In Situ TEM Observation of the Electrochemical Process of Individual

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In Situ TEM Observation of the Electrochemical Process of Individual CeO2/Graphene Anode for Lithium Ion Battery Qingmei Su,† Ling Chang,‡ Jun Zhang,‡ Gaohui Du,*,†,‡ and Bingshe Xu*,† †

College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China

‡

ABSTRACT: The reaction mechanism of ceria as an anode in a lithium ion battery (LIB) is unknown. To solve this issue, a nano-LIB was constructed inside a transmission electron microscope (TEM) using an individual CeO2/graphene composite as the anode. The lithiation/delithiation cycles of the CeO2/graphene composite were conducted inside the TEM, and the electrochemical process was in situ monitored by simultaneous determination of the microstructure with high-resolution TEM, electron diffraction, and electron energy loss spectroscopy. The surfaces of the graphene nanosheets and ceria nanoparticles were covered by a nanocrystalline Li2O layer after lithiation, and the Li2O layer shrank and showed partially reversible changes after delithiation. The CeO2 nanoparticles showed imperceptible volumetric and morphological changes, while comprehensive analysis revealed a fully reversible phase transformation between fluorite CeO2 and cubic Ce2O3 during the electrochemical process. These results give direct evidence and profound insights into the lithiation/delithiation mechanism of CeO2/graphene anode in a LIB.

1. INTRODUCTION Lithium ion batteries (LIBs) have attracted extensive attention as one of the most important energy storage devices.1−3 In order to meet the ever-growing need for high capacity and high power, various metal oxides for LIBs have been widely investigated due to their low cost, high theoretical capacity (>500 mAh/g), and environmental friendliness.4−6 However, the main challenges preventing the implementation of metal oxide electrodes are their large volume change and poor electronic conductivity that occur during the charge/discharge process in practical LIBs. The strategy is to incorporate nanostructure carbon based materials with the metal oxides to conquer these obstacles and achieve good cyclability as well as to enhance the electrochemical performance.7 Recently, graphene-based composites with metal oxides, such as Co3O4,8 MnO2,9 NiO,10 Fe3O4,11 and TiO2,12 have been fabricated as anode materials for rechargeable LIBs. These composites showed high reversible capacity, long cycle life, and good rate performance owing to their outstanding combined electrochemical and physical properties between heterogeneous components. Moreover, the graphene can act as a buffer to accommodate the volume change of metal oxides and keep the electron transport active.13 Typical techniques to evaluate the electrochemical performances of anode materials involve the coin cell assembly, cyclic voltammetry, and galvanostatic charge/discharge measurements. These traditional methods can provide the key parameters for the electrode materials, e.g., reversible capacity, cycle life, and rate performance. However, they lack the ability to reveal the electrochemical process and the lithiation © 2013 American Chemical Society

mechanism, such as the volume expansion, the formation of cracking and fracture, and the influence of structural change on the capacity fading and failure of batteries.14 A profound understanding of the structure evolution of electrode materials during charge/discharge cycling is quite important, but the ex situ methods cannot solve these issues due to the dynamic nature of the process. Recently, in situ methods based on nuclear magnetic resonance spectroscopy,15 Raman spectroscopy,16 X-ray diffraction,17 and scanning electron microscopy18 have provided some useful information regarding the structural evolution of the electrode materials. Compared with these techniques, in situ transmission electron microscopy (TEM) and associated spectroscopic techniques have the unique merit of providing direct insights into the electrochemical process, and have the ability to investigate the microstructure evolution of the electrode materials at high spatial resolution. To build a nano-LIB inside a TEM, allowing for real time and atomic scale observation of battery lithiation and delithiation processes, two types of designs have been demonstrated: one was based on ionic liquid electrolytes19−22 and the other was based on all solid components,23−25 involving a solid-state lithium oxide (Li2O) electrolyte and a Li metal as the counter electrode. CeO2 has a fluorite structure, and the oxidation state of cerium can mutate quickly and expediently between Ce(III) and Ce(IV), leading to its wide application in solid-state fuel cells, oxygen storage materials, superconductors, and so on. Received: December 10, 2012 Revised: February 7, 2013 Published: February 8, 2013 4292

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3. RESULTS AND DISCUSSION A schematic illustration of our experiment setup of the nanoLIB device constructed inside the TEM is shown in Figure1a.

