Investigation the electrochemical performance of layered cathode material Li1.2Ni0.2Mn0.6O2 coated with Li4Ti5O12

Advanced Powder Technology xxx (2016) xxx–xxx

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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Investigation the electrochemical performance of layered cathode material Li1.2Ni0.2Mn0.6O2 coated with Li4Ti5O12 Yunjian Liu a,b, Qiliang Wang a, Zhiqiang Zhang a, Aichun Dou a, Jun Pan b, Mingru Su a,b,⇑ a b

School of Material Science and Technology, Jiangsu University, Zhenjiang 212013, China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

a r t i c l e

i n f o

Article history: Received 6 August 2015 Received in revised form 11 January 2016 Accepted 11 May 2016 Available online xxxx Keywords: Li-rich layered cathode Coating Electrochemical performance Voltage decay

a b s t r a c t Layered cathode material Li1.2Ni0.2Mn0.6O2 has been synthesized and coated with different content of Li4Ti5O12 (1, 3, 5 wt%) by a sol–gel method. The effect of Li4Ti5O12 coating on the physical and electrochemical properties of Li1.2Ni0.2Mn0.6O2 material has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS), cycling and rate performance tests. The XRD, SEM and TEM results show that the Li1.2Ni0.2Mn0.6O2 has been coated with the Li4Ti5O12 particles effectively. The electrochemical performance results indicate that the 3 wt% Li4Ti5O12-coated sample has the best electrochemical performance, showing initial discharge capacity of 258.5 mA h g 1 at 0.1 C, capacity retention of 98.7% after 50 cycles at 0.2 C, and a high discharge capacity of 110.8 mA h g 1 at 10 C. The voltage decay of Li1.2Ni0.2Mn0.6O2 is mitigated remarkably after 3 wt% Li4Ti5O12 coating. EIS results show that the Rct of Li1.2Ni0.2Mn0.6O2 electrode decreases after coating, which is responsible for superior rate capability. As a result, the surface coating of Li1.2Ni0.2Mn0.6O2 with Li4Ti5O12 is a beneficial way to improve the electrochemical performances of Li1.2Ni0.2Mn0.6O2, which is due to Li4Ti5O12 layer acts as a relatively stable protective barrier as well as an excellent lithium-ion conductor. Ó 2016 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan.

1. Introduction In recent years, lithium-rich layered oxide materials xLi2MnO3
(1 x)LiMO2 (M = Mn, Ni, Co, Cr, etc.) have been deeply studied as a promising material for lithium-ion battery cathodes because of their ability to deliver capacities of 250 mA h g 1 or more between 2.0 V and 4.8 V, a lower cost and better safety than LiCoO2 cathode [1–4]. However, there are still many issues for the further application of these materials, such as its low initial coulomb efficiency, poor rate capability, which still need to be improved to meet the ever-increasing demand of high-power and high capacity for hybrid electric vehicles [5]. Surface coating has been proved to be an effective method to improve the electrochemical properties of cathode materials. It is reported that many metal oxides and metal phosphate have been coated on the surface of layered solid solution materials to improve the electrochemical properties, such as Al2O3 [6], AlPO4 [7], ZrO2 [8], AlF3 [9,10], TiO2 [11], ZnO [12] and MnO2 [13]. However, due to the low lithium-ion conductivity, these coating materials are ⇑ Corresponding author at: School of Material Science and Technology, Jiangsu University, Zhenjiang 212013, China. Tel.: +86 511 88790190. E-mail address: [email protected] (M. Su).

generally unfavorable for both lithium-ion conduction of cathode materials and interfacial charge transfer of the electrode [14]. It is hard to improve the rate capability of cathode material adequately. Hence, the coating material needs high lithium-ion conductivity. Recently, coating of lithium-ion conductive oxides has been reported as an effective modification technique, such as LiAlO2 [15,16] and Li2ZrO3 [17]. Li4Ti5O12 has been reported as a kind of fast lithium-ion conductor because of its high mobility of lithium ions in the structure. In addition, it is also a zero-strain material, which means that there is no structural change during the insertion/extraction process of lithium-ion [18,19]. It exhibits excellent lithium-ion insertion/ extraction reversibility and flat discharge–charge plateau and high cycle stability during charge–discharge cycling. Moreover, Li4Ti5O12 may not form a solid electrolyte interface (SEI) that protects against electrolyte reactivity when fully charged [20–23]. Based on the features mentioned above, it is concluded that Li4Ti5O12 is suitable for the surface coating of layered solid solution materials. In this paper, Li1.2Ni0.2Mn0.6O2 has been coated with Li4Ti5O12 using sol–gel method with different contents (1, 3, 5 wt%) successfully. The influence of the Li4Ti5O12 coating layer on the physical and electrochemical performance of the Li1.2Ni0.2Mn0.6O2 material has been investigated in detail.

http://dx.doi.org/10.1016/j.apt.2016.05.008 0921-8831/Ó 2016 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan.

