Temperature dependence of the electrochemical behavior of LiCoO2 in quaternary ammonium-based ionic liquid electrolyte

Solid State Ionics 176 (2005) 2219 – 2226 www.elsevier.com/locate/ssi

Temperature dependence of the electrochemical behavior of LiCoO2 in quaternary ammonium-based ionic liquid electrolyte Honghe Zheng a,*, Jianhua Qin a, Yang Zhao a, Takeshi Abe b, Zempachi Ogumi b b

a College of Chemistry and Environmental Sciences, Henan Normal University, Xinxiang, 453007, P.R. China Department of Energy and Hydrocarbon Chem. Graduate School of Engineering, Kyoto University, Kyoto, 615-8510, Japan

Received 23 February 2005; received in revised form 7 June 2005; accepted 24 June 2005

Abstract Temperature significantly influences the physiochemical characteristics of trimethyl-n-hexylammonium bis(trifluoromethanesulfone) imide (TMHA-TFSI) room temperature ionic liquid containing differing concentrations of LiTFSI salt. Electrochemical behavior of a LiCoO2 electrode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte was investigated under different temperatures by using cyclic voltammetry, galvanostatic measurements and electrochemical impedance spectroscopy. A discharge capacity of 128.2 mAh/g and 92.1% coulombic efficiency was obtained in the first electrochemical cycle at an optimum temperature of 35 -C. Decreasing temperature resulted in lower reversible capacity whereas increasing temperature led to serious electrochemical decay with electrochemical cycles. Mechanism related to the obvious temperature dependence of the LiCoO2 electrochemical behavior in the ionic electrolyte was discussed. High internal resistance and the resultant large IR drop between the cut-off voltage are dominant factors responsible for the low reversible capacity under low temperature. At elevated temperature, irreversible structural degradation of the LiCoO2 during electrochemical process interprets the electrochemical deterioration with electrochemical cycles. D 2005 Elsevier B.V. All rights reserved. Keywords: Room temperature ionic liquid; Lithium ion batteries; LiCoO2 cathode; Electrochemical behavior; Activation energy

1. Introduction As advanced functional non-aqueous liquid systems, room temperature ionic liquids (RTILs) have been recently attracting considerable interest in the field of battery research due to the advantages of nonflammability, high thermal stability, wide liquid phase range, and negligible vapor pressure etc. [1– 5]. Ionic electrolytes are promising for production of lithium (ion) batteries based on the concepts of safety and Fgreenness_. This makes the advanced battery systems possible for practical use in large-scale power systems such as electric vehicles, and even for operation over a wide range of temperature. Recently, a lot of RTILs are emerging every year with various physical and chemical properties suitable for various scientific and technical purposes. Combination of 1-ethyl-3* Corresponding author. Tel.: +86 373 3328168; fax: +86 373 3326544. E-mail address: [email protected] (H. Zheng). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.06.020

methyl-imidazolium cation with various anions produces RTILs with low viscosity and high conductivity. Application of these ionic liquids into different energy storage devices such as electric double layer capacitor and solar cells has been recently attracting considerable interest. However, practical application of imidazolium-based ionic electrolytes into lithium (ion) batteries seems to be difficult. The greatest difficulty is associated the poor cathodic stability of the ionic liquid. Lithium and carbon is excluded because imidazolium cations are prone to be reduced at the surface of lithium metal or at carbon surface when the electrode is polarized to 0.7 V vs. Li/Li+ [6– 9]. Combining more stable cations with some anions produces ionic liquids with wider electrochemical windows. RTILs based on tetraalkyl ammonium, pyrrolidinium or piperidinium cations are satisfactorily stable against cathodic reduction on lithium metal or carbon surface. Application of these ionic electrolytes into lithium (ion) batteries has shown better prospects [2,10,11].

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H. Zheng et al. / Solid State Ionics 176 (2005) 2219 – 2226

Although with excellent thermal stability and wide liquid phase range, physiochemical properties of ionic liquids such as viscosity, conductivity, density, adsorption and electrochemical stability are strongly temperature dependent [12,13]. When used as the electrolyte into lithium (ion) batteries, the remarkable variation of physiochemical properties with temperature will inevitably influence the battery’s performance. However, the influence of temperature on the electrochemical behavior of LiCoO2 cathode in ionic electrolyte has rarely been reported so far. This present work addresses the temperature dependence of some physiochemical properties of trimethylhexylammonium bistrifluoromethane sulfonylimide (TMHA-TFSI) ionic liquid containing differing concentrations of LiTFSI salt. Temperature effects on the electrochemical behavior of a LiCoO2 cathode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte were investigated. Possible mechanisms related to the temperature dependence of LiCoO2 electrochemical behavior in the ionic electrolyte were discussed.