CeO2 is also one of the excellent candidates for metal oxide anode materials in LIBs.26 As for many metal oxide anodes, their reaction mechanisms have been proposed. Taking CuO and Co3O4 for examples, we know the oxidation states of the metal cations reduce to zero from the highest value after electrochemical reaction with Li. However, the basic electrochemical reaction of CeO2 in LIBs is still unknown. Supposing the electrochemical process of CeO2 is similar to that of these metal oxides, the lithiation/delithiation reactions would involve a reversible transformation of 4e− + CeO2 + 4Li+ ↔ Ce + 2Li 2O

Certainly, there are other possible mechanisms, e.g., a reversible transformation between CeO2 and Ce2O3. Thus, it would be significant to reveal the lithiation/delithiation process using new in situ techniques. In this paper, we constructed an all-solid nano-LIB using an individual CeO2/graphene nanocomposite as anode inside a TEM, and we directly observed the electrochemical processes for the first time. The evolution of CeO2 nanoparticles was monitored by simultaneous determination of the microstructure with high-resolution TEM (HRTEM), electron diffraction (ED), and electron energy loss spectroscopy (EELS). An in-depth understanding of the electrochemical process of CeO2/graphene nanocomposite in LIB has been achieved.

Figure 1. (a) Schematic illustration of the in situ experimental setup and (b) corresponding TEM image of the nano-LIB constructed inside TEM.

The CeO2/graphene composite was attached to a gold wire and used as the working electrode. A tungsten probe was used to scratch Li metal inside a glovebox filled with argon; the Li metal attached to the tungsten wire served as the counter electrode. During the sample loading process, the exposure of the Li metal to air led to the natural formation of Li2O layer on the surface of the Li metal, which was used as a solid electrolyte allowing Li+ transport. Figure 1b shows the corresponding TEM image of the nano-LIB. The Li2O/Li electrode sitting on the mobile STM probe was driven to contact the CeO2/graphene electrode. Once a contact was established, a potential of −2 V was applied to the CeO2/graphene with respect to the Li counter electrode to drive the flow of electrons and Li+ ions across the circuit to initiate the lithiation, and the bias was reversed to +2 V to facilitate the delithiation. The morphology evolution of CeO2/graphene by lithium insertion is shown in Figure 2. Figure 2a is a TEM image of the pristine CeO2/graphene anode before contacting with the Li2O solid electrolyte; the initial width of the graphene sheet was 236 nm (indicated with arrowheads). Figure 2b shows the TEM image of the CeO2/graphene after 15 min of lithiation. It was found that the width of the graphene tip increased to 279 nm, corresponding to a 17.4% expansion. It has been reported that the spacing of the (101̅0) plane in graphene remained unchanged after lithiation.25 We infer that the observed lateral size change was not a real lattice expansion but a result of tilting and expanding from corrugated graphene to a flat one due to the large stress caused by the lithium intercalation into the (0002) planes. The experiments were conducted with the electron beam blanked except for short time imaging, so the electron beam irradiation effect during the lithiation process could be excluded. From Figure 2c,d, it can be seen clearly that the surfaces and edges of the CeO2/graphene were coated by a uniform layer of crystallites with a thickness of about 10 nm, which were identified to be Li2O by the comparison of electron diffraction patterns (EDPs) of pristine and lithiated CeO2/

2. EXPERIMENTAL SECTION Graphene nanosheets were prepared via the H2 reduction of exfoliated graphite oxide materials at 400 °C for 1 h. To synthesize the CeO2/graphene composites, graphene (0.2 g), Ce(NO3)·6H2O (0.15 g), CH3COONa (5.0 g), and urea (0.3 g) were add to 30 mL of deionized water. After sonication for 30 min, the mixture was heated using a microwave oven at a power of 500 W for 15 min and then cooled to room temperature naturally. Subsequently, the products were filtered, washed with distilled water and ethanol several times, and dried at 60 °C for 12 h in an oven. The in situ electrochemical experiments were conducted using the Nanofactory TEM-STM holder in a JEOL JEM2100F TEM. The TEM-STM holder integrates a fully functional STM probe. Once the holder is inserted into the TEM column, the STM probe can be driven by the piezopositioner to approach the other electrode at fine steps. The electrochemical nano-LIB device consisted of a CeO2/graphene electrode, a layer of Li2O solid electrolyte, and a bulk Li metal counter electrode inside the TEM to conduct real-time observation of the structural evolution of the CeO2/graphene during the electrochemical reaction. To minimize the electron beam irradiation effect during the reaction, the experiments were conducted with the electron beam blanked except for short time beam exposure for imaging. The EELS measurement was performed in the image mode using a Gatan Enfina parellel electron energy loss spectrometer attached to the JEM-2100F TEM. We used the intensity ratio of the Ce M4,5 white lines (M5/M4 ratio) to judge the oxidation state changes of cerium atoms, which was calculated by integrating the background subtracted intensities for both M5 and M4 edges and then dividing to get the ratio. The backgrounds were subtracted using the standard AE−r model. 4293