Please cite this article in press as: Y. Liu et al., Investigation the electrochemical performance of layered cathode material Li1.2Ni0.2Mn0.6O2 coated with Li4Ti5O12, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.05.008

3. Results and discussion Fig. 1 shows the XRD patterns of the pristine and Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 materials. As shown in Fig. 1, all of the samples display nearly the same XRD patterns, exhibiting layered characteristics. And all peaks can be indexed on the basis of a-NaFeO2 structure. In the modified samples, no peak for Li4Ti5O12 phase is detected, which may be attributed to the low content and poor crystallinity of Li4Ti5O12. Features located in the 20–25° regions for all samples indicate the presence of Li2MnO3-type C2/ m phase [24,25]. The lattice parameters of these samples are calculated and summarized in Table 1. The c/a ratios of all samples are greater than 4.99 which imply a well layered structure. Compared with pristine one, the lattice parameters of Li4Ti5O12-coated samples increase slightly. That may be because Ti4+ ions enter the crystal lattice of Li1.2Ni0.2Mn0.6O2 through the interaction of the coated

(018) (110) (013)

(107)

Indensity (a.u.)

The layered oxide solid solutions were all synthesized by coprecipitation method as following. Required amounts of the transition metal acetates were dissolved in deionized water and then added drop by drop into a 0.1 M NaOH solution to form the coprecipitated hydroxides of Ni and Mn, and dried overnight at 110 °C in an air-oven to get the material Ni0.25Mn0.75(OH)2. Then a certain amount of Ni0.25Mn0.75(OH)2 was mixed with a required amount of lithium carbonate, heated in air at 500 °C for 5 h and 850 °C for 12 h followed by furnace cooling, get the final material Li1.2Ni0.2Mn0.6O2 (indicated as ‘a’). The Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 materials with different content were prepared via a sol–gel method. Firstly, a certain quantity of Ti(C4H9O)4 and CH3COOLi
2H2O were dissolved in ethanol and distilled water with a cationic ratio of Li:Ti = 4:5 to form a clear solution. Secondly, a certain quantity of citric acid was added into the above solution by strong stirring to get a clear yellow sol. Then the prepared Li1.2Ni0.2Mn0.6O2 powders were dispersed uniformly in this sol. After stirring for 6 h, a viscous black gel was obtained. Finally, the gel was dried at 120 °C for 1 h to get a black precursor, and calcinated in air at 600 °C for 8 h to yield the final powders. The content of Li4Ti5O12 in the final product was 1, 3, 5 wt% (indicated as ‘b’, ‘c’, ‘d’ respectively). Phase identification studies of the samples were carried out by X-ray diffraction (XRD) (Rigaku D/MAX-cB) with monochromated Cu Ka radiation (45 kV, 50 mA) between 10° and 80° at 2° min 1. The surface morphology of the samples was observed using scanning electron microscopy (SEM, JEOL, JSM-5600LV), the microstructure of the samples was observed using transmission electron microscopy (TEM, Hitachi, 7650). The electrochemical characterizations were performed using a CR2025 coin cell. The cells were constructed by mixing the active material, poly (vinylidene fluoride) and carbon black in a weight ratio of 8:1:1. The mixture was dispersed in N-pyrrolidinone (NMP) and the resultant slurry was coated onto Al foil using the Doctor-Blade technique. After drying at 110 °C under vacuum for 10 h. The electrodes were assembled with the positive electrodes as-prepared, metallic lithium foil as counter electrode, Cellgard 2400 as separator, and 1 M LiPF6 dissolved in ethyl carbonate (EC) and dimethyl carbonate (DMC) (1:1 in volume) as electrolyte in an argon-filled glove box. Charge–discharge experiments were performed galvanostatically at 0.1 C (1 C = 250 mA h g 1) between 2.0 and 4.8 V on a Neware battery tester (Neware, Shenzhen). The cyclic performances of synthesized samples were tested at 0.2 C. The electrochemical impedance spectroscopy (EIS) has been carried out with a CHI660A electrochemical analyzer (Chenghua Instrument Co. Ltd, Shanghai).