2. Experimental TMHA-TFSI ionic liquid was prepared, thoroughly purified and dehydrated as described in the literature [14]. Water content of the TMHA-TFSI sample was determined less than 20 ppm. The ionic electrolyte solution was prepared by adding different amounts of LiTFSI salt as the supporting electrolyte. Electrolyte viscosities were determined by using a suspended level Ubbelohde viscometer with a thermostatic oil bath. Flow time measurements were performed by a Schott AVS 310 photoelectric time unit with a resolution of 0.01 s. The viscometer was calibrated using the efflux time of water. Solution viscosity, g, is calculated from the following equation: q=g ¼ ct  k=t

ð1Þ

where q is the solution density, c and k are the viscometer constants, and t is the efflux time. Solution densities were determined with an Anton Paar DMA 60/602 vibrating-tube digital densimeter with the same temperature precision as the viscosity measurements. The estimated error of the experimental viscosity is T 0.5%, and of the experimental density is T 1 10 5 gIcm 3. The details of the experimental procedure are given elsewhere [15]. The electrolyte conductivity was measured with the ac impedance method in the frequencies range of 10 MHz to 100 Hz, using a conductivity cell (cell constant = 1.02). The cell was calibrated with 0.1 M KCl solution. The temperature was controlled to be as precise as the viscosity measurements. The estimated error of the experimental conductivity is T 1%. Electrochemical windows of the neat TMHA-TFSI ionic liquid were measured with cyclic voltammetry. Lithium foil was used as both counter and reference electrode. Pt plate,

Cu foil, Ni foil, Al foil and actylene black were employed as the working electrode, respectively. The electrode was polarized in the range of 0¨5 V vs. Li/Li+ with a scan rate of 0.1 mV/s. LiCoO2 electrode was prepared by mixing LiCoO2 powder with binder (PVDF) and conductor (acetylene black) by a weight ratio of 85 : 5 : 10. We then rolled the mixture onto Al foil and cut it into pellets. The loading of active material was ca. 2 mg/cm2. Three electrode cells were assembled in an argon-filled glove box with the dew point below  80 -C. 1 M LiTFSI/TMHA-TFSI ionic electrolyte was used as the electrolyte. Lithium foil was used as counter and reference electrode, respectively. Cyclic voltammetry of the LiCoO2 cathode was conducted over the potential range of 3.0 –4.5 V at 0.1 mV/s scan rate unless otherwise specified. The constant current charge– discharge experiment was carried out with the current density of 15 mA/g. The operating voltage was set between 3.3 and 4.3 V. The impedance spectra (100 kHz to 10 mHz, 5 mV perturbation) were recorded by using Sorlatron 1255 with a three-electrode cell. Prior to AC impedance measurements, the LiCoO2 electrode was held at 3.98 V for at least 1 h to attain the condition of sufficiently low residual current. Structural characterization of the LiCoO2 electrode after electrochemical cycles in the ionic electrolyte under different temperatures was performed by using X-ray diffraction (XRD) with an automated Rigaku X-ray diffractometer using a Cu Ka radiation. The diffraction angle (2h) was measured between 10- and 70- with an increment of 1-/min.

3. Results and discussion 3.1. Physiochemical properties of TMHA-TFSI ionic liquid containing differing concentrations of LiTFSI salt Since many applications require the density data, density is a most often reported physical property of room temperature ionic liquids. However, many of the reported density values were given at a room temperature of 25 -C. In order to estimate the volume property of an electrolyte under different temperatures, the variation of electrolyte density with temperature should be addressed. Temperature dependence of the TMHA-TFSI ionic liquid containing differing concentrations of LiTFSI salt was shown in Fig. 1. Experimental density data shows a linear dependence on temperature. A linear equation of the following form is confirmed: q ¼ a þ bT

ð2Þ

where q is the density, a and b are constants which are variable according to the LiTFSI concentration, and T is the absolute temperature. This equation is useful in predicting the volume property of the ionic electrolyte within a battery under different temperatures.

