Rapid Self-Assembly Spherical Li1.2Mn0.56Ni0.16Co0.08O2 with

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Rapidly 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01683 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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ACS Applied Materials & Interfaces

Rapidly 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, Gang Yang∗ Jiangsu Lab of Advanced Functional Material, Changshu Institute of Technology, Changshu, 215500, China

∗

Corresponding author: Tel.: +86 512 52251895; fax: +86 512 52251842. E-mail address: [email protected] (G. Yang). 1

ACS Paragon Plus Environment


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Abstract Spherical Li-rich Li1.2Mn0.56Ni0.16Co0.08O2 compound is rapidly synthesized through a facile microwave hydrothermal method followed by a high temperature solid-state reaction. Homogenous spherical precursor can be precipitated through microwave hydrothermal (MH) method within 30 minutes without rigorous co-precipitation 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

Brunaurer-emmett-teller (BET) tests prove well-formed layered structure and large specific surface area containing pores are obtained through MH method. Such structure is 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 high discharge capacity of 235.6 mAh g−1 at 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; Co-precipitation; Cathode material; Lithium ion battery; 2


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1. Introduction Recently, as the exhaustion of fossil fuels, energy storage with excellent properties for clean regenerated energies such as solar energy, wind energy, is significantly required to solve the energy crisis, as well as the environment problems. Lithium ion battery (LIBs) is considered to be one of the most promising energy storage due to its favorable benefits such as high capacity, long cycle life, high energy density, low cost and safety. Cathode material is considered as the key part of LIBs and has been widely investigated, from the early LiCoO2 1 to nowadays LiFePO4 2, 3,4,5, 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. 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 3



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state reactions 29, co-precipitation 30-33, freezing 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, co-precipitation method is applied widely as the main way for commercial production. However, this method is very rigorous, which need not only precise control of the pH value, but also both equipment and time cost. Furthermore, there is space and time difference inevitably during the process of precipitation for transition metal ions

41

. Homogeneous traditional solvo/hydrothermal synthesis

42,43

can effectively overcome some disadvantages of co-precipitation method. And 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 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 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 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 4


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30 minutes, which is much faster than the traditional hydrothermal method and co-precipitation

method.

The

as

synthesized

Li-rich

compound

Li1.2Mn0.56Co0.16Ni0.08O2 reveals excellent electrochemical performances due to the homogenous 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 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 distilled water with continually 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 Fig. 1. A little amount of the precipitate appeared due to the inevitable dissolution of NH4HCO3. 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 Fig. 1. The condition of the reaction was set as 200 oC under 30 atm. After a fast reaction within 30 minutes, the system was cooled to room temperature. And then, 5



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the precipitate was collected through centrifugation, followed by washing with distilled water and ethanol several times. After dried in a vacuum oven to remove the water, the precursors was mixed with stiochiometric amounts of LiOH·H2O using ethanol as the medium. Finally, the mixture was dried and calcined at 800 oC under air atmosphere

for

16

h

to

synthesize

the

Li-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 co-precipitation method. The synthesis processes were as follows: Initially, 200 mL de-ionized 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. 2M aqueous solution with stoichiometric amounts of NiSO4·6H2O, MnSO4·H2O and CoSO4·7H2O was prepared as the metal sources. At the same time, 2M ammonia solution and 4M 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 co-precipitation reaction. After reacting at 50 ºC for 12 h, the precipitates were filtered, washed and dried in a vacuum oven at 60 ºC. 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 oC 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 0.02o and counting time duration of 8.0 s for each step using Cu Kα radiation from 10° to 80° on 6


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X-ray diffractometer (D/max-2200-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 thermo gravimetric analyzer (TGA/SDTA851) and thermal analyzer (STA 449 F3). The morphologies and structures of the as-prepared compounds were characterized by field emission scanning electron microscopy (SEM, SIGMA, ZEISS microscope), high-resolution transmission electron microscopy (TEM, TECNAI G20) and X-ray photo-electron

spectroscopy

(XPS,

Thermo

scientific,

Escalab250Xi).

