Rapid Amorphization in Metastable CoSeO3Â·H2O Nanosheets for

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Rapid Amorphization in Metastable CoSeO3·H2O Nanosheets for Ultrafast Lithiation Kinetics Yingchang Jiang, Yun Song, Zhichang Pan, Yu Meng, Le Jiang, Zeyi Wu, Peiyu Yang, Qinfen Gu, Dalin Sun, and Linfeng Hu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b02352 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Rapid Amorphization in Metastable CoSeO3·H2O Nanosheets for Ultrafast Lithiation Kinetics Yingchang Jiang1, Yun Song1, Zhichang Pan1, Yu Meng1, Le Jiang1, Zeyi Wu1, Peiyu Yang1, Qinfen Gu2*, Dalin Sun1*, Linfeng Hu1*

*Corresponding author: E-mail: [email protected]; [email protected]; [email protected] 1 Department of Materials Science, Fudan University, Shanghai 200433, China 2 Australia Synchrotron (ANSTO), 800 Blackburn Road, Clayton, 3168, Australia

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ABSTRACT: The realization of high-performance anode materials with high capacity at fast lithiation kinetics and excellent cycle stability remains a significant but critical challenge for high power applications such as electric vehicles. Two-dimensional nanostructures have attracted considerable research interest in electrochemical energy storage devices owing to their intriguing surface effect and significantly decreased ion-diffusion pathway. Here we describe rationally designed metastable CoSeO3·H2O (M-CSO) nanosheets synthesized by a facile hydrothermal method for use as a Li ion battery (LIB) anode. This crystalline nanosheet can be steadily converted into amorphous phase at the beginning of the first Li+ discharge cycling, leading to ultrahigh reversible capacities of 1,100 and 515 mAh g-1 after 1, 000 cycles at a high rate of 3 and 10 A g-1, respectively. The as-obtained amorphous structure experiences an isotropic stress which can significantly reduce the risk of fracture during electrochemical cycling. Our study offers a precious opportunity to reveal the ultrafast lithiation kinetics associated with the rapid amorphization mechanism in the layered cobalt selenide nanosheets.

KEYWORDS: CoSeO3·H2O nanosheets, metastable, rapid amorphization, ultrafast lithiation kinetics, lithium storage

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The development of improved rechargeable Lithium-ion batteries (LIBs) continues to be a major challenge nowadays, as LIBs constitute the critical components in the shift from the internal combustion engine to electric powered vehicles, while also enabling the storage of intermittent renewable energy.1 Given that conventional anodes using graphite are significantly hindered by a limited theoretical capacity of 372 mAh g-1. Therefore, synthesising high capacity anode materials is seen as the best way to achieve the aforementioned applications.2, 3 In the past decades, various anode materials, including intercalation type grapheme,4 alloy type silicon, and conversion type metal oxides/sulfates/selenides/nitrides,5 have been widely explored. Although the lithium storage capacity has been greatly enhanced through the development of these materials, none of them showed satisfactory high-rate capacity with long-cycle stability, an indispensable pre-requisite for the use with high power batteries. The poor cycle performance mainly originates from the large volume change and cyclic electrochemical stress in the electrode, e.g. anode, during the lithiation/delithiation process. For example, although silicon has emerged as one of the most promising highenergy anode materials with high theoretical specific capacity up to ~ 4200 mAh g-1,6 it suffers from ~ 300% volume change and consequent anode pulverization during Li+ insertion and extraction, which leads to rapid loss of capacity and only a low power capability.7 In recent years, the incorporation of nanomaterials with abundant grain boundaries and a much shortened Li+ diffusion distance into various carbon bases were investigated as an attractive strategy to overcome the fracture problem and enhance the cycling stability.8, 9 However, these complex designs have a high production cost with complicated procedures, hindering widespread application and industrial production. Thus, it is highly desirable to design a facile and low-cost strategy to develop these highperformance anodes. Our previous work has demonstrated improved rate performance for LIB anodes in two-dimensional (2D) γ-FeOOH and NiCo2S4 nanosheets.10,

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Following this, we

propose that high-rate performance with long-term cyclability for LIB anodes is likely achievable by solving two key issues: (1) modifying the 2D morphology: a suitable spacing between the 2D nanosheet layers which allows a large amount of Li+ storage and 3


