Li1.2Ni0.13Co0.13Mn0.54O2 with Controllable Morphology and Size

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Li1.2Ni0.13Co0.13Mn0.54O2 with Controllable Morphology and Size for High Performance Lithium-Ion Batteries Gang Wang,† Liling Yi,† Ruizhi Yu,† Xianyou Wang,*,† Yu Wang,† Zhongshu Liu,† Bing Wu,† Min Liu,† Xiaohui Zhang,† Xiukang Yang,† Xunhui Xiong,‡ and Meilin Liu‡,§ †

National Base for International Science & Technology Cooperation, National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key Laboratory of Electrochemical Energy Storage and Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, China ‡ Key Laboratory of Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, China § School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States ABSTRACT: The controllable morphology and size Li-rich Mn-based layered oxide Li1.2Ni0.13Co0.13Mn0.54O2 with micro/nano structure is successfully prepared through a simple coprecipitation route followed by subsequent annealing treatment process. By rationally regulating and controlling the volume ratio of ethylene glycol (EG) in hydroalcoholic solution, the morphology and size of the final products can be reasonably designed and tailored from rod-like to olive-like, and further evolved into shuttle-like with the assistance of surfactant. Further, the structures and electrochemical properties of the Li-rich layered oxide with various morphology and size are systematically investigated. The galvanostatic testing demonstrates that the electrochemical performances of lithium ion batteries (LIBs) are highly dependent on the morphology and size of Li1.2Ni0.13Co0.13Mn0.54O2 cathode materials. In particular, the olive-like morphology cathode material with suitable size exhibits much better electrochemical performances compared with the other two cathode materials in terms of initial reversible capacity (297.0 mAh g−1) and cycle performance (95.4% capacity retention after 100 cycles at 0.5 C), as well as rate capacity (142.8 mAh g−1 at 10 C). The excellent electrochemical performances of the as-prepared materials could be related to the synergistic effect of well-regulated morphology and appropriate size as well as their micro/nano structure. KEYWORDS: Li-rich Mn-based layered oxide, micro/nano structure, controllable morphology and size, lithium-ion batteries, electrochemical performance

1. INTRODUCTION

(M = Mn, Ni, Co, Fe, Cr, etc.), as one of the most promising cathode materials for future generations of LIBs due to their extremely high reversible capacities (>250 mAh g−1) and high operating voltages (≥4.6 V vs Li/Li+), have attracted enormous interests in recent years.7,8 Despite their excellent capacities and high energy densities, several fundamental challenges for Li-rich Mn-based layered oxides still need to be settled before largescale commercial application. First of all, the huge irreversible capacity loss of Li-rich Mn-based layered oxides could be related to the irreversible active reaction of the Li2MnO3 during the initial charge, which displays an irreversible removal of Li2O accompany with a production of the oxygen vacancies from the crystal lattice at high potential range.9 Second, the poor cycle performance and rate capacity could be related to intricate structural evolution and formed solid-electrolyte interfacial (SEI) layer, caused by the undesirable reaction between the

Rechargeable LIBs have been extensively applied in portable electronic equipment and considered as the leading candidate of the power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) because of their high energy density, long cycling life, fast charging/discharging rate, superior safety, and good environmental benignancy.1−4 The electrochemical performances of LIBs strongly depend on the electrode materials, especially for cathode materials, which plays a decisive role in the determination of electrochemical performance, energy density and cost of the whole cell.5 However, the conventional cathode materials are restricted by their theoretical capacity (170 mA h−1 for olivine LiFePO4, 140 mA h−1 for spinel LiMn2O4, 140−160 mA h−1 for layered LiCoO2), and cannot meet the increasing requirements in EVs and energy storage.6 Consequently, it is highly urgent to exploit advanced electrode materials with potentially large specific capacity and eminent rate capability to fulfill the requirements for high energy density and power density LIBs. During recent years, Li-rich Mn-based layered oxides xLi2MnO3·(1−x)LiMO2 © 2017 American Chemical Society

Received: May 19, 2017 Accepted: July 11, 2017 Published: July 11, 2017 25358

DOI: 10.1021/acsami.7b07095 ACS Appl. Mater. Interfaces 2017, 9, 25358−25368

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic diagram illustrating the formation of the Li-rich Mn-based layered oxides with various morphologies.

