal thin films in a future study. For more detailed analysis of crystal orientation, we performed EBSD mapping of the Cu sheet from the vertical and horizontal directions. The patterns were recorded per 10 nm and 25 nm, respectively. When measuring the EBSD patterns of the roughened-Cu as it is, free spaces formed between the Cu sheets, causing noise in the patterns. To solve this problem, an


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amorphous Ni−P coating by electroplating or a resin coating was applied. Then, the surfaces being measured were polished using ion milling. Figure 3 depicts FE-SEM images and corresponding EBSD patterns of the Cu sheets observed from the vertical and horizontal directions. The inverse pole figure (IPF) map, shown in the inset of the EBSD image does not apparently express orientation commonalities in the Cu crystal. Because the Cu sheets did not grow completely on the substrate along the vertical direction, various crystal planes were inevitably exposed by polishing using ion milling (even when Cu electrochemically grew in a specific orientation). Therefore, the EBSD mapping images do not reflect the entire structure (Figure S2, Supporting Information). However, the respective Cu sheets observed from the images show the same crystal orientation in the analysis from the vertical direction as well as from the horizontal direction, which means that each one is a single crystal. In addition, we conducted quantitative evaluation of the crystal orientation of the Cu sheets by the calculation of relative texture coefficient RTC(hkl) according to the following equation40: RTC (hkl ) =

o I hkl / I hkl 3

∑I

hkl

/I

× 100%

(1)

o hkl

1

where Ihkl denotes the diffraction intensities of the (hkl) lines measured for the deposited Cu from PAA-containing electroplating bath, and Iohkl is defined as the corresponding intensities of the standard Cu powder sample which is randomly oriented. The denominator was the summation of three dominant peaks for Cu (111), (100), and (110). Therefore, a preferred orientation through an axis [hkl] is indicated by values of RTC ≥ 33.3%, and the preferred orientation is absolute when RTC approaches the value of 100%. Figure 4 shows variation of relative texture coefficient for Cu (111), (100), and (110) diffraction peaks as a function of charge amount in electroplating bath (0.85 CuSO4 + 0.55 H2SO4 with 3.0 × 10–4 M PAA). The values of RTC(110) and RTC(111)


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exceeded 33.3% at any charge amount, and RTC(110) became the maximum when the charge amount was 29.5 C cm–2. From the results, it is indicated that [110] is the preferred orientation. Meanwhile, the Cu (110) plane shows greater surface energy than those of the Cu (100) and (111) planes because of its surface atoms with a larger number of missing nearest-neighbors per unit area. In contrast, the most densely packed Cu (111) plane has the lowest surface energy.41 Considering from the characteristic, it is reasonable to assume that the preferred orientation results in [111] or [100] due to the suppression of crystal growth by PAA adsorption on Cu (110) plane. Given the results obtained from the XRD and EBSD analyses and the cathodic polarization behaviors, however, it is suggested that PAA molecules probably adsorbed onto the crystal plane of Cu (100) to suppress the Cu growth on the plane, thereby leading to the formation of Cu sheets.


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Figure 2. (a) Cross-sectional FE-SEM images and (b) XRD patterns of roughened-Cu substrates obtained from an electroplating bath with 3.0×10−4 M PAA at a current density of 20 mA cm−2. (c) Cathodic linear sweep voltammograms in aqueous solutions including PAA of various concentrations.


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Figure 3. FE-SEM image and corresponding EBSD patterns of Cu sheets grown on a substrate. The EBSD maps were obtained from (a) vertical and (b) horizontal directions.


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Figure 4. ariation of relative texture coefficient for Cu (110), (100), and (111) diffraction peaks as a function of charge amount in electroplating bath (0.85 CuSO4 + 0.55 H2SO4 with 3.0 × 10–4 M PAA). FE-SEM image of commercially available Cu powder as a standard sample which is randomly oriented. Inset: XRD pattern of the Cu powder.

