Effective Interlayer Engineering of Two-Dimensional VOPO4

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Effective Interlayer Engineering of Two-Dimensional VOPO4 Nanosheets via Controlled Organic Intercalation for Improving Alkali Ion Storage Lele Peng, Yue Zhu, Xu Peng, Zhiwei Fang, Wangsheng Chu, Yu Wang, Yujun Xie, Yafei Li, Judy J. Cha, and Guihua Yu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02958 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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Effective Interlayer Engineering of Two-Dimensional VOPO4 Nanosheets via Controlled Organic Intercalation for Improving Alkali Ion Storage Lele Peng,†,‡ Yue Zhu,†, ‡ Xu Peng,§ Zhiwei Fang,† Wangsheng Chu,§ Yu Wang,┴ Yujun Xie,¶ Yafei Li,┴ Judy J. Cha,¶ and Guihua Yu†,* †

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

University of Texas at Austin, Austin, Texas 78712, United States. §

National Synchrotron Radiation Laboratory, University of Science and Technology of China,

Hefei, Anhui 230026, China. ┴

College of Chemistry and Materials Science, Nanjing Normal University, Nanjing, Jiangsu

210023, China. ¶

Department of Mechanical Engineering and Materials Science, Yale University, New Haven,

Connecticut 06520, United States.

KEYWORDS: VOPO4 nanosheets, organic intercalation, interlayer engineering, sodium ion battery, energy storage

ABSTRACT:

Two-dimensional

(2D)

energy

materials

have

shown

the

promising

electrochemical characteristics for lithium ion storage. However, the decreased active surfaces

ACS Paragon Plus Environment

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and the sluggish charge/mass transport for beyond-lithium ion storage that has potential for large-scale energy storage systems, such as sodium or potassium ion storage, caused by the irreversible restacking of 2D materials during electrode processing remain a major challenge. Here we develop a general interlayer engineering strategy to address the above-mentioned challenges by using 2D ultrathin vanadyl phosphate (VOPO4) nanosheets as a model material, for challenging sodium ion storage. Via controlled intercalation of organic molecules, such as triethylene glycol (TEG) and tetrahydrofuran (THF), the sodium ion transport in VOPO4 nanosheets has been significantly improved. In addition to advanced characterization including XRD, HRTEM, and XAFS to characterize the interlayer and the chemical bonding/configuration between the organic intercalants and the VOPO4 host layers, DFT calculations are also performed to understand the diffusion behavior of sodium ions in the pure and TEG intercalated VOPO4 nanosheets. Due to the expanded interlayer spacing in combination with the decreased energy barriers for sodium ion diffusion, intercalated VOPO4 nanosheets show much improved sodium ion transport kinetics and greatly enhanced rate capability and cycling stability for sodium ion storage. Our results afford deeper understanding of the interlayer-engineering strategy to improve the sodium ion storage performance of the VOPO4 nanosheets. Our results may also shed light on possible multivalent-ion based energy storage such as Mg2+ and Al3+.

Sodium ion batteries (SIBs) are of great interest as a potentially low-cost and safe alternative to the prevailing lithium ion battery technology owing to the high abundance of sodium in earth crust, its even distribution in nature and favorable redox potential (only ~0.3 V above that of lithium). 1−7 Figures of merit for future SIBs call for a breakthrough in energy (>200 Wh kg−1)


