3O2 cathode of lithium ion battery by electrochemical impedance spectroscopy”

Journal of Electroanalytical Chemistry 688 (2013) 393–402

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Reprint of ‘‘Investigation of layered LiNi1/3Co1/3Mn1/3O2 cathode of lithium ion battery by electrochemical impedance spectroscopy’’ q Xiang-Yun Qiu a,b, Quan-Chao Zhuang a,⇑, Qian-Qian Zhang a, Ru Cao a, Ying-Huai Qiang a, Peng-Zhan Ying b, Shi-Gang Sun c,⇑ a b c

Li-ion Batteries Lab, School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China State Key Lab of Physical Chemistry of Solid Surfaces, Department of Chemistry College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

a r t i c l e

i n f o

Article history: Available online 21 February 2013 Keywords: Lithium-ion battery LiNi1/3Co1/3Mn1/3O2 EIS Electrical conductivity Kinetic parameters

a b s t r a c t The present study applies electrochemical impedance spectra (EIS) to study the interfacial processes of lithium ion batteries (LIBs) and determine the corresponding kinetic parameters. The EIS of the insertion and extraction of lithium ions in layered LiNi1/3Co1/3Mn1/3O2 materials as cathode of LIBs are obtained at different potentials during the first charge/discharge cycle and at different temperatures after 10 charge/ discharge cycles. The EIS spectra exhibit three semicircles and a slightly inclined line that appear successively along with decrease in frequency. The high-frequency, the middle-frequency, and the low-frequency semicircles can be attributed respectively to the migration of the lithium ions through the SEI film, the electronic properties of the material and the charge transfer step. The slightly inclined line arises from the solid state diffusion process. The electrical conductivity of the layered LiNi1/3Mn1/3Co1/3O2 changes dramatically at early delithiation as a result of an insulator-to-metal transition. In an electrolyte solution of 1 mol L1 LiPF6–EC(ethylene carbonate): DEC(diethyl carbonate): DMC(dimethyl carbonate), the activation energy of the ion jump which is related to the migration of the lithium ions through the SEI film, the thermal activation energy of the electrical conductivity and the activation energy of the intercalation/deintercalation reaction are determined 23.1, 44.0 and 66.5 kJ mol1, respectively. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The lithium-ion battery is the most popular rechargeable power source for use in cell phones, laptop computers, and other mobile computing and communication devices [1–3]. In addition, it is the most promising power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) [4]. Existing and emerging technologies demand even better performance in terms of the energy density, power, safety, price and environmental impact. Layered LiNi1/ 3Mn1/3Co1/3O2 that was first introduced by Ohzuku and Makimura [5] in 2001, and zLi2MnO3-(1z)LiNi1/3Mn1/3Co1/3O2 are now widely investigated because of its favourable features, including a relative high capacity, structural stability, thermal stability, low cost, and safety [4,6–17]. LiNi1/3Co1/3Mn1/3O2 integrates the properties of

DOI of original article: http://dx.doi.org/10.1016/j.jelechem.2012.09.027 q

This article is a reprint of a previously published article. For citation purposes, please use the original publication details; Journal of Electroanalytical Chemistry, 687, pp. 35-44. ⇑ Corresponding authors. Tel./fax: +86 516 83591870. E-mail addresses: [email protected] (Q.-C. Zhuang), sgsun@xmu. edu.cn (S.-G. Sun). 1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.02.009

LiCoO2, LiNiO2, and LiMnO2; however, its electrical conductivity is worse than that of LiCoO2 [18]. The poor electrical conductivity of the LiNi1/3Co1/3Mn1/3O2 declines its electrochemical performance and limits its practical applications. The reversible capacity of the LiNi1/3Co1/3Mn1/3O2 could be increased by raising the charging cut-off voltage to 4.5 V. However, charging the electrodes at such a high voltage can deteriorate the cycle performance [6,19]. This capacity fade is caused by an increase in the surface reactivity between the highly delithiated cathode and the electrolyte solution, which increases the interfacial resistance. To overcome these problems, it is necessary to understand the electronic and ionic transport properties of the electrode as well as the charge transfer reaction at the electrode/electrolyte interface of LIBs, since these properties govern the charge capacity and the rate capability of the material, which are vital for improving the performance of the device. Recently, efforts have been made to boost the electrical conductivity of this material by coating it with metal hydroxides, doping it with other guest ions or synthesising it using different technologies [20–23]. However, few studies have examined the electronic and ionic transport properties of LiNi1/3Mn1/3Co1/3O2 or the charge transfer reaction that occurs at the electrode/electrolyte interface. Electrochemical impedance spectroscopy (EIS) is one of the most

X.-Y. Qiu et al. / Journal of Electroanalytical Chemistry 688 (2013) 393–402

electrode were measured in 1 mol L1 LiPF6–EC(ethylene carbonate): DEC(diethyl carbonate): DMC(dimethyl carbonate) electrolyte solution and at the temperatures ranging from 20 to 25 °C. Based on the EIS data the kinetic parameters of the insertion/extraction process of lithium ions were determined.

