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Two-Dimensional Holey Co3O4 Nanosheets for High-Rate Alkali-Ion Batteries: from Rational Synthesis to In-Situ Probing Dahong Chen, Lele Peng, Yifei Yuan, Yue Zhu, Zhiwei Fang, Chunshuang Yan, Gang Chen, Reza Shahbazian-Yassar, Jun Lu, Khalil Amine, and Guihua Yu Nano Lett., Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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Two-Dimensional Holey Co3O4 Nanosheets for High-Rate Alkali-Ion Batteries: from Rational Synthesis to In-Situ Probing Dahong Chen,†,‡,# Lele Peng,†,# Yifei Yuan,¶,⊥ ,# Yue Zhu,† Zhiwei Fang,† Chunshuang Yan,†,‡ Gang Chen,‡ Reza Shahbazian-Yassar,⊥ Jun Lu,¶ Khalil Amine,¶ and Guihua Yu*,† †

Materials Science and Engineering Program and Department of Mechanical Engineering, The

University of Texas at Austin, Austin, Texas 78712, United States ‡

Department of Chemistry, Harbin Institute of Technology, Harbin, Heilongjiang 150001, P. R.

China ¶

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois

60439, United States ⊥

Mechanical and Industrial Engineering Department, University of Illinois at Chicago, Chicago,

Illinois 60607, United States

KEYWORDS: 2D holey assembly of nanoparticles; Co3O4; sodium-ion batteries; nanosheets; energy storage;

ACS Paragon Plus Environment

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ABSTRACT: A general template-directed strategy is developed for the controlled synthesis of two-dimensional (2D) assembly of Co3O4 nanoparticles (ACN) with unique holey architecture and tunable hole sizes that enable greatly improved alkali-ion storage properties (demonstrated for both Li and Na ion storage). The as-synthesized holey ACN with 10 nm holes exhibit excellent reversible capacities of 1324 mAh/g at 0.4 A/g and 566 mAh/g at 0.1 A/g, for Li and Na ion storage, respectively. The improved alkali-ion storage properties are attributed to the unique interconnected holey framework that enables efficient charge/mass transport as well as accommodates volume expansion. In-situ TEM characterization is employed to depict the structural evolution and further understand the structural stability of 2D holey ACN during the sodiation process. The results show that 2D holey ACN maintained the holey morphology at different sodiation stages because Co3O4 are converted to extremely small interconnected Co nanoparticles and these Co nanoparticles could be well dispersed in a Na2O matrix. These extremely small Co nanoparticles are interconnected to provide good electron pathway. In addition, 2D holey Co3O4 exhibits small volume expansion (~6%) compared to the conventional Co3O4 particles. The 2D holey nano-architecture represents a promising structural platform to address the restacking and accommodate the volume expansion of 2D nanosheets for superior alkali-ion storage.


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2D nanomaterials have attracted a great deal of attention due to the novel and intriguing properties brought by the atomic thickness and high aspect ratios.1,2 As an important research topic, 2D nanosheets materials have been explored as electrodes for energy storage.3-6 However, the irreversible restacking of the individual 2D nanosheets during materials processing or device fabrication is inevitable, leading to the decrease of active surfaces and longer diffusion lengths for charge storage.7 Many efforts have been devoted to alleviate the restacking problems of 2D nanosheets and to open up the unavailable surfaces.8,9,10 For instance, intercalation of various compounds, such as carbonaceous nanomaterials7,11,12, oxides13 and polymers14, into 2D nanosheets has been reported to open up the interlayer space for improved charge storage, while it involves complicated synthesis processes. Hierarchical structures via a solvothermal process or surfactant-directed synthesis15-17 have been shown useful for energy storage, but they have inferior energy densities because of the relatively low tap densities of the resulting structures. Therefore, designing novel 2D nanostructures to minimize the restacking problem still remains a challenge for more efficient energy storage. In the rich family of 2D nanomaterials, transition metal oxide (TMO) nanosheets show promising potentials in both fundamental research and technological applications due to their crystal structures, tunable chemical compositions, and a variety of material properties.18 Among these TMO materials, Co3O4 has attracted extensive interest in the fields of lithium ion batteries (LIBs) and sodium ion batteries (SIBs) research due to its high theoretical capacity.19-23 However, its large volume expansion/contraction associated with the alkali ion insertion and extraction process lead to electrode pulverization and loss of particle contact and, consequently, result in a large irreversible capacity loss and poor cycling stability.24 This disadvantage is more prominent in SIBs due to the large ionic radii of sodium compared with lithium.21,25 Various