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a high Li+ transfer rate at the electrolyte/electrode interface. Additionally, a pseudocapacitance effect coupled with the short Li+ pathways which existing in 2D layered nanosheets of high aspect ratios, suggests that a 2D morphology is highly promising for anode candidates in terms of high-rate capability;12 (2) Non-crystallinity: metallic glass materials,13, 14 a disordered structure at the atomic level, show excellent electron conductivity, thermal stability, and elastic modulus. The amorphous structure experiences an isotropic stress which can reduce the risk of fracture during electrochemical cycling because of the disordered arrangement of electrochemically active atoms. Previous research suggests that amorphous anodes resulted in enhanced electrochemical stability during cycling than that in the corresponding crystalline one.15 Thus, synthesis of an amorphous anode capable of minimizing volumetric change and electrochemical strain during charge/discharge process would be attractive. However, it remains a challenge to achieve a stable amorphous anode through high-temperature solid reactions or low-temperature solution-based methods due to the spontaneous crystallization of most inorganic compounds or the transformation of nanostructures into bulk materials, respectively. Typically, hydrated layered structures are metastable and easily collapsed by the loss of bound water, gradually transforming into amorphous phases under conditions such as a very low humidity.16, 17 This allows production of an amorphous anode from an easily metastable layered compound. Recently, cobalt-based selenides, including CoSe, Co0.85Se, CoSe2, etc. have attracted considerable attention and exhibit excellent performance in energy storage devices owing to their very high theoretical capacity.18, 19 However, there is still no report on a metastable, layered cobalt selenides nanosheets as yet. If we can realize a facile synthesis of this soft matter with intrinsic electrochemical properties and rapidly convert it into an amorphous like state after Li+ insertion/extraction, it would be highly promising to enhance the reversible capacity accompanied with ultrafast lithiation kinetics. Herein, we report a rationally designed CoSeO3·H2O (M-CSO) nanosheets with metastable, layered structure for use as a LIB anode. As expected, these M-CSO nanosheets can be steadily converted into amorphous phase at the beginning of the first 4


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Li+ discharge cycling, leading to ultrahigh reversible capacities of 1,100 and 515 mAh g-1 after 1, 000 cycles at a high rate of 3 and 10 A g-1, respectively, which is superior to all reported values in conversion anodes. The resulting amorphous structure should be responsible for the as-observed ultrafast Li+ transport kinetics in our M-CSO/graphene composite due to the as-expected minimization on volume change and cyclic electrochemical stress. Our study offers a precious opportunity to reveal the ultrafast lithiation kinetics associated with the rapid amorphization mechanism in the layered CoSeO3·H2O nanosheets.

RESULTS AND DISCUSSION Synthesis and characterization of M-CSO. M-CSO bulk sample was synthesized by a facile hydrothermal reaction of an aqueous mixture of Co(Ac)2 and SeO2. After the hydrothermal reaction, acicular purple crystals were produced in a large scale at the bottom of the autoclave. A series of contrast experiments at different hydrothermal temperatures for different times were performed to optimize the synthetic conditions (Figure S1). The XRD pattern was initially measured in reflection geometry using Cu Kα radiation (λ = 1.5405 Å) with a graphite monochromator. The laboratory diffraction data showed intense and sharp (0 k 0) (k = 1, 2, 3, 4) diffraction peaks with a basic d-spacing of ~6.64 Å, suggesting a characteristic layered structure with strong preferred orientation along [010] direction (Figure S2). During this sample preparation for laboratory diffraction, the powder sample has been ground carefully. However, the strong preferred orientation along [010] direction was not eliminated significantly. The purple bulk sample was further examined in a spinning 0.5 mm quartz capillary with transmission configuration via synchrotron X-ray powder Diffraction (XRPD). The diffraction peaks from this high resolution synchrotron diffraction data can be indexed into a monoclinic unit cell with the refined lattice parameters of a = 4.7705(1) Å, b = 13.2295(1) Å, c = 5.6849(1) Å, β = 90.484(2)°with space group P21/n (Figure 1a). There were no other peaks present in the pattern, indicating high purity of the product. Accordingly, the reaction scheme for this hydrothermal synthesis could be written as: 5


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SeO H O → H SeO

(1)

H SeO → 2H SeO

(2)

Co SeO H O → CoSeO ∙ H O (3)

Figure 1. (a) Rietveld refinement of synchrotron XRPD pattern and 3D atomic configuration of a CoSeO3·H2O unit cell. (b)The local coordination of strongly distorted [CoO6] octahedra and [SeO3] trigonal pyramidal. (c) The top-view and (d) side-view of 6