3O2

2. Experimental The cathode was prepared from a solution that contained an 80:7:3:10 (w/w) ratio of the commercially LiNi1/3Mn1/3Co1/3O2 active material (B&M Ltd., Co., Tianjin, China), graphite, carbon black and a polyvinylidene fluoride binder in an N-methyl pyrrolidinone solvent. Aluminium foil was used as the current collector. The phase was identified using powder XRD with Cu Ka radiation, and the measurements were performed on a Rigaku D/Max3B diffractometer. Diffraction data were collected by step scanning over an angular range of 15–80° with a step width of 0.02° (35 kV, 30 mA). The particle morphologies of the samples were examined using a scanning electron microscope (SEM, S-4800). The electrochemical impedance of the first charge/discharge cycle was studied in a three-electrode glass cell with auxiliary and reference electrodes that were both composed of Li foils. The experiment was performed using an electrochemical work station (CHI660D, Chenhua Ltd., Co., Shanghai, China) with an electrolyte solution that was composed of 1 M LiPF6 in a 1:1:1 (v/v/v) mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC). The thickness (l) and the area (S) of the electrode film were 100 lm and 4 cm2, respectively. A two-electrode button cell was used to investigate the changes in the impedance spectra as a function of temperature. The spectra were collected at temperatures ranging from 20 to 25 °C in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution. The measurements were taken after 10 cycles to reduce the negative impact of lithium on the EIS [51]. The amplitude of the AC perturbation signal was 5 mV and the frequency ranged from 105 to 102 Hz. The electrode was equilibrated for 1 h before the EIS measurements were taken. The impedance data were analysed using Zview software. The charge/discharge cycles were carried out at 0.1 C over a potential range of 2.8–4.3 V, and lithium metal was used as the second electrode. 3. Results and discussion 3.1. Characterisation of the LiNi1/3Co1/3Mn1/3O2 cathode The XRD patterns of the LiNi1/3Co1/3Mn1/3O2 cathode shown in Fig. 1 reveal that the sample has an a-NaFeO2-type layered strucCommercial LiNi1/3Co1/3Mn1/3O2

(003)

(104) 20

30

40

50

60

70

(201)

(108) (110) (113)

(107)

(105)

(101) 10

(102)

(006)

powerful tools for analysing the electrochemical processes that occur at electrode/electrolyte interfaces, and has been widely used to study the electrochemical intercalation of lithium into carbonaceous materials and transition metal oxides [24–39]. Nobili et al. [25–30] used EIS to investigate LixCoO2 and other members of the LixNi1yCoyO2 family of electrodes; their results suggest that the finite electrical conductivity of materials should be reflected in the Nyquist plots. In this case, the Nyquist plots of the electrochemical intercalation of lithium into LiCoO2 and LixNi1yCoyO2 electrodes should exhibit three semicircles and an inclined line. The three semicircles arise from the migration of the lithium ions through the surface of the film (also called the solid electrolyte interphase, which is abbreviated SEI), the electronic properties of the material and the charge transfer step, and the inclined line arises from the solid state diffusion. The crystal structure of LiNi1/3Co1/3Mn1/3O2 is the same as that of LiCoO2; it has a a-NaFeO2-type structure, and the Co–O distance of LiNi1/3Co1/3Mn1/3O2 is nearly identical to that of LiCoO2. These similarities suggest that the local electronic structure of Co and the mobilities of the electrons and holes in LiNi1/3Co1/3Mn1/3O2 should be comparable to those of LiCoO2 [40,41]. As a result, the EIS features of LiNi1/3Co1/3Mn1/3O2 should be similar to those of LiCoO2, and the Nyquist plots of the electrochemical intercalation of lithium into LiNi1/3Co1/3Mn1/3O2 should exhibit three semicircles and an inclined line. However, the EIS spectra of LiNi1/3Co1/3Mn1/3O2 that were measured in previous studies [42–44] exhibited only two semicircles and an inclined line. The two semicircles were attributed to the migration of lithium ions through the SEI film and the charge transfer step, and the inclined line was attributed to the solid state diffusion. The cathodes of LIBs are composed of composite materials, whose active mass particles are bound to an aluminium current collector that contains a polymeric binder such as polyvinylidene difluoride (PVdF). The composite electrodes must also contain a conductive additive, which usually consists of carbon particles (e.g., carbon black or graphite). The electrodes are usually prepared from slurry that contains the particles and the binder in an organic solvent. The slurry is spread onto the current collector and then dried. The final shape is obtained by applying pressure to the electrode. The amount of the conductive additive in the composite material and the contact between the cathode film and the aluminium current collector can have a significant impact on the electrical conductivity of the electrode [45]. Therefore, the changes in the electrical conductivity of the active mass that occur in response to the changes in the electrode potential can only be only detected when the cathode contains a sufficient amount of the conductive additive and there is good contact between the cathode film and the aluminium current collector [46,47]. In the current study, the LiNi1/3Co1/3Mn1/3O2 electrode was prepared with a high weight percent of the conductive additive and a PVdF binder. After drying, the LiNi1/3Co1/3Mn1/3O2 electrode was compressed using a rolling machine (between iron wheels) to ensure a good contact between the LiNi1/3Co1/3Mn1/3O2 electrode film and the aluminium current collector. EIS was used to investigate the first delithiation/lithiation process in the layered LiNi1/3Mn1/ 3Co1/3O2 electrode. At intermediate stages of the first delithiation/lithiation process, the Nyquist plots exhibit four distinct features: three semicircles and an inclined line, which appear successively as the frequency decreases. The high-frequency, the middle-frequency, and the low-frequency semicircles can be attributed to the migration of lithium ions through the SEI film, the electronic properties of the material and the charge transfer step, respectively. The slightly inclined line arises from the solid state diffusion process. A drastic change in the electrical conductivity of LiCoO2 and the layered LiNi1/3Mn1/3Co1/3O2 occurs at an early stage of delithiation [48–50] due to the insulator-to-metal transition. In addition, the impedance spectra of the LiNi1/3Co1/3Mn1/