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Co3O4 structures were investigated to overcome the volume expansion during electrochemical reactions in SIBs. For instance, sodium ion electrodes made of hybrid graphene networks exhibited delivered capacity of 525.3 mAh/g and 82.3 mAh/g at current rates of 25 mA/g and 500 mA/g, respectively26. Recently, Su et al. investigated the sodiation reaction mechanism of NiO nanosheets through the in-situ TEM observation and MD simulation.27 They proved that the sodiation process was blocked by a passivation layer of Na2O which formed at an early stage and resulted in sluggish kinetics. This may explain the low rate capability of transition metal oxides for sodium ion batteries. Here we develop a template-directed strategy for controlled synthesis of 2D transition metal oxide Co3O4 nanosheets with an unique holey architecture and tunable hole sizes by self-linking of oxide nanoparticles on graphene oxide templates. This unique holey structure can minimize the restacking of 2D nanosheets and provide more active sites for lithium and sodium ion storage. Due to the enhanced active surface/interface and the improved charge transport characteristics, 2D holey ACN demonstrate much improved overall electrochemical properties for both lithium and sodium ion storage, especially rate capability and cycling stability. Holey ACN with 10 nm holes (HACN-10) have exhibited excellent reversible capacities of 1324 mAh/g at 0.4 A/g and 566 mAh/g at 0.1 A/g, for Li and Na ion storage respectively. Moreover, the holey structure can sustain the volume expansion and prevent the pulverization during lithiation and sodiation. In-situ transmission electron microscopy (TEM) is employed to monitor the morphology evolution of the 2D holey ACN during sodiation process in real time. The results show that 2D holey ACN composed of chemically interconnected Co3O4 nanoparticles can maintain the holey nano-architecture and display minimal structural changes during the sodiation process, due to the excellent mechanical properties inherited from graphene oxide. Before and


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after sodiation, 2D holey ACN show very small volume expansion (6%), indicating that the unique holey structures can effectively buffer the volume change. The finds reported here show that these 2D holey transition metal oxide nanosheets are a promising structural platform to minimize the restacking and the volume expansion of 2D nanosheets, for improving their electrochemical characteristics as alkali-ion battery electrodes. The holey ACN are formed by conjugating Co3O4 nanocrystals into a free-standing 2D structure. Graphene oxide (GO) has a 2D structure modified with sufficient oxygen-containing functional groups, which promotes the growth of metal ion on its surface (Figure S1). During the refluxing process (experimental details in supporting information), cobalt ions were anchored on the GO nanosheets through residual functional group and formed the cobalt precursors integrated on GO as shown in Figure S2. During the calcination process, the cobalt precursors self-linked with each other to form the holey ACN due to the thin and highly flexible of GO template. The unique structure of the holey ACN satisfies several critical requirements for an ideal lithium and sodium ion battery electrode (Scheme 1). First, the nanosized Co3O4 particles in the 2D structure are highly interconnected together to prevent them from electrode pulverization and loss of particle contact. Maintaining the holey framework can further boost the specific surface area and increase the activated site between electrode and electrolyte.28 Second, compared with the nanoparticles, this 2D holey structure can provide direct paths that the electron can be transported continuously between the interconnected nanocrystals. The electrical conductivity of electrodes can be efficiently increased, which helps enable better rate capability for fast charging/discharging.29 Third, unlike the conventional 2D nanostructures with flat and smooth surface that are tend to restack and thus difficult to be accessed by electrolyte ions, holey ACN can be integrated into a hierarchical porous architecture to form the open channels for efficient


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ion transport throughout the entire electrode. Together, these combined features make it possible for 2D holey ACN to achieve both high specific capacity and excellent rate capability.