Brunaurer-emmett-teller (BET) surface area analyzer (ASAP 2020) was performed 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 assemblymen 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 micro-porous film as the separator. The whole processes were performed in an argon-filled glove box 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). And the rate capability is evaluated under the current densities between 20 and 2000 mA g−1. Galvanostatic intermittent titration technique 7



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(GITT) was performed to calculate the diffusion of Li+ on this apparatus, too. Electrochemical workstation with the type of CHI660E was used to perform the Cyclic voltammetry (CV) test and Electrochemical impedance spectroscopy (EIS) measurements. The potential window and scan rate for CV tests were set as 2.5 V 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 Fig. 2 to analyze the heat-treatment processes of microwave hydrothermal precursors mixed with Li sources. The mixture is heated from room temperature to 1000 oC with a speed of 5 oC min−1. There is an exothermal peak below 200 oC 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 oC to 500 oC 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 oC, there is still a mild loss of mass, which is attributed to Li and O loss at high temperature. Thus, 800 oC is chosen as the reaction temperature for the synthesis of 8


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Li-rich compounds. On one side, the particle size will become too large at higher temperature above 800 oC, 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 Fig. 3. All the diffraction peaks of the carbonate precursor in Fig. 3 (a) reflect a similar lattice character as rhombohedral phase of MnCO3 which belong to the space group of R-3c, based on the hexagonal structure47, 48. Because of the substitution among Mn, Co, Ni each other, the XRD peaks have a small offset according to the normal MnCO3, CoCO3, NiCO3 peaks shown in Fig. 3(a). The XRD pattern is similar as other mixed carbonate precursor (such as Ni1/6Co1/6Mn4/6CO3) reported 49,50. No distinct impurity is observed, indicating high purity of the carbonate precursor. Fig. 3(b) and (c) displays the Rietveld refinement results of the finally prepared Li-rich

compounds

by

microwave

hydrothermal

method

(MH-800)

and

co-precipitation method (CP-800), respectively. All the diffraction peaks of both MH-800 and CP-800 patterns can be indexed belonging to the α-NaFeO2 structure (R-3m), 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 9



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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 structure53. They are listed in Table 1. MH-800 has a larger value of I003/I104 than that of CP-800. It indicates better ion arrangement for MH-800 than CP-800. 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, larger value of c/a indicates better channel for lithium-ion transfer 53. It seems that 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 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 Fig. 4.

Fig. 4 (a) and (b) shows the SEM images of the carbonate precursor at low 10


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and high magnification, respectively. The precursor appears a homogenous spheriform secondary particle with particle size of about 2-3 µm without any impurity. The insert in Fig. 4(b) is the magnification of the spherical surface, which indicates that the secondary sphere is consist of 20-30 nm primary particles. Fig. 4 (c) and (d) shows the SEM images of MH-800. After calcination at 800 oC, the secondary sphere morphology is still maintained, 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 Fig.4 (e) and (f), it also exhibits a hierarchical secondary particle consist of small primary particles. However, the agglomeration of the primary particles becomes much severe. In addition, the particle size of CP-800 is less homogenous 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 Fig. 5. The results of EDS (Table 2) show that Mn, Co, Ni elements exist in the final product with an approximate 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. Fig. 6 shows the TEM images of MH-800. The image at low magnification, as shown in Fig. 6(a), manifests that the hierarchical secondary sphere is solid without hollow structure, which is benefit for the tap density. HRTEM image (in the insert of 11



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Fig. 6b) demonstrates that the lattice distance is 0.47 nm, which agrees well with both the {003} planes for rhombohedral phase (R-3m) 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 Fig.7. It demonstrates that the binding energy of Mn 2p1/2 and Mn 2p3/2 peaks of MH-800 are around 654.1 eV 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 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 eV and 780.3 eV with their satellite peaks locating at around 789.7 eV and 804.9 eV. The Co 2p signal is consistent with that in Co3+ chemical environment

58

. It demonstrates that the

transition metal elements of as-prepared 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 Fig. 8. The Brunauer, Emmett, and Teller (BET) surface area are 5.8830 and 3.0950 m2 g−1 for MH-800 and CP-800, respectively. The BET specific surface area of MH-800 is about twice as that of CP-800, which may be attributed to the less agglomeration and the pores appear among particles. The data of pore size distribution is shown in the insert of Fig. 7. MH-800 has remarkable pore distribution at pore size of 1-2 nm, while CP-800 doesn’t. The calculated total pore 12


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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. Fig. 9 (a) 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 V-4.40 V, 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 R-3m) of Li-rich layered compounds. The latter one is ascribed 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 Li from Li2MnO3-like region cannot be re-inserted 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 84.04 % due to the irreversible activation at about 4.5 V. For comparing, 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 more ordered ionic arrangement and higher hexagonal ordering, which is proved from the XRD data. 13



