Ultrathin Hexagonal 2D Co2GeO4 Nanosheets: Excellent Li-Storage

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Ultrathin Hexagonal 2D Co2GeO4 Nanosheets: Excellent Li-Storage Performance and Ex Situ Investigation of Electrochemical Mechanism Shuaixing Jin, Gongzheng Yang, Huawei Song, Hao Cui, and Chengxin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08446 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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ACS Applied Materials & Interfaces

Ultrathin Hexagonal 2D Co2GeO4 Nanosheets: Excellent Li-Storage Performance and Ex Situ Investigation of Electrochemical Mechanism Shuaixing Jin,†,‡,# Gongzheng Yang,†,‡,# Huawei Song,†,‡,# Hao Cui,†,‡,# and Chengxin Wang*,†,‡,# †

The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, ‡State Key Laboratory of Optoelectronic Materials and Technologies, #School of Physics Science and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China Abstract Two-dimensional nanostructures are desirable configuration for lithium ion battery electrode due to their large open surface and short pathway for lithium ions. Therefore, exploring new anode materials with 2D structure could be a promising direction to develop high-performance LIBs. Herein, we synthesized a new type of 2D Ge-based double metal oxides for lithium storage. Ultrathin hexagonal Co2GeO4 nanosheets with nanochannels are prepared by simple hydrothermal method. When used as LIB anode, the sample delivers excellent cyclability and rate capability. A highly stable capacity of 1026 mAhg-1 was recorded after 150 cycles. Detailed morphology and phase evolutions were detected by TEM and EELS measurements. It is found Co2GeO4 decomposed into Ge NPs which are evenly dispersed in amorphous Co/Li2O matrix during the cycling process. Interestingly, the in-situ formed Co matrix could serve as a conductive network for electrochemical process of Ge. Moreover, aggregations of Ge NPs could be restricted by the ultrathin configuration and Co/Li2O skeleton, leading to unique structure stability. Hence, the large surface areas, ultrathin thickness and the atomically metal matrix finally bring the superior electrochemical performance.

KEYWORDS: 2D materials, Co2GeO4, lithium storage mechanism, Ge, double metal oxides *Corresponding author: Fax: +86-20-8411-3901; e-mail: [email protected]

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Introduction Recently, two dimensional (2D) nanostructures have gained great interests for their potential applications in electronics1-3, optoelectronics4-6 and especially, energy devices7-9. As an important energy storage device, high performance lithium ion battery (LIB) needs electrode materials with well-designed configuration. The large open surface and nano-sized thickness of 2D nanostructures could provide abundant reaction activity sites as well as short diffusion pathway for Li+, which is essential factor for high power density LIB10-12. In addition, a most recent report shows 2D structure could efficiently prevent the aggregation of nanograins emerging upon lithiation/delithiation process, suggesting 2D nanostructures also have an advantage to realize high cyclability electrode13. Germanium (Ge) is a typical Li-alloying material with high theoretical capacity (1624 mAhg-1), which could be a promising candidate to commercial graphite14. It reacts with Li by the following equation: Ge + 4.4Li ↔ Li4.4Ge

(1)

Currently, the practical application of Ge has still been hampered by several obstacles, for instance, capacity fading caused by volume expansion, poor rate performance related to low electrical conductivity and lithium diffusivity14-16. Inspired by the unique properties of 2D nanostructures, we can anticipate the electrochemical performance of Ge could be improved by utilizing 2D-structured Ge anode. However, so far the mostly reported 2D nanosheets are graphene or graphene-like materials which have layered structure. In contrast, due to the three-dimensional close-packed structure of Ge, it tends to stack into 3D nanostructure rather than 2D sheet17-18. Thus it still remains a challenge to prepare Ge or Ge-based 2D materials. Compared with elemental Ge, Ge-based double metal oxides (DMOs) could exhibit more microstructure changes due to incorporating of the second metal, which provides new direction to prepare 2D Ge-based electrodes. So far, several Ge-based DMOs with different morphologies have been studied, for example, Fe2GeO4 nanoparticles19, CuGeO3 nanowires20, Zn2GeO4 nanorods21, Co2GeO4 nanoplates22, et, al. Besides the morphology diversity, the introduction of the second metal could bring 2


