Building Honeycomb-Like Hollow Microsphere Architecture in a

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Building Honeycomb-Like Hollow Microsphere Architecture in a Bubble Template Reaction for High-Performance Lithium-Rich Layered Oxide Cathode Materials Zhaoyong Chen,* Xiaoyan Yan, Ming Xu, Kaifeng Cao, Huali Zhu, Lingjun Li, and Junfei Duan College of Materials Science and Engineering, Changsha University of Science and Technology, Changsha 410004, P. R. China S Supporting Information *

ABSTRACT: In the family of high-performance cathode materials for lithium-ion batteries, lithium-rich layered oxides come out in front because of a high reversible capacity exceeding 250 mAh g−1. However, the long-term energy retention and high energy densities for lithium-rich layered oxide cathode materials require a stable structure with large surface areas. Here we propose a “bubble template” reaction to build “honeycomb-like” hollow microsphere architecture for a Li1.2Mn0.52Ni0.2Co0.08O2 cathode material. Our material is designed with ca. 8-μm-sized secondary particles with hollow and highly exposed porous structures that promise a large flexible volume to achieve superior structure stability and high rate capability. Our preliminary electrochemical experiments show a high capacity of 287 mAh g−1 at 0.1 C and a capacity retention of 96% after 100 cycles at 1.0 C. Furthermore, the rate capability is superior without any other modifications, reaching 197 mAh g−1 at 3.0 C with a capacity retention of 94% after 100 cycles. This approach may shed light on a new material engineering for high-performance cathode materials. KEYWORDS: lithium-rich layered oxide, bubble-bath reaction, hollow microspheres, cathode materials, lithium-ion batteries Many approaches, such as surface coating (AlF3,24,25 Al2O3,26,27 graphene,28 etc.29−33), doping (aluminum,34,35 titanium,36,37 magnesium,38,39 etc.40−43), and heterostructuring,44,45 have been developed and yielded noticeable improvements. Nevertheless, the LRLO cathode materials have still faced many fundamental challenges and require long-term energy retention and high energy densities.46,47 From the structural point of view, Li2MnO3 phases in the LRLO cathode materials require an activation process during electrochemical cycling,19,20,22 after which it delivers most of the capacities. In view of this, the particle size plays a crucial role in activation of the Li2MnO3 phase. Many efforts have been devoted to synthesizing nanosized LRLO cathode materials to achieve higher capacity with a fully activated Li2MnO3 phase in a short time.48−51 Note that deterioration of the LRLO cathode materials is ascribed to two major mechanisms: (1) side

1. INTRODUCTION The flourishing development of lithium-ion batteries (LIBs) has accelerated the manufacturing of potable electric devices, electric vehicles (EVs), and hybrid electric vehicles (HEVs).1−3 However, the unsatisfied performance of cathode materials in LIBs, such as low energy density, etc.,4−6 limits its extensive application in EVs and HEVs in the future; for example, the energy density of current LIBs with LiCoO2,2,6,7 LiMn2O4,8,9 LiFePO4,10−12 and LiMO2 (M = Ni, Co, Mn)13−15 cathode materials are all below 300 Wh kg−1. In this context, scientists have switched their attention to high-energy and low-cost cathode materials.16,17 Lithium-rich layered oxide (LRLO) cathode materials are receiving international attention because they can deliver an exceptionally high reversible capacity of 250 mAh g−1 between 2.0 and 4.8 V.18−22 Besides, the inexpensive manganese element serves as a substitute for the costly nickel and cobalt elements, which can reduce production costs. In the past decades, numerous studies have focused on stabilizing the structure of LRLO cathode materials to overcome their intrinsically poor cycling stability and rate capabilities.21,23 © 2017 American Chemical Society

Received: May 27, 2017 Accepted: August 22, 2017 Published: August 22, 2017 30617