H. Zheng et al. / Solid State Ionics 176 (2005) 2219 – 2226

1.44

1.0 M

Density/g.cm

-3

1.42 1.40

0.5 M

1.38 0.25 M

1.36

0M

1.34 280

290

300

310

320

330

340

Temperature/K Fig. 1. Variation of solution density with temperature for the TMHA-TFSI ionic liquid containing differing concentrations of LiTFSI salt.

Viscosity is an important physical property affecting many solution properties including the solution fluidity, wettability, and diffusion of ions. It therefore affects the practical issues. Ionic liquid systems are known much more viscous than traditional organic solvents commonly used in lithium (ion) batteries. For pure TMHA-TFSI ionic liquid, the dynamic viscosity is determined to be 120.3 mPaIs under room temperature (30 -C). Addition of LiTFSI salt significantly increases the electrolyte viscosity. When 1 M LiTFSI salt was dissolved into the ionic liquid, a viscosity of 687.4 mPaIs was obtained. Compared with the viscosity of EC-DEC of around 10 mPaIs at room temperature, this viscosity value of 1 M LiTFSI/TMHA-TFSI ionic electrolyte is considerably high. The temperature dependence of the dynamic viscosity is also very complicated compared with that of non-aqueous organic electrolyte. As shown in Fig. 2, viscosity of the TMHA-TFSI solution increases as the temperature decreases with tendencies depending on the LiTFSI concentration. The higher the LiTFSI concentration is in the liquid, the more rapidly the viscosity increases with 1600

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decreasing temperature. For the pure TMHA-TFSI, there is about 5-fold increase of the viscosity when decreasing temperature from 60 to 20 -C. On the other hand, for 1 M LiTFSI/TMHA-TFSI ionic solution, nearly 15-fold increase was obtained in the same temperature range. This fact implies that TMHA-TFSI containing more LiTFSI salt has a higher activation energy of viscosity. The large activation energy is often attributed to the strong association between cations and anions within the ionic system [12]. Fig. 3 displays the Arrhenius plots of the conductivity of the TMHA-TFSI ionic liquid containing differing concentrations of LiTFSI salt. At room temperature (30 -C), 14.1 10 4 S cm 1 is measured for the neat TMHA-TFSI ionic liquid. Addition of LiTFSI salt significantly decreased the electrolyte conductivity. When 1 M LiTFSI salt was dissolved into the ionic liquid, the conductivity was determined to be 2.56  10 4 S cm 1. Temperature dependence of the conductivity shows an inverse trend with that of viscosity. Elevating temperature increases the electrolyte conductivity with growing tendencies for the solution containing more LiTFSI salt. In the 10 -C to 60 -C temperature range, conductivity of the pure TMHA-TFSI ionic liquid varies between 4.5 and 42  10 4 S cm 1 (about 10-fold increase). For 1 M LiTFSI/TMHA-TFAI ionic electrolyte, the conductivity varies from 0.61 to 13.6  10 4 S cm 1 (more than 20-fold increase) in the same temperature range. These variations are well consistent with the change in the viscosity and present strong interactions between ions within the ionic electrolyte. Compared with the conductivity of many conventional organic electrolytes of around 10 2¨10 3 S cm 1 and around 10 4¨10 5 S cm 1 for many polymer electrolytes at room temperature, these conductivity values we got fall in the expected range for lithium (ion) battery applications. The electrochemical windows of the TMHA-TFSI ionic liquid on the surfaces of different materials were determined by using cyclic voltammetry with a scan rate of 0.1 mV/s as

4

1.0 M

1400 3 0.5 M

ln (σ / 10-4 cm-1)

Viscosity/mPa s

1200 1000 800 600 400

0.25 M

0M

2 0M 0.25 M

1

0.5 M

0

200

1.0 M

0 -1

280

290

300

310

320

330

340

Temperature/K Fig. 2. Variation of the viscosity of the TMHA-TFSI ionic liquid containing differing concentrations of LiTFSI with temperature.

3.0

3.1

3.2

3.3

3.4

3.5

3.6

-1

1000/T (K ) Fig. 3. Arrhenius plot of the conductivity with temperature for TMHA-TFSI containing differing concentrations of LiTFSI salt.