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the buckled CeSeO3·H2O layers. (e) Band structure for CoSeO3·H2O by DFT calculation studies. (f) Total density of states (DOS) of CoSeO3·H2O crystal. The local atomic arrangement and crystal structure of CoSeO3·H2O phase, which is built on chains of corner-shared [CoO6] octahedra and [SeO3] trigonal pyramidal that run parallel to the b-axis, are shown in Figure 1b-d. The crystallographic thickness (D0) of a [CoSeO4] atomic monolayer projected along [0k0] can be calculated as: D0= dSe-Se + 2rSe = (1─ 2*yse) *b + 2 rSe = 1.31 nm. The density functional theory (DFT) calculation was performed to understand the electronic structure and intrinsic strong interaction within the intra-layer (Figure 1e-f, Figure S3, S4).The crystallographic water content in this compound was verified as n = 1 by thermogravimetric analysis (TGA) in Figure S5, which also showed that it was stable up to 350︒C where dehydration occurred. The chemical states of Co, Se in the compound were examined by X-ray Photoelectron Spectroscopy (XPS) characterization (Figure S6). Using a Gaussian fitting, two obvious shakeup satellites (indicated as “Sat”) close to two spin-orbit doublets at 800.2 and 784.4eV can be identified as Co 2p1/2 and Co 2p3/2 signals of Co2+, respectively. The apparent peak intensity difference in the corresponding two satellite peaks (noted as “Sat”.) suggests the main presence of Co2+. Two main peaks at binding energies of 51.2 and 60.1 eV are respectively assigned to the Se 3d5/2 and Se 3d3/2signals of Se4+.

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Figure 2. (a)

In-situ synchrotron XRPD patterns of the M-CSO sample at high

temperatures from room temperature to 300 °C. (b) The expansion of lattice parameters, a, b, c and unit-cell volume (v) as a function of temperature. (c) Rietveld refinement of synchrotron XRPD pattern and (d) 3D atomic configuration of a dehydrated CoSeO3 unitcell after annealed at 500 °C.

Rietveld refinement of in-situ XRPD patterns at high temperatures revealed that the lattice parameters and unit cell volume of CeSeO3·H2O increased linearly with the temperature before dehydration (Figure 2a, b). Note that the expansion of b lattice is apparently much larger than that of a and c lattices, further verifying the weak Van del Waals interactions among [CeSeO3·H2O] buckled layers. After annealing at 500︒C, the crystallographic water was completely removed and a phase transformation was identified from the layered structure into a 3D structure of CoSeO3 phase (JCPDS 510053) with a monoclinic unit cell (Figure 2c, d). The dehydrated sample retained the original microrod-like morphology, and the corresponding SEM image is shown in Figure S7.

Optical microscope (OM) image in Figure 3a of the M-CSO bulk sample shows numerous ultra-long rod-like objects with length of several hundred nanometers, with a few even up to millimeter scale. The statistical size distribution and average aspect ratio 8


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measured from more than 200 ultra-long microrods were statistically estimated to be 600 µm and 8.5, respectively (Figure S8). Interestingly, the scanning electron microscopy (SEM) image in Figure 3b, c of an individual microrod (thickness: ~9.4µm) demonstrates a hierarchical superstructure of stacks of very thin nanosheets. One typical ultra-long microrod sample was further examined under transmission electron microscopy (TEM) (Figure 3d). The selected area electron diffraction (SAED) pattern taken from the edge portion of this rod exhibited very sharp diffraction spots with rectangular pattern (Figure 3e). However, it was very difficult to get the high-resolution TEM image due to the metastable behavior of the material under e-beam irradiation. The material gradually transformed to an amorphous structure with several broadened diffraction rings (Figure 3f) under these conditions. This behavior is expected due to the weak inter-layer interactions and strongly distorted corner-sharing octahedra within the buckled layered structure. This buckled structure can be easily distorted or twisted under external forces, which also demonstrated by its exfoliation behaviour from a mechanical exfoliation method using scotch tape as shown in Figure 4. The as-obtained nanosheets show an average thickness of ∼1.7 nm, which is closed to the crystallographic thickness (1.31 nm) of a single atomic M-CSO layer (Figure 1d), demonstrating their unilamellar nature. The slight discrepancy between this experimental thickness and the experimentally obtained value should be attributed the hydration of the nanosheets, which is frequently observed in other oxide/hydroxide nanosheets.20

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Figure 3. (a) OM and (b, c) SEM images of the as-prepared M-CSO sample. (d) A typical TEM image of an individual M-CSO microrod and (e) the corresponding SAED pattern. (f) The corresponding SAED pattern after a short time e-beam irradiation.

