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Tuning Shell Numbers of Transition Metal Oxide Hollow Microspheres toward Durable and Superior Lithium Storage Dan Luo,†,⊥ Ya-Ping Deng,†,⊥ Xiaolei Wang,‡ Gaoran Li,† Juan Wu,§ Jing Fu,† Wen Lei,† Ruilin Liang,† Yangshuai Liu,† Yuanli Ding,† Aiping Yu,† and Zhongwei Chen*,† †
Department of Chemical Engineering, University of Waterloo 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada Department of Chemical and Materials Engineering, Concordia University 1455 De Maisonneuve Boulevard West, Montreal, Quebec H3G 1M8, Canada § Department of Material Science and Engineering, McMaster University 1280 Main Street West, Hamilton, Ontario L8S 4L8, Canada ‡
S Supporting Information *
ABSTRACT: Multishelled hollow structured transition metal oxides (TMOs) are highly potential materials for high energy density energy storage due to their high volumetric energy density, reduced aggregation of nanosized subunits, and excellent capacity and durability. However, traditional synthetic methods of TMOs generally require complicated steps and lack compositional/morphological adjustability. Herein, a general and straightforward strategy is developed to synthesize multishelled porous hollow microspheres, which is constituted of nanosize primary TMO particles, using metal acetate polysaccharide microspheres as the precursor. This universal method can be applied to design TMOs’ hollow spheres with tunable shell numbers and composition. The hierarchical porous quadruple-shelled hollow microspheres with nanosized Ni−Co−Mn oxide demonstrate an increased number of active sites, boosted rate capability, enhanced volumetric energy density, and showed great tolerance toward volume expansion upon cycling, thus exhibiting excellent Li+ storage capability with high specific capacity (1470 mAh g−1 at 0.2 A g−1 and 1073.6 mAh g−1 at 5.0 A g−1) and excellent cycle retention (1097 mAh g−1 after 250 cycles at 0.2 A g−1) among TMO anode materials for lithium-ion batteries. KEYWORDS: transition metal oxide, hollow sphere, shell-controlled, micro-nano structure, lithium-ion battery the conversion mechanism.9 In comparison to unitary TMOs, ternary TMOs, which involve different metal elements in a single structure such as Ni−Co−Mn oxide and ZnCoMnO4, exhibit potentially superior electrochemical performance due to their complex chemical composition, abundant defects, and synergistic effects of multiple metal species.10,11 The exceptionally high capacity and rate capability of ternary TMOs make them extremely compelling as electrode materials for lithium
T
he realization of high energy density storage material is one of the greatest scientific and engineering challenges in the 21st century.1−3 Energy storage devices are required to be able to store large quantities of electrical energy in a small space, which would never be realized without rational component design of appropriate electrode materials.4,5 Transition metal oxides (TMOs) with various compositions have been extensively studied during the past decades as intriguing electrode materials for inexpensive energy storage systems, such as rechargeable lithium-ion batteries (LIBs), fuel cells, and supercapacitors.6−8 Particularly, TMOs are of great interest because of their superior lithium storage capability by © 2017 American Chemical Society
Received: September 4, 2017 Accepted: November 1, 2017 Published: November 1, 2017 11521
DOI: 10.1021/acsnano.7b06296 ACS Nano 2017, 11, 11521−11530
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Cite This: ACS Nano 2017, 11, 11521-11530
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ACS Nano Scheme 1. Formation Process of TMO Quadruple-Shelled Hollow Spheres
storage applications.12 However, ternary TMOs suffer from structure instability caused by large volumetric expansion during the lithiation process and poor electrical/ionic conductivity.13 These distinct structural weaknesses of ternary TMOs have greatly impeded their practical application. Structural design of ternary TMOs offers a promising route to enhance Li+ storage properties.14 The construction of a porous and hollow structure is capable of facilitating ion access to the electrode matrix by offering sufficient ion transport pathways.15 Compared to single-shelled hollow spheres, a multishelled hollow structure offers additional benefits such as high volumetric energy density, shell permeability, and reduced aggregation of nanosized subunits.16−18 In addition, the high porosity and large surface area of multishell spheres facilitates electrolyte infiltration and reduces Li-ion diffusion distance, resulting in improved active material utilization and fast electrochemical kinetics.19,20 Moreover, multishelled