Intensity (a.u)

394

80

2θ (Degree) Fig. 1. XRD patterns of the commercially available LiNi1/3Co1/3Mn1/3O2.

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 No impurity peaks were observed in ture with space group of R3m. the XRD pattern. The clear splitting of the lines that have Miller indices of (006, 102) and (108, 110) in Fig. 1 indicate that the layered structure has good characteristics. Fig. 2 shows typical SEM micrographs and EDS analysis results (Cu as substrate) of the commercially available LiNi1/3Co1/3Mn1/3O2. The active material of the electrodes consists of two types of particles that are agglomerated together, i.e., particles that have a diameter of 1–2 lm and particles that have a diameter of 3–4 lm. The atomic ratios confirm the ratios of Mn, Co and Ni is 1/3: 1/3: 1/3. Fig. 3 displays the first 10 charge/discharge curves, the variations in the discharge capacities, the voltage as a function of x curves as well as the differential capacitance curves of LiNi1/3Co1/ 1 . If 3Mn1/3O2 electrode. The initial charge capacity is 160 mA h g we assume that all charge consumed during the first charge process was due to lithium extraction, almost 0.5 mol of lithium was extracted from the LiNi1/3Co1/3Mn1/3O2 electrode. The initial specific discharge capacity is close to 139 mA h g1 (0.44 mol Li), and after 50 cycles the capacity retention is above 99%, demonstrating an excellent electrochemical performance. The differential capacitance curves of LiNi1/3Co1/3Mn1/3O2 electrode demonstrate that the current peak at 3.8 V corresponds to the flat domain in the charge/ discharge curves. It is well known [15,16] that the material re mains a layered structure until 4.4 V with space group R3m. That is to say, a Li-rare hexagonal phase (H2) begins to replace a Li-rich hexagonal phase (H1), and traces of the H1 phase can be observed until 4.16 V; The H2 phase last until 4.4 V. 3.2. The first delithiation/lithiation process of the LiNi1/3Co1/3Mn1/3O2 cathode The Nyquist plots of the LiNi1/3Co1/3Mn1/3O2 cathode from 3.5 to 4.4 V during the first charge process are illustrated in Fig. 4. Fig. 5 shows the Nyquist plots of the discharge process, which follow a converse pathway. At the open circuit potential, 3.5 V, the Nyquist

