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Preparation and rate capability of carbon coated LiNi1/3Co1/3Mn1/3O2 as cathode material in lithium ion batteries Chaofan Yang, Xiaosong Zhang, Mengyi Huang, Junjie Huang, and Zebo Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16741 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017

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ACS Applied Materials & Interfaces

Preparation and rate capability of carbon coated LiNi1/3Co1/3Mn1/3O2 as cathode material in lithium ion batteries Chaofan Yanga, Xiaosong Zhanga, Mengyi Huanga, Junjie Huang*a, Zebo Fangc a: College of Chemistry & Chemical Engineering, Shaoxing University, Shaoxing 312000, PR China c: Mathematic Information College, Shaoxing University, Shaoxing 312000, PR China ∗Corresponding author. Tel.: +86 575 88342606; fax: +86 575 88342606. E-mail address: [email protected] (J. Huang). Keywords: LiNi1/3Co1/3Mn1/3O2, active carbon, coating, cathode, lithium ion battery. Abstract LiNi1/3Co1/3Mn1/3O2 (NCM) is regarded as a promising material for next-generation lithium ion batteries due to the high capacity, but its practical applications are limited by the poor electronic conductivity. Here, a one-step method is used to prepare carbon coated LiNi1/3Co1/3Mn1/3O2 (NCM/C) by applying active carbon as reaction matrix. TEM shows

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LiNi1/3Co1/3Mn1/3O2 particles are homogeneously coated by carbon with a thickness about 10 nm. NCM/C delivers the discharge capacity of 191.2 mAh g-1 at 0.5 C (85 mA g-1) with a columbic efficiency of 91.1%. At 40 C (6800 mA g-1), the discharge capacity of NCM/C is 54.6 mAh g-1, while NCM prepared through sol-gel route only delivers 13.2 mAh g-1. After 100 charge and discharge cycles at 1 C (170 mA g-1) the capacity retention is 90.3% for NCM/C, while it is only 72.4% for NCM. The superior charge/discharge performance of NCM/C owes much to the carbon coating layer, which is not only helpful to increase the electronic conductivity, but also contributive to inhibit the side reactions between LiNi1/3Co1/3Mn1/3O2 and the liquid electrolyte. 1. Introduction The rapid development of electric vehicles and renewable energy storage brings a growing demand for lithium ion batteries (LIBs) with high power performance and high energy density 12

. To meet the high requirements, electrode materials with high capacities are needed. Among the

cathode materials 3-5, LiNi(1-x-y)CoxMnyO2 have been regarded as the promising cathode materials used in the next-generation LIBs due to their high specific capacities

6-9

. In LiNi(1-x-y)CoxMnyO2

group, LiNi1/3Co1/3Mn1/3O2 (NCM) has been hotly studied due to the superior charge/discharge properties. It is known that NCM is almost zero volume or phase change when cycled among 2.50-4.40 V because Mn4+ does not conduct the redox reaction, which helps to stabilize the crystal structure

10-14

. However, the electronic conductivity of LiNi1/3Co1/3Mn1/3O2 is poor

15

,

which results in a bad rate capability, limiting its practical applications. With the aim to increase the electronic conductivity of LiNi1/3Co1/3Mn1/3O2, the attempts of the addition of dopants

16-18

and surface decoration by coating with various materials like metal


2

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oxides (Al2O3

19-20

, Cr2O3

21

, Nb2O5

22

, ZrO2

23

), and metal phosphate (AlPO4

24

) have been

directed. Carbon is excellent in electronic conductivity, which is routinely used to coat electrode materials like Li4Ti5O12 25, LiFePO4 26-27 and silicon 28. It is proved that the carbon coating layer with an appropriate thickness is very useful in increasing the conductivity and inhibiting the particles from reacting with the electrolyte, resulting in a great enhancement in electrochemical performance, especially for high-rate capability 25-28. Normally, NCM is synthesized under air or oxygen atmosphere at high temperature (> 800 ºC). At such high calcination temperature, organic compound is likely to oxidize to CO in air/oxygen atmosphere during the synthesizing period, so it is not easy to form carbon on NCM surface. With the aim to coat carbon on NCM surface, a two-step method has been designed by using chemical vapor deposition (VCD) pyrolysis of adsorbed organic compounds

