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Polypyrrole-coated LiCoO2 nanocomposite with enhanced electrochemical properties at high voltage for lithium-ion batteries
Journal of Power Sources 281 (2015) 49e55
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Polypyrrole-coated LiCoO2 nanocomposite with enhanced electrochemical properties at high voltage for lithium-ion batteries Jingchao Cao, Guorong Hu, Zhongdong Peng, Ke Du, Yanbing Cao* School of Metallurgy and Environment, Central South University, Changsha 410083, China
h i g h l i g h t s PPy-coated LiCoO2 particles are prepared by a chemical polymerization method. The PPy film, like a capsule shell, avoids LiCoO2 corrosion from HF attacking. The PPy film builds a conductive network and increases the electronic conductivity. The electrochemical properties of LiCoO2 are enhanced after coating with PPy.
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Article history: Received 24 November 2014 Received in revised form 8 January 2015 Accepted 29 January 2015 Available online 29 January 2015
A conducting polypyrrole thin film is successfully coated onto the surface of LiCoO2 by a simple chemical polymerization method. The structure and morphology of pristine LiCoO2 and PPy-coated LiCoO2 are investigated by the techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Energy dispersive X-ray spectroscopy (EDXS), Fourier transform infrared spectrometry (FTIR) and thermogravimetric analysis (TGA) further demonstrate the existence of PPy. The electrochemical properties of the composites are investigated by galvanostatic chargeedischarge test and AC impedance measurements, which show that the conductive PPy film on the surface significantly decrease the charge-transfer resistance of LiCoO2. The PPy-coated LiCoO2 exhibits a good electrochemical performance, showing initial discharge capacity of 182 mAh g1 and retains 94.3% after 170 cycles. However, the retention of pristine LiCoO2 is only 83.5%. The rate capability results show that the reversible capacity retention (10C/0.2C) of LiCoO2 increases from 52.4% to 80.1% after being coated with PPy. The continuously coated thin PPy film is just like a capsule shell, which can protect the core (LiCoO2) from corrosion causing by the HF attacking and greatly reduce the dissolution of Co into electrolyte. © 2015 Elsevier B.V. All rights reserved.
Keywords: PPy-coated LiCoO2 Chemical polymerization Electrochemical properties Lithium-ion batteries
1. Introduction Lithium-ion batteries (LIBs) have been extensively used for a wide variety of applications, for instance, from portable electronic equipments to hybrid and electric vehicles. The performance of the batteries is often decided by the properties of cathode material. Among various cathode materials, lithium cobalt oxide (LiCoO2) is currently one of the most popular cathode materials in batteries for portable devices due to its high energy density, ease of preparation, good C-rate capacity and long cycle life [1e5]. LiCoO2 has a specific capacity of 137 mAh g1 during the (de)lithiation process from 2.75
* Corresponding author. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.jpowsour.2015.01.174 0378-7753/© 2015 Elsevier B.V. All rights reserved.
to 4.2 V vs. Li/Liþ, only 0.5 Liþ per molecule of LiCoO2 can be extractable [6,7]. Increasing the reversible capacity is strongly demand with rapidly growing demands for higher energy density lithium-ion batteries. One promising attempt to increase the capacity of LiCoO2 is charging cells above 4.2 V [8,9]. However, the structural instability and capacity fading is more severely which is caused by easy dissolution of Co4þ into the acidic electrolyte medium, as well as a large number of side reactions on the surface of the cathode particles [4,10]. Meanwhile, attempts have been made to resolve the problems by doping non-transition and transition metal ions into the LiCoO2 structure by partial substitution of cobalt, but capacity fading has been still observed even though some improvement has been achieved [5,11]. As an alternative, the surface modification of LiCoO2 with inorganic materials such as LiFePO4 [12], LiMgPO4 [13],
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LiNi0.5Mn1.5O2 [8], LiNbO3 [14], LiMn2O4 [15], Li4Ti5O12 [16], Li [Li0.2Mn0.6Ni0.2]O2 [17], Al2O3 [18], ZnO [19], LiF [20], MgF2 [21] and FePO4 [22] has been extensively investigated and achieved good effect on inhibiting capacity fading. However, the inorganic materials are discontinuously deposited onto the surface of LiCoO2, tending to act as an inert layer regarding ionic conduction and also require complex coating process. Polypyrrole (PPy), a typical soft conducting polymer with good mechanical flexibility and chemical stability during the electrochemical process, has been attracted much attention. PPy can act as a host material for Liþ-ion insertion/extraction in the voltage range of 2.0e4.5 V versus Li/Liþ, with a theoretical capacity of 72 mAh g1 [23]. Therefore, PPy additives can be used as both conductive agents and cathode materials. The application of PPy to improve electrochemical performance of electrode materials for lithium ion batteries have been reported, such as Fe3O4/PPy [24], Fe2O3/PPy [25], LiMn2O4/PPy [26], LiV3O8/PPy [27,28], LiFeO2/PPy [29], LiFePO4/PPy [30] and LiNi1/3Co1/3Mn1/3O2/PPy [31]. But up till now, using PPy as a coating layer to improve the electrochemical properties of LiCoO2 at high voltage has not been reported. In this study, LiCoO2-PPy nanocomposite was synthesized using a chemical polymerization method. The structure, morphology and electrochemical performance of the LiCoO2-PPy are discussed and compared with the performance of bare LiCoO2 cathode material. 2. Experimental 2.1. Synthesis of nanoarchitectured PPy-coated LiCoO2 LiCoO2 powder with an average particle size of 10 mm was prepared in our previous work [17]. The PPy-coated LiCoO2 composite powder was prepared by the chemical polymerization with sodium p-toluenesulfonate (AR, 99%) as the dopant. In order to avoid the introduction of impurity ions such as Fe3þ, H2O2 (AR, H2O2 30%) was chosen to use as the oxidant. LiCoO2 (5 g), H2O2 (2 mL) and p-toluenesulfonate (0.5 g) were dispersed into alcohol solution (50 mL), pyrrole (0.1 g) was diluted into 10 mL with alcohol, and then was slowly added to the mixture with continuously magnetically stirred for 15 h. The final products were filtered and washed with deionized water several times and dried at 60 C in a vacuum oven for 12 h. In order to calculate the PPy content, pure PPy powder was also prepared using the same method above.