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synergetic effects for anode performance. For instance, Li et al. studied Ca2Ge7O16 anode and found the formed Ge nanoparticles are highly dispersed in the CaO matrix, which provides an elastic medium to buffer volume changes23. Kim et al. studied the role of the in-situ generated Cu nanoparticles in CuGeO3 electrode and found the Cu NPs could improve electrical conduction of Ge NPs24. However, neither Ca nor Cu is electrochemical activity material, which means capacity of the anodes will sacrifice in some extent. Zhang et al. reported electrochemical performance of Co2GeO4 nanoplates for the first time and obtained remarkable results25. However, lithium storage mechanism and morphology evolutions upon lithiation/delithiation remains unclear yet, thus, the lithium storage process of 2D Co2GeO4 nanostructure still need more researches. In this paper, we prepared ultrathin hexagonal Co2GeO4 nanosheets, and in detail studied their performance in LIB anode. The sample shows a thickness less than 10 nm and contains numerous nanochannels, which could provide sufficient space to accommodate volume expansion during the lithium insertion process. Electrochemical tests show Co2GeO4 could deliver a highly stable capacity of 1026 mAhg-1 for 150 cycles. Morphology and phase evolutions of Co2GeO4 nanosheets are in detail investigated by TEM and EELS to understand the lithium storage mechanism. It was found Co2GeO4 directly decomposed into Ge nanoparticles which are dispersed in amorphous Co/Li2O matrix during the first discharge process. Upon the subsequent cycling, since the voltage window for Ge alloying/dealloying with Li+ is below that of Co reduction/reoxidation, the elemental Co matrix forms a highly conductive network for the electrochemical reaction of Ge, which could intrinsically enhance the anode kinetics. Additionally, aggregations of Ge NPs are restricted by the ultrathin configuration as well as the Co/Li2O matrix, leading to excellent structure stability. Therefore, our results reveal the 2D Co2GeO4 anode can be a promising candidate for LIB. Results and discussions The sample synthesis includes two steps. Firstly, precursor nanosheets, denoted as Co2GeO4·nH2O, are prepared by hydrothermal methods with CoCl2·6H2O and 3



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GeO2 as raw materials. After being annealed in air at 500 oC for 6 h, ultrathin Co2GeO4 nanosheets are obtained. Morphologies of the pristine nanosheets are shown in Figure 1. As displayed by SEM image (Figure 1(a)), the nanosheets exhibit regular edges and most of them have hexagonal outlines. TEM image (Figure 1(b)) reveals that the sample has smooth surface. It should be noted that the outlines of the nanosheets which are partially covered by the uppers could still be observed, suggesting thickness of the nanosheets is very small. Length of the sample is measured to be ~150 nm, as Figure 1 (c) shows. High resolution TEM (HRTEM) displays well-defined lattice fringes, indicating the precursor sample possesses single crystalline structure, which is consistent with the corresponding selected area electron diffraction (SAED) result (inset image). To verify the sample phase, we measured the XRD pattern of the precursor nanosheets, as shown in Figure S1(a). Unexpectedly, we cannot index the pattern to any Co- or Ge- included phase, suggesting it should be a new material. Based on the results of energy-dispersive X-ray spectroscopy (EDS, Figure S1(b)), it is confirmed the presence of cobalt, germanium and oxide in the sample. And the atom ratio of Co/Ge is approximately 2. EDS element mapping was used to characterize elemental distribution in the nanosheets. The area chosen to be scanned is marked by orange rectangular in Figure 1(e). It could be seen the elements Co, Ge and O are evenly distributed in the nanosheets (Figure 1(f)). It is reasonable to speculate that the sample might be crystallization water including material or Co and Ge based hydroxide. According to the thermogravimetric (TG) curve measured under air atmosphere (Figure S2), it reveals only a weight loss of 5.5% below 500 oC. Combing these results, we tend to believe the precursor sample is Co2GeO4·nH2O (n≈ 1). Figure 2(a) shows XRD pattern of the final products. All the diffraction peaks are in good agreement with the standard data for cubic Co2GeO4 (JCPDS no. 10-0464, a=b=c=0.83 nm), and no signals from impurity phases are detected. Inset is the crystallographic structure of cubic Co2GeO4. It could be found the crystal structure is composed of GeO4 tetrahedral (green) and CoO6 octahedral (blue). X-ray photoelectron spectroscopy (XPS) was carried out to analyze elemental composition 4


