In Situ TEM Investigation of the Electrochemical Behavior in CNTs

Aug 14, 2017 - Transition metal oxides have attracted much interest owing to their ability to provide high power density in lithium batteries; therefo...
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In-situ TEM Investigation of the Electrochemical Behavior in CNTs/MnO Based Energy Storage Devices 2

Tsung-Chun Tsai, Guan-Min Huang, Chun-Wei Huang, Jui-Yuan Chen, Chih-Chieh Yang, Tseung-Yuen Tseng, and Wen-Wei Wu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b00958 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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In-situ TEM Investigation of the Electrochemical Behavior in CNTs/MnO2 Based Energy Storage Devices Tsung-Chun Tsai,a Guan-Min Huang,a Chun-Wei Huang,a Jui-Yuan Chen,a Chih-Chieh Yang,b TseungYuen Tsengb and Wen-Wei Wu*a a

Department of Materials Science and Engineering, National Chiao Tung University, No.1001, University Rd., East Dist., Hsinchu City 30010, Taiwan. E-mail: [email protected]; Tel: +886-3-5712121-55395 b Department of Electronics Engineering, National Chiao Tung University, No.1001, University Rd., East Dist., Hsinchu City 30010, Taiwan. *Corresponding authors ABSTRACT: Transition metal oxides have attracted much interest owing to their ability to provide high power density in lithium batteries; therefore, it is important to understand the electrochemical behavior and mechanism of lithiation-delithiation processes. In this study, we successfully and directly observed the structural evolution of CNTs/MnO2 during the lithiation process using transmission electron microscopy (TEM). CNTs/MnO2 were selected due to their high surface area and capacitance effect, and the lithiation mechanism of the CNT wall expansion was systematically analyzed. Interestingly, the wall spacings of CNTs/MnO2 and CNTs were obviously expanded by 10.92 % and 2.59 %, respectively. The MnO2 layer caused structural defects on the CNTs surface that could allow penetration of Li+ and Mn4+ through the tube wall and hence improve the ionic transportation speed. This study provided direct evidence for understanding the role of CNTs/MnO2 in the lithiation process used in lithium ion batteries and also offers potential benefits for applications and development of supercapacitors.

Owing to their advantages of high power density, long lifecycle, and short charging time,1-4 supercapacitors have attracted considerable attention in recent years, and these materials can be used in wide ranges of energy applications, such as mobile electronic devices,5 hybrid electric transportation, etc.6 However, to meet the increasing demands for better electrochemical performance, improving the specific capacities of electrode materials is a crucial goal that has recently emerged. Compared with the 372 F/g specific capacity of conventional graphite material,7,8 transition metal oxides materials such as cobalt oxides,9,10 iron oxides,11,12 tin oxides,13,14 and titanium oxides,15,16 have greater reversible capacities due to the conversion reaction mechanism. Among these materials, MnO2 offers the advantages of low-cost and environmental compatibility as well as a theoretical capacitance value that can reach 1370 F/g in an aqueous electrolyte.17,18 Nevertheless, poor electron transportation between the MnO2 electrode and the aqueous electrolyte limit the conductivity. Accordingly, it is important to develop superior electrochemical performance, better electric conductivity, and larger reaction surface area. By combining MnO2 with conductive materials such as carbon nanotubes (CNTs)19-21 or graphene22,23 to form composite electrodes, the electrode performances and the specific capacity could be enhanced. CNTs were selected since their advantages, such as thermal stability, electric conductivity, structural strength, and elasticity; and the most important thing is that CNTs are more easily fabricated and higher e-beam radiation tolerance than graphene. Although many studies have investigated CNTs coated with transition metal oxides, few have reported direct observations and examination of the dynamic mechanism

responsible for the charging behavior of CNTs/MnO2, and the reactions between lithium and carbon nanotubes are unclear. Over the past few years, TEM has become one of the most powerful nanostructure-analysis instruments for the study of electrochemical reactions and growth kinetics.24-30 In this study, we successfully constructed a simple specimen to observe the lithiation progress in situ and noted several observations that were illustrated in the schematic diagrams. More importantly, we focused on the role of MnO2 in the supercapacitor. With the assistance of MnO2, we found that the notably large volume changes of the tube wall and the inner spacing of the CNTs/MnO2 were dramatically different with CNTs during lithiation. These observed phenomena provided us further motivation to study the related electrical field and how to prevent these types of materials from breakdown.