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Fig. 9(b) displays the cycle performances of MH-800 and CP-800 at current densities of 200 mA g−1, 1000 mA g−1 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 a cycling tests at 1000 mA g−1 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. And 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 continue 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 cut-off 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 current density of 200 mA g−1, much higher than that of CP-800. Such phenomenon may be attributed to the 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 14


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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 remained 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 Fig. 9 (c). The better rate capability of MH-800 can also be ascribed to the larger specific 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) Li-rich layered compounds synthesized by other methods reported 14-16, 59-62. Setting the discharge current density 15



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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 V 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 Fig. 9 (d). 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 Fig. 9 (a). 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 re-arrangement 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 Fig. 9 (d), there are three peaks during the whole oxidation processes, at 4.4 V, 3.85 V 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. 16


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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 other63. 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 accordant with the results of the electrochemical performances. EIS tests are performed to study the reasons of the better electrochemical performances for MH-800. Before the tests, the cells are first charged and discharged for 5 cycles. And then, the EIS tests are taken at the charge state of 4.5 V. The Nyquist plots of the Li-rich layered oxides shown in Fig. 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. And 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 Nyquist plots are fitted by using an

equivalent circuit. Besides the Re, Rf, Rct and Zw, the left units for fitting are CEPct 17



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and CEPf, which represent the non-ideal capacitance of the double-layer and the surface layer, respectively. The results of fitting are shown as the real line in Fig. 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 as 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 benefit 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 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 Fig. 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 Fig. 11 (b) at a high magnification. This is may be due to the side-reactions between the electrolyte and the active material surface. The maintaining of the hierarchical morphology 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 18


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electrochemical performances. GITT

tests

are

performed

to

study

the

Li+

diffusion

in

the

Li1.2Mn0.56Ni0.16Co0.08O2 Li-rich layered oxides. Fig. 12 (a) and (b) show the GITT patterns of MH-800 and CP-800 at a current density of 20 mA g−1 between 2.5 V to 4.8 V for the initial cycle. Here, the chemical diffusion coefficient of Li+ (DLi+) in the Li-rich layered oxides is calculated according to Eq. (1) as follows 67: DLi +

2  ∆Es 4  mVM   =     π  MA   τ ( dEτ / d τ ) 

2

 τ ≤ L2   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 follows67: 2

DLi +

4  mVM   ∆Es  =   πτ  MA   ∆Eτ 

2

(2)

Based on Eq. (2) and the data of the GITT tests, the calculated DLi+ at various potentials are patterned in Figs. 12 (c) and (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, which is corresponding to Li ions extracting from the layered structure (R-3m) of LiMO2 (M= Mn, Ni, Co). And 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 ions diffusion in Li2MnO3 phase is much slower compared with that in 19



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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 Li-rich compounds are extremely complicated. Thus, the DLi+ calculated here is 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 extends 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 maintain 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 20


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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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High-Voltage Cathode for Batteries Working at Elevated Temperatures. Electrochim. Acta 2012, 81, 283-291. (59) Liu, J.-L.; Wang, J.; Xia, Y.-Y., A New Rechargeable Lithium-Ion Battery with a xLi2MnO3·(1−x)LiMn0.4Ni0.4Co0.2O2 Cathode and a Hard Carbon Anode. Electrochim. Acta 2011, 56, 7392-7396. (60) Wang, C.; Zhou, F.; Chen, K.; Kong, J.; Jiang, Y.; Yan, G.; Li, J.; Yu, C.; Tang, W.-P., Electrochemical Properties of Α-MoO3-Coated Li[Li0.2Mn0.54Ni0.13Co0.13]O2 Cathode Material for Li-Ion Batteries. Electrochim. Acta 2015, 176, 1171-1181. (61) Nayak, P. K.; Grinblat, J.; Levi, M.; Aurbach, D., Electrochemical and Structural Characterization of Carbon Coated Li1.2Mn0.56Ni0.16Co0.08O2 and Li1.2Mn0.6Ni0.2O2 as Cathode Materials for Li-Ion Batteries. Electrochim. Acta 2014, 137, 546-556. (62) Xiang, Y.; Yin, Z.; Zhang, Y.; Li, X., Effects of Synthesis Conditions on the Structural

and

Electrochemical

Properties

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Material

Li[Li0.2Ni0.17Co0.16Mn0.47]O2 Via the Solid-State Method. Electrochim. Acta 2013, 91, 214-218. (63) Johnson, C. S.; Li, N.; Lefief, C.; Vaughey, J. T.; Thackeray, M. M., Synthesis, Characterization

and

Electrochemistry

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Electrodes:

xLi2MnO3·(1−x)LiMn0.333Ni0.333Co0.333O2 (0 ≤ x ≤ 0.7). Chem. Mater. 2008, 20, 6095-6106. (64) Li, H. H.; Yabuuchi, N.; Meng, Y. S.; Kumar, S.; Breger, J.; Grey, C. P.; Shao-Horn, Y., Changes in the Cation Ordering of Layered O3 LixNi0.5Mn0.5O2 During Electrochemical Cycling to High Voltages: An Electron Diffraction Study. Chem. 30