DOI: 10.1021/acsami.7b07542 ACS Appl. Mater. Interfaces 2017, 9, 30617−30625

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ACS Applied Materials & Interfaces

the organic impurities, and then dried inside a vacuum oven at 120 °C for 12 h. Control experiments based on different heating temperatures (140, 160, and 180 °C), different heating rates (5 and 7 °C min−1), and HMT amounts (1.42 and 2.83 g) were also performed (the corresponding results can be seen in the Supporting Information). The as-prepared precursors were treated at 450 °C for 5 h and mixed with LiOH (99%, Sinopharm Chemical Reagent), subsequently calcining at 800 °C for 12 h in air to obtain the “honeycomb-like” Li1.2Mn0.52Ni0.2Co0.08O2 hollow microspheres. 2.2. Additional Experiments. The Li1.2Mn0.52Ni0.2Co0.08O2 nonmicrosphere cathode material was synthesized by a sol−gel method. Stoichiometric amounts of C2H3O2Li·4H2O (99%, Sinopharm Chemical Reagent), C4H6NiO4·4H2O (99%, Sinopharm Chemical Reagent), C4H6CoO4·4H2O (99%, Sinopharm Chemical Reagent), and C4H6MnO4·4H2O (99%, Sinopharm Chemical Reagent) were dissolved in a hybrid solvent (ethanol and deionized water in a volume ratio of 1:4) and mixed with an aqueous solution of 0.5 mol L−1 chelating agent at a ratio of the transition-metal (TM) ions (Ni2+, Co2+, and Mn2+) to chelating agent = 1.5. The resulting solution was stirred at 80 °C for 4 h to obtain a clear viscous gel and then dried in an oven at 120 °C for 12 h to obtain a solid mixture. The dried mixture was precalcined at 450 °C for 5 h in air and cooled to room temperature natually. The final product was formed by subsequent heating of the precursor at 800 °C for 12 h in air. For comparison, the Li1.2Mn0.52Ni0.2Co0.08O2 microsphere cathode material was also synthesized by a coprecipitation method. Stoichiometric amounts of C4H6NiO4·4H2O (99%, Sinopharm Chemical Reagent), C4H6CoO4·4H2O (99%, Sinopharm Chemical Reagent), and C4 H6 MnO4 ·4H2O (99%, Sinopharm Chemical Reagent) were dissolved in deionized water and stirred for 30 min to obtain a homogeneous solution. At the same time, a solution of 1.65 mol L−1 Na2CO3 and the desired amount of NH4OH solution were separately added to the mixed solution and kept stirring at 55 °C for 4 h. The pH value was carefully controlled at 7.5 during the reaction. The obtained carbonate precursor was then filtered, washed, and dried at 120 °C. Finally, the as-prepared precursor was treated at 450 °C for 5 h and mixed with LiOH (99%, Sinopharm Chemical Reagent), subsequently calcining at 800 °C for 12 h in air to obtain the Li1.2Mn0.52Ni0.2Co0.08O2 microspheres. 2.3. Characterizations. The X-ray diffraction (XRD) patterns of all samples were collected with Cu Kα radiation at 30 kV and 30 mA in the 2θ range from 10° to 90°. The morphology was characterized by field-emission scanning electron microscopy (SEM; FEI QUANTA 250), equipped with energy-dispersive X-ray analysis. The elemental compositions were characterized using the energy-dispersive spectroscopy (EDS; Oxford INCA, Oxford instruments). The sol−gel chemistry of the cathode materials was investigated by X-ray photoelectron spectroscopy (XPS; VG Multilab 2000). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were carried out on a JEOL JEM-2100 instrument. The Barrett− Joyner−Halenda (BJH) pore-size distribution was characterized by N2 adsorption/desorption isotherms (Quadrasorb S1-3MP). 2.4. Electrochemical Measurements. The electrochemical performance of the materials was tested using a coin-type cell (2025). The as-prepared cathode materials were mixed with Super P conductive carbon black and poly(vinylidene difluoride) in N-methyl2-pyrrolidone with a weight ratio of 85:10:5 and coated on aluminum foil. The 2025 coin-type cells were assembled with lithium metals as anodes in a glovebox filled with argon. The electrolyte was 1 M LiPF6 dissolved in ethyl carbonate and dimethyl carbonate (1:1 in volume), and the separator was a Celgard 2400 membrane. The electrodes were punched into disks with ø = 1.0 cm and thickness of 0.025 mm. and the active material loading was ca. 3.0 mg·cm−2. Galvanostatic charge and discharge tests were carried out on a LAND system (Wuhan, China) in a cutoff voltage range within 2.0−4.8 V versus Li/Li+ for cathode materials. The electrochemical impedance spectroscopy (EIS) of the cells was conducted using the PARSTAT 2273 electrochemical measurement system in a frequency range from 1 mHz to 100 kHz.