Experimental section Preparation of CNTs/MnO2 Composite Electrode Material In this study, the chosen electrode material was a CNTs/MnO2 composite for use in a supercapacitor. This material was synthesized using a redox titration method at room temperature. The CNTs were placed in a 0.005 M KMnO4 solution and dispersed by ultrasonic vibration for 30 min, and 0.016 M MnSO4 was subsequently added into the CNTs/KMnO4 solution in a drop-by-drop manner at room temperature. This solution was stirred for 6 h until the purple color disappeared. Finally, the composites were obtained by filtering the solution and were cleaned with DI water and dried in an oven at 110ºC for 12 h.

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Specimen Preparation for in-situ TEM Observation The CNTs/MnO2 composites were directly deposited onto 4 mm X 5.8 mm specimens via electrophoretic deposition (EPD). First, the CNTs/MnO2 composite powder was well dispersed in the solution by ultrasonic vibration. The specimen and platinum foil were placed in this solution at a distance of 1 cm to act as the cathode and anode electrodes, respectively. The CNTs/MnO2 composite was electrophoretically deposited on the electrode under a direct current (DC) voltage of 30 V for 15 seconds. After EPD, the composites formed on one side of the membrane electrode were dried in an oven at 80ºC for 12 h and served as the working electrode. The LiPF6 electrolyte was absorbed by a glass probe and transferred onto the other side of the membrane electrode. The overall schematics of the fabrication processes are presented in Figure S1. In-situ Electrical Biasing Holder for TEM observations The Protochips Aduro 300 in-situ TEM holder enhances the analytical capabilities of electron microscopes by providing the ability to conduct in-situ electrical analysis and to heat up to a temperature of 1200 °C while maintaining the maximum resolution of the instrument. The applying voltage of the Electrochemical sample was used by Keithley 2616A system. Transmission Electron Microscopy (TEM) High-resolution imaging and elemental analysis were performed using a Cs-corrected scanning transmission electron microscope (STEM, JEOL ARM 200F) with an energy dispersive spectrometer (EDS). Electrochemical Properties Measurement CH Instruments 618B electrochemical analyzer was used to measure electrochemical properties of electrodes, including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). All properties were measured at room temperature in a threeelectrode system, which consists of a working electrode, a Pt counter electrode and a SCE reference electrode. CV data were recorded at different scan rates (5, 20, 50, 100, 200 mV/s), and GCD data were recorded at varying current densities (1, 3, 5, 10 A/g) in a working potential window ranging from 0 to 1 V in 1 M Na2SO4 electrolyte. EIS data were recorded at different alternating current frequencies from 0.1 to 0.1M Hz.

Results and Discussion In this section, we describe the lithiation behavior in carbon nanotube coated with manganese oxide (CNTs/MnO2) composite material electrode as observed via transmission electron microscopy (TEM) during charging-discharging with colloidal electrolyte, which was 1.0 M LiPF6 in propylene carbonate. Figure 1(a) shows a schematic illustration of the experimental specimen. The CNTs/MnO2 composites were first deposited on one side of the membrane via electrophoresis to serve as the working electrode, and a glass probe was used to dip a tiny drop of the electrolyte (LiPF6) directly onto the other side. The LiPF6 is an inorganic compound that is used in commercial secondary batteries; therefore, we used it as the colloidal electrolyte to form a “Li battery” on our TEM specimens. After the in-situ TEM experiments, the LiPF6 still remained on the specimens under the vacuum environment, showing that LiPF6 was independent with vacuum and the source of lithium ions was sufficient.31 The more detailed schematic diagrams of the specimen fabrication processes are provided in supporting

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Figure 1. Schematic illustration of the sample configuration of the Charging-Discharging process in CNTs/MnO2 and the TEM image with EDS analysis. (a) Schematic diagram of in-situ TEM sample. Note the partially enlarged picture from the red frame. (b) The TEM image shows the carbon nanotubes coated with MnO2 (CNTs/MnO2). (C) The labels of the EDS spectra and table correspond to Figure 1(b). The table shows the atomic % of C, O, and Mn elements.