hollow spheres with more shells offer freedom to manipulate the materials properties through the design and construction of multicomponents with complex architectures to achieve enhanced properties.21−24 Therefore, a synergistic combination of well-designed composition and structure of the TMOs’ hollow sphere is promising to realize greatly improved lithium storage performance.25 Currently, few papers have reported the synthesis of TMOs’ multishelled hollow sphere by vesicles, gas bubble, or hard templating methods.26−28 However, these strategies usually involve time-consuming processes and struggle to control the structures and components of the products. The development of versatile methodologies to synthesize porous hollow structures with multicomponents and tunable number of shells remains a great challenge.29 Herein, we present a straightforward strategy for the synthesis of multishelled TMO microspheres with a tunable number of shells, which involves the one-pot synthesis of metal acetate polysaccharides (MAPs) and subsequent thermal treatment. Multiple coordinate bonds between polysaccharides and transition metal ions allow them to cross-link with each other to construct a metal−polysaccharide network. Porous multishelled hollow microsize structure with nanosized subunits can be subsequently obtained by oxidizing the metal ions and removing carbon through thermal treatment.30−32 This economic and facile synthetic method delivers great universality and strong controllability on the morphology and composition of the obtained hollow microspheres by varying concentration and ratio of metal ions, which makes it possible to systematically investigate the structure−electrochemical
property relationships. The as-developed Ni−Co−Mn oxide quadruple-shelled hollow microspheres (QS-HS) offer abundant electrode/electrolyte interfaces, which minimizes the lithium-ion diffusion path and provides numerous active sites for the Li+ conversion reaction. Furthermore, the quadrupleshelled structure with empty interiors is flexible toward volume changes and is capable of diverting strain and stress during charge−discharge processes. Benefiting from the structural integrity and compositional advantages, QS-HS demonstrated an extremely high specific capacity of 1470 mAh g−1 at 0.2 A g−1 with high cycle stability over 250 cycles and extremely high gravimetric and volumetric energy density.
RESULTS AND DISCUSSION The conventional syntheses of multishelled TMOs are complicated and time-consuming and usually require processes such as template synthesis, ion adsorption, and template removal.33−36 In comparison, we developed a convenient route to prepare TMO hollow microspheres with a tunable number of shells and composition by using metal acetate polysaccharides as the self-template. The synthesis method of TMO QSHS is illustrated in Scheme 1. QS-HS with a uniform gap between adjacent thin shells are synthesized by a hydrothermal method followed by thermal treatment. A homogeneously mixed solution of metal acetates and glucose is prepared for hydrothermal reaction. During the glucose polymerization process, metal cations (Ni2+, Co2+, and Mn2+) are adsorbed onto the polysaccharides to form MAPs via an in situ coordinating process that involves intermolecular dehydration of glucose and simultaneous coordination of transition metal ions.37 The adsorbed metal ions on the MAPs’ framework will further promote the growth of microspheres by acting as a cross-linking agent and forming coordination bonds with freeflowing glucose, resulting in carbonaceous spheres (CS) with uniformly distributed metal ions and expanded volume.38,39 Figure S1A (Supporting Information) shows scanning electron microscopy (SEM) images of MAP spheres. Compared to pure CS directly synthesized without metal acetates by a hydrothermal method (Figure S1B, Supporting Information), the diameter of MAPs’ spheres dramatically increased from ∼300 nm to ∼6 μm. A transmission electron microscopy (TEM) image (Figure S2A, Supporting Information) confirmed the formation of solid MAP spheres without a hollow interior, indicating that the huge volume expansion results from the absorbed metal ions and their coordination with glucose. From the energy-dispersive X-ray (EDX) line scan of MAPs (Figure 11522
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Figure 1. SEM images of (A) single Ni−Co−Mn oxide QS-HS and (B) multiple of Ni−Co−Mn oxide QS-HS. (C) TEM image of Ni−Co−Mn oxide QS-HS. (D) HRTEM of single particle on Ni−Co−Mn oxide QS-HS (small figure is fast Fourier transform image of Ni−Co−Mn oxide particle). (E) EDX element mappings of Ni−Co−Mn oxide QS-HS. (F) HAADF-STEM image of Ni−Co−Mn oxide QS-HS with mixed element distribution of Ni, Co, and Mn.
