Nature-Inspired 2D-Mosaic 3D-Gradient Mesoporous Framework

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Nature-Inspired 2D-Mosaic 3D-Gradient Mesoporous Framework: Bimetal Oxide DualComposite Strategy toward Ultrastable and High-Capacity Lithium Storage Jia Yu,†,‡ Yanlei Wang,† Lihui Mou,†,‡ Daliang Fang,†,‡ Shimou Chen,*,†,‡ and Suojiang Zhang*,†,‡ †

Beijing Key Laboratory of Ionic Liquid Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: In allusion to traditional transition-metal oxide (TMO) anodes for lithium-ion batteries, which face severe volume variation and poor conductivity, herein a bimetal oxide dual-composite strategy based on twodimensional (2D)-mosaic three-dimensional (3D)-gradient design is proposed. Inspired by natural mosaic dominance phenomena, Zn1−xCoxO/ZnCo2O4 2D-mosaic-hybrid mesoporous ultrathin nanosheets serve as building blocks to assemble into a 3D Zn−Co hierarchical framework. Moreover, a series of derivative frameworks with high evolution are controllably synthesized, based on which a facile one-pot synthesis process can be developed. From a component-composite perspective, both Zn1−xCoxO and ZnCo2O4 provide superior conductivity due to bimetal doping effect, which is verified by density functional theory calculations. From a structure-composite perspective, 2D-mosaic-hybrid mode gives rise to ladder-type buffering and electrochemical synergistic effect, thus realizing mutual stabilization and activation between the mosaic pair, especially for Zn1−xCoxO with higher capacity yet higher expansion. Moreover, the inside-out Zn−Co concentration gradient in 3D framework and rich oxygen vacancies further greatly enhance Li storage capability and stability. As a result, a high reversible capacity (1010 mA h g−1) and areal capacity (1.48 mA h cm−2) are attained, while ultrastable cyclability is obtained during high-rate and long-term cycles, rending great potential of our 2D-mosaic 3D-gradient design together with facile synthesis. KEYWORDS: bimetal oxide dual-composite strategy, Zn1−xCoxO/ZnCo2O4, 2D-mosaic hybrid nanosheet, 3D concentration-gradient framework, morphology and structure evolution, lithium-ion battery

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nanosized 2D building blocks give rise to effective conductive networks and shortened diffusion pathways for Li+/e− as well as easier access to electrolytes, being key factors to rapid kinetics and high capacity.2,7,14 On the other hand, microsized 3D higher-level assembly largely enhances tap density and provides sufficient free space and stable mechanics frame to alleviate structural strain, while avoiding excessive formation of solid electrolyte interphase (SEI) layer.15 Furthermore, certain mesoporosity has also proved to bring richer Li storage sites and extra free volume.16−18 Therefore, in order to realize

ecently, two-dimensional (2D) architectural materials have attracted increasing attention owing to special surface properties, kinetics characterization and application performance.1−3 Especially, attributing to rapid electron transport and easy accessibility to lithium ions, they have emerged as promising electrode materials for lithium-ion batteries (LIBs).4−7 Meanwhile, building a three-dimensional (3D) nanomicro framework as a higher-level superstructure has also become an effective strategy in wide energy-related fields,8−10 classical examples include pomegranate,11 watermelon,12 and peapod structure electrodes for LIBs.13 Since most of reported nanomicro frameworks were composed of zero-dimensional (0D) nanoparticles, it was speculated that those assembled from 2D nanosheets would hold additional advantages as LIB electrodes. On one hand, interconnected © XXXX American Chemical Society

Received: January 8, 2018 Accepted: February 9, 2018 Published: February 9, 2018 A

DOI: 10.1021/acsnano.8b00168 ACS Nano XXXX, XXX, XXX−XXX

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Cite This: ACS Nano XXXX, XXX, XXX−XXX

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a

Left: schematic illustration of the evolution lines of 2D-3D Zn−Co frameworks. From top left to bottom right, Zn−Co-x (x = 0, 0.5, 1, and 2) series products with increasing Co/Zn ratio; from bottom left to top right, Zn−Co-1-NF, Zn−Co-1, Zn−Co-1-HF, and Zn−Co-1-UF series products with enhancing fluorine level. Intersection of the two evolution line was Zn−Co-1 2D-mosaic 3D-gradient mesoporous framework, which was assembled from 2D-mosaic-hybrid Zn1−xCoxO/ZnCo2O4 mesoporous nanosheets and possessed an inside-out Zn−Co concentration-gradient. Right: schematic of Zn1−xCoxO/ZnCo2O4 nanosheet as the building block, exhibiting a 2D-mimic-hybrid mode which was inspired by natural mosaic dominance phenomena.

simultaneously satisfy the demands for high capacity, conductivity, and stability. First, from a component-composite perspective, bimetal oxides (like Co-doped ZnO or Zn-doped Co3O4) possess superior conductivity owing to doping effect and are usually accompanied by more oxygen vacancies.25,26 Subsequently, from a structure-composite perspective, elaborately integrating distinct TMOs was an efficient design to obtain better performance than any single phase, such as Zn− Fe-based microcubes,27 Zn−Co-based fibrous root arrays,23 Fe−Mn-based nanotube arrays,28 owing to electrochemical synergistic effect. More importantly, as a natural hybrid trait expression, the mosaic dominance phenomena inspired us with a 2D-mosaic structural composite mode (Scheme 1). Differing from traditional modes like 3D packing or core−shell, herein Zn-based oxide together with Co-based oxide could serve as a pair of mosaic blocks to fabricate into 2D-mosaic-hybrid nanosheets. Because of distinct voltage platforms of the mosaicblock pair, their highly heterogeneous dispersion and lowdimensional assembly characteristics would give rise to a ladder-type buffering effect during lithiation process, which realizes effective mutual stabilization, especially for the zincbased matrix with higher capacity yet higher expansion. Therefore, after the 2D-mosaic-hybrid nanosheets further assemble into a 3D framework, advanced Li storage performance should be anticipated, nevertheless a facile synthesis approach remains a challenge. In this work, basing on above strategy, Zn1−xCoxO/ZnCo2O4 mesoporous ultrathin nanosheets with a 2D-mosaic-hybrid mode were assembled into a 3D hierarchical framework. Besides, a series of Zn−Co frameworks with evolving components and structures were controllably obtained through regulating the Co/Zn ratio and fluorine level, accordingly we developed a quite facile one-pot, fluorine-induced solvothermal route. Moreover, because of the relatively easier nucleation of