H. Zheng et al. / Solid State Ionics 176 (2005) 2219 – 2226

3.2. Temperature dependence of the electrochemical behavior of LiCoO2 cathode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte

o

80 C

0.8 0.6

o

50 C

0.4

o

35 C o

20 C

0.2 0.0 -0.2 -0.4 -0.6 -0.8 3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

Voltage/ V Fig. 5. Temperature effects on the cyclic voltammograms of the LiCoO2 cathode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte at the scan rate of 0.1 mV/s.

ing cathodic peaks demonstrate that the lithium extraction/insertion reaction is facilitated by an increase of temperature. Rate capability of the LiCoO2 cathode was examined by using cyclic voltammetry at various scan rates. Fig. 6 shows the cyclic voltammograms of the LiCoO2 cathode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte under 20 -C at 0.02 and 0.1 mV/s scan rate, respectively. At the scan rate of 0.02 mV/s, there exists a great potential difference between the anodic peak (4.15 V) and the cathodic peak (3.55 V) indicating a large potential hysteresis during the electrochemical process. Increasing the scan rate to 0.1 mV/s produced a very different CV profile. Instead of the appearance of anodic and cathodic peaks, a straight anodic line in the potential range from 4.1 to 4.5 V and a cathodic line at potential below 3.75 V were observed. The appearance of anodic and cathodic lines implies that there exists a speed limit in the lithium extraction and insertion

0.05mA/cm

0.20

Al 0.15

Pt

0.1 mV/s 2

Ni

Current/ mA/cm

Current Density / mA/cm

2

2

Temperature plays an important role on many physiochemical properties of the quaternary ammonium-based ionic electrolyte. The electrochemical behavior of an electrode in the ionic electrolyte must thus be strongly temperature dependent. Fig. 5 shows the cyclic voltammograms of a LiCoO2 cathode in 1 M LiTFSI/TMHATFSI ionic electrolyte at the scan rate of 0.1 mV/s under different temperatures. At low temperature (20 -C), peaks are broadened and the peak intensity is very weak indicating the lithium extraction and insertion are strongly hindered due to slower kinetics. With increasing temperature, the peak intensity grows rapidly and the peaks are narrowed. The fast growing peak intensity and the approaching between the anodic and the correspond-

1.0

2

shown in Fig. 4. The electrochemical stability of the TMHA-TFSI ionic liquid was found strongly dependent upon the electrode materials. On the surfaces of Pt plate and Al foil, there is no obvious reduction and oxidation reaction over the potential range from 0 to 5 V vs. Li/Li+. However, on the surfaces of Ni foil, Cu foil and acetylene black, noticeable reduction and/or oxidation reactions were observed. If we define the potential where reduction reaction occurs on a material as the cathodic limit, we get the cathodic limit of the TMHA-TFSI ionic liquid becomes negative in the order: acetylene black > Ni > Cu > Al å Pt. On the other hand, the anodic limit becomes positive in the order: Cu < acetylene black < Ni
Current density/ mA/cm

2222

Cu Acetylene Black

0.10

0.02 mV/s

0.05 0.00 -0.05 -0.10

0

1

2

3

4

5

Voltage / V Fig. 4. Electrochemical windows of TMHA-TFSI ionic liquid on the surfaces of different materials.

-0.15 3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

Voltage/V Fig. 6. Influence of the scan rate on the cyclic voltammograms of the LiCoO2 cathode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte under 20 -C.

H. Zheng et al. / Solid State Ionics 176 (2005) 2219 – 2226

o

o

o 20 C 35 C 50 C

4.2

Voltage/V

4.0 3.8 3.6 3.4 3.2 3.0 2.8 0

30

60

90

120

150

180

Capacity/(mAh/g) Fig. 7. Charge-discharge profiles of the LiCoO2 cathode in 1 M LiTFSI/ TMHA-TFSI ionic electrolyte under different temperatures.