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and power density (>2000 W kg−1) as well as the cycle life (>4000 cycles) by rational material design and engineering as well as identifying new chemistries, to meet the requirements of many potential applications ranging from ubiquitous portable electronics to grid-scale energy storage.8−12 2D energy materials especially those that undergo ion insertion/extraction reactions have shown the promising electrochemical characteristics, especially high redox activity and rate capability for alkali ion storage.13−15 Vanadium-based phosphates, especially 2D vanadyl phosphate (VOPO4) with a long history in intercalation chemistry, have been recently explored as promising cathode materials for electrochemical energy storage, such as pseudocapacitors, and lithium batteries, due to the rich redox reactions, high theoretical capacity, high redox reaction potential as well as unique structural features.16−22 When one sodium ion is considered to take part in the insertion/extraction process, 2D VOPO4 nanosheets output a voltage plateau at ~3.5 V vs Na/Na+ for V4+/V5+ redox couple and a high theoretical capacity of 166 mAh g−1. For example, Manthiram and co-workers prepared the VOPO4 layered structures by chemical delithiation of the tetragonal αI-LiVOPO4 at room temperature.23 In sodium ion cells, the VOPO4 cathodes delivered a reversible capacity of 110 mAh g−1, most of which are contributed by a long plateau at 3.4~3.5 V. 2D VOPO4 nanosheets also show advantageous structural features for energy storage. For instance, 2D ultrathin VOPO4 nanosheets possess open 2D ion transport channels and short ion diffusion pathways, which enable a highly reversible intercalation pseudocapacitance of lithium ions in the ultrathin VOPO4 nanosheets synthesized by isopropanol exfoliation.24 2D ultrathin VOPO4 nanosheets possess adjustable interlayer spacings by controllably intercalating various intercalants such as small organic molecules and polyelectrolytes. This feature will improve the ion transport kinetics and maintain the structural integrity of the electrode during the electrochemical cycles.25−27


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Meanwhile, a general concern about 2D energy materials has been raised recently. 2D open framework materials have proven effective in constructing kinetically favorable sodium ion channels, but 2D nanomaterials also suffer from significantly decreased active surfaces and sluggish sodium ion transport due to the irreversible restacking of the isolated 2D nanosheets during materials processing or device fabrication.28−31 To address this issue, a potential solution to better accommodate sodium ions is to increase the interlayer spacing to facilitate sodium ion transport by creating a lower energy barrier for sodium ion transport through the interlayer space.32,33 Like many other layered materials, VOPO4 has been explored as host materials for studying intercalation chemistry involving various organic molecules, such as alcohols, ethers, aldehydes, carboxylic acids, amines and heterocyclic N- and S-donors. Corresponding intercalation mechanisms, species of intercalants and the possible bonding between the intercalants and the host have been studied.34−36 Recently, intercalation of 2D materials has emerged as a useful modification strategy towards tuning their physical and chemical properties, such as proton conduction, current and energy transport.37-40 Despite many exciting results reported, the electrochemical energy storage characteristics of the intercalated 2D nanomaterials remain to be further explored. Here we develop an effective interlayer engineering strategy to improve the sodium ion transport in VOPO4 nanosheets via controlled organic intercalation. As a proof-of-concept, triethylene glycol (TEG) and tetrahydrofuran (THF) are chosen as the intercalants to demonstrate the feasibility of this interlayer engineering strategy and its advantageous features for improving sodium ion transport and storage. The as-obtained VOPO4 nanosheets show similar 2D morphology with ultrathin thickness and expanded interlayer distances, as identified by the Xray diffractometry (XRD) patterns and the cross-sectional high resolution transmission electron


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microscopy (HR-TEM). The intercalation process can be extended to various organic molecules and the interlayer distance of the VOPO4 nanosheets can be readily tuned by intercalating different organic molecules, such as the TEG and THF molecules adopted in the current report. We also use the X-ray absorption fine structure (XAFS), for the first time, to study the chemical bonding between the organic intercalants and the VOPO4 host layers. The XAFS results show that the organic intercalants are successfully intercalated into the individual VOPO4 layer. Density functional theory (DFT) calculations are performed to better understand the diffusion behavior of sodium ions in the pure and TEG intercalated VOPO4 nanosheets. Post-cycling characterization including XRD analysis and STEM imaging confirm the stability of the intercalants in the interlayers. Due to the expanded interlayer spacing in combination with the decreased energy barriers for sodium ion diffusion, the interlayer expanded VOPO4 nanosheets show much improved sodium ion transport kinetics and much improved rate capability and cycling stability for sodium ion storage.


