Tuning Shell Numbers of Transition Metal Oxide Hollow Microspheres

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Tuning Shell Numbers of Transition Metal Oxide Hollow Microspheres towards 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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06296 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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Tuning Shell Numbers of Transition Metal Oxide Hollow Microspheres towards Durable and Superior Lithium Storage

Dan Luo a ‡, Ya-Ping Deng a ‡, Xiaolei Wang b, Gaoran Li a, Juan Wu c, Jing Fu a, Wen Lei a

, Ruilin Liang a, Yangshuai Liu a, Yuanli Ding a, Aiping Yu a and Zhongwei Chen a*

a. Department of Chemical Engineering, University of Waterloo 200 University Ave West, Waterloo, Ontario, N2L 3G1, Canada

b. Department of Chemical and Materials Engineering, Concordia University 1455 De Maisonneuve Blvd. West, Montreal, Quebec, H3G 1M8, Canada

c. Department of Material Science and Engineering, McMaster University 1280 Main Street West, Hamilton, Ontario, L8S 4L8, Canada

Abstract Multi-shelled 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 multi-shelled porous hollow microspheres, which is constituted of nano-size primary TMOs particles, using metal acetate polysaccharides microspheres as precursor. This universal method can be applied to design TMOs hollow spheres with tunable shell numbers and composition. The hierarchical porous quadruple-

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shelled hollow microspheres with nano-sized Ni-Co-Mn oxide demonstrate increased number of active sites, boosted rate capability, enhanced volumetric energy density and great tolerance toward volume expansion upon cycling, thus exhibit 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 TMOs anode materials for lithium ion batteries. Keywords: transition metal oxide, hollow sphere, shell-controlled, micro-nano structure, lithium ion battery

The realization of high energy density storage material is one of the greatest scientific and engineering challenges in the twenty-first century.1-3 Energy storage devices are required to be able to store large quantities of electrical energy in 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 storages system 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 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 it extremely compelling as electrode


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materials for lithium 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 instinct 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 porous and hollow structure is capable of facilitating ion access to the electrode matrix by offering sufficient ion transport pathways.15 Comparing to single-shelled hollow sphere, multi-shelled hollow structure offers additional benefits such as high volumetric energy density, shell permeability and reduced aggregation of nanosized subunits.16-18 Besides, the high porosity and large surface area of multi-shells sphere facilitates electrolyte infiltration and reduces Li-ion diffusion distance, resulting in improved active material utilization and fast electrochemical kinetics.19,

20

Moreover,

multi-shelled hollow spheres with more shells offers freedom to manipulate the materials properties through the design and construction of multi-components with complex architectures to achieve enhanced properties.21-24 Therefore, a synergistic combination of well-designed composition and structure of TMOs hollow sphere is promising to realize greatly improved lithium storage performance.25 Currently, few literatures have reported the synthesis of TMOs multi-shelled 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 multi-components and tunable number of shell remain to be a great challenge.29


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Herein, we present a straightforward strategy for the synthesis of multi-shelled TMOs microspheres with tunable number of shells, which involves one-pot synthesis of metal acetate polysaccharides (MAPs) and subsequent thermal treatment. Multiple coordinate bonds between polysaccharides and transition metal ions allow them to crosslink with each other to construct a metal-polysaccharides network. Porous multi-shelled hollow micro-size structure with nano-sized 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-CoMn

oxide quadruple-shelled

hollow

microspheres

(QS-HS)

offers

abundant

electrode/electrolyte interfaces, which minimizes lithium ion diffusion path and provides numerous active sites for Li+ conversion reaction. Furthermore, the quadruple-shelled structure with empty interiors is flexible toward volume changes and is capable of diverting strain and stress during charge-discharge process. 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/discussion The conventional syntheses of multi-shelled TMOs are complicated and time consuming which usually require process such as template synthesis, ion-adsorption and template