reactions with the electrolytes cause electrolyte exhaustion and hydrogen fluoride attack;24−27 (2) phase transitions from a layered to a spinel-like phase, which propagate from the surface into the bulk of the materials, decrease the working voltage and capacity retention, severely lowering the energy density in the electrodes.27,47,48,52 Accordingly, the nanosized LRLO cathode materials with large surface areas cause lower volumetric energy densities and degrade long-term cycling performance. For this reason, instead of simple modifications such as surface coating or doping, building a large secondary hollow microsphere architecture consisting of small primary particles for LRLO cathode materials is essential to overcoming these limitations. One possible way to improve the structure stability of LRLO cathode materials and to alleviate the above-mentioned obstacles would be to build a hollow microsphere architecture with highly exposed pores on its surface, like a “honeycomb”. The design of this “honeycomb” of microspheres for the LRLO cathode materials starts with several advantages: (1) enhanced specific capacities, which originate from full activation of the Li2MnO3 phase in small primary particles,19,20,50,53 (2) an improved rate capability,54−56 which results from the large surface area of the porous structure, and (3) an enhanced structural stability due to the void space within each microsphere, which can buffer the local volume changes upon repeated lithium-ion insertion/extraction processes.57 Considering the particular advantages mentioned above, here we attempted to use hexamethylenetetramine as a bubble maker to produce size-controlled Ni0.25Co0.1Mn0.65CO3 microspheres in a “bubble-bath” reaction. Furthermore, this “bubble-bath” reaction (rather than alkaline coprecipitation and sol−gel methods) avoids introducing metal ion (Na+, K+, etc.) impurities into the final cathode materials. Through our green and simple “bubble-bath” reaction, Li1.2Mn0.52Ni0.2Co0.08O2 microspheres with highly exposed porous and hollow (“honeycomb-like”) architecture were obtained by further calcination at appropriate temperatures. Upon use as cathode materials in LIBs, the as-synthesized “honeycomb-like” Li1.2Mn0.52Ni0.2Co0.08O2 microspheres demonstrated a considerably improved electrochemical performance and cycling stability compared with those of coprecipitation-prepared microspheres and sol−gel-synthesized nonmicrosphere cathode materials. Therefore, we believe that such “honeycomb-like” Li1.2Mn0.52Ni0.2Co0.08O2 microspheres potentially promotes the practical applications of LIBs, whereas the as-developed “bubble-bath” reaction pathway is being applied in the synthesis of other Ni−Co−Mn-based high-performance cathode materials.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. The size-controlled Ni0.25Co0.1Mn0.65CO3 microsphere precursors were synthesized via a “bubble-bath” reaction using hexamethylenetetramine (HMT) as a “bubble maker”. Specifically, a 2.12 g amount of HMT was dissolved in 70 mL of deionized water and stirred for 30 min to obtain a homogeneous and transparent solution. After that, 0.63 g of C4H6NiO4·4H2O (99%, Sinopharm Chemical Reagent), 0.25 g of C4H6CoO4·4H2O (99%, Sinopharm Chemical Reagent), and 1.61 g of C4H6MnO4·4H2O (99%, Sinopharm Chemical Reagent) were added in the HMT solution and stirred for 2 h under a nitrogen atmosphere. This mixed solution was then put into a Teflon-lined stainless steel autoclave of 100 mL capacity. After the autoclave was maintained at 160 °C for 24 h without shaking or stirring during the heating period, it was cooled to room temperature naturally. The precipitate was collected using a centrifugal machine, washed with distilled water and ethanol to remove 30618


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Figure 1. SEM images of Ni0.25Co0.1Mn0.65CO3 precursors (insets are the corresponding Li1.2Mn0.52Ni0.2Co0.08O2 cathode materials) obtained at (a) 140, (b) 180, and (c) 160 °C with a mole ratio of HMT:TM ions = 1.5:1, (d) HMT:TM ions = 1:1, and (e) HMT:TM ions = 2:1 at 160 °C. A schematic view of the preparation of carbonate precursor and cathode material in the “bubble-template” reaction is also provided.