microspheres,24 core−shell structure,25 porous fusiformis,19 nanotubes,26 and hollow microspheres,27 etc. In general, the special morphology of particles can play a decisive role in determining the physical and chemical properties for LIBs and its impact goes far beyond aesthetic appeal.28,29 Moreover, electrode material with optimal morphology can efficiently shorten the transfer distance of lithium ion and rationally enlarge the contact areas between the electrode and the electrolyte, thus remarkably enhancing the electrochemical performance.30 Moreover, the cathodes with regular morphologies are helpful for realizing high energy density and power density in LIBs due to their high packing density and specific volumetric capacities.31 Meanwhile, the electrode material with consistent morphology and optimal size particles could ensure uniform reversible insertion/extraction during repeated charge/ discharge procedure.32 Notwithstanding the above superiority, the design, synthesis and manipulation of Li-rich Mn-based layered oxides as cathode material for high performance LIBs still remain a huge challenge. The versatile EG possesses special physical and chemical performances such as viscosity, chelation, vapor pressure and high boiling point, and thus can usually be used as a shape directing agent.33 Apparently, EG can play a vital role in controlling the morphology and size of the final products. Meanwhile, the cationic surfactant cetyltrimethylammonium bromide (CTAB) can induce the micellar particles gathered into long-chain structure by decreasing the surface charge density of ionic micelles.34,35 Most significantly, the metal oxalates acts as a kind of coordination polymer and tends to form one-dimensional chain molecular structure in hydroalcoholic mixed solution.17 Therefore, by adjusting the volume ratio of EG in mixed solvent, the Li-rich Mn-based layered oxides with various morphologies (rod-like, olive-like, and shuttle-like) can be controllably synthesized with the assistance of surfactant CTAB. This efficient approach is conducive to developing high performance LIBs and expected to be extended to the synthesis of other micro/nano structured materials with controlled morphology and size. Herein, we deliberately and rationally design a simple strategy fabricating micro/nanostructured Li-rich Mn-based layered oxides with controllable morphology and size in hydroalcoholic solution. The influences of diverse morphologies and sizes on the structures and electrochemical properties of the as-prepared Li-rich Mn-based layered oxide are systematically investigated.

electrode materials and the electrolyte at the high operating potential.10,11 Tailoring the particle size of the electrode material to nanoscale has been testified to be an efficient approach to enhance the electrochemical performance of Li-rich Mn-based layered oxides.12,13 Nanosized structures endow the cathode materials a shorter pathway of electron and lithium ion transport as well as a larger surface area for good penetration of the electrolyte, thus improving the rate capacity of Li-rich Mnbased layered oxides. However, the nanosized materials are thermodynamically unstable and tend to agglomerate upon electrochemical cycling processes. Meanwhile, the high surface area of nanosized materials could increase the risk of side reactions between cathode and the electrolyte, which leads to irreversible capacity loss and poor cyclic stability.14 Therefore, employing electrode with micro/nano structured materials is regarding as a efficient approach for developing advanced LIBs. The electrode materials with micro/nano structure can faultlessly configure and combine the merit of both nanoparticles and microstructure with shorter diffusion pathways and better electrochemical reversibility.15,16 Recently, considerable efforts have been expended on the synthesis of Li-rich layered oxides with micro/nano structure. For examples, Ma et al.17 reported a general and mild approach involving mixed solvent coprecipitation to selectively prepare an extensive series of 1D micro- and nanostructured manganese-based electrode materials, which exhibits outstanding rate capability, cycle performance and high discharge capacities. Zhang et al.18 reported an easy and versatile method to fabricate lithium-rich cathode material with peanut-like hierarchical micro/nanostructure, which reaches a charge capacity as high as 229.9 mA h g−1 at a current density of 200 mA g−1 and maintains a high capacity of 216.5 mA h g−1 after 100 cycles in the voltage range of 2.0−4.8 V. Besides, in our previous work, we have designed rationally and prepared successfully a new-type fusiform porous micronano structure 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O 2 cathode material through a facile coprecipitation strategy, and the as-prepared sample shows a high Coulombic efficiency of 87.2% and a remarkable capacity retention of 87.1% after 200 cycles at current rate of 0.5 C.19 Besides, it have been demonstrated that the control of particle morphology for electrode material was another feasible approach in modulating the electrochemical properties of Lirich Mn-based layered oxides.20,21 Much excellent work has been devoted to improving the cycle performance and rate capacity of Li-rich layered oxides through fabricating various typical morphologies, such as hollow cubes,22 nanorods,23 25359