prepared by dispersing the 85 wt. % cathode materials, 10 wt. % acetylene black and 5 wt. % polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidinone (NMP). The slurry was coated on an aluminum foil and then dried at 120 C for 20 h under vacuum. The cells were assembled in an argon filled glove box with electrolyte of 1 mol L1 LiPF6 in EC-DMC-EMC (1:1:1 volume ratio) solution, metallic Li as anodes and polypropylene separators (Celgard 2400). The cells galvanostatically charged and discharged in the potential range of 3.0e4.5 V (vs. Li/Liþ) at different current densities using LAND CT2001A test system. Cyclic voltammograms measurements were done by a Solartron 1287 electrochemical interface between 2.8 and 4.6 V at a scan rate of 0.1 mV s1. Electrochemical impedance spectroscopy (EIS) analysis was carried out with a three electrode cell system from 1 MHz to 1 mHz by using the potentiostat/galvanostat Model 2273A potentiostat and the Model 5210 lock-in amplifier with 5 mV ac excitation. All tests were performed at room temperature.
3. Results and discussion The XRD patterns of pristine LiCoO2 and PPy-coated LiCoO2 are presented in Fig. 1. It is observed that the pristine LiCoO2 shows a well-defined a-NaFeO2 structure with no minor phases. The PPycoated LiCoO2 composite also conform to a single-phase hexagonal structure of a-NaFeO2 structure is attributed to LiCoO2 structure, and PPy phase is not observed in this pattern. SEM images of the pristine LiCoO2 and the PPy-coated LiCoO2 composite are shown in Fig. 2(a) and (b). The particle size of pristine LiCoO2 powder is about10 mm. It can be seen from Fig. 2(a) the surface of pristine LiCoO2 is smooth and clean. After intruding the PPy, the PPy-coated LiCoO2 exhibits an anomalous coating layer, which is featured with a thin and highly continuous morphology. TEM photograph of PPy-coated LiCoO2 exhibits the thickness of PPy coating layer is approximately 20 nm. In order to further confirm the presence of PPy, energy dispersive X-ray (EDX) mapping was used to observe the distribution of PPy (Fig. 2(e)e(g)). The element of N and C are only existed in PPy. It obviously can be seen from Fig. 2(f) and (g) that the N and C are distributed uniformly throughout the whole area, which indicates that the PPy film had uniformly coated on the surface of LiCoO2 powders. FT-IR spectra obtained for the KBr diluted pellets of pristine
2.2. Materials characterization X-ray diffraction (XRD, D/max-r A type Cu Ka, 40Kv, 300 mA) was employed to identify the crystalline phase of the products. The scan range was 10e80 with a scan speed of 0.02 min1. The morphological and compositional characterization of the asprepared compound was performed by means of scanning electron microscopy (SEM, JEOLJSM-6360LV), transmission electron microscope (TEM, JEOL, 2010) and energy dispersive X-ray spectroscopy (EDXS). Fourier transform infrared spectrometry (FTIR) measurements were carried out on a Hitachi FTIR-8900 spectrometer (Japan) in wave-numbers ranges of 400e4000 cm1 using a KBr wafer. Thermogravimetric analysis (TGA) was carried out on a TG 209 F1 Iris under air atmosphere in the temperature range of 30e800 C at a heating rate of 5 C/min. The dissolution of Co into electrolyte was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 4300DV). 2.3. Electrochemical measurements Electrochemical properties of the materials were investigated by using coin cells (2025 type). The working electrodes were
Fig. 1. XRD patterns of LiCoO2 and PPy-coated LiCoO2 composite.
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Fig. 2. SEM images of LiCoO2 (a), PPy-coated LiCoO2 (b), TEM of PPy-coated LiCoO2 (c), and corresponding EDX mapping for the PPy-coated LiCoO2 image (d) as follows: Co (e), N (f) and C (g).