information Figure S1. The composite electrode (CNTs/MnO2) was fabricated using a redox titration method. The morphology of the CNTs/MnO2 is shown in Figure1(b), which demonstrates that the MnO2 particles were coated on the CNTs to roughen the outside surface. The atomic percentage relationships of the C, O and Mn in the composite electrode according to EDS analysis are shown in Figure 1(c). The C-V curves of CNTs/MnO2 at different scan rates are shown in Figure S2(a), and the rectangular shapes indicate the ideal electrochemical double-layer behavior. The charge-discharge curves with different current densities for the as-prepared electrode were obtained from 0 V to 1 V, as shown in Figure S2(b). Figure S2(c) presents the specific capacitance of CNTs/MnO2 as a function of scan rate calculated based on the C-V curves. The highest specific capacitance was 97 F/g, and approximately 53 % of the capacitance was retained as the scan rate increased from 5 mV/s to 200 mV/s. The in-situ TEM observations have been confirmed as immediate and effective methods for investigating the morphological and structural evolution of materials. In this work, we constructed a simple TEM sample using CNTs/MnO2 as the working electrodes to examine their structural changes and the influences of the MnO2 during lithiation. Above all, to rule out the properbility of electron beam effect, we let the chips expose under electron beam for 30 minutes. As this result shown in Figure S8, the morphology nearly preserved same as that at initial state. It can demonstrated that our experiment is indepenent from electron beam. Figure 2 shows a series of TEM and HRTEM images showing the lithiation reactions of CNTs/MnO2 during the charging process. As shown in Figure 2(a), the average diameter of the pristine MnO2 nanoparticles (NPs) is approximately 25 nm, and they are coated on the surface of CNTs. Moreover, the widths of the tube wall and the inner spacing as measured in the Gatan software are 4.86 and 3.60 nm in Figure 2(d), respectively. After the lithiation process, the MnO2 NPs had shrunk and vanished (Figure 2c

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Figure 3. Series of images showing the lithiation process of CNTs/MnO2. (a) The initial CNTs/MnO2 with a 10.74 nm tube wall and 8.41 nm inner spacing. (b) The solid electrolyte interface (SEI), which consisted of the Li2O layer, appears with a thickness of 2.1 nm. (c) As the lithiation process proceeds, the SEI layer becomes thicker to restrain the increment of the tube wall; however, the inner spacing continues to decrease.

Figure 2. Lithiation process and the HRTEM images of CNTs/MnO2 and the change rate. (a, b, c) The MnO2 particles (blue arrow) shrank and diminished during the lithiation process. (d, e, f) HRTEM images of the other CNTs/MnO2 sample. During lithiation process, the tube wall (black) increased and the inner spacing (red) decreased. (g) The amount of change (in percent) with time of the inner spacing and tube wall was measured from Figure (a-c) and Figure S3 at intervals of every 30 seconds. (h) The total diameter of the CNTs/MnO2 indicates that the volume expansion occurred mainly primarily in the internal CNTs.

and Movie S1), and furthermore, the width of the tube wall was increased to 5.83 nm, but the inner spacing was simultaneously decreased to 1.66 nm (Figure 2f and Movie S2). During this process, the lithium ions from the LiPF6 electrolyte reacted with the MnO2 NPs to form Li2O and Mn4+ ions, and therefore, the MnO2 NPs were diminished and faded away. At the same time, both of the Li+ and Mn4+ ions entered the interval of the tube wall to enlarge the width during the lithiation process. The presentative EELS spectra of the Li Kedge and Mn M-edge (the corresponding area indicated by a red dot in Figure 2f) was shown as the Figure S3(a). The MnO2 NPs caused the originally smooth CNTs surface to become rough and porous, which benefited the insertion of Li+ and Mn4+ ions. The wall interval of the carbon nanotubes possesses notably good ionic conductivity for transportation in the axial direction because CNTs have a cylindrical sp2bonded tube wall.32 The thin Li2O solid electrolyte interface (SEI) that formed on the surface of the CNTs acted as an ion channel that facilitated greater penetration into the tube wall spacing, which occurred more rapidly and easily.33 Interestingly, the results of CNTs without the MnO2 were distinct. The variation of the tube wall was 11.42 to 12.27 nm, and the change in the inner spacing was 9.23 to 8.52 nm (Figure S4). During the lithiation process, the CNTs undergo a smaller volume change because without the support of MnO2 NPs and the Li2O product, it is more difficult for Li+ ions to enter the tube wall through the surface of the CNTs. Figure 2(g) illustrates the change in percentage. The blue lines and the green lines indicate CNTs/MnO2 and CNTs, respectively. Due to either the inner spacing or the tube wall, the tendencies of the amount of change are larger in the CNTs/MnO2 sample. The different slopes of the trend line represent different rates