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Figure 4. (a) The illustrative procedure of scotch-tape based micromechanical cleavage of the M-CSO bulk. (b) OM image of CoSeO3·H2O nanosheets by the micromechanical exfoliation shows lots of 2D ultrathin sheet-like objects. (c) AFM image and height information of as-exfoliated CoSeO3·H2O nanosheets. A traditional electrical measurement of an individual M-CSO microrod indicated its insulating nature (Figure S9). Thus, highly conductive graphene prepared by a physical method (sheet size: 0.5 ~ 2 µm, thickness: 0.8 nm, monolayer: 80%) was adopted as a conductive additive for preparation of battery anode. 80 wt % of M-CSO sample and 20 wt % of graphene was homogeneously mixed by a ball-milling treatment. Due to the layered nature from both M-CSO and graphene structures, the ball-milling treated sample can form a well compatible stacked nanosheets composite. The M-CSO/graphene composite after ball-milling showed the similar XRD pattern with the pristine M-CSO sample (Figure 5a, b), and the very slight change in basic d-spacing should be attributed to the deviation caused by the different environmental humidity during the measurement. 11


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These XRD patterns suggest that the phase structure and crystallographic water well maintain during the ball-milling treatment. The Fourier transform infrared spectroscopy (FTIR) spectrum also confirmed the existence of crystallographic water in this composite (Figure 5c). However, the microrod morphology was completely lost and converted into massive M-CSO/graphene ultrathin nanosheets after the ball-milling process (Figure 5d, Figure S10-11). Note that the real sample after ball-milling should be ultrathin sheets but not single-layer structure.

Figure 5. (a) XRD pattern of pure M-CSO sample and M-CSO/graphene composite, (b) enlarged view of (010) peaks at a range of 12-15°, and (c) the FTIR spectra of pure MCSO as-synthesized sample and M-CSO/graphene composite after ball-milling. (d) Typical TEM image of M-CSO/graphene composite after ball-milling process. The long microrods generally disappear, and massive 2D M-CSO nanosheets stacked among with graphene layers are observed. 12


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Ultrafast Li+ kinetics in M-CSO/graphene. The electrochemical properties of M-CSO nanosheets/graphene composite were investigated in a half-cell with Li foil as the counter electrode. The load of active material (the total mass of the M-CSO and graphene) is about 1.68 mg cm-2, and the thickness (≈ 3 µm) was scaled by the cross-section SEM image of the electrode in Figure S12. A reversible conversion reaction of M-CSO/graphene during Li ion storage was verified by the cyclic voltammetry (CV) in Figure 6a, which was collected at a scan rate of 0.1 mV s1

in a potential window of 0.01−3.00 V versus Li+/Li. Several pairs of cathodic/anodic

peaks were detected in this CV curve. Since the sample was steadily converted into amorphous composite at the beginning of the first Li+ discharge cycling (discussed below), it is difficult to reveal the real origin of every cathodic peaks using XRD characterization. The intense cathodic peaks located at 1.61 V was only observed in the first cycle, which maybe is the insertion of the Li+ into the interlayer of CoSeO3·H2O nanosheets, and this phenomenon also appears in other layered materials.21 From the second cycle onward, the distinct peak at 1.61 V disappeared. Most recently, Wang et al. developed CoSnO3/ grapheme/carbon nanotubes composite with enhanced lithium storage capabilities.22 In their work, the redox peak at around ∼0.5/∼1.2 V in the first cycle, is attributed to the reduction reaction of CoSnO3 to transform into metal Co, Sn and Li2O, and also the formation of a solid electrolyte interface (SEI) and electrolyte decomposition. Consideration the composition similarity between the CoSnO3 and CoSeO3·H2O, we propose that the peaks located at 0.67 V and 1.37 V might be attributed to the formation reaction of Co and Se, as well as the reductive decomposition of the organic electrolyte to form a solid-electrolyte interphase (SEI) layer at the electrode/electrolyte interface.22-24 The minor cathodic peak around 0.25 V should correspond to the formation of LixSe alloys,25 The anodic peaks at 1.05, 1.27, 1.84 and 2.36V in the anodic process should be contributed to the extraction of Li+ in the dealloying reaction.25-27 Figure 6b presents the galvanostatic discharge-charge (GDC) voltage profiles of the electrode in the potential window of 0.01−3 V at a current density of 0.2 A g-1. In the initial cycle, the M-CSO/graphene electrode displayed a high discharge capacity of 13