beneficial Li storage functionalities, 2D−3D hierarchical framework with rational porosity is highly desirable. Besides the significant role of structure design, specific components of active material are another key factor. Benefiting from high theoretical capacity and rich structural adjustability in preparation process, transition-metal oxides (TMOs) should be considered as the preferred material for constructing desired 2D-3D mesoporous frameworks as LIB anodes.19,20 Among them, owing to unique dual Li storage mechanism (extra LixZn alloying reaction at low potential apart from conversion into Zn(0)), ZnO possess a higher theoretical capacity (978 mA h g−1), lower discharge voltage platform (∼0.5 V vs Li+/Li) and larger Li+ diffusion coefficient.21 Besides, zinc-based oxide is relatively easier to nucleate in solvothermal processes, while additional main advantages include low cost and environmental friendliness.10,22 Unfortunately, the extra alloying stage inevitably results in extraordinarily severe expansion/contraction during lithiation/delithiation process followed by rapid pulverization, accompanied with poor conductivity. Therefore, relying only on structure optimization, like 2D-3D framework, may be hardly enough to address the above intrinsic issues, and more sophisticated componental and structural design should be adopted. Initially, combining with another metal matrix (Co, Mn, etc.) to form spinel bimetal oxide was found to deliver higher conductivity, while partially alleviating the huge volume variation of the Zn matrix, for example, ZnMn2O4 onedimensional (1D) architecture as a high-performance LIB anode.20,23 However, since a large proportion of Zn was replaced, corresponding theoretical capacity would decrease to some extent and the voltage platform would become higher (∼1.0 V).24 Herein, we proposed a bimetal oxide dual-composite strategy based on 2D-mosaic-hybrid mode, for example, Co-doped-Znbased/Zn-doped-Co-based TMO hybrid system, aiming to B


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Figure 1. (a) SEM images of Zn−Co-x (x = 0, 0.5, 1, and 2) precursors with 2D-3D hierarchical frameworks. (b) HRSEM images of 2D building blocks of Zn−Co-x (x = 0, 0.5, 1, and 2) after calcination. (c) XRD patterns of Zn−Co-x series. (d) Shift of (100), (002), and (101) peak positions with the introduction of Co2+ (inset: fine XRD pattern of Zn−Co-x series between 30° and 38°) . (e) TG curves under air atmosphere of Zn−Co-x precursor series. (h) N2 adsorption−desorption isotherms of Zn−Co-x series (inset: specific surface area comparison). (g) 2D and 3D AFM micrographs of 2D building blocks of Zn−Co-1 before and after calcination, respectively, with height profiles.

agent, regulating the fluorine level had an obvious effect on assembly mode of 2D building blocks. Besides the abovementioned Zn−Co-x series obtained from a low-fluorine system (F/Zn molar ratio of 1), herein products obtained from none- (F/Zn ratio of 0), high- (F/Zn ratio of 2), and ultrahigh- (F/Zn ratio of 10) fluorine systems were denoted as Zn−Co-x-NF, Zn−Co-x-HF and Zn−Co-x-UF (x = 0, 0.5, 1, and 2), respectively. Fluorine-induced effect played a significant role in the one-pot solvothermal process and morphology control of products. Higher fluorine level resulted in more compact 3D assembly of 2D building blocks, like the evolution from divergent Zn−Co-x-NF to convergent Zn−Co-x-HF, yet an ultrahigh fluorine level gave rise to distinct star-shaped microplate products (Zn−Co-x-UF). A rationally compact assembly would benefit tap density and mechanics frame stability. Obviously, regulating the Co/Zn ratio could efficiently control the framework component and building block size, while the fluorine level mainly influenced the assembly density of building blocks. Based on above two evolution lines, we successfully developed a one-pot, template-free approach, using a solvothermal system with Co/Zn ratio of 1 and low-fluorine level. Desired 2D-3D precursors were efficiently synthesized, which could be converted into Zn1−xCoxO/ZnCo2O4 bimetal oxide dual-composite mesoporous frameworks after simple calcination, offering facile and versatile strategy for synthesis of two-dimensional heterostructures.33 Zn−Co 2D-3D Hierarchical Mesoporous Framework. Building desired 2D-3D hierarchical precursors was the key step in this work. Scanning electron microscopy (SEM) observation clearly demonstrated the morphology evolution

zinc-based matrix during the bimetal coprecipitation process, we obtained a Zn−Co concentration gradient within 3D framework, in which a higher Zn proportion inside benefited specific capacity and higher Co proportion outside improved cycling stability.29,30 In addition, accompanying abundant oxygen vacancies could provide more electrochemical active sites and facilitated Li+/e− transport.31 As expected, the Zn1−xCoxO/ZnCo2O4 exhibited significant advantages on both capacity and stability, especially for high-rate and longterm cycles.

RESULTS AND DISCUSSION Evolution Rules and Facile Synthesis of Bimetal Oxide Dual-Composite Framework. In previous reports, most hierarchical hybrid architectures based on TMO were prepared using a complex multistep route or extra surfactants, thus a facile synthesis approach was expected.10,32 As schemetically illustrated in Scheme 1, through regulating solvothermal parameters including fluorine level and Co/Zn ratio, a series of derivative 2D-3D Zn−Co frameworks with high evolution in component, building block, and assembly mode were controllably obtained. On one hand, with zinc and cobalt as the bimetal source, by regulating predesigned Co/Zn ratio (0:1, 0.5:1, 1:1 and 2:1) in solvothermal systems, obtained products (denoted as Zn−Cox, x = 0, 0.5, 1 and 2, respectively) demonstrated evolving components from ZnO to ZnCo 2O4 going through a Zn1−xCoxO/ZnCo2O4 intermediate stage and evolving 2D-3D structures from flower-shape to sphere-shape with progressively smaller and narrower nanosheet build blocks. On the other hand, with ammonium fluoride as the morphology-controlling C


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Co-1 appeared distinctly deteriorating as Co2+ was introduced, suggesting a heavy doping for ZnO phase.37 Similarly with their precursors, Zn−Co-x series also exhibited obvious color transition as shown in Figure S5. In addition, thermal behaviors of the precursors under air atmosphere were studied by thermogravimetric (TG) analysis (Figure 1e). The prominent weight loss from about 250 °C should be attributed to the conversion of metal carbonate hydroxide to metal oxide. The weight loss percent of Zn−Co-0 precursor was 27.9%, being slightly larger than its theoretical value due to existence of crystalline water. With raising the Co proportion, the weight loss percentage became lower and finally decreased to 24.0% for Zn−Co-2 precursor, because the formation of cobalt(III) oxide gave rise to lower weight loss. Moreover, two weight loss events could be distinguished along the multiple downward slope of Zn−Co-0.5 or Zn−Co-1 precursor, indicating formation of multiple-phase products, which were distinct from Zn−Co-0 or Zn−Co-2 with single downward slope. N2 adsorption−desorption analyses were carried out to investigate specific surface areas and porous texture. As shown in Figure 1f, all isotherms of Zn−Co-x series belonged to type IV, and their hysteresis loops at relative pressure ranges of 0.8− 0.95 were ascribed to type-H3 which suggested pore diameters of 15−25 nm.32,38 Herein, detail BET and Langmuir specific surface area (SBET and SLangmuir), pore volume (Vpore), and pore size (Rpore) data were summarized in Table 1 and inset of