3. RESULTS AND DISCUSSION 3.1. Formation Mechanism and Structural Characterization. In this work, we introduce a “bubble-bath” reaction to realize the size control in producing Ni0.25Co0.1Mn0.65CO3 microspheres so as to harvest a high-performance “honeycomb-like” Li1.2Mn0.52Ni0.2Co0.08O2 microsphere cathode material (Figure 1). The key and prerequisite to achieving the desirably designed “honeycomb-like” cathode material depend on the synthesis of size-controlled Ni 0.25Co0.1Mn0.65CO3 microsphere precursors. First, Ni0.25Co0.1Mn0.65CO3 precursors are prepared through a single-step “bubble-bath” reaction. Specifically, during this reaction, the decomposition of HMT at temperatures >130 °C released CO2 at the rate of good diffusion, producing large amounts of CO2 bubbles. Because TM ions (Ni2+, Co2+, and Mn2+) can react with CO2 to produce precipitates, acetate molecules would be attracted to the periphery of CO2 bubbles, in which acetic groups are oriented toward the outside and metal ions toward the inside. Subsequently, these instantaneously produced Ni0.25Co0.1Mn0.65CO3 precipitates around CO2 bubbles would attach to each other to form a carbonate nucleus. After several hours of heat preservation, the initially formed carbonate nucleus continuously grows into Ni0.25Co0.1Mn0.65CO3 microspheres under strong steric hindrance from acetic groups. After chemical lithiation, the final “honeycomb-like” Li1.2Mn0.52Ni0.2Co0.08O2 microsphere cathode material is obtained (see the Experimental Section for details of the synthesis). When it was seen that HMT may play a crucial role in building such a “honeycomb-like” architecture in “bubbletemplate” reaction, several control experiments were conducted to determine the effect of HMT amounts and heating

temperatures (the producing rate of CO2 bubbles) on the formation of Ni0.25Co0.1Mn0.65CO3 precursors. First, it was found that the Ni0.25Co0.1Mn0.65CO3 microspheres with rough surfaces were obtained at 140 °C (Figure 1a), indicating low decomposition rates of HMT and reaction kinetics for the formation of carbonate seeds. Meanwhile, the large hexahedron-shaped building-block-assembled Ni0.25Co0.1Mn0.65CO3 microspheres were obtained at 180 °C (Figure 1b). Second, when a small amount of HMT (containing TM ions and HMT in a mole ratio of 1:1) was added, an olive-like-shaped Ni0.25Co0.1Mn0.65CO3 particle with a smooth surface was obtained (Figure 1d). By comparison, the large primaryparticle-assembled Ni0.25Co0.1Mn0.65CO3 microspheres were obtained when TM ions and HMT in a mole ratio of 1:2 were contained (Figure 1e). Significantly, temperatures, heating rates, and HMT amounts have a strong effect on the producing rate of CO2 bubbles. It apparently changes the concentration of CO2 bubbles and thus alters the kinetics of carbonate nucleation. More details of the morphology and chemical composition for as-prepared precursors are illustrated in the Supporting Information (Figures S1−S5). Comparatively, the Ni0.25Co0.1Mn0.65CO3 microspheres obtained at 160 °C with a mole ratio of HMT:TM ions = 1.5:1 (Figure 1c) are composed of primary nanoparticles with hexahedron shape in the range of 100−200 nm. After chemical lithiation, these Li1.2Mn0.52Ni0.2Co0.08O2 microspheres present an expected “honeycomb-like” structure (inset in Figure 1c). Therefore, more detailed characterizations were performed to deeply investigate their structures and performance. Thermal decomposition of the as-prepared Ni0.25Co0.1Mn0.65CO3 precursor was characterized by thermal gravimetry (TG), as shown in Figure 2a. On the basis of the 30619


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Figure 2. (a) TG and corresponding dM/dT curves (insets are XRD patterns referring to two weight-loss regions, respectively). (b) XRD pattern of the Li1.2Mn0.52Ni0.2Co0.08O2 cathode material (the inset refers to the Ni0.25Co0.1Mn0.65CO3 precursor). (c) XPS spectra of Ni 2p, Co 2p, and Mn 2p for the cathode material. (d) N2 adsorption/desorption isotherms (the inset refers to the BJH pore-size distribution).