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2. Experimental

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Y. Liu et al. / Advanced Powder Technology xxx (2016) xxx–xxx

(101) (006) (012) (104)

2

d c b a

20

40

60

80

2θ , degree Fig. 1. XRD patterns of the pristine and coated Li1.2Ni0.2Mn0.6O2.

Table 1 Lattice parameters of pristine and coated samples. Electrodes

a (Å)

C (Å)

c/a

a b c d

2.8625 2.8612 2.8596 2.8588

14.2842 14.2811 14.2768 14.2680

4.9901 4.9913 4.9926 4.9909

Li4Ti5O12 with the substrate Li1.2Ni0.2Mn0.6O2 during the coating process. These variations are attributed to the ionic radius differences among Li+ (0.059 nm), Ni2+ (0.069 nm), Mn4+ (0.053 nm), and Ti4+ (0.0605 nm) [20], revealing the incorporation of Ti atoms onto the Ni sites. The results suggest that Li4Ti5O12 is only modified the surface of the active material without changing the layered crystal structure. The scanning electron microscopy (SEM) images and EDS results of the pristine and coated Li1.2Ni0.2Mn0.6O2 materials are shown in Fig. 2. In Fig. 2(a), it can be seen that the pristine sample exhibits an extremely smooth and clean surface. In contrast, as shown in Fig. 2(b)–(d), after surface modification treatment with different contents of Li4Ti5O12, the surface of Li4Ti5O12-coated samples becomes a little coarser. And a lot of small particles are observed on the surface of Li4Ti5O12-coated samples. As seen in the EDS results, the Ti element has been detected on the surface of Li4Ti5O12-coated samples while the pristine Li1.2Ni0.2Mn0.6O2 does not exhibit any Ti element. Therefore, it can be speculated that the surface of the prepared Li1.2Ni0.2Mn0.6O2 is covered with small Li4Ti5O12 particles. It can be expected that the coated Li4Ti5O12 will decrease the direct contact area between the cathode and electrolyte, and thus suppress the surface side reaction between Li1.2Ni0.2Mn0.6O2 and electrolyte, which may result in better electrochemical performance. The microstructure of the pristine and Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 materials has been further confirmed by TEM analysis as shown in Fig. 3. As seen Fig. 3A, it is clear that the pristine Li1.2Ni0.2Mn0.6O2 sample has no visible fringes, which means that the crystals of Li1.2Ni0.2Mn0.6O2 grow very well and have good crystallinity. As compared, the TEM image of 3 wt% Li4Ti5O12coated sample is shown in Fig. 3B, which reveals that the surface of Li1.2Ni0.2Mn0.6O2 particles have been coated by a thin amorphous coating layer formed of distinguished bright translucent particles. The coating can provides a good physical protecting layer, preventing undesired surface side reactions and therefore improving the cycling performance and structural stability of the active material. Fig. 4 shows the initial charge/discharge curves of the pristine and Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 electrodes in the voltage range of 2.0–4.8 V with a current density of 0.1 C. The constant current–constant voltage (CC–CV) method is used. Table 1 compares

Please cite this article in press as: Y. Liu et al., Investigation the electrochemical performance of layered cathode material Li1.2Ni0.2Mn0.6O2 coated with Li4Ti5O12, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.05.008

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3

Fig. 2. SEM images combined with the EDS results for the pristine and coated Li1.2Ni0.2Mn0.6O2 powders (a) 0%; (b) 1%; (c) 3%; (d) 5%.