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Figure 2. Electrochemical lithiation of CeO2/graphene anode. (a) Pristine CeO2/graphene. (b) Lithiated CeO2/graphene. (c, d) High-magnification TEM images showing the lithiated CeO2/graphene covered with a uniform Li2O layer. The black line marks the profile of the graphene (d). (e, f) EDPs of (e) pristine and (f) lithiated CeO2/graphene. (g, h) EELS spectra of (g) pristine and (h) lithiated CeO2/graphene.

CeO2/graphene corresponds to the π* + σ* mode of graphite. In comparison, the plasmon peaks of the lithiated CeO2/ graphene shifted to 13.2 and 20.5 eV, which was mainly caused by the formation of Li2O and the intercalation of Li+ into the graphene sheet. Obviously, two sharp peaks at 61.6 and 66.6 eV appeared in the EELS spectrum of the lithiated CeO2/ graphene, which were attributed to the Li K edge peaks. The EELS results are consistent with the EDPs and HRTEM analysis. Theoretically, it has been proposed that Li+ ions can be adsorbed on both sides of the graphene, leading to two layers of lithium for each graphene sheet, with a theoretical capacity of 744 mAh/g through the formation of Li2C6.28 The Li2O formation around ceria particles is interpretable by the reaction

graphene (Figure 2e,f). A notable phenomenon is that there was no obvious morphological change observed for the CeO2 particles on the graphene sheet after the lithiation (Figure 2a,b). After comparing the EDPs in Figure 2e,f, we found CeO2 nanoparticles completely transformed to Ce2O3 nanoparticles after the lithiation. The EDP in Figure 2e could be well indexed to the fluorite CeO2 with the lattice constant of a = 5.41 Å (JCPDS No. 81-0792), and the EDP in Figure 2f could be perfectly indexed as two phases containing cubic Ce2O3 with a lattice constant of a = 11.41 Å27 (ICSD No. 96202) and cubic Li2O with a lattice constant of a = 4.62 Å (JCPDS No. 772144). The change in the EDPs indicates the formation of Li2O and Ce2O3 crystals through the electrochemical reaction of CeO2/graphene with Li. Prolonged lithiation did not lead to further changes in morphology or diffraction patterns, which revealed that the nature of the lithiation process of the CeO2 anode in LIB was an electrochemical reduction to Ce2O3. The low-loss EELS regions of the pristine and lithiated CeO2/ graphene composites are shown in parts g and h, respectively, of Figure 2. The plasmon peak at 25.6 eV from the pristine

2CeO2 + 2Li → Ce2O3 + Li 2O

while the Li2O on the graphene sheet seems to be controversial. Similar in situ experiments on carbon nanotubes24 and graphene nanoribbons25 also revealed the formation of Li2O, and the authors proposed that it is an intrinsic property of the 4294

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Figure 3. Microstructure evolution of CeO2/graphene electrode in the lithiation and delithiation processes during the first two cycles. (a) Prisitine CeO2/graphene composite. (b) First lithiated and (c) first delithiated CeO2/graphene. (d) Second lithiated and (e) second delithiated CeO2/ graphene. (a1−e1) HRTEM images of the composite in (a)−(e), and the insets are enlarged HRTEM images. (a2−e2) FFT images corresponding to the enlarged HRTEM images in (a1)−(e1). (a3−e3) EDPs of the electrode in (a)−(e) revealing the reversible transformation between CeO2 and Ce2O3.