of Zn−Co-x precursor series under low-fluorine level. First, Zn−Co-0 precursor exhibited uniform 3D flower-like framework, which was assembled from radially standing smooth 2D ultrathin nanosheets accompanied with a sufficient interval (Figure 1a). As the Co/Zn ratio gradually increased, it evolved from flower-shape to sphere-shape on the aspect of 3D hierarchical framework, meanwhile smaller and narrower nanosheets on the aspect of 2D building unit, which was further showed by high-resolution SEM (HRSEM) observation (Figure S1, Supporting Information). Besides, Figure S2 exhibited the full-view SEM image and particle size distribution of Zn−Co-1 precursor, in which 10−12 μm range occupied the highest percentage, while other three kinds of precursors exhibited similar particle sizes. After calcination, original 2D-3D hierarchical frameworks were well-preserved, but HRSEM observations revealed masses of nanopores and rough surface emerged for the nanosheets (Figures 1b and S3), which was also evidenced by atomic force microscopy (AFM) micrographs and corresponding height profiles (Figure 1g). It mainly resulted from the H2O and CO2 releasing process as well as crystallization during calcination, which significantly favored the Li storage performance.27 Serving as the building block, the ultrathin (10−20 nm) and interconnected nanosheets contributed to rapid Li+ diffusion kinetics and efficient conductive network, accompanied with sufficient free space.2 Crystalline structures were comprehensively investigated by X-ray diffraction (XRD), to study the chemical component and doping effect. As shown in Figure S4, diffraction peaks of Zn− Co-0 precursor were in good agreement with monoclinic hydrozincite Zn5(Co3)2(OH)6 (JCPDS card no. 19-1458, s.g.: C2/m).34 After introducing Co2+ in the solvothermal system, other peaks indexed to orthorhombic Co(CO3)0.5(OH)· 0.11H2O (JCPDS card no. 48-0083, s.g.: P2212) emerged, whose relative peak intensity was gradually enhanced as the Co/Zn ratio increased.23 As shown in Figure S5, the color transition (from white to dark pink) of the Zn−Co-x precursor series evidenced the intensifying coprecipitation of Co2+ into Zn-based matrix. Concerning calcinated products, identified peaks of Zn−Co-0 and Zn−Co-2 were consistent with hexagonal wurtzite ZnO (JCPDS card no. 36-1451, s.g.: P63mc) and cubic spinel ZnCo2O4 (JCPDS card no. 23-1390, s.g.: Fd3m), respectively.24,34 Differing from the above homogeneous single-phase components, Zn−Co-0.5 and Zn− Co-1 exhibited intermediate XRD patterns revealing ZnO/ ZnCo2O4 heterogeneous compositions, coinciding with the morphology evolution behavior (Figure 1c). The distinct and sharp peaks indicated high crystallinity for Zn−Co-x series, while their average crystallite sizes were inferred to become smaller with the Co/Zn ratio via the Scherrer equation.35,36 Since Co2+ (0.72 Å) and Zn2+ (0.74 Å) had comparative ionic radii which promoted mutual doping, closer observation on crystalline structures was performed to verify the successful fabrication of a bimetal-oxide dual-composite system.37 For Zn−Co-0, 0.5, and 1 samples containing ZnO phase, as the Co/Zn ratio increased, it was worth noting that the three most intense ZnO peaks ((100), (002), and (101)) obviously shifted toward higher angles which indicated a reduction of unit cell (Figure 1d). It revealed an intensifying isomorphous substitution of Co2+ into ZnO lattices to form Zn1−xCoxO, since the ionic radius of Co2+ is slightly lower than that of Zn2+. Meanwhile, the positions of two most intense ZnCo2O4 peaks ((220) and (311)) remain relatively steady, for Zn−Co-0.5, 1, and 2 samples. Besides, the ZnO peaks of Zn−Co-0.5 and Zn−

Table 1. SBET, SLangmuir, Vpore, and Rpore for Zn−Co-x (x = 0, 0.5, 1 and 2) Mesoporous Frameworks Zn−Co-x Series

SBET (m2 g−1)

SLangmuir (m2 g−1)

VPore (cm3 g−1)

Rpore (nm)

Zn−Co-0 Zn−Co-0.5 Zn−Co-1 Zn−Co-2

32.7 26.0 39.2 84.1

44.5 36.0 53.5 114.7

0.180 0.173 0.201 0.471

18.6 26.2 22.1 21.2

Figure 1f. It was observed that with increasing Co/Zn ratio, surface areas and pore volumes of Zn−Co-x series basically maintained stable first and then increased, which might be due to evolving building block sizes and whole framework. Rational surface area could provide rapid kinetics and easy access to electrolyte and Li ions, while avoiding excessive formation of SEI layer accompanying with high irreversible capacity loss.39−41 Besides, Barrett−Joyner−Halenda (BJH) pore-size distributions revealed average pore sizes around 20 nm, clearly demonstrating their mesoporous nature, which would be favorable for improving electrochemical interface and mitigating electrode volume expansion (Figure S6).16 2D-Mosaic Building Block and 3D-Gradient Framework. TEM observations were used to further study the fine structure, especially on the 2D building blocks. As shown in Figures 2a and S7, 3D flower-like Zn−Co-0 was assembled from large porous 2D nanosheets, which were further composed of numerous ultrafine grains and voids with sizes around tens of nanometers. In the high-resolution TEM (HRTEM) image, dominant lattice fringes with interplanar spacing of 0.28 and 0.26 nm were assigned to (100) and (002) planes of hexagonal ZnO, respectively, accompanied with corresponding selected area electron diffraction (SAED) patterns (Figure 2b).10,40 D


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Figure 2. (a) TEM images of Zn−Co-0 and (b) HRTEM image of its 2D building block, inset refers to SEAD pattern. (c,d) TEM images of Zn−Co-1 and (e) HRTEM images of its 2D building block, inset refers to SEAD pattern. Blue and green regions refer to ZnCo2O4 and Zn1−xCoxO phases in a 2D-mosaic-mode, respectively. (f) EDS region-mapping results of Zn−Co-1 building block on various positions. (g) EPMA results of Zn−Co-1. (h) Schematic diagram of 3D concentration-gradient framework for Zn−Co-1, with gradual increasing Co/Zn ratio from the inside out obtained from EDS line scan.

