Significantly Improving Lithium-Ion Transport via Conjugated Anion

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Significantly Improving Lithium-Ion Transport via Conjugated Anion Intercalation in Inorganic Layered Hosts Chunshuang Yan,†,‡,§ Zhiwei Fang,†,§ Chade Lv,†,‡ Xin Zhou,‡ Gang Chen,*,‡ and Guihua Yu*,†

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†

Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States ‡ MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, PR China S Supporting Information *

ABSTRACT: Layered hydroxides (LHs) have emerged as an important class of functional materials owing to their unusual physicochemical properties induced by various intercalated species. While both the electrochemistry and interlayer engineering of the materials have been reported, the role of interlayer engineering in improving the Li-ion storage of these materials remains unclear. Here, we rationally introduce pillar ions with conjugated anion dicarboxylate groups, cobalt oxalate ions ([CoOx2]2−), into the interlayers of Co(OH)2 nanosheets [denoted as ICo(OH)2 NSs]. The pillar ion guarantees excellent structural stability, high electrical conductivity, and accelerated Li-ion diffusion. The structure delivers highrate cycling performance for lithium-ion batteries. This work provides insights for the design of LH-based highperformance electrode materials by a rational interlayer-engineering strategy. KEYWORDS: layered hydroxides, oxalate ions, interlayer engineering, pillar effect, lithium-ion battery

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low electrical conductivity and limited reaction kinetics prevent their widespread use. Moreover, serious structural collapse during Li-ion insertion and extraction leads to the limited cycle lifetime. Tackling these problems requires a strategic material and structural design, which satisfies the following criteria: (a) high conductivity, (b) fast Li-ion transfer kinetics, and (c) enough buffering space for volume change.17−19 The interlayer engineering is considered a promising strategy by which to overcome the above-mentioned issues. The intercalation ions can expand the interlayer spacing to facilitate the ion diffusion.20 For instance, the various intercalation ions (e.g., sulfate, nitrate, chloride, acetate, etc.) as pillars have been intercalated into the interlayers of LHs to improve the electrochemical performance of supercapacitors.21−24 In addition, the intercalation can also influence the electrical structure to tailor the physicochemical properties of materials.25,26 Unfortunately, most intercalation ions are unstable and only have weak bonding with the hydroxide

ayered hydroxides (LHs), an emerging class of important functional anionic clays, consist of positively charged metal-hydroxyl host layers and charge-balancing interlayer anions.1,2 The most-intriguing characteristic is the ability to accommodate various ions and molecules between the layers, a phenomenon known as intercalation.3 By the proper control of anionic species between layers, LHs can be endowed with different properties for wide-ranging applications.4−10 For example, Kanatzidis’s group introduced MoS42− ions into the MgAl-LH gallery as an adsorbent to capture toxic heavy metals due to the strong covalent bonds between intercalated sulfides and heavy metals.6,7 He et al. reported that the organometallic complexes can also be intercalated in the interlayer space of LHs as heterogeneous catalysts to increase the enantioselectivity owing to the steric hindrance of the LH layer.8 Electrochemical energy storage devices that can operate at high rates (fast charging and discharging) are urgently needed in modern society. Developing valuable electrodes to enable high-rate capability is essential.11−14 LHs emerge as attractive anode materials for high-energy lithium ion batteries (LIBs) on account of their high theoretical capacities.15,16 However, the © XXXX American Chemical Society

Received: June 17, 2018 Accepted: July 18, 2018 Published: July 18, 2018 A

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Figure 1. Schematic diagram of the preparation of I-Co(OH)2 and β-Co(OH)2 NSs.

layers.21 Compared with the common intercalation ions, the metallic oxalate complex ions with conjugated dicarboxylate groups can strongly bond with the hydroxide layers by hydrogen bonding, resulting in excellent structural stability.27 Besides, the conjugated carboxylate family can offer extra Liion storage sites to enhance specific capacity due to the π−π orbital delocalization.28,29 Therefore, metallic oxalate complex ions are expected to be excellent candidates as the pillars of LHs. To the best of our knowledge, the rational intercalated structure with highly effective pillar ion has not been explored for LHs to improve Li-ion storage performance thus far. Herein, we present a design of conjugated anion intercalation in inorganic layered hosts, taking Co(OH)2 as a model material. Cobalt oxalate ions ([CoOx2]2−) intercalated Co(OH)2 nanosheets (I-Co(OH)2 NSs) were synthesized via a one-step solvothermal method using excess Na2C2O4 as both oxalate source and intercalation species. The achieved ICo(OH)2 NSs demonstrate enlarged interlayer spacing and abundant oxygen vacancies and nanopores. The proposed synthesis method is illustrated in Figure 1. As a comparison, βCo(OH)2 NSs as control sample were obtained without adding Na2C2O4 (Figure S1). The structural differences between them are studied in detail, shown below.