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removal.33-36 In comparison, we developed a convenient route to prepare TMOs hollow microspheres with tunable number of shells and composition by using metal acetate polysaccharides as self-template. The synthesis method of TMOs QS-HS is well illustrated in Scheme 1. QS-HS with uniform gap between adjacent thin shells are synthesized by hydrothermal method followed with 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 frameworks will further promote the growth of microspheres by acting as cross-linking agent and forming coordination bonds with free flowing 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 MAPs spheres. Comparing to pure CS directly synthesized without metal acetates by hydrothermal method (Figure S1B, Supporting Information), the diameter of MAPs spheres dramatically enlarged from ~300 nm to ~6 µm. Transmission electron microscopy (TEM) image (Figure S2A, Supporting Information) confirmed the formation of solid MAPs sphere without hollow interior, indicating that the huge volume expansion is resulted from the absorbed metal ions and their coordination with glucose. From Energy Disperse X-ray (EDX) line scanning of MAPs (Figure S3, Supporting Information), it can be observed that Ni, Co and Mn elements are uniformly distributed within the carbon sphere. Comparing to metal ion CS prepared by ion adsorption method


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reported in other literature25 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+ in comparison to Mn2+, more Ni2+ and Co2+ ions are hydrolyzed and 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, Fourier Transform Infrared (FTIR) spectrum of MAPs shows characteristic adsorption peaks at 1695 cm-1 and 1576 cm-1, contributed by COO- and C=C group, respectively (Figure S4A, Supporting Information). Comparing 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 surface were transformed into a metal oxide shell which was induced by the large temperature gradient along the radical 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 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° 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 and B, Supporting Information) show the separation of the microspheres and the formation of the porous


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surface. The X-ray diffraction (XRD) peak indicated the formation of nano-sized Ni-CoMn oxide (Figure S6C, Supporting Information). Furthermore, the inner carbon sphere decomposes with increasing temperature, which is accelerated by as formed TMOs catalytic combustion process,45 and result in a drastic weight loss of 38.8% as shown in TGA. At around 400°C, the morphology of the outer shell did not change while inner shell has formed, as confirmed by SEM and STEM image (Figure S6D and E, Supporting Information). The increase of 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 C=C and COO- are observed from FTIR spectra while 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, 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 Optical Emission Spectrometry (ICP) in Table S1 (Supporting Information). SEM images of QS-HS in different resolutions are shown in Figure 1A, B and Fig S15 (Supporting Information). The QS-HS was 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 HRTEM (Figure 1D) with an interplanar spacing of


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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 sphere at different depths. The detail 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 micro pores. 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 and F) indicates the uniform distribution of each element in every shell of Ni-Co-Mn oxide hollow microspheres. Figure 3A shows XRD result of Ni-Co-Mn oxide QS-HS. The diffraction peaks can be ascribed to the cubic spinel phase which has the Fd3m space group in consistent with previous reports.49 Besides, 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 micro pores built into the microstructures are beneficial to sufficient electrochemical redox and fast ion diffusion. X-ray 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 eV and


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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 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 sub peaks of Mn2+ are located at 642.2 and 654.0 eV and the peaks of Mn3+ can be clearly seen 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 Ni-Co-Mn oxide QS-HS sample.57 To demonstrate the generality of our strategy, we successfully prepared other quadrupleshelled 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, respectively. Moreover, this general method is also capable of producing Ni-Co-Mn oxide hollow microspheres with different number of shell by only varying the concentration of metal acetate. The SEM and TEM images in Figure 5 demonstrate the successful synthesis of double-shelled and tripleshelled Ni-Co-Mn oxide hollow microspheres by the same method (DS-HS and TS-HS). These evidences indicate that this universal and facile method can also be used to creatively control component and number of shells to design multi-shelled TMOs hollow microspheres. Notably, with an even higher concentration of metal source, quintupleshelled 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


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Ternary metal oxides have been regarded as very promising electrode materials for high-performance energy storage devices. Through investigating the influence of Li+ storage with different number of shells, the Ni-Co-Mn oxide QS-HS delievers outstanding capacity and posseses superiror Li+ storage properties comparing to 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 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 stabilized at a capacity of around 1400 mAh g-1 during the subsequent cycles. The irreversible capacity loss in second discharge cycle is attributed to electrolyte decomposition and solid electrolyte interface (SEI) formation.60 Figure 6B presents the typical cyclic voltammetry (CV) curves of QSHS, which shows good consistency with its voltage profile. For the initial cathodic scan, two sharp peaks can be observed at 0.83 V and 0.59 V, which correspond to the reduction of high valance Ni, Co and Mn, the formation of SEI layer and Li2O.61 Two broad anodic peaks can be observed at 1.50 V and 2.18 V, corresponding to the reformation of high valence TMOs. During the subsequent scans, 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