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Figure 1. Schematic illustration of the intercalation process of 2D VOPO4 nanosheets. (a) Crystal structure of the intrinsic VOPO4∙2H2O nanosheets. (b) Chemical structure of the TEG and THF intercalants. (c-d) Schematic of intercalation process and the intercalated structure. (e) The bonding structure of the TEG and THF in VOPO4 nanosheets. The concept for the developed general synthesis of 2D TEG and THF intercalated VOPO4 nanosheets is illustrated in Figure 1. The intercalation compounds were obtained by a facile displacement reaction, and the intercalation process was suitable for large-scale production (see Supporting Information for experimental details). In brief, VOPO4·2H2O bulk chunk was first employed as the starting material for the intercalation (Figure 1a), and mixed with IPA and the organic intercalants, i.e. TEG or THF solvents (Figure 1b). The as-obtained VOPO4 nanosheets with organic molecule intercalation exhibited different and controllable interlayer distances, due to the different chemical structures of the TEG and THF molecules. TEG and THF can uniformly replace the water molecules in the bulk chunk and intercalate in the layers through the hydrogen bonds formed between the intercalants and VOPO4 layers (Figure 1d). The intercalated molecules can be stabilized by these hydrogen bonds, as observed the similar case of intercalants in graphic intercalation compounds.41,42 We proposed the possible bonding structures of the TEG and THF molecules with the VOPO4 layers in Figure 1e. TEG molecules were aligned between the VOPO4 layers because the final product exhibited the large interlayer distance.


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Figure 2. Structural characterization of the organic molecule intercalated VOPO4 nanosheets. (a) XRD patterns of pure VOPO4·2H2O bulk chunk, THF and TEG intercalated VOPO4 nanosheets. (b) Typical STEM image of the TEG-VOPO4 nanosheets. Scale bar: 500 nm. (c) Cross-sectional HR-TEM image of 2D TEG-VOPO4 nanosheets, showing a characteristically layered structure. Scale bar: 20 nm. (d) Enlarged picture of the cross-sectional TEM, showing an interlayer distance of ~1 nm. A crystallographic model of the layered structure is shown here, highlighting the (001) planes. The black dashed circle correlates these planes to the lattice fringes shown in cross-sectional TEM image. Scale bar: 5 nm. (e)

K-edge X-ray absorption near-edge

spectroscopy (XANES) of the VOPO4 nanosheet intercalated with different molecules. (f) Extended X-ray absorption fine structure (EXAFS) at vanadium K-edge of the VOPO4 nanosheet intercalated with different molecules. The interlayer distances of TEG and THF intercalated VOPO4 were studied with XRD and cross-sectional HR-TEM. As shown in Figure 2a, the diffraction peaks corresponding to the interlayer spacing shift to lower angles after the intercalation of TEG and THF molecules. This


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shift indicates a gradually increased interlayer distance according to Bragg’s formula (d= 0.5λ/sin(θ)). The interlayer distance of pristine VOPO4·2H2O bulk chunk is calculated to be 0.74 nm.43,44 Upon intercalation of organic molecules, the spacings further increase to 0.88 nm for THF intercalated VOPO4 nanosheets (THF-VOPO4) and 1.06 nm for TEG intercalated VOPO4 nanosheets (TEG-VOPO4), corresponding to 20 and 43 % increase, respectively. The calculated spacing values were cross-validated by HR-TEM observation, as shown in Figure 2b-d. Figure 2b showed the typical STEM image of TEG-VOPO4 nanosheets with flat surface and ultrathin thickness. Similar morphology and thickness can be obtained in the THF-VOPO4 nanosheets (Figure S1). Focused ion beam (FIB) was used to cut the nanosheets for the cross-sectional TEM characterization. Cross-sectional TEM of the TEG-VOPO4 nanosheets in Figure 2c and 2d showed the well-defined layered structures with ~7 atomic layers and an interlayer distance of ~1 nm, which is consistent with the value obtained from the XRD patterns. Further characterizations give insights into the chemical composition and structural properties of the interlayer expanded VOPO4 nanosheets. Raman spectra shown in Figure S2 verified the structural properties of the pure VOPO4·2H2O bulk chunk, THF and TEG intercalated VOPO4 nanosheets. The bands at 937 cm−1 in the curve of pure sample was assigned to the symmetric O–P–O stretching modes.45,46 Obviously, the symmetry of O–P–O stretching modes is strongly correlated to regional microstructures and very sensitive to the hydrogen bonding between the interlayered H2O molecules and V atoms. With the intercalation of THF and TEG molecules and removal of the interlayered H2O, the hydrogen bonds formed between the O atoms of P–O bond in the layer and H2O were broken, inducing the mitigation of the steric hindrance and the shift of O–P–O stretching modes to lower energy (lower wavenumbers). While the peaks of the symmetric bending vibrations of O–P–O, V–O and V=O stretching mode