ordering and layered characteristics, as concluded by other researchers.22,37,38 Typical XPS spectra for Ni 2p, Co 2p, and Mn 2p of the Li1.2Mn0.52Ni0.2Co0.08O2 cathode material are shown in Figure 2c. The observed binding energies of 854.2, 780.3, and 654 eV coincide well with those of Ni2+, Co3+, and Mn4+, respectively.26,39,45,48 The BJH pore-size distribution was performed to analyze the porous structure of the cathode material. As can be seen in Figure 2d, the curve exhibits a typical characteristic of mesoporous materials, showing a hysteresis loop in isotherms due to the capillary condensation of N2 gas in the pore. The BJH average pore diameter is around 10 nm, and the specific surface area is 87 m2 g−1, indicating that the release of CO2 during thermal decomposition of the carbonate precursor causes a highly porous structure. SEM and TEM were performed to further analyze the microstructures of the Li1.2Mn0.52Ni0.2Co0.08O2 cathode material (Figure 3a−h). As can be seen, the Li1.2Mn0.52Ni0.2Co0.08O2 cathode material shows the 8-μm-sized secondary particle, made up of primary particles with sizes in the range of 200−800 nm (Figure 3a). Notably, it exhibits a “honeycomb-like” structure with homogeneously and loosely distributed pores on the surface of microspheres. Furthermore, it also confirms a hollow structure from the cross-sectional SEM image (inset in Figure 3a), in which the surface and interior of the secondary particles are well connected through a number of primary particles. The TEM (Figure 3b,c) and HRTEM (Figure 3d) images combined with the corresponding selected-area fast Fourier transform (FFT) pattern (inset in Figure 3d) further

observation, three weight-loss steps are clearly seen in the TG curve. The first weight-loss step occurs between 25 and 125 °C with a weight loss of 3.2%, which could be ascribed to the evaporation of free water. The second one is presented in a temperature range of 125−480 °C with a weight loss of 18.5%, which is attributed to the decomposition of Ni0.25Co0.1Mn0.65CO3 and the formation of TM oxides (inset: XRD pattern in Figure 2a). The last part is observed between 500 and 600 °C with a weight loss of 13.8%, which is due to the formation of a spinel structure (inset: XRD pattern in Figure 2a). To better understand the thermal decomposition of Ni0.25Co0.1Mn0.65CO3, we plotted the differential mass loss as a function of the temperature, as seen in Figure 2a. From the dM/dT data, each downward peak corresponds to a mass-loss region in the TG curve, which could be attributed to a series of thermal decompositions of Ni0.25Co0.1Mn0.65CO3 with increasing temperature. The XRD patterns shown in Figure 2b indicate that the precursor is a mixture of TM carbonates because all diffraction peaks of the precursor match well with those of the standard XRD patterns for NiCO3 (PDF 78-0210), CoCO3 (PDF 01-1020), and MnCO3 (PDF 01-0981). M e a n w h i l e , a l l s t r o n g d i ff r a c t i o n p e ak s o f t h e Li1.2Mn0.52Ni0.2Co0.08O2 cathode material are well indexed in terms of the R3m ̅ structure of hexagonal α-NaFeO2, while weak peaks observed at 2θ of 20−25° are attributed to an integrated monoclinic Li2MnO3 phase (C2/m).19,20,27,48 It should be noted that the pair reflections (006)/(102) and (018)/(110) are well split for all samples, demonstrating the good hexagonal 30620


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Figure 3. (a) Magnified SEM image (the inset refers to the cross-sectional SEM image). (b) TEM image. (c) Single-crystal stripped from part b. (d) HRTEM image (the inset refers to FFT from the selected region) of the Li1.2Mn0.52Ni0.2Co0.08O2 cathode material. SEM images of the precursors obtained from (e) sol−gel and (f) coprecipitation methods (the insets refer to the corresponding cathode materials). (g) EDS spectra and (h) element mapping of the Ni0.25Co0.1Mn0.65CO3 precursor (obtained at a mole ratio of HMT:TM ions = 1.5:1 at 160 °C).

all of the peaks in the XRD patterns (Figure 4a) could be indexed on the basis of LRLO structures, in good agreement with the results of the Li1.2Mn0.52Ni0.2Co0.08O2 cathode material prepared in a “bubble-template” reaction and other reports.19,20 The initial charge/discharge profiles at 0.1 C rate are shown in Figure 4b, presenting a typical slope-charging voltage region below 4.45 V (referring to Ni2+ to Ni3+/4+ and Co3+ to Co4+) and the subsequent voltage plateau at ca. 4.5 V (referring to activation of the Li2MnO3 component).19,20,44,45,47−50 The sol− gel-prepared nonmicrospheres deliver discharge capacities approaching 275 mAh g−1. However, for the coprecipitationprepared microspheres, the discharge capacity is only 261 mAh g−1. Specifically, the charge capacity above 4.45 V is 143 mAh g−1 for the sol−gel-prepared nonmicrospheres, which is ca. 10% higher than those extracted from the coprecipitation-prepared microspheres, implying a higher content of the activated Li2MnO3 component available during lithium-ion extraction in this voltage region.50,51,53,54 The rate capability combined with the corresponding charge/discharge profiles at various rates (Figure 4c and insets) shows a superiority of interconnected particles with pores in achieving high rate capabilities over those of coprecipitation-prepared microspheres. However, the cycling performance shown in Figure 4d indicates that the coprecipitation-prepared microspheres exhibit better cycling stability with 90% capacity retention after 100 cycles, which is

