Compound-Hierarchical-Sphere LiNi0.5Co0.2Mn0.3O2: Synthesis

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Compound-Hierarchical-Sphere LiNi0.5Co0.2Mn0.3O2: Synthesis, Structure and Electrochemical Characterization Lecai Wang, Li Li, Xiaoxiao Zhang, Feng Wu, and Renjie Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09985 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Applied Materials & Interfaces

Compound-Hierarchical-Sphere LiNi0.5Co0.2Mn0.3O2: Synthesis, Structure and Electrochemical Characterization Lecai Wang†, Li Li†,‡, Xiaoxiao Zhang,† Feng Wu†,‡, Renjie Chen †,‡,* †

Beijing Key Laboratory of Environmental Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ‡

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China

Abstract: Compound-hierarchical-sphere structured LiNi0.5Co0.2Mn0.3O2 was synthesized to improve the electrochemical performance of this material in lithium-ion battery cathodes. The product was found to have a large specific surface area, good electron and ion conductivities, a stable interface and a robust nano-micro hierarchical structure, all of which improved the rate capability, capacity and cycling stability of this material. When cycled between 3.0 and 4.3 V, a high discharge capacity of 180.8 mA h g−1 was obtained at 0.2 C with 94.0% capacity retention after 100 cycles. In addition, a superior discharge capacity of 148.9 mA h g−1 was observed at a high current density of 1600 mA g−1. This compound-hierarchical-sphere LiNi0.5Co0.2Mn0.3O2 is readily prepared using our ternary coprecipitation method. We also propose an effector unit theory to explain the enhanced cycling stability of this substance and believe that the present results will assist in the design of cathode materials for lithium-ion batteries.

Keywords: Compound-hierarchical-sphere, Lithium-ion battery, Cathode, LiNi0.5Co0.2Mn0.3O2, Effector unit

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1. Introduction

Clean, high performance energy storage will play a key role in mitigating environmental concerns associated with present-day energy structures.1-4 Notably, lithium-ion batteries (LIBs) have already made it possible to reduce road CO2 emissions by replacing combustion engines with battery-powered electric motors. In fact, many modern electronic devices also depend heavily on progress in LIBs and other high-end energy storage technologies. Further improvements in LIB technology will require the exploration of high performance cathode materials.5-9 In recent years, the LiNixCoyMn1−x−yO2 family, characterized by high discharge capacities, moderate voltage platforms, low cost, and low environmental toxicity, have become of interest in research and applied studies.10-14 As with other members of this family, LiNi0.5Co0.2Mn0.3O2 adopts a rhombohedral α-NaFeO2 structure (R3m) with alternating [MO2]– (M = Ni, Co, Mn) and Li+ layers. Ni2+/Ni4+ and Co3+/Co4+ redox reactions contribute to Li+ transfer in this material, and so the discharge capacity of LiNi0.5Co0.2Mn0.3O2 is relatively high considering its increased proportion of Ni+. In addition, the Mn4+ proportion in this compound tends to stabilize the crystal structure while reducing cation dissolution and John–Teller distortion.15-16 Since the discovery of LiNi0.5Co0.2Mn0.3O2, chemists and material scientists have attempted to optimize its electrochemical performance both by altering the synthesis process to achieve the appropriate crystalline properties and by combining LiNi0.5Co0.2Mn0.3O2 with various cations or compounds.13, 17-22 In our opinion, the best strategy is to produce fine crystals in conjunction with nano/micro structural designs that promote electrochemical conversion, such as increased cation ACS Paragon Plus Environment

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ordering and appropriately enlarged surface area. In the present work, we developed a facile ternary coprecipitation method for the synthesis of compound-hierarchical-sphere LiNi0.5Co0.2Mn0.3O2 (h-LNCM). In this process, spherical micro particles are initially prepared via a co-precipitation process. These particles are porous and have a nano-micro hierarchical structure as a result of a contraction effect during the oxidative decomposition of the co-precipitate precursors. These structural features are expected to provide a large specific surface area, short lithium diffusion pathways, fast lithium transfer channels and good mechanical strength. In addition, a conductive compound layer is added to provide a stable interface and rapid electron transfer to the particle centers. This elaborate design is illustrated in Figure 1. An effector unit theory was also developed during this study to provide a better understanding of the design of electrode material structures for use in LIBs.

