Rapid Self-Assembly Spherical Li1.2Mn0.56

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Rapid Self-Assembly Spherical Li1.2Mn0.56Ni0.16Co0.08O2 with Improved Performances by Microwave Hydrothermal Method as Cathode for Lithium-Ion Batteries Shaojun Shi, Ting Wang, Min Cao, Jiawei Wang, Mengxi Zhao, and Gang Yang* Jiangsu Lab of Advanced Functional Material, Changshu Institute of Technology, Changshu, 215500, China ABSTRACT: Spherical Li-rich Li1.2Mn0.56Ni0.16Co0.08O2 compound is rapidly synthesized through a facile microwave hydrothermal method followed by a high-temperature solidstate reaction. Homogenous spherical precursor can be precipitated through the microwave hydrothermal (MH) method within 30 min without rigorous coprecipitation condition. The as-prepared Li-rich compound exhibits a hierarchical structure composed of spherical secondary particles (2−3 μm) and small primary particles (150−250 nm) with pores. X-ray diffractometry (XRD) and Brunauer− Emmett−Teller (BET) tests prove that a well-formed layered structure and a large specific surface area containing pores are obtained through the MH method. Such structure is a benefit for the thorough contact between active materials and electrolyte to increase the reactive points. Thus, the as-prepared Li-rich compound exhibits perfect electrochemical performances with a high discharge capacity of 235.6 mAh g−1 at a current density of 200 mA g−1. Even at higher current densities of 1000 and 2000 mA g−1, discharge capacities of 168.6 and 131.2 mAh g−1 are still maintained, respectively. Furthermore, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT) are carried out to study the material prepared by microwave hydrothermal method. It is considered as an efficient way to synthesize Li-rich compound as cathode material for applications. KEYWORDS: microwave hydrothermal method, Li-rich compound, coprecipitation, cathode material, lithium ion battery

1. INTRODUCTION Recently, with the depletion of fossil fuels, energy storage with excellent properties for clean regenerated energies, such as solar energy and wind energy, is significantly required to solve the energy crisis, as well as the environment problems. Lithium ion battery (LIBs) are considered to be one of the most promising energy storage materials due to its favorable benefits such as high capacity, long cycle life, high energy density, low cost, and safety. The cathode material is considered as the key part of LIBs and has been widely investigated, from the early LiCoO21 to nowadays LiFePO4,2−5 LiMn2O4,6,7 LiMnxNiyCo1−x−yO2,8−12 and Li-rich layered compounds.13−23 Especially, Li-rich layered compounds with notations as xLi2MnO3·(1−x)LiMO2(M = Mn, Co, Ni) exhibit remarkable high specific capacities, over 250 mAh g−1 at a low current density (0.1C). It is considered as a new generation of positive electrode materials with great prospect applied in hybrid electric vehicles (HEVs) and electric vehicles (EVs). However, the poor conductivity itself and the rearrangement of manganese, nickel, and cobalt ions during the initial activation lead to unsatisfactory rate capability. In addition, commercial organic electrolyte containing F element will dissolve the surface transition metal ions during charging and discharging, resulting in attenuation of the reversible capacity.24−27 © 2016 American Chemical Society

It has been reported that different synthesis will result in cathode materials with different electrochemical performances.28 Proper synthesis method may efficiently improve the electrochemical performances of the cathode materials. Nowadays, lots of methods have been performed to obtain Li-rich layered compounds, such as solid-state reactions,29 coprecipitation,30−33 freeze-drying method,34 sol−gel method,35,36 combustion method,37,38 molten salt method,39 microwave heating process,40 and so on. Among them, the coprecipitation method is applied widely as the main way for commercial production. However, this method is very rigorous and needs not only precise control of the pH value but also specialized equipment and sufficient time. Furthermore, there is an inevitable space and time difference during the process of precipitation for transition metal ions.41 Homogeneous traditional solvo/hydrothermal synthesis42,43 can effectively overcome some disadvantages of the coprecipitation method. In addition, the Li-rich layered compounds obtained through such solvo/hydrothermal method will exhibit specific morphology, such as secondary sphere, hollow sphere, and so on,41,42 with improved electrochemical performances. However, it is still Received: February 8, 2016 Accepted: April 21, 2016 Published: April 21, 2016 11476