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Thermo Fisher Scientific, UK) with a K-Alpha 1063 using mono Al Kα. 2.3. Electrochemical Measurements. Electrochemical measurements were performed using a CR2025 type coin cells assembled in an argon-filled glovebox. The working electrode is consisted of active material, polyvinylidene fluoride (PVDF) binder and acetylene black in N-methyl-2-pyrrolidone (NMP) with a weight ratio of 80:10:10. The as-prepared electrodes were cut into circular electrodes with a diameter of 10 mm and dried at 110 °C in vacuum oven for 12 h before to apply. Metallic lithium was served as both the counter electrode, 1 M LiPF6 solution in a mixture of ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1, V:V) as the electrolyte and porous polypropylene based membrane (Celgard 2400) as the separator. Generally, the typical mass loading of the active substances is approximately 2−3 mg cm−2.Cells assembly were carried out in an argon-filled glovebox with moisture and oxygen concentrations below 1 ppm. The galvanostatic charge/discharge tests were performed on a Neware battery test system (Shenzhen, China) at different rate densities between 2.0 and 4.6 V (vs Li+/Li). Cyclic voltammetry (CV) measurements were performed on a CHI660e electrochemical workstation. Moreover, the electrochemical impedance spectroscopy (EIS) measurements were carried out on an electrochemical workstation (VersaSTAT3, Princeton Applied Research, USA) with an AC voltage of 5 mV amplitude over a frequency range from 100 kHz to 1 Hz. All electrochemical measurements were performed at room temperature.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. All reagents in this study were of analytical grade and used without further purification. The Li-rich Mnbased layered oxide Li1.2Ni0.13Co0.13Mn0.54O2 with different morphology and size were prepared through a facile coprecipitation method in hydroalcoholic solution, and the fabrication process is schematically illustrated in Figure 1. Actually, the metal oxalates acts as a kind of coordination polymer and tends to form long-chain molecular structure in hydroalcoholic solution.17 Meanwhile, the viscous EG and robust surfactant CTAB are generally regarded as critical solvent and directing agent in the mixed solution since they can provide the effective way to control the morphology, size, and grown crystals.33−35 Moreover, the gradually increased hydrogen bond formed from EG molecule, which can drive the fringe of micro/nanorods to be more tenuous and rounded, and thus, the olive-like and shuttle-like micro/ nano-structure materials appeared consecutively with the increasing contents.36,37 Consequently, the morphology of the as-prepared material could be related to the synergistic effect of the CTAB and solvent effect as well as oxalate coprecipitation. Afterward, the asprepared precursors were calcinated in air to gain the final products. 2.1.1. Synthesis of MC2O4·xH2O (M = Ni, Co, Mn) Micro/ Nanorods. In a typical procedure, 1.5 mmol CTAB was first dissolved in hydroalcoholic solution composed of water (15 mL), EG, and ethanol with volume ratio of 3:1:13. Stoichiometric amounts of Ni(CH3COO)2·4H2O (0.5 mmol), Co(CH3COO)2·4H2O (0.5 mmol), and Mn(CH3COO)2·4H2O (2 mmol) were added into the above mixed solvents. The solution was stirred for 20 min until it became transparent. Similarly, H2C2O4 (6 mmol) was served as a precipitant agent and dissolved in another identical hydroalcoholic solution. The oxalate solution was added into the mixed metal−acetate solution drop by drop under consecutive magnetic stirring at 25 °C for 12 h. After the reaction completed, the MC2O4·xH2O precipitate was obtained by filtering and washing several times with deionized water and absolute ethanol, and then dried at 80 °C for 10 h. Subsequently, the as-synthesized precursor was presintered for 6 h at 500 °C to converting into oxide powder in a muffle furnace. Finally, the oxide powder was well-mixed with an appropriate amount of Li2CO3 and then calcined at 800 °C for 12 h under an air atmosphere to obtain the porous Li1.2Ni0.13Co0.13Mn0.54O2 micro/nanorods cathode material, denoted by LNCM-R. 2.1.2. Synthesis of MC2O4·xH2O Micro/Nanoolives. The preparation was carried out as above, except that the volume ratio of water, EG and ethanol (V:V:V = 1:1:3) in hydroalcoholic solution. Moreover, under this condition, the obtained porous Li1.2Ni0.13Co0.13Mn0.54O2 micro/nanoolives cathode material was denoted by LNCM-O. 2.1.3. Synthesis of MC2O4·xH2O Micro/Nanoshuttles. The preparation was carried out as above, except that the volume ratio of water, EG, and ethanol (V:V:V = 3:5:5) in hydroalcoholic solution. Moreover, under this condition, the obtained porous Li1.2Ni0.13Co0.13Mn0.54O2 micro/nanoshuttles cathode material was denoted by LNCM-S. 2.2. Material Characterization. The chemical composition and elemental distribution of the Li-rich Mn-based layered oxides were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 7300 DV, PerkinElmer Co., USA) and energydispersive X-ray spectroscope (EDX, JEOL JSM-6100LV). The structure and phase purity of the samples were characterized by using a Rigaku D/Max-3C X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.54178 Å) and a graphite monochromator operated at 40 mA and 40 kV at a scan rate of 4° min−1 from 10° to 80° (2θ). The morphology and structure of the samples were examined by field emission scanning electron microscopy (FESEM, Quanta FEG 250) and transmission electron microscopy (TEM, TEM, JEOL, JEM2100F) with a high-resolution transmission electron microscopy. The surface area (SBET) of the materials was determined by Brunauer− Emmett−Teller (BET) method (TriStar II 3020, Micromeritics USA) with nitrogen as adsorption/desorption gas. The metal oxidation states were detected by way of X-ray photoelectron spectroscopy (XPS,