3.2 Effect of Substrate Geometry on Charge–Discharge Performance of Sn Electrodes. The roughness degree and thickness of the Cu sheets could be arbitrarily changed by changing either the current density on electroplating or the PAA concentration in the electroplating bath, respectively (Figure S3, Supporting Information). When the space formed by the Cu sheets is too small, active material and conductive additive powders cannot get into the space, which results in the deterioration of adhesion strength. In the present study, we therefore set the current density and PAA concentration to 5 mA cm−2 and 1.0×10−4 M, respectively. In addition, to reveal the effect of substrate geometry on the charge−discharge performance of the Sn electrodes, roughened current collectors were prepared by using electroplating baths with 1.0×10−5 M and 5.0×10−5 M PAA (Figure 5). The root mean square roughness of the substrate surface (Rrms), an index parameter of the degree of irregularity, was analyzed by CLSM. The Rrms value increased


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with PAA concentration (Figure S4, Supporting Information). In the case of a PAA concentration of 1.0×10−5 M, micrometer-sized Cu crystals formed on the substrate and the substrate surface was slightly roughened. The height difference was approximately 100 nm at maximum. The use of 5.0×10−5 M PAA resulted in granulated Cu with a 2 µm diameter. On the other hand, in an aqueous solution including 1.0×10−4 M PAA, Cu grew as a sheet on the substrate to a height of about 3 µm, and sheets formed the space capable of accommodating an active material layer. In fact, after slurry casting, it was confirmed that the Cu sheets extended to inside the active material layer, like a support pillar. The ideal structure is expected to suppress peeling of the electrode material (electrical isolation) from the substrate, even during significant volume expansion/contraction of Sn during charge and discharge reactions. Thus, the achievement of good electrode performance is expected.


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Figure 5. Surface and cross-sectional FE-SEM images of roughened-Cu current collectors (middle) before and (bottom) after casting of slurry consisting of Sn nano powder (70 wt.%), AB (15 wt.%), CMC (10 wt.%), and SBR (5 wt.%), with ethanol as the dispersant for the Sn powder. Inset values are root mean square roughness (Rrms) values analyzed by CLSM.

Before evaluating the Cu sheets as a current collector for the negative electrode, we conducted cyclic voltammetry measurements in the range of 0.0–3.0 V vs. Na/Na+ with a sweep rate of 1.0 mV s–1.32 Figure 6 compares the electrochemical durability of the roughened-Cu current collector (PAA: 1.0×10−4 M) under the electrolyte of 1 M NaPF6/EC:DEC with FEC additive, which is frequently used in NIB studies. The cathodic peak observed at approximately 2.0 V originated from the reduction of CuO to Cu2O, although the Cu-based oxides were not confirmed from their XRD patterns (not shown here). The peak located at 1.42 V originated from


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the irreversible decomposition of the electrolyte and additive.32 The peak at 0.36 V comes from the reduction process to Cu associated with the Na-storage reaction as Na2O. The stepwise oxidations at 1.5 V and 3.0 V on the anodic side are the reverse reactions of those shown in the cathodic process, that is, the conversion reaction of CuO.42 Although the current response increased compared to those of the flat-Cu substrate, the increase is probably due to its large surface area, and it does not matter when operated using an active material.

Figure 6. Cyclic voltammograms of roughened-Cu (PAA: 1.0×10−4 M PAA, 2.7 C cm−2) and flat-Cu current collectors in 1 M NaPF6/EC:DEC with 5 vol.% FEC. The potential range is 0−3.0 V vs. Na/Na+. Sweep rate was set to 1 mV s−1. Adhesion strength, defined as the force required for peeling the electrode mixture layer from the substrate, was investigated by a tensile tester (Figure 7a). The adhesion strength was enhanced with increasing roughness degree of the substrate: the holding capability of the


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electrode mixture for the roughened-Cu current collector obtained from the aqueous solution including 1.0×10−4 M PAA improved 1.8 times as much as that of flat-Cu (Figure 7b). Figure 7c displays FE-SEM images of the flat and roughened-Cu substrate surfaces after the tensile tests. Although there was almost no electrode material powder on either substrate, a residual component was observed on the roughened-Cu. Note that Cu sheets remained as they were on the substrate, which indicates the high structural integrity of the interface between the substrate and the grown Cu sheets.

Figure 7. (a) Diagram of a tensile test. (b) Summary of adhesion strength defined as the force required to peel the electrode mixture layer from a substrate. (c) FE-SEM images of flat and roughened-Cu substrates after the tensile test.