395

plots show a small semicircle in the high-frequency (HF) region, a large semicircle in the middle-frequency (MF) region and a line with a slight incline in the low-frequency (LF) region. These results are different from what has been reported in the literature [42–44], but are in agreement with the results that we obtained for LiMn2O4 and LiCoO2 electrodes in our previous studies [31,32]; they illustrate that the amount of the conductive additive that is present in the composite material and the contact between the cathode film and the aluminium current collector can have a significant impact on the EIS features. As the electrode polarisation potential increases, the small HF semicircle does not change significantly; however, below 3.9 V, the diameter of the large MF semicircle decreases rapidly. The slightly inclined line in the LF region, which is strongly dependent on the potential, shows an increasing tendency to move toward the real axis. Finally, at 4 V, another semicircle and a steep sloping line appear in the low frequency region. On further charging to 4.4 V, for example at 4.25 V, the Nyquist plot clearly contains four distinct features, which consist of three semicircles and one line; these results are in accordance with the findings of Nobili and coworkers [25–30]. Therefore, the three semicircles and the inclined line that appear at intermediate degrees of delithiation can be attributed to the migration of lithium ions through the SEI films, the electronic properties of the material, the charge transfer step and the solid state diffusion of the lithium ions, respectively. Based on these experimental results, an equivalent circuit was proposed to fit the impedance spectra of the electrode during the first charge/discharge cycle as shown in Fig. 6, which is equivalent to one that was used in our previous studies [31,32]. In the equivalent circuit, Rs is the ohmic resistance, RSEI is the resistance of the SEI film and Rct is the resistance of the charge transfer reaction. The capacitances of the SEI film and the double layer are represented by the constant phase elements (CPEs) QSEI and Qdl, respectively. The very low frequency region, however, cannot be properly modelled with a finite Warburg element. Therefore, we replaced the finite diffusion term with a CPE, i.e., QD. This approach has been

Fig. 2. SEM images and EDS analysis results of the commercially available LiNi1/3Co1/3Mn1/3O2.

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165

4.4 (a) First 10 circles

(b)

Discharge Capacity /mAhg

-1

4.2

Potential /V

4.0 3.8 3.6 3.4 3.2 3.0

150

135

120

105

2.8 2.6 -20

0

20

40

60

80

90

100 120 140 160 180

0

10

-1

4.4

(c)

800

4.2

Charge

(dQ/dV) /(mAg-1 V-1 )

Potential /V

4.0 3.8

Discharge

3.6

20

30

40

50

Cycling Number

Specific Capacity /mAhg

3.4 3.2 3.0 2.8

(d)

1st charge

600

2st charge 400 200 0 -200

1st discharge 2st discharge

-400

2.6 0.5

0.6

0.7

0.8

0.9

2.8

1.0

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

E /V(vs. Li + /Li)

x in LixNi1/3Co1/3Mn1/3O2

Fig. 3. (a) Charge/discharge curves of the Li/LiNi1/3Co1/3Mn1/3O2 cells. (b) Variation in the discharge capacity as a function of the cycle number. (c) The voltage as a function of x curves of the LiNi1/3Co1/3Mn1/3O2 electrode. (d) Differential capacitance curves of LiNi1/3Co1/3Mn1/3O2 electrode at 1st and 2nd cycles.

used to characterise graphite electrodes [52] and gave a good fit to the experimental data. The electronic properties of the material are characterised by the electrical resistance and the capacitance, which are represented by Re and the constant phase element Qe. The expression for the admittance response of the CPE (Q) is

Y ¼ Y 0 xn cos

np np þ jY 0 xn sin 2 2

ð1Þ

where x is the angular frequency and j is the imaginary unit. A CPE represents a resistor when n = 0, a capacitor with capacitance of C when n = 1, an inductor when n = 1, and a Warburg resistance when n = 0.5. Fig. 7 compares the simulated impedance spectra with the experimental EIS data at 4.25 V, and the parameters values are listed in Table 1. The relative standard deviations of most of the parameters that were obtained from fitting the experimental impedance spectra are less than 15%, which indicates that the proposed model provides a satisfactory description of the experimental data. The variation of the RSEI during the first charge/discharge cycle as a function of the electrode polarisation potential is plotted in Fig. 8, which was calculated by fitting the experimental impedance spectra of the LiNi1/3Co1/3Mn1/3O2 electrode. In the charge process, RSEI remains nearly constant below 3.8 V and above 3.95 V, but decreases rapidly between 3.8 and 3.95 V. This phenomenon is commonly attributed to the breakdown or dissolution of the resistive SEI film, which is caused by the spontaneous reactions between the cathode active materials and the non-aqueous organic electrolytes that occur at the surface of the cathodes [53–55]. Because there is a limited amount of research into these reactions, the microscopic picture of this breakdown mechanism remains unknown