Figure 2. HAADF-STEM 3D reconstruction of QS-HS slicing at different lengths.
absorbed by polysaccharides,40 resulting in a high concentration of Ni2+ and Co2+ inside the particle, which is also confirmed by EDX line scan and element mapping. Meanwhile, the Fourier transform infrared (FTIR) spectrum of MAPs shows characteristic adsorption peaks at 1695 and 1576 cm−1, contributed by COO− and CC group, respectively (Figure S4A, Supporting Information). Compared to the pure CS, the COO− stretching vibration of MAPs shifts from 1706 to 1695 cm −1 , demonstrating the coordination of carboxylate groups of CS to metal cations.41,42 During the annealing of MAPs in air, the microspheres began to shrink near 300 °C, and their surfaces were transformed into a metal oxide shell which was induced by
S3, Supporting Information), it can be observed that Ni, Co, and Mn elements are uniformly distributed within the carbon sphere. Compared to the metal ion CS prepared by the ion adsorption method reported in other literature,25 both particle size and M2+/C intensity ratio drastically increased in the case of MAPs, indicating more metal ions have adsorbed inside particles. EDX element mapping obtained by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) also confirms uniform element distribution in MAPs (Figure S2B, Supporting Information). Due to the relatively high hydrolysis rate of Ni2+ and Co2+ compared to that of Mn2+, more Ni2+ and Co2+ ions are hydrolyzed and 11523
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Figure 3. (A) XRD of Ni−Co−Mn oxide QS-HS. (B) Nitrogen adsorption−desorption isotherms at 77 K of Ni−Co−Mn oxide QS-HS (small figure is the pore size distribution of Ni−Co−Mn oxide QS-HS). XPS spectra of the (C) Ni 2p, (D) Co 2p, and (E) Mn 2p regions for Ni− Co−Mn oxide QS-HS.
the large temperature gradient along the radial direction.43 In this progress, two forces, contraction force and adhesion force, are acting simultaneously on the shell interfaces in opposing directions. The contraction force, which causes the inward shrinkage of the inner sphere, is induced by the considerable weight loss (∼47.7 wt %) during the oxidative degradation, as verified by thermogravimetric analysis (TGA) (Figure S5, Supporting Information) with a ramping rate of 1 °C min−1.14,44 On the contrary, the adhesion force hinders the inward contraction of the core from the relatively rigid shell.43 SEM and HAADF-STEM images (Figure S6A,B, Supporting Information) show the separation of the microspheres and the formation of the porous surface. The X-ray diffraction (XRD) peak indicated the formation of nanosized Ni−Co−Mn oxide (Figure S6C, Supporting Information). Furthermore, the inner carbon sphere decomposes with increasing temperature, which is accelerated by as-formed TMOs’ catalytic combustion process45 and result in a drastic weight loss of 38.8%, as shown in TGA. At 400 °C, the morphology of the outer shell did not change, whereas the inner shell formed, as confirmed by SEM and STEM images (Figure S6D,E, Supporting Information). The increase of the XRD diffraction peak intensity revealed the continuing crystallization of Ni−Co−Mn oxide during the annealing progress (Figure S6F, Supporting Information). At ∼500 °C, Ni−Co−Mn oxide QS-HS was formed with prolonged annealing. No vibration peaks of CC and COO− are observed from the FTIR spectra, whereas broad peaks appear in the range of 400−700 cm−1, which are assigned to characteristic vibrational modes of M−O bonds, indicating the formation of Ni−Co−Mn oxide after heat treatment.46,47 Also, a Raman spectrum was collected, as shown in Figure S4B (Supporting Information). Clear peaks appear in the range of 400−700 cm−1 corresponding to the vibrational modes of M− O bonds,48 which confirms the formation of Ni−Co−Mn oxide and the removal of carbon. The chemical composition is
confirmed by inductively coupled plasma (ICP) optical emission spectrometry in Table S1 (Supporting Information). SEM images of QS-HS in different resolutions are shown in Figure 1A,B and Figure S15 (Supporting Information). The QS-HS were obtained with porous surface and uniform primary particle size. Figure 1C shows a typical TEM image of Ni−Co− Mn oxide QS-HS, which clearly reveals its sophisticated interior structure. Ni−Co−Mn oxide nanocrystal subunits on the shell are shown in the high-resolution TEM (Figure 1D) with an interplanar spacing of 0.48 nm, corresponding to the (110) crystallographic plane of spinel NiCoMnO4. Moreover, Figure 2 shows reconstructed 3D HAADF-STEM images of QS-HS visualized by slicing the sphere at different depths. The detailed reconstruction animation of the outer surface, inner structure, and representative slices (one voxel thickness, 10 nm) through the 3D volume of the hollow microspheres is shown in Video S1 (Supporting Information). The sophisticated inner structure of QS-HS can be seen from the 3D reconstruction where primary metal oxide nanoparticles unite together and form a solid shell with micropores. This designed robust architecture facilitates electron and ion transportation and improves kinetics, which makes it highly promising for improved performance in energy storage applications. Meanwhile, EDX element mapping images of Ni, Co, Mn, O, and mixed elements (Figure 1E,F) indicates the uniform distribution of each element in every shell of Ni−Co−Mn oxide hollow microspheres. Figure 3A shows the XRD result of Ni−Co−Mn oxide QS-HS. The diffraction peaks can be ascribed to the cubic spinel phase, which has the Fd3̅m space group, consistent with previous reports.49 In addition, the obtained QS-HS possess a high Brunauer−Emmett−Teller (BET) surface area of 69.5 m2 g−1 with a hierarchical porosity which is confirmed by the pore distribution in Figure 3B. The high surface area and abundant micropores built into the microstructures are beneficial for sufficient electrochemical redox and fast ion diffusion. X-ray 11524
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Figure 4. (A,D) SEM image, (B,E) HAADF-STEM image, and (C,F) XRD pattern of Co3O4 QS-HS and NiCo2O4 QS-HS.