from/into LiCoO2 in 1 M LiTFSI/TMHA-TFSI ionic electrolyte. This result shows that the rate capability of the Li/ionic electrolyte/LiCoO2 cell is poor. LiCoO2 cathode is not capable of heavy current density in the ionic electrolyte. Therefore, a current density of 15 mA/g was applied in the galvanostatic measurements of this study. The influence of temperature on the charge– discharge properties of the Li/ionic electrolyte/LiCoO2 cell at a constant current of 15 mA/g is shown in Fig. 7. As shown in Fig. 7, LiCoO2 cathode can be effectively cycled in 1 M LiTFSI/TMHA-TFSI ionic electrolyte. Similar with LiCoO2 electrode in conventional organic liquid electrolyte, charge and discharge potential plateaus of around 4 V are observed in the ionic liquid. At a temperature of 35 -C, the LiCoO2 electrode performance is almost equivalent to it in a conventional liquid electrolyte. In the first electrochemical cycle, a discharge capacity of 128.2 mAh/g is obtained with 92.1% coulombic efficiency. This result is well compared with that of LiCoO2 in other different ionic electrolytes reported in the literature. A Li –Al/LiCoO2 battery using a 1-methyl-3-ethylimidazolium chloride room temperature molten salt with the addition of 0.05 mol/kg C6H5SO2Cl showed satisfactory results with a discharge capacity of 112 mAh/g and better than 90% coulombic efficiency [6]. Nakagawa et al. [7] examined the electrochemical properties of a LiCoO2 electrode in LiBF4/EMI-BF4 ionic electrolyte with Li4Ti5O12 as the counter electrode. A discharge capacity of the LiCoO2 electrode was obtained to be around 120 mAh/g with 71.4% coulombic efficiency in the first cycle. Most recently, Garcia et al. [8] presented a lithium-ion cell using LiCoO2 and Li4Ti5O12 as the electrode materials with EMI-TFSI ionic electrolyte. LiCoO2 cathode retained a reversible capacity of 106 mAh/g after 200 cycles. Sakaebe et al. [11] investigated the electrochemical properties of a LiCoO2 cathode in N-

methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) imide (PP13-TFSI) ionic electrolyte in which more than 130 mAh/g reversible capacity was obtained at C/10 current rate. Temperature greatly influences the Charge– discharge performances of the LiCoO2 cathode. On the one hand, the charge potential plateau is obviously higher than the discharge plateau under low temperature reflecting a large potential hysteresis. The charge and discharge potential plateau approaches with increasing temperature demonstrating the decrease of overpotential. This is consistent with the approaching anodic and cathodic peaks with increasing temperature obtained in cyclic voltammograms. On the other hand, the electrode capacity cannot attain a satisfactory value under low temperature. Despite with satisfactory Coulombic efficiency of 88.6%, a discharge capacity of only 108.9 mAh/g was delivered in the first cycle at 20 -C. At an elevated temperature (50 -C), a charge capacity of 157.1 mAh/g was obtained with a discharge capacity of only 120.2 mAh/g resulting in 76.5% coulombic efficiency. An optimum temperature of 35 -C was obtained at which LiCoO2 electrode exhibits satisfactory electrochemical performances in terms of discharge capacity and coulombic efficiency in the first cycle. The cycling behavior of the LiCoO2 cathode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte is significantly influenced by temperature too. As shown in Fig. 8, satisfactory cycleability of the LiCoO2 cathode in the ionic electrolyte was obtained at 20 -C. Under this temperature, no obvious capacity loss was observed during the initial 10 electrochemical cycles. With increasing temperature, a distinctive loss of capacity occurs. At 35 -C, a noticeable capacity loss was observed in the initial 10 electrochemical cycles. When the temperature was set at 50 -C, the electrode capacity fades rapidly with increasing electrochemical cycles. After 10 electrochemical cycles, the LiCoO2 electrode lost almost half of its initial capacity.

140

Capacity/(mAh/g)

4.4

2223

120

o

20 C

100

o

35 C 80

o

50 C 60 0

2

4

6

8

10

Cycle Number Fig. 8. Cycleability of the LiCoO2 cathode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte under different temperatures.

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H. Zheng et al. / Solid State Ionics 176 (2005) 2219 – 2226 200

3.3. Mechanisms affecting the electrochemical behavior of the LiCoO2 cathode in the ionic electrolyte under different temperatures To explain the low reversible capacity of the LiCoO2 cathode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte under low temperature, electrochemical impedance spectroscopy (EIS) was employed because this technique provides kinetic information of the intercalation compound and allows the resolution of various physical processes. Fig. 9 shows a sequence of characteristic Nyquist plots obtained at different deintercalation potentials at 35 -C. The high frequency limit refers to the bulk electrolyte resistance. The high-to-medium frequency semicircle, which didn_t show obvious evolution with the electrode potential, is ascribed to the contact resistance at the oxide particle/current collector interface or oxide particle/particle interface [16 – 18]. The semicircle in the middle frequency region, which is strongly potential dependent, is attributed to the charge transfer process at the interface of the LiCoO2/ionic electrolyte [17,18]. The sloping line at low frequencies is ascribed to a semi-infinite diffusion of lithium ions in the LiCoO2 electrode, which is not clearly distinguishable. The overall impedance varies greatly as the lithium deintercalation proceeds. The overall impedance at any potential is several times larger than that obtained in conventional organic electrolyte. The large electrolyte resistance, is associated with the low electrolyte conductivity. The high contact resistance at the oxide particle/current collector interface or oxide particle/ particle interface and the large charge transfer resistance at electrode /electrolyte interface are likely to be the origin of the poor wettability of the ionic electrolyte with the active materials. The oxide particles covered by PVDF will not be