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and valence state in the sample. The existence of Co and Ge elements was verified by overall spectrum in Figure 4 (b). The high resolution spectra of Ge 3d shows an intense peak centered at 32 eV, which is usually a location for germanium oxide and consistent with the results for Ge(IV) in previously reported germanate

26-27

.Co 2p

spectrum shows two main peaks centered at 781.3 and 797.2 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively. It also could be seen each main peak is accompanied by a shakeup satellite peak (787.3, 803.6 eV), which is a fingerprint to Co2+, indicating Co element in the nanosheets has +2 valence28-30. Figure 3 presents morphologies of the final products. SEM image in Figure 3(a) shows a similar view to precursor. However, it could be noted from TEM image (Figure 3(b)) that nanosheets changed to rougher surfaces. The magnified image in Figure 3(c) clearly shows that the nanosheets evolved into a structure composed of numerous interconnected nanoparticles with various nanochannels. Clearer images are shown in Figure 3(d) and Figure S3, the inset SAED pattern reveals polycrystalline nature of the nanosheet and could be well-indexed to Co2GeO4. It also could be seen nanochannels are disorder but evenly distributed in the nanosheets. During the annealing process, precursors decompose and successively release H2O, which leads to the Formation of these channels31-32. EDS mapping of the annealed nanosheets was also conducted. As Figure 3(e) shows, the element Co, Ge and O keeps uniform distribution，However, some dark spots appeared in the images, especially in Co image. It could be observed these positions correspond to particle voids, which is consistent with the nanochannel morphology of the nanosheet. To further study the specific surface areas and porous nature of Co2GeO4 nanosheets,

nitrogen

physisorption

measurement

was

conducted.

The

N2

adsorption-desorption isotherm of the sample are shown in Figure S4(a). Brunauer-Emmett-Teller (BET) specific surface area for the nanosheets is 77 m2g-1. The corresponding density functional theory (DFT) pore size distribution in Figure S4(b) indicates most of the pores is about 8~10 nm. The existence of these voids is an advantage for anode application. In one hand, they offer buffer spaces for volume expansion to keep the material stable. In the other hand, the nanochannels could 5



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facilitate Li+ diffusion by holding more electrolyte nanodrops, which is quite feasible for rate performances10, 33-34. As TEM and STEM images all suggest the samples are very thin nanosheets, cross-sectional atomic force microscopy (AFM) was carried to accurately measure the thickness. Figure 4 (a) is the AFM image of an individual nanosheet and (b) shows three stacked nanosheets. All sheets exhibit thickness below 10 nm. The sheet in Figure 4(a) shows 7.5 nm in height. And in Figure 4(b), the height from top to bottom is 4.5, 6.5, 6.6 nm, respectively. When used as anode, these ultrathin nanosheets could provide shortest possible depth for Li+ insertion, which perfectly meets the requirement of fast lithium storage. Revealed by the above discussions, the Co2GeO4 nanosheets possess ultrathin thickness, large surface area and nanochannels structure, which can offer a series of advantages for lithium storage application such as short lithium ion pathway, abundant reaction activity spots, etc. Therefore, excellent electrochemical performance could be anticipated when these nanosheets are used as electrode materials. Galvanostatic discharge-charge tests at different current density were carried out to investigate electrochemical performance of the Co2GeO4 nanosheets. The testing voltage window was set between 0.01 and 3 V. Figure 5(a) shows voltage profiles for 1, 2, 10, 150 cycles at a current density of 0.22 Ag-1. The initial discharge and charge capacity are 1275 and 992 mAhg-1, respectively, indicating a Coulombic efficiency of 78%. The capacity loss in the first cycle is not only due to formation of SEI film, but also related to the irreversible formation of Li2O35-37. We noted the initial CE of Co2GeO4 is higher than many Cobalt oxides or germanium oxides, suggesting there are fewer side reactions in our sample38-41. From the second cycle, Co2GeO4 nanosheets anode exhibits a stable cycling performance with Coulombic efficiency more than 98%. After 150 cycles, the recorded capacity is 1026 mAhg-1, indicating a capacity retention near to 100% (compared with the second discharge capacity). Besides the above results, Co2GeO4 nanosheets also show outstanding performance at larger current density. As shown in Figure S5(a), the anode delivers a 6