photoelectron spectroscopy (XPS) spectrum of the obtained Ni−Co−Mn oxide QS-HS is presented in Figure S7 (Supporting Information). In the Ni 2p spectrum (Figure 3C), two peaks corresponding to Ni 2p3/2 and 2p1/2 levels are located at 854.8 and 872.6 eV. The four other peaks associated with Ni2+ can be seen at 854.8, 872.3, 856.4, and 874.0 eV.50,51 The Co 2p spectrum (Figure 3D) exhibited two peaks at 780 and 795.6 eV associated with Co 2p3/2 and Co 2p1/2.52 The satellite peaks confirmed the existence of Co2+ and Co3+ species at the surface.53,54 Two main peaks of the Mn 2p spectrum (Figure 3E) are located at 642.3 and 653.9 eV, corresponding to the Mn 2p3/2 and 2p1/2 levels. The subpeaks of Mn2+ are located at 642.2 and 654.0 eV, and the peaks of Mn3+ can be clearly seen at 644.5 and 657.3 eV.55,56 XPS analysis confirms the existence and reveals the chemical status of Ni2+/Ni3+, Co2+/Co3+, and Mn2+/Mn3+ on the surface of the Ni−Co−Mn oxide QS-HS sample.57 To demonstrate the generality of our strategy, we successfully prepared other quadruple-shelled metal oxides with tunable composition. Figure 4 shows the morphology, interior architecture, and crystal structure of Co3O4 and NiCo2O4 QS-HS. SEM and TEM images confirm that these types of QS-HS possess similar quadruple-shelled structure morphology and size uniformity. The crystal structure and chemical composition of each mixed metal oxide material are verified by XRD. Moreover, this general method is also capable of producing Ni−Co−Mn oxide hollow microspheres with a different number of shells by only varying the concentration of metal acetate. The SEM and TEM images in Figure 5 demonstrate the successful synthesis of double-shelled and triple-shelled Ni−Co−Mn oxide hollow microspheres by the same method (DS-HS and TS-HS). This evidence indicates that this universal and facile method can also be used to creatively control component and number of shells to design multishelled TMO hollow microspheres. Notably, with an even
Figure 5. SEM and TEM morphology of (A,B) Ni−Co−Mn oxide rigid microspheres, (C,D) double-shelled hollow microspheres, (E,F) triple-shelled hollow microspheres, and (G,H) quintupleshelled hollow microspheres. 11525
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Figure 6. (A) Galvanostatic discharge−charge curves of Ni−Co−Mn oxide QS-HS at 0.1 A g−1. (B) CV scan of Ni−Co−Mn oxide QS-HS from 0.01 to 3 V at a scan rate of 0.1 mV s−1. (C) Rate performance of Ni−Co−Mn oxide QS-HS, TS-HS, DS-HS, and rigid sphere. (D) Li-ion diffusion coefficient of Ni−Co−Mn oxide QS-HS, TS-HS, DS-HS, and rigid sphere obtained from discharge and charge process. (E) Specific capacity and Coulombic efficiency obtained from Ni−Co−Mn oxide QS-HS, TS-HS, DS-HS, and rigid sphere after cycling for 250 cycles at 0.2 A g−1.