confirm that the as-prepared microspheres are composed of a well-crystallized layered structure. Additionally, the Li1.2Mn0.52Ni0.2Co0.08O2 cathode materials with microsphere and nonmicrosphere characteristics were prepared by coprecipitation and sol−gel methods to further compare and demonstrate the superiority of “honeycomb-like” architecture. As shown in Figure 3e,f, the cathode materials obtained via sol−gel and coprecipitation methods show the very different morphologies. The well-interconnected particles with sizes in the range of 300−600 nm were obtained for the sol−gelprepared nonmicrospheres. Comparatively, the coprecipitationprepared cathode material (Figure 3f) displays a microsphere architecture without any pores on the surface. The Brunauer− Emmett−Teller results (Figure S6) also demonstrate that there are almost no mesopores in these two kinds of particles. Moreover, element mapping of the Ni0.25Co0.1Mn0.65CO3 precursor shows that nickel, cobalt, and manganese elements distributed uniformly across the microspheres. The atomic ratios of nickel, cobalt, and manganese detected from the EDS spectrum are close to the theoretical ratio of the elements in Ni0.25Co0.1Mn0.65CO3. 3.2. Electrochemical Performance. The XRD patterns and electrochemical performance of Li1.2Mn0.52Ni0.2Co0.08O2 cathode materials prepared by sol−gel and coprecipitation methods were evaluated and are shown in Figure 4. Obviously, 30621


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Figure 4. (a) XRD pattern. (b) Initial charge/discharge profiles at 0.1 C in the voltage range of 2.0−4.8 V. (c) Rate performance (the insets refer to charge/discharge profiles at various rates). (d) Cycling performance at 1.0 C rate (the insets refer to charge/discharge profiles evolution during cycling) of cathode materials prepared via sol−gel and coprecipitation methods.

mAh g−1, respectively. After 100 cycles, the cathode material still has capacities of 247, 218, and 185 mAh g−1 with capacity retentions of 95%, 96%, and 94%, respectively. The SEM images of the electrode after different cycles at 1 C rate are exhibited in Figure S12. Most of the “honeycomb-like” architectures still remained integral even after 40 cycles, which further confirms the structural stability. Comparatively, this test at 1.0 C rate is also applied for the cathode material that suffered from heavy grinding for 1 h, which could destroy the “honeycomb-like” structure (inset of Figure 5b). After the grinding treatment, the cathode material shows a rapid capacity fade from 211 to 126 mAh g−1 after 100 cycles. The great difference in the cycling stability before and after the grinding treatment demonstrates that the “honeycomb-like” architecture plays an important role in achieving good structure stability. Such a superior cycling performance is in good agreement with its differential capacity dQ/dV plots (Figure 5c). It can be further confirmed from Figure 5d that the Li1.2Mn0.52Ni0.2Co0.08O2 microspheres exhibit good rate capability. When the rate is decreased from 5 to 0.1 C, its reversible capacity can recover back to 287 mAh g−1. The electrochemical impedance of the “honeycomb-like” hollow microspheres (Figure 5d) after 100 cycles is smaller than that of the samples prepared by the sol−gel and coprecipitation methods (Figure S13 and Table S1), which is combined with the aforementioned

