Ultra-thin bismuth nanosheets for stable Na-ion batteries: clarification

3 days ago - Yaxin Huang , Chongyang Zhu , Shengli Zhang , Xuemin Hu , Kan Zhang , Wenhan Zhou , Shiying Guo , Feng Xu , and Haibo Zeng. Nano Lett...
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Ultra-thin bismuth nanosheets for stable Na-ion batteries: clarification of structure and phase transition by in situ observation Yaxin Huang, Chongyang Zhu, Shengli Zhang, Xuemin Hu, Kan Zhang, Wenhan Zhou, Shiying Guo, Feng Xu, and Haibo Zeng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04417 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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Ultra-thin bismuth nanosheets for stable Na-ion batteries: clarification of structure and phase transition by in situ observation Yaxin Huang1, Chongyang Zhu2, Shengli Zhang*,1, Xuemin Hu1, Kan Zhang1, Wenhan Zhou1, Shiying Guo1, Feng Xu*,2, Haibo Zeng*,1 1Key

Laboratory of Advanced Display Materials and Devices, Ministry of Industry and

Information Technology, College of Material Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China 2SEU-FEI

Nano-Pico Center, Key Laboratory of MEMS of the Ministry of Education, Southeast

University, Nanjing 210096, China *Correspondence and requests for materials should be addressed ([email protected]), F.X. ([email protected]), H. Z. ([email protected])

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Abstract Bismuth has garnered tremendous interests for Na-ion batteries (NIBs) due to potentially high volumetric capacity. Yet, the bismuth upon sodiation/desodiation experiencing structure and phase transitions remains unclear, which set a challenge for accessing nanotechnology and nanofabrication to achieve its applicability. Here, we use in situ transmission electron microscopy (TEM) to disclose the structure and phase transitions of layered bismuth (few-layer bismuth nanosheets) during Na+ intercalation and alloying processes. A multi-step phase transitions from Bi→NaBi→c-Na3Bi (cubic)→h-Na3Bi (hexagonal) are clearly identified, during which the Na+ migration from interlayer to in-plane evokes the structure transition from ABCABC staking type of c-Na3Bi to ABABAB staking type of h-Na3Bi. It is found that the metastable c-Na3Bi devotes to buffer the dramatic structure changes from thermodynamic stable h-Na3Bi, which unveils the origin of volume expansion for bismuth and has important consequences for 2D in-plane structure. As the lateral ductility can efficiently alleviate the in-plane mechanical strain caused by the Na+ migration, the few-layer bismuth nanosheet exhibits a potential cyclability for NIBs. Our findings will encourage more attentions into bismuthene as a novel anode material for secondary batteries.

Keywords: Bismuthene, sodiation, desodiation, phase transition, in situ transmission electron microscopy

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Currently, Na-ion batteries (NIBs) have been considering as potential alternatives to Li-ion battery (LIBs) due to abundant Na sources are in natural world that are widely available without the geopolitical issues.1-8 Bismuth (Bi), as an alternately stacked layer-by-layer material with interlayer spacing of 3.95 Å along the c-axis, possesses a similar gravimetric capacity of 385 mAh g-1 to commonly used graphite (372 mAh g-1) in LIBs,9 but a high theoretical volumetric capacity of 3800 mAh cm-3, enables its potential application in the NIBs.10 Although the volume expansion of Bi upon sodiation to Na3Bi is 250%,11 the bulk Bi can demonstrate 94.4% capacity retention after 2000 cycles by using NaPF6-diglyme electrolyte.12 The impressive result was proposed as gradual structure evolution from the layer to a porous integrity, which favors Na+ access and transport. In fact, apart from the structure evolution, the phase transition of Bi upon sodiation/desodiation has witnessed an increasing attention, because the reaction path is central to understand the underlying mechanism of structure transformation. In the past years, the sodiation mechanism of Bi has been generally considered as two-step