[51,56,57]. In the discharge process, RSEI increases slowly as the electrode polarisation potential decreases; this increase is probably caused by the electrolyte oxidation, which is accompanied by the insertion of lithium ions [58]. The electrical conductivities of the component materials can have a significant impact on the rate performance of batteries [59]. Measurements of the conductivity during the lithium insertion and extraction reactions can also be used to study the variation in the electronic structure of the materials as a function of the lithium content [60,61]. Fig. 9 illustrates the variation in the Re as a function of the electrode polarisation potential, which was calculated by fitting the experimental impedance spectra of the LiNi1/3Co1/3Mn1/3O2 electrode during the first charge/discharge cycle. The electrical conductivity was calculated from the Re using the following equation:

R ¼ l=ðrSÞ

ð2Þ

where r is the conductivity, l is the thickness of the electrode film (100 lm), and S is the area of the electrode film (4 cm2). The variation in the conductivity (r) as a function of the electrode polarisation potential during the first charge/discharge cycle is shown in Fig. 10. The behaviour of r during the first charge process can be divided into three parts: (i) below 3.8 V, the electrical conductivity increases slowly; (ii) between 3.8 and 3.95 V, the electrical conductivity increases drastically; and (iii) above 3.95 V, the electrical conductivity increases slowly. The potential-conductivity profile shows a decent reversibility when the scan of the potentials is reversed, which is in agreement with the potential-conductivity profile of LiCoO2 that was measured by Shibubuya et al. [62] and Sauvage et al. [63]. In addition, we calculated the electrical conductivities of LiNi1/3Co1/3Mn1/3O2 between 3.5 V (r = 1.3  106 -

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-400

-200

-100

0 0

100

-240

200

-12

-120

-8

-80

-4

2.69kHz

0

-40

4

0

10 15 20 25 30

300

-16

-160

100kHz 5

-20

-200

2.69kHz

0

3.85 V 3.9 V 3.95 V

-280

Z" /Ω

Z" /Ω

-300

-25 -20 -15 -10 -5 0 5

-320

3.5 V 3.55 V 3.6 V 3.65 V 3.7 V 3.75 V 3.8 V

0.01Hz

400

0

40

80

14.68Hz 4

120

Z' /Ω

8

160

12

200

240

16

20

280

320

Z' /Ω -16

-25

4.05 V

4V -20

-12

-8

Z" /Ω

Z" /Ω

-15 -10

0.56Hz -4

-5

17.82kHz

2.69kHz 0

0.03Hz

0

14.68Hz

14.68Hz 5 0

5

10

15

20

25

4

30

4

8

12

4.15 V

24

Experimental data 4.25 V

-12

Simulation result

-12

-10 -8

-8

Z" /Ω

Z" /Ω

20

-14

-16

0.01Hz -4

2.69kHz

LFL

-6 -4

LFS

MFS

HFS

-2

0

0.06Hz

14.68Hz 4

16

Z' /Ω

Z' / Ω

4

8

12

16

0

20

2

24

2.69kHz 4

6

8

10

Z' / Ω

0.07Hz

17.78Hz 12

14

16

18

20

Z' /Ω -10

4.4 V

4.35 V -8

-8

Z" /Ω

Z" /Ω

-6 -4 0.46kHz 21.48kHz

2.15Hz

0.01Hz

-4

3.91kHz

-2 0.02Hz

82.52kHz

0

0

21.54Hz 4

8

12

Z' / Ω

16

2

4

6

8

10

0.46Hz 12

14

16

Z' / Ω

Fig. 4. Nyquist plots of the LiNi1/3Co1/3Mn1/3O2 electrode at a series of potentials ranging from 3.5 to 4.4 V during the first delithiation.