DOI: 10.1021/acsami.6b01683 ACS Appl. Mater. Interfaces 2016, 8, 11476−11487

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration for preparation of microwave hydrothermal method. rich layered compound Li[Li0.2Mn0.56Ni0.16Co0.08]O2 which is named as MH-800 for short. In order to display a comparison, Li-rich layered compound with the same component was also synthesized by traditional coprecipitation method. The synthesis processes were as follows: Initially, 200 mL of deionized water was taken as a base solution. Ammonia was used to adjust the pH value of the base solution to 11. Then, the base solution was fed into a reactor. A 2 M aqueous solution with stoichiometric amounts of NiSO4·6H2O, MnSO4·H2O, and CoSO4·7H2O was prepared as the metal sources. At the same time, a 2 M ammonia solution and a 4 M NaOH aqueous solution were prepared as the precipitant. Both the solutions were fed into the reactor at various speeds by a wriggle pump to control the pH value during the coprecipitation reaction. After reacting at 50 °C for 12 h, the precipitates were filtered, washed, and dried in a vacuum oven at 60 °C. Then, the precipitate precursors were mixed with stoichiometric LiOH·H2O (3% excess) thoroughly also using ethanol as the medium. Finally, a heat-treatment at a temperature of 800 °C was performed in air in a furnace for 16 h to get Li-rich layered compound Li[Li0.2Mn0.56Ni0.16Co0.08]O2, which was named as CP-800 for short. X-ray diffractometry (XRD) data was collected with a fixed step width of 0.02° and counting time duration of 8.0 s for each step using Cu Kα radiation from 10° to 80° on X-ray diffractometer (D/max2200-PC). The X-ray diffraction profiles were fitted through Rietveld refinement to calculate the atomic distribution and cell parameters with the Rietveld Program (GSAS). TG/DSC tests were performed on a thermogravimetric analyzer (TGA/SDTA851) and a thermal analyzer (STA 449 F3). The morphologies and structures of the asprepared compounds were characterized by field emission scanning electron microscopy (SEM, SIGMA, ZEISS microscope), highresolution transmission electron microscopy (TEM, TECNAI G20), and X-ray photoelectron spectroscopy (XPS, Thermo scientific, Escalab250Xi). A Brunauer−Emmett−Teller (BET) surface area analyzer (ASAP 2020) was used to characterize the surface area and pore condition. The as-prepared Li-rich layered oxides were first made into a slurry mixed with carbon conductive agent (10 wt %) as conductive agent and polyvinylidene fluoride (PVDF) (10 wt %) as binder. The slurry was then coated onto an aluminum foil. The electrolyte used for cell assembly contains 1 M LiPF6 as Li source and ethylene carbonate (EC)-dimethyl carbonate (DMC) with a volume ratio of 1:1 as solvent. The coin cells with the type of 2016 were assembled using metallic lithium foil as counter electrode and a polypropylene microporous film as the separator. The whole processes were performed in an argon-filled glovebox in which the amount of water and oxygen concentration was below 1 ppm. The electrochemical window for galvanostatic tests was set from 2.5 to 4.8 V on a LAND battery program-control test system (Wuhan, China). The rate capability is evaluated under the current densities between 20 and 2000 mA g−1. Galvanostatic intermittent titration technique (GITT) was performed to calculate the diffusion of Li+ on this apparatus. An electrochemical workstation with the type of CHI660E was used to perform the cyclic voltammetry (CV) test and electrochemical

time-consuming. Usually 12 to 24 h is necessary for a traditional solvo/hydrothermal process to obtain the precursors.41,42,44,45 In addition, it is difficult to obtain homogeneous precursors containing Mn, Ni, and Co with an atomic mixed level through only solvo/hydrothermal process without any organic complexing agents. It is reported that a binary precursor is usually first obtained through the hydrothermal process, and then the third element (Ni or Co) will be added by another way to finally reach the Li-rich layered compounds.44,46 It is difficult to mix the third added element (Ni or Co) well with the other two elements at an atomic level. Here, in this present work, we use the microwave hydrothermal method to get ternary precursor containing Mn, Co, and Ni without any organic complexing agents. The microwave irradiation magically helps to finish the hydrothermal process within 30 min, which is much faster than the traditional hydrothermal method and coprecipitation method. The as synthesized Li-rich compound Li1.2Mn0.56Co0.16Ni0.08O2 reveals excellent electrochemical performances due to the homogeneous spherical secondary particles composing of small primary particles. It is revealed that such microwave hydrothermal method is a promising method which achieves the controllable growth of Li-rich layered compound particles without any organic complexing agents and within less time for application.