Scheme 1. Schematic illustration of the formation of 2D holey ACN and their advantages for ion transport.

The crystal structures of holey ACN before and after calcination are confirmed by XRD as displayed in Figure S3. After calcination, all of the diffraction peaks can be assigned to facecentered cubic phase Co3O4 (JCPDS No. 76-1802). The pore sizes and particle sizes of holey ACN can be facilely tuned by changing several parameters such as the annealing temperature, the refluxing time, and the amount of Co precursors etc. Holey ACN with different pore sizes reported here was synthesized by altering the amount of Co precursors. Figure 1a-c display the STEM images of holey ACN synthesized with 0.3 mmol, 0.5 mmol, and 5 mmol Co precursors. The as-prepared Co3O4 products are 2D holey nanosheets interconnected by nanocrystals. The typical length of nanosheets is between 500 nm to 1 µm. Figure S4 and 1c show the STEM


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images of ACN obtained with 0.3 and 5.0 mmol Co precursors, respectively. By increasing the amount of Co precursors, both the particle sizes and pore sizes increase, as shown in Figure 1e-f. The hole sizes of 2D holey ACN obtained with different amounts of Co precursors are verified by the BET characterization (Figure S5). The BET results show the average hole sizes of ~7, ~10, and ~20 nm for the HACN-5, HACN-10, and HACN-20 samples, respectively, which are consistent with the values obtained from the STEM images. The particle size of Co3O4 synthesized with 0.5 mmol cobalt precursors is nearly 9 nm and the pore size is ~ 10 nm. When the amount of precursor increases to 5 mmol, the particle size can reach nearly 45 nm. However, the pore size is only about 20 nm. The holey ACN with pore sizes of 5 nm, 10 nm, and 20 nm are marked as HACN-5, HACN-10, HACN-20, respectively, as shown in Table 1. The ratio of pore size to particle size (RPP) of HACN-10 is 1.11, while the RPP of HACN-5 and HACN-10 are both below 1. Holey 2D Co3O4 nansoheets with different RPP show different electrochemical characteristics for lithium and sodium ion storage. Figure 1d shows the high-resolution transmission electron microscopy (HR-TEM) image and electron diffraction (ED) pattern of HACN-10, demonstrating the single crystalline nature of the nanoparticles. A lattice spacing of 0.28 nm was observed, which is in good agreement with the inter-plane spacing of spinel Co3O4 (111) planes. The selected-area electron diffraction (SAED) pattern (inset of Figure 1d) shows well-defined diffraction rings, which correspond to the (220) and (331) planes, demonstrating that the nanosheets are polycrystalline with d-spacing consistent with the XRD results (Figure S3). In addition to Co3O4 shown above, this strategy can be also applied to the synthesis of other simple TMOs, such as Fe2O3, and Mn2O3, and mixed TMOs, such as ZnMn2O4, and NiCo2O4 (Figure S6).


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Figure 1. Morphological characterization of holey ACN (HACN): (a-c) STEM images of Co3O4 HACN-5, HACN-10 and HACN-20, respectively; (d) HRTEM image of HACN-10, inset is the ED pattern of HACN-10; Corresponding (e) pore size and (f) particle size distribution obtained by statistical analysis of the STEM images. The scale bars in a-c are 200 nm. The scale bar in d is 10 nm.