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Mater. 2007, 19, 2551-2565. (65) Hu, S.-K.; Cheng, G.-H.; Cheng, M.-Y.; Hwang, B.-J.; Santhanam, R., Cycle Life Improvement of ZrO2-coated spherical LiNi1/3Co1/3Mn1/3O2 Cathode Material for Lithium Ion Batteries. J. Power Sources 2009, 188, 564-569. (66) Liu, J.; Wang, Q.; Reeja-Jayan, B.; Manthiram, A., Carbon-Coated High Capacity Layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 Cathodes. Electrochem. Commun. 2010, 12, 750-753. (67) Weppner, W.; Huggins, R. A., Determination of the Kinetic Parameters of Mixed‐Conducting Electrodes and Application to the System Li3Sb. J. Electrochem. Soc. 1977, 124, 1569-1578. (68) Li, Z.; Du, F.; Bie, X.; Zhang, D.; Cai, Y.; Cui, X.; Wang, C.; Chen, G.; Wei, Y., Electrochemical Kinetics of the Li[Li0.23Co0.3Mn0.47]O2 Cathode Material Studied by Gitt and Eis. J. Phys. Chem. C 2010, 114, 22751-22757.

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Table 1 The 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

2.8661

14.3048

4.991

3.06

2.27

1.211

0.2229

0.0175

CP-800

2.8634

14.2862

4.989

5.59

3.94

1.125

0.2366

0.0424

Table 2 The results of EDS for MH-800. Element

Weight ratio /%

Atomic ratio /%

Mn

68.86

70.30

Co

11.14

10.60

Ni

20.00

19.10

Table 3 The results of the fitting for the Nyquist plots. Sample

Re /ohm

Rf /ohm

Rct /ohm

Chi squared /10−4

MH-800

3.535

29.81

157.8

6.817

CP-800

6.942

26.91

257.2

6.304

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Figures captions Fig. 1 The schematic illustration for preparation of Microwave hydrothermal method. Fig. 2 TG/DSC curves of the mixture containing precursor of MH-800 and Lithium source. Fig.3 XRD patterns of (a) the precursor of MH-800; Rietveld refinement results for XRD patterns of (b) MH-800 and (c) CP-800. Fig. 4 SEM images of (a), (b) the precursor of MH-800; (c), (d) MH-800 and (e), (f) CP-800; the inserts in (b), (d), (f) are the magnification of themselves. Fig.5 EDS tests of MH-800. Fig.6 TEM images of MH-800 at (a) low magnification and (b) high magnification. Fig. 7 XPS spectra of MH-800 Fig. 8 Nitrogen adsorption–desorption isotherms and pore size distribution of MH-800 and CP-800 powders. Fig. 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 mA g−1, 1000 mA g−1 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 V−5.0 V for the initial three cycles. Fig. 10 Nyquist plots of Mh-800 and CP-800 powders after 5 cycles at the charge state of 4.5 V, the insert is the equivalent circuit performed to fit the curves and the solid lines are the results of the fitting. Fig. 11 SEM images of MH-800 at (a) low magnification and (b) high magnification 33



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(mixed with PVDF and carbon conductive agent) after 100 cycles. Fig. 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.

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Figures

Fig. 1 The schematic illustration for preparation of Microwave hydrothermal method.

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

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

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

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Fig.5 EDS tests of MH-800.

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

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Fig. 7 XPS spectra of MH-800

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

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Fig. 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 mA g−1, 1000 mA g−1 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 V−5.0 V for the initial three cycles.

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Fig. 10 Nyquist plots of Mh-800 and CP-800 powders after 5 cycles at the charge state of 4.5 V, the insert is the equivalent circuit performed to fit the curves and the solid lines are the results of the fitting.

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

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Fig. 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.

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Table of Contents Graphic

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Rapid Self-Assembly Spherical Li1.2Mn0.56Ni0.16Co0.08O2 with

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