(HRTEM) images, lattice orders are severely damaged and accompanied by the formation of atomic vacancies due to the intercalation of [CoOx2]2− ions (Figure 2d,f,h). Energydispersive X-ray (EDX) mapping affirms the presence of elements Co, O, and C (Figure 2e). High-resolution TEM (HRTEM) images recorded from the edge region (Figures 2g and S6) reveals an enlarged lattice spacing of 9.8 Å, which is in good agreement with the (003) plane of I-Co(OH)2 NSs. The crystal structures of the products were examined by powder X-ray diffraction (XRD). Figure 3a shows the comparison of β-Co(OH)2 and I-Co(OH)2 NSs. The XRD pattern of β-Co(OH)2 NSs is consistent with a typical hexagonal β-LH structure (JPCDS no. 30-0443), of which basal spacing (dbasal) is 0.46 nm. For I-Co(OH)2 NSs, the two prominent low-angle reflections at 8.6° and 17.4° can be assigned to the (003) and (006) reflections of the α-Co(OH)2 phase. No peaks of any other phases related to CoC2O4 are detected, indicating the high purity of the final products. The weakened (003) peak relative to the stronger (006) peak results from the heavy nature of intercalation ions, implying the presence of [CoOx2]2− ions.27,32−34 Fourier transform infrared spectroscopy (FTIR) analysis further verifies the formation of I-Co(OH)2 NSs (Figure 3b). Both samples exhibit broad and intense bands at around 3455 cm−1, which correspond to the O−H stretching vibrations of H-bound OH groups. The sharp peak of β-Co(OH)2 NSs at 3636 cm−1 is attributed to the O−H stretching mode of the free OH groups.35 However, for I-Co(OH)2 NSs, the corresponding peak disappears due to the existence of hydrogen bonding between the hydrogen atoms and intercalation ions.24,36 There are three obvious peaks at 1360, 1317, and 822 cm−1 that can be assigned to the O− C−O stretching bands of CoC2O4·2H2O NRs (Figures S7 and S8 and Table S1).37 In I-Co(OH)2 NSs, these bands are shifted because of the interaction between the intercalated [CoOx2]2− ions and the hydroxide layer.32,38 The interaction impacts accordingly on the thermal decomposition behaviors of I-Co(OH)2 NSs, reflecting in the delay of dehydroxylation (Figures 3c and S9). Meanwhile, the thermogravimetric (TG)

RESULTS AND DISCUSSION A representative sample of the as-prepared I-Co(OH)2 NSs is comprehensively characterized as shown below (Figures S2 and S3). The typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images in Figures 2a and S4 clearly show the sheet-like morphology of I-Co(OH)2 NSs. The average thickness of individual nanosheets is ∼5 nm (Figure 2i−l). Nanopores with an average size of ∼3 nm are well-distributed on the surface of nanosheets (Figure 2b). The corresponding selected-area electron diffraction (SAED) pattern (Figure 2c) shows apparent single-crystalline characteristics with hexagonal symmetry (Figure S5). Additionally, it should be noted that all the diffraction spots are enlarged, which arises from the structural defects and disorder on the exposed surface.30,31 Indeed, from the high-resolution TEM B

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Figure 2. Structural and morphology characterization of I-Co(OH)2 NSs. (a, b) TEM images (nanopores are highlighted by the blue dotted line), (c) SAED pattern, (d, f, h) HRTEM images, (e) STEM image and the corresponding EDX mapping, (g) HRTEM image and the line profile, (i) AFM image, and (j, k, l) the nanosheet thickness distribution obtained by statistical analysis of the AFM images and corresponding height profiles in panel i.