(Figure 2h), which was also observed by electron-probe X-ray microanalysis (EPMA) and EDS element mapping (Figures 2g and S12). Besides, similar phenomena was observed for Zn− Co-0.5 (Figure S13). A relatively high proportion of Zn1−xCoxO internally could provide higher capacity owing to dual Li-storage mechanism of Zn-based oxide, while the external part rich in stable ZnCo2O4 significantly improved cycling capability.21 The observed Zn−Co relative concentration gradient might be mainly attributed to the preferential nucleation of zinc-based matrix during the bimetal coprecipitation process.22 In addition, accompanying mapping quantitative analysis revealed the Co/Zn molar ratios of Zn−Co-x (x = 0, 0.5, 1, and 2) to be 0, 0.44, 0.86, and 1.81, respectively, which basically agreed with predesigned solvothermal systems. DFT Calculations on Bimetal Mutual-Doping Effect. As a component-composite system, Co2+ and Zn2+ were mutually doped into ZnO and Co3O4 lattices to form Zn1−xCoxO and ZnCo2O4 phases, respectively, due to coprecipitation effect in bimetal solvothermal process. In order to study the influence of doping on conductivity, density functional theory (DFT) calculations were carried out. As shown in Figure 3a, total and projected density of states (DOS) of pristine ZnO and dopedZnO (Zn1−xCoxO and Zn1−2xCo2xO, x = 0.125) were compared. It was found that pristine ZnO showed a semiconducting characteristic. However, after replacing partial Zn2+ with Co2+, it showed obviously enhanced conductivity which approached conductor, owing to emerging new electronic states.45 The schematic of Co-doping into hexagonal wurtzite ZnO with P63mc space group was also provided. On the other hand, a similarly change tendency was observed for pristine/doped Co3O4 system with Fd3m spacing group, through comparing their DOS (Figure 3b). When the divalent Co 2+ occupying the tetrahedral voids was partially (ZnxCo3−xO4, x = 0.25) or entirely (ZnCo2O4) substituted by Zn2+, the band gap width of pristine Co3O4 decreased obviously

After the Co component was introduced, Zn−Co-1 exhibited a whole framework closer to the sphere assembled from smaller nanosheets, which possessed a similar mesoporous nature to Zn−Co-0 (Figures 2c,d and S8). In HRTEM images of nanosheets, (002) planes belonging to hexagonal Zn1−xCoxO phase, together with (111) and (222) planes belonging to cubic ZnCo2O4 phase were observed to coexist in the same area (Figure 2e).42 Meanwhile the SAED pattern indicated its polycrystalline characteristic. More relevant HRTEM images were shown in Figure S9. It distinctly demonstrated a 2Dmosaic-hybrid mode for the Zn1−xCoxO/ZnCo2O4 heterogeneous nanosheets, based on which a bimetal-oxide dualcomposite system was successfully built. This design was inspired by mosaic dominance phenomena, which was a classic form of hybrid trait expression in nature, like mosaic color ladybirds, butterflies, leaves, etc. (Figure S10).43 Taking the ladybird (harmonia axyridis) as an example, black stains (melanin) and nonblack stains (carotenoids) are results of mosaic dominance expression of a series of alleles. Multiple stains were synergistically expressed to form mosaic color phenotype with various distributions, which provided a wide variety of protective patterns and helped to adapt to habitats.44 Besides, energy dispersive X-ray spectrometer (EDS) mapping analyses on certain fine regions was performed for the nanosheet of Zn−Co-1. As shown in Figure 2f, adjacent positions possessed distinct Co/Zn element ratios (3.3, 0.6, and 1.1 for Positions 1, 2, and 3), which is also observed in Figure S11, further verifying the 2D-mosaic-mode of Zn1−xCoxO/ ZnCo2O4 hybrid building blocks. Besides the 2D building blocks, component and distribution characteristics within the whole 3D framework were also studied. From EDS line-scan analysis, a gradual increasing Co/ Zn ratio from center toward surface was observed for Zn−Co1, indicating relative Zn-rich and Co-rich components for the internal and external part of the 2D-3D framework, respectively E


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as new electronic states appeared within the conduction band, indicating an enhanced conductivity. Moreover, change became more obvious with increasing Zn-doping degree. Therefore, bimetal oxide (Zn1−xCoxO and ZnCo2O4) possessed superior electrical conductivity to pristine singlemetal oxide, which greatly benefited Li-storage performance especially for rate capability. In addition, according to the calculation results, the lattice constants and volume of ZnO became lower as the Co-doping degree enhanced, which was consistent with above fine XRD patterns, because of the smaller radius of Co2+ than that of Zn2+ (Table S1). Composition and Surface Investigation. X-ray photoelectron spectroscopy (XPS) measurements were performed to further provide component information, especially the oxygen vacancy. Figure 4a showed the survey spectra of Zn−Co-x (x = 0, 0.5, 1, and 2) which all contained typical peaks of Zn, Co, and O except for Zn−Co-0 without Co, accompanied with gradually increasing relative intensity ratio of Co 2p peaks to Zn 2p peaks. In high-resolution XPS spectra of O 1s state, the peak Oa with highest binding energy (BE) located around 532.4 eV was attributed to the surface adsorbed oxygen from O2, H2O, etc., and the lowest one (Ol) corresponded to the lattice oxide (Figure 4b).24 Meanwhile, the middle-located peak (Od) was mainly assigned to defect sites which had low oxygen coordination.46,47 It is worth noting that based on the peakfitting area data, the relative atomic ratio of Od/Ol for representative Zn−Co-1 (3.19) was far higher than those of Zn−Co-0 (0.34) and Zn−Co-2 (0.47). Obviously, Zn1−xCoxO/ ZnCo2O4 dual-composite system had much more oxygen vacancies than any single phase, which would create more electrochemically active sites and facilitate lithium ion transport.46 Besides, relative DFT calculations proved improved conductivity attributing to the delocalization tendency of

Figure 3. Left: Density of states of (a) Co-doped ZnO system (ZnO, Zn1−xCoxO, and Zn1−2xCo2xO, x = 0.125) and (b) Zn-doped Co3O4 system (Co3O4, ZnxCo3−xO4 and ZnCo2O4, x = 0.25). The green, blue, and red imaginary lines referred to projected density of states of Zn, Co, and O elements, respectively. Right: Hexagonal wurtzite superstructures of ZnO system before and after Codoping, cubic spinel structures of Co3O4 system before and after Zn-doping. Gray, blue, and red balls represent Zn, Co, and O atoms, respectively.