30

29

or

, in which the first step involves the synthesis of

NCM, the second step is to coat carbon on the particles at the high temperature. For simplifying the process, N. Sinha designed a one-step method to synthesize NCM/C by an inverse microemulsion route

31

, but the complex procedure makes this method difficult to prepare carbon-

coated NCM in large scale. In the present study, a facile way is introduced to prepare carbon-coated LiNi1/3Co1/3Mn1/3O2 (NCM/C) by using porous active carbon as reaction matrix. In this method active carbon is used as absorbent to absorb the solution containing the ions of Li+ and [M(NH3)x]2+ (M: Co, Ni and Mn), so the pores in active carbon are helpful to control the sizes of NCM particles, making them in nano scale. Finally, part of carbon possibly leaves on NCM particles’ surfaces to form a uniform coating layer. NCM/C presents an excellent high-rate capability; at 40 C it can deliver a discharge capacity of 54.6 mAh g-1 while that of NCM is only


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13.2 mAh g-1. Here, the fabrication and electrochemical behavior of NCM/C are detailly discussed. 2. Experimental 2.1 Materials synthesis Preparation of NCM/C: The compounds of LiAc·2H2O, Ni(Ac)2·2H2O, Co(Ac)2·2H2O and Mn(Ac)2·2H2O with stoichiometric amounts were dissolved in concentrated ammonium hydroxide, while the ions were in the form of [M(NH3)x]2+ (M=Ni, Co and Mn). Then the active carbon was used to absorb the obtained solution, as the way we used in our previous work

32

.

Here, the weight ratio of active carbon and the calculated amounts of LiNi1/3Co1/3Mn1/3O2 is 2:1. After then, the precursor was sintered to 450 ºC for 2 hours with a ramp of 5 ºC min-1, and 850 ºC for another 10 hours in air. Finally, NCM/C was obtained. NCM synthesized through a sol-gel route was used as a reference. The compounds of LiAc·2H2O, Ni(Ac)2·2H2O, Co(Ac)2·2H2O and Mn(Ac)2·2H2O with stoichiometric amounts were dissolved in the distilled water, forming a solution. The mixed solution of citric acid and ethylene glycol was dropped in with the rate of 1 d s-1, and the pH value was kept at ~7.0 by NH3 H2O solution. After stirring at 80 ºC for 10 hours by water bath, the sol was dried and turned into viscous transparent gel. The gel was heat-treated at 450 ºC for 2 hours and 850 ºC for 10 hours in air. Finally, NCM was obtained. All the reagents used were obtained from Sino pharm Group Company. 2.2 Materials characterization


4

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The crystal structures of NCM and NCM/C were characterized by using X-ray diffract meter (Bruker D8, Cu Kα) in the 2θ range of 10º-80º. The data was recorded at a step width of 0.026º and a scan rate of 0.58º min-1. The morphologies were analyzed by TEM (transmission electron microscopy) (JEOL JEM-1011) and the elemental distributions were characterized by EDS (energy dispersive X-ray spectrometry) (JEOL JSM-6360LV). The contents of Li, Ni, Co and Mn were also tested with inductively coupled plasma-atomic (ICP) equipment (Prodigy xp, American Leeman Company). The content of Carbon was analyzed with EA (element analysis equipment) (EA3000, Italy Euro Vector Company). Electrochemical performances were measured by using a coin-cell, which is composed of a metallic lithium anode and a cathode separated by a polyethylene ﬁlm. The cathode was fabricated by mixing the active material with PTFE and conductive carbon in a weight ratio of 80:5:15. The mixture was pressed onto aluminum foil (20 µm in thickness). Finally, the electrode was dried under vacuum at 100 ºC for 10 hours. The electrode has the area of about 1 cm2 with the active material loading of about 5 mg. The coin-cell was assembled in the glove box ﬁlled with argon, and the electrolyte solution is 1 M LiPF6-EC/DMC (1:1, v/v) (BASF, America). The charge and discharge performance was carried out galvanostatically at different current densities on electrochemical equipment (Wuhan Land electro-chemical equipment company, China, Land 2001A), and the cut-off voltages are 4.50 and 2.50 V respectively. Electrochemical impedance spectroscopy was conducted on the electrochemical equipment (Germany, Zahner Ennium). All the tests were conducted under 30 ºC. 3. Results and Discussions