2. Material and methods

2.1 Materials preparation

NiCl2·6H2O (99.9%), CoCl2·6H2O (99.9%), MnCl2·4H2O (99.9%) and NaHCO3 (99.9%), were purchased from Macklin, Inc. (Shanghai, China) and LiNO3 (99.98 %) was purchased from Aladdin (Shanghai, China). A sample of Ni0.5Co0.2Mn0.3CO3 was synthesized by slowly adding 100 mL of an aqueous solution containing NiCl2·6H2O, CoCl2·6H2O and MnCl2·4H2O (c(Ni2+) = 1 M, c(Co2+) = 0.4 M, c(Mn2+) = 0.6 M) to 400 mL of a saturated NaHCO3 aqueous solution with continuous stirring while bubbling



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CO2 through the mixture. After 2 h of reaction at 0 °C, the product was removed by filtration, washed with ethanol several times to remove ions, and dried in air overnight. The Ni0.5Co0.2Mn0.3CO3 sample synthesized in this manner was subsequently calcined at 600 °C for 5 h, with a temperature increase rate of 5 °C min-1 to take advantage of volumetric shrinkage. The resulting transition element oxide was subsequently mixed with LiNO3 and calcined (using a procedure described in our previous study and changing the calcination temperature to 850 °C) to produce

hierarchical-sphere

LiNi0.5Co0.2Mn0.3O2.23

Finally,

the

hierarchical-sphere

LiNi0.5Co0.2Mn0.3O2 was mixed with glucose and sintered at 700 °C under a N2 atmosphere for 6 h at a heating rate of 3 °C min-1 to form h-LNCM. For comparison purposes, bulk LiNi0.5Co0.2Mn0.3O2 (b-LNCM) was prepared using a process described in the literature, changing the starting materials to chlorides and changing the calcination temperature to 850 °C.23

2.2 Characterization

Crystallographic analyses of the prepared samples were performed using X-ray diffraction (XRD; Ultima IV-185, Rigaku Corporation, Tokyo, Japan) with a Cu Kα radiation source. Scanning electron microscopy (SEM; Quanta 200f, EFI Technology Inc., California, USA), specific surface area and pore size analysis (ASIQM000-1-MP, Thermo Fisher Scientific Inc., Massachusetts, USA), transmission electron microscopy (TEM; JEM-2100f, JEOL Ltd., Tokyo, Japan) were used to characterize the morphology of the prepared samples. Elemental mapping was carried out with an energy dispersive X-ray (EDX) detector on the TEM equipment mentioned above. An X-ray photoelectron spectrometer (XPS; ESCALAB MKII, Thermo Fisher Scientific Inc., Massachusetts, ACS Paragon Plus Environment

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USA) was used to determine the ion valence states in the metal oxides.

2.3 Electrochemical tests

Electrochemical tests were performed in two-electrode coin cells (type: 2025) at room temperature, with lithium metal as both the reference electrode and counter electrode. The working electrode was composed of 80 wt% active material, 10 wt% conductive additive (acetylene black) and 10 wt% binder (polyvinylidene difluoride, PVDF), with an aluminum foil current collector. A lithium salt solution consisting of 1 M LiPF6 in a mixture of equal volumes of dimethyl carbonate (DMC), ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte. Cell assembly was performed in a glovebox filled with argon, with oxygen and water levels below 1 ppm. Galvanostatic charge/discharge data were obtained using a battery tester (LAND-CT2001A, Shenglan Inc. Wuhan, China) with a voltage window of 3.0 − 4.3 V at selected current rates (1 C = 160 mA g-1). Cyclic voltammetry (CV) tests were carried out with a EC-LAB SP-150 electrochemical workstation (Bio-Logic Science Instruments, Vaucanson, France) over the voltage range of 3.0 − 4.3 V. Electrochemical impedance spectroscopy (EIS) tests were also performed with this same instrument over the frequency range of 0.1 MHz to 0.01 Hz with an AC perturbation signal of 5 mV.