decomposition and solid electrolyte interface (SEI) formation.60 Figure 6B presents the typical cyclic voltammetry (CV) curves of QS-HS, which shows good consistency with its voltage profile. For the initial cathodic scan, two sharp peaks can be observed at 0.83 and 0.59 V, which correspond to the reduction of high valence Ni, Co, and Mn and the formation of the SEI layer and Li2O.61 Two broad anodic peaks can be observed at 1.50 and 2.18 V, corresponding to the re-formation of high valence TMOs. During the subsequent scans, a single major cathodic peak can be observed at 0.92 V, representing the reduction of TMOs.62 The rate performance and voltage profiles of hollow spheres with different shell numbers are shown in Figure 6C and Figure S8 (Supporting Information), respectively. The QS-HS electrode exhibits the best rate capability among the different shelled hollow microspheres with a highly reversible capacity of 1073.6 mAh g−1 at a very high current of 5 A g−1, which corresponds to approximately ∼70% of its reversible capacity at 0.1 A g−1, indicating its fast reaction kinetics and robust electrode integrity. The superior rate
higher concentration of metal source, quintuple-shelled hollow microspheres (FIS-HS) can also be synthesized but possess low structural stability and size uniformity after annealing due to the decrease of mechanical strength.58,59 Ternary metal oxides have been regarded as very promising electrode materials for high-performance energy storage devices. By investigating the influence of Li+ storage with a different number of shells, the Ni−Co−Mn oxide QS-HS delivers outstanding capacity and possesses superior Li+ storage properties compared to those of FIS-HS, TS-HS, DS-HS, and rigid microspheres. The lithium storage properties are evaluated by galvanostatic charge−discharge cycling with a voltage window of 0.01−3 V vs Li/Li+ to determine the specific capacity values and capacity retentions. Figure 6A shows the voltage profiles of the Ni−Co−Mn oxide QS-HS electrode. It delivers a very high initial discharge capacity of 1761.8 mAh g−1 at 0.2 A g−1 and is stabilized at a capacity of around 1400 mAh g−1 during the subsequent cycles. The irreversible capacity loss in the second discharge cycle is attributed to electrolyte 11526
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Figure 7. Gravimetric and volumetric energy density of Ni−Co−Mn oxide DS-HS, TS-HS, and QS-HS at different discharge current density. (Note: Gravimetric energy density and volumetric energy density are calculated by the mass and volume of active material, respectively.)
published reports based on TMO electrodes (Table S2, Supporting Information). As shown in Figure 6D,E, the first cycle Coulombic efficiency of QS-HS, TS-HS, DS-HS, and the rigid sphere is 72.3, 70.7, 67.7, and 64.3%, respectively, which correspond to SEI formation and irreversible reactions. The first cycle Coulombic efficiency slightly increased with the increased number of shells. This is because the high surface area and porous structure of the multishelled hollow sphere reduce the ion and electron transport pathway, which facilitates the reaction of metal with Li2O and thus increases the charge capacity in the first cycle. Moreover, even at a raised current rate of 1 A g−1, QS-HS still shows the best cycle stability compared to that of TS-HS, DS-HS, and rigid microspheres (Figure S10, Supporting Information). However, because of the structure instability of FIS-HS, its capacity and cycle stability is much lower than that of QS-HS. The successful synthesis of these multishelled hollow microspheres is also highly promising for practical use of metal oxide anode material. By designing a multishelled microsphere with a nanosize subunit, this material efficiency utilizes the hollow interior, achieves excellent capacity and structural stability, and minimizes particle agglomeration and safety/environmental hazards associated with nanomaterials. Table S3 (Supporting Information) compares the tap density of multishelled Ni−Co−Mn oxide with two to four shells. As demonstrated, the tap density greatly improves with increasing number of shells and reaches a high value of 2.85 g cm−3 when the number of shells increased to four. Figure 7 compares gravimetric and volumetric energy density of Ni−Co−Mn oxide DS-HS, TS-HS, and QS-HS at different discharge current densities. Although the gravimetric energy density only increases slightly when the number of shells is increased, higher shell number materials form a relatively dense and stable structure, maximize volume utilization and material compactness, and thus achieve excellent volumetric energy density. Overall, QS-HS obtained the highest gravimetric energy density (1261.7 Wh kg−1) and volumetric energy density (3595.9 Wh L−1) at a current density of 0.1A g−1, and QS-HS has the highest energy density at all discharge current density compared to that of DS-HS and TS-HS. The great capacity, excellent rate capability, good cycle stability, and high specific energy density of the Ni−Co−Mn oxide QS-HS hollow microspheres provides this nanomaterial with unlimited potentials in practical applications.