2. EXPERIMENTAL SECTION To prepare the precursor of Li-rich layered compound Li[Li0.2Mn0.56Ni0.16Co0.08]O2, stiochiometric amounts of CoSO4·7H2O, NiSO4·6H2O, and MnSO4·H2O (total 6 mmol) were added into 20 mL of distilled water with continual stirring to form a transparent solution, which was named as solution A. The precipitant NH4HCO3 was added into a mixed solution of ethanol and distilled water (1:1) to form solution B. After that, solution A and B were pulled into a microwave hydrothermal system (WX-6000) together, as shown in Figure 1. A little amount of the precipitate appeared due to the inevitable dissolution of NH 4HCO3. However, such primary precipitate would offer a core for the carbonate precipitate to grow up during the microwave hydrothermal reactions to finally form a sphere. The microwave irradiation would be absorbed homogeneously around the core, and then the carbonate precipitate would form at the surface of the core spontaneously, as shown in Figure 1. The condition of the reaction was set as 200 °C under 30 atm. After a fast reaction within 30 min, the system was cooled to room temperature. Subsequently, the precipitate was collected through centrifugation, followed by washing with distilled water and ethanol several times. After the precipitate was dried in a vacuum oven to remove the water, the precursors were mixed with stiochiometric amounts of LiOH·H2O using ethanol as the medium. Finally, the mixture was dried and calcined at 800 °C under air atmosphere for 16 h to synthesize the Li11477


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ACS Applied Materials & Interfaces impedance spectroscopy (EIS) measurements. The potential window and scan rate for CV tests were set as 2.5 to 5.0 V and 0.1 mV s−1, respectively. EIS tests were conducted with a frequency range from 100 kHz to 10 mHz and the amplitude of the AC signal of 5 mV in a three-electrode system.

3. RESULTS AND DISCUSSION 3.1. Material Characterization. DSC and TG curves are shown in Figure 2 to analyze the heat-treatment processes of

Figure 2. TG/DSC curves of the mixture of precursor of MH-800 and lithium source.

microwave hydrothermal precursors mixed with Li sources. The mixture is heated from room temperature to 1000 °C with a speed of 5 °C min−1. There is an exothermal peak below 200 °C with mass loss, which represents the loss of absorbed water molecules and the crystal water of LiOH·H2O. The dissolution of carbonate and the primary forming of Li-rich compounds are observed from 200 to 500 °C as the second step. Thus, there is an obvious mass loss resulting from the loss of CO2. Furthermore, because of the primary forming of Li-rich compounds, O2 attends the oxidation of metal elements, such as Mn and Co, resulting in a mass increase to counteract the mass loss of CO2. On the final step, the layered structure continues to form accompanying with metal ions transfer. When the temperature increases above 700 °C, there is still a mild loss of mass, which is attributed to Li and O loss at high temperature. Thus, 800 °C is chosen as the reaction temperature for the synthesis of Li-rich compounds. On one side, the particle size will become too large at higher temperature above 800 °C, resulting in long diffusion distance of Li+. On the other side, the severe loss of Li2O and the chemical state change of Mn may destroy the layered structure. XRD patterns of the as-prepared carbonate precursor and the final Li-rich layered compound Li1.2Mn0.56Ni0.16Co0.08O2 are shown in Figure 3. All the diffraction peaks of the carbonate precursor in Figure 3a reflect a similar lattice character as rhombohedral phase of MnCO3 which belong to the space group of R3̅c, based on the hexagonal structure.47,48 Because of the substitution among Mn, Co, and Ni, the XRD peaks have a small offset according to the normal MnCO3, CoCO3, NiCO3 peaks shown in Figure 3a. The XRD pattern is similar to other mixed carbonate precursors (such as Ni1/6Co1/6Mn4/6CO3) reported.49,50 No distinct impurity is observed, indicating high purity of the carbonate precursor. Figure 3b,c displays the Rietveld refinement results of the finally prepared Li-rich compounds by microwave hydrothermal

Figure 3. XRD patterns of (a) the precursor of MH-800; Rietveld refinement results for XRD patterns of (b) MH-800 and (c) CP-800.