3. Results and Discussion

3.1 Morphology characterization

Scanning electron microscopy (SEM) images of the carbonate precursor and of typical a b-LNCM and h-LNCM particles are shown in Figure 2. As a result of the proper control of the synthesis, both



the carbonate precursor particles and the h-LNCM were almost perfect spheres with uniform diameters about 1.2 µm (Figure 2 (a-d)). In addition, nano-sized primary particles and pore-channel structures can be easily identified on the h-LNCM particles (Figure 2 (c, d)). In the case of the b-LNCM, the primary particles appear bulky and no pores are evident (Figure 2 (f)). Nitrogen adsorption/desorption data were acquired to assess the porosity of the h-LNCM, and the average Barrett-Joyner-Halenda (BJH) theory pore diameter and Brunauer–Emmett–Teller (BET) theory surface area of the sample was 9.2 nm and 78.5 m2 g-1, respectively.

3.2 Crystalline and compositional analysis

Figure 3 presents the XRD patterns generated by the h-LNCM and b-LNCM. In both cases, all diffraction peaks can be indexed to a hexagonal α-NaFeO2 structure with the R3m space group. The (006)/(102) and (108)/(110) splitting peaks are characteristic of a highly ordered hexagonal layered structure. In addition, the ratio of the (003) and (104) peak intensities (I(003)/I(104)) in the h-LNCM pattern (1.42) is higher than that in the b-LNCM pattern, indicating less cation mixing in the former material. The lattice parameters of the h-LNCM were determined to be a = 2.867 Å, c = 14.241 Å and c/a = 4.97, in good agreement with former studies.24-27 A high c/a value indicates well-ordered transition metal ions in the metal layer, along with a minimal degree of cation mixing. TEM images of the samples are provided in Figure 4. The alternating dark and light bands in the bright-field image of the h-LNCM particle (Figure 4 (a)) suggest that the material is porous, while the high-resolution image at the particle edge (Figure 4 (b)) clearly shows a crystal phase and an amorphous carbon layer, which indicates successful coating. In addition, the well-defined lattice fringes produced by the h-LNCM demonstrate good crystallization. The inter-planar distances ACS Paragon Plus Environment

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obtained from the diffraction pattern (Figure 4 (b, c), d003 = 4.73 Å, d101 = 2.45 Å) fully agree with the data obtained from XRD analyses (d003 = 4.73 Å, d101 = 2.45 Å). Figure 4 (d) summarizes the elemental proportions in the specimen, which are suitably close to those expected for LiNi0.5Co0.2Mn0.3O2. The elemental maps in Figure 4 (e) show that Ni, Co and Mn were distributed uniformly throughout the h-LNCM, which is characteristic of the ternary co-precipitation method. The valences of elements in the h-LNCM were determined by XPS, and Figure 5 summarizes the results, based on calibration using the C 1s binding energy value of 284.6 eV. The dominant peaks at 854.3, 780.0 and 642.8 eV are attributed to Ni2+, Co3+ and Mn4+, respectively. The shake-up satellite peak accompanying the Ni 2p3/2 peak (Figure 5 (a)) is usually assigned to the O (2p) → Ni (3d) charge-transfer transition, which is characteristic of nickel oxides. After deconvolution, a minor Ni 2p3/2 peak was identified at 864.1 eV, corresponding to Ni3+. In addition, the shake-up Co 2p3/2 peak at 780.1 eV (Figure 5(b)) can be unequivocally assigned to Co3+. The Mn 2p spectrum contains two main spin-orbit lines at approximately 641.9 eV (assigned to 2p3/2) and 653.5 eV (2p1/2) with a separation of 11.6 eV, indicated that Mn4+ is the majority species. However, the pair of less prominent peaks at about 636.9 and 644.7 eV also demonstrate the presence of Mn3+. According to a previous report, the presence of Ni3+ and Mn3+ can be ascribed to the charge transfer effect at the sample surface.28 In the O 1s spectrum (Figure 5 (d)), the 528.9, 529.5 and 531.4 eV peaks correspond to Mn-O, Co-O and Ni-O bonding, respectively.