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stable capacity of 670 mAhg-1 for 100 cycles at a current density of 0.88 Ag-1. The corresponding capacity retention is 95%. Moreover, when tested at a large current density of 1.8 Ag-1(Figure S5(b)), the sample could still deliver a good capacity retention of 91% after 200 cycles, demonstrating excellent cyclability of the Co2GeO4 nanosheets. Rate performance of the Co2GeO4 nanosheets was also tested and the result is shown in Figure 5(d). Current density was stepwise increased from 0.22 Ag-1 to 6.95 Ag-1. Upon each rate stage, the sample could deliver stable capacity. When current density returned back to 0.22 Ag-1, the capacity could also recover, indicating good rate stability. Table S1 shows comparison of Co2GeO4 nanosheets and the reprorted Co2GeO4/RGO composites. It could be seen our sample possesses lower specific capacity, but exhibits longer cycling life and higher capacity retention. To understand the excellent performance of the Co2GeO4 nanosheets anode, we carried out a series of measurements to make clear its lithium storage mechanism. Firstly, cyclic voltammogram (CV) for several initial cycles was measured and the result is shown in Figure S6(a). The CV curve of CoO reported by Guan et al. is also displayed in Figure S6(b) for comparison42. Upon cathodic scan, it could be easily found the curve of the first cycle is significantly different from the followings. There is only one intense cathodic peak in first cycle and three peaks upon subsequent cycles beyond the 2nd. For anodic process, it shows identical curve from the first process and there are also 3 anodic peaks. According to the CV result, we had chosen 6 representative points at different cycling stages to observe the morphology and structure changes of Co2GeO4 nanosheets. As shown in Figure 6(a), these points are located at 0.75, 0.21 and 0V upon the first discharge process and 0.8, 1.6 and 3 V upon the subsequent charge process, respectively. Ex-situ TEM and electron energy loss spectroscopy (EELS) were used to analyze the electrode evolution. Figure 6 (b) shows low-magnification TEM image of the anode discharging from open circuit potential to 0.75 V (stage 1). The most notable change is the 7



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disappearance of nanochannels, which is caused by volume expansion of pristine nanoparticles upon lithium insertion process. It also could be seen the sheet edge become rougher while the nanosheets keep intact. HRTEM image reveals detail structure changes. As Figure 6 (c) shows, clear lattice fringes with spacing distance of 0.2 and 0.23 nm could be assigned to (220) and (211) planes of Ge. Fast Fourier transformation (FFT) pattern in Figure 6(d) further confirm these crystalline fringes derive from cubic Ge. As pointed by many previous reports, it is conceivable that pristine Co2GeO4 could decompose into Ge during lithiation43-45. However, one question has still remained unclear whether intermediate substance such as GeO2 emerges during the decomposition process? In other word, the process of Ge-based DMOs reduction to Ge is direct or indirect? Actually, Pena et al. have demonstrated the voltage for GeO2 reduction to Ge is below 0.55 V46. Taking into account we could observe Ge phase at 0.75 V, therefore, our result tends to support that Co2GeO4 directly decomposed into Ge upon first lithiation process. When discharged to 0.21 V, low magnification TEM image of the nanosheet shows a similar morphology to stage 1, yet lattice fringes in HRTEM with a distance of 0.27 nm could be appointed to (007) plane of Li22Ge5, which is further confirmed by FFT image (Figure 6(e-g)) . This result is consistent with the previous study on Ge anode where Li-Ge alloying process commonly occurred at voltage level below0.3 V47-49. As expected, HRTEM and FFT reveal the crystalline structure still keeps Li22Ge5 phase when discharged to 0 V (Figure 6(i, j)). Figure 7 presents anode evolutions during the subsequent delithiation process. When charged to 0.8 V, the nanosheet still keeps similar morphology under low-magnification TEM (Figure 7(a)), but the diffraction rings in FFT image (Figure 7(c)) are perfectly indexed to tetragonal Ge, indicating the dealloying process of Li22Ge5 to Ge. It is also confirmed by ex-situ XRD (as Figure S7 shows), which could be well assigned to tetragonal Ge (JCPDF, no.18-0549). The next two stages at higher voltage show the crystalline nanoparticles stay in the same phase. It should be noted that though the particle phase unchanged above 0.8 V, slight morphology changes of 8


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nanosheets could be distinguished. Especially at 3 V, where the anode is fully delithiated, it could be noted numerous quantum dots are scattered on an amorphous matrix (Figure 7(g)), suggesting the crystallinity of Ge NPs enhanced at high voltage level. From the above discussions we can understand morphology and Ge phase evolutions

during

the

cycling

process.