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LiCoO2 and LiCoO2-PPy composite are shown in Fig. 3. For pristine LiCoO2, the characteristic absorption bands of CoeO and OeCoeCo vibrations are observed at approximately 526 and 613 cm1, respectively [32]. The PPy-coated LiCoO2 has absorption peaks of PPy. The band at 1504 cm1 is due to aromatic C]C in PPy. CeN and CeH show peaks around 1304 and 1001 cm1, respectively. The peak at around 2235 cm1 is attributed to the OeH stretching vibration [33e35]. This result confirms that the PPy has been successfully coated onto the surface of LiCoO2 powder. To quantify the amount of PPy in the PPy-coated LiCoO2 composite, TGA analysis was carried out in air. Fig. 4 shows the TGA curves of the PPy-coated LiCoO2 composite along with those of pristine LiCoO2 and pure PPy powders when heated from room temperature to 800 C with a rate of 5 C min1 in air. It can be seen from Fig. 4 that the pristine LiCoO2 shows a constant weight throughout the sintering process. The pure PPy burns off at 550 C, and there is about 5.5% oxidation remaining. For PPy-coated LiCoO2, there is light mass loss below 220 and 450 C, which is corresponding to the burning of PPy. According the TGA curve, the mass of fraction PPy in PPy-coated LiCoO2 is 1.67%. Fig. 5(a) and (b) shows the typical chargeedischarge curves for different cycles of pristine LiCoO2 and PPy-coated LiCoO2 electrodes in coin cells using lithium as the counter and reference electrode between 3.0 and 4.5 V at 0.2C (75 mA g1). It can be seen that there is no difference in the operation voltage of the first cycle on charge and discharge between pristine LiCoO2 and PPy-coated LiCoO2. Both of the cells have stable and smooth chargeedischarge curves even after 170 cycles. The initial charge and discharge capacity of pristine LiCoO2 are 201.5 and 189 mAh g1, the corresponding columbic efficiency is 93.8%. However, the initial columbic efficiency of PPy-coated LiCoO2 is 92.9% (initial charge capacity is 196.5 and discharge 182 mAh g1), which is slightly lower than that of pristine LiCoO2. This is might be caused by two reasons: one is a small amount of Liþ lost during the process of PPy coating; the other may be that a small amount of Liþ inserted into PPy in the initial charge/discharge process which leaded to irreversible loss of lithium. The comparison of cycle performance for pristine LiCoO2 and PPy-coated LiCoO2 is shown in Fig. 5(c). It is clearly noticed that the PPy-coated LiCoO2 presented superior cycling stability with an initial capacity of 182 mAh g1 and 94.3% of this initial capacity was
Fig. 3. FT-IR spectra measured on the KBr-diluted pellets of pristine LiCoO2 and LiCoO2-PPy composite.
Fig. 4. TGA curves of PPy, LiCoO2 and PPy-coated LiCoO2 composite.
retained after 170 cycles. However, the discharge capacity of pristine LiCoO2 faded quickly and only 83.5% of the original capacity remained after 170 cycles. The conductive PPy thin film on the surface builds the conductive network, which is beneficial to Liþ ions diffusion and reduces the electrode resistance during chargeedischarge processes. What’ more, the continuously coated thin film can avoid direct contact of LiCoO2 and the electrolyte and suppress their reaction on the surface. Both of these two factors are benefit to improving the electrochemical properties of LiCoO2. To further investigate the electrochemical performance of the pristine LiCoO2 and PPy-coated LiCoO2 composite electrodes, the rate capability was tested and shown in Fig. 6. Both of the composite electrodes were measured at different rates from 0.2C to 10C, followed by a return to 0.2C. For each stage, the process was taken by 10 cycles. It is found from Fig. 6(a) that pristine LiCoO2 exhibits poor rate performance and the discharge capacity drop sharply with the increase of chargeedischarge rate especially at 10C. Meanwhile, the discharge plateau voltage decreased from 3.96 V to 3.2 V when the chargeedischarge current rate increased from 0.2C to 10C. For PPy-coated LiCoO2 shown in Fig. 6(b), the discharge plateau voltage decreases from 3.97 V to 3.45 V and the discharge curves are also very smooth even at 10C, suggesting the electrode structures are stable and provide a favorable network for fast lithium kinetics [36]. Fig. 6(c) shows the comparison of rate performance for pristine LiCoO2 and PPy-coated LiCoO2. The discharge capacity pristine LiCoO2 at 0.2C, 0.5C, 1C, 2C, 5C and 10C are 188.5, 181.1, 172.5, 162.5, 134.2 and 98.7 mAh g1, respectively. The corresponding initial discharge capacity retention (xC/0.2C, x ¼ 0.5, 1, 2, 5 and10) are 96.1%, 91.5%, 86.2%, 71.2% and 52.4%. For PPy-coated LiCoO2, the discharge capacity keeps the value of 183, 180.5, 179.4, 177.4, 159.1 and 146.6 mAh g1, and the corresponding initial discharge capacity retention are 98.6%, 98%, 96.9%, 86.9% and 80.1%, respectively. The excellent rate capability is mainly due to the PPy coating greatly reduced the electrode resistance, which ensured higher discharge plateau voltage even at 10C. In addition, the continuously thin PPy coating on the LiCoO2 protects the surface of the active material by suppressing the generation of oxygen significantly, thereby improving the thermal stability at a higher potential range. The capacity fading of LiCoO2 during cycling process is associated with structural transition from a hexagonal to monoclinic to
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(CeV) of pristine and PPy-coated LiCoO2. All cells were cycled in the
Fig. 5. Charge-discharge curves for selected cycles for electrodes of (a) pristine LiCoO2, and (b) PPy-coated LiCoO2 composite; (c) cycling behavior of pristine LiCoO2 and PPycoated LiCoO2 electrodes.
hexagonal modifications at potentials above 4.1 V vs Liþ/Li [37e39]. Thus, differential curves were plotted in order to examine the effect of the coating on the phase transitions that accompany the chargeedischarge processes. Fig. 7 shows the cyclic voltammograms
Fig. 6. Discharge curves of (a) pristine LiCoO2 and (b) PPy-coated LiCoO2 composite at different rate; (c) rate capability for pristine LiCoO2 and PPy-coated LiCoO2 composite electrodes.