3.3 Electrochemical characteristics

The cyclic voltammetry curve generated by the h-LNCM is very similar to that of b-LNCM, as shown in Figures 6 (a) and (b). Using a voltage range of 3.0 − 4.3 V and a scan rate of 0.1 mV s-1, ACS Paragon Plus Environment


both the h-LNCM and b-LNCM electrodes produced a pair of redox peaks in the potential range of 3.6 – 3.9 V. This result corresponds well with previously-reported studies concerning LiNi0.5Co0.2Mn0.3O2.18,

29-30

Ni2+/Ni4+ and Co3+/Co4+ redox reactions evidently take place during

cycling and are responsible for these current peaks. Ideally, Mn4+ would not undergo a valance change during the cycling of a LiNi0.5Co0.2Mn0.3O2 electrode to ensure the stability of the metal oxide lattice. Indeed, a peak Mn3+/Mn4+ peak does not appear at 3.2 V in the curves for either material, suggesting good cation ordering in both samples. Notably, the h-LNCM curves exhibit good repeatability, suggesting that this material has superior cycling stability. In addition, high peak intensities of h-LNCM might indicate better kinetics performance. Galvanostatic charge/discharge curves (Figures 6 (c) and (d)) are useful in assessing energy density, and the voltage changes of the present materials were found to be quite smooth between 3.7 to 4.3 V, which is characteristic of LiNi0.5Co0.2Mn0.3O2. This is also a very important property when considering applications in devices that require a stable power supply. The h-LNCM exhibits superior performance, as reflected in a higher capacity and low voltage decay, which guarantee long term high energy density. In a typical test, the discharge capacity values for the h-LNCM after the first, third and tenth cycles were 179.9, 179.2 and 178.5 mA h g-1, respectively, and so only 0.78% of the discharge capacity was lost after ten cycles. Except for the smaller discharge capacity (164.9, 164.2 and 160.7 mA h g-1 for the b-LNCM), the b-LNCM lost 2.55% of its discharge capacity after ten cycles. The EIS results (Figure 7 (a)) show that the h-LNCM produced a much smaller semicircle, indicating lower charge transfer impedance at the electrode/electrolyte interface. This reduced


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resistance of the h-LNCM suggests better kinetics performance and is attributed to the compound-hierarchical-sphere structure design. The large specific surface area, short Li+ diffusion pathways in the nano primary particles and electrolyte-flooded pores in this material allow rapid Li+ exchange between the h-LNCM and the electrolyte. The presence of conductive carbon on the h-LNCM surface enables fast electron access, even deep within the particle. In addition, polarization can be substantially reduced based on the rapid convergence of Li+ ions and electrons at the reaction sites. These designed structural advantages were also evident in the rate performance tests (Figure 7 (b)). Here, the h-LNCM delivered values of 180.8, 180.2, 178.3, 174.8, 165.6 and 148.9 mA h g-1, respectively, at 0.1, 0.2, 0.5, 1, 3 and 10 C. The capacity retention at 10 C was 82.4 %. The b-LNCM produced values of 165.6, 161.8, 159.4, 152.8, 132.9 and 97.3 mA h g-1, respectively, at 0.1, 0.2, 0.5, 1, 3 and 10 C, but with a capacity retention at 10 C of only 58.8 %. Thus, at higher discharge rates, the contrast between the h-LNCM and b-LNCM is more obvious. According to Table 1, high current performance of h-LNCM is also among the best compared to previous studies. The specific capacity of the h-LNCM was also higher than that of the b-LNCM, as shown in Figure 7 (c), in keeping with the earlier galvanostatic and rate performance trials. According to previous studies, this behavior can be attributed to pseudo-capacitive charging and additional active sites resulting from the size effect.31-32 These results suggest the possibility of a new class of high capacity electrode materials, and it is of particular interest that h-LNCM is more stable than its bulk counterpart during cycling. As shown in Figure 7 (c), the discharge capacity curve of the h-LNCM is much more horizontal and the capacity retention of this material was 94.0% after 100 cycles at 0.2 C,