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showed little shift in peak positions. Pure VOPO4 spectrum showed a sharp band at 995 cm−1 corresponding to a vanadyl V=O stretching vibration, which appeared to be particularly sensitive to the atoms coordinated to vanadium within an octahedral arrangement in the host lattice structure. The position of V=O stretching vibration shifted from 995 cm−1 in the dihydrate form to 1031 and 1021 cm−1 respectively, after the intercalation of TEG and THF molecules into VOPO4. These positions observed in these bands correspond to the coordination of oxygen atoms of TEG and THF to vanadium, respectively.47 The Raman results indicated that the intercalated VOPO4 samples are found most probably not to contain trapped water, and the intercalated VOPO4 nanosheets maintained the integrity of the in-plane layered VOPO4 structure without obvious structural deformations. Thermogravimetric analyses (TGA) of the pure, TEG and THF intercalated VOPO4 samples reveal the weight loss (Figure S3). Pure VOPO4 bulk chunk showed a weight loss of ~18%, indicating the loss of the two structural water molecules in the layers. The intercalated samples showed weight loss of ~25% and ~30% for THF and TEG intercalated VOPO4 nanosheets, indicating that 0.3 THF molecule and 0.4 TEG molecule were intercalated in VOPO4.48 To further unravel the chemical bonding/configuration between the organic intercalants and the VOPO4 host layers, XAFS tests were conducted. Considering the polarity or the dielectric property of the ligand can affect the chemical value of the center metal, synchrotron-based X-ray absorption spectroscopy (XAS), that is sensitive to the partial electronic structure and the local geometry around the selected absorber, has been used to monitor the change of the chemical configuration of the VOPO4 nanosheet to determine the interaction between vanadium and the different intercalants, H2O, TEG and THF. Figure 2e shows the vanadium K-edge X-ray absorption near-edge spectroscopy (XANES) of the VOPO4 nanosheet intercalated with different


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molecules. The XANES spectra of the vanadium foil and VO2 were also measured to calibrate the X-ray energy. It is easy to find the edge shifts to the lower energy with the order of H2O > TEG > THF. That means the chemical value of vanadium in the samples gradually decreases with the same order. This is further verified by the XPS results of the three samples (Figure S4). The chemical value changes of these samples were also reflected by the pre-edge feature of the XANES. The XANES of VOPO4·2H2O presents an obvious pre-edge peak at 5472.0 eV, labeled by b. Besides the peak b, the other pre-edge peak, labeled by a, appeared at 5470.5 eV for the VOPO4-THF. Pre-edge peak at this position is a typical fingerprint of the V4+ oxidation state, as shown by the pre-edge peak of VO2 reference. The energy shift of 1.5 eV indicates the V oxidation state in VOPO4·2H2O sample is about V5+. Higher oxidation state of the vanadium means the stronger electronegativity of the coordinated molecule for this case. It’s known that the electronegativity of TEG is less than that of H2O and stronger than THF. This coherence between the oxidation state of the center vanadium and the electronegativity of the ligand molecules suggests the organic molecules TEG and THF have been successfully intercalated into the VOPO4 nanosheet and coordinated into vanadium. Extended X-ray absorption fine structure (EXAFS) at vanadium K-edge was also performed to check if any rearrangement occurs for the local atomic environment around the vanadium ion. Figure 2f shows the k3-weighted Fourier transforms (FTs) of the EXAFS oscillations of three prepared samples. All of these FT curves present similar pair distribution functions until the distance of 6 Å, which indicates the integrity of the local structure around vanadium ion conserved while the organic molecules TEG and THF were intercalated into the VOPO4 nanosheet layers. There are three types of coordinated oxygen atoms in the intercalated VOPO4 nanosheet: 1) the singly coordinated oxygen atom, with a short V-O bond of ~1.57 Å along the c