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Fig. 7. Cyclic voltammograms of (a) pristine LiCoO2 and (b) PPy-coated LiCoO2 electrodes measured at room temperature with a scanning rate of 0.1 mV s1.
range of 2.8e4.6 V at a scan rate of 0.1 mV s1. It is clearly to be seen from Fig. 7(a) and (b) that both of two groups of the curves have two strong oxidation peaks, a major oxidation peak at about3.98 V and a minor one at 4.56 V with their corresponding to reduction peak at 3.86 V and 4.45 V, respectively, which can be attributed to the redox couple Co3þ/4þ [40]. The intensity of PPy-coated LiCoO2 decreases much less than that of LiCoO2 with the cycle increases. In addition, for LiCoO2, the anodic peak potentials shift toward positive obviously with the increase of cycle number. However, the curves of PPy-coated LiCoO2 are nearly identical and indicate good reversibility. This is also an evident that better chargeedischarge performance is achieved after coating with PPy. In order to verify that the conductive PPy coating is responsible to the good electrochemical performance of the cell with the PPycoated LiCoO2, electrochemical impedance spectroscopy (EIS) was carried out at the charged state of 4.5 V before cycle and after 50 cycles at room temperature. The simplified equivalent circuit model is constructed to analyze the impedance spectra and the Nyquist plots of cells are presented in Fig. 8. In this model, Rs represents electrolyte resistance; Rsf and CPE1 mean the surface film resistance which are reflected by the semi-circle at the high frequency region; Rct and CPE2 are the charge-transfer resistance of the electrochemical reactions which are associated with the second semi-
Fig. 8. Equivalent circuit model and Nyquist plots of pristine LiCoO2 and PPy-coated LiCoO2: (a) before cycle, (b) after 50 cycles.
circle appearing at medium frequency region; Warburg impedance Rw is presented where the 45 lines exist in the plots [41e43]. Table 1 lists the Rct of pristine LiCoO2 and PPy-coated LiCoO2 samples. It can be seen that the value of the pristine LiCoO2 cells before cycle is 73.5 U but considerably increases after 50 cycles reached to 558 U. For the PPy-coated LiCoO2, the Rct increases fairly slowly, increasing from 28.4U before cycle to 238.5U after 50 cycles. It can be assumed that the electrochemical impedance is greatly suppressed by the presence of PPy.
Table 1 Charge-transfer resistance (Rct) for pristine LiCoO2 and PPy-coated LiCoO2. Samples
Pristine LiCoO2 PPy-coated LiCoO2
Rct (U) Before cycle
After 50 cycles
73.5 28.4
558 238.5
J. Cao et al. / Journal of Power Sources 281 (2015) 49e55 Table 2 The amount of Co dissolved into electrolyte from the pristine and PPy-coated LiCoO2. Storage (h)
12 24 36 72
Dissolved Co amount (ppm) Pristine
PPy-coated
1.118 2.043 3.516 5.288
0.034 0.175 0.572 0.965
The decrease of Rct is attributed to that the PPy thin film on the surface builds the conductive network, which is beneficial to Liþ ions diffusion during chargeedischarge processes. Therefore, a good rate capability and cycling performance of the PPy-coated LiCoO2 is achieved when the high accessibility of Liþ ions is coupled with the high electronic conductivity of the PPy. Co dissolution into the electrolyte is widely considered as one of the major reasons for capacity fading at high cut off voltages [44]. The amount of Co dissolved into the electrolytes was examined by AAS to further investigate the effect of PPy coating on LiCoO2. The half cells were charged to 4.5 V and were carefully disassembled in a glove box with the concentration of H2O was below 1 ppm, then the cathode materials were stored in electrolyte at room temperature for different time. It can be seen from Table 2 that the amount of Co dissolved from PPy-coated LiCoO2 into the electrolyte is much lower than that from the pristine LiCoO2. This indicates that the continuously capsule-like shell (PPy) can effectively avoid the direct contact of core (LiCoO2) and electrolyte and prevent corrosion caused by HF, which in turn, reduce the dissolution of Co into the electrolyte. 4. Conclusions The LiCoO2 particles coated with conducting polypyrrole has been successfully synthesized by a sample chemical oxidation polymerization method. The initial discharge capacity of PPycoated LiCoO2 cathode was 182 mAh g1 and remained to be 169.9 mAh g1 after 170 cycles, while the capacity of pristine LiCoO2 was only 157.4 mAh g1 remained after 170 cycles. The rate capability was also greatly improved. The reversible capacity retention (10C/0.2C) of LiCoO2 increased from 52.4% to 80.1% after being coated with PPy. The remarkable improvement of rate capability of the PPy-coated LiCoO2 is due to that the PPy thin film on the surface builds a conductive network and increases the electronic conductivity, which reduces the charge-transfer resistance of electrochemical reactions (the value of Rct reduces from 558 to 238.5U). The continuously coated thin PPy film is just like a capsule shell, which can protect the core (LiCoO2) from corrosion causing by the HF attacking, and suppress the dissolution of Co into electrolyte in turn. The two above aspects is beneficial to improving the structural stability and cycling performance of PPy-coated LiCoO2 electrode. References [1] T. Ohzuku, A. Ueda, J. Electrochem. Soc. 141 (1994) 2972e2977. [2] C. Delmas, M. Menetrier, L. Croguennec, I. Saadoune, A. Rougier, C. Pouillerie, G. Prado, M. Grune, L. Fournes, Electrochim. Acta 45 (1999) 243e253.