reaction

mechanisms,

intercalation

and

alloying

process

(Bi↔NaBi↔Na3Bi).11,13 Two years ago, through the quasi-simultaneous in operando synchrotron XRD/XAS and DFT calculations, Sottmann et al. pointed out that the phase transition upon sodiation is depending on the initial size of the Bi grain.14 The Na-Bi alloying presented hexagonal Na3Bi (h-Na3Bi) and cubic Na3Bi (c-Na3Bi) respectively in microsized Bi grain and nanosized Bi grain. On the other hand, Sun et al claimed a single-step intercalation mechanism for nanosized Bi particles by using ex-situ XRD, which was also evidenced by DFT calculation on Na+ accommodation in c-axis of Bi crystals.15 The contradictive results illustrate that further studies are needed to understand the sodiation/desodiation mechanism of Bi by clearly visualizing phase transition processes. Indeed, due to metastable NaBi and Na3Bi alloys, the ex-situ condition may provide inaccurate massages as a result of decomposition of these compounds. Moreover, the anisotropic Na+ diffusion in Bi

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crystal might also lead to desynchronized reaction occurring on different atomic arrangements. As a result, a clarification of structure and phase transition of Bi-NIBs, by in situ transmission electron microscopy (TEM) for visualizing sodiation of ultra-thin bismuth nanosheets (NSs), is of utmost importance. Considering the anisotropic behaviors of Na+ insertion/extraction and the crystallographic basal plane favored sodiation/desodiation, 2D flake with the structural advantages of easy ion access and long-range order basal plane would be a best sampling for the in situ observing intercalation and alloying processes.16,17 Bismuth, as the last member of group-VA materials, exhibits a layer structure resembling to As and Sb18-23 which can be easily exfoliated into 2D nanosheets.24-26 Ultra-thin bismuth material (few-layer bismuth nanosheets) may provide an effective to tackle these problems. In this work, we successfully fabricate the few-layer and monolayer bismuth NSs through ice-bath sonication liquid phase exfoliation for disclosing the intercalation and alloying processes for Na+ storage, motivated by aforementioned advantages. We then visualize the sodiation mechanism through in situ TEM, which clearly distinguish the intercalation to alloying processes with a gradual evolution of NaBi (111), cubic-Na3Bi (111) and hexagonal-Na3Bi (002) planes on bismuth NS (003). The Bi atoms in planes of the four crystal phases with similar atomic arrangement observed by high-resolution TEM indicate the alloying process with Na ions inserting the interlayer or in-plane space of Bi (003) planes. Meanwhile, the 2D feature of bismuth NSs can still retain after repeated sodiation/desodiation processes, suggesting that bismuth NSs basal plan can alleviate the huge volume expansion of alloying processes. Bi crystal, as a typical semimetal material, exhibits a layered hexagonal structure with the space group R3m (No. 166), as shown in Supporting Information Figure S1. Ultra-thin bismuth NSs were successfully prepared by a simple liquid-phase exfoliation technique that involves the grinding of crystalline Bi powder and the subsequent ultrasound probe sonication in an ice-bath. The SEM images of the freeze-dried ultra-thin bismuth NSs powders are shown in Figure 1a and Figure S2.

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Intriguingly, ultra-thin bismuth NSs exhibit flower-like structures during treatment, which can be ascribed to the feasibility of the ultrasound probe sonication in preparing plenty of ultra-thin bismuth NSs. Detailed methods (Supporting Information) and the schematic illustration of the liquid-exfoliation operation are shown in Figure S3.

Figure 1. Characterization of ultra-thin bismuth NSs. (a) Scanning electron microscope (SEM) image of freeze-drying ultra-thin bismuth nanosheets powders. (b) HRTEM image of ultra-thin bismuth NSs with the lattice fringes of (012) and (110) planes, upper inset is FFT. (c) AFM image of as-obtained ultra-thin bismuth NSs and the corresponding height profiles. (d) XRD patterns, (003) peak upper insert and (e) Raman spectra of bulk Bi crystals and bismuth nanosheets. (f) XPS of bismuth nanosheets.