3.80V

600

500 3.90V

-Z''/Ω

400

300 4.00V 200 4.05V 100 4.15V 0 0

100

200

300

400

500

Z'/Ω Fig. 9. Series of Nyquist plots of the LiCoO2 cathode in the ionic electrolyte at some significant potentials under 30 -C.

-Z'(Ω)

150

100 o

50

26 C o 30 C o 35 C o 40 C o 45 C o 50 C o 55 C o 60 C

0 0

100

200

300

400

Z(Ω) Fig. 10. Nyquist plots of the LiCoO2 cathode in the ionic electrolyte at 3.98 V under different temperatures.

effectively wetted by the ionic solution. Temperature dependence of the Nyquist plots for the LiCoO2 cathode at a potential of 3.98 V is shown in Fig. 10. Decreasing temperature results in the increase in the electrolyte viscosity, the decrease in the conductivity and the poor wettibility of the electrolyte with active materials. The overall impedance increases swiftly with decreasing temperature. The higher internal resistance under low temperature produces large potential hysteresis. From this point of view, the increasing IR drop for the electrochemical cycling carried out between 4.3 –3.3 cut-off voltage is thus an important factor leading to the low reversible capacity of the LiCoO2 electrode in the ionic electrolyte under low temperature [19,20]. With increasing temperature, the high frequency limit shift in the negative direction on the real axis illustrating the decrease of the electrolyte resistance. Shrinkage of the two semicircles with increasing temperature shows that all the reaction steps of the lithium intercalation are thermally activated processes. Of the two internal resistances, the charge transfer resistance at the electrolyte/electrode interface is strongly dependent on temperature and is therefore a rate-controlling process of the lithium intercalation into the LiCoO2 electrode. The activation energy for the interfacial Li ion transfer at 3.98 V was evaluated from the temperature dependence of the charge transfer resistances as shown in Fig. 11. The apparent activation energy of the charge transfer was calculated from the slope of the plot. As a result, a value of 0.6 T 0.05 eV was obtained for the LiCoO2 cathode in the ionic electrolyte. This value of activation energy is considerably large in comparison with that of 0.3 eV for lithium ion conduction through LiCoO2 reported by Nakamura et al. [21] and that of 0.2 eV at the interface of LiCoO2 in a conventional organic electrolyte reported by Iriyama et al. [22]. The large activation energy indicates that a high energy barrier exists at the LiCoO2 /ionic electrolyte interface. The high energy barrier hinders lithium ion

H. Zheng et al. / Solid State Ionics 176 (2005) 2219 – 2226 -1.2 -1.4

Log( 1/R)

-1.6 0.6 eV -1.8 -2.0 -2.2 -2.4 2.9

3.0

3.1

3.2

1/Tx10

3.3

3.4

3.5

3

Fig. 11. Temperature dependence of charge transfer resistance at the LiCoO2/ionic electrolyte interface.

transfer at the interface between LiCoO2 electrode and the ionic electrolyte. The high charge transfer resistance and the large activation energy control the electrode reaction rate [23]. This result may account for the poor rate capability of the LiCoO2 electrode in the ionic electrolyte under low temperature. At elevated temperatures, electrochemical deterioration of the LiCoO2 cathode in 1 M LiTFSI/TMHA-TFSI ionic electrolyte may be controlled by different mechanisms. To elucidate the mechanisms for the serious capacity loss of the LiCoO2 cathode in the ionic electrolyte, the electrolyte stability and the structural stability of the LiCoO2 in the ionic electrolyte were examined under different temperatures. As shown in Fig. 12, increasing temperature promotes the oxidation reaction of TMHA-TFSI on acetylene black. The onset of oxidation potential becomes negative and the intensity of oxidation current gets stronger. As it is generally accepted, oxidation of electrolyte accumulates a resistive surface layer on the cathode active