Figure 4. (a) XPS survey spectra of Zn−Co-x (x = 0, 0.5, 1, and 2). (b) O 1s, (c) Co 2p3/2, (d) Zn 2p3/2 high-resolution XPS spectra and inset of (a) PL spectra for Zn−Co-0, 1, and 2, respectively. (e) FT-IR and (f) Raman spectra of Zn−Co-x series. F


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Figure 5. (a) Growth mechanism of the 2D-3D hierarchical framework, through a series of Zn−Co-1 precursors at various solvothermal reaction stages: (I) 30 min; (II) 60 min; (III) 90 min, (IV) 180 min, and (V) 360 min. (b−d) Morphologies of Zn−Co-1-NF, HF, and UF precursors obtained under various fluorine levels. (e) XRD patterns of Zn−Co-x-UF precursor series. (f) EDS mapping analysis of Zn−Co-1UF precursor.

electrons at defect states.48 As shown in inset of Figure 4a, photoluminescence (PL) spectra demonstrated that Zn−Co-1 delivered the highest luminescence intensity, further verifying its sufficient oxygen vacancies.49 Furthermore, the chemical status variation of Co and Zn were investigated in detail. As shown in Figure S14a, Zn−Co-1 and Zn−Co-2 showed similar Co 2p3/2 and Co 2p1/2 peaks around 779.7 and 794.8 eV, respectively, accompanied with satellite peaks at higher BE.50 Generally, the spin−orbit splitting (from Co 2p3/2 peak to Co 2p1/2 peak) of about 15.0 eV and energy gap (from main peak to satellite peak) of about 9.8 eV verified the dominant existence of the Co3+ oxidation state.24 According to the peak-fitting data of Co 2p3/2 peaks in Figure 4c, the cobalt valence distribution revealed that relative atomic ratio of Co2+/Co3+ for Zn−Co-1 (0.42) was much higher than that for Zn−Co-2 (0.15). The lower cobalt average valence of Zn−Co-1 evidenced an intensified unsaturated coordination of Co atoms which agreed with the above O 1s state analysis, meanwhile was also caused by existence of the Zn1−xCoxO phase. Concerning Zn 2p states, Zn 2p1/2 and Zn 2p3/2 peaks were clearly observed in Figure S14b. In high-resolution spectra, Zn−Co-0 and Zn−Co-2 possessed typical symmetrical Zn 2p3/2 peaks located at 1021.4 and 1020.9 eV, which were ascribed to the divalent Zn in ZnO and ZnCo2O4, respectively (Figure 4d).27 However, the main peak of Zn−Co-1 shifted toward the high BE side (1022.3 eV), indicating a decreased electronic density of zinc and less zinc were bound to oxygen atoms. Fourier transform infrared (FT-IR) spectra of Zn−Co-x (x = 0, 0.5, 1 and 2) further confirmed the successful fabrication of dual-composite system (Figure 4e). The sharp peaks at 570 and 665 cm−1 indexed to the Co−O and Zn−O stretching vibration were observed since Zn−Co-0.5, whose intensity enhanced with the content of Co2+, confirmed the formation of ZnCo2O4

spinel structure.51 Concerning the Zn1−xCoxO phase, a main peak around 423 cm−1 was observed for all samples except for Zn−Co-2.10 As expected, both kinds of absorption peaks coexisted for Zn−Co-0.5 and Zn−Co-1. Raman spectra at room temperature are further shown in Figure 4f. The P63mc space group of hexagonal wurtzite ZnO suggested a Raman active mode of polar A1 + E1 + nonpolar 2E2 for Zn−Co-0, by group theory.52 The dominant peak around 437 cm−1 and small peaks around 332, 380, and 582 cm−1 were assigned to E2high, E2high−E2low, A1 and E1 modes, respectively. In respect to Zn− Co-2 belonging to Fd3m space group, peaks around 476, 511, and 612 cm−1 were assigned to Eg, F2g, and F2g modes, respectively.51 And the splitting of A1g into two bands at 676 and 689 cm−1 evidenced the ZnCo2O4 phase. Since the Raman spectrum of ZnO was more sensitive to doping and structural defects, Zn−Co-0.5 and Zn−Co-1 embodied more Raman characteristics of spinel phase.53 Growth Mechanism and Derivative Frameworks. In order to better understand growth mechanism of the 2D-3D hierarchical framework, a series of Zn−Co-1 precursors with various reaction times were observed, which could be divided into four stages. Initially, in a gradual temperature-rising process, F− first tended to combine with Zn2+ and Co2+ to form ZnF+ and CoF+ complexes.54 Meanwhile the urea was hydrolyzed to generate sufficient hydroxyl (OH−) and carbonate (CO32−) anions, serving as the slow release pH adjuster as well as carbonate provider (eqs S1 and S2).23 Because of strong affinity to metal cations, released CO32− combined with ZnF+/CoF+ and OH− to initiate the nucleation of zinc and cobalt carbonate hydroxides, as illustrated in eqs S3 and S4, and began to form chip-like nanocrystals after about 30 min of reaction time (Figure 5aI).55 During the second stage, these fresh nanonuclei tended to aggregate for decreasing interfacial energy which was a G


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Figure 6. (a) Upper: Multistep Li storage mechanism of 2D-mosaic-hybrid Zn−Co-1, showing a ladder-type buffering effect which relieves the expansion stress during electrode lithiation. Lower: Initial discharge profile of Zn−Co-1 with a current density of 100 mA g−1. (b) Upper: Li storage mechanism of Zn−Co-0. Lower: Initial discharge profiles of Zn−Co-0 and Zn−Co-2 with a current density of 100 mA g−1. (c) Crosssectional SEM images of pristine and fully lithiated Zn−Co-1 electrodes. (d) Ex situ XRD patterns of Zn−Co-1 electrodes at various discharge states. (e) Initial discharge/charge profiles of Zn−Co-1 with various current densities. (f) CV curves of Zn−Co-1 with a scan rate of 0.1 mV s−1.

thermodynamic unstable process, thus leading to formation of nonuniform primary nanosheets (Figure 5aII).56 Subsequently, as the reaction proceeded, the reactant concentration became lower to enter a kinetic-controlled stage. It provided more nucleation sites for nanonucleus growing on the surface of initial-formed precursors, which promoted the primary nanosheets to self-assemble in a 3D mode in about 90 min (Figure 5aIII).56 As shown in Figure 5aIV, an undeveloped 3D framework was then formed through Ostwald ripening and recrystallization process, which evolved into complete 2D-3D frameworks during the final epitaxial growth stage, after a total of 360 min of solvothermal process (Figure 5aV). In general, a fluorine-induced effect within gradual temperature-rising process was the key to construction of desired 2D3D structure. When compared with metal ions, the formation of ZnF+ and CoF+ complexes could slow their reaction rate with precipitants, thus facilitating the one-pot, gradual precipitation process and morphology control of products.57 More derivative Zn−Co frameworks were obtained through regulating the fluorine level of the solvothermal system, which could control the assembly mode of 2D building blocks. Figure 5b,c show the morphologies of Zn−Co-1-NF and Zn−Co-1HF precursors, which were obtained from nonfluorine (F/Zn ratio of 0) and high-fluorine (F/Zn ratio of 2) systems. When compared with the Zn−Co-1 precursor corresponding to F/Zn ratio of 1, it was found that higher fluorine level would result in more compact assembly of 2D nanosheets within 3D hierarchical framework. This tendency from divergent to convergent was further confirmed by morphology evolution of Zn−Co-x-NF precursor series with various Zn/Co ratios as well as Zn−Co-0 precursor series with various fluorine levels (Figures S15 and S16). A rational assembly density greatly contributed to high tap density and stable mechanics frame. XRD patterns of Zn−Co-x-NF (x = 0, 0.5, 1, and 2) precursors revealed that although being able to control the assembly mode of building blocks, moderate fluorine levels had