Atomically thin bismuth NSs can be observed from a low-magnification TEM image with extremely lower contrast even than carbon films, despite some small and thick bismuth NSs on its surface, as shown in Figure 1b. The homogenously exfoliated ultra-thin bismuth NSs are confirmed by a series of TEM images in Figure S4. Besides, its high-resolution TEM (HRTEM) image in Figure 1c shows the prefect lattice fringes of 3.27 and 2.26 Å, corresponding to the prominent (012) and (110)

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planes of the hexagonal Bi crystal. Inset is the corresponding fast Fourier transformation (FFT) pattern, where the symmetric hexagonal spots further confirm the fabrication of ultra-thin bismuth NSs. The AFM images of bismuth NSs are shown in Figure 1c and Figure S5. Height profiles reveal a thickness of 0.65 nm and 0.63 nm for green line and blue line. The theoretical monolayer bismuth NS thickness is 0.395 nm. Because the thickness of 2D material measured by the AFM is generally overestimated due to the substrate roughness,27-30 the thickness calculated that is smaller than 2-fold theoretical value could be considered monolayer bismuth NS. To examine the quality of the obtained bismuth NSs, the crystal structure and the surface chemical composition are further acquired by XRD, Raman spectrum and XPS. All diffraction peaks in the XRD pattern of the bismuth NSs can match well with those of bulk Bi crystals in Figure 1d, indicating good crystallinity of bismuth NSs even after the severe treatment of sonication. Notably, compared to Bi crystal, the (003) peak of bismuth NSs shifts to a smaller angle and the corresponding full-width at half-maximum (FWHM) broadens (Figure S6), directly indicating the decrease of layers and expanding of c-axis interlayer distance. Raman spectra are used to confirm the ultra-thin bismuth NSs. Two prominent peaks locate at 69 and 98 cm-1, which correspond to the in-plane Eg peak and the out-plane A1g peak, respectively.31 In addition, we note that no signal of bismuth oxides is detected from the XRD, Raman and XPS spectra,32 indicating high quality of ultra-thin bismuth NSs (Figure 1e,f and Figure S7). We next explore the sodiation dynamics of ultra-thin bismuth NS by the real-time observation of its sodiation process inside TEM. To this end, an all-solid nanosized NIB that enables the in situ electrochemical experiments of an individual bismuth NS is constructed, as schematically shown in Figure 2a.33,34 Briefly, the nanosized electrochemical cell consists of a bismuth NS working electrode that is dispersed on the carbon film-supported half TEM grid, a metal Na counter electrode on sharp tungsten probe, and a solid electrolyte of Na2O layer naturally grown on the surface of metal Na. The tungsten probe can be driven by a piezo-positioner inside TEM to make the Na/Na2O layer contact with bismuth NS. Afterwards, constant negative potential is applied to the bismuth NS with respect to sodium, so as to drive the

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transport of sodium ions through the Na2O layer, consequently initiating the electrochemical sodiation of bismuth NS. Figure 2b presents the corresponding TEM image of the nanosized NIBs using bismuth NS as electrode material.

Figure 2. In situ observation of the multi-step sodiation process of a bismuth NS. (a) Schematic illustration of the experimental setup for in situ electrochemical sodiation/desodiation of bismuth NS. (b) A corresponding TEM image of the nanosized NIBs using bismuth NS as electrode material. (c-k) Time-sequences HRTEM images of a bismuth NS during sodiation, scale bar: 10 nm. (l) Graph of the projected area of each sodiated phase as a function of sodiation time. Colors used in (l) and (m) represent a pristine phase (red), a phase during the intercalation reaction (blue), and a phase during the alloying reaction (green). (m) Graph showing the areal change rates of the intercalation and conversion regions as a function of sodiation time.

Figure 2c-k show the time-sequences of HRTEM images of a bismuth NS during sodiation, where the morphological and structural evolutions are observed

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(Supporting Information Movie S1). Upon sodiation, the intrinsic contrast of the bismuth NS immediately became light, as indicated by the white dashed line. This may be caused by the insertion of Na ions into the host structure of the bismuth NS. However, we note that the lattice fringes at the locations where Na ions have swept remained the same as the original, meaning that these Na ions are only intercalating into the interlayers of bismuth NS but do not collapse its crystal structure. Similar phenomenon was also observed in Fe2O3 and Cu2S nanoplates, in which the intercalation of Li or Na ions led to significant changes in contrast.16,