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Fig. 1a presents the XRD spectra of NCM and NCM/C, and the crystal parameters are listed in Table 1. All the peaks in NCM and NCM/C can be indexed to α-NaFeO2 structure 33-35, and no impurity phases can be found in XRD spectra. The well resolved planes of 006/102 and 018/110 indicate NCM and NCM/C possess a typical hexagonal layered structure (R=(I(006)+I(012))/I(101) is inversely proportional to the hexagonal ordering

11

36-37

. R-factor

, the calculated

values of NCM and NCM/C are 0.48 and 0.46 respectively, which infers NCM/C has a higher hexagonal ordering. From Table 1, the ratio of c/a is 4.9771 for NCM and 4.9777 for NCM/C, which means the well-defined layered structure for both NCM and NCM/C 33,38. The crystal structure of NCM/C and NCM are analyzed by Rietveld refinements, and the curves and lattice parameters are presented in Fig 1b-d and Table 1, respectively. The lattice parameter a corresponds to the average intra-slab distance of metal-metal (M-M), while c is associated with the average M-M inter-slab distance. Since the radius of Li+ is bigger than that of Ni2+, Li+ ions on 3a sites lead to the increase of the planar parameter a

39

. From Table 1,

parameter a for NCM and NCM/C are 2.8653±0.00004 Å and 2.8635±0.00009 Å respectively, indicating NCM/C with a lower Li+/Ni2+ mixing. Furthermore, the ratio of I(003) and I(104) peaks is also applied to judge the cation mixing degree in LiNi1/3Co1/3Mn1/3O2 40-41, and the value of 1.2 is supposed to be the boundary of cation disordering, and the higher value means better ordering for the cations. The I(003)/I(104) ratios of NCM and NCM/C are 1.4822 and 1.6838 respectively, which also means the better cation ordering of NCM/C. The refined values of Ni in Li site of NCM and NCM/C are 0.0306 and 0.0284 respectively, indicating the better Ni/Li ordering in NCM/C. The cation mixing greatly influences to the electrochemical performance, especially for the capacity and cycling stability, therefore the lower cation mixing in NCM/C infers a better electrochemical performance 11,42.


6

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Table 1 Refined Lattice parameters of NCM and NCM/C c(Å)

c/a

I(003)/I(104)

Ni in Li site

Rp(%)

Rwp(%)

14.2608 2.8653

4.9771

1.4822

0.0306

3.02

5.68

NCM/C 14.2542 2.8635

4.9777

1.6838

0.0284

2.46

5.72

NCM

a(Å)

Fig. 1 XRD spectra of NCM and NCM/C (a); Rievelt refinement XRD spectra of NCM and NCM/C (b-c); the schematic micro-structure of NCM/C (d). Fig. 2 shows the SEM and TEM images of NCM and NCM/C. As shown in the SEM images, NCM and NCM/C display the similar morphology with irregular particle shape, and their particle sizes are very small. From TEM images, almost every particle in NCM can be