However,

throughout

the

whole

discharge/charge process, there is not found any Co-related crystalline structures, indicating the Co composites have amorphous nature. To make clear the phase evolutions of Co, electron energy loss spectroscopy (EELS) was tested at every stage. It is well known that for cobalt and cobalt oxides, the transition of an electron from 2p state to 3d state can form white lines. Specifically, electron transition from 2p3/2 to 3d3/2 and 3d5/2 leads to L3 lines, while 2p1/2 to 3d3/2 leads to L2 lines50-51. It has been well studied that the intensity ratio of the two white lines (L3/L2) is an effective fingerprints to indicate the valence state of Co since this data are related to the unoccupied states in the 3d bands52-55. Therefore, L3/L2 ratios at different stage were detected to track Co phase changes. Figure 8(a) shows EELS spectrum of Co in the pristine sample. The intensity ratio is 4.04, which is in agreement with the value of CoO56, indicating the valence state of Co is +2. When discharged to 0.75 V (Figure 8(b)), the L2/L3 ratio decreased to 3.2, which is related to Co0, indicating cobalt in the sample has been reduced from oxidation state to elemental state57-58. Logically, we obtained a ratio of 3.21 and 3.19 when the terminal voltage is 0.21 and 0 V (Figure 8(c,d)), respectively, suggesting Co0 state unchanged upon the following lithiation process. During delithiation process, the ratio was measured to be 3.14 at 0.8 V, indicating Co still stay in elemental state (Figure 8(e)). Then charging the anode to 1.6 V, the ratio shows a same value (3.11, Figure 8(f)). According to CV curve, there is an anodic peak between 0.8 and 1.6 V, which also appeared in previous reported CoO anodes59-61. Given to our result reveals Co still stays in elemental state at 1.6 V, we ascribe this peak to deformation of the gel-like films, which is consistent with previous reports62-64. Finally, when charge voltage reached 3 V, the ratio increased to 9



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4.11, indicating Co0 was reoxidized to CoO (Figure 8(g)). More information of the nanosheet at this full delithiated state is shown in Figure S10. From STEM image it could be noted the nanosheet keeps intact morphology and the corresponding EDS mapping reveals the elements Co, Ge, O are uniformly distributed on the sheet plane. Combined with the above results, we can propose a thorough reaction mechanism of Co2GeO4 with Li+. At the beginning stage, the ultrathin nanosheets provide sufficient activity spots for Li+ due to their large open surface and nanochannels. Then, accompanying with Li+ insertion, Co2GeO4 directly decomposes into crystalline Ge NPs and amorphous Co/Li2O mixed composites. Subsequently, Ge NPs alloys with Li to form Li22Ge5 NPs which are highly dispersed in Co/Li2O matrix. Upon inversed charging process, Li22Ge5 phase decomposes below 0.8 V to form Ge NPs and when voltage reaches higher than 1.6 V, elemental Co reoxidizes to CoO. The complete chemical reaction process is depicted in eq. (2-4): Co2GeO4 + 4Li →2Co + Ge + 2Li2O

(2)

Ge + 4.4Li ↔ Li4.4Ge

(3)

Co + Li2O ↔ CoO + 2Li

(4)

It could be noted that since the voltage window for Ge alloying/dealloying with Li+ is below that of Co reduction/reoxidation, the elemental Co matrix could serve as a highly conductive network during the electrochemical reaction of Ge, which intrinsically enhance the anode kinetics. This point was confirmed by the electrochemical impedance spectroscopy (EIS). Figure S8 shows Nyquist plots of a same battery at the open circuit potential (OCP) and after 10 cycles. As the image shows, the diameter of the semicircle for the battery after 10 cycles is much smaller than that at the OCP, which means the charge transfer resistance has greatly decreased after 10 cycles. Thus, it indicates the conductivity of the electrode enhanced during the cycling process. We noted that the Co2GeO4 nanosheets kept integrity upon the initial discharging/charging process, indicating their tough mechanical stability. To further confirm that point, anode morphology after 100 cycles and 155 cycles at full charge state were observed. As Figure S9(a) shows, after 100 cycles, the sample still exhibits 10