much higher than those of nonmicrospheres (only 70%). It is definitely demonstrated that the coprecipitation-prepared microspheres show better structure stability than those of sol−gel-synthesized nonmicrospheres. Meanwhile, the interconnected particles with pore structure make more contributions to the Li2MnO3 activation process, thus delivering high capacities. Accordingly, these results indicate that the interconnected primary-particle-assembled microspheres with pores on the surface (honeycomb-like) are an essential consideration when constructing high-performance cathode materials for LIBs. To demonstrate the application value of the “honeycomblike” architecture, our designed Li1.2Mn0.52Ni0.2Co0.08O2 microspheres were investigated as cathode materials for LIBs. The initial discharge capacity of Li1.2Mn0.52Ni0.2Co0.08O2 microspheres with “honeycomb-like” architecture reached 287 mAh g−1 with an initial Coulombic efficiency of 84.5% at a rate of 0.1 C (Figure 5a), which is remarkably enhanced compared with other cathode materials (Figures S7−S11). The capacities span from 260 to 180 mAh g−1 in a current density range of 0.2−5.0 C, and superior reproducibility of the voltage plateaus and charge/discharge profiles is also exhibited. Figure 5b shows the cycling performance of the Li1.2Mn0.52Ni0.2Co0.08O2 microspheres at current rates of 0.2, 1.0, and 3.0 C. The initial capacities at 0.2 C, 1.0 and 3.0 C rate are 260, 228, and 197 30622


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Figure 5. (a) Charge/discharge profiles at various rates in the voltage range of 2.0−4.8 V. (b) Cycling performances at 0.2, 1.0, and 3.0 C and in comparison with the ground cathode material (inset). (c) Differential capacity dQ/dV plot evolution upon cycling (Q = capacity and V = voltage of the cells; the inset is the corresponding voltage evolution). (d) Rate capability at different rates (the inset is a EIS plot after 100 cycles at 1.0 C rate, where 1 C = 250 mAh g−1) of the Li1.2Mn0.52Ni0.2Co0.08O2 cathode material with “honeycomb-like” architecture.

results further confirms that the Li1.2Mn0.52Ni0.2Co0.08O2 microspheres with “honeycomb-like” architecture shows great potential in achieving high capacity and rate capability as well as cycling stability. It is believed that the outstanding performance mainly originated from the “honeycomb-like” architecture. The interconnected pores spanning from the surface to the bulk of microspheres provide a significantly reduced distance for Li+ ions diffusion and a large electrode−electrolyte contact area for Li+ ion flux across the interface, leading to improved rate capability. Importantly, the hollow structure promises an enhanced structural stability because of the void space within each microsphere, which can buffer the local volume changes upon repeated lithium-ion insertion/extraction processes. Compared with the above-mentioned results, the high quality of the Li1.2Mn0.52Ni0.2Co0.08O2 microspheres with “honeycomblike” architecture achieved by the “bubble-template ” reaction is finally demonstrated.

Ni0.25Co0.1Mn0.65CO3 microspheres by the aid of HMT, which is defined as a “bubble maker”. After chemical lithiation, Li1.2Mn0.52Ni0.2Co0.08O2 hollow microspheres with “honeycomb-like” architecture were obtained. Upon using as cathode materials in LIBs, the combination of the hollow structure and the highly exposed porous surface has synergistically improved the structure stability and rate capability of these “honeycomb” microspheres. Furthermore, in a significant departure from conventional approaches such as coprecipitation and sol−gel, the present method is green, flexible, and efficient in preparing high-performance microsphere cathode materials. The asdeveloped methodology has made LRLO cathode materials more suitable for next-generation LIBs and should therefore inspire further attempts at additional spectral applications in future.

4. CONCLUSIONS In summary, we have developed a “bubble-template” approach that is simple, scalable, and effective in preparing size-controlled

* Supporting Information

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07542. 30623


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SEM images, EDS spectra, XRD pattern and electrochemical performance of samples at different heating temperatures (140, 160, and 180 °C), different heating rates (5 and 7 °C min−1), and HMT amounts (1.42 and 2.83 g), N2 adsorption/desorption isotherms of sol−gel and coprecipitation samples, SEM images for cycled electrodes of a “honeycomb-like” hollow structure, EIS plots and the corresponding data for sol−gel and coprecipitation samples after the 100th cycle at 1 C rate, and Li+-ion migration (Rsei) and charge-transfer (Rct) resistances of different samples (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.C.). ORCID

Zhaoyong Chen: 0000-0001-7191-4625 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are thankful for financial support from the Hunan Provincial Key Research and Development Plan (Grant 2016GK2012), a project supported by the Research Foundation of Education Bureau of Hunan Province (Grant 16A001). This project was supported by the Hunan Provincial Natural Science Foundation of China (Grant 2016JJ3008) and National Science Foundation for Young Scientists of China (Grants 51604042 and 21601020).

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