performance is mainly originated from the micropores on surface of the thin shell, which facilitates both the electron and ion transportation. The Li+-ion storage performance greatly improves with the increasing shell numbers, and the discharge capacity reaches a maximum value. However, the FIS-HS electrode undergoes a fast capacity fading compared to that of QS-HS electrode due to its relatively lower structural stability.63 To further investigate the kinetics of the lithiation/ delithiation process, electrochemical impedance spectroscopy (EIS) and potentiostatic intermittent titration technique (PITT) are used to estimate the chemical diffusion coefficients of Li+ in solid electrodes. Figure S9 (Supporting Information) shows a representative Nyquists plot of Ni−Co−Mn oxide QSHS. All the Nyquist plots contain a depressed semicircle in the high- to medium-frequency region and a linear Warburg tail in a low-frequency region. The intercept of the plot with the real axis at high frequency corresponds to the bulk resistance of the electrolyte, whereas the semicircle reflects the resistance of the migration through the SEI layer.64 The Warburg tail usually represents the ion diffusion resistance in the electrode materials and structure.65 When the cycle number increases from first to 250th, the charge transfer impedance slightly increased, which means the electrode experienced a larger interface impedance and capacity fading during the cycle life testing. Moreover, the lithium-ion diffusion coefficient was obtained from PITT analysis. The plot of ln DLi+ as a function of lithiation level during the discharge and charge process is shown in Figure 6D. It can be clearly seen that the QS-HS electrode possesses constantly the highest DLi+ during the overall discharge and charge process compared to that of TS-HS, DS-HS, and the rigid sphere. Specifically, a significantly lower DLi+ can be observed during the lithiation and delithiation, which are assigned to the material phase transition.66 The diffusion coefficients in charge and discharge processes are in good correspondence, which indicates the good reversibility of these materials. The high DLi+ of QS-HS is the benefit from their multishelled hollow structure and high porosity on each shell, which offer favorable electrolyte wetting and abundant Li+ pathways to enhance the Li+ diffusion rate. The excellent cycling stability can be observed in Figure 6E, where the electrode was galvanostatically cycled at a current density of 0.2 A g−1. Comparatively, the cycling capacity increases along with the number of shell. The QS-HS delivers the best cycling performance with the highest capacity retention of 1097 mAh g−1 after 250 cycles, which is superior to those in recently 11527
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of 1 °C min−1 from room temperature to 600 °C. Nitrogen sorption (ASAP 2020 micromeritics) was used to retrieve data, which were analyzed using BET theory to calculate and, in return, characterize the surface area. The SEM images were collected with a FEI Quanta Feg 250 SEM. The TEM images were collected using Phillips CM 12 TEM, JEOL 2010F TEM, and FEI Titan 80-300 LB TEM instruments. X-ray diffraction (MiniFlex 600 Rigaku) experiments were performed to determine the crystal structure. XPS was performed using a K-Alpha XPS spectrometer. The tomographic reconstruction and the analysis were performed using a software package known as Mantis.67 Three-dimensional reconstruction data were further processed by Avizo 9.0.1 for visualization. Electrode Fabrication. The active material powders, conductive agent (Super P), and binding agent (poly(vinylidene fluoride) were grinded and homogenized in N-methyl-2-pyrrolidone in a 6:3:1 ratio by ultrasonication and vigorous stirring overnight. Cu foil substrates (MTI Corporation) were used as the current collector and coated with the homogeneous slurries, dried at 80 °C, and cut into small pieces with certain dimensions as electrodes. The mass loading was controlled to be 1−2 mg cm−2. Electrochemical Characterization. The lithium-ion battery tests were performed as follows: Coin-type battery cells were assembled in a glovebox (Labstar MB10 compact, mBraun) with water and oxygen levels below 0.5 ppm. Next, 1.0 M LiPF6 solution in 1:1 (v/v) mixture of dimethyl carbonate and ethylene carbonate was used as electrolyte. The counter and reference electrode used was a lithium metal chip, whereas Celgard 2500 was used as the separator. The cells were tested using a Land CT2001A battery tester in the voltage range of 0.01 to 3 V vs Li/Li+. The EIS were measured in the frequency range from 1000000 to 0.01 Hz, with an amplitude of 10 mV. The PITT method was used to calculate lithium-ion diffusion coefficient based on a one-dimensional finite-space diffusion model. The lithium-ion diffusion coefficient (DLi+) can be calculated based on eq 1.68
The excellent rate capability and cycle stability of QS-HS are attributed to the structure integrity, shortened distances for mass/charge transfer, and hollow structure to accommodate volume variation. From the TEM images in Figure S11 (Supporting Information), the thicknesses of each shell are observed to be ∼100 nm before lithiation and ∼220 nm after lithiation, indicating good structure integrity after lithiation. Moreover, after cycle life testing, the hollow sphere maintained its size and shape without major structure damage, revealing good resistance to volume expansion (Figure S12A, Supporting Information). EDX element mapping result (Figure S12B−D, Supporting Information) also shows that Ni, Co, and Mn are uniformly distributed in each shell with a well-reserved multishelled hollow structure.63 Also, TS-HS and DS-HS TMO particles basically maintain their size and morphology after cycling, as shown in Figure S14 (Supporting Information), indicating their good structure stability during the cycling. The large voids between the shells are helpful for the alleviation of the mechanical strain and accommodate volume effect during the lithiation/delithiation process. However, the rigid sphere undergoes large morphology change and partially pulverizes after cycling, indicating its structure instability, which is consistent with the fast capacity fading during cycle life testing. Overall, due to these favorable features, the QS-HS electrode achieves outstanding rate capability, high cycle stability, and excellent structure integrity for long-life lithium storage.