34

With the

sodiation proceeding, the intercalation reaction front gradually propagates forward, with a trend from the outside to the inside of the nanosheet, suggesting isotropic sodium diffusion in the in-plane of bismuth NS. It is worthy to note that before this front propagation finishes, another reaction front marked by yellow dotted line follows closely, indicating that two different reactions occur simultaneously. From Figures 2c-k, the lattice fringes of bismuth NS behind the yellow front gradually vanish and show obvious volume expansion. The newly formed polycrystalline structure directly reveals the bismuth NS alloying reaction. To quantitatively analyze the sodiation dynamics of bismuth NS, the areal changes of the aforementioned two reaction regions are measured from time-lapsed HRTEM images (Figure 2l). Upon Na insertion, the intercalation reaction begins with its area increasing gradually. The intercalation kinetics is regarded to be controlled by the diffusion of Na ions in bismuth NS according to the arc-shaped propagation path of the intercalation front (Figure 2c‒g). Subsequently, the alloying reaction area also starts to increase. Based on the results from Figure 2l, the areal change rates of two reactions are obtained (Figure 2m). Clearly, the initial intercalation speed is two times higher than that of the alloying reaction. However, as the intercalation front moves away from the sodium source, the intercalation velocity dramatically decreases due to the rapid consumption of sodium by the fast-moving alloying reaction front. Ultimately, the intercalation reaction comes to an end and the bismuth NS undergoes the alloying reaction with a relatively stable speed.

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Figure 3. Structural evolution of bismuth NS at different sodiation stages. HRTEM images and the corresponding FFT patterns of (a) bismuth NS, (b) NaBi, (c) cubic Na3Bi, (d) hexagonal Na3Bi, scale bar: 2 nm. (e) Atomic structure models for different stages of phase evolutions and their plane distances corresponding to HRTEM (Vacuum of bismuth NS > 15 Å).

To deeply understand the phase evolutions of bismuth NS during alloying reactions, we performed real-time HRTEM to examine the atomic structures of all the reaction phases (Movie S1). As shown in Figure 3 and Figure S8, HRTEM images are obtained at different stages of sodiation with the corresponding FFT patterns and atomic model schematics. Before reaction, pristine bismuth NS exhibits a hexagonal structure. The lattice fringe of 3.96 Å can be well indexed to the Bi (003) plane. Sodium ions transport along the [001] direction into bismuth NS and generate multiple phases. At the first reaction step, sodium ions react with the bismuth NS and form NaBi alloy with a tetragonal structure (P4/mmm, No. 123). Both HRTEM image and the FFT pattern indicate the formation of NaBi phase because the lattice fringe and diffraction spots match well with the (111), (001), and (002) planes of the NaBi. Meanwhile, the diffraction spot of (003) planes of pristine bismuth NS are also

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observed in the FFT pattern, suggesting the co-existence of bismuth NS and NaBi at this stage. With sodiation reaction going on, the NaBi phase finally evolves into the Na3Bi phase. However, promisingly, two types of Na3Bi, cubic and hexagonal Na3Bi phases are found in our work (Figure 3e). In particular, the cubic Na3Bi phase distributes over a very small area and only exists within a short time scale. At the final reaction step, the metastable cubic Na3Bi phase further transforms into the hexagonal Na3Bi phase which shows lower formation energy and acts as the ultimate sodiation product. Therefore, the entire sodiation mechanism of bismuth NS can be expressed as follows: Bi (hexagonal) + xNa+ + xe- → NaxBi (hexagonal), NaxBi (hexagonal) + (1-x)Na+ + (1-x) e- → NaBi (tetragonal), NaBi (tetragonal) + 2Na+ + 2e- → Na3Bi (cubic), Na3Bi (cubic) → Na3Bi (hexagonal).

Figure 4. Further schematic demonstrating of structure evolution during the different stages of change, the Bi atoms located in the HRTEM related planes are highlighted exhibiting (a-c) ABCABC… or (d) ABAB… stacking. Red, green and blue spheres represent Bi atoms in A, B and C layers, respectively, and yellow spheres represent Na ions. Top views of (e) Bi (003), (f) NaBi (111), (g) c-Na3Bi (001) and (h) h-Na3Bi (002) planes.