Table 1. Summary of HACNs with different pores HACN-5

HACN-10

HACN-20

Pore sizes (nm)

5

10

20

Particle size (nm)

8

9

45

Ratio of Pore size to Particle size

0.63

1.11

0.44

(RPP)

The lithium ion storage properties of holey ACN were further investigated. Figure 2a shows the cyclic voltammograms (CV) of HACN-10 electrode from 0.01 to 3 V at the scan rate of 0.05 mV/s. In spite of the first cycle, all peaks are stable and reproducible in the following cycles, implying that the HACN-10 exhibits good electrochemical reversibility. Typical charge–


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discharge voltage profiles at a current density of 0.4 A/g in the voltage range of 0.01–3 V versus Li+/Li are shown in Figure S7a. A long flat voltage plateau is clearly identified at ~ 0.9 V in the initial discharge process, and slightly shifts upward in the subsequent discharge cycle, which is consistent with the CV curves.30-32 To demonstrate the hypothesis that better electrochemical performance could be achieved by this unique holey ACN, Co3O4 nanoplates with no pores (Figure S8, Supporting Information) were used as control sample. The discharge capacities were measured at different current densities for holey ACN with different pores (Figure 2b). At the current density of 0.4 A/g, the discharge capacity of HACN-10 can reach nearly 1350 mAh/g, even at a high density of 3.2 A/g, the discharge capacity can deliver a capacity of ~ 550 mAh/g, which is far higher than the 159 mAh/g observed for the control Co3O4 nanoplates. The chargedischarge profiles of HACN-10 measured between 0.01 and 3 V at different current densities from 0.4 A/g to 5 A/g are presented in Figure S7b. Figure 2c highlights the cycling performance of holey ACN with respect to that of the control Co3O4 nanoplates, wherein HACN-10 shows the highest specific capacity and best cycling performance. After 200 cycles at a current density of 1 A/g, the specific capacity of HACN-5, HACN-10 and HACN-20 remained 1000, 450, and 400 mAh/g, respectively, whereas that of the control Co3O4 nanoplates with no pores was only 300 mAh/g. The cyclic stability and rate capability of the reported holey ACN are compared with the reported Co3O4-based anodes in Table S1.


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Figure 2. Electrochemical characterization of holey ACN for lithium ion battery: (a) CV curves of HACN-10 at a sweep rate of 0.05 mV/s; (b) Rate capabilities for the holey ACN with different pores; (c) Cycling performance of the holey ACN with different pores at 1 A/g.

The LIB performance of the HACN-10 was clearly superior to that of HACN-5 and HACN20. It should be noted that holey ACN with higher RPP presents higher electrochemical performance including the capacity, cyclic stability and rate performance. This phenomenon may be explained by the different volume-occupying rates of the different holey ACN after Li uptake.19,33 A smaller volume-occupying rate may cause the low volumetric specific capacity, whereas the larger volume-occupying rate may destroy the structure stability of electrode and result in the poor cycling performance. 10,11 In this study, the electrodes composed of HACN-10 with high RPP displays significantly better electrochemical performance than those made from