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-12

-10

4.25 V

4.35 V -10

-8

-8

Z" /Ω

Z" /Ω

-6 -4 0.03Hz

-6 -4 0.38kHz

3.17kHz

-2

-2

0

0

0.46Hz

21.54Hz

2.15Hz

82.52kHz

2

2 4

6

8

10

12

14

16

4

6

8

10

12

14

16

18

Z' /Ω

Z' / Ω -16

-12 4.15 V

4.05 V

-14

-10

-12 -10

-6

Z" /Ω

Z" /Ω

-8

0.01Hz

-4 3.91kHz

-8 -6 -4

0.38kHz

-2 0

0.02Hz

0

0.18Hz

21.54Hz

1.47Hz

21.48kHz

-2

2

2 4

6

8

10

12

14

16

4

18

6

8

10

12

14

16

18

20

22

Z' /Ω

Z' /Ω -140

-200 3.8 V

-120

3.75 V -160

-100 -120

-60

Z" /Ω

Z" /Ω

-80 0.02Hz

-40

-80 0.03Hz -40

-20

3.17kHz

2.69kHz 0

0

0.32kHz

10Hz

20 -20

0

20

40

60

80

100

120

40 -40

140

0

40

80

Z' /Ω

120

160

200

Z' /Ω

-240

-280 3.7 V

3.6 V

-240

-200 0.01Hz

-200

-160

Z" /Ω

Z" /Ω

-160 -120 -80 -40

-120 0.02Hz -80 -40

0.1Hz

82.52kHz

0.26kHz

0

0 0

40

80

120

Z' /Ω

160

200

240

40 -40

0.26Hz 5.62Hz

0

40

80

120

160

200

240

280

Z' /Ω

Fig. 5. Nyquist plots of the LiNi1/3Co1/3Mn1/3O2 electrode at a series of potentials ranging from 4.35 to 3.6 V during the first lithiation.

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7

Charge 6

Discharge

Fig. 6. The equivalent circuit that was proposed for the analysis of the charge/ discharge process of the LiNi1/3Co1/3Mn1/3O2 electrode.

RSEI /Ω

5

4

3

2 -14 1

Experimental data 4.25 V

-12

Simulation result 0 3.4

-10

3.6

3.8

Z" /Ω

-8

4.2

4.4

Fig. 8. The variation in RSEI as a function of the electrode potential, which was calculated by fitting the experimental impedance spectra of the LiNi1/3Co1/3Mn1/3O2 electrode during the first charge/discharge cycle.

LFL

-6 -4

LFS

MFS

HFS

-2 0 2

4.0

E /V (vs. Li + /Li)

4

6

8

10

12

14

16

18

20

Z' /Ω Fig. 7. A comparison of the EIS Data that were collected at 4.25 V during the first charge process with the data that were simulated using the equivalent circuit that is pictured in Fig. 6.

Table 1 The equivalent circuit parameters that were obtained from a fit of the experimental impedance spectra of the first charge process at 4.05 V. Parameters

Values

Uncertainty (%)

Rs RSEI QSEI  Y0 QSEI  n Re Qe  Y0 Qe  n Rct Qdl  Y0 Qdl  n QD  Y0 QD  n

6.8 0.9 3.3E06 0.96 4.9 1.1E3 0.7 8.4 3.0 0.9 5.1E2 0.8

0.6 8.8 12.1 6.4 5.2 10.9 4.3 2.8 12.4 4.9 2.5 2.1

In this equation, f denotes the electrochemical constant (which is equal to F/RT, where F is the Faraday constant, R is the gas constant and T is the absolute temperature), ks denotes the heterogeneous rate constant, and A denotes the total electroactive surface area. Eq. (3) predicts that a rapid increase in Rct with the decrease of x as x < 0.5, a rapid decrease in Rct with the decrease of x as x > 0.5. Therefore the minimum value of Rct should occur at x = 0.5, which corresponds to 4.4 V in this study. Fig. 11 shows that in the charge process, lnRct decreases rapidly as the electrode polarisation potential decreases, and the minimum value of lnRct occurs at 4.4 V [66–68]. A similar behaviour is observed in the discharge process. These results confirmed that Eq. (3) correctly interprets the experimental data. Therefore, the LF semicircle can be attributed to the charge-transfer process.

3.3. The temperature dependence of kinetic parameters To further elucidate the intercalation mechanism of the lithium ions, EIS was used to measure the variation in the impedance spec-

225 200

Charge

175

Discharge

150

Rct ¼

1 fFAks x0:5 ð1  xÞ0:5

ð3Þ

125

Re /Ω

S cm1) and 4.4 V (r = 7.3  104 S cm1), which varied by two orders of magnitude in the first charge cycle. These results indicate that LiNi1/3Co1/3Mn1/3O2 is a p-type semiconductor which conducts by holes. In the charge process, with the decreasing of Li+ content, the number of hole carriers increase proportionally. So, the conductivity will increase with the degree of delithiation and the discharge process follows a converse pathway, i.e., the presence of an ‘‘insulator-to-metal’’ transition was found in LiNi1/3Co1/3Mn1/3O2 as the same as LiCO2 observed in our previous [48–50]. LiNi1/3Co1/3Mn1/3O2 is a material that exhibits a good reversibility; therefore, the Rct versus E plot should behave according to the following classical equation [64,65]:

100 75 50 25 0 -25 3.4

3.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5

E /V (vs. Li + /Li) Fig. 9. The variation in Re as a function of the electrode potential, which was calculated by fitting the experimental impedance spectra of the LiNi1/3Co1/3Mn1/3O2 electrode during the first charge/discharge cycle.