whereas the b-LNCM lost 14.8% of its capacity after 100 cycles. The coulombic efficiency of the h-LNCM is also higher. However, the large specific surface area of this material should lead to more side reactions as well as greater capacity loss, thus there must be another mechanism responsible for this behavior that has yet to be discovered. Based on literatures and our own experiment, hierarchical particles tend to exhibit greater cycling stability than their bulk counterparts, even without surface modification.23, 33-38 Thus, we attempted to develop an effector unit theory to explain this phenomenon, as a means of providing guidance to future studies. We adopted two hypotheses, as summarized by Equations (1) and (2), below. − ⁄ = ⁄ + ⁄ (1) ⁄ ≥ 0 (2) Here, C is capacity, CD is capacity loss by crystalline decay, CS is capacity loss by side reactions and n is the number of cycles. In this proposed theory, crystalline decay in the material itself and side reactions at the solid-electrolyte interface are responsible for the loss of capacity in crystalline cathode materials such as LiNi0.5Co0.2Mn0.3O2. There is also a non-negative correlation between capacity loss caused by crystalline decay and the charge/discharge cycle number. Conceptually, we can constantly reduce the size of a particle until there is only one Li+ insertion/extrusion unit. Meanwhile, hypothesis (2) still applies. That is to say, there is a non-negative correlation between the functionality loss of a Li+ site (that is, an effector unit) and the number of insertion/extrusion events it undergoes. This conclusion is summarized by Equation (3), below. ⁄ ≥ 0 (3) Here, d is the functionality loss of a Li+ site in the crystal and nu is the number of


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insertion/extrusion events it undergoes. The capacity loss of a crystal cathode is therefore an accumulative process based on the decay of individual effector units. Figure 8 illustrates this idea with material morphology difference in consideration. In short, particles with shorter length of Li+ diffusion pathways could decay with lower rates. While cycling, nearby surface Li+ sites are used as transfer stations for the Li+ sites in the particle center. If the distance from the material surface to the center along one transportation channel is x Li+ site units, (1 + ) ∗ / 2 insertion/extrusion events would occur in a single charge/discharge cycle in this channel. As such, there is a quadratic relationship between ⁄ and the length of Li+ pathway. Meanwhile, surface area growth can be far slower than quadratic, if we don’t create separate small particles to achieve short Li+ pathways. This explains the improved cycling stability of hierarchical materials such as h-LNCM quite well. Materials with short Li+ diffusion pathways, including h-LNCM, would simply experience less wear than their bulk counterparts during the same charge/discharge operation. Although side reactions would be more frequent at the surface in the case of a larger specific surface area (likely with a linear correlation), this change would be less than the quadratic crystalline decay. In addition, surface side reactions could be mitigated by surface modification, thus further decreasing the value of − ⁄. This effector unit theory also suggests that crystal structure decay of a solid particle would be more severe near the surface area than at the particle center. This inference will be investigated further in our laboratory in the future, to develop a better understanding of the battery failure mechanism and to allow innovation in the development of cathode materials.