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Figure 8 shows charge−discharge (Na-insertion/extraction) curves of Sn electrodes operated in the voltage range of 0.005−2.000 V (vs. Na/Na+). Wang et al. studied the adequate particle size of Sn on the basis of results obtained by in-situ hard X-ray nanotomography analysis.43 The results showed that the use of medium size (1−2 µm) and larger size (5−10 µm) Sn particles caused mild structural degradation and significant morphology changes, such as large cracks and pulverization. On the other hand, there is no obvious microstructural degradation in the case of Sn particles below 0.5 µm. Considering the size-dependent structural change, we used Sn powder with a size of 500 nm or less as the active material powder (Figure S5, Supporting Information). The initial coulombic efficiencies were 83% for the flat-Cu/Sn and 80% for the roughened-Cu/Sn electrode. In the first cycle for both Sn electrodes, the shoulder observed at 0.8−1.3 V is attributed to the decomposition of the electrolyte that included the FEC additive, which is the main reason for the irreversible capacity.21 It was reported that graphite as a conductive additive suppresses the decomposition of the electrolyte because of its low surface area.21 Therefore, the application should lead to improved reversibility of the electrode reaction. Clear plateaus at 0.32−0.45 V and 0.15−0.20 V come from the sodiation to the NaSn and Na9Sn4 phases, respectively. Based on the charge capacity obtained from the Sn electrodes, fully sodiated Na15Sn4 was thought to form at the lower cut-off voltage of 0.005 V. Although the roughened-Sn electrode maintained a higher capacity after the 20th cycle, the discharge capacities quickly decreased, contrary to expectation. The volumetric change from Sn to fully sodiated Na15Sn4 reaches 420%4,5, and the change ratio is larger than other materials showing high theoretical capacities (i.e., Na3P/2596 mA h g−1/290%, Na3Sb/660 mA h g−1/290%).8,11 The significant volume expansion and contraction during alloying/dealloying reactions with Na result in the peeling of electrode mixture layer from the substrate or the electrical isolation (breakage of


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electrochemical network) of the active material powder inside the electrode.6 In the present study, because relatively small Sn particles were used to avoid the shape change such as pulverization and formation of cracks, the occasion of the poor cyclability is considered to be in the former. In fact, from the cross-sectional FE-SEM images of Sn electrodes after cycling (Figure S6, Supporting Information), detachment of electrode mixture layer from the substrates was confirmed, regardless of substrate geometry. Komaba et al. succeeded in a major upgrade of the cycling performance of a Sn electrode containing a graphite conductive additive and a sodium polyacrylate (PAANa) binder by limiting the upper cut-off voltage to 0.65 V.21 They found that the FEC-derived surface layer was decomposed by the electrocatalytic activity of β-Sn to form a thick surface layer, which caused deterioration in the cycling performance when the cut-off voltage was 0.70 V. For the purpose of reducing the volume change ratio of Sn and improving the stability of the surface layer, we again conducted charge−discharge tests under an upper cutoff voltage of 0.65 V (Figure S7, Supporting Information). We expected the advantage of the using the roughened-Cu substrate to become pronounced. Figure 9a exhibits the charge−discharge behavior of Sn electrodes operated in the voltage range of 0.005−0.65 V (vs. Na/Na+). The initial discharge capacities for the flat-Cu/Sn and the roughened-Cu/Sn electrodes were 450 mA h g−1 and 510 mA h g−1, respectively. It is important to emphasize that the capacity obtained from the roughened-Cu/Sn electrodes at the second cycle remarkably increased as compared to the flat-Cu/Sn electrode: for the flat-Cu/Sn electrode, the capacity rose by 100 mA h g−1, whereas the roughened-Cu/Sn electrode (PAA: 1.0×10−4 M) resulted in an increase of 210 mA h g−1. The noticeable enhancement indicates the high currentcollecting ability of the roughened-Cu substrate, that is, the Cu sheets grown on the substrate acted as a conductive network inside the electrode mixture layer. The favorable function was


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demonstrated by the results of electrochemical impedance spectroscopic (EIS) measurements. Figure 9b depicts a typical Nyquist plot of a cell including a negative electrode at the charged state. In the present study, it is the symmetric cell consisted of two identical electrode sheets charged up to 0.005 V (vs. Na/Na+), and we can assign the two semicircles in the high and low frequency regions to the interfacial resistance (Rif) and the charge transfer resistance (Rct), respectively.44 The interfacial resistance originates from the interfacial Na-ion transfer through a surface layer induced by a reductive decomposition of electrolyte. The charge transfer is interpreted as the sodiation of active material (Na−Sn alloying reactions). The roughened-Cu substrate made charge transfer reactions smooth; the resistance (8 Ω) resulted in the less than that of flat-Cu substrate (14 Ω). The lower resistance indicates that the Cu sheets piercing inside active material layer allows the improvement of the utilization of Sn to exert its potential capacity. In general, the utilization tends to decrease as thickness of composite material layer increases. However, the use of the roughened-Cu current collector developed in this study would lead to the realization of potential high capacity and high cyclability even in the case of thicken composite material layer.