400

X.-Y. Qiu et al. / Journal of Electroanalytical Chemistry 688 (2013) 393–402

significantly as the temperature increases. For the sake of clarity, each plot is shifted by 50 X along the imaginary axis. The Nyquist plots that were recorded at 3.85, 3.95, 4.05 and 4.25 V are shown in Supplementary data Fig. S1–S4. Below the freezing point, the Nyquist plots consist of three semicircles, namely the HF semicircle (HFS), MF semicircle (MFS) and LF semicircle (LFS); these results are in agreement with those that were obtained in the three-electrode glass cell. As the temperature increases, the HFS and the MFS begin to overlap, and when the temperature reaches 15 °C, the HFS and the MFS merge into a single semicircle. These results indicate that there are two different physical phenomena that have a different temperature dependence because they have different time constants [27]. The equivalent circuit, as shown in Fig. 6, was proposed to fit the impedance spectra of the electrode in a two-electrode button cell.

-4

Conducivity /S cm-1

× 10 8 7

Charge

6

Discharge

5 4

1.0

3

0.8

2

0.6 0.4

1

0.2

0 -1 3.4

3.5

3.6

3.7

3.8

0.0 3.4

3.5

4.0

4.1

3.9

3.6

4.2

3.7

4.3

3.8

4.4

4.5

ln RSEI ¼ lnðRTl=4z2 F 2 a2 cv Þ þ W=RT

ð4Þ

E /V (vs. Li + /Li) Fig. 10. The variation in the conductivity, which was derived from Re, as a function of the electrode potential. The values of the Re were calculated by fitting the experimental impedance spectra of the LiNi1/3Co1/3Mn1/3O2 electrode during the first charge/discharge cycle.

9

Charge Discharge

8 7

lnRct /Ω

6 5 4 3 2 1 0 -1 3.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5

E /V (vs. Li + /Li) Fig. 11. The variation in the logarithm of Rct as a function of the electrode potential, which was calculated by fitting the experimental impedance spectra of the LiNi1/ 3Co1/3Mn1/3O2 electrode during the first charge/discharge cycle.

ln Re ¼ ln

l ðEa  kB Þ þ þ1 Sr 0 kB T

ln Rct ¼ ln

ð5Þ

R n2e F 2 cmax Af ðM Liþ Þð1aÞ ð1  xÞð1aÞ xa

þ

ðDG  RÞ þ1 RT

ð6Þ

Eqs. (4)–(6), the temperature dependences of RSEI, Re and Rct, which are deduced in the Supporting Information (SI), indicate that there are linear relationships between lnR and 1/T. The barrier energy for jumping (W), the thermal activation energy of the electrical conductivity (Ea) and the activation energy of the intercalation/deintercalation reaction (DG) can be obtained from the slope of the line. Fig. 13 demonstrates the variation in lnRSEI as a function of 1/T at different potentials in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution. The relationship between lnRSEI and 1/T is always strongly linear. The W was calculated to be 18.5, 24.0, 23.3, 26.2 and 23.4 kJ mol1 at 3.85, 3.95, 4.05, 4.15 and 4.25 V, respectively, and the average value was 23.1 kJ mol1. These results indicate that after 10 charge/discharge cycles, W changes less as a function of the potential when there is a stable SEI film on the surface of the LiNi1/3Co1/3Mn1/3O2 electrode. The variation of lnRe as a function of 1/T at different potentials in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution is shown in Fig. 14. The relationship between lnRe and 1/T is always strongly linear. The Ea was calculated to be 49.4, 41.1, 39.7, 45.5 and 44.3 kJ mol1 at 3.85, 3.95, 4.05, 4.15 and 4.25 V, respectively, and the average value was 44.0 kJ mol1.

-600

-500

-20 -15 -10 -5 0 5 10 15 20 25

-400

Z" /Ω

tra of the LiNi1/3Co1/3Mn1/3O2 electrodes in a two-electrode button cell after 10 cycles. The measurements were taken at temperatures ranging from 20 to 25 °C in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution. In a two-electrode button cell, the impedances of the LiNi1/3Co1/3Mn1/3O2 electrode, the electrolyte, and the Li electrode all contribute to the EIS. However, during the lithiation and delithiation cycle, the contributions of the impedances of the electrolyte and the lithium electrode to the EIS are reduced; this reduction is a result of the instant stripping and plating of lithium, which keeps the lithium surface fresh [51]. Therefore, any change in the EIS during the cycling process arises from the LiNi1/3Co1/ 3Mn1/3O2. As a result, we used a two-electrode Li/LiNi1/3Co1/3Mn1/ 3O2 cell to study the temperature dependence of the kinetic parameters. The Nyquist plots of Li/LiNi1/3Co1/3Mn1/3O2 in 1 mol L1 LiPF6– EC: DEC: DMC electrolyte solution were recorded at various temperatures and at potentials of 3.85, 3.95, 4.05, 4.15 and 4.25 V. As an example, the Nyquist plots that were recorded at 4.15 V are shown in Fig. 12; they show that the absolute impedance decreases

-300

-200

-100

0 0

100

200

300

400

500

600

Z' /Ω Fig. 12. The variation in the impedance spectra of the LiNi1/3Co1/3Mn1/3 electrode as a function of temperature in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution.