With further analysis of the structure evolutions, we find that Bi atoms in the planes corresponding to the HRTEM lattice fringes of each stage stay in the similar trigonal arrangement. From Bi → NaBi → c-Na3Bi, Figure 4a-c,e-g show that the trigonal planes remain the similar ABCABC type, and only the distances of Bi atoms and the interlayers spaces change. Noted that the Na ions only exist in the interlayers of (111) planes of NaBi and c-Na3Bi phases. In Figure 4d,h, when the metastable c-Na3Bi

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phase further changes into the h-Na3Bi phase, the stacking type changes from ABCABC to ABABAB with the interlayer expansion of trigonal plane. Also, one third of Na ions insert in the trigonal Bi atoms planes (Figure 4d). In consequence, the structure evolution of NaBi→c-Na3Bi is milder than c-Na3Bi→h-Na3Bi or NaBi directly changed into h-Na3Bi, which explains the reason that NaBi changes into c-Na3Bi before h-Na3Bi. Also, by means of first-principles study, the h-Na3Bi possesses higher stability with its formation energy (-0.377 eV/atom) lower than c-Na3Bi (-0.364 eV/atom). So that c-Na3Bi transforms into h-Na3Bi, ultimately. More interestingly, c-Na3Bi and h-Na3Bi have the same value of charge transfer of 2.11 e/atom. At the same time, c-Na3Bi has higher density of 4.036 g/cm3 than that of h-Na3Bi of 3.63 g/cm3. These mean that c-Na3Bi can contain the same electrical energy with less volume expanding during the sodiation process, indicating that the c-Na3Bi could buffer the intense volume expansion of NaBi→h-Na3Bi. Our study also shows that major variation of the structure evolution in the first two steps is Na ions insertion in the interlayers of Bi (003) plane corresponding to the bismuth NS surface. So that the flexibility of the ultra-thin bismuth NSs may lead to more c-Na3Bi which could buffer the intense volume expansion of sodiation. We believe this finding would lead to the precise understanding on the sodiation process of bismuth NS electrode.

Figure 5. Morphological evolution of bismuth NS during multiple electrochemical sodiation/desodiation cycles

For bismuth NS electrode, cycling performance is of great importance when used

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for rechargeable batteries. Thus the structural changes of bismuth NS during electrochemical sodiation/desodiation cycles have been recorded in Figure 5 (Movie S2). It can be seen that the bismuth NS shows good structural stability after repeated sodiation/desodiation processes as no obvious fractures are recognized on its surface. This phenomenon differs from the case of bulk bismuth material where large volume expansion and severe structural collapse would occur upon numerous electrochemical cycles. This is mainly attributed to the nonuniform strain induced by the sodium insertion and has been thought to be responsible for the fast fading in specific capacity. On the contrary, sodium ions have lower diffusion barrier in few-layer bismuth NSs, which facilitates the rate performance of batteries. Moreover, bismuth NSs show small lateral expansion of around 13% after four cycles. Cyclic performances also show better cyclic stabilities of bismuth NSs compared with bulk bismuth (Figure S9), indicating that the ultra-thin bismuth NSs are promising electrodes for rechargeable batteries. In summary, by means of in situ HRTEM, we clearly observed the whole sodiation process of ultra-thin bismuth NSs, fabricated by a simple probe liquid exfoliation technique, from insertion to conversion. The structure evolution of conversion process was mainly carried out with Na ions insert in the interlayer of Bi (003) planes which possess more space in few-layer bismuth NSs. Moreover, bismuth NSs show good structural stability after sodiation/desodiation cycles. These findings provide an important guidance for the development of bismuthene and its potential battery applications.

ASSOCIATED CONTENT Supporting Information Details of the fabrication and the characterization of the bismuth NS, calculation results and in situ TEM measurement.

AUTHOR INFORMATION Corresponding Author Zhang, S. ([email protected])

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Xu, F. ([email protected]) Zeng, H. ([email protected])

Author Contributions Huang, Y. and Zhu. C contributed equally to this work.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Funds for Distinguished Young Scholar (61725402), Natural Science Foundation of Jiangsu Province (BK20180071), the National Basic Research Program of China (2015CB352106), the National Natural Science Foundation of China (11774051, 61574034) and PAPD of Jiangsu Higher Education Institutions.

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