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Figure 8. Charge−discharge curves of Sn electrodes prepared by using flat-Cu and roughened-Cu (PAA: 1.0×10−4 M) current collectors. The voltage range is 0.005−2.000 V (vs. Na/Na+).


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Figure 9. (a) Charge−discharge properties of Sn electrodes using roughened current collectors. For comparison, flat-Cu substrate is also shown. The PAA concentrations are 1.0×10−5 M, 5.0×10−5 M, and 1.0×10−4 M, respectively. (b) Cell configuration, typical Nyquist plot, the result of fitting by equivalent circuits for impedance analysis, and impedance spectra of [electrode|electrolyte|electrode] symmetric cells.

Figure 10 represents the cycling performance of the Sn electrodes and cross-sectional FE-SEM images of the electrode after cycling. Although the flat-Cu/Sn electrode showed a reversible capacity of 530 mA h g−1 after the 80th cycle with a stable capacity retention, the utilization ratio of the active material to the theoretical capacity of Sn (Na15Sn4: 847 mA h g−1) was still only a fraction of 62%. In contrast, all the roughened-Cu/Sn electrodes achieved high capacities,


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irrespective of roughness degree. In particular, the roughened-Cu/Sn electrode (PAA: 1.0×10−4 M) provided the much higher capacity of 685 mA h g−1 (utilization ratio: 80%), which is more than double the capacity obtained from carbon-based materials.13,14 At the 80th Na-extraction, there were great vertical cracks in the electrode mixture layer in the flat-Cu/Sn electrode, whereas such disastrous destruction was not observed for the roughened-Cu/Sn electrode. Based on the discharge capacities derived from the electrodes, the volume change ratio of Sn during alloying and dealloying reactions with Na was larger in the roughened-Cu/Sn electrode. Nevertheless, the fact that electrode disintegration was smaller demonstrates that the Cu sheets effectively suppressed the detachment of the electrode mixture layer from the substrate surface. Thus, we succeeded in obtaining the potential high capacity of Sn by designing the optimal roughness degree on the substrate surface from an electroplating bath that included PAA. The developed roughened substrates would thus be promising candidates as current collectors for expansion/contraction-type active materials (i.e., a Si electrode for LIBs, Li3.75Si: 3580 mA h g−1, volume expansion ratio: 280%).45−47 Although there was concern that the fabrication of roughened-Cu on a substrate by electroplating using a PAA-containing solution would decrease the weight energy density of a battery, it was never a problem. Also electroplating is employed as one of the manufacturing methods of Cu foil.48 This concern should be readily solved by adopting the technology developed in this study to the final stage of the usual process. Furthermore, it will be very useful for adhesiveless heterogeneous interfacial bonding, which has become a hot topic in recent years.49


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Figure 10. (a) Dependence of charge and discharge capacities on cycle number for Sn electrodes in 1 M NaPF6/EC:DEC with 5 vol.% FEC at a rate of 0.1C (ca. 85 mA g−1). (b) Cross-sectional FE-SEM images of Sn electrodes (flat-Cu, roughened-Cu/PAA: 1.0×10−4 M) after the 10th cycle.

4. Conclusions The influence of PAA molecules on the growth orientation and morphology of electrochemically deposited Cu on a substrate was studied by XRD and FE-SEM equipped with an EBSD camera. The obtained results indicated that PAA molecules adsorbed onto the Cu (100) to suppress the Cu growth on the planes, resulting in to the formation of Cu sheets. Roughed Cu substrates prepared using aqueous solution with different PAA concentrations were applied to a current collector of a Sn negative electrode for NIBs, and the effect of substrate geometry on


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charge−discharge performance was investigated. The flat-Cu/Sn electrode showed discharge capacities of 450 mA h g−1 and 550 mA h g−1 at the first cycle and the second cycle, respectively. On the other hand, Cu sheets (PAA: 1.0×10−4 M) grown on a substrate effectively acted as a conductive network inside the electrode to exhibit a noticeable increase in reversible capacity by 210 mA h g−1 from the first to the second cycle. In addition, the Cu sheets suppressed detachment of the active material layer from the substrate, and the roughened-Cu/Sn electrode achieved a capacity of 685 mA h g−1 at the 80th cycle with a high capacity retention of more than 90% by the anchor effect. These results suggest that the developed roughened-Cu substrate is a promising candidate as the current collector for rechargeable batteries.