method (MH-800) and coprecipitation method (CP-800), respectively. All the diffraction peaks of both MH-800 and CP800 patterns can be indexed belonging to the α-NaFeO2 structure (R3̅m), except for a series of weak peaks between 20° and 25°. Such super lattice peaks can be indexed to the monoclinic unit cell C2/m51,52 which indicate the existence of Li-rich phase. They are consistent with the LiMn6 cation arrangement in the transition metal layers of Li2MnO3 region. No impurity peak appears. (006)/(102) and (108)/(110) peaks appear with distinct splitting, which is ascribed to the formation of layered oxides with good structure.53 It has been reported that intensity ratios of I003/I104 and (I006 + I012)/I101, which is also called R factor, show the ordering of the oxide structure.53 They are listed in Table 1. MH-800 has a larger value of I003/I104 than that of CP-800. It indicates better ion 11478


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ACS Applied Materials & Interfaces Table 1. Result of Rietveld Refinement for XRD Patterns of MH-800 and CP-800 samples

a /Å

c /Å

c/a

Rwp /%

Rp /%

I(003)/(104)

I(006+012)/(101)

Ni2+ in 3a site

MH-800 CP-800

2.8661 2.8634

14.3048 14.2862

4.991 4.989

3.06 5.59

2.27 3.94

1.211 1.125

0.2229 0.2366

0.0175 0.0424

Figure 4. SEM images of (a),(b) the precursor of MH-800; (c),(d) MH-800 and (e),(f) CP-800; the insets in (b),(d),(f) are the magnification of themselves.

arrangement for MH-800 than CP-800. A smaller value of R factor indicates better hexagonal ordering. Likewise, the R factors of MH-800 are smaller than that of CP-800. In addition, a larger value of c/a indicates a better channel for lithium-ion transfer.53 It seems that a c/a value of MH-800 is a little larger than that of CP-800. The results of Rietveld refinement also reveal the ion mixing, especially for Ni2+ and Li+ due to their similar ion radii. The Ni2+ will occupy the 3a site of Li+, which will block the Li+ transfer during charge and discharge leading to unsatisfactory rate capability. It demonstrates that MH-800 exhibits less ion mixing between Ni2+ and Li+. Furthermore, there is a minor difference on the superlattice peaks presented in the refined XRD patterns. Because the fast ionic transfer and rapid preparation are the intrinsic characteristic of microwave hydrothermal method, MH-800 should be grown with ordered superlattice structure and better crystallization of monoclinic

phase. Although both samples display well-formed Li-rich layered structure and pure phase after high-temperature heat treatment, the analysis above indicates that MH-800 may have better electrochemical performances due to better ion arrangement (less ion mixing between Ni2+ and Li+), hexagonal ordering, and crystallization. The morphology of the precursor and the finally Li-rich compounds are shown on Figure 4. Figure 4a,b show the SEM images of the carbonate precursor at low and high magnification, respectively. The precursor appears as a homogeneous spheriform secondary particle with particle size of about 2−3 μm without any impurity. The inset in Figure 4b is the magnification of the spherical surface, which indicates that the secondary sphere consists of 20−30 nm primary particles. Figure 4c,d show the SEM images of MH-800. After calcination at 800 °C, the secondary sphere morphology is still 11479


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Figure 5. EDS tests of MH-800.

ratio as that of Li1.2Mn0.56Ni0.16Co0.08O2. Furthermore, element mapping demonstrates that Mn, Co, Ni elements are distributed homogeneously in the Li-rich layered compound sphere, indicating successful synthesis of microwave hydrothermal method. Figure 6 shows the TEM images of MH-800. The image at low magnification, as shown in Figure 6a, manifests that the hierarchical secondary sphere is solid without hollow structure, which is a benefit for the tap density. HRTEM image (in the inset of Figure 6b) demonstrates that the lattice distance is 0.47 nm, which agrees well with both the {003} planes for rhombohedral phase (R3̅m) and the {001} planes for monoclinic phase (Li2MnO3).24,52 The clear lattice fringes also indicate formation of high crystallinity. XPS test is performed to analyze the chemical environments of transition metal elements in MH-800, as shown in Figure 7. It demonstrates that the binding energy of Mn 2p1/2 and Mn 2p3/2 peaks of MH-800 are around 654.1 and 642.6 eV, respectively, which are consistent with the value reported for Mn4+ in MnO2.54 No traces of any satellite peaks were observed in the Mn 2p pattern. The binding energy of Ni 2p3/2 is around 855.0 eV with a satellite peak at about 863.1 eV. The sites of Ni