the first charge/discharge capacities, irreversible capacity values and coulomb efficiency of all samples. As shown in Fig. 4, all these electrodes display two distinguished voltage regions during the initial charge process. The voltage plateau region located at 4.5 V is mainly due to the removal of oxygen and lithium ions from the structure of Li2MnO3 [26,27], which means that the Li4Ti5O12-coating does not change the intrinsic lithium de/intercalation properties of Li1.2Ni0.2Mn0.6O2, such as charge/discharge behavior. As shown in Fig. 4, the charge/discharge curves of Li4Ti5O12-coated sample are similar with pristine Li1.2Ni0.2Mn0.6O2, indicating that Li4Ti5O12 is stable during charge and discharge. The phenomena have also been reported in Refs. [21,23,28]. As shown in Table 2, it can be seen that the first discharge capacities are increased with the amounts of coated Li4Ti5O12 increasing except

of the sample ‘d’. The reason may be that the relatively thick Li4Ti5O12 coating layer of the sample ‘d’ forms a barrier for the movement of lithium ions. Among the coated samples, the 3 wt% Li4Ti5O12-coated sample shows a higher capacity (258.5 mA h g 1) compared to the pristine Li1.2Ni0.2Mn0.6O2 (235.2 mA h g 1). And the coulomb efficiencies are 82.1% and 69.7%, respectively. The result shows that Li4Ti5O12 coating can lead to the enhanced reversible properties for Li-rich cathode material Li1.2Ni0.2Mn0.6O2 effectively. And this is because that the coating layer can suppress the oxidation of the electrolyte, the dissolution of the transition metals, and then restrain the simultaneous removal of Li+ and O2 during the first cycle. Fig. 5 compares the cycling performance of the pristine and Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 electrodes at 0.2 C in the voltage

Please cite this article in press as: Y. Liu et al., Investigation the electrochemical performance of layered cathode material Li1.2Ni0.2Mn0.6O2 coated with Li4Ti5O12, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.05.008

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Fig. 3. TEM images of the pristine (A) and 3 wt% Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 (B).

5.0

260

4.5

240

Voltage (V)

4.0

Capacity (mAh/g)

0.2 C

0.1C

3.5

3.0 a b c d

2.5

2.0 0

50

220

200

180

a b c d

160

140 100

150

200

250

300

0

350

10

Capacity (mAh/g) Fig. 4. First charge/discharge curves of Li1.2Ni0.2Mn0.6O2 electrodes with different contents of Li4Ti5O12 coating.

Discharge capacity (mA h g 1)

Irreversible capacity loss (mA h g 1)

Coulomb efficiency (%)

a b c d

337.34 321.58 314.68 320.97

235.18 237.54 258.48 228.88

102.16 84.04 56.2 92.09

69.7 73.8 82.1 71.3

range of 2.0–4.8 V at room temperature. After 50 cycles at 0.2 C, the retained discharge capacities of the pristine and 1 wt%, 3 wt%, 5 wt % Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 decrease to 198.5, 213.6, 229.2 and 191.9 mA h g 1, respectively. The pristine Li1.2Ni0.2Mn0.6O2 only retained 90.4% of its initial discharge capacity, while the capacity retentions of 1, 3 and 5 wt% Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 are 95.6%, 98.7% and 93.2%, respectively. Obviously, Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 materials exhibit a better cycling performance, and the sample coated with 3 wt% Li4Ti5O12 shows the best discharge capacity and cycling stability. The cyclic performance of Li4Ti5O12-coated sample is better than previous report [29]. These results indicate that the cyclic performance of Li1.2Ni0.2Mn0.6O2 is improved by Li4Ti5O12 coating effectively. The reason can be ascribed to the Li4Ti5O12 coating layer, which not only avoids the direct contact between the electrode and electrolyte to prevent the Li1.2Ni0.2Mn0.6O2 from dissolving into the

40

50

105

100

Coulomb efficiency (%)

Charge capacity (mA h g 1)

30

Fig. 5. Cyclic performance of Li1.2Ni0.2Mn0.6O2 electrodes with different content of Li4Ti5O12 coating.

Table 2 First charge–discharge capacity and coulomb efficiency of the samples. Samples

20

Cycle number

95

90

85

Li1.2Ni0.2 Mn0.6O2 3% wt% Li4 Ti 5O12-coated Li1.2Ni0.2Mn 0.6O2

80 0

10

20

30

40

50

Cycle number Fig. 6. Coulomb efficiency of the pristine and 3 wt% Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 at 0.2 C rate in 50 cycles.

electrolyte, but also reduces the oxidation of the electrolyte on the surface of cathode at charge state [30]. Fig. 6 shows the coulomb efficiency of the pristine and 3 wt% Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 electrodes at 0.2 C rate in the voltage range of 2.0–4.8 V. It is clear that the 3 wt% Li4Ti5O12coated electrode has a higher mean coulomb efficiency than that

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3.6

3.4

Voltage(V)

3.2

3.0

2.8 Li 1.2 Ni 0.2 Mn0.6O2

2.6

3% wt% Li 4 Ti 5O12-coated Li1.2 Ni 0.2Mn 0.6O2

0

5

10

15

20

25

30

35

40

45

50

55

Cycle number Fig. 7. The middle voltage of the pristine and 3 wt% Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 in 55 cycles.