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identified clearly, and the particle sizes range from 50 nm to 200 nm. Different from that, it looks like the particles aggregating together in NCM/C, which is because they are packaged by a layer of carbon as proved by EDS spectra in Fig. 4. The outline of the particles in NCM/C can be distinguished; the distribution of the particle’s size is homogeneous with the diameter of about 150 nm. We speculate the formation of NCM/C should be related to the active carbon, the synthesis process is illustrated in Fig. 3. It is well known that many pores exist in active carbon, these pores are able to absorb the solution containing the ions of Li+ and [M(NH3)x]2+ and these positive ions can interact with the organic groups of -OH and -COO- on carbon by electrostatic interaction. When the solvent evaporated, the compounds related to Li+, Ni2+, Co2+ and Mn2+ deposited on the pores’ walls. Active carbon can be stable even at the temperature above 500 oC, in such state the compounds related to Li+, Ni2+, Co2+ and Mn2+ are changed to metal oxides, leading to inorganic oxides coated carbon composite. Finally, a part of carbon left in LiNi1/3Co1/3Mn1/3O2 after calcination step, resulting in carbon coated LiNi1/3Co1/3Mn1/3O2 material. Of course, the calcination temperature and annealing time are two important factors for the formation of carbon layer. For example, when the calcination time extends much longer, carbon will react with oxygen in air and disappear 43. The influence of calcination temperature on carbon forming was also conducted, and the obtained samples were observed by TEM as shown in Figure S.1. TEM images show the average thickness of carbon layer become thinner from 16 nm to 10 nm as the calcination temperature increasing from 800 oC to 850 oC, moreover, when increasing the temperature to 900 oC, the carbon coating layer disappeared. It is well known that the calcination temperature greatly influenced the crystallinity of LiNi1/3Co1/3Mn1/3O2, commonly

the

higher

calcination

temperature

implies

the

better

crystallinity

of

LiNi1/3Co1/3Mn1/3O2. Based on the crystallinity of NCM and the carbon forming information, we


8

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selected NCM/C calcinated at 850 oC to study the improved charge and discharge performance in this study.

Fig. 2 SEM images of NCM (a) and NCM/C (b); TEM images of NCM (c) and NCM/C (d)


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Fig. 3 Schematic illustration of the synthesis process of NCM/C Fig. 4 shows the EDS spectra and mapping of NCM and NCM/C. It can be found the elements of Co, Mn and Ni are homogenously distributed in NCM and NCM/C. Carbon is only detected in NCM/C while not in NCM, which is mainly caused by the different thermal behaviors of ethylene glycol, citric acid and active carbon. In air atmosphere, when the temperature is higher than 200 oC ethylene glycol will evaporate and citric acid will be decomposed, but active carbon can keep stable even at the temperature higher than 500 oC. From the EDS mapping of NCM/C in Fig. 4, only carbon is detected in the coating layer. Table 2 shows the atomic ratio of Co, Mn, Ni and Li analyzed by ICP-AES, the atomic ratio in the two samples are all equivalent to the expected values. The carbon content in NCM/C is about 4.10% (wt%).


10

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Fig. 4 EDS spectra and mapping of NCM and NCM/C Table 2 Stoichiometric molar composition of NCM and NCM/C measured by ICP-AES Expected Li/ Co / Ni /Mn Coefficient

Measured Li/ Co / Ni /Mn Coefficient

NCM

1.0/0.33/0.33/0.33

1.01/0.32/0.35/0.32

NCM/C

1.0/0.33/0.33/0.33

1.03/0.33/0.32/0.34

Fig. 5 displays the initial charge/discharge performances of NCM and NCM/C recorded at 0.5 C (85 mA g-1). The curves correspond to the typical charge/discharge behavior of layered LiNi1/3Co1/3Mn1/3O2, the charge capacity about 80 mAh g-1 in 3.75-3.90 V relates to the redox couple of Ni2+/Ni3+; the subsequent voltage ranges of 3.90-4.10 V and 4.10-4.50 V correspond to the redox couples of Ni3+/Ni4+ and Co3+/Co4+ respectively, which is agreeable with the previous


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reports by K.M. Shaju 44 and T. Ohzuku 13. The initial charge capacities of NCM and NCM/C are 213.6 and 209.9 mAh g-1 at 0.5 C respectively, and the initial discharge capacities of NCM and NCM/C are 175.7 and 191.2 mAh g-1. The higher specific capacity of NCM/C is attributed to the lower cation mixing and the improved electronic conductivity by carbon coating. The initial columbic efficiency of NCM is 82.3% while 91.1% for NCM/C as listed in Table 3. The initial capacity loss was caused by SEI forming and electrolyte decomposing. The higher initial columbic efficiency of NCM/C is due to the carbon coating, which inhibits the side reactions between LiNi1/3Co1/3Mn1/3O2 and liquid electrolyte, leading to a reduction in the irreversible charge capacity. The capacity of NCM/C is higher than that of carbon coated LiNi1/3Co1/3Mn1/3O2 prepared by the two-step method in literatures 45-47, which is possibly due to carbon coating was fabricated in the process of NCM fabrication, leading to the better contacting between carbon and NCM, therefore the electronic conductivity was improved greatly.