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2D structure in which there are large amount of dispersed nanoparticles. Compared with the first cycle, these nanoparticles have larger size and clear round edges, suggesting smaller Ge nanograins gradually migrated together during the cycling. However, these particles only have diameters around 5 nm and are separated to each other (Figure S9(b)). The nanosheets shows a similar morphology to the 100th one after 155 cycles (Figure S9(c-d)). This phenomenon should be ascribed to two factors. Firstly, the ultrathin 2D structure of nanosheets results in much fewer nanograins generated in vertical direction than did in horizontal surface, which restricts the particle aggregation mainly in sheet plane. Additionally, migration of Ge nanograins in sheet surface was further prevented by Co/Li2O matrix. Therefore, Co/Li2O composites exhibit like a framework which embraces the Ge nanoparticles, leading to the excellent structure stability. Conclusions In conclusion, we have synthesized a new type of 2D nanostructures, ultrathin Co2GeO4 nanosheets with nanochannels, and for the first time investigated their electrochemical performance. Detailed TEM and EELS measurements were carried out to understand the lithium storage mechanism. During the cycling process, Co2GeO4 decomposed into crystalline Ge nanoparticles which are highly dispersed in amorphous Co/Li2O matrix. The unique configuration and composite advantages, such as the large open surface, ultrathin thickness and conductive Co framework lead to superior mechanical stability and enhanced electrical/ionic conductivity. Thanks to these intrinsic advantages, the Co2GeO4 anode exhibits excellent cycling stability and high-rate capability, which indicates it could be a promising candidate for future high-performance lithium ion batteries.

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Sample synthesis: A typical synthesis process is depicted as follow: 20 mg (0.5 mmol) NaOH was dissolved into 30 mL deionized water at room temperature. Then, 52 mg (0.5 mmol) GeO2 powders (Aldrich, 99.99%) was added to the alkali solution under magnetic stirring. When the solution became transparent, 238 mg (1 mmol) CoCl·6H2O was added. After continuous stirring for 10 min, the pristine suspension was transferred to a 50 mL Teflon-lined autocalve and heated at 180 oC for 6 h. The synthesized light purple precipitations were washed by deionized water and ethanol for several times, and then dried at 80 oC for 3 h. Subsequently, the powders were annealed at 500 oC under air ambient for 6 h to obtain the final products. To obtain the optimal sample, different reaction factors were tested, such as the amount of NaOH, synthesis time, temperature and Co sources, and the results could be seen in Figure S11 ~ S13.

Characterization: Morphology observations were conducted by field-emitting scanning electron microscope (FESEM; JEOL, JSM-6700F, 15KV) and transmission electron microscopy (TEM, FEI, Tecnai G2 F30, 300KV). Crystal structure, chemical compositions and elemental analysis were characterized by X-ray diffraction (XRD, D/Max 2200 VPC), X-ray photoelectron spectroscopy (XPS, ESCA Lab 250) and energy dispersive X-ray spectroscopy (EDS). N2 adsorption experiments were conducted on an ASAP 2020 surface area analyzer (Micrometeritics Co., USA). The specific

surface

area

of

the

samples

was

calculated

according

to

the

Brunauer-Emmett-Teller equation from the adsorption data. AFM images of the products on a fresh silicon surface were taken with a Nanoscope III in tapping mode using an NSC14/no Al probe (MikroMash,Wislsonville, OR, USA).