ASSOCIATED CONTENT Supporting information Zeta potential, FE-SEM images, photographs, CLSM images, and XRD patterns of roughenedCu current collectors prepared by electrodeposition using PAA; FE-SEM images of Sn powder used in this study; XRD patterns of Sn electrode during the first sodiation/desodiation cycle.

AUTHOR INFORMATION Corresponding Author: Masahiro Shimizu, Susumu Arai *E-mail: [email protected], [email protected]

Acknowledgments This work was supported by a Grant-in-Aid for Research Activity Start-up (16H06838) from the Japan Society for the Promotion of Science (JSPS). This work was supported in part by the


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Alumni Association “Wakasatokai” of the Faculty of Engineering, Shinshu University. The authors thank Ms. M. Ueda and Mr. T. Ogasawara for their kind assistance during the tensile tests and zeta potential measurements.

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Figure captions Figure 1. (a) Schematic illustration of proposed growth process for Cu sheets by electroplating using polyacrylic acid (PAA; average molecular weight: 5000) as a three-dimensional structure former. (b) STEM images of Cu sheets and an interface between the substrate and the desposited Cu.

Figure 2. (a) Cross-sectional FE-SEM images and (b) XRD patterns of roughened-Cu substrates obtained from an electroplating bath with 3.0×10−4 M PAA at a current density of 20 mA cm−2. (c) Cathodic linear sweep voltammograms in aqueous solutions including PAA of various concentrations.

Figure 3. FE-SEM image and corresponding EBSD patterns of Cu sheets grown on a substrate. The EBSD maps were obtained from (a) vertical and (b) horizontal directions.

Figure 4. Variation of relative texture coefficient for Cu (110), (100), and (111) diffraction peaks as a function of charge amount in electroplating bath (0.85 M CuSO4 + 0.55 M H2SO4 with 3.0 × 10–4 M PAA). FE-SEM image of commercially available Cu powder as a standard sample which is randomly oriented. Inset: XRD pattern of the Cu powder.

Figure 5. Surface and cross-sectional FE-SEM images of roughened-Cu current collectors (middle) before and (bottom) after casting of slurry consisting of Sn nano powder (70 wt.%), AB (15 wt.%), CMC (10 wt.%), and SBR (5 wt.%), with ethanol as the dispersant for the Sn powder. Inset values are root mean square roughness (Rrms) values analyzed by CLSM. Figure 6. Cyclic voltammograms of roughened-Cu (PAA: 1.0×10−4 M PAA, 2.7 C cm−2) and flat-Cu current collectors in 1 M NaPF6/EC:DEC with 5 vol.% FEC. The potential range is 0−3.0 V vs. Na/Na+. Sweep rate was set to 1 mV s−1.


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Figure 7. (a) Diagram of a tensile test. (b) Summary of adhesion strength defined as the force required to peel the electrode mixture layer from a substrate. (c) FE-SEM images of flat and roughened-Cu substrates after the tensile test.

Figure 8. Charge−discharge curves of Sn electrodes prepared by using flat-Cu and roughened-Cu (PAA: 1.0×10−4 M) current collectors. The voltage range is 0.005−2.000 V (vs. Na/Na+). Figure 9. (a) Charge−discharge properties of Sn electrodes using roughened current collectors. For comparison, flat-Cu substrate is also shown. The PAA concentrations are 1.0×10−5 M, 5.0×10−5 M, and 1.0×10−4 M, respectively. (b) Cell configuration, typical Nyquist plot, the result of fitting by equivalent circuits for impedance analysis, and impedance spectra of [electrode |electrolyte| electrode] symmetric cells.

Figure 10. (a) Dependence of charge and discharge capacities on cycle number for Sn electrodes in 1 M NaPF6/EC:DEC with 5 vol.% FEC at a rate of 0.1C (ca. 85 mA g −1). (b) Cross-sectional FE-SEM images of Sn electrodes (flat-Cu, roughened-Cu/PAA: 1.0×10−4 M) after the 10th cycle.


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TOC Graphic


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(a)

Adsorption of PAA molecules PAA (A.M.W: 5000)

Adsorption of PAA molecules on Cu formed on the substrate

After

(b) Deposited Cu

Nucleus formation

Before

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Crystal growth of Cu as sheets (Roughed current collector)

Substrate 300 nm

Cu sheet

growth direction 150 nm

Figure 1 ACS Paragon Plus Environment

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(a)

3.8 C cm−2

9.5 C cm−2

28.5 C cm−2

19 C cm−2

5 mm

5 mm

5 mm

5 mm

Current density / mA cm2

Charge amount 28.5 C cm−2

ICSD (Cu): 00-004-0836

(2 2 0)

(c)

(2 0 0)

(1 1 1)

0

Intensity / arb. unit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


19 C cm−2 9.5 C

cm−2

3.8 C

cm−2

Ni−P film

20

30

40

50

60

2 (Cu-K) / deg.