CONCLUSIONS In summary, we developed a universal and facile strategy to design hierachical TMO porous hollow microspheres with controlable shell number and chemical composition. The high porosity and multishell architecture favors Li+ transfer and volumetric accommodation. These features contribute the great rate capability up to 5 A g−1 and high cyclability over 250 cycles, presenting good potential in future lithium storage application. This method offers an appealing recipe for rational design of multishelled TMOs with tailored composition and number of shells to meet specific requirements of potential applications such as LIBs, supercapacitors, and electrochemical catalysis. This method also provided a facile and eclectic way to prepare nanosize TMO material for high energy density lithium-ion batteries.
DLi + = −
d ln l 4L2 × 2 dt π
(1)
The ln I−t plots can be plotted, and the thickness of the electrode can be measured from SEM cross-sectional images (Figure S13 Supporting Information). DLi+ can be calculated with eq 1, where DLi+ is the lithium-ion diffusion coefficient, I represents the step current, t refers to the step time, and L reflects the diffusion distance, which can be approximated to the thickness of the anode. PITT is obtained from Ni−Co−Mn oxide hollow microspheres with a different number of shells in the second cycle after being charged to 2.5 V at 0.2 A g−1, at a step potential difference of 100 mV to the anode material, and then a chronoamperometric curve can be obtained. One step test lasted for 1 h, corresponding to the second discharge and charge processes.
METHODS Synthesis of Ni−Co−Mn Oxide QS-HS. All reagents were of analytical grade and were purchased from Sigma-Aldrich without any purification. In a typical preparation process, 0.056 mol glucose, 0.02 mol Ni(CH3COO)2·6H2O, 0.03 mol Co(CH3COO)2·6H2O, and 0.03 mol Mn(CH3COO)2·6H2O were dissolved into 100 mL of distilled water under magnetic stirring. Subsequently, the obtained solution was transferred into a 200 mL Teflon container, sealed in an autoclave, and hydrothermally reacted at 180 °C for 400 min. The black precipitate was collected, washed, and centrifuged with distilled water and anhydrous ethanol three times, and the obtained powders were dried at 70 °C for 12 h and collected as the precursor. Finally, the final Ni− Co−Mn oxide multishelled hollow microspheres were obtained by annealing MAPs at 500 °C for 1 h in air, with a ramping rate of 1 °C min−1. Synthesis of Co3O4 and NiCo2O4 QS-HS. The synthesis procedures of Co3O4 and NiCo2O4 QS-HS are similar to that of Ni−Co−Mn oxide QS-HS except for the replacement of cobalt acetate as reactant for Co3O4 and nickel acetate and cobalt acetate as reactants for NiCo2O4. Material Characterizations. TGA (TA Instruments Q500) was conducted under air atmosphere. The protocol entailed a heating rate
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06296. Video S1 (AVI) Additional TEM, SEM, XRD, FTIR, Raman, TGA, XPS, and electrochemical characterizations, and additional tables (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Xiaolei Wang: 0000-0002-7751-4849 Aiping Yu: 0000-0002-7422-7537 11528
DOI: 10.1021/acsnano.7b06296 ACS Nano 2017, 11, 11521−11530
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ACS Nano
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Zhongwei Chen: 0000-0003-3463-5509 Author Contributions ⊥