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Polypyrrole-coated LiCoO2 nanocomposite with enhanced electrochemical properties at high voltage for lithium-ion batteries Jingchao Cao, Guorong Hu, Zhongdong Peng, Ke Du, Yanbing Cao* School of Metallurgy and Environment, Central South University, Changsha 410083, China
h i g h l i g h t s PPy-coated LiCoO2 particles are prepared by a chemical polymerization method. The PPy film, like a capsule shell, avoids LiCoO2 corrosion from HF attacking. The PPy film builds a conductive network and increases the electronic conductivity. The electrochemical properties of LiCoO2 are enhanced after coating with PPy.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 November 2014 Received in revised form 8 January 2015 Accepted 29 January 2015 Available online 29 January 2015
A conducting polypyrrole thin film is successfully coated onto the surface of LiCoO2 by a simple chemical polymerization method. The structure and morphology of pristine LiCoO2 and PPy-coated LiCoO2 are investigated by the techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Energy dispersive X-ray spectroscopy (EDXS), Fourier transform infrared spectrometry (FTIR) and thermogravimetric analysis (TGA) further demonstrate the existence of PPy. The electrochemical properties of the composites are investigated by galvanostatic chargeedischarge test and AC impedance measurements, which show that the conductive PPy film on the surface significantly decrease the charge-transfer resistance of LiCoO2. The PPy-coated LiCoO2 exhibits a good electrochemical performance, showing initial discharge capacity of 182 mAh g1 and retains 94.3% after 170 cycles. However, the retention of pristine LiCoO2 is only 83.5%. The rate capability results show that the reversible capacity retention (10C/0.2C) of LiCoO2 increases from 52.4% to 80.1% after being coated with PPy. The continuously coated thin PPy film is just like a capsule shell, which can protect the core (LiCoO2) from corrosion causing by the HF attacking and greatly reduce the dissolution of Co into electrolyte. © 2015 Elsevier B.V. All rights reserved.
Keywords: PPy-coated LiCoO2 Chemical polymerization Electrochemical properties Lithium-ion batteries
1. Introduction Lithium-ion batteries (LIBs) have been extensively used for a wide variety of applications, for instance, from portable electronic equipments to hybrid and electric vehicles. The performance of the batteries is often decided by the properties of cathode material. Among various cathode materials, lithium cobalt oxide (LiCoO2) is currently one of the most popular cathode materials in batteries for portable devices due to its high energy density, ease of preparation, good C-rate capacity and long cycle life [1e5]. LiCoO2 has a specific capacity of 137 mAh g1 during the (de)lithiation process from 2.75
* Corresponding author. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.jpowsour.2015.01.174 0378-7753/© 2015 Elsevier B.V. All rights reserved.
to 4.2 V vs. Li/Liþ, only 0.5 Liþ per molecule of LiCoO2 can be extractable [6,7]. Increasing the reversible capacity is strongly demand with rapidly growing demands for higher energy density lithium-ion batteries. One promising attempt to increase the capacity of LiCoO2 is charging cells above 4.2 V [8,9]. However, the structural instability and capacity fading is more severely which is caused by easy dissolution of Co4þ into the acidic electrolyte medium, as well as a large number of side reactions on the surface of the cathode particles [4,10]. Meanwhile, attempts have been made to resolve the problems by doping non-transition and transition metal ions into the LiCoO2 structure by partial substitution of cobalt, but capacity fading has been still observed even though some improvement has been achieved [5,11]. As an alternative, the surface modification of LiCoO2 with inorganic materials such as LiFePO4 [12], LiMgPO4 [13],
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LiNi0.5Mn1.5O2 [8], LiNbO3 [14], LiMn2O4 [15], Li4Ti5O12 [16], Li [Li0.2Mn0.6Ni0.2]O2 [17], Al2O3 [18], ZnO [19], LiF [20], MgF2 [21] and FePO4 [22] has been extensively investigated and achieved good effect on inhibiting capacity fading. However, the inorganic materials are discontinuously deposited onto the surface of LiCoO2, tending to act as an inert layer regarding ionic conduction and also require complex coating process. Polypyrrole (PPy), a typical soft conducting polymer with good mechanical flexibility and chemical stability during the electrochemical process, has been attracted much attention. PPy can act as a host material for Liþ-ion insertion/extraction in the voltage range of 2.0e4.5 V versus Li/Liþ, with a theoretical capacity of 72 mAh g1 [23]. Therefore, PPy additives can be used as both conductive agents and cathode materials. The application of PPy to improve electrochemical performance of electrode materials for lithium ion batteries have been reported, such as Fe3O4/PPy [24], Fe2O3/PPy [25], LiMn2O4/PPy [26], LiV3O8/PPy [27,28], LiFeO2/PPy [29], LiFePO4/PPy [30] and LiNi1/3Co1/3Mn1/3O2/PPy [31]. But up till now, using PPy as a coating layer to improve the electrochemical properties of LiCoO2 at high voltage has not been reported. In this study, LiCoO2-PPy nanocomposite was synthesized using a chemical polymerization method. The structure, morphology and electrochemical performance of the LiCoO2-PPy are discussed and compared with the performance of bare LiCoO2 cathode material. 2. Experimental 2.1. Synthesis of nanoarchitectured PPy-coated LiCoO2 LiCoO2 powder with an average particle size of 10 mm was prepared in our previous work [17]. The PPy-coated LiCoO2 composite powder was prepared by the chemical polymerization with sodium p-toluenesulfonate (AR, 99%) as the dopant. In order to avoid the introduction of impurity ions such as Fe3þ, H2O2 (AR, H2O2 30%) was chosen to use as the oxidant. LiCoO2 (5 g), H2O2 (2 mL) and p-toluenesulfonate (0.5 g) were dispersed into alcohol solution (50 mL), pyrrole (0.1 g) was diluted into 10 mL with alcohol, and then was slowly added to the mixture with continuously magnetically stirred for 15 h. The final products were filtered and washed with deionized water several times and dried at 60 C in a vacuum oven for 12 h. In order to calculate the PPy content, pure PPy powder was also prepared using the same method above.