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axis (perpendicular to the layer); 2) the four-fold coordinated oxygen atoms bridged phosphorus atoms, with V-O bond of ~1.97 Å; 3) an oxygen atom from water or organic molecules out of layers, with a large bond distance of ~2.23 Å. FT curves of these samples only show a strong peak at ~1.79 Å (no phase corrections) and show the four-fold coordinated oxygen atoms from PO43- dominates the V-O pair distribution. The position of this peak almost does not change, but its intensity gradually decreases with the order of H2O > TEG > THF. It means the average distances of V-O bonds of these samples are the same, but the distortion of V-O bonds becomes larger and larger with the intercalation of TEG and THF. This trend is consistent with the change of the chemical value of vanadium ions revealed by XANES spectra discussed above. This is due to the weaker electronegativity of TEG and THF results in the larger dynamics of the coordinated oxygen atoms from the organic group.


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Figure 3. Sodium ion storage properties of 2D TEG and THF intercalated VOPO4 nanosheets. (a) CV curves of the TEG, THF intercalated VOPO4 nanosheets and the control pure VOPO4 nanosheets at the scan rate of 0.1 mV s−1. (b) Rate performance of the TEG, THF intercalated VOPO4 nanosheets and the control pure VOPO4 nanosheets from the C rates of 0.1 C to 20 C. (c) Long-term cycling stability and Coulombic efficiency of the TEG, THF intercalated VOPO4 nanosheets and the control pure VOPO4 nanosheets at 5C for over 500 cycles. To examine electrochemical properties of the 2D TEG and THF intercalated VOPO4 nanosheets, cyclic voltammetry (CV) curves, rate capability and cycling stability were conducted. Figure 3a shows the CV curves of the TEG, THF intercalated VOPO4 nanosheets and the control pure VOPO4 nanosheets at the scan rate of 0.1 mV s−1. All of the three samples exhibited two sharp redox peaks, corresponding to the sodiation/desodiation process, in the scanning range of 2.5V~4.3V. Figure 3b shows the rate performance of the TEG and THF intercalated VOPO4 nanosheets and the pure VOPO4 control sample. Three electrodes were able to deliver similar capacities of ~151, 149 and 151 mAh g−1 at the current density of 0.1C. The capacities are similar because the major contribution to the capacity is from diffusion controlled process. As the charge/discharge rates increase, three electrodes delivered different capacities indicating the different rate capabilities. As shown in Figure 3b, the TEG intercalated VOPO4 nanosheets were able to deliver reversible capacities of 155, 143, 132, 119, 107, 100, 89 and 74 mAh g−1 at C rates of 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20, and the THF intercalated VOPO4 nanosheets were able to deliver reversible capacities of 149, 140, 126, 114, 103, 89, 78 and 68 mAh g−1 at C rates of 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20. Both samples showed better rate performance than the pure VOPO4 nanosheets based control electrodes demonstrated in this work. The charge-discharge curves of the TEG, THF intercalated VOPO4 nanosheets at various


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C rates were shown in Figure S5. The reasons are attributed to the expanded interlayer distances of the TEG and THF intercalated VOPO4 nanosheets, which opens up the inner surfaces for sodium ion storage and facilitates the sodium ion transport between the layers. In addition, Figure 3c shows the cycling stability of the three nanosheet electrodes. TEG intercalated VOPO4 nanosheets delivered an average reversible capacity of approximately 88 mAh g−1 at a current rate of 5 C and sustained for 500 cycles with no obvious capacity decay, showing a capacity retention rate of 88%. THF intercalated sample showed a capacity of ~ 68 mAh g−1 at a current rate of 5 C, and can be maintained for 500 cycles with a capacity retention rate of 76%. Those values were higher than that of the pure VOPO4 nanosheets (51%). It should be noted that the intercalated VOPO4 nanosheets show reasonably good stability in terms of the organic intercalants and morphology, which were verified by the XRD patterns and morphology characterization before and after 500 cycles test (Figure S6). This further confirms the inherent stability of the intercalants in the electrodes.