Table 2. Results of EDS for MH-800 element

weight ratio /%

atomic ratio /%

Mn Co Ni

68.86 11.14 20.00

70.30 10.60 19.10

maintained, and only the particle size of the primary particles changes. After the high-temperature reaction, the primary particle size increases to 150−250 nm. Furthermore, a few pores between the primary particles are observed, which may enlarge the specific surface area of the material, resulting in more reactive points during charge−discharge processes. For CP-800, as shown in Figure 4e,f, it also exhibits a hierarchical secondary particle consist of small primary particles. However, the agglomeration of the primary particles becomes much more severe. In addition, the particle size of CP-800 is less homogeneous with an average value of 150−450 nm. In order to analyze the component of the Li-rich layered oxides, EDS and element mapping of MH-800 are performed, as shown in Figure 5. The results of EDS (Table 2) show that Mn, Co, Ni elements exist in the final product with an approximate 11480


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Figure 6. TEM images of MH-800 at (a) low magnification and (b) high magnification.

Figure 7. XPS spectra of MH-800.

MH-800 stay at a nearly perfect chemical state for Li-rich compounds. Nitrogen adsorption/desorption isotherms recorded for Li1.2Mn0.56Ni0.16Co0.08O2 powders are shown in Figure 8. The BET surface areas are 5.8830 and 3.0950 m2 g−1 for MH-800 and CP-800, respectively. The BET specific surface areas of MH-800 is about twice that of CP-800, which may be attributed to the reduced agglomeration and the pores appearing among particles. The data of pore size distribution is shown in the inset of Figure 7. MH-800 has a remarkable pore distribution at a pore size of 1−2 nm, whereas CP-800 does not. The calculated total pore volume of MH-800 is 3.4371 × 10−2 cm3 g−1, much larger than that of CP-800, 1.1615 × 10−2 cm3 g−1. The larger surface area and pore volume will increase the reactive points for MH-800 particles, resulting in accelerated electrochemical kinetics. 3.2. Electrochemical Properties. Both the electrochemical performances of MH-800 and CP-800 are tested to make a contrast. Figure 9a shows the initial charge−discharge curves of MH-800 and CP-800. The test is performed at a current density of 20 mA g−1 between 2.5−4.8 V at room temperature. As we can find, two charge platforms appear in evidence. One is from about 3.75−4.40 V, and the other is at about 4.5 V. The former one is attributed to the oxidation of mainly Ni2+/Ni4+ and partly Co3+/Co4+, accompanying with Li ion removing from electrochemical active part (space group R3̅m) of Li-rich layered compounds. The latter one is ascribed

Figure 8. Nitrogen adsorption−desorption isotherms and pore size distribution of MH-800 and CP-800 powders.

2p3/2 and the satellite peaks indicate that Ni ions are at the state of +2.55−57 In addition, the binding energy of Co 2p1/2 and Co 2p3/2 are at around 795.2 and 780.3 eV with their satellite peaks locating at around 789.7 and 804.9 eV. The Co 2p signal is consistent with that in the Co3+ chemical environment.58 It demonstrates that the transition metal elements of as-prepared 11481


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Figure 9. (a) Initial charge−discharge curves of MH-800 and CP-800; (b) Cycle performances of MH-800 and CP-800 at current densities of 200, 1000, and 2000 mA g−1; (c) Rate capability of MH-800 and CP-800 at different current densities; (d) CV curves of MH-800 at a scan speed of 0.1 mV s−1 from 2.5−5.0 V for the initial three cycles.