3. RESULTS AND DISCUSSION ICP-OES analysis technique was used to determine the average chemical compositions of the as-prepared materials, and the results were listed in Table 1. The atomic ratios of Li, Ni, Co, Table 1. Measured and Designed Chemical Compositions of the As-Prepared Materials normalized element content (use Mn = 0.54 for all samples) samples

designed ratio of Li/Ni/Co/ Mn

measured ratio of Li/Ni/Co/ Mn

LNCM-R LNCM-O LNCM-S

Li1.2Ni0.13Co0.13Mn0.54 Li1.2Ni0.13Co0.13Mn0.54 Li1.2Ni0.13Co0.13Mn0.54

Li1.213Ni0.127Co0.130Mn0.540 Li1.184Ni0.134Co0.112Mn0.540 Li1.179Ni0.131Co0.128Mn0.540

and Mn for all samples are normalized by fixing the atomic ratio of Mn in each sample at 0.54. On the basis of the results of ICP-OES analysis, it can be found that the measured compositions of all the samples are in good agreement with the designed value. The morphologies and microstructures of the as-prepared precursors and their corresponding Li-rich Mn-based layered oxides were examined by FESEM. Figure 2a−c show the FESEM images of the LNCM-R, LNCM-O, and LNCM-S precursors, it can be seen that the shapes of the MC2O4·xH2O precursors vary sequentially from rod-like to olive-like, and finally transform into shuttle-like with the increase of EG content, and the surfaces of the particles are highly smooth. Among all samples, the LNCM-R sample (Figure 2a) exhibits a rod-like morphology with widths of about 300 nm and lengths of 1−2 μm. For the LNCM-O sample, there is regular and uniform olive-like morphology with size of 4−5 μm in Figure 2b. Meanwhile, the LNCM-S sample (Figure 2c) possesses a well-regulated shuttle-like morphology with lengths of 7−8 μm. Figure 2d-j display the SEM images of the Li1.2Ni0.13Co0.13Mn0.54O2 layered oxides with diverse morphology at different magnification. Apparently, it can be seen that all the as-prepared materials essentially maintained their respective 25360


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Figure 2. FESEM images of (a) LNCM-R, (b) LNCM-O, and (c) LCNM-S precursors; corresponding (d and h) LNCM-R, (e and i) LNCM-O, and (f and j) LCNM-S cathode materials at different magnification.

results. Besides, the elemental mappings verify that the Ni, Co, and Mn elements are uniformly distributed throughout the particles for these samples, which implies that the elements coprecipitated in each particle, rather than progressively separated in different particles. The XRD patterns of MC 2O4·xH2O precursors and Li1.2Ni0.13Co0.13Mn0.54O2 layered oxides are revealed in Figure 4. The main diffraction peaks of MC2O4·xH2O precursors in Figure 4a can be indexed to NiC2O4·2H2O (JCPDS No. 010299), CoC2O4·2H2O (JCPDS No. 25-0250), and MnC2O4· 2H2O (JCPDS No. 25-0544) phases, indicating that the precursors were successfully prepared via coprecipitation method.17,42 After the further calcination of the mixture of the metal oxide powder with Li2CO3 at high temperature, the diffraction peaks are getting more and more narrow (Figure 4b), signifying that all the final products have a higher crystallinity. Moreover, all the diffraction peaks of all samples can be well-indexed to α-NaFeO2 hexagonal type structure with a space group symmetry of R3̅m, except for several weak peaks around 20−25° (2θ), which can be ascribed to the existence of monoclinic Li2MnO3 component with space group C/2m and the short-range Li, Ni, and Mn cations ordering in the transition metal layers.43,44 The related crystal structure models of Li2MnO3 and LiMO2 (M = Ni, Co, and Mn) are list in Figure 4c and 4d. Meanwhile, the distinct splitting of peak

precursor morphology during the calcination process. Moreover, all the particles with different shapes are comprised of densely aggregated primary grains. The surfaces of Li1.2Ni0.13Co0.13Mn0.54O2 layered oxides are very rough and there are many small size pores or holes resulting from the emission of CO2 gas during the calcination process. The porous structure and rough surface can markedly shorten the diffusion distance for lithium ion and enhance the contact areas between cathode material and the electrolyte. The structures and element distributions of the as-prepared Li-rich Mn-based layered oxides with various shapes are further examined by TEM and EDXs as shown in Figure 3. From the typical TEM images in Figure 3, it can be obviously observed that the LNCM-R, LNCM-O, and LNCM-S materials present obvious rod-like, olive-like and shuttle-like morphologies. Notably, TEM images exhibit that all the as-prepared micro matrixes were consisted of many interconnected nanosized subunits with highly porous structure. Moreover, the distance of the lattice fringe with interlayer space of 0.47 nm can be observed from the HRTEM image, which is a typical value for (003) plane of LiMO2 (R3̅m) and (001) plane of Li2MnO3 (C2/m).38,39 In addition, the corresponding fast Fourier transformation (FFT) patterns taken from HRTEM image further reveal the high crystallinity of Li-rich Mn-based layered oxides,40,41 which are further confirmed by the following XRD 25361


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Figure 3. TEM images and corresponding HRTEM of (a and d) LNCM-R, (b and f) LNCM-O, and (c and h) LCNM-S samples; FFT (e, g, and i) patterns showing sets of reflections that were assigned to the monoclinic phase; SEM images and element mappings (Ni, Co and Mn) of (j) LNCMR, (k) LNCM-O, and (l) LCNM-S samples.