2225

material causing voltage drop and a slowdown in the electrode reaction rate [24]. The oxidation products may also bring about Co dissolution [25]. From this point of view, electrolyte oxidation on the conductor at elevated temperature may be a factor responsible for the electrochemical deterioration of LiCoO2 electrode. Aurbach et al. [24] pointed out that the apparent capacity fading of LiCoO2 in 1 M LiPF6/EC-DMC electrolyte is attributing mostly to the surface phenomena at the electrode/electrolyte interface. However, considering that the electrolyte oxidation occurs mainly at around 5 V potential, which is considerably higher than the upper limit value of 4.3 V. The electrolyte oxidation should not be the predominant factor for the LiCoO2 capacity fading. Because increasing temperature significantly decreases the electrolyte viscosity, improves the wettability with the electrode, and increases the electrolyte conductivity, the internal resistance of the Li/ionic electrolyte/ LiCoO2 cell is greatly reduced by heating. The decrease of internal resistance contributes to the lower electrode overpotential during electrochemical process. Under such a situation, the charge becomes deeper than the case of low temperature at which there is a higher overpotential when the same upper limit value is applied. Excess lithium ions would be extracted from the LiCoO2 cathode. A charge capacity of 157 mAh/g was obtained for lithium extraction at 50 -C. Upon repeated removal and insertion of Li, LiCoO2 undergoes a sequence of structural degradation accompanied by changes in crystal symmetry and a variety of defects, especially when the electrode is polarized up to higher voltages (e.g., 4.3 V) or excess lithium is extracted [26,27]. Fig. 13 shows the X-ray diffraction profiles of the LiCoO2 electrode after 10 electrochemical cycles in the ionic electrolyte at different temperatures. With increasing temperature, the typical diffraction peak (003) shifts toward

(110)

(107)

o

40 C

(101) (006)

o

60 C 0.10

(108)

Intensity/(arbitary)

2

Current/(mA/cm )

0.15

(104)

o

80 C

(003)

0.20

a

0.05 o

20 C

b

0.00

-0.05 3.0

A

10

3.5

4.0

4.5

5.0

5.5

20

B

C

c 30

40

50

60

70

2θ/angle

Voltage/V Fig. 12. Oxidation behavior of the TMHA-TFSI ionic liquid on the surface of acetylene black under different temperatures.

Fig. 13. XRD patterns of the LiCoO2 electrode after 10 electrochemical cycles in 1 M LiTFSI/TMHA-TFSI ionic electrolyte at (a) 20, (b) 35, (c) 50 -C.

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H. Zheng et al. / Solid State Ionics 176 (2005) 2219 – 2226

a low angle. The peak is broadened and the intensity is decreased. This result indicates a change of the LiCoO2 structural parameter, which is most probably associated with the changes in crystal symmetry. In addition, new peaks designated by A, B and C appeared in the XRD pattern when the electrode was cycled at 50 -C. The appearance of these new peaks suggests the formation of new phase within the electrode. The structural degradation of LiCoO 2 correlates with the observed electrochemical performance. From this viewpoint, the irreversible phase transformation of LiCoO2 in the ionic electrolyte interprets the electrochemical decay under high temperature.

4. Conclusion Although with wide liquid phase temperature range, the physiochemical properties of the quaternary ammoniumbased ionic electrolyte are strongly temperature dependent. The electrochemical performance of LiCoO2 in 1 M LiTFSI/ TMHA-TFSI ionic electrolyte was greatly affected by temperature. An optimum operating temperature of around 35 -C was obtained for the Li/ionic electrolyte/LiCoO2 cell in terms of reversible capacity and coulombic efficiency in the first cycle. The high internal resistance aroused from the high electrolyte viscosity and poor wettability with the active material results in a serious voltage hysteresis. A large IR drop between the 4.3– 3.3 cut-off voltage accounts for the low electrode capacity under low temperature. The irreversible structural conversion of LiCoO2 in the ionic electrolyte interprets the electrochemical decay of LiCoO2 electrode under high temperature.

Acknowledgments The authors are greatly indebted to the funding of Natural Science Foundation of China (NSFC, contract no. 20273019) and the young researcher program of Henan Province (04120001100), China.

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