defects with a low oxygen coordination, and O3 (532.3 eV) represents the O−C−O peak of [CoOx2]2− ions (Figure S10b). The higher O2 content suggests an enrichment of oxygen vacancies on the surface of I-Co(OH)2 NSs. Figure 3g clearly shows the characteristic electron paramagnetic resonance (EPR) signal at g = 2.001, manifesting the electron trapping at oxygen vacancies (Vo).33 Based on the above measurement results, the introduction of [CoOx2]2− ions is confirmed and can be beneficial for I-Co(OH)2 NSs to improve rate capacity and cycling stability. The I-Co(OH)2 NSs were evaluated as anode materials for LIBs. The Li-ion storage property is first investigated using cyclic voltammetry (CV). The CV profiles of I-Co(OH)2 NSs look quite different than ones reported for β-Co(OH)2 NSs (Figure 4a).15,16 In the first scan process, there are two cathodic peaks (peak 1, at 0.58 V, and peak 2, at 0.80 V) for ICo(OH)2 NSs, which results from the formation of Li2O, Li2C2O4, and solid electrolyte interphase (SEI) films. In the anodic scan, two broad anodic peaks, 3 and 4, are ascribed to the delithiation of Li2O and Li2C2O4.15,16,45 The former, peak

curve of I-Co(OH)2 NSs can trace the step in the range of 250−350 °C for the decomposition of metallic oxalate complex.27,33 The X-ray photoelectron spectroscopy (XPS) was conducted to provide further insight on the surface chemical compositions and valence states of elements. In the C 1s spectrum of I−Co(OH)2 NSs (Figures 3d and S10a), an obvious peak at 288.7 eV is identified as O−C−O bond of [CoOx2]2− ions.39,40 The spectra of Co 2p3/2 (Figures 3e and S10c) display the strong satellite peaks, which are the characteristic of Co2+ ions.41,42 Meanwhile, it is noticed that the Co2+ peak of I-Co(OH)2 NSs at 780.8 eV shifts toward a higher binding energy compared to that of β-Co(OH)2 NSs. This may be ascribed to the coordination of tetrahedral Co(II) with higher electronegativity in the presence of [CoOx2]2− ions.41 Ultraviolet−visible diffuse reflectance spectroscopy (UV−vis DRS) provides complementary evidence for tetrahedral Co(II) complex (Figure 3h).41,43 The O 1s spectrum of I-Co(OH)2 NSs (Figure 3f) can be divided into three peaks: O1, O2, and O3.44 O1 (530.8 eV) is the peak of metal−oxygen bonds, O2 (531.5 eV) is considered to be C

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Figure 3. Structural characterization of I-Co(OH)2 and β-Co(OH)2 NSs. (a) XRD patterns, (b) FTIR spectra; (c) TG curves; (d, e, f) highresolution XPS spectra of C 1s, Co 2p, and O 1s; (g) EPR spectra; and (h) UV−vis diffuse reflectance spectra. υ3(Td) and υ3(Oh) are attributed to the tetrahedral Co(II) coordination and those of Co(II) in octahedral geometry.

were also tested for comparison. Their first CEs only reach ∼59% and ∼43%, with a discharge and charge capacity of ∼755/449 and ∼1587/675 mA h g−1, respectively (Figures S11b, S12a and S13b, c). As could be seen, the capacities of βCo(OH)2 NSs and CoC2O4·2H2O NRs are very low, showing a serious decay in the initial few cycles and a final value at ∼284 and ∼321 mAh g−1 after 250 cycles (Figures 4c and S12b). In sharp contrast, during the initial cycles, the capacity of I-Co(OH)2 NSs suffers only slight fluctuation, likely due to activation step of the interlayered [CoOx2]2− ions.48−50 After 250 cycles, I-Co(OH)2 NSs can still reach a capacity of 870 mA h g−1, which is the one of highest value among LHs-based anodes (Table S2). The conjugated structure of [CoOx2]2− ions contributes to the high capacity of I-Co(OH)2 NSs.28,46 The FTIR, XPS, and SEM techniques were used to verify the chemical environments and morphology of the cycled ICo(OH)2 NSs. FTIR spectra (Figure S15) demonstrate significant absorptions located at 1653, 1337, and 775 cm−1 after discharge and fully charged states, which are characteristic of Li2C2O4.51 The XPS C 1s spectra further prove the presence of metallic oxalate complex (Figure S16). These results indicate that the [CoOx2]2− ions play an important role in realizing the stable cycle performance. The well-preserved morphology of I-Co(OH)2 NSs after cycling (Figures S17 and S18) confirms the pillar effect of [CoOx2]2− ions. The rate performance of I-Co(OH)2 and βCo(OH)2 NSs at different current densities are also studied and shown in Figure 4d. A pair of electrodes deliver similar capacities at a low current density of 0.05 and 0.1 A g−1 because the major contribution to the capacity is from diffusion-controlled process. As the charge−discharge rates increase, two electrodes demonstrate different rate capabilities. The discharge capacities of I-Co(OH)2 NSs are 1149, 1017,