the tap densities of the LNCM-R, LNCM-O, and LNCM-S materials are about 1.29, 1.38, and 1.44 g cm−3, respectively. Usually, appropriate specific surface area and porous structure can not only enlarge the contact area between the cathode material and electrolyte, but also facilitate the transportation of Li+ and electron, thus further improving the high-rate capacity of Li-rich Mn-based layered oxides. Besides, the porous structure can buffer the volume change of the cathode material during the repeated Li+ insertion/extraction process, thus keeping the structural integrity of Li-rich Mn-based layered oxides and resulting in a long cyclic life. XPS technique was performed to confirm the oxidation states of each transition metal elements (Ni, Co and Mn) for LNCMO sample and the corresponding spectra of Ni 2p, Co 2p, and Mn 2p are appeared in Figure 6. It can be observed in Figure 6a that the Ni 2p3/2 binding energies for LNCM-O sample are in accordance with the standard Ni2+ and Ni3+, which are theoretically assigned to 854.1 and 855.1 eV.46 Additionally, the binding energies in Figure 6b and 6c located at 780.1 (Co 2p3/2) and 642.3 eV (Mn 2p3/2) for the as-prepared sample coincide well with the standard Co3+ (779.9 eV) and Mn4+ (642.2 eV).44,47 Consequently, the XPS results manifest that the oxidation states of Ni, Co, and Mn elements in the asprepared material are principally +2, + 3, and +4, respectively. Figure 6d displays the first charge/discharge curves of the

between the adjacent peaks of (006)/(102) and (018)/(110) can be obviously observed, indicating a typical layered structure.18,21 In general, the integrated intensity ratios of I(003)/I(104) and (I006 + I012)/I101 (R factor) can be used as the evaluation of layered structure and cation mixing degree, the I(003)/I(104) value higher than 1.2 and R factor lower than 0.4 indicate better layered structure and lower cation mixing of the as-prepared material.19,45 The intensity ratio of I(003)/I(104) for the as-prepared LNCM-R, LNCM-O, and LNCM-S samples were 1.53, 1.81, and 1.69, the corresponding R factor were 0.396, 0.381, and 0.389, respectively, implying that all the samples have a favorable layered structure without undesirable cation mixing. Figure 5 shows the nitrogen adsorption/desorption isotherms and Barrett−Joyner−Halenda (BJH) pore-size distribution (inserts) analysis of LNCM-R, LNCM-O, and LNCM-S samples. As being seen in the nitrogen adsorption/desorption isotherms and the pore size distribution curve, all the samples appear a type III category, indicating the existence of mesoporous in the as-prepared materials. Correspondingly, the average pore size of LNCM-R, LNCM-O, and LNCM-S samples was 10.24, 11.98, and 9.05 nm, respectively. Notably, the LNCM-O sample possesses a BET specific surface area of 8.024 m2 g−1, which is slight higher than that of LNCM-R (6.564 m2 g−1) and LNCM-S samples (5.689 m2 g−1). Besides, 25362


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Figure 4. XRD diffraction patterns of the LNCM-R, LNCM-O, and LCNM-S (a) precursors and (b) cathode materials; crystal structure of the (c) monoclinic Li2MnO3 structure and (d) rhombohedral LiMO2 (M = Ni, Co, Mn) structure.

Figure 5. Nitrogen adsorption/desorption isotherms of (a) LNCM-R, (b) LNCM-O, and (c) LNCM-S samples; the inset is the pore size distribution.

charge voltage above 4.4 V.51 In addition, the decomposed electrolyte and the formed SEI film also can lead to irreversible lithium loss.52 Figure 6e presents the CV of the LNCM-O sample at a scan rate of 0.1 mV s−1 in a voltage range of 2−4.6 V. It can be apparently found that the peaks of CV curves are completely consistent with charge/discharge profiles. The anodic peak at ∼4.0 V is assigned primarily to the oxidation of Ni2+/Co3+ to Ni4+/Co4+, meanwhile, the strong anodic peak around 4.6 V is principally associated with the extraction of Li+ from Li2MnO3 component accompanying with the extraction of oxygen and structural rearrangement. 53−55 Besides, the followed curves were overlapped well with each other, confirming that LNCM-O sample exhibits an eminent structural reversibility and good cycle performance. To investigate the capacity retention of the micro/nano structured Li-rich Mn-based layered oxides, the LNCM-R, LNCM-O, and LNCM-S electrodes were tested at 0.5 C in a voltage range of 2.0−4.6 V after cycling at 0.1 C for 3 times. Figure 7a−c shows the continuous charge/discharge profiles and cycle performance of all the electrodes. It can be found that the LNCM-O sample presents inconspicuous polarization than the other two samples after 100 cycles, which are in accordance