prepared by dispersing the 85 wt. % cathode materials, 10 wt. % acetylene black and 5 wt. % polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidinone (NMP). The slurry was coated on an aluminum foil and then dried at 120 C for 20 h under vacuum. The cells were assembled in an argon filled glove box with electrolyte of 1 mol L1 LiPF6 in EC-DMC-EMC (1:1:1 volume ratio) solution, metallic Li as anodes and polypropylene separators (Celgard 2400). The cells galvanostatically charged and discharged in the potential range of 3.0e4.5 V (vs. Li/Liþ) at different current densities using LAND CT2001A test system. Cyclic voltammograms measurements were done by a Solartron 1287 electrochemical interface between 2.8 and 4.6 V at a scan rate of 0.1 mV s1. Electrochemical impedance spectroscopy (EIS) analysis was carried out with a three electrode cell system from 1 MHz to 1 mHz by using the potentiostat/galvanostat Model 2273A potentiostat and the Model 5210 lock-in amplifier with 5 mV ac excitation. All tests were performed at room temperature.
3. Results and discussion The XRD patterns of pristine LiCoO2 and PPy-coated LiCoO2 are presented in Fig. 1. It is observed that the pristine LiCoO2 shows a well-defined a-NaFeO2 structure with no minor phases. The PPycoated LiCoO2 composite also conform to a single-phase hexagonal structure of a-NaFeO2 structure is attributed to LiCoO2 structure, and PPy phase is not observed in this pattern. SEM images of the pristine LiCoO2 and the PPy-coated LiCoO2 composite are shown in Fig. 2(a) and (b). The particle size of pristine LiCoO2 powder is about10 mm. It can be seen from Fig. 2(a) the surface of pristine LiCoO2 is smooth and clean. After intruding the PPy, the PPy-coated LiCoO2 exhibits an anomalous coating layer, which is featured with a thin and highly continuous morphology. TEM photograph of PPy-coated LiCoO2 exhibits the thickness of PPy coating layer is approximately 20 nm. In order to further confirm the presence of PPy, energy dispersive X-ray (EDX) mapping was used to observe the distribution of PPy (Fig. 2(e)e(g)). The element of N and C are only existed in PPy. It obviously can be seen from Fig. 2(f) and (g) that the N and C are distributed uniformly throughout the whole area, which indicates that the PPy film had uniformly coated on the surface of LiCoO2 powders. FT-IR spectra obtained for the KBr diluted pellets of pristine
2.2. Materials characterization X-ray diffraction (XRD, D/max-r A type Cu Ka, 40Kv, 300 mA) was employed to identify the crystalline phase of the products. The scan range was 10e80 with a scan speed of 0.02 min1. The morphological and compositional characterization of the asprepared compound was performed by means of scanning electron microscopy (SEM, JEOLJSM-6360LV), transmission electron microscope (TEM, JEOL, 2010) and energy dispersive X-ray spectroscopy (EDXS). Fourier transform infrared spectrometry (FTIR) measurements were carried out on a Hitachi FTIR-8900 spectrometer (Japan) in wave-numbers ranges of 400e4000 cm1 using a KBr wafer. Thermogravimetric analysis (TGA) was carried out on a TG 209 F1 Iris under air atmosphere in the temperature range of 30e800 C at a heating rate of 5 C/min. The dissolution of Co into electrolyte was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 4300DV). 2.3. Electrochemical measurements Electrochemical properties of the materials were investigated by using coin cells (2025 type). The working electrodes were
Fig. 1. XRD patterns of LiCoO2 and PPy-coated LiCoO2 composite.
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Fig. 2. SEM images of LiCoO2 (a), PPy-coated LiCoO2 (b), TEM of PPy-coated LiCoO2 (c), and corresponding EDX mapping for the PPy-coated LiCoO2 image (d) as follows: Co (e), N (f) and C (g).