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Figure 4. Kinetics analysis of the 2D TEG intercalated VOPO4 nanosheets for sodium ion storage. (a) CV curves at various scan rates from 0.02 to 0.2 mV s−1 for TEG intercalated VOPO4 nanosheets. (b) b-value evaluation using the relationship between peak current and scan rate. (c) Separation of the capacitive and diffusion currents at a scan rate of 0.5 mV s−1. (d) Contribution ratio of the capacitive and diffusion-controlled charge at various scan rates. To explore the reasons for the improved electrochemical characteristics for sodium ion storage of the TEG and THF intercalated VOPO4 nanosheets, CV measurements at various scanning rates were carried out to further understand the electrochemical kinetics. CV curves at various scan rates from 0.02 to 0.2 mV s−1 displayed similar shapes and a gradual broadening of the peaks can be observed (Figure 4a). For an electrochemical energy storage device, the capacity is always contributed by the diffusion controlled and capacitive controlled processes and these two processes can be measured by testing the CV data at various scan rates according to

i = av b

(1)

where the measured peak current i obeys a power law relationship with the sweep rate υ and a and b are adjustable parameters. In particular, the b-value of 0.5 indicates a total diffusion controlled process, whereas 1.0 represents a total capacitive process.49 By plotting log i vs log υ, b-values determined as the slopes and 0.82 and 0.87 were calculated for cathodic and anodic peaks (Figure 4b), indicating that the majority of the current at the peak potential is capacitive. The total capacitive contribution at a certain scan rate could be quantified by separating the specific contribution from the capacitive and diffusion-controlled process at a particular voltage according to


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i(V) = k1v + k2v1/ 2

(2)

where k1 and k2 are constants for a specific potential. By plotting i(V)/υ1/2 versus υ1/2, k1 is the slope and k2 is the intersection. Therefore k2 can be used to calculate the capacitive and diffusion contributions. For example, at scan rate 0.5 mV s−1 (Figure 4c), ∼70.6% of the capacity is contributed by the capacitive process. Figure 4d showed the ratio between the capacitive and diffusion currents at other scan rates. The quantified results showed that the capacitive contribution gradually improved with the scan rate increasing. For instance, a significantly high contribution (∼57.5%) of the capacity is from the capacitive process at the low scan rate of 0.1 mV s−1. When scan rate increasing to 10 mV s−1, the contribution increases to 98.7%. The electrochemical kinetic study for THF-intercalated VOPO4 nanosheets are shown in Figure S7. The b-values for the cathodic and anodic peaks in THF-intercalated VOPO4 nanosheet electrodes were calculated to be 0.73 and 0.79, respectively. At the scan rate of 0.1 mV s−1, ∼52.7% of the total capacity is capacitive in nature. The capacitive contribution increases to 96.3% as the scan rate improve to 10 mV s−1. The capacity contribution ratios at various scan rates were also showed in Figure S7. Both the calculated b-value and capacitive contribution clearly demonstrate the unique characteristics as a result of intercalation pseudocapacitance (Table S1). This unique storage behavior verifies the reasons for the improved rate capability and cycling stability in TEG and THF intercalated VOPO4 nanosheets. Table S2 summarizes the recently reported sodium ion cathode materials, where some major electrochemical parameters such as capacity, capacity retention and rate capability are included. In comparison, the organic intercalated VOPO4 nanosheets show excellent electrochemical performance.


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Figure 5. Diffusion barrier profiles of sodium ion transport in pure VOPO4, and TEG intercalated VOPO4 nanosheets. (a) Views of geometric structures of VOPO4 nanosheets (left) and sodium ion diffusion pathways (right). (b) Diffusion barrier (minimum energy path) profiles of sodium ion transport in pure VOPO4 nanosheets, and the TEG intercalated VOPO4 nanosheets. To validate our material and structural design and gain fundamental insight into the intercalation effect on the enhanced rate and cycling performance, we further performed DFT calculations for the diffusion behavior of sodium ions in the pure and TEG intercalated VOPO4 nanosheets. Computationally, the mobility of sodium ions in VOPO4 nanosheets can be deduced from the migration barriers. Thus, sodium ion migration simulations based on the TEG intercalated VOPO4 and the control pure VOPO4 nanosheets were carried out using the climbingimage nudged elastic band (CI-NEB) method as implemented in VASP software.50,51 Note that two possible diffusion pathways (P1 and P2) were identified for the diffusion of sodium ions in VOPO4 nanosheets (Figure 5). Figure 5a shows the minimum migration pathways of the pure and TEG intercalated VOPO4 nanosheets. According to our calculations, the pure VOPO4 nanosheets prefer the P1 pathway for sodium ion migration, whereas the TEG intercalated VOPO4 nanosheets with enlarged interlayer distance is in favor of P2. Specially, compared with the pure VOPO4 nanosheets (Eba = ~0.72 eV), sodium ion diffusion over TEG intercalated