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kinetics and robust electrode intergity. The superior rate performance is mainly originated from the micro pores on surface of the thin shell which facilitate 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 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 representative Nyquists plot of Ni-Co-Mn oxide QSHS. All the Nyquist plots contain a depressed semicircle in the high to medium frenquency region and a linear Warburg tail in low frequency region. The intercept of the plot with the real axis at high frequency corresponds to the bulk resistance of the electrolyte, while the semi-circle 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 1st to 250th, the charge transfer impedance slightly increased, which means the electrode experienced a larger interface impdeance and capacity fading during the cylce 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 comparing to TS-HS, DS-HS and Rigid sphere. Specifically, a significantly lower DLi+ can be observed during the lithiation and


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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 multi-shelled hollow structure and high porosity on each shell which offer favorable electrolyte wetting and abundant Li+ pathways for enhance 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 published literatures based on TMOs electrode (Table S2, Supporting Information). As shown in Figure 6D and 6E, the first cycle coulombic efficiency of QS-HS, TS-HS, DS-HS and 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 number of shells increase. This is because the high surface area and porous structure of multi-shelled hollow sphere reduce the ion and electron transport pathway, which facilitates 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 comparing to 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 QS-HS. The successful synthesis of these multi-shelled hollow microspheres is also highly promising for practical use of metal oxide anode material. By designing a multi-shelled


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microsphere with nano-size subunit, this material efficiency utilizes hollow interior, achieves

excellent capacity and structural

stability,

and minimizes particles

agglomeration and safety/environmental hazards associated with nanomaterial. Table S3 (Supporting Information) compares the tap density of multi-shelled 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 NiCo-Mn oxide DS-HS, TS-HS and QS-HS at different discharge current density. Although the gravimetric energy density only increases slightly when the number of shells is raised, higher shell numbers materials forms a relatively dense and stable structure, maximizes volume utilization and material compactness, and thus achieved 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 comparing to 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. 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 thickness 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 sustained its size and shape without major


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structure damage, revealing good resistance to volume expansion (Figure S12A, Supporting Information). EDX element mapping result (Figure S12B, C and D, Supporting Information) also shows that Ni, Co and Mn are uniformly distributed in each shell with a well-reserved multi-shelled hollow structure.63 Also, TS-HS and DS-HS TMOs 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, accommodate volume effect during the lithiation/delithiation process. However, rigid sphere undergoes large morphology change and partly pulverizes after cycling, indicating its structure instability, which is consist with the fast capacity fading during cycle life testing. Overall, due to these favorable features, 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 TMOs porous hollow microspheres with controlable shell number and chemical composition. The high porosity and multi-shell 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 multi-shelled TMOs with tailored composition and number of shells to meet specific requirements of potential applications such as LIBs, supercapacitors, and electrochemical catalysis. This


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method also provided a facile and eclectic way to prepare nano-size TMOs material for high energy density lithium ion batteries.

Methods/Experimental Synthesis of Ni-Co-Mn oxide QS-HS: All reagents were analytical graded 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 distilled water under magnetic stirring, respectively. Subsequently, the obtained solution was transferred into a 200ml Teflon container, sealed in autoclave and hydrothermal reacted at 180 degree for 400 min. The black precipitation was collected, washed and centrifuged with distilled water and anhydrous ethanol for 3 times, the obtained powder were dried at 70 °C for 12 h and collected as precursor. Finally, the final product Ni-Co-Mn oxide multi-shelled hollow microspheres were obtained by annealing MAPs at 500 degree for 1h 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 is similar to that of Ni-Co-Mn oxide QS-HS except for the replacement of cobalt acetate as reactant for Co3O4 and nickel acetate, cobalt acetate as reactants for NiCo2O4.