3, corresponds to reactions 2 and 3, and the latter, peak 4, is mainly due to the reaction 1. After the initial scan process, the anodic peak position remains the same. In the subsequent cathodic scan, peak 6, which is associated with reactions 1 and 3, appears and remains invariable, suggesting a reversible charge−discharge process for the Co(OH)2 matrix. However, peak 5 shifts to a lower voltage with the scanning. This may be due to the activation process of [CoOx2]2− ions.45 In previous studies, the carbonyl groups in organics have electrochemical activity that can be further promoted by a conjugated structure. Therefore, Li2C2O4 may react with up to two Li-ion under the catalytic effect of metal Co nanoparticles.28,46 As a comparison, the main cathodic peak of β-Co(OH)2 NSs electrode is at 0.58 V upon first discharge, which can be attributed to the reduction from Co2+ to Co0 and the formation of SEI films (Figure 4b). Correspondingly, there are two anodic peaks in the charge process at 2.15 and 1.37 V.15,16 The emerging cathodic peaks at 1.17 and 1.53 V can be observed from the second cycle onward. The result implies that the original structure of β-Co(OH)2 NSs cannot be reversibly recovered after the first discharge process:47 Co(OH)2 + 2Li+ + 2e− ↔ 2LiOH + Co0

(1)

[Co(C2O4 )2 ]2 − + 2Li+ + 6e− ↔ 2[Li 2C2O4 ]2 − + Co0 (2)

LiOH + 2Li+ + 2e− ↔ Li 2O + LiH

(3)

Figure 4c depicts the cycling stability of I-Co(OH)2 NSs at a constant current density of 1 A g−1. The first discharge and charge capacities are ∼1178 and ∼779 mAh g−1, giving a Coulombic efficiency (CE) of ∼66% (Figures S11a and S13a). The performance of β-Co(OH)2 NSs and CoC2O4·2H2O NRs D

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Figure 4. Lithium-ion storage properties and electronic structure calculations of I-Co(OH)2 and β-Co(OH)2 NSs. (a, b) CV curves for the first four cycles. (c) Cycling performance and (d) rate capabilities. (e) Schematic illustration highlights the structural benefits of anode during Li-ion diffusion. (f) Optimized atomic configurations, (g) density of states, and (h) charge-density difference (yellow and green surfaces represent charge accumulation and charge depletion, respectively). For the spheres in models, blue is cobalt, red is oxygen, and gray is carbon.

880, 694, and 566 mAh g−1 at 0.2, 0.5, 1, 2, and 5 A g−1. The ICo(OH)2 NSs exhibit better rate performance than βCo(OH)2 NSs and significantly outperform the previous reported Co(OH)2-based anodes (Table S2). To explore the superiority of the I-Co(OH)2 NSs electrode in electrochemical kinetics, density functional theory calculations were carried out. The optimized atomic configuration of Co(OH) 2 interlayered with [CoOx2]2− ions is presented in Figure 4f. In view of the importance of charge transport kinetics on LIBs, the density of states (DOS) of β-Co(OH)2 and I-Co(OH)2 are first plotted in Figure 4g. The intercalation of [CoOx2]2− ions, which exhibit continuous states, leads to a higher dispersion of band structure in Co(OH)2, realizing the high electrical conductivity. Moreover, the interlayer electron−electron coupling in the I-Co(OH)2 NSs is further analyzed (Figure 4h). The charge density distribution plot clearly manifests the unbalance charge distribution between [CoOx2]2− and hydroxide layers, which induces an interfacial electric-field, thus boosting the Li-ion diffusion kinetics and improving the high-rate performance. Overall, the enhanced electrical conductivity and built electric-field would promote ion diffusion/electron transport, which can be confirmed by much lower charge-transfer impedance and higher Li-ion diffusion coefficient (DLi) (Figures S19 and S20).