84.04% due to the irreversible activation at about 4.5 V. For comparison, CP-800 only delivers a low initial discharge capacity of 246.7 mAh g−1, much lower than that of MH-800. The high discharge capacity obtained at 20 mA g−1 for MH-800 may be ascribed to the better structure formed, such as the more ordered ionic arrangement and higher hexagonal ordering, which is proved from the XRD data. Figure 9b displays the cycle performances of MH-800 and CP-800 at current densities of 200, 1000, and 2000 mA g−1, respectively. The cells are directly charged and discharged at the moderate current density of 200 mA g−1. However, in order to make a thorough activation of the cells for high current density charge/discharge, the cells are first performed at a low current density of 20 mA g−1 for one cycle, and then, take cycling tests at 1000 and 2000 mA g−1, respectively. MH-800 reveals a high initial discharge capacity of 229.8 mAh g−1 at 200 mA g−1, and reaches a maximum value of 235.6 mAh g−1 after several cycles. The increment of discharge capacity is inconspicuous. For comparison, the initial discharge capacity of CP-800 at 200 mA g−1 is much lower, only 157.8 mAh g−1. Furthermore, there is a distinct increment of discharge capacity during the initial several cycles. The maximum value of the discharge capacity is 182.4 mAh g−1 after 12 cycles, 24.6 mAh g−1 higher than that of the initial cycle. The reason for this can be attributed to the incomplete activation of the Li2MnO3-like region, which leads to continued increased capacity on the initial several cycles. Such phenomenon will disappear if the cells are first charge−discharged at a low current density. It has been reported that the discharge capacity of such Li-rich layered compounds will continually decrease at high cutoff voltage of 4.8 V.14−16,26 After 100 cycles, the discharge capacity of MH-800 is still maintained at a high value of 185.4 mAh g−1 at a current density of 200 mA g−1, much higher than that of CP-800. Such phenomenon may be attributed to the

Figure 10. Nyquist plots of Mh-800 and CP-800 powders after five cycles at the charge state of 4.5 V; the inset is the equivalent circuit performed to fit the curves, and the solid lines are the results of the fitting.

Table 3. Results of the Fitting for the Nyquist Plots sample

Re /ohm

Rf /ohm

Rct /ohm

chi-squared/10−4

MH-800 CP-800

3.535 6.942

29.81 26.91

157.8 257.2

6.817 6.304

to the activation of Li2MnO3-like phase. The process can only be observed in the initial cycle, resulting from its irreversibility.24,32,37 Thus, the extraction of Li from the Li2MnO3-like region cannot be reinserted into the structure leading to the low initial Coulomb efficiency. High initial charge and discharge capacity of 317.1 and 266.5 mAh g−1 are obtained for MH-800, only with a low Coulombic efficiency of 11482


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surface area, the spherical morphology, and the pores between particles. The electrochemical performance of MH-800 here is better than that of other pristine (without any modification) Lirich layered compounds synthesized by other methods reported.14−16,59−62 Setting the discharge current density back to 20 mA g−1, high discharge capacity of 256.5 mAh g−1 still can be reached for MH-800, indicating an excellent rate capability. CV tests were performed at a scan speed of 0.1 mV s−1 from 2.5 to 4.8 V to further study the electrochemical processes during charge and discharge for MH-800. The initial three CV curves of MH-800 are shown in Figure 9d. The anodic curve of the first cycle is much different from other ones. Two main anodic peaks appear during the first anodic process. One appears at about 4.05 V, and the other appears above 4.5 V. The low potential peak is related to the oxidation with Ni2+ and Co3+, accompanying with Li ions extracting from the layered structure of LiMO2 (M = Mn, Ni, Co). It is corresponding to the first platform in the initial charge curve, shown in Figure 9a. The other anodic peak at above 4.5 V (vs Li/Li+) is ascribed to the activation of the Li2MnO3-like region, accompanying with Li ions extracting from the structure of Li2MnO3.63 It is corresponding to the second platform at high voltage in the initial charge curve. During the activation, there is not only extraction of Li ions from the structure but also rearrangement of the metal ions.64 Thus, after activation, the inactive Li[Li1/3Mn2/3]O2 amazingly becomes electrochemical active [MnO2], leading to an extremely high discharge capacity. It reveals that CV curves of the following anodic processes (the second and third cycles) are much different from the first one, indicating that the activation processes are irreversible. As shown in Figure 9d, there are three peaks during the whole oxidation processes, at 4.4, 3.85, and 3.15 V. They are considered to be the main anodic peaks for the new phase formed after initial activation, which can be marked as [M]O2 (M = Mn, Ni, Co).64 There are only two large broad peaks across the whole reduction processes. They are distinctly regarded as the main cathodic peaks for the new phase. Because of the complexity of the reduction processes, it is difficult to differentiate the individual cathodic peaks of Mn, Ni, and Co from each other.63 However, such cathodic processes are reversible, which still appear during the second and third cycles. It indicates that a high reversible capacity is obtained, which is consistent with the results of the electrochemical performances. EIS tests are performed to study the reasons for the better electrochemical performances of MH-800. Before the tests, the cells are first charged and discharged for five cycles. Subsequently, the EIS tests are taken at the charge state of 4.5 V. The Nyquist plots of the Li-rich layered oxides shown in Figure 10 are similar for both MH-800 and CP-800. The shape of the Nyquist plots can be divided into three parts. A small interrupt and a semicircle make up the first part in the high frequency. The small interrupt is corresponds to the solution impedance (solution resistance Re), which is similar for both the electrodes. The other small semicircle can be assigned to the Li+ diffusion impedance (Li+ diffusion resistance Rf) in the surface layer. The second part is composed of only one semicircle, which can be assigned to the charge transfer impedance (charge transfer resistance Rct) in the high to medium frequency. The last part is a short quasi-straight line observed in the low frequency. It is reported that the line is related to the solid-state diffusion of Li+ in the active materials, which is also called Warburg impedance (Zw).26,65,66 The