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2713.7 mAh g−1 (based the total mass of the M-CSO and grapheme, the same below) and a charge capacity of 1888.4 mAh g−1, with a low corresponding coulombic efficiency of 69.5 %. The irreversible capacity in the first cycle may be attributed to the formation of the solid-electrolyte interphase (SEI) film on the surface of the electrode material and the destruction of its layered structure. From the 2nd cycle onward, the discharge /charge curves are in high coincidence with each other. This result is in good agreement with the CV curves in Figure 6a.

Figure 6.

(a) Representative CV curve of M-CSO/graphene electrode for the first to

third cycles at a scan rate of 0.1 mV s-1. (b) Galvanostatic charge-discharge voltage 14


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profiles at a current density of 0.2 A g-1. (c) Rate properties at different current densities. (d) Long-cycle performance at current densities of 3 A g-1 and 10 A g-1. (e) Comparison of capacity of some reported cobalt or selenium based composites as anode materials and the present work at different current densities30-39, 11.

Rate capability is an important indicator for evaluating lithium storage system. Thus, our M-CSO/graphene composite electrode in the range of 0.01−3.00 V was evaluated at various current densities (0.2−2 A g-1) as shown in Figure 5c. The electrode showed very high reversible capacity of 2000, 1870, 1750, 1640, and 1530 mAh g-1 at current densities of 0.2, 0.4, 0.8, 1, and 2 A g-1, respectively. When the current density was decreased back to 0.2 A g-1, the capacity returned to 2000 mAh g-1. The specific capacity gradually decreased from 1470 to 960 mA h g-1 during the first 95 cycles and then started to increase until 300 cycles, maintaining a high specific capacity of 1100 mA h g-1 after 1000 cycles. Such a capacity increase during the early stage of cycling has also been commonly observed in other oxide anodes, and this capacity recovery behaviour should be attributed to the formation of a gel-like polymer arising from electrolyte degradation28. Unusually, at a higher current density of 3, 10 A g−1 as shown in the long-term cycling of Figure 6d, the electrode still exhibited a reversible capacity of 1100, 515 mA h g−1 after 1000 cycles, respectively. Figure 6e depicts Li+ specific capacity plot comparing the different current densities of our sample with alloyed Si, Si-C composites and other LIB anode materials. 29, 31 The Li+ specific capacity of our M-CSO/graphene anode is superior to all reported values of conversion anodes and comparable with the best performance of nanostructured silicon anodes (e. g. Si@SiO2@C cluster) recently reported.29, 31 It should be noted that our hydrothermal synthetic procedure of the M-CSO nanosheets is much more facile than the reported Si@SiO2@C cluster, which was prepared by complex fabrication processes including secondary structure design and hierarchical bottom-up growth. Undoubtedly, such a high-rate performance and facile preparation of our M-CSO sample is very promising for the potential application on high power batteries in electric vehicle/hybrid electric vehicle systems.

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Amorphous phase enhanced mechanism. To obtain a deeper understanding of the electrochemical reaction mechanisms and ultrafast lithiation kinetics in M-CSO/graphene composite anode, we performed inoperando synchrotron XRPD measurement with key reflection peaks as shown in Figure 7a, b. The XRPD patterns were collected continuously every three mins with charge/discharge cycles at 0.5 A g-1 (Figure S13). The structure and phase evolution of the M-CSO/graphene composite electrode during the first two cycles was investigated. The as-assembled coin cell showed M-CSO diffraction peaks in addition to Cu foil peaks and lithium counter electrode peaks, demonstrating the prepared M-CSO/graphene multilaminated composite anode was stable in the assembled cell. During the first discharge under constant current mode, the M-CSO peak intensities decreased gradually with nearly no peak movement in two-theta angles. The peaks had completely disappeared when the voltage was below 1.6 V. This describes the continuous break down of the M-CSO nanosheets from a multi-laminated structure to amorphous clusters during 1st discharge cycle. In the following two cycles, no new peaks were detectable in the XRPD patterns, indicating our M-CSO/graphene anode remained amorphous state. The amorphous structure has also been confirmed by the TEM observation after the 1st and 2nd cycle as shown in Figure S14-15, while the corresponding SAED patterns display faint and broadened diffraction rings and spots. The crystal structure analysis shows that the corner shared [CoO6] octahedra in M-CSO can easily twist against each other and lose the long range order during the first discharge. The resulting amorphous structure should experience an isotropic stress which can significantly reduce the risk of fracture during electrochemical cycling, and be responsible for the as-observed ultrafast Li+ transport kinetics in our M-CSO/graphene composite due to the as-expected minimization on volume change and cyclic electrochemical stress (Figure 7c).15 Note that FTIR of the M-CSO/graphene composite shows it is still hydrated after the ball-milling process (Figure 5c). In most cases, the bonding crystallographic water is considered harmful as it can react with high-voltage window aprotic electrolytes such as LiPF6 to form HF, which is damaging to LIBs. Surprisingly, the excellent 16