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LiCoO2 and LiCoO2-PPy composite are shown in Fig. 3. For pristine LiCoO2, the characteristic absorption bands of CoeO and OeCoeCo vibrations are observed at approximately 526 and 613 cm1, respectively [32]. The PPy-coated LiCoO2 has absorption peaks of PPy. The band at 1504 cm1 is due to aromatic C]C in PPy. CeN and CeH show peaks around 1304 and 1001 cm1, respectively. The peak at around 2235 cm1 is attributed to the OeH stretching vibration [33e35]. This result confirms that the PPy has been successfully coated onto the surface of LiCoO2 powder. To quantify the amount of PPy in the PPy-coated LiCoO2 composite, TGA analysis was carried out in air. Fig. 4 shows the TGA curves of the PPy-coated LiCoO2 composite along with those of pristine LiCoO2 and pure PPy powders when heated from room temperature to 800 C with a rate of 5 C min1 in air. It can be seen from Fig. 4 that the pristine LiCoO2 shows a constant weight throughout the sintering process. The pure PPy burns off at 550 C, and there is about 5.5% oxidation remaining. For PPy-coated LiCoO2, there is light mass loss below 220 and 450 C, which is corresponding to the burning of PPy. According the TGA curve, the mass of fraction PPy in PPy-coated LiCoO2 is 1.67%. Fig. 5(a) and (b) shows the typical chargeedischarge curves for different cycles of pristine LiCoO2 and PPy-coated LiCoO2 electrodes in coin cells using lithium as the counter and reference electrode between 3.0 and 4.5 V at 0.2C (75 mA g1). It can be seen that there is no difference in the operation voltage of the first cycle on charge and discharge between pristine LiCoO2 and PPy-coated LiCoO2. Both of the cells have stable and smooth chargeedischarge curves even after 170 cycles. The initial charge and discharge capacity of pristine LiCoO2 are 201.5 and 189 mAh g1, the corresponding columbic efficiency is 93.8%. However, the initial columbic efficiency of PPy-coated LiCoO2 is 92.9% (initial charge capacity is 196.5 and discharge 182 mAh g1), which is slightly lower than that of pristine LiCoO2. This is might be caused by two reasons: one is a small amount of Liþ lost during the process of PPy coating; the other may be that a small amount of Liþ inserted into PPy in the initial charge/discharge process which leaded to irreversible loss of lithium. The comparison of cycle performance for pristine LiCoO2 and PPy-coated LiCoO2 is shown in Fig. 5(c). It is clearly noticed that the PPy-coated LiCoO2 presented superior cycling stability with an initial capacity of 182 mAh g1 and 94.3% of this initial capacity was
Fig. 3. FT-IR spectra measured on the KBr-diluted pellets of pristine LiCoO2 and LiCoO2-PPy composite.
Fig. 4. TGA curves of PPy, LiCoO2 and PPy-coated LiCoO2 composite.
retained after 170 cycles. However, the discharge capacity of pristine LiCoO2 faded quickly and only 83.5% of the original capacity remained after 170 cycles. The conductive PPy thin film on the surface builds the conductive network, which is beneficial to Liþ ions diffusion and reduces the electrode resistance during chargeedischarge processes. What’ more, the continuously coated thin film can avoid direct contact of LiCoO2 and the electrolyte and suppress their reaction on the surface. Both of these two factors are benefit to improving the electrochemical properties of LiCoO2. To further investigate the electrochemical performance of the pristine LiCoO2 and PPy-coated LiCoO2 composite electrodes, the rate capability was tested and shown in Fig. 6. Both of the composite electrodes were measured at different rates from 0.2C to 10C, followed by a return to 0.2C. For each stage, the process was taken by 10 cycles. It is found from Fig. 6(a) that pristine LiCoO2 exhibits poor rate performance and the discharge capacity drop sharply with the increase of chargeedischarge rate especially at 10C. Meanwhile, the discharge plateau voltage decreased from 3.96 V to 3.2 V when the chargeedischarge current rate increased from 0.2C to 10C. For PPy-coated LiCoO2 shown in Fig. 6(b), the discharge plateau voltage decreases from 3.97 V to 3.45 V and the discharge curves are also very smooth even at 10C, suggesting the electrode structures are stable and provide a favorable network for fast lithium kinetics [36]. Fig. 6(c) shows the comparison of rate performance for pristine LiCoO2 and PPy-coated LiCoO2. The discharge capacity pristine LiCoO2 at 0.2C, 0.5C, 1C, 2C, 5C and 10C are 188.5, 181.1, 172.5, 162.5, 134.2 and 98.7 mAh g1, respectively. The corresponding initial discharge capacity retention (xC/0.2C, x ¼ 0.5, 1, 2, 5 and10) are 96.1%, 91.5%, 86.2%, 71.2% and 52.4%. For PPy-coated LiCoO2, the discharge capacity keeps the value of 183, 180.5, 179.4, 177.4, 159.1 and 146.6 mAh g1, and the corresponding initial discharge capacity retention are 98.6%, 98%, 96.9%, 86.9% and 80.1%, respectively. The excellent rate capability is mainly due to the PPy coating greatly reduced the electrode resistance, which ensured higher discharge plateau voltage even at 10C. In addition, the continuously thin PPy coating on the LiCoO2 protects the surface of the active material by suppressing the generation of oxygen significantly, thereby improving the thermal stability at a higher potential range. The capacity fading of LiCoO2 during cycling process is associated with structural transition from a hexagonal to monoclinic to
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(CeV) of pristine and PPy-coated LiCoO2. All cells were cycled in the
Fig. 5. Charge-discharge curves for selected cycles for electrodes of (a) pristine LiCoO2, and (b) PPy-coated LiCoO2 composite; (c) cycling behavior of pristine LiCoO2 and PPycoated LiCoO2 electrodes.
hexagonal modifications at potentials above 4.1 V vs Liþ/Li [37e39]. Thus, differential curves were plotted in order to examine the effect of the coating on the phase transitions that accompany the chargeedischarge processes. Fig. 7 shows the cyclic voltammograms
Fig. 6. Discharge curves of (a) pristine LiCoO2 and (b) PPy-coated LiCoO2 composite at different rate; (c) rate capability for pristine LiCoO2 and PPy-coated LiCoO2 composite electrodes.