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control Co3O4 nanoplates. The possible reasons can be multi-faceted, and discussed as follows. First, the number of surface atoms would increase when decreasing the particle sizes, and as a result, the Co3O4 particles would be more active for the lithium electrochemical reaction. Second, this unique 2D holey nanostructure can lead to an increase in the electrolyte/Co3O4 contact area, a decrease in the effective diffusion distance for both lithium ions and electrons, and thus better rate capabilities. Third, during the Li ion insertion, the volume expansion factor of Co3O4, namely from Co3O4 to Co and Li2O, is 2.13.34,35 The pore structures with high RPP can improve the cycling performance by providing more spaces and buffering against the local volume change. Due to the higher radius of sodium ion (0.102 nm) than that of lithium ion (0.059 nm), SIBs exhibit more severe volume expansion and much lower kinetics in conversion reactions.36,37 The sodium storage behavior of holey ACN was also investigated. Figure 3a shows the CV of HACN-10 electrode from 0.01 to 3 V (vs Na+/Na) at the scan rate of 0.05 mV/s. In spite of the first cycle, all peaks are also stable and repeatable in the following cycles, showing good electrochemical reversibility of the HACN-10 as SIB electrodes. A sharp peak at ~ 0.55 V in the initial discharge process corresponds to the formation of SEI. The peak at ~0.65 V at the second and third cycles corresponds to the sodiation reaction of the HACN-10.26,38 The HACN-10 electrode delivers a charge and discharge capacity of 553 and 305 mAh/g, respectively, as shown in the charge/discharge curves (Figure S9). Figure 3b displays the discharge–charge voltage profiles of HACN-10 at current densities from 0.1 A/g to 1.6 A/g. The reversible capacity of HACN-10 varies from 566 mAh/g to 175 mAh/g at current rates of 0.1 A/g and 1.6 A/g, respectively. The control sample can only reach about 75 mAh/g at 0.8 A/g (blue dash line in Figure 3b). It can be seen that the HACN-10 electrode exhibits much better cycling performance


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than that of other samples. It is obvious that the HACN-10 possesses the highest sodium storage capacity, and the reversible capacity reaches nearly 300 mAh/g after 100 cycles at current density of 0.8 A/g, which is superior among the Co3O4 works reported (Table S1). For HACN-5 electrodes, the reversible capacity can maintain ~154 mAh/g after 100 cycles.

Figure 3. Electrochemical characterization of holey ACN for sodium ion battery: (a) CV curves of HACN-10 at a sweep rate of 0.05 mV/s for SIB; (b) Charge/discharge curves of HACN-10 at various current densities, the blue dash line is charge/discharge curve of the control sample at 0.8 A/g; (c) Cycling performance of holey ACN with different pores at 0.8 A/g for 100 cycles.

As discussed before, the improved capacity and cycle life of HACN-10 electrodes may be attributed to high RPP nature of nanosheets which is more accessible for electrolyte diffusion and insertion of sodium ions into the active phases. Meanwhile, such holey structures could


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efficiently buffer volume changes during the charge/discharge process. Furthermore, holey ACN with high crystallinity and structural stability would also help retain the pristine nanostructure and composition upon cycling (Figure S10). The high Na+ storage capacity, high rate capability and cycling stability of 2D holey ACN can be ascribed to the two-dimensional interconnected holey framework that provides large active surface areas, bicontinuous Na+ and electron pathways and excellent structural stability. To further understand the structural stability of 2D holey ACN, structural evolution during the sodiation process is carried out by in-situ TEM imaging on holey ACN. In-situ TEM is well known as a powerful technique in real-time study of reaction dynamics, and particularly in rechargeable alkali ion (Li+, Na+, etc.) batteries.39-42 The first-cycle sodiation process of 2D holey ACN was examined, as shown in Figure 4. In the nanobattery set-up, the conversion reaction between Co3O4 and inserted Na happens in the local interface of the Co3O4 and the Na/Na2O electrodes. The reaction products, i.e. Co nanoparticles embedded inside the Na2O nanocrystal matrix, form a low-contrast matrix on the 2D holey ACN (Figure 4a-h). As the sodiation time increases, the low-contrast region enlarges. As shown in Figure 4a-h, the red dash line shows the sodiation front (conversion phase boundary) at different sodiation time. During the sodiation process, 2D holey ACN maintained the holey morphology at different sodiation stages/ times, for example 4 mins, 15 mins, 45 mins and 65 mins. After full sodiation, the overall shape of the initial nanosheet can be preserved in 2D geometry, and the overall morphology remains as holey/porous structures because Co3O4 are converted to extremely small interconnected Co nanoparticles and these Co nanoparticles would be well dispersed in a Na2O matrix (Figure 4i and Figure S11). To be noted, these small Co nanoparticles are interconnected to provide good electron pathway. In addition to the maintained holey structures during cycling, 2D Co3O4 also


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exhibits very small volume expansion compared to conventional Co3O4 nanoparticles. As show in Figure 4b and 4h, the overall volume change of 2D holey ACN is only ~6% (assuming no expansion along the normal direction), which is much smaller than the theoretical values of Co3O4 (See supporting information for calculation details). These results show that 2D holy structure can greatly buffer the volume change during electrochemical processes.