LNCM-R, LNCM-O and LNCM-S electrodes at a rate of 0.1 C within a potential range from 2.0 to 4.6 V. Markedly, all the electrodes have similar initial charge/discharge profiles, which exhibits a smooth voltage slope below 4.5 V and a long flat plateau around 4.5 V. The sloping curve (4.5 V) was in accordance with the irreversible removal of Li2O from the Li2MnO3 component, which commonly leads to larger initial irreversible capacity and lower Coulombic efficiency.23 Besides, the LNCM-O sample displays a significantly high initial reversible capacity of 297.0 mAh g−1 (290.7 mAh g−1 for LNCM-R and 270.9 mAh g−1 for LNCM-S) and a high Coulombic efficiency of 86.1% (83.9% for LNCM-R and 82.5% for LNCM-S) among all the electrodes, which is obviously enhanced in comparison with the previous reported materials.22,48 As we known, the low Coulombic efficiency during the first charge process is a drawback of Li-rich layered cathode materials and has been proved in a lot of literatures.49,50 The low Coulombic efficiency can be ascribed to the irreversible removal of Li and O atoms from the Li2MnO3 phase while 25363


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Figure 6. XPS spectra of the LNCM-O material: (a) Ni 2p, (b) Co 2p, and (c) Mn 2p; (d) Initial charge/discharge profiles of LNCM-R, LNCM-O, and LNCM-S electrodes at a rate of 0.1 C and corresponding (e) cyclic voltammetry curves of the LNCM-O material at a scan rate of 0.1 mV·s−1 between 2.0 and 4.6 V.

the discharge rate changes from 10 to 0.1 C, the capacity recovers substantially to 295.4 mAh g−1, implying the excellent structural integrity and electrochemical reversibility of LNCMO sample even at high rate. Furthermore, LNCM-O sample (Figure 7j) can maintain a reversible capacity of 200.3 and 151.9 mAh g−1 with capacity retentions of 90.1% and 79.4% at a rate of 2 and 5 C, respectively, further demonstrating that the LNCM-O sample exhibits remarkable cycle performance and good rate capacity. Therefore, the excellent electrochemical properties of LNCM-O sample could be related to the regular morphology and appropriate size, which can maintain homogeneous intercalation reversibility for Li+ and efficient contact areas between the cathode material and electrolyte during the repeated cycles. Meanwhile, the micro matrix assembled with nanoscale primary particles can consolidate the integrity of the host structure and shorten the transfer distance of Li+. Accordingly, the abundant pore structures not only accelerate the penetration of electrolyte but also suppress the volume changes upon repeated Li+ insertion/extraction procedures, thus improving the rate capacity and keeping the structure stability of Li-rich Mn-based layered oxides. To get insight into the origin of the difference in the electrochemical properties among the LNCM-R, LNCM-O and

with the results of dQ/dV (Figure 7d−f). The mitigated voltage decay further suggest that LNCM-O sample with uniform morphology and suitable size can effectively maintain their structural integrality during repeated cycling processes. As being seen in Figure 7b and 7g, the LNCM-O electrode delivers a first reversible capacity of 249.7 mAh g−1 at 0.5 C and maintains a stable reversible capacity of 238.3 mAh g−1 with higher capacity retention of 95.4%. On the contrast, the LNCM-R and LNCM-S electrodes exhibit relatively stable albeit low capacity retention (91.2% for LNCM-S and 83.9% for LNCM-R). Moreover, the LNCM-O electrode displays remarkable cyclic stability and capacity retention, which is far more exceedingly excellent than the previous reported materials.18,22 Figure 7i further compares the rate capabilities of the three different Li-rich Mn-based layered oxide electrodes under varying rate densities. As being seen in Figure 7h and 7i, the LNCM-O electrode delivers discharge capacities of 297.2, 273.7, 251.3, 239.8, 223.4, 188.9, and 142.8 mAh g−1 at a rate density of 0.1, 0.2, 0.5, 1, 2, 5, and 10 C, respectively. Overall, the Li-rich Mn-based layered oxides exhibit the improved rate capability compared to other previous reported,42,56,57 in particular, the capacity of LNCM-O is higher than that of both LNCM-R and LNCM-S samples. Most importantly, when 25364


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Figure 7. continued

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Figure 7. Continuous charge/discharge profiles of the 1st, 50th, and 100th cycles for the (a) LNCM-R, (b) LNCM-O and (c) LNCM-S; the corresponding dQ/dV curves of (d) LNCM-R, (e) LNCM-O, and (f) LNCM-S at particular cycles; (g) Capacity retention of the LNCM-R, LNCMO and LNCM-S at a rate of 0.5 C; (h) Charge−discharge profiles of LNCM-O in the potential range of 2.0 to 4.6 V at various rates; (i) rate capacity at different rates for LNCM-R, LNCM-O and LNCM-S materials; (j) cycle performances of the as-prepared LNCM-O at 2 and 5 C.