D.L. and Y.-P.-D. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This research was financially supported by the National Science and Engineering Research Council of Canada (NSERC) and the University of Waterloo. The authors would like to thank Dr. Carmen Andei, Dr. Andreas Korinek, and Dr. Gianluigi Botton from the Canadian Centre for Electron Microscopy at McMaster University for their great help with TEM characterizations. REFERENCES (1) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A battery of Choices. Science 2011, 334, 928−935. (2) Wang, X.; Li, G.; Seo, M. H.; Lui, G.; Hassan, F. M.; Feng, K.; Xiao, X.; Chen, Z. Carbon-Coated Silicon Nanowires on Carbon Fabric as Self-Supported Electrodes for Flexible Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9, 9551−9558. (3) Hassan, F. M.; Batmaz, R.; Li, J.; Wang, X.; Xiao, X.; Yu, A.; Chen, Z. Evidence of Covalent Synergy in Silicon-Sulfur-Graphene Yielding Highly Efficient and Long-Life Lithium-Ion Batteries. Nat. Commun. 2015, 6, 8597. (4) Wang, X.; Li, G.; Li, J.; Zhang, Y.; Wook, A.; Yu, A.; Chen, Z. Structural and Chemical Synergistic Encapsulation of Polysulfides Enables Ultralong-Life Lithium-Sulfur Batteries. Energy Environ. Sci. 2016, 9, 2533−2538. (5) Wang, X.; Li, G.; Seo, M. H.; Hassan, F. M.; Hoque, M. A.; Chen, Z. Sulfur Atoms Bridging Few-Layered MoS2 with S-Doped Graphene Enable Highly Robust Anode for Lithium-Ion Batteries. Adv. Energy Mater. 2015, 5, 1501106. (6) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Tin Based Amorphous Oxide: A high-Capacity Lithium-Ion-Storage Material. Science 1997, 276, 1395−1397. (7) Zhao, Y.; Li, X.; Yan, B.; Xiong, D.; Li, D.; Lawes, S.; Sun, X. Recent Developments and Understanding of Novel Mixed TransitionMetal Oxides as Anodes in Lithium Ion Batteries. Adv. Energy Mater. 2016, 6, 1502175. (8) Yang, X.; Shi, K.; Zhitomirsky, I.; Cranston, E. D. Cellulose Nanocrystal Aerogels as Universal 3D Lightweight Substrates for Supercapacitor Materials. Adv. Mater. 2015, 27, 6104−6109. (9) Ren, Y.; Ma, Z.; Bruce, P. G. Ordered Mesoporous Metal Oxides: Synthesis and Applications. Chem. Soc. Rev. 2012, 41, 4909−4927. (10) Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J. Rapid Room-Temperature Synthesis of Nanocrystalline Spinels as Oxygen Reduction and Evolution Electrocatalysts. Nat. Chem. 2011, 3, 79−84. (11) Yuan, C.; Wu, H. B.; Xie, Y.; Lou, X. W. Mixed Transition-Metal Oxides: Design, Synthesis, and Energy-Related Applications. Angew. Chem., Int. Ed. 2014, 53, 1488−1504. (12) Pendashteh, A.; Palma, J.; Anderson, M.; Marcilla, R. Ni-Co-Mn Oxide Nanoparticles on N-Doped Graphene: Highly Efficient Bifunctional Electrocatalyst for Oxygen Reduction/Evolution Reactions. Appl. Catal., B 2017, 201, 241−252. (13) Li, N.; Patrissi, C. J.; Che, G.; Martin, C. R. Rate Capabilities of Nanostructured LiMn2O4 Electrodes in Aqueous Electrolyte. J. Electrochem. Soc. 2000, 147, 2044−2049. (14) Zhang, G.; Lou, X. W. General Synthesis of Multi-Shelled Mixed Metal Oxide Hollow Spheres with Superior Lithium Storage Properties. Angew. Chem., Int. Ed. 2014, 53, 9041−9044. (15) Jiang, J.; Li, Y.; Liu, J.; Huang, X.; Yuan, C.; Lou, X. W. Recent Advances in Metal Oxide-Based Electrode Architecture Design for Electrochemical Energy Storage. Adv. Mater. 2012, 24, 5166−5180. (16) Wang, G. H.; Hilgert, J.; Richter, F. H.; Wang, F.; Bongard, H. J.; Spliethoff, B.; Weidenthaler, C.; Schuth, F. Platinum-Cobalt 11529
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DOI: 10.1021/acsnano.7b06296 ACS Nano 2017, 11, 11521−11530