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Materials Characterizations: TGA (TA instrument Q500) was conducted under air atmosphere. The protocol entailed a heating rate of 1°C min-1 from room temperature to 600 °C. Nitrogen sorption (ASAP 2020 micromeritics) was used to retrieve data which was analyzed using Brunauer– Emmett–Teller theory to calculate and in return, characterize the surface area. The SEM images was 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. 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 3D reconstruction data is 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, (PVDF)) were grinded and homogenized in N-Methyl-2pyrrolidone (NMP) in a 6:3:1 ratio by ultrasonication and vigorous stirring overnight. Cu foil substrates (MTI Corporation) were used as the current collect and coated with the homogenous slurries, dried at 80 degree and cut into small pieces with certain dimensions as electrodes. The mass loading was controlled to be 1~2 mg cm-2.


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Electrochemical characterization: The lithium ion battery tests were performed as follows: Coin-type battery cells were assembled in glovebox (Labstar MB10 compact, mBraun) with water and oxygen levels below 0.5 part per million. 1.0 M LiPF6 solution in 1:1 (V:V) mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) was used as electrolyte. The counter and reference electrode used was a lithium metal chip while Celgard 2500 was used as the separator. The cell were tested using a Land CT2001A battery tester in the voltage range of 0.01V - 3V vs. Li/Li+. The EIS were measured in the frequency range from 1000000 Hz to 0.01 Hz, with an amplitude of 10 mV. The PITT method were 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 Equation 1.68

= −

∙

(1)

The ln I − t plots can be plotted and the thickness of electrode can be measured from SEM cross-section images (Figure S13 Supporting Information). DLi+ can be calculated through Equation 1 where DLi+ is the lithium ion diffusion coefficient, I represents the step current, t refers the step time, and L reflects the diffusion distance, which can be approximated to the thickness of anode. PITT is obtained from Ni-Co-Mn oxide hollow microspheres with different numbr of shells in the 2nd cycle after being charged to 2.5V at 0.2 A g-1, at a step potential difference of 100 mV to the anode material and then


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chronoamperometric curve can be obtained. One step test lasted for 1h, corresponding to the second discharge and charge processes, respectively.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publication Website. Additional TEM, SEM, XRD, FTIR, Raman, TGA, XPS and electrochemical characterizations, and additional tables (PDF)

Author Information Corresponding Authors *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally to this work.

Acknowledgements This research was financially supported by the National Science and Engineering Research Council of Canada (NSERC) and the University of Waterloo. The author would


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like to thank Dr. Carmen Andei, Dr. Andreas Korinek and Dr. Gianluigi Botton from Canadian Centre for Electron Microscopy at McMaster University for their great help with TEM characterizations.

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Scheme 1. Formation process of TMOs quadruple-shelled hollow sphere (QS-HS).


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Figure 1. SEM images of (A) single Ni-Co-Mn oxide QS-HS and (B) multiple of Ni-CoMn 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: Fast Fourier Transform image of NiCo-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, Mn.


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Figure 2. HAADF – STEM 3D reconstruction of QS-HS slicing at different lengths.


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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: pore size distribution of NiCo-Mn oxide QS-HS). XPS spectra of the (C) Ni 2p, (D) Co 2p, (E) Mn 2p regions for Ni-Co-Mn oxide QS-HS.


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Figure 4. (A, B, C) SEM image, HAADF-STEM image and XRD pattern of Co3O4 QSHS and (D, E, F) NiCo2O4 QS-HS.


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Figure 5. (A, B) SEM and TEM morphology of Ni-Co-Mn oxide rigid microspheres (C,D) double-shelled hollow microspheres (DS-HS), (E, F) triple-shelled hollow microspheres (TS-HS) and (G, H) quintuple-shelled hollow microspheres (FIS-HS).


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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 0.01V to 3V at a scan rate of 0.1 mV s1 . (C) Rate performance of Ni-Co-Mn oxide QS-HS, TS-HS, DS-HS and rigid sphere. (D) The 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.


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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.)


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Tuning Shell Numbers of Transition Metal Oxide Hollow Microspheres

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