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Fig. 7. Cyclic voltammograms of (a) pristine LiCoO2 and (b) PPy-coated LiCoO2 electrodes measured at room temperature with a scanning rate of 0.1 mV s1.
range of 2.8e4.6 V at a scan rate of 0.1 mV s1. It is clearly to be seen from Fig. 7(a) and (b) that both of two groups of the curves have two strong oxidation peaks, a major oxidation peak at about3.98 V and a minor one at 4.56 V with their corresponding to reduction peak at 3.86 V and 4.45 V, respectively, which can be attributed to the redox couple Co3þ/4þ [40]. The intensity of PPy-coated LiCoO2 decreases much less than that of LiCoO2 with the cycle increases. In addition, for LiCoO2, the anodic peak potentials shift toward positive obviously with the increase of cycle number. However, the curves of PPy-coated LiCoO2 are nearly identical and indicate good reversibility. This is also an evident that better chargeedischarge performance is achieved after coating with PPy. In order to verify that the conductive PPy coating is responsible to the good electrochemical performance of the cell with the PPycoated LiCoO2, electrochemical impedance spectroscopy (EIS) was carried out at the charged state of 4.5 V before cycle and after 50 cycles at room temperature. The simplified equivalent circuit model is constructed to analyze the impedance spectra and the Nyquist plots of cells are presented in Fig. 8. In this model, Rs represents electrolyte resistance; Rsf and CPE1 mean the surface film resistance which are reflected by the semi-circle at the high frequency region; Rct and CPE2 are the charge-transfer resistance of the electrochemical reactions which are associated with the second semi-
Fig. 8. Equivalent circuit model and Nyquist plots of pristine LiCoO2 and PPy-coated LiCoO2: (a) before cycle, (b) after 50 cycles.
circle appearing at medium frequency region; Warburg impedance Rw is presented where the 45 lines exist in the plots [41e43]. Table 1 lists the Rct of pristine LiCoO2 and PPy-coated LiCoO2 samples. It can be seen that the value of the pristine LiCoO2 cells before cycle is 73.5 U but considerably increases after 50 cycles reached to 558 U. For the PPy-coated LiCoO2, the Rct increases fairly slowly, increasing from 28.4U before cycle to 238.5U after 50 cycles. It can be assumed that the electrochemical impedance is greatly suppressed by the presence of PPy.
Table 1 Charge-transfer resistance (Rct) for pristine LiCoO2 and PPy-coated LiCoO2. Samples
Pristine LiCoO2 PPy-coated LiCoO2
Rct (U) Before cycle
After 50 cycles
73.5 28.4
558 238.5
J. Cao et al. / Journal of Power Sources 281 (2015) 49e55 Table 2 The amount of Co dissolved into electrolyte from the pristine and PPy-coated LiCoO2. Storage (h)
12 24 36 72
Dissolved Co amount (ppm) Pristine
PPy-coated
1.118 2.043 3.516 5.288
0.034 0.175 0.572 0.965
The decrease of Rct is attributed to that the PPy thin film on the surface builds the conductive network, which is beneficial to Liþ ions diffusion during chargeedischarge processes. Therefore, a good rate capability and cycling performance of the PPy-coated LiCoO2 is achieved when the high accessibility of Liþ ions is coupled with the high electronic conductivity of the PPy. Co dissolution into the electrolyte is widely considered as one of the major reasons for capacity fading at high cut off voltages [44]. The amount of Co dissolved into the electrolytes was examined by AAS to further investigate the effect of PPy coating on LiCoO2. The half cells were charged to 4.5 V and were carefully disassembled in a glove box with the concentration of H2O was below 1 ppm, then the cathode materials were stored in electrolyte at room temperature for different time. It can be seen from Table 2 that the amount of Co dissolved from PPy-coated LiCoO2 into the electrolyte is much lower than that from the pristine LiCoO2. This indicates that the continuously capsule-like shell (PPy) can effectively avoid the direct contact of core (LiCoO2) and electrolyte and prevent corrosion caused by HF, which in turn, reduce the dissolution of Co into the electrolyte. 4. Conclusions The LiCoO2 particles coated with conducting polypyrrole has been successfully synthesized by a sample chemical oxidation polymerization method. The initial discharge capacity of PPycoated LiCoO2 cathode was 182 mAh g1 and remained to be 169.9 mAh g1 after 170 cycles, while the capacity of pristine LiCoO2 was only 157.4 mAh g1 remained after 170 cycles. The rate capability was also greatly improved. The reversible capacity retention (10C/0.2C) of LiCoO2 increased from 52.4% to 80.1% after being coated with PPy. The remarkable improvement of rate capability of the PPy-coated LiCoO2 is due to that the PPy thin film on the surface builds a conductive network and increases the electronic conductivity, which reduces the charge-transfer resistance of electrochemical reactions (the value of Rct reduces from 558 to 238.5U). The continuously coated thin PPy film is just like a capsule shell, which can protect the core (LiCoO2) from corrosion causing by the HF attacking, and suppress the dissolution of Co into electrolyte in turn. The two above aspects is beneficial to improving the structural stability and cycling performance of PPy-coated LiCoO2 electrode. References [1] T. Ohzuku, A. Ueda, J. Electrochem. Soc. 141 (1994) 2972e2977. [2] C. Delmas, M. Menetrier, L. Croguennec, I. Saadoune, A. Rougier, C. Pouillerie, G. Prado, M. Grune, L. Fournes, Electrochim. Acta 45 (1999) 243e253.
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