little impact on the chemical component of products (Figure S17). As shown in Figure S18, TG curves also reflected a highly similar thermal behavior between products from nonfluorine and low-fluorine systems. By contrast, an ultrahigh fluorine level (F/Zn ratio of 10) gave rise to a completely different Zn− Co-x-UF precursor series with star-shaped microplate structure, rather than 2D-3D hierarchical structure (Figures 5d and S16). From XRD patterns, diffraction peaks of Zn−Co-0-UF precursor were assigned to orthorhombic Zn(OH)F (JCPDS card no. 32-1469, s.g.: Pna21), and peaks of orthorhombic Co(OH)F (JCPDS card no. 50-0827, s.g.: Pna21) emerged with the introduction of Co2+ (Figure 5e).58 Moreover, for Zn−Co1-UF precursor, two kinds of microplate morphologies could be distinguished. Combined with corresponding EDS mapping and line-scan analyses, relatively smooth ones were found to be zinc-based microplates while coarser ones referred to cobaltbased microplates, showing a structure differentiation phenomenon in the ultrahigh-fluorine system (Figures 5f and S19). This phenomenon was also partly observed for Zn−Co-1-NF, further verifying the significant role of fluorine-induced effect. Figure S20 gives TG curves of the Zn−Co-x-UF series. When compared with other precursor series, they exhibited significantly delayed decomposition temperature and lower weight loss percent, due to distinct chemical component. Besides, influence of solvothermal temperature was studied. As shown in Figure S21a, when the reaction temperature was decreased to 160 °C, the obtained Zn−Co-1 precursor remained in a similar 2D-3D hierarchical framework, except for more incompact assembly of building blocks. By contrast, after the temperature furthermore decreased to 140 °C, the precursor became an unshaped aggregation of nanosheets without obvious 3D assembly (Figure S21b). Similar phenomena was also observed for Zn−Co-0 (Figures S21c,d). Multistep Li Storage Mechanism and Ladder-Type Buffering Effect. As a proof of concept for our bimetal-oxide dual-composite strategy, the 2D-mosaic 3D-gradient H


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Figure 7. Cycling performance of Zn−Co-x (x = 0, 1, and 2) at (a) 100 mA g−1 and (b) 1000 mA g−1, respectively, accompanied with corresponding CEs. Inset of (b) referred to the morphology of Zn−Co-1 electrode after cycles. (c) Rate properties of Zn−Co-x (x = 0, 1, and 2) electrodes. (d) Nyquist impedance plots of the electrodes. (e) Schematic illustration of the lithiation/delithiation process for Zn−Co-1 electrode.

electrode pulverization and capacity fade were greatly inhibited, which would contribute to ultrastable cyclability compared with Zn−Co-0 or Zn−Co-2 (Figure 6b). In addition, the 2Dmosaic-hybrid prevented Co/Zn nanograins from aggregation to some extent, ensuring enhanced interfacial Li-storage.59 Figure S22 also showed initial discharge/charge profiles of Zn−Co-0 and Zn−Co-2 electrodes. Concerning Zn−Co-0, a voltage plateau from ∼0.5 V followed by a downward sloping matched with a characteristic conversion reaction for ZnO, while curves below 0.3 V were mainly related to formation of LixZn alloy and SEI layer.21 Meanwhile Zn−Co-2 showed one main plateau at ∼1.0 V which agreed with the conversion of ZnCo2O4.24 Initial discharge capacities of Zn−Co-0 and Zn− Co-2 anodes were 1265 and 1236 mA g−1, respectively. Initial Coulombic efficiency (CE) of Zn−Co-1 was about 71%, being close to that of Zn−Co-2 (71%) and higher than that of Zn− Co-0 (59%). The irreversible capacities were mainly due to the incomplete decomposition of SEI layer as well as intrinsic kinetic limit to Li+ extraction.23 Figure 6e compared the initial discharge/charge profiles of Zn−Co-1 with various current densities, exhibiting gradual decreasing initial capacities as rate increased. As shown in Figure 6c, we observed a relatively low expansion degree (∼35%) for Zn−Co-1 electrode after the first full lithiation, owing to 2D-3D mesoporous framework that provided sufficient free space to buffer the drastic volume expansion, which would prevent electrode pulverization and improve cycle stability. Moreover, to confirm this multistep Li storage mechanism, ex situ XRD patterns of the Zn−Co-1 electrode piece at various discharge states was obtained, which agreed well with above initial discharge profile (Figure 6d). Initially, the electrode clearly exhibited coexistence of ZnO (partial Co-doping) and ZnCo2O4 phases until 1.9 V. Subsequently, when the potential further declined to 1.2 and 0.7 V successively, the ZnCo2O4 and ZnO peaks began to

Zn1−xCoxO/ZnCo2O4 mesoporous framework was evaluated as a LIB anode. Figure S22 showed the initial galvanostatic discharge/charge profile of representative Zn−Co-1, with a current density of 100 mA g−1 and potential range of 0.01−3.0 V (vs Li+/Li). The discharge capacity of 1389 mA h g−1 was higher than the theoretical capacity of either ZnO or ZnCo2O4, which should be attributed to a harmonious multistep Li storage mechanism containing three stages (Figure 6a). First, at the first voltage platform which was relatively higher (∼1.0 V), ZnCo2O4 was converted into nano Co(0) and Zn(0) as eq 1, while the concomitant Zn1−xCoxO phase remained unreacted.55 ZnCo2O4 + 8Li+ + 8e− → Zn + 2Co + Li 2O

(1)

Subsequently, when the voltage was decreased to the second voltage platform (∼0.5 V), Zn1−xCoxO began lithiation and formed nano Zn(0) in Li2O matrices (accompanied with a little Co(0)), as eq 2, while the previously formed Co(0)−Zn(0)− Li2O region remained stable. Zn1 − xCox O + 2Li+ + 2e− → 1 − x Zn + xCo + Li 2O (2)

Finally, at the low potential region, there existed multiple Li storage mechanisms which further enhanced the capacity, including generation of SEI layer, extra interfacial reaction (storage of Li+ and electrons on the Li2O and Co sides, respectively),59 and alloying reaction between Zn (0) and Li (eq 3).40 Zn + x Li+ + x e− → LixZn(x ≤ 1)

(3)

It is noticeable, benefiting from the 2D-mosaic-mode hybrid, that there existed a beneficial ladder-type buffering effect during stages I and II of the multistep Li storage (Figure 6a). At a certain voltage, the inactive component could function as a buffer zone, to efficiently relieve the expansion stress of the surrounding active component resulting from lithiation. Thus, I