Fig. 5 Initial charge/discharge performances of NCM and NCM/C, current density: 85 mA g-1 Table 3 Initial charge and discharge capacities and columbic efficiencies of NCM and NCM/C Initial charge capacity (mAh g-1)

Initial discharge capacity (mAh g-1)

Columbic Efficiency (%)


12

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NCM

213.6

175.7

82.3

NCM/C

209.9

191.2

91.1

Fig. 6 shows the rate performance of NCM and NCM/C conducted from 0.5 C (85 mA g-1) to 40 C (6800 mA g-1). As increasing the charge and discharge rates, the specific capacities decrease regularly. At 0.5, 1, 2, 5, 10, 20 and 40 C, the discharge capacities of NCM are 175.7, 154.8, 143.5, 101.7, 71.5, 44.5 and 13.2 mAh g-1 respectively, but that of NCM/C are 191.2, 178.9, 163.5, 135.8, 109.4, 81.9 and 54.6 mAh g-1 respectively. NCM/C exhibits better rate performance than NCM, which is mainly due to the coating carbon with excellent electronic conductivity, resulting in lower electrochemical impedance. The electrochemical impedance of NCM and NCM/C was analyzed by EIS, which was conducted after the 1st cycle (0.5 C) at 30 ºC.

Fig. 6 Rate capabilities of NCM and NCM/C from 0.5 C to 40 C (2.50-4.50 V). The Nyquist plots are fitted and presented in Fig. 7a, in the medium frequency region a semicircle can be found, which is related to the charge-transfer resistance of the electrode. A line at low frequency region with about 45o to the real axis is assigned to the Li+ diffusion within the


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electrode

32

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. In the equivalent circuits inserted in Fig. 7c, Re represents the uncompensated

ohmic resistance, which includes the ohmic resistance in NCM or NCM/C materials and the tab of cathode/anode. Rs1 represents the diffusion resistance of Li+ in the surface layer (including SEI film). Rs2 represents the resistance of Li+ diffusion in carbon coating layer, Rct refers to the charge-transfer resistance. CPEs1 and CPEs2 refer to the nonideal capacitance of the carbon coating layer and the surface layer, and CPEct is assigned to charge transfer capacitance, Zw refers to the Warburg impedance, which represents the diffusion behavior of Li+ in the bulk

48

.

The relationship of Z’ (the real part impedance) and ω-1/2 (ω: the angular frequency, ω=2πf) in the low frequency region are also displayed in Fig. 7b. The Warburg impedance coefficients (σ) and DLi+ (lithium diffusion coefficients) were evaluated according to Eq.1 and Eq.2

32, 48

. The

simulated values after the 1st cycle are shown in Table 4; the resistances of NCM/C are lower than that of NCM, implying a better high-rate performance in NCM/C than NCM. Moreover, the lower value of Rs1 for NCM/C (65.728 Ω) indicates a smaller electrochemical impedance in SEI of NCM/C, which is similar to the reported results of graphite after carbon coating

49-50

. The

lithium diffusion coefficients (DLi+) are 1.232×10-14 cm2 s-1 for NCM and 1.513×10-14 cm2 s-1 for NCM/C. NCM/C also has a better high-rate performance compared to the literature results as listed in Table 5, which is due to the coated carbon with excellent electron conductivity. Z′ = Rs + Rct + σ߱ ିଵ/ଶ ୖ మ ୘మ

‫ܦ‬௅௜ = ଶ୅మ ୬ర ୊ర େమ σమ

Eq. 1 Eq. 2


14

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Fig. 7 Fitted Nyquist plots (a), relationships between Z’ and ω-1/2 at low frequency region (b) and corresponding equivalent circuits (c) of NCM and NCM/C Table 4 EIS parameters of NCM and NCM/C after the first cycle Re(Ω)

Rs1(Ω)

Rs2(Ω)

Rct(Ω)

σ(Ω cm2) DLi+(cm2 s-1)