The excellent electrochemical performance of I-Co(OH)2 NSs is mainly attributed to the following merits: (1) fast charge transfer by the enhanced electrical conductivity and built-in electric field; (2) shortened Li-ion diffusion distance as a result of nanopores; and (3) mitigated volume change buffered through the ample interlayer space (Figure 4e). To further understand the origin of the superior electrochemical performance of I-Co(OH)2 NSs, the Li-ion storage mechanism was investigated by sweep voltammetry. Figure S21 shows the log i versus log v plots at oxidation and reduction states. The b values of I-Co(OH)2 NSs for cathodic and anodic peaks were calculated to be near 0.9, indicating the predominant capacitive-like nature of the kinetics (Figure 5a). In contrast, the b values of β-Co(OH)2 NSs are both close to 0.7, meaning that more charge storage is affected by the diffusion processes (Figure 5d).52,53 To be more precise, the capacitive (κ1) and diffusion (κ2) contributions are separated at a particular potential according to eq 4:54,55 i(V ) = κ1ν + κ2ν1/2

(4) −1

Typically, at a scan rate of 1 mV s , 76.7% of capacities is contributed by the capacitive process for I-Co(OH)2 NSs (Figure 5b). Contribution ratios between the two different processes are calculated at other scan rates, as shown in Figure 5f. The quantified results show that the capacitive contribution E

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Figure 5. Kinetics analysis of I-Co(OH)2 and β-Co(OH)2 NSs for Li-ion storage. (a, d) CV curves at various scan rates from 0.05 to 0.5 mV s−1 and the calculated b values for cathodic and anodic peaks. (b, e) Separation of the capacitive and diffusion currents at a scan rate of 1.0 mV s−1 and (c, f) contribution ratio of the capacitive and diffusion-controlled charge at various scan rates. Synthesis of Co2C2O4·2H2O Nanorods. A total of 2 mmol of CoCl2·6H2O and 2 mmol of Na2C2O4 were dissolved in 40 mL of ethylene glycol to form a homogeneous solution. The solution was then transferred to a 50 mL Teflon-lined autoclave and kept at 200 °C for 16 h in the oven. After the reaction was completed, the precipitate was collected and washed with DI water several times. Last, the samples were dried in a vacuum oven at 60 °C for 12 h. Material Characterization. XRD was performed on a Rigaku D/ max-2000 diffractometer with Cu−Kα radiation (λ = 0.15406 nm). The SEM and TEM images of the samples were taken on a Hitachi scanning electron microspcope (S-5500), and a transmission electron microscopy image was obtained by a JEOL JEM-2100 (RH) operating at an accelerating voltage of 200 kV, respectively. The thermogravimetric results were obtained using a PerkinElmer TGA4000 thermogravimetric analyzer at a heating rate of 5 °C min−1 in air. An X-ray photoelectron spectrometer (XPS, Kratos Analytical, Axis Ultra DLD) was used to analyze the elemental composition of the powders. Fourier transform infrared spectra were recorded by a Fourier transform infrared spectrometer (Thermo Mattson, Infinity Gold FTIR) equipped with a liquid nitrogen cooled narrow-band MCT detector using an attenuated total reflection cell equipped with a Ge crystal. Ultraviolet-visble diffuse reflectance spectroscopy results were recorded by a spectrophotometer (HITACHI UH-4150), and BaSO4 was used as the reflectance standard. Electron paramagnetic resonance (EPR) spectra were conducted on a Bruker A200 EPR spectrometer operating at room temperature with a microfrequency of 9.86 GHz. Electrochemical Measurements. Electrochemical performance was characterized on CR2032 coin-type cells. The electrodes were prepared by mixing the active material of I-Co(OH)2 NSs (80 wt %), conducting agent (Super P, 10 wt %) and polyvinylidene fluoride (10 wt %) dissolved in N-methyl-2-pyrrolidone as a binder. The slurry was coated onto Cu foils and dried at 120 °C for 12 h. A polypropylene film (Celgard 2500; USA) was used as a separator, and Li metal was used as the counter electrode. The 1 M LiPF6 was used with 1:1 ethylene carbonate and diethyl carbonate as the electrolyte. The cyclic voltammetry (CV) tests were performed by biologic potentiostat (VMP3) at 0.5 mV s−1 in the voltage window between 0.01 and 3.0 V.

in I-Co(OH)2 NSs is higher than that in β-Co(OH)2 NSs at each scan rate. This clearly implies that I-Co(OH)2 NSs possess faster Li-ion transfer kinetics than β-Co(OH)2 NSs owing to its distinctive structure.