300

0.1C 0.2C

250

Capacity (mAh/g)

of the pristine Li1.2Ni0.2Mn0.6O2 electrode. It can be attributed to the formation of protection layer on the electrode surface, implying that the coating is beneficial to the reversible intercalation and de-intercalation of Li+, which may be speculated that the surface coating of Li4Ti5O12 suppress the formation of passivation on the surface of the Li1.2Ni0.2Mn0.6O2, preventing the electrolyte decomposition and the metal ions dissolution [31]. Fig. 7 shows the discharge middle voltage of the pristine and 3 wt% Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 electrodes during 55 cycles. Middle voltage refers to the voltage when the discharge specific capacity is 50%. As shown in Fig. 7, it is clear that the 3 wt% Li4Ti5O12-coated sample shows higher middle voltage compared to the pristine Li1.2Ni0.2Mn0.6O2. The middle voltage value of pristine Li1.2Ni0.2Mn0.6O2 is decreased from 3.451 V to 3.147 V, while the 3 wt% Li4Ti5O12-coated sample is decreased from 3.476 V to 3.251 V. These results indicate that the Li4Ti5O12 coating layer is helpful to maintain the higher middle voltage of lithium ion batteries. As reported in Refs. [9,32,33], the phenomenon of voltage decay is result from the undesired spinel growth in the layered host structure usually occurs during the long-term cycle. The results show that the spinel phase maybe controlled in the Li4Ti5O12 coated Li1.2Ni0.2Mn0.6O2 material during the charge/discharge cycling. As reported [34], a substitutional compound Li1.2NixMnyTi1 x yO2 has formed on the surface of coated Li1.2Ni0.2Mn0.6O2 through the interaction of the coated oxide with the substrate, which improves the stability of the Li1.2Ni0.2Mn0.6O2 because of the strong bond of Ti–O. Fig. 8 shows the rate performance of the pristine and Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 electrodes in the voltage range of 2.0–4.8 V with different current density. The tested cells are charged at 0.1 C and then progressively discharged at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and 10 C rates, respectively. As shown in Fig. 8, the discharge specific capacities of all the samples show gradual decreases with increasing current density. Compared with pristine Li1.2Ni0.2Mn0.6O2 electrode, the rate capabilities of the Li4Ti5O12coated samples except of 5 wt% coated have been improved significantly. Among the coated samples, the sample coated with 3 wt% Li4Ti5O12 obviously exhibits a much better rate capability at any rates, especially at high rates 5 C and 10 C. The discharge capacity at 5 C can reach 150.7 mA h g 1, while that of pristine Li1.2Ni0.2Mn0.6O2 is only 67.1 mA h g 1. The 3 wt% Li4Ti5O12-coated sample still can reach 110.8 mA h g 1 at 10 C rate, while the pristine sample only sustains 23.6 mA h g 1. The results show that the rate capability of Li1.2Ni0.2Mn0.6O2 has been improved with Li4Ti5O12

0.5C 1C

200

2C 5C

150

10C 100

a b c d

50

0 0

5

10

15

20

25

30

35

Cycle number Fig. 8. Rate performance of Li1.2Ni0.2Mn0.6O2 electrodes with different content of Li4Ti5O12 coating.