NCM

11.213

98.436

/

100.602

450.415

1.232×10-14

NCM/C

9.686

65.728

6.621

73.124

406.543

1.513×10-14

Table 5 Rate capability of the reported LiNi1/3Co1/3Mn1/3O2 by surface modification

Coating layer

Carbon (this study) Cr2O3 21

Discharge Capacity

Discharge Capacity

(mAh g-1) at 0.5 C

(mAh g-1) at high rate

2.5-4.5

191.2

54.6 (40 C)

2.8-4.5

196 (0.1 C)

/

Voltage range (V)


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ZrO2 23

3.0-4.5

~180

160 (1 C)

AlPO4 24

2.7-4.3

135

/

Carbon 29

2.0-4.3

141

118 (2 C)

Carbon 30

2.8-4.3

153.7 (0.2 C)

134.3 (5 C)

Carbon 31

2.5-4.3

158 (1/7 C)

125 (~1 C)

Nb2O5 50

2.8-4.6

~180

~5 (10 C)

Sb2O3 51

3.0-4.6

~168

~134 (4 C)

LiF 52

2.5-4.5

179

137 (10 C)

Al2O3 53

2.75-4.5

176

100 (6 C)

SrF2 54

2.5-4.6

~178

/

ZrFx 55

3.0-4.6

~177 (1 C)

~50 (12 C)

FePO4 56

2.8-4.5

165

127 (5 C)

Phosphate 57

2.5-4.5

~160

90 (5 C)

Fig. 8 Cycle performances of NCM and NCM/C at 1 C (2.50-4.50 V)


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The cycling stabilities of NCM and NCM/C were investigated at 1 C (170 mAh g-1), as shown in Fig. 8. At 100 cycles, NCM has a discharge capacity of 113 mAh g-1, and the capacity retention is 72.4%. Meanwhile, the discharge capacity and the capacity retention of NCM/C are 162.7 mAh g-1 and 90.3% respectively. This indicates NCM/C has a superior cycling stability than NCM. The improved cycling performance of NCM/C is mainly ascribed to the carbon coating layer, which can inhibit the side reaction between LiNi1/3Co1/3Mn1/3O2 and liquid electrolyte

47, 58

. Although the coating of carbon brings a great improvement for the

electrochemical performance of LiNi1/3Co1/3Mn1/3O2, the reason for carbon forming is not clear and needs a further study. 4. Conclusion NCM/C has been synthesized through a facile way by using active carbon as the absorbent to absorb the solution containing the ions of Li+ and [M(NH3)x]2+ (M: Co, Ni and Mn). This cathode material presents a high specific capacity, an excellent cycling stability and high-rate capability, which is mainly due to the carbon coating. Carbon has the advantage in electronic conductivity, which helps to enhance the high-rate performance and as a coat also benefits to the reduction of the side reactions between LiNi1/3Co1/3Mn1/3O2 and liquid electrolyte. This method is easy and low cost for the production of carbon coated LiNi1/3Co1/3Mn1/3O2 in large scale, and is also suitable to synthesize other cathode materials. Acknowledgements Financial supports from the program of the National Natural Science Foundation of China ( 51272159), the Natural Science Foundation of Zhejiang province (LY15A040001),

Zhejiang


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Figure captions: Fig. 1 XRD spectra of NCM and NCM/C (a); Rievelt refinement XRD spectra of NCM and NCM/C (b-c); the schematic micro-structure of NCM/C (d). Fig. 2 SEM images of NCM (a) and NCM/C (b); TEM images of NCM (c) and NCM/C (d). Fig. 3 Schematic illustration of the synthesis process of NCM/C. Fig. 4 EDS spectra and mapping of NCM and NCM/C. Fig. 5 Initial charge/discharge performances of NCM and NCM/C, current density: 85 mA g-1. Fig. 6 Rate capabilities of NCM and NCM/C from 0.5 C to 40 C (2.50-4.50 V). Fig. 7 Fitted Nyquist plots (a), relationships between Z’ and ω-1/2 at low frequency region (b) and corresponding equivalent circuits (c) of NCM and NCM/C. Fig. 8 Cycle performances of NCM and NCM/C at 1 C (2.50-4.50 V).


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