of change. The two-stage slopes of the blue lines show that the lithiation speed is more rapid at the initial stage of the lithiation process because MnO2 can actually enhance the ionic insertion. As the reaction proceeds, the CNTs/MnO2 become saturated, and the ionic insertion is slowed. However, the green line shows that CNTs displayed a slower rate and smaller amount of change since the lacks of MnO2, the efficiency of lithiation is lower and the saturation state is easily reached. As a result, the change of slopes of green curve is hard to be distinguished. Figure 2(h) shows that the total diameter of the CNTs/MnO2 was changed, which indicated that the CNTs undergo greater structural confinement to inhibit the outward expansion19,34 but extrude inward and affect the inner spacing. As mentioned previously, we found that a small amount of MnO2 (Figure S5a) could increase the lithiation rate and the insertion quantity of Li+ ions, which could actually enhance the charge-discharge rate and the electrochemical properties of the supercapacitors. However, if the amount of MnO2 is much larger (Figure S5b), the SEI layer becomes much thicker and smoother, which influenced the results of the lithiation process (Figure 3 and Movie S3).31,35-38 The initial CNTs/MnO2 have a 10.74 nm tube wall and a 8.41 nm inner spacing (Figure 3a). After 25 seconds, the SEI was clearly observed on the surface of the CNTs and consisted of Li2O with a thickness of 2.1 nm, whereas the tube wall grew to only 10.76 nm (Figure 3b) because the Li+ ions preferred to react with MnO2 to form a Li2O SEI layer first rather than insert into the tube wall. According to the lithiation process shown in Figure 3(c), the SEI layer evolved and became thicker, but the tube wall was maintained with a constant thickness of approximately 10.77 nm instead. The EELS spectra of charged CNTs/MnO2 material (indicated by a red dot in Figure 3c) displayed in Figure S3(b) also expressd the status at full charging. The reason for this phenomenon was that the MnO2 robbed the Li+ ions to restrain the increment of the tube wall, and the colloidal electrolyte on the in-situ TEM sample was tiny and limited such that the supply of Li+ ions from LiPF6 was finite, but in this sample, the amount of MnO2 was relatively adequate. Interestingly, the inner spacing continued to decrease during lithiation. The smooth and continuous SEI layer acted as an outer shell to confine the radial extension of CNTs such that the CNTs tended to compress the inner spacing to release the pressure of the volume expansion caused by Li2O growth.33 Therefore, the Li2O SEI layer as well as the CNTs give the supercapacitors better structural stability. We performed another similar experiment with a

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Figure 5. Schematic diagram for the lithiation mechanism in CNTs/MnO2. (a) The schematic diagram of the native multiwalled CNTs coated with rough MnO2. (b) At the beginning of the lithiation, the lithium reacts with the MnO2 to form a smooth Li2O oxidation layer and a portion of the reduction, i.e., manganese, inserts into the CNTs. (c) As the redox reaction between MnO2 and Li+ completes, the remaining Li+ also inserts into the interlayer of the CNTs to enlarge the tube wall spacing. (d) At the end of the lithiation process, the large amount of Mn and Li increases the wall spacing. Figure 4. CNTs lattice expansion ratio between CNTs and CNTs/MnO2 during lithiation behavior. (a) The schematic diagrams show the charging-recharging experimental installation. The CNTs and CNTs/MnO2 were first adhered on a nickel plate and subsequently placed in the LiPF6 electrolyte. After lithiation, the products were transferred to copper mesh for high-resolution TEM (HRTEM) observation. (b) The average wall spacing per layer of CNTs and CNTs/MnO2. (c, d) The single-wall spacing expansion of CNTs only after charging was 2.59%. (e, f) The single wall expansion of CNTs/MnO2 was 10.92%.