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finished in about 16 min, while a capacity far higher than the theoretical value of commercial graphite (370 mA h g−1) was obtained. Moreover, its capacity recovered to more than 800 mA h g−1 when the rate recovered to 100 mA g−1, demonstrating good capacity retention ability. As comparison, Zn−Co-2 and Zn−Co-0 faced more dramatic capacity fade as the rate increased, with discharge capacities of only 377 and 94 mA h g−1 at 2000 mA g−1, respectively. In addition, the inset of Figures 7b and S24 showed that 2D-3D framework of Zn−Co1 was well-preserved after long-term cycles, revealing excellent structural stability. The Nyquist impedance plots of the three kinds of electrodes are compared in Figure 7d, wherein the depressed semicircles at high-frequency regions and inclined lines at low-frequency regions were indexed to charge impedance and lithium diffusion impedance, respectively.16 Among them, Zn−Co-1 exhibited the lowest impedance, indicating better conductivity for the bimetal oxide dual-composite system which was consistent with above DFT calculations, laying the foundation for its good rate capability.

decline respectively, suggesting the ordered conversion of two phases. At the final cutoff potential (0.01 V), neither peak belonging to ZnCo2O4 nor ZnO was observed, revealing complete conversion. During the discharge process, XRD peaks of Co(0), Zn(0), and Li2O were not observed, indicating their amorphous nature.50 As shown in Figure 6f, initial five cycles of cyclic voltammetry (CV) curves of Zn−Co-1 electrode were carried out, at 0.1 mV s−1 within 0.01−3.0 V. There were obvious peaks at 0.75 and 0.33 V in the first cathodic scan, while the former mainly derived from the conversion of ZnCo2O4 (eq 1), and the latter was attributed to reduction of ZnO together with further alloying reaction (eqs 2 and 3).24,40 Subsequently, two small anodic peaks nearby 0.32 and 0.68 V referred to the multistep LixZn dealloying, meanwhile large peaks at 1.7 and 2.1 V originated from oxidation of Zn(0) and Co(0) to ZnO and Co3O4, respectively, accompanied with decomposition of SEI layer (eqs S3−S5).24 From the second scans, the major cathodic peak shifted to 1.02 V due to changes in structure and crystallinity of electrodes, while indicating irreversible reaction of SEI formation in the first cycle.60 In subsequent cycles, the hybrid oxide took part in electrochemical reactions as a form of ZnO-Co3O4, which possessed a ladder-type buffering effect based on the Zn/ZnO and Co/Co3O4 redox couples with distinct conversion potentials. In the following cycles, similar peak shapes and intensities suggested good reversibility of Listorage process. Electrochemical Performance. Owing to the bimetal oxide dual-composite strategy based on 2D-mosaic 3D-gradient mesoporous framework, Zn−Co-1 was expected to exhibit superior Li storage performance. Figure 7a compared the cycling performances of Zn−Co-x (x = 0, 1, and 2) as LIB anodes at a current density of 100 mA g−1. It is found that after initial dozens of cycles, Zn−Co-1 showed an upward capacity curve with a CE staying above 99%. The capacity rising trend was normally observed for conversion mechanism anodes, mainly due to electrode kinetic activation and extra interfacial Li storage.14,27 Thereafter, the discharge capacity remained stable and reached as high as 1010 mA h g−1 over 200 cycles. By contrast, Zn−Co-2 showed a similar capacity trend but delivered a lower capacity of 665 mA h g−1, while Zn−Co-0 suffered from a severe capacity decay with only 241 mA h g −1 left. More strikingly, benefiting from a microsized 3D framework, Zn−Co-1 anode possessed an excellent areal specific capacity. Its above gravimetric special capacity corresponded to a regular mass loading density of active material (0.78 mg cm−2), and when the areal current density remained unchanged but mass loading was increased to 1.69 mg cm−2, a high areal capacity of 1.48 mA h cm−2 was obtained (Figure S23). This was superior to most of the nanosized TMO-based anodes and could be comparative with Si-based anodes, rending its great potential in practical applications.12 Furthermore, the performance advantage of Zn−Co-1 became more obvious in high-rate and long-term cycles. It maintained an ultrastable reversible capacity when the current density was increased to 1000 mA g−1 and reached 741 mA h g−1 after 800 cycles with a high CE, working much better than the other two samples (Figure 7b). Discharge capacities at various current densities for Zn−Co-0, 1, and 2 were further compared in Figure 7c, and Zn−Co-1 delivered superior rate capability to other two samples. Even at a high rate of 2000 mA g−1, Zn−Co-1 still delivered a discharge capacity of 558 mA h g−1. That meant the discharge or charge process could be

DISCUSSION As is well-known, biological hybrid structures usually showed exquisite control in multiple composition and architectural hierarchy and possessed exceptional properties. Thus, building a task-specific biomimetic structure has become a promising strategy; classical examples include nacre-mimic hierarchically ordered structure with good ultimate strength,61 watermelonmimic Si/C microspheres for densely compacted LIB,12 and fibrous-root-inspired synergistic nanoarray with enhanced Li storage,23 etc. Herein, inspired by natural mosaic dominance phenomena, a Zn1−xCoxO/ZnCo2O4 2D-mosaic 3D-gradient mesoporous framework was built. Functioning as a bimetal oxide dual-composite system, it exhibited excellent Li storage performance, especially for high-rate and long-term cycles, accompanied by a high areal specific capacity, which should be attributed to the synergistic effect of several factors (Figure 7e). From a componental perspective: (1) Zn1−xCoxO/ZnCo2O4 hybrid integrated both advantages of ZnO (higher capacity, lower voltage platform) and ZnCo2O4 (higher stability). (2) Bimetal oxide possessed higher conductivity due to doping effect. (3) Rich oxygen vacancies provided more electrochemical active sites and facilitated the transport of Li+/e. From a structural perspective: (1) 2D-mosaic-hybrid building blocks enabled ladder-type buffering and electrochemical synergistic effect, greatly relieving the expansion stress and benefiting the cycling stability. (2) Zn−Co concentration gradient within 3D framework helped to satisfy the demands for high capacity and high cyclability. (3) 2D-3D hierarchical mesoporous framework efficiently facilitated rapid diffusion kinetics, stable mechanics frame, and high areal mass loading. CONCLUSIONS In summary, inspired by natural mosaic dominance phenomena, we have developed a bimetal oxide dual-composite strategy based on 2D-mosaic 3D-gradient mesoporous framework, for building TMO-based anodes simultaneously possessing high capacity, conductivity, and stability. Following this strategy, a 3D Zn−Co framework assembled from Zn1−xCoxO/ZnCo2O4 2D-mosaic-hybrid mesoporous nanosheets was controllably synthesized, more importantly, via a quite facile one-pot solvothermal route followed by calcination. The componentJ


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ACS Nano composite provided superior conductivity due to bimetal doping effect, which is verified by DFT calculations; the 2Dmosaic structure-composite realizes effective mutual stabilization and activation owing to ladder-type buffering and electrochemical synergistic effect. Moreover, the inside-out Zn−Co concentration gradient and rich oxygen vacancies further enhanced Li storage capability. When used as a LIB anode, the Zn1−xCoxO/ZnCo2O4 delivered superior capacity (1010 mA h g−1, 1.48 mA h cm−2) as well as ultrastable cyclability (very little capacity loss after 800 cycles at a current density of 1000 mA g−1). This 2D-mosaic 3D-gradient framework accompanied with facile preparation provide one versatile design, which may be extended to synthesis of TMObased materials for wider electrochemical applications.