Figure 11. SEM images of MH-800 at (a) low magnification and (b) high magnification (mixed with PVDF and carbon conductive agent) after 100 cycles.

dissolution of the organic electrolyte during cycling, especially at high voltage. The organic electrolyte will react with the surface metal ions resulting in the destruction of the surface and irreversible capacity. It is reported that surface modification is an efficient way to protect the active material.16,20,26 When the discharge current density increases to 1000 and 2000 mA g−1, high discharge capacity can still be reached for MH-800 (168.6 and 131.2 mAh g−1, respectively). However, only 115.0 and 53.5 mAh g−1 are maintained for CP-800 at current densities of 1000 and 2000 mA g−1. The discharge capacity even fades to about 18 mAh g−1 at 2000 mA g−1 for CP-800. The situation of MH-800 is much different. The discharge capacities of MH-800 at 1000 mA g−1 and 2000 mA g−1 are still maintained at a high level, 164.3 mAh g−1 (97.4%) and 120.9 mAh g−1 (92.1%), respectively, after 100 cycles. Such excellent performance is mainly attributed to the enlarged specific surface area, which increases the reactive points of the active material and accelerates the kinetics. The spherical morphology and the pores formed among the particles also contribute a lot for the excellent capability at high current density. Rate capability of the MH-800 and CP-800 are also performed from current densities of 20 mA g−1 to 2000 mA g−1. The discharge capacities are about 266.5, 227.6, 204, 178.2, 148.2, and 128.6 mAh g−1 at 20, 100, 200, 400, 1000, and 2000 mA g−1, respectively, as shown in Figure 9c. The better rate capability of MH-800 can also be ascribed to the larger specific 11483


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Figure 12. GITT curves of (a) MH-800 and (b) CP-800; Diffusion coefficients of Li+ in MH-800 and CP-800 at different (c) charge and (d) discharge states.

Nyquist plots are fitted by using an equivalent circuit. Besides the Re, Rf, Rct, and Zw, the left units for fitting are CEPct and CEPf, which represent the nonideal capacitance of the doublelayer and the surface layer, respectively. The results of fitting are shown as the real line in Figure 10, with a low chi-squared value below 10−3 as shown in Table 3. The values of Rf and Rct are 29.81 Ω and 157.80 Ω, respectively, for MH-800. It seems that the Rf value of CP-800 (26.91) is similar to that of MH-800. The main difference between these two electrodes is the charge transfer resistant (Rct). The value of Rct for CP-800 (257.2 Ω) is much higher than that of MH-800, which can be attributed to the secondary spherical morphology with large pores between primary particles. It reveals that on one side, the pores among the particles are beneficial for the electrolyte to thoroughly contact with the particle surface, which will facilitate the Li+ to travel from the electrolyte to the solid material. On the other side, there are still connected points between the primary particles (due to the formation of the secondary sphere), resulting in a facile transfer of Li+ and the electron in the solid active material. Thus, it is also one of the main reasons for the improved rate capability of MH-800. The SEM images of the MH-800 electrode after 100 cycles at a current density of 200 mA g−1 are shown in Figure 11. It is observed that after cycling, the morphology of the secondary sphere is still maintained without any pulverization to small independent particles. Only the surface of the particles changes, as shown in Figure 11 b at a high magnification. This may be due to the side-reactions between the electrolyte and the active material surface. The maintaining of the hierarchical morphol-

ogy after cycles ensures that high reversible discharge capacity can still be obtained even at high current density, which is accordant with the results of the electrochemical performances. GITT tests are performed to study the Li+ diffusion in the Li1.2Mn0.56Ni0.16Co0.08O2 Li-rich layered oxides. Figure 12a,b show the GITT patterns of MH-800 and CP-800 at a current density of 20 mA g−1 between 2.5 to 4.8 V for the initial cycle. Here, the chemical diffusion coefficient of Li+ (DLi+) in the Lirich layered oxides is calculated according to eq 1 as follows:67 DLi+ =