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4. Conclusions

We

successfully

designed

a

facile

ternary

coprecipitation

method

to

synthesize

compound-hierarchical-sphere LiNi0.5Co0.2Mn0.3O2 as a cathode material. The product shows a high capacity, excellent cycling performance and good rate capability in comparison with the bulk counterpart. These superior properties are attributed to the large specific surface area, good electron and ion conductivities, stable interface, uniform ion distribution and robust nano-micro hierarchical structure. The enhanced cycling stabilities of structured cathode materials such as this are explained well by our effector unit theory, which suggests a positive correlation between the total insertion/extrusion times of Li+ sites and capacity loss. Materials with short Li+ ion diffusion pathways simply experience less wear than their bulk counterparts during charge/discharge operations. The compound-hierarchical-sphere cathode structure design and effector unit theory introduced herein are expected to represent helpful strategies that will promote innovations in the development of lithium-ion battery cathode materials.

Acknowledgement The experimental work of this study was supported by National Key R&D Program of China (2017YFB0102104), the Joint Funds of the National Natural Science Foundation of China (U1564206), the National Natural Science Foundation of China (51772030) and the Major Achievements Transformation Project for Central University in Beijing.


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Figures and Tables

Figure 1. Demonstration of the compound-hierarchical-sphere LiNi0.5Co0.2Mn0.3O2 design



Figure 2. SEM and BET characterization of as synthesized samples. High (a) and low (b) resolution SEM images of the carbonate precursor of h-LNCM; high (c) and low (d) resolution SEM images of h-LNCM particles; (e) Nitrogen adsorption/desorption isotherms and BJH pore size distribution of h-LNCM; (f) SEM image of a typical h-LNCM particle.


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Figure 3. XRD patterns of as-synthesized h-LNCM and b-LNCM.



Figure 4. TEM and EDX test for h-LNCM. (a) A bright-field image of a single h-LNCM particle; (b) A high resolution image at the particle surface of h-LNCM; (c) Diffraction pattern of the LiNi0.5Co0.2Mn0.3O2 phase; (d) EDX analysis of h-LNCM; (e) Element mapping of a h-LNCM particle.


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Figure 5. XPS spectra of (a) Ni 2p3/2, (b) Co 2p3/2, (c) Mn 2p3/2, and (d) O 1s for h-LNCM.



Figure 6. Electrochemical behavior of h-LNCM and b-LNCM. Cyclic voltammetry (CV) curves of h-LNCM (a) and b-LNCM (b); Voltage-capacity curves of h-LNCM (c) and b-LNCM (d).


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Figure 7. Electrochemical performances of h-LNCM and b-LNCM. (a) Electrochemical impedance spectroscopy (EIS) test in the frequency range of 0.1 MHz to 0.01 Hz with an AC perturbation signal of 5 mV; (b) Rate performance test at 0.1 C, 0.2 C, 0.5 C, 1 C, 3 C, and 10 C; (c) Cycle stability test at a low rate of 0.2 C.



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Table 1. A brief comparison of LiNi0.5Co0.2Mn0.3O2 samples in previous articles and h-LNCM.

LiNi0.5Co0.2Mn0.3O2 Structure Cathode

Current Capacity Capacity Operating Reference Rate at the Retention Voltage (mA/g) Given Compared (V) Current to 1/10 of Rate the Given (mAh/g) Rate (%) 1600 148.9 85.18 3.0 – 4.3 This Work

h-LNCM

Compound Hierarchical Sphere

T-NCM

Hollow Microsphere

1000

148.5

85.79

3.0 – 4.3

(39)

3 wt.% LMP

LiMnPO4 Coated Sphere

1000

124.4

76.51

2.8 – 4.4

(26)

Al-NCM

Al Coated Sphere

1000

126.9

79.15

3.0 – 4.3

(40)

NCM523-pristine

Bulk

1000

～136

～81.93

2.0 – 4.3

(41)

Nb1

Nb Doped Bulk

1000

125.5

77.46

3.0 – 4.3

(42)

N5@(0.5+0.5)% LT

Li2TiO3 Coated Bulk

800

130.0

78.41

2.8 – 4.3

(43)


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Figure 8. Demonstration of the effector unit theory. Particles with shorter Li+ diffusion path experience less torment than their bulk counterparts during charge/discharge operation.



TOC


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Compound-Hierarchical-Sphere LiNi0.5Co0.2Mn0.3O2: Synthesis

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