70

80

-10

(b) Scan rate: 10 mV s−1

-20 3.0×10−4 M

-30

1.0×10−4 M 5.0×10−5 M 1.0×10−5 M

-40

PAA free

-50 -0.3

-0.2

-0.1

0

Potential / V vs. SCE

Figure 2


0.1

TD

EBSD analysis

RD

Page 38 of 45

(a)

From vertical direction amorphous Ni−P coating Cu sheet

1 mm

(b)

Cu substrate Exfoliation & Flip vertical Surface polishing (a) using ion beam

Figure 3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


From horizontal direction resin

Cu sheet

(b) Cu substrate

111

1 mm ACS Paragon Plus Environment

001

101

Page 39 of 45

60

Cu powder (standard sample)

20 Cu(110) Cu(111) Cu(100)

10 0

5

10

30

15

20

25

Charge amount / C cm

2

(220)

30

(200)

Intensity / arb. unit

40

0

(111)

50

RTC(hkl) (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


40

50

60

70

2 (Cu-K) / deg.

30

Figure 4


80

2 mm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 40 of 45

Flat-Cu

PAA: 1.0×10–5 M

PAA: 5.0×10–4 M

PAA: 1.0×10–4 M

Rrms: 0.087

Rrms: 0.17

Rrms: 0.26

Rrms: 0.53

5 mm

5 mm

5 mm

5 mm

5 mm

5 mm

5 mm

5 mm

5 mm

5 mm

5 mm

5 mm

Figure 5


Page 41 of 45

0.1 1st

Current / mA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


0 Flat-Cu Roughened-Cu

-0.1

W.E.: Cu substrate (Φ 16 mm) C.E., R.E.: Na metal

-0.2

0

1

2

Potential / V vs. Na/Na+ Figure 6 ACS Paragon Plus Environment

3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

(a)

Page 42 of 45

(b) Backing plate Epoxy resin Sn electrode

Flat-Cu

Active material layer Epoxy resin Stud pin (Φ 2.7 mm)

1.0×10−5 M

Roughed -Cu

1.0×10−4 M

0

Tensile force (2 kg s−1)

(c)

5.0×10−5 M

1000

2000

2

Adhesion strength / N cm

Flat-Cu

Roughened-Cu (1.0×10−4 M)

2 mm

Figure 7 ACS Paragon Plus Environment

2 mm

3000

Page 43 of 45

F-Cu/Sn R-Cu/Sn (PAA: 1.0×10−4 M) 20th

2

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


1st

1

0 0

200

400

600

800

Capacity / mA h g1 Figure 8 ACS Paragon Plus Environment

1000


1 0.8 1st

0.6

Flat-Cu Cell configuration

2, 5, 10th

0.4 Identical electrode Separator (Electrolyte)

0.2

0.6

1st

2, 5, 10th

Typical Nyquist plot

0.4 0.2

Imag Z / 

PAA: 5.0×10−5 M

0.8

1st

0.6

2, 5, 10th

0.4

CPE1

CPE2

R1

R2

Figure 9

Voltage / V

PAA: 1.0×10−5 M

0.8

0

Voltage / V

Page 44 of 45

(b)

0

Rsol

0.2

Zw

Real Z / 

0

20

PAA: 1.0×10−4 M

0.8 1st

0.6

2, 5, 10th

0.4 0.2

Imag Z / 

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Voltage / V

(a)

0

R-Cu/Sn (PAA: 1.0×10−4 M)

10

0 0

200

400

600

800

1

Capacity / mA h g

1000

F-Cu/Sn

0

10


20

Real Z / 

30

40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42


Dischage capacity / mA h g1

Page 45 of 45

1000

(a)

(b) Flat-Cu

Roughened-Cu/Sn PAA: 1.0×10−4 M PAA: 5.0×10−5 M PAA: 1.0×10−5 M

800 600

10 mm

400

R-Cu / PAA: 1.0×10−4 M

Flat-Cu/Sn

200 0 0

10 mm

20

40

60

80

Cycle number Figure 10 ACS Paragon Plus Environment

Design of Roughened Current Collector by Bottom- up Approach

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