high-resolution transmission electron microscope (HRTEM), and the schematic diagram of the experimental setup is shown in Figure 4(a). First, the CNTs/MnO2 composite materials were deposited on a nickel sheet via electrophoretic deposition. Two Ni-plates were placed in the LiPF6 electrolyte to serve as the electrodes after applying a voltage of ±1 V with 21 cycles. Figure 4(c, d) shows the HRTEM images and dspacing before and after the cycle experiment on the CNTs, and the calculated average tube wall d-spacings are 0.3666 and 0.3761 nm, respectively. Similarly, Figure 4(e, f) also shows the HRTEM images before and after the cycle experiment on CNTs/MnO2 with average d-spacings of 0.3443 and 0.3819 nm. These average numbers were measured using 31 counts of each sample shown in Figure S6. The overall d-spacing and the amount of change are integrated and illustrated in Figure 4(b). The respective amounts of wall expansion for the CNTs and CNTs/MnO2 are 2.59 % and 10.92%, and these large differences indicated that the CNTs/MnO2 composites clearly enhance the lithiation efficiency and reinforce the properties of the supercapacitors. The large wall expansion of CNTs/MnO2 (12.1%) could also be measured from Figure 2(d-f), as shown in Figure S7. The increased spacing of the wall interlayers between the CNTs and CNTs/MnO2 are dramatically different according to the HRTEM data. In this section, we provide selected observations and illustrate the relevant theorems in the schematic diagram of Figure 5. The initial CNTs/MnO2 material is shown in Figure 5(a). First, the spacing of the CNTs and MnO2 ensures great ion transportation speed, which benefits the insertion of ions. Similarly, the other advantages of CNTs, such as notably good electronic conductivity and large surface area, also increased the reaction surface area to accelerate the lithiation process. The reaction of Li+ ions with

MnO2 to form Li2O and Mn4+ ions also occurs at the same time (Figure 5b). Moreover, discontinuous MnO2 can cause additional defects on the outer surface of the CNTs, serving as entrance vacancies that allow Li ions to insert into the interlayer of CNTs more rapidly and easily. The continuous Li2O grows outside of the CNTs and shapes into the SEI. Together with the consumption of MnO2, the remaining Li ions also insert into the interlayer of CNTs to enlarge the tube wall spacing (Figure 5c). Interestingly, the radii of Mn4+ and Li+ ion are 53 and 76 pm, respectively, and this slight difference was not the main reason for the lattice expansion of CNTs/MnO2 of 10.92 %, a value that is rather distinct from that of CNTs only (2.78 %). The main reason for the excessively different result is that the assistance of MnO2 serves as a coadjutant to accelerate the ionic intercalation. At the end of the lithiation process, the large amount of Mn4+ and Li+ ions increase the wall spacing (Figure 5d). From these observations, we speculate that MnO2 speeds up and increases the insertion of Li ions, which could further reinforce the capacitance value and the performance of the supercapacitors. In brief, carbon nanotubes have been widely used in electrode materials for lithium-ion batteries and provide outstanding electrical conductivity because the one-dimensional CNTs form a network structure. In addition, the mechanism of CNTs/MnO2 lithiation can be classified into three steps. Firstly, the reaction between Li+ ions and MnO2 formed the Li2O and Mn4+ ions. Secondly, the surface defects of CNTs caused by MnO2 allowed Li+ and Mn4+ penetrating through the tube wall easily. Finally, more Li+ and Mn4+ ions could insert into CNTs, which enhance the rate of lithiation. The CNTs/MnO2 composites have high endurance capacity, high rate capability, and notably good structural stability, and these advantages reinforce the supercapacitor performance.

Conclusions In this research, we successfully fabricated composite electrode materials (CNTs/MnO2) on TEM chips and observed their in-situ evolution. The results from in-situ TEM show the directly observed dynamic volume variation of CNTs and CNTs/MnO2 during the lithiation process and the dynamic mechanism of the lithiation process and also explain the major role of MnO2 in the composite electrode. Large volume

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changes in the tube wall and the inner spacing of the CNTs/MnO2 after lithiation were observed, and the lattice expansion measured from HRTEM images reached 10.92 %, which is notably different from the 2.59 % of CNTs only. This result proved that MnO2 actually enhances the lithium storage capacity and cycle rate. Our results indicate that the CNTs/MnO2 composites, which has incredible performance and properties, is a promising electrode material for supercapacitors.

ASSOCIATED CONTENT Supporting Information The schematic diagram of sample fabrication, the electrochemical properties, lithiation process of CNTs, the EDS spectra for CNTs/MnO2, the statistical diagrams of the average d-spacing distributions, and lattice expansion ratio provide evidence of direct observation of the lithiation process. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Tel: +886-3-5712121-55395

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the support by the Ministry of Science and Technology through grants 103-2221-E-009-222-MY3, 1042221-E-009-050-MY4, and 105-3113-E-009-002.

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