Electrochemical measurements: To fabricate the working electrode, active materials were mixed with acetylene black (Sigma-Aldrich) and sodium alginater (SA, Sigma-Aldrich) complied with weight ratio of 8:1:1. Then the mixtures were coated on a clean copper foil (18 µm thick). When the slurry are fully dried (90 oC, 6 h), the foil was cut into round pieces. Coin-like half cells (CR 2032) were assembled in an 12


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argon filled glove box (Mikrouna Co., Ltd. Universal) with lithium metal as counter electrode. The electrolyte was a solution of 1 M LiPF6 in mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) with a mass ratio of 1:1. A polypropylene membrane was used as separator. The electrode mass was weighted by a microbalance with readability of 0.1µg (Mettle-Toled AG, XP2U, Switzerland). Galvanostatic discharge-charge tests were carried out on a BT-2000 battery tester (Arbin Instruments, College station, TX) at different current densities, all capacities are calculated based on the total mass of the electrochemical active materials. Cyclic voltammetry was recorded by a Solartron 1286 Electrochemical workstation.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51125008, 11274392, U1401241). The following contents could be found in Supporting Information. The XRD pattern, EDS spectrum and the TGA curve of the precursor, the HRTEM images, N2 adsorption/desorption isotherm and Pore size distribution of the Co2GeO4 nanosheets, cycling performance of Co2GeO4 nanosheets anode at current density of 0.88Ag-1 and 1.8 Ag-1, CV curves of Co2GeO4 and CoO, Nyquist plots, Morphologies of nanosheets cycling for 100 and 150 cycles, STEM image and the corresponding EDS mapping after the first cycle, sample prepared under different reaction condition, comparison between our sample and the reported Co2GeO4 nanoplate/RGO nanocomposites. This material is available free of charge via the internet at http://pubs.acs.org/.

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Zn2GeO4/N-Doped

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Figure 1 Morphologies of the pristine nanosheets. (a) SEM and (b,c) TEM images of the as-prepared pristine nanosheets. (d) HRTEM image shows single crystalline nature of the nanosheets, inset is the selected area electron diffraction (SAED) image. (e) Scanning TEM image of several pristine nanosheets. (f) EDS mapping of a selected nanosheet, it shows evenly distribution of element Co, Ge and O. Figure 2 (a) XRD pattern of the Co2GeO4 nanosheets, inset is the schematic visualization of crystalline structure for cubic Co2GeO4, in which GeO4 tetrahedral and CoO6 octahedral is represented by green and blue color, respectively. (b) Full XPS spectrum of Co2GeO4 nanosheets. (c) and (d) high resolution XPS spectra of Ge 3d and Co 2p peaks, respectively. Figure 3 Morphologies of the as-annealed nanosheets. (a) SEM image shows similar morphology to pristine nanosheets. (b) and (c) TEM image of the nanosheets exhibit nanochannels in the sample. (d) HRTEM image of the nanosheets, inset is the SAED pattern. (e) STEM image of several Co2GeO4 nanosheets, the corresponding EDS mapping are shown in the right images, from top to bottom is Co, Ge and O, respectively. Figure 4 AFM images of the ultrathin Co2GeO4 nanosheets. (a) An individual nanosheet. (b) Three stacked nanosheets. Inset pictures are ‘height-width’ profiles corresponding to lines mark with red in the main image. Figure 5 Electrochemical performances of the ultrathin Co2GeO4 nanosheets. (a) Voltage profiles of the 1, 2, 10, 150 cycles. (b) Cycling performance at current density of 0.22 Ag-1. (c) Voltage profiles of the anode at different rates. (d) Rate capability of Co2GeO4 nanosheets. Figure 6 Structure and phase evolutions of Co2GeO4 nanosheets in the initial discharge process. (a) Based on CV curve and voltage profile of the first cycle, 6 cutoff voltages were selected. (b, e, h) TEM images of the Co2GeO4 nanosheets discharging to 0.75, 0.21 and 0 V, respectively. (c, f, i) Representative HRTEM images of the Co2GeO4 nanosheets at 0.75, 0.21 and 0 V, respectively. (d, g, j) Fast Fourier transformation (FFT) patterns of Co2GeO4 nanosheets at different discharging stage. 21



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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7 Structure and phase evolutions of Co2GeO4 nanosheets in the first charging process. (a, d, g) TEM images of the Co2GeO4 nanosheets charging to 0.8, 1.6 and 3 V, respectively. (b, e, h) Representative HRTEM images at 0.8, 1.6 and 3 V, respectively. (c, f) Fast Fourier transformation (FFT) patterns of Co2GeO4 nanosheets at stage 4 and 5. (i) SAED pattern of Co2GeO4 nanosheets at 3 V. Figure 8 EELS spectra of Co white lines at different discharging/charging process. (a) The pristine sample. (b-d) Spectra obtain at cutoff voltage of 0.75, 0.21 and 0 V during initial discharging process, respectively. (e-g) Spectra obtain at cutoff voltage of 0.8, 1.6 and 3 V during the first charging process, respectively.