Figure 8. EIS spectra of the LNCM-R, LNCM-O, and LNCM-S electrodes (a) before cycling and (b) after 100 cycles.

electrodes were increased with cycling and it is worth noting that the extents of increase for LNCM-R and LNCM-S electrodes are still more. After 100 cycles, the values of Rct for the LNCM-R and LNCM-S samples increase to 336.9 and 205.0 Ω with an increment of 195.7 and 115.4 Ω. Instead, the value of Rct for LNCM-O sample only increases to 144.5 Ω with an increment of 68.7 Ω. The LNCM-O sample has the smallest increment of Rct value, demonstrating that the transfer of electrons and Li+ is more easily due to the suitable shape and porous structure, which can shorten the transfer pathway of Li+ and facilitate the penetration of electrolyte, thus leading to high rate capacity.

LNCM-S samples, EIS analysis was carried out in a fully discharged states after 3 cycles at 0.1 C (Figure 8a) and after100 cycles at 0.5 C (Figure 8b). In a typical Nyquist plot, all the impedance spectra are similar shape and consisted of a semicircle at high frequency and a slope line at low frequency. In general, the semicircle at the higher frequency range is associated with the charge transfer resistance (Rct), which represents the electrochemical kinetics of the cell reactions; while the slop line at low frequency range reflects the Warburg impedance (Zw), which is related to Li+ diffusion in the particles of the electrode material.58,59 The intercept of the semicircle in the high frequency region with the Z′ real axis corresponds to uncompensated ohmic resistance (Rs), meaning the resistance of electrolyte, Li metal anode and Al foil current collector. On the basis of above analysis, the equivalent circuit for all the samples are inserted in Figure 8b and the corresponding simulated electrochemical parameters are displayed in Table 2. As shown in Figure 8a and Table 2, the calculated charge transfer resistances (Rct) for LNCM-R, LNCM-O and LNCM-S are 141.2, 75.8, and 89.6 Ω, respectively. The Rct values of all

4. CONCLUSIONS The rod-shaped, olive-shaped, and shuttle-shaped Li-rich Mnbased layered oxides Li1.2Ni0.13Co0.13Mn0.54O2 with micro/nano structure for the application of advanced LIBs have been conveniently prepared by a simple coprecipitation strategy in the hydroalcoholic solution combined with high-temperature solid state process. The morphology and size of the assynthesized materials can be easily tuned by adjusting the volume ratio of EG in the mixed solvent. The Li-rich Mn-based layered oxide with different morphologies and sizes used as cathode material for LIBs show eminent electrochemical performances. Noticeably, the LNCM-O sample exhibits optimal electrochemical performance among all samples, it delivers a high reversible capacity of 297.0 mAh g−1 at 0.1 C with high Coulombic efficiency of 86.1% and excellent cycling stability with 95.4% capacity retention after 100 cycles at 0.5 C. These results indicate that the remarkable electrochemical

Table 2. Simulated Results from Electrochemical Impedance Spectra of LNCM-R, LNCM-O, and LNCM-S Electrodes Rs (Ω)

Rct (Ω)