form a slurry, and pasted onto copper foil. After drying, the foil was cut into electrode pieces with a diameter of 14 mm. Normally, the mass loading of active material was about 1.2 mg/piece, while a high-loading piece contained 2.6 mg. Lithium foil was used as the counter electrode, and Celgard 2400 membrane was used to separate two electrodes. One M LiPF6 in EC/DEC/DMC (1:1:1 wt.) was used as electrolyte. Coin cells were cycled between 0.01 and 3 V (vs Li/Li+) on a Neware battery testing system with various current densities (100−2000 mA g−1). CV data were recorded on an Autolab (PGSTAT302N) electrochemical workstation, with a scanning rate of 0.1 mV s−1 and a voltage range of from 0.01 to 3.0 V. EIS data were carried out on the Autolab electrochemical workstation under a frequency range from 100 kHz to 0.01 Hz.

EXPERIMENTAL DETAILS

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b00168. Supplementary SEM/HRSEM images, particle size distribution analysis, XRD patterns, photographs, poresize distribution curves, TEM/HRTEM images, EDS element mapping, EDS line-scan, XPS spectra, TG curves, discharge/charge profiles, areal specific capacity profiles, lattice constant and volume data, equations for growth mechanism, equations for delithiation process, etc. (PDF)

ASSOCIATED CONTENT S Supporting Information *

Synthesis of Zn−Co-x Series 2D-3D Mesoporous Framework. In a typical synthesis, first zinc nitrate hexahydrate (Zn(NO3)2· 6H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), urea (CO(NH2)2), and ammonium fluoride (NH4F) were dissolved in 40 mL of deionized water with a certain molar ratio of 1:x:5:1 (unit: mmol), corresponding to the low-fluorine level system. Subsequently, the solution was transferred into Teflon-lined stainless-steel autoclave, which was sealed and gradually heated to 180 °C, and maintained for a certain time up to 6 h. After cooling to room temperature, obtained precursors were washed for several times and dried at 80 °C and then calcined at 400 °C for 3 h to obtain final Zn−Co-x (x = 0, 0.5, 1 and 2) series products. Synthesis of Zn−Co-x-NF, Zn−Co-x-HF, and Zn−Co-x-UF Series. For Zn−Co-x-NF, Zn−Co-x-HF, and Zn−Co-x-UF (x = 0, 0.5, 1, and 2), the synthetic conditions were similar to above, except for the amount of NH4F was 0, 2, and 10 mmol, respectively. In addition, the calcination temperature for Zn−Co-x-UF series was enhanced to 600 °C. DFT Calculations. DFT calculations were performed using planewave based Vienna ab Initio Simulation Package (VASP). US pseudopotentials were used for core−valence electron interaction, and GGA-PBE was used for exchange−correlation interactions. A 2 × 2 × 2 supercell was employed for ZnO, Zn1−xCoxO, and Zn1−2xCo2xO (x = 0.125), while an energy cutoff of 400 eV was used for plane-wave basis sets and 3 × 3 × 3 mesh grid for sampling k points was set up for Brillouin zone integration. A primary cell was employed for Co3O4 and ZnxCo3−xO4 (x = 0.25 and 1), and an energy cutoff of 500 and 2 × 2 × 2 Monkhorst−Pack mesh grid were used. A total energy convergence threshold was set to be 1 meV/atom. The geometrical relaxation was carried out until the convergence threshold of force on atom was 0.01 eV/Å.62 Materials Characterizations. SEM observations were carried out on a Hitachi SU-8020 microscope operated at 5.0 kV. TEM observations and SAED measurements were performed with a JEOL JEM-2100F microscope operated at 200 kV. AFM micrographs were obtained using a Bruker Multimode 8. XRD patterns were recorded using a Rigaku SmartLab 9 kW X-ray diffractometer equipped with a Ni-filtered Cu−Kα radiation (λ = 0.15406 nm) source. EDS analyses were performed using Oxford X-MaxN attached to Hitachi SU8020 and JEOL JEM-2100F microscopes. XPS spectra were tested with a Thermo Fisher Scientific ESCALAB 250Xi instrument. Raman spectra were recorded using a Rensihaw inVia confocal Raman microscope. TG analyses were measured using a Shimadzu DTG-60H under air atmosphere with a heating rate of 5 °C min−1. EPMA results were obtained with a Shimadzu EPMA-1720H. PL spectra were carried out using a Hitachi F4600 fluorescence spectrophotometer. FT-IR spectra were recorded on a NICOLET FT-IR spectrometer in KBr tablets. Electrochemical Measurements. The electrochemical characterizations of Zn−Co-0, Zn−Co-1, and Zn−Co-2 were tested at room temperature using CR2025 coin-type half-cells. Active material, super P, and polyvinylidene fluoride (PVDF) were mixed with a weight ratio of 7:1.5:1.5 using 1-methyl-2-pyrrolidinone (NMP) as the solvent, to

AUTHOR INFORMATION Corresponding Authors

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

Shimou Chen: 0000-0002-2533-4010 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the support of National Natural Science Foundation of China (nos. 91534109 and 91434203), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (no. XDA09010103), National Key Projects for Fundamental Research and Development of China (no. 2016YFB0100100), and Beijing Natural Science Foundation (no. 2184124). REFERENCES (1) Zhang, Z. W.; Chen, P.; Duan, X. D.; Zang, K. T.; Luo, J.; Duan, X. F. Robust Epitaxial Growth of Two-Dimensional Heterostructures, Multiheterostructures, and Superlattices. Science 2017, 357, 788−792. (2) Wang, X.; Weng, Q. H.; Yang, Y. J.; Bando, Y.; Gotberg, D. Hybrid Two-Dimensional Materials in Rechargeable Battery Applications and Their Microscopic Mechanisms. Chem. Soc. Rev. 2016, 45, 4042−4073. (3) Ran, J. R.; Gao, G. P.; Li, F. T.; Ma, T. Y.; Du, A. J.; Qiao, S. Z. Ti3C2 Mxene Co-Catalyst on Metal Sulfide Photo-Absorbers for Enhanced Visible-Light Photocatalytic Hydrogen Production. Nat. Commun. 2017, 8, 13907. (4) Pomerantseva, E.; Gogotsi, Y. Two-Dimensional Heterostructures for Energy Storage. Nat. Energy 2017, 2, 17089. (5) Mei, J.; Liao, T.; Kou, L. Z.; Sun, Z. Q. Two-Dimensional Metal Oxide Nanomaterials for Next-Generation Rechargeable Batteries. Adv. Mater. 2017, 29, 1700176. K


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