22


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Page 23 of 31

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)


(b)

(c)

(f)

(e)

(d)

Co

Ge

Figure 1

23


O


(a)

(b)

440

220

O 1s

Co 2p

Intensity (a.u.)

Intensity (a.u.)

311

222 400 511

Ge 3d

422

20

30

40

50

60

1200

70

1000

(c)

800

32

200

0

Co 2p3/2 Intensity (a.u.)

30

400

(d)

Ge 3d

28

600

Binding energy (eV)

2θ (degree)

Intensity (a.u.)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

34

36

Co 2p1/2

775

38

Sat.

Sat.

780

785

790

795

800

Binding energy (eV)

Binding energy (eV)

Figure 2

24


805

810

Page 25 of 31

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(c)

(b)

(d)

(e)

Co

Ge

O

Figure 3

25



(a) 6

(b)

15 Height (nm)

Height (nm)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

4 2 7.5 nm 0

5

6.5 nm

0

6.6 nm

-5

-2 0

100 200 Width (nm)

300

Figure 4

26


4.6 nm

10

0

100 200 Width (nm)

300

(a) 3.0

(b) 2000

100

2.0 1

1.5

2

st nd

10th th 150

1.0 0.5

1600 -1

0.22 Ag 1200

50 800 400 0

0.0 0

200

400

600

800

-1 1000

1200

0

1400

Capacity (mAhg )

(c)

3.5

-1

20

30

40

50

60

70

80

90

0 100 110 120 130 140 150

Cycle number -1

6.95 Ag

10

0.22 Ag

(d)

100

1600 Current denstiy: A/g

-1

Capacity (mAhg )

3.0 2.5

0.22

1200

2.0 1.5 1.0 0.5

0.22 0.58 0.58

800

1.16

1.16

2.32 4.63

400

2.32 6.95

4.63

0

0.0 0

200

400

600 800 1000 1200 1400 -1 Capacity (mAhg )

50

0 0

10

20

30 Cycle number

Figure 5

27


40

50

Coulombic efficiency (%)

Voltage (V)

-1

Capacity (mAhg )

2.5

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 27 of 31


(a) 3

0.2 Current (mA)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2

-0.2

2# 0.21V -0.4

-0.8

1# 0.75V 3# 0V

0.0

0.5

5# 1.6V 1.0

1# 0.75V

0

6# 3V

6# 3V

5# 1.6V

0.0

-0.6

1

4# 0.8V

Page 28 of 31

1.5 2.0 Voltage (V)

2.5

3.0

4# 0.8V

2# 0.21V

3# 0V

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

Figure 6

28


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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figure 7

29



(a)

Pristine Co2GeO4

L3

780

1# 0.75V

800

820

L2

760

840

Energy loss (eV)

2# 0.21V

Intensity (a.u.)

L2

800

820

840

L2

760

780

(f)

820

760

840

780

6# 3V

L3

L3/L2=4.11 L2

800

820

5# 1.6V

800

820

Energy loss (eV)

(g)

780

840

L2

Energy loss (eV)

760

820

L3/L2=3.11

Intensity (a.u.)

Intensity (a.u.)

L2

800

800

L3

4# 0.8V L3/L2=3.14

780

3# 0V

Energy loss (eV)

(e) L3

840

L3/L2=3.19

Energy loss (eV)

760

820

L3

L3/L2=3.21

780

800

Energy loss (eV)

(d)

L3

760

780

Intensity (a.u.)

(c)

L3/L2=3.2

Intensity (a.u.)

Intensity (a.u.)

L2

760

(b) L3

L3/L2=4.04

Intensity (a.u.)

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

840

Energy loss (eV)

Figure 8

30


840

Page 31 of 31

2000 -1

100

1600 -1

0.22 Ag 1200

50 800 400 0 0

10

20

30

40

50

60

70

80

90

0 100 110 120 130 140 150

Cycle number

31



TOC

Capacity (mAhg )

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ultrathin Hexagonal 2D Co2GeO4 Nanosheets: Excellent Li-Storage

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