CONCLUSIONS In summary, cobalt oxalate ion ([CoOx2]2−)-intercalated Co(OH)2 nanosheets [I-Co(OH)2 NSs] were designed to significantly improve structural stability, enhance electrical conductivity, and accelerate Li-ion diffusion. When used as anode materials for LIBs, the as-prepared I-Co(OH)2 NSs exhibit high specific capacity, outstanding rate capability (566 mAh g−1 at 5 A g−1), and excellent cycling stability (870 mAh g−1 at 1 A g−1 after 250 cycles). This work affords an in-depth insight into show the critical role of intercalated conjugated anion for improving the Li-ion storage properties of LHs-based electrode materials, which is applicable to many other twodimensional layered electrode materials. METHODS Synthesis of I-Co(OH)2 Nanosheets. In a typical solvothermal route, 2 mmol Co(CH3COOH)2·4H2O (Co(OAc)2) and 14 mmol Na2C2O4 were dissolved in 40 mL of ethylene glycol (EG) to form a homogeneous solution under vigorous stirring. The solution was then transferred into a 50 mL Teflon-lined stainless autoclave with 2 mmol NaOH and maintained at 200 °C for 16 h in the oven. Next, the reaction system was allowed to cool to room temperature naturally. The obtained products were collected by centrifugation, washed with DI water several times, and dried in a vacuum oven at 60 °C for 12 h. A series of I-Co(OH)2 nanosheets were obtained by adding different amount of Na2C2O4 while keeping 2 mmol of Co(OAc)2 constant. The samples with molar ratios [Na2C2O4 to Co(OAc)2] of 1, 3, 5, and 9 were denoted as I-Co(OH)2-1, I-Co(OH)2-3, I-Co(OH)2-5, and ICo(OH)2-9, respectively. For β-Co(OH)2 nanosheets, its synthesis condition is same with that of I-Co(OH)2 nanosheets except for the absence of Na2C2O4. F

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Galvanostatic charge and discharge experiments were carried out at different current densities on a LAND battery cycler (CT2001A). Calculation Details. The Vienna ab initio simulation package (VASP) with the generalized gradient approximation (GGA) was used. In the optimization procedure, the force and energy were converged to 10−5 eV/atom and 10−2 eV/Å, respectively.56−58 The k points were set to 3 × 3 × 3 for the structure relaxation and increased to 8 × 8 × 8 for the electronic structure calculations.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04614. Additional XRD patterns, SEM and TEM images, FTIR spectra, TG curves, XPS spectra, SAED patterns, electrochemical characterizations, and calculation results. (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhiwei Fang: 0000-0001-8826-8834 Gang Chen: 0000-0003-1502-0330 Guihua Yu: 0000-0002-3253-0749 Author Contributions §

C.Y. and Z.F. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS G.Y. acknowledges financial support from the Welch Foundation award no. F-1861, the Sloan Research Fellowship, and the Camille Dreyfus Teacher−Scholar Award. G.C. acknowledges financial support from the National Natural Science Foundation of China (grant no. 21471040). C.Y. acknowledges financial support from the China Scholarship Council (grant no. 201606120229). REFERENCES (1) Pinnavaia, T. J. Intercalated Clay Catalysts. Science 1983, 220, 365−371. (2) Podsiadlo, P.; Kaushik, A. L.; Arruda, E. M.; Waas, A. M.; Shim, B. S.; Xu, J.; Nandivada, H.; Pumplin, B. G.; Lahann, J.; Ramamoorthy, A.; Kotov, N. A. Ultrastrong and Stiff Layered Polymer Nanocomposites. Science 2007, 318, 80−83. (3) Wang, Q.; O’Hare, D. Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets. Chem. Rev. 2012, 112, 4124−4155. (4) Khan, A. I.; O’Hare, D. Intercalation Chemistry of Layered Double Hydroxides: Recent Developments and Applications. J. Mater. Chem. 2002, 12, 3191−3198. (5) Sun, P.; Ma, R.; Bai, X.; Wang, K.; Zhu, H.; Sasaki, T. SingleLayer Nanosheets with Exceptionally High and Anisotropic Hydroxyl Ion Conductivity. Sci. Adv. 2017, 3, e1602629. (6) Ma, L.; Islam, S. M.; Xiao, C.; Zhao, J.; Liu, H.; Yuan, M.; Sun, G.; Li, H.; Ma, S.; Kanatzidis, M. G. Rapid Simultaneous Removal of Toxic Anions [HSeO3]−, [SeO3]2−, and [SeO4]2−, and Metals Hg2+, Cu2+, and Cd2+ by MoS42− Intercalated Layered Double Hydroxide. J. Am. Chem. Soc. 2017, 139, 12745−12757. (7) Ma, L.; Wang, Q.; Islam, S. M.; Liu, Y.; Ma, S.; Kanatzidis, M. G. Highly Selective and Efficient Removal of Heavy Metals by Layered G

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