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electrochemical performance of the present M-CSO/graphene composite implies the absence of such a detrimental reaction with LiPF6. Most recently, Wang et al. found, for the first time, that water in intercalated lithium titanate hydrates may be not bad actor for Li+ storage in aprotic electrolyte if the water is trapped in the lattice and not freely contact with electrolyte.40 While in our case, M-CSO nanosheets were completely decomposed after the first discharge/charge cycle, and the crystallographic water molecules were dissociated from the CoSeO3·H2O crystal lattice according to CV curves in Figure 5a. The presence of water molecules do not damage the cycling ability in conversion typed anode was observed for the first time, and the real reason is still unclear. We speculate that these dissociated water molecules were trapped among multilaminated graphene layers in M-CSO/graphene composite. Recently, as reported by Urban et al., graphene sheets have been shown to function as protective layers by preventing the permeation of H2O into hydrogen storage materials.41 In our case, according to the SEM images of the sample after lithiation in Figure S16, the graphene can completely wrap the amorphous M-CSO compound and might act as protective layers. This protective layer only allows Li+ diffusion through the layers, and the water molecules should be trapped and isolated with LiPF6 electrolyte. The lithium-ion storage behavior of the dehydrated sample was further measured as a comparison. Figure S17a presents the rate-properties at different current densities of the CoSeO3/graphene electrode. Compared with the hydrated M-CSO/graphene anode, the CoSeO3/graphene sample shows a much lower capability of 900 mAh/g at 3 A g-1. As cycling continues, following an initial dip, the specific capability increases progressively from the 31th cycle, and reaches a stable value of ~1500 mAh/g at the 780th cycle (Figure S17b). The CoSeO3/graphene sample exhibits superior cycling stability than that of the hydrated M-CSO. The conductivity and lithium-ion diffusion of these anodes was also identified by the electrochemical impedance measurements before the charge/discharge process as shown in Figure S18. Apparently, CoSeO3/graphene sample possesses a much lower inherent resistance compared with that of hydrated MCSO/graphene anode, suggesting that the removal of crystallographic water facilitates

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the lithium ion transport. Further study is still carried out to make a deep understanding of this phenomenon.

Figure 7. (a) Schematic of a typical modified coin cell for synchrotron in-situ XRPD experiment setup. (b) 3D plot of in-situ XRPD patterns of M-CSO/graphene composite anode under the first discharge cycle. (c) The possible Li+ storage mechanism of the present M-CSO/graphene composite anode. 18


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CONCLUSION We have developed a rapid amorphization strategy in metastable CoSeO3·H2O nanosheets for ultrafast lithiation kinetics. The in-situ XRPD characterization indicates that the M-CSO nanosheets are completely converted into amorphous phase in the first charge/discharge cycle. The rapid amorphization induces high-rate specific capacity and ultrafast kinetics for Li+ storage, as high as 1,100 mAh g-1 at 3 A g-1 after 1, 000 cycles, which is dramatically higher than all of the other conversion anodes previously reported. The presence of crystallographic water molecules shows no significant damage on the cycling ability. This is the first observation of this phenomenon in conversion typed anodes. In contrast to conventional concept of LIB anode design, our study provides a general strategy to gain ultrafast lithiation kinetics from metastable nanosheets induced by a rapid amorphization mechanism. In the next work, it should be interesting to further study other deuterogenic compounds (such as NiSeO3·H2O, MnSeO3·H2O, ZnSeO3·H2O) with outstanding electrochemical properties on energy storage.