Figure 4. In-situ sodiation process of 2D holey ACN: (a-h) typical morphological evolution of 2D holey ACN during the sodiation process; the red dash line shows the sodiation front


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(conversion phase boundary) and the red arrows in b and o show the volume expansion. (i) Diffraction pattern of the phase after full sodiation. The scale bar in a-h is 200 nm.

The sodiated holey ACN remained its overall morphology and holey nature when the ACN was under pressing during in-situ stress tests, as shown in Figure 5a-f. After first pressing and second pressing, the soliated ACN can return to the initial state. Such structural flexibility is of great importance for long cycling stability of battery electrodes, especially when conversionbased charge storage mechanism dominates the electrochemistry. The pressing experiments, in combination with the in-situ sodiation results, demonstrated that the 2D holey ACN showed the mechanically robust structures that can maintain the unique structure and buffer the volume expansion during sodiation process. These results were consistent with the afore-mentioned improved electrochemical characteristics for Li/Na ion storage.


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Figure 5. Morphological evolution under different pressing states: (a-c) morphology evolution in the first press and recover states; (d-f) morphology evolution in the second press and recover states. The scale bar in a-d is 200 nm.

In summary, we have demonstrated a general and scalable template-directed synthesis of 2D transition metal oxide nanosheets with unique holey architecture and tunable pore sizes. These novel structures were formed by thermally induced self-assembly of oxide nanoparticles on the graphene template. The as-prepared holey ACN with 10 nm pore size exhibited high specific capacities and excellent cycling performance when evaluated as lithium and sodium ion battery anode materials. The superior cyclic stability and rate capacity can be attributed to the small diffusion lengths in nanoparticle building blocks and sufficient void space to buffer the volume expansion. We anticipate that the approach presented in this work could be extended to the fabrication of other holey nanosheet structures, and the holey nanosheet material platform can open up other promising applications in other electronics and electrochemical energy devices.

ASSOCIATED CONTENT Supporting Information. Experimental details, XRD patterns, SEM and STEM images, EIS spectra, and additional electrochemical characterizations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(G.Y.) E-mail: [email protected].


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Author Contributions # D.C., L.P., and Y.Y. contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT G.Y. acknowledges the funding support from the Welch Foundation Grant F-1861, ACS-PRF award (55884-DNI10), and Sloan Research Fellowship. . J.L. and K.A. were supported by the U.S. Department of Energy under Contract DE-AC0206CH11357 with the main support provided by the Vehicle Technologies Office, Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE). R. Shahbazian-Yassar acknowledge the NSF CMMI1619743 to utilize the in-situ TEM facility in order to perform this work. R. Shahbazian-Yassar and Y. Yuan acknowledge NSF DMR-1620901 for the TEM characterization services, and Argonne National Laboratory award No. 4J-30361. REFERENCES (1) Bonaccorso, F.; Colombo, L.; Yu, G.; Stoller, M.; Tozzini, V.; Ferrari, A. C.; Ruoff, R. S.; Pellegrini, V. Science 2015, 347, 41. (2) Peng, L.; Zhu, Y.; Li, H.; Yu, G. Small, 2016, 12, 6183−6199. (3) Guo, Y.; Xu, K.; Wu, C.; Zhao, J.; Xie, Y. Chem. Soc. Rev. 2015, 44, 637−646. (4) Zhang, Q.; Wang, Y.; Seh, Z. W.; Fu, Z.; Zhang, R.; Cui, Y. Nano Lett. 2015, 15, 3780−3786.


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