Fig. 9. AC impedance of the pristine and Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 samples (A) and equivalent circuit (B).

coating effectively. It is well known that the faster ionic diffusion ability of Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 particles contributed to the better rate capability [22,23]. Electrochemical impedance spectroscopy (EIS) profiles of the pristine and Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 samples as positive electrode materials after charge–discharge cycle tests are shown in Fig. 9A. The measurements have been carried out in the charged state of 4.4 V after 50 cycles at 0.2 C. The shapes of the EIS curves for all samples are similar. All of the plots are mainly composed of one semicircle in the high-frequency region and a sloping line in the low-frequency region. The semicircle is correlated with the surface charge transfer resistance and the linear portion represents the Warburg diffusion impedance [35,36]. The obtained EIS spectra are simulated using the equivalent circuit exhibited in Fig. 9B. In the equivalent circuit, Rs refers to the uncompensated ohm resistance between the working electrode and the reference electrode, Rct refers to the charge transfer resistance, CPE1 represents the constant phase-angle element depicting the non-ideal capacitance of the surface layer, W refers to the Warburg impedance describing the lithium-ion diffusion in the bulk material [14]. Each impedance spectrum is fitted with suggested equivalent circuit model by ZView 2.0 to give simulation of the ohm resistance (Rs) and charge transfer resistance (Rct), as summarized in Table 3. As seen in

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Table 3 Impedance parameters of equipment circuit. Electrodes

Rs (X)

Rct (X)

CPE-T

a b c d

7.9 5.2 2.1 9.8

236.7 205.8 159.2 286.5

1.42E 3.95E 1.26E 1.88E

5 6 6 7

CPE-P

Ws-T

Ws-P

Ws-R

0.87 0.82 0.78 0.76

110.2 108.5 196.7 78.3

0.371 0.406 0.4 0.255

1143 1295 2049 409

Table 3, a rapid decrease of the surface charge transfer resistance and ohm resistance has been observed in the Li4Ti5O12 coated Li1.2Ni0.2Mn0.6O2 except of the 5 wt% coated sample. The thicker coating layer and the poorer electrically conductivity may be the reasons of the highest both ohmic and charge transfer resistance of 5% coated sample. The 3 wt% Li4Ti5O12 coated Li1.2Ni0.2Mn0.6O2 cathode shows the lowest charge transfer resistance (Rct). The surface charge transfer resistance for the pristine Li1.2Ni0.2Mn0.6O2 is 236.7 X. While, for the 3% Li4Ti5O12-coated sample, the value is 159.2 X. The surface coating decreases Rct by reducing the SEI layer thickness due to a suppressed interaction between the cathode surface and electrolyte while maintaining a micro-porous structure allowing lithium-ions to diffuse through [37]. Furthermore, the excellent lithium-ion conductivity of Li4Ti5O12 should be another reason. This result suggests that Li4Ti5O12 coating is effective to decrease the interfacial resistance; and the lower Rct is responsible for the better rate performance of Li1.2Ni0.2Mn0.6O2 with Li4Ti5O12 coating. 4. Conclusion The Li1.2Ni0.2Mn0.6O2 has been coated with different contents of Li4Ti5O12 particles by a sol–gel method successfully. All the samples keep well-defined layered characteristics. The electrochemical tests show that the electrochemical performance of pristine Li1.2Ni0.2Mn0.6O2 has been improved greatly after Li4Ti5O12 coating. Among the coated samples, the 3 wt% Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 exhibits the best electrochemical performances. The first discharge capacity is increased from 235.2 to 258.5 mA h g 1, and the first coulomb efficiency is increased from 69.7% to 82.1%. The capacity retention is 98.7% after 50 cycles and the discharge capacity at 10 C rate can retain 110.8 mA h g 1, while that of pristine sample only shows 90.4% for 50 cycles and 23.6 mA h g 1 for 10 C rate. The voltage decay phenomenon of Li1.2Ni0.2Mn0.6O2 has been mitigated after Li4Ti5O12 coating. EIS results show that the Rct of the pristine Li1.2Ni0.2Mn0.6O2 electrode after cycles is decreased after Li4Ti5O12 coating. Those results suggest that Li4Ti5O12 surface coating can probably be an effective way in improving the electrochemical performances of Li1.2Ni0.2Mn0.6O2 cathode material. Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (51304081), Natural Science Foundation of Jiangsu Province (BK20140581) and Postgraduate Research, Innovation Plan Project of Jiangsu Province (SJLX_0463) and State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. References [1] J.R. Croy, S.-H. Kang, M. Balasubramanian, M.M. Thackeray, Li2MnO3-based composite cathodes for lithium batteries: a novel synthesis approach and new structures, Electrochem. Commun. 13 (2011) 1063–1066.

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Please cite this article in press as: Y. Liu et al., Investigation the electrochemical performance of layered cathode material Li1.2Ni0.2Mn0.6O2 coated with Li4Ti5O12, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.05.008