EXPERIMENTAL SECTION Materials. Graphene (sheet size: 0.5 ~ 2 µm, thickness: 0.8 nm, monolayer rate: 80%) was purchased from Nanjing XFNANO Materials Tech Co.,Ltd, Co(Ac)2·4H2O and SeO2 (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd and directly used without further purification. Synthesis of M-CSO sample. The M-CSO sample was synthesized by a hydrothermal method. The solvent was deionized water with the molar ratio of Co:Se atom 1:1. In a typical procedure, Co(Ac)2·4H2O (2.0 mmol) and SeO2 (2.0 mmol) were added into the deionized water (70 mL) while stirring. The mixed solution was then transferred to a 100 mL Teflon container and heated in a sealed autoclave at 180oC for 15 h. Upon cooling to room temperature, the product was washed with deionized water and ethanol several times and dried under vacuum at 70oC. This M-CSO powder was exfoliated into single layer nanosheets using scotch tape. After repeatedly peeling flakes of M-CSO off the 19


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bulk, the M-CSO layer becomes thinner and thinner as one repeats this numerous times. Thin flakes left in the tape were transferred to the silicon substrate for further characterization. For dehydrated CoSeO3 sample, the M-CSO sample was put in a quartz boat in a muffle furnace under atmosphere at 500 oC for 1 h to realize the complete dehydration. Material Characterization. Sample morphologies were characterized using an Olympus BX51M optical microscope, a FEI Nova Nano SEM 450 field-emission scanning electron microscopy, a FEI Tecnai G2F20 S-Twin transmission electron microscopy, and a Bruker Dimension Icon atomic force microscopy. The crystal structure characteristics were studied using a Bruker D8-A25 diffractometer using Cu Kα radiation (λ = 1.5406 Å). For phase identification and structure determination, the synthesized sample was loaded into 0.7 mm quartz capillaries and synchrotron XRPD data were collected using a Mythen-II detector at Powder diffraction beamline, Australian Synchrotron. Two separate collections were undertaken at two wavelengths: 0.6884 Å or 0.5986 Å, determined using NIST SRM 660b LaB6. The high temperature stability was studied using a hot air blower heating up to 500 ºC at a ramp rate of 10 ºC min-1. In-operando XRPD measurements with cycling of the modified 2032 coin cell in transmission mode was performed using a Neware battery tester system (China). Cells were cycled under a constant current mode with a rate of 0.5 A g-1. Data were collected contiguously with an exposure time of 180 s. The elemental composition and chemical state of the sample were measured using a PHI 5000C EACA system X-ray photo electron spectroscopy (XPS), with a C1s peak at 284.6 eV as the standard signal. Thermogravimetric (TGA) measurements were carried out using a SDT Q600 instrument in a temperature range of room temperature to 900 oC at a heating rate of 10 oC min-1 under air flow. FT-IR spectra were recorded on Nicolet iS50 FT-IR spectrometer. Battery fabrication and electrochemical measurements. To fabricate working electrodes, firstly, the graphene was fully mixed with the M-CSO sample by ball milling. Accordingly, the raw materials(80% M-CSO sample and 20% graphene) and grinding medium (steel balls in different diameter, Φ5 mm, Φ10 mm, Φ20 mm, with weight ratio 7:2:1 in sequence) with a mass ratio of 60 : 1 were put into a sealed stainless steel jar, and 20


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ACS Nano

milled in a high-energy planetary ball mill system (YXQM-0.4L, MITR) with an optimized combination of revolution speed and autorotation speed of 700 rpm and 600 rpm at room temperature for 6h. Then, active powder (the above ball-milled powder), conductive carbon black (EQ-Lib-Super P) and binder (polyvinylidene fluoride, PVDF) were mixed in a weight ratio of 80:10:10; 1-methyl-2-pyrrolidinone solvent (Aldrich, 99%) was added to form a slurry. The well-mixed slurry was coated onto a Cu foil current collector (thickness: 12 µm, diameter: 12 mm) by a doctor blade technique, and after dried under vacuum at 120oC for 12 h, we usually got the load and thickness of active material coated on the Co foil before being assembled. Coin-type half-cells were assembled inside an argon-filled glove box (

Rapid Amorphization in Metastable CoSeO3Â·H2O Nanosheets for

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