composite is shown in Figure 3. Figure 3a is a TEM image of the pristine CeO2/graphene, revealing that the CeO2 nanoparticles with diameters of 10−40 nm were dispersed randomly on the basal plane of the few-layer graphene nanosheet. The HRTEM image (the inset in Figure 3a1) comprises perfect lattice fringes and indicates the crystalline nature of the nanoparticles. Figure 3a2 shows the corresponding fast Fourier transform (FFT) image of the HRTEM, revealing that the nanoparticles were fluorite CeO2. After a contact was established, a −2 V potential was applied to the CeO2/ graphene to initiate the first lithiation. As the lithiation process proceeded, a thin layer of Li2O crystallites was formed on the graphene sheet (Figure 3b). The delithiation process was initiated by reversing the polarity of the bias (i.e., applying a +2 V potential to the CeO2/graphene), and the Li2O layers gradually shrank and almost disappeared after full delithiation (Figure 3c). However, a thin layer of Li2O was left on the surface of the graphene after delithiation, which could be due to the formation of a stable solid electrolyte interphase (SEI)

interaction between lithium and exposed graphite basal planes. In this work, we suggest that the formation of Li2O could be partially attributed to the electrochemical reaction of Li with the residual oxidic groups (carboxylic acid group, epoxy group, hydroxyl group, etc.) on graphene, which were introduced during the preparation of graphite oxide through an improved Hummer method29 and could not be completely eliminated in the reduction process. In real batteries, other Li-containing compounds such as carbonates and alkylates could be formed due to the decomposition of electrolyte.30 More importantly, the Li2O layer on the graphene sheet was found to shrink during the subsequent delithiation process and to show reversible changes during lithiation/delithiation cycles (Figure 3), further supporting that the Li2O was an inherent product from the electrochemical reaction between the graphene sheet and Li. To further reveal the reaction mechanism, two cycles of lithiation and delithiation processes were conducted inside the TEM, and the structure evolution of the CeO2/graphene 4295

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membrane. The remains of Li2O crystallites can account for the loss of capacity in the first cycle due to irreversibility. The second lithiation process proceeded with the −2 V potential again (Figure 3d), and the Li2O layer thickened and continued to cover the graphene surface like the first lithiation. The Li2O layer reduced again in the second delithiation process (Figure 3e), showing the reversible change. Figure 3b1−e1 depicts the microstructure evolution of the composite in the two sequential lithiation/delithiation cycles, and the insets are the HRTEM images of the nanoparticles. The corresponding FFT images are shown in Figure 3b2−e2, and these FFT images reveal that the reversible phase transformation between CeO2 and Ce2O3 occurred during the cycling. The reversible phase transformation in the electrochemical reaction was further demonstrated by the EDPs shown in Figure 3a3−e3. These in situ TEM results display that the lithiation and delithiation of CeO2/graphene composite was irreversible in the first cycle due to the formation of a thin stable Li2O layer (or SEI layer). However, the transformation between CeO2 and Ce2O3 phases remains reversible during all the cycles. The graphene sheet suffered from large stress and strain during the lithiation and delithiation process, and we found that the tip of the graphene sheet could crack after several cycles. The EELS technique is ideally suited for studying core-loss edges below the energy loss of 2 keV. The M4,5 white lines of Ce reflect the transitions of 3d core electrons to unoccupied states of p- and f-like symmetries. The sharp M5 and M4 peaks near the edge onsets arise from quasi-atomic, dipole-allowed transitions from an initial state of the form 3d104fn to final 3d94fn+1 states. The M4,5 edges are suitable for studying Ce because they exhibit distinct valence-specific shapes. Here, further insights on the evolution of the ceria nanoparticles in the lithiation and delithiation process during the first two cycles can be obtained from the inspection of the corresponding EELS spectra shown in Figure 4. The IM5/IM4 intensity ratio of Ce at initial state was 0.65, corresponding to the valence state of 4+ (Figure 4a). A reversal in the intensity of Ce M4,5 white lines was observed in Figure 4b. The IM5/IM4 ratio increased to 0.80 in Figure 4b after first lithiation process of the CeO2/graphene composite. The changes in the relative intensity of the M4,5 edge peaks indicated a transformation of the Ce oxidation state from 4+ to 3+.31,32 Figure 4c show the Ce M4,5 edge peaks of the composite after the first delithiation process, and the IM5/ IM4 intensity ratio reversed to 0.64. In the second cycle of lithiation and delithiation process, a similar reversal of the IM5/ IM4 intensity ratio was also observed. The repeated changes in the intensity ratio suggest that Ce underwent a reversible transition between Ce4+ and Ce3+ in the lithiation/delithiation process. Moreover, the shoulder peaks (indicated by arrowheads in Figure 4a) suggest the strong covalent hybridization between the Ce 4f and O 2p states, and are a feature of Ce4+. The shoulder peaks disappear after lithiation and reappear after delithiation, further confirming the reversible transition between Ce4+ and Ce3+ in the lithiation/delithiation process. Combining with the HRTEM and ED analysis in Figure 3, we can conclude that the lithiation and delithiation process of ceria is a reversible phase transformation between CeO2 and Ce2O3. The electrochemical process of CeO2 can be expressed as the reaction