2 ⎞2 ⎛ τ ≤ L2 ⎞ ΔEs 4 ⎛ mVM ⎞ ⎛ ⎜ ⎟ ⎜ ⎟ ⎟⎜ π ⎝ MA ⎠ ⎝ τ(dEτ /d τ ) ⎠ ⎝ DLi+ ⎠

(1)

Here, m and M are the mass and molecular weight of the Li1.2Mn0.56Ni0.16Co0.08O2, respectively. VM is the molar volume of the Li-rich layered compounds deduced from the crystallographic data. L is the radius of the active particle, and A is the active surface of the electrode. If the relationship of E and τ1/2 exhibits a behavior of beeline across the whole time period, the above equation can be simplified as follows:67 DLi+

2 2 4 ⎛ mVM ⎞ ⎛ ΔEs ⎞ ⎜ ⎟ = ⎜ ⎟ πτ ⎝ MA ⎠ ⎝ ΔEτ ⎠

(2)

Based on eq 2 and the data of the GITT tests, the calculated DLi+ at various potentials are patterned in Figure 12c,d. The results of the charge sections exhibit similar rules as those of normal Li-rich layered oxides.37 Both MH-800 and CP-800 have a steady DLi+ of about 10−13 cm2 s−1 from 3.2 to 4.3 V, 11484


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which is corresponding to Li ions extracting from the layered structure (R3̅m) of LiMO2 (M = Mn, Ni, Co). Then, the DLi+ decreases to a minimum at about 4.5 V, which is corresponding to the Li ions extraction from the Li2MnO3-like domain. It reveals that lithium ion diffusion in Li2MnO3 phase is much slower compared with that in LiMO2 (M = Mn, Ni, Co) domain. However, there are not distinct differences between MH-800 and CP-800. DLi+ of MH-800 is a little larger than that of CP-800, which may be due to better layered structure of MH-800. Similarly, during the whole discharge processes, DLi+ of MH-800 is also a little larger than that of CP-800, between 10−13 cm2 s−1 to 10−15 cm2 s−1. It has been reported that the electrochemical processes of Lirich compounds are extremely complicated. Thus, the DLi+ values calculated here are considered as apparent diffusion coefficients.68 The structure and morphology of MH-800 may not affect the DLi+ a lot. The excellent electrochemical capability of MH-800 is probably ascribed to the hierarchical particles obtained, which reveals high specific surface area and pore volume to efficiently extend the reactive interface between the particles of active material and electrolyte, as well as shorten the diffusion distance of Li+.

4. CONCLUSION Here, in this work, spherical Li-rich layered compound Li1.2Mn0.56Ni0.16Co0.08O2 is synthesized by a facile microwave hydrothermal method followed by high-temperature solid-state reactions. Such method is facile, time-economic without any organic chelating agent. The as-prepared material exhibits a well-formed layered structure and hierarchical morphology. Spherical secondary particles consisting of small primary particles are observed, which not only keep pores among the particles but also maintains good contact between the particles. It reveals that on one side, the pores among the particles are benefit for the electrolyte to thoroughly enfold the particles, which will facilitate the Li+ to travel from the electrolyte to the solid material. On the other side, the maintained connected points between the primary particles result in a facile transfer of Li+ and electron in the solid active material. Thus, excellent electrochemical performances of Li1.2Mn0.56Ni0.16Co0.08O2 are obtained, especially the rate capability. High discharge capacity can still be reached at high current densities of 1000 and 2000 mA g−1. The capacity retention is also very high, above 90% under such high current densities after 100 cycles. EIS tests show that such hierarchical morphology mainly decreases the charge transfer resistance to facilitate the electrochemical kinetics. However, the Li+ diffusion coefficient calculated by GITT tests only change a little. It seems that the excellent electrochemical performances are mainly attributed to the short diffusion distance resulting from such hierarchical morphology.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 512 52251895. Fax: +86 512 52251842. Notes

The authors declare no competing financial interest.

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REFERENCES

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Rapid Self-Assembly Spherical Li1.2Mn0.56

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