X.-Y. Qiu et al. / Journal of Electroanalytical Chemistry 688 (2013) 393–402

Fig. 15 depicts the variation in lnRct as a function of 1/T at different potentials in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution. The relationship between lnRct and 1/T is always strongly linear. The DG was calculated to be 67.4, 65.0, 66.5, 66.1 and 67.6 kJ mol1 at 3.85, 3.95, 4.05, 4.15 and 4.25 V, respectively, and the average value was 66.5 kJ mol1. These results indicate that the activation energy of the intercalation/deintercalation reaction remained nearly constant after the insulator-to-metal transition.

3.6

3.2

lnRSEI /Ω

2.8

3.85 V 3.95 V 4.05 V 4.15 V 4.25 V

2.4

2.0

4. Conclusions

1.6

EIS was used to characterise the electronic and ionic transport properties of commercial available layered LiNi1/3Co1/3Mn1/3O2 materials as cathode of lithium ion battery, as well as to study the charge transfer reaction at the electrode/electrolyte interface. The EIS spectra were collected as a function of electrode potential and temperature. The first delithiation/lithiation process was conducted in a three-electrode glass cell. The EIS spectra exhibit three semicircles and a slightly inclined line that appear successively as the frequency decreases. The high-frequency, the middle-frequency, and the low-frequency semicircles were attributed to the migration of the lithium ions through the SEI films, the electronic properties of the material and the charge transfer step, respectively, and the inclined line as ascribed to the solid state diffusion process. At early delithiation there was a dramatic change in the electrical conductivity of the layered LiNi1/3Mn1/3Co1/3O2 that was caused by an insulator-to-metal transition, as the same pattern observed in LiCoO2. Along with raising potential from 3.5 to 4.4 V, the electrical conductivity of the LiNi1/3Co1/3Mn1/3O2 increased by two orders of magnitude, i.e. from 1.3  106 to 7.3  104 S cm1. The characteristics of the impedance spectra of the LiNi1/3Co1/ 3Mn1/3O2 electrode are strongly influenced by the temperature. Linear relationships between lnRSEI and 1/T, lnRe and 1/T, as well as lnRct and 1/T are confirmed. As a consequence, the kinetic parameters W, Ea and DG could be calculated from the slopes of these linear variations. In an electrolyte solution of 1 mol L1 LiPF6–EC: DEC: DMC, the values of W, Ea and DG have been determined quantitatively 23.1, 44.0 and 66.5 kJ mol1, respectively.

1.2 3.3

3.4

3.5

3.6

3.7 -1

1000T /K

3.8

3.9

4.0

-1

Fig. 13. The variation in the logarithm of the RSEI of the LiNi1/3Co1/3Mn1/3 electrode as a function of 1/T in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution.

4.8 4.2 3.6

lnR e /Ω

401

3.85 V 3.95 V 4.05 V 4.15 V 4.25 V

3.0 2.4 1.8 1.2 0.6 3.3

3.4

3.5

3.6

3.7 -1

1000T /K

3.8

3.9

4.0

-1

Fig. 14. The variation in the logarithm of the Re of the LiNi1/3Co1/3Mn1/3 electrode as a function of 1/T in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution.

Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (2010LKHX03, 2010QNB04, 2010 QNB05) and major State Basic Research Development Program of China (2009CB220102).

6

lnRct /Ω

5

4

3.85 V 3.95 V 4.05 V 4.15 V 4.25 V

Appendix A. Supplementary material This file includes derivations about the temperature dependences of RSEI, Re and Rct, and some results for Nyquist plots after 10 cycles, at various temperatures, obtained with a two-electrode button cell in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution recorded at 3.85, 3.95, 4.05 and 4.25 V, including Figs. S1–S4. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jelechem.2012.09. 027.

3

2

References 1 3.3

3.4

3.5

3.6

3.7 -1

1000T /K

3.8

3.9

4.0

-1

Fig. 15. The variation in the logarithm of the Rct of the LiNi1/3Co1/3Mn1/3 electrode as a function of 1/T in 1 mol L1 LiPF6–EC: DEC: DMC electrolyte solution.

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