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VOPO4 nanosheets with enlarged interlayer spacing exhibits a much lower Eba of ~0.22 eV (Figure 5b). Since the diffusion energy barrier describes the minimum energy which must be available for the sodium ion diffusion in the VOPO4 layers, the values of the energy barriers are closely related to the difficulty of the sodium ion transport. Therefore the increased interlayer distances should be the essential reason for the enhanced electrochemical performance in the intercalated VOPO4 nanosheets. These simulations not only provide theoretical evidence for the much improved rate capability and cycling stability in the TEG and THF intercalated VOPO4 nanosheets, but also strongly suggest that interlayer-expansion strategy is effective to improve the diffusion kinetics of large cations (such as Na+) in layer-structured hosts. To conclude, we have demonstrated controlled interlayer engineering of ultrathin VOPO4 nanosheets with significantly improved sodium ion transport and storage characteristics achieved by an effective organic intercalation. The general synthesis of the organic intercalated VOPO4 nanosheets can be extended to other organic molecules, such as amines and alcohols, as well as to other 2D nanomaterials. The intercalated VOPO4 nanosheets show expanded interlayer distance due to the different structures of the organic molecular intercalants. The TEG and THF intercalated VOPO4 nanosheets exhibit expanded interlayer distance with 1.06 nm and 0.88 nm, respectively. We use the XAFS, for the first time, to study the chemical bonding between the organic intercalants and the VOPO4 host layers. The XAFS results show that the organic intercalants are successfully intercalated into individual VOPO4 layer. Due to the expanded interlayer spacing in combination with the uniform intercalation of intercalants in VOPO4, the interlayer-engineered VOPO4 nanosheets show much improved sodium ion transport kinetics and much improved rate capability and cycling stability for sodium ion storage, compared with the pure VOPO4 nanosheets without organic molecules intercalation. DFT calculations of the energy


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barriers are performed to validate the observation of improved sodium ion transport in 2D TEG intercalated VOPO4 nanosheets. The results demonstrate that the energy barrier of sodium ion transport in TEG intercalated VOPO4 nanosheets is 0.22 eV, which is much lower than that of the pure VOPO4 nanosheets (0.72 eV). The simulations not only provide theoretical evidence for the much improved rate capability and cycling stability in the TEG and THF intercalated VOPO4 nanosheets, but also strongly suggest that interlayer engineering is very effective to improve the diffusion kinetics of large cations (such as Na+) in layer-structured hosts. Our results may also bring a unique perspective in structural design of energy storage electrode materials for enabling future generation of large-scale energy storage systems beyond Li+, such as Na+, K+, Mg2+, and Al3+.

ASSOCIATED CONTENT Supporting Information. Material synthesis, material characterization, such as STEM, Raman, TGA, XPS and electrochemical characterizations of the TEG and THF intercalated VOPO4 nanosheets, and DFT calculation details of the sodium ion transport. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions


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L.P. and Y.Z. equally contributed to this work. Notes The authors declare no financial competing interest. ACKNOWLEDGMENT We thank Prof. John B. Goodenough at the University of Texas at Austin for valuable discussion. G.Y. acknowledges the funding support from the Welch Foundation Award F-1861, ACS-PRF Young Investigator award (55884-DNI10), Sloan Research Fellowship, and Camille Dreyfus Teacher-Scholar Award.

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Table of Contents Graphic


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Effective Interlayer Engineering of Two-Dimensional VOPO4

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