sample

3rd

100th

3rd

100th

LNCM-R LNCM-O LNCM-S

5.43 4.29 4.94

8.12 6.57 7.76

141.2 75.8 89.6

336.9 144.5 205.0 25366


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(11) Lu, J.; Peng, Q.; Wang, W. Y.; Nan, C. Y.; Li, L. H.; Li, Y. D. Nanoscale Coating of LiMO2 (M = Ni, Co, Mn) Nanobelts with Li+Conductive Li2TiO3: Toward Better Rate Capabilities for Li-Ion Batteries. J. Am. Chem. Soc. 2013, 135, 1649−1652. (12) Song, H.-K.; Lee, K. T.; Kim, M. G.; Nazar, L. F.; Cho, J. Recent Progress in Nanostructured Cathode Materials for Lithium Secondary Batteries. Adv. Funct. Mater. 2010, 20, 3818−3834. (13) Wei, G. Z.; Lu, X.; Ke, F. S.; Huang, L.; Li, J. T.; Wang, Z. X.; Zhou, Z. Y.; Sun, S. G. Crystal Habit-Tuned Nanoplate Material of Li[Li1/3−2x/3NixMn2/3‑x/3]O2 for High-Rate Performance Lithium-Ion Batteries. Adv. Mater. 2010, 22, 4364−4367. (14) Yu, L.; Zhang, L.; Wu, H. B.; Zhang, G.; Lou, X. W. Controlled Synthesis of Hierarchical CoxMn3‑xO4 Array Micro-/Nanostructures with Tunable Morphology and Composition as Integrated Electrodes for Lithium-Ion Batteries. Energy Environ. Sci. 2013, 6, 2664−2671. (15) Li, X.; Liu, J.; Meng, X.; Tang, Y.; Banis, M. N.; Yang, J.; Hu, Y.; Li, R.; Cai, M.; Sun, X. Significant Impact on Cathode Performance of Lithium-Ion Batteries by Precisely Controlled Metal Oxide Nanocoatings via Atomic Layer Deposition. J. Power Sources 2014, 247, 57− 69. (16) Dong, Y.; Li, S.; Zhao, K.; Han, C.; Chen, W.; Wang, B.; Wang, L.; Xu, B.; Wei, Q.; Zhang, L.; Xu, X.; Mai, L. Hierarchical Zigzag Na1.25V3O8 Nanowires with Topotactically Encoded Superior Performance for Sodium-Ion Battery Cathodes. Energy Environ. Sci. 2015, 8, 1267−1275. (17) Ma, G.; Li, S.; Zhang, W.; Yang, Z.; Liu, S.; Fan, X.; Chen, F.; Tian, Y.; Zhang, W.; Yang, S.; Li, M. A General and Mild Approach to Controllable Preparation of Manganese-Based Micro- and Nanostructured Bars for High Performance Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2016, 55, 3667−3671. (18) Zhang, Y.; Li, Y.; Niu, X.; Wang, D.; Zhou, D.; Wang, X.; Gu, C . ; T u , J . A P e a n u t - L i k e H i e r a r c h i c a l M i c r o / Na n o Li1.2Mn0.54Ni0.18Co0.08O2 Cathode Material for Lithium-Ion Batteries with Enhanced Electrochemical Performance. J. Mater. Chem. A 2015, 3, 14291−14297. (19) Wang, G.; Wang, X.; Yi, L.; Yu, R.; Liu, M.; Yang, X. Preparation and Performance of 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2 with a Fusiform Porous Micro-Nano Structure. J. Mater. Chem. A 2016, 4, 15929−15939. (20) Shi, S. J.; Tu, J. P.; Tang, Y. Y.; Liu, X. Y.; Zhao, X. Y.; Wang, X. L.; Gu, C. D. Morphology and Electrochemical Performance of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 Cathode Materials Treated in Molten Salts. J. Power Sources 2013, 241, 186−195. (21) Song, B.; Liu, H.; Liu, Z.; Xiao, P.; Lai, M. O.; Lu, L. High Rate Capability Caused by Surface Cubic Spinels in Li-rich Layer-Structured Cathodes for Li-Ion Batteries. Sci. Rep. 2013, 3, 3094. (22) Shi, Z. S. J.; Lou, R.; Xia, T. F.; Wang, X. L.; Gu, C. D.; Tu, J. P. Hollow Li1.2Mn0.5Co0.25Ni0.05O2 Microcube Prepared by Binary Template as a Cathode Material for Lithium Ion Batteries. J. Power Sources 2014, 257, 198−204. (23) Yang, J.; Cheng, F.; Zhang, X.; Gao, H.; Tao, Z.; Chen, J. Porous 0.2Li2MnO3·0.8LiNi0.5Mn0.5O2 Nanorods as Cathode Materials for Lithium-Ion Batteries. J. Mater. Chem. A 2014, 2, 1636−1640. (24) Yu, R.; Wang, X.; Wang, D.; Ge, L.; Shu, H.; Yang, X. SelfAssembly Synthesis and Electrochemical Performance of Li1.5Mn0.75Ni0.15Co0.10O2+δ Microspheres with Multilayer Shells. J. Mater. Chem. A 2015, 3, 3120−3129. (25) Yang, X.; Wang, X.; Zou, G.; Hu, L.; Shu, H.; Yang, S.; Liu, L.; Hu, H.; Yuan, H.; Hu, B.; Wei, Q.; Yi, L. Spherical Lithium-rich Layered Li1.13[Mn0.534Ni0.233Co0.233]0.87O2 with Concentration-Gradient Outer Layer as High-Performance Cathodes for Lithium Ion Batteries. J. Power Sources 2013, 232, 338−347. (26) Ma, D.; Li, Y.; Zhang, P.; Cooper, A. J.; Abdelkader, A. M.; Ren, X.; Deng, L. Mesoporous Li1.2Mn0.54Ni0.13Co0.13O2 Nanotubes for High-Performance Cathodes in Li-Ion Batteries. J. Power Sources 2016, 311, 35−41. (27) Jiang, Y.; Yang, Z.; Luo, W.; Hu, X.; Huang, Y. Hollow 0.3Li2MnO3·0.7LiNi0.5Mn0.5O2 Microspheres as a High-Performance

properties can be mainly attributed to the regular shape and suitable size as well as the porous micro matrix, which can maintain the structure stability and shorten the diffusion pathway of Li+. Potentially, the facile synthesis of functional Lirich cathode material with controllable morphology and size may provide broad prospects for the preparation of advanced electrodes for high performance LIBs.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 731 58293377. Fax: +86 731 58292695. E-mail: [email protected]. ORCID

Xianyou Wang: 0000-0001-8888-6405 Meilin Liu: 0000-0002-6188-2372 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported financially by the National Natural Science Foundation of China under project No. 51272221, Key Project of Strategic New Industry of Hunan Province under project No. 2016GK4005 and 2016GK4030, the Hunan Provincial Innovation Foundation for Postgraduate (No. CX2016B229), the Natural Science Foundation of Hunan Province (No. 2015JJ6103 and 2015JJ2137), the Scientific Research Foundation of Education Department of Hunan Province (No. 15C1313), and the Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200).

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