Figure 4. EELS spectra of Ce M4, 5 edges in the lithiation and delithiation processes during the first two lithiation/delithiation cycles. (a) Initial stage; (b, c) after the first (b) lithiation and (c) delithiation process; (d, e) after the second (d) lithiation and (e) delithiation process.

It is noticeable that CeO2 nanoparticles showed imperceptible volumetric and morphological changes after the transformation to the Ce2O3 phase. Based on the theory calculation from their crystal structures, a volume increase of 17.18% accompanied the lithiation process since Ce2O3 phase has a lower density than CeO2. The mean size change in radial direction would be about 5.42%, supposing the volumetric expansion is isotropic. Since the ceria nanoparticles were 10− 40 nm in diameter, the radial changes would be in the range of 0.5−2 nm. The real measured changes from the TEM images were anisotropic but basically agreed with the calculation. Therefore, the real volumetric changes for the dynamic phase transformation between CeO2 and Ce3O2 were too small to be visually observed from the TEM images. Recently, a reversible phase transformation of ceria taking place at 1003 K in a hydrogen atmosphere has been revealed by using in situ environmental TEM;32 the reduction reaction under electron beam illumination33 or an electrical field34 was also investigated using in situ TEM. All the results indicated that the reduction process of ceria was involved with the introduction of oxygen vacancies and structural reconstruction. Our electrochemical redox process of CeO2 during lithiation and delithiation is similar to that at high temperature. During the lithiation process, some oxygen anions bonded with Li+ to form Li2O on the nanoparticle surface, leading to the formation of oxygen vacancies; simultaneously the electrons transferred from the electrode to adjacent Ce cations around the vacancies, reducing the oxidation state of Ce from 4+ to 3+ (as proved by the EELS measurements). With the increase of oxygen vacancy concentration, CeO2 gradually transformed to Ce2O3. Vice versa, during the delithiation process, the decomposition of

2CeO2 + 2Li+ + 2e− ↔ Ce2O3 + Li 2O 4296

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Li2O leads to the formation of oxygen anions; accordingly, the vacancies were recovered and the extra electrons were transferred to the electrode from Ce sites. Therefore, the Ce oxidation state reverted to 4+, and the Ce2O3 transformed to CeO2. The reversible structure and valence state changes during the lithiation/delithiation process have been confirmed by our HRTEM and EELS measurements.

4. CONCLUSIONS In summary, the electrochemical lithiation and delithiation cycles of CeO2/graphene were studied for the first time by constructing a nano-LIB using CeO2/graphene and metal Li as two electrodes inside the TEM. The electrochemical process of CeO2/graphene was monitored by simultaneous determination of the structure with HRTEM and ED and of the chemistry with EELS. The results revealed that a Li2O layer formed on the surface of the CeO2 nanoparticles and the graphene sheet during the lithiation. After the full delithiation process, most of the Li2O disappeared. The remains of a thin Li2O layer indicate the possible formation of a stable SEI layer on the electrode, accounting for the capacity loss in the first cycle. During the electrochemical cycles, the dispersed CeO2 nanoparticles showed imperceptible volumetric and morphological changes. However, the comprehensive analysis revealed that these nanoparticles underwent a fully reversible phase transformation between fluorite CeO2 and cubic Ce2O3 during the electrochemical process. The results provide direct evidence and a profound understanding of the reaction mechanism governing the CeO2/graphene anode performance in LIBs.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 86-579-82282234 (G.D.). E-mail: [email protected] (G.D.); [email protected] (B.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Nos. 10904129 and 21203168), and the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-11-1081).

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp312169j | J. Phys. Chem. C 2013, 117, 4292−4298