Electrosprayed polyaniline as cathode material for lithium secondary batteries

Electrosprayed polyaniline as cathode material for lithium secondary batteries

Materials Research Bulletin 45 (2010) 265–268 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.c...

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Materials Research Bulletin 45 (2010) 265–268

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Electrosprayed polyaniline as cathode material for lithium secondary batteries James Manuel a, Prasanth Raghavan a, Chorong Shin a, Min-Yeong Heo a, Jou-Hyeon Ahn a,*, Jung-Pil Noh b, Gyu-Bong Cho b, Ho-Suk Ryu b, Hyo-Jun Ahn b a b

Department of Chemical and Biological Engineering and Engineering Research Institute, Gyeongsang National University, 900, Gajwa-dong, Jinju 660-701, Republic of Korea School of Nano & Advanced Materials Engineering, Gyeongsang National University, 900, Gajwa-dong, Jinju 660-701, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history: Received 31 December 2008 Accepted 29 June 2009 Available online 28 December 2009

Doped polyaniline with LiPF6 is electrosprayed onto aluminum foil using electrospinning technique, and evaluated as cathode active material for application in room-temperature lithium batteries. Doping level is characterized using FTIR and UV–vis spectroscopy. In FTIR Spectra, characteristic peaks of PANI are shifted to lower bands as a result of doping which indicates the effectiveness of doping. Doping level is also confirmed by UV–vis spectra. Surface morphology of the cathode is studied using scanning electron microscope. Electrochemical evaluation of the cell using electrosprayed PANI as cathode show good cycling properties. The cell delivers a high discharge value of 142.5 mAh/g which is about 100% of theoretical capacity, and the capacity is lowered during cycle and reached 61% of theoretical capacity after 50 cycles. The cell delivers a stable but lower discharge capacity at higher C-rates. ß 2009 Elsevier Ltd. All rights reserved.

Keywords: A. Polymers B. Chemical synthesis C. Electron microscopy D. Electrochemical properties D. Energy storage

1. Introduction Until three decades ago, all carbon based polymers were, in general, regarded as electrically non-conducting. Under certain circumstances, polymers can indeed be made to function very much like a metal, which is already proven by Shirakawa, MacDiarmid, and Heeger for which they received Nobel prize in chemistry in 2000 [1]. Unique combination of ion exchange characteristics and optical properties of intrinsically conducting polymers makes them so attractive. In addition, polymeric materials are light weight, processable and flexible. At low potentials, they are readily oxidized and reduced, the redox process is reversible and accompanied by large change in composition, color and conductivity of the material. These polymers are made conducting by doping process through the reaction of conjugated semiconducting polymer with an oxidizing agent, reducing agent, or a protonic acid resulting in highly delocalized polycations or polyanions [2]. Polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh) are some of important conducting polymers which are stable in air and possess good electrochemical properties. PANI has become of special interest because of a number of important reasons like inexpensive monomer, straightforward polymerization of monomer, polymerization reaction with high yield, and high stability of

* Corresponding author. Fax: +82 55 753 1806. E-mail address: [email protected] (J.-H. Ahn). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.12.021

conductive form of PANI. PANI is unique among all conducting polymers, and its electrical properties can be reversibly controlled by both changing the oxidation state of the main chain and protonation of the imine nitrogen atoms [3]. PANI has been extensively studied for many potential applications including secondary batteries [4], light-emitting or electrochromic devices [5], electromagnetic radiation absorbers [6], gas sensors [7], separation membranes [8], antistatic coatings [9], conducting molecular wires [10], super capacitors [11] and anticorrosion coatings [12]. Various dopants have been used to improve physical and chemical properties of PANI. Among them, lithium ionic salts such as LiClO4, LiBF4, LiPF6 and Zn(ClO4)2 were received much more attention [13] and their application in rechargeable lithium ion batteries has been extensively studied [14,15]. Nanostructured PANI offers a possibility of enhanced performance wherever the interfacial area between PANI and its environment is important [16]. Recently researchers are focusing towards the enhancement of electrochemical property by changing the structure of active material used in the battery. In the present study, PANI doped with LiPF6 was mixed with the binder and conductor, and electrosprayed directly onto current collector, aluminum foil. The electrosprayed material is used as a cathode instead of conventional casting method. FTIR and UV–vis spectroscopy were used for the characterization of doped PANI, and surface morphology was studied using SEM. The electrochemical behavior and discharge/charge performance of electrosprayed PANI (ES-PANI) have been also investigated.

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2. Experimental All chemicals were used as received from Aldrich. PANI was doped with LiPF6 using 1 M solution of LiPF6 in an equivolume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). PANI powder was immersed in 1 M LiPF6 solution for 72 h at room temperature. The doped powder was washed with ethyl ether several times and dried under vacuum at 50 8C for 24 h. Infrared spectra of the polymers were recorded using a VERTEX 80v, Bruker Optics Fourier transform infrared spectrophotometer (FTIR). A pinch of the sample was mixed with KBr and compressed into a pellet for analysis and the spectra were recorded in the range of 400–4000 cm 1. UV–vis spectra of the PANI samples were recorded using LAMBDA-900, Perkin-Elmer UV–vis-NIR spectrometer. 20 mg of sample was dissolved in 2 ml of N-methylpyrrolidone (NMP), coated on glass plate and dried. UV–vis spectra of the polymer films were recorded using a plain glass plate as reference. The solution for electrospraying was prepared by mixing LiPF6 doped PANI with conductive agent carbon black (Super-P) and binder PVdF in 70:20:10 weight ratio. The ingredients were mixed together in NMP in a high energy mixer mill at room temperature for 45 min to get homogenous solution having a concentration of 10 wt%. The solution was fed into a syringe, and air pressure was applied to the syringe. A high DC voltage of 20 kV was applied between the needle and aluminum foil fixed on stainless steel rotating drum. The distance between the needle and the drum was set to be 20 cm. The ES-PANI onto aluminum was vacuum dried at 60 8C for 12 h before use. The electrosprayed film was cut into circular discs of 0.95 cm2 area and 0.5 mg mass for use as cathode. Two-electrode electrochemical Swagelok cells were assembled with lithium metal (300 mm thickness, Cyprus Foote Mineral Co.) as anode, Celgard1 2200 separator 1 M LiPF6 in EC/DMC (1:1, v/v) electrolyte and ES-PANI with as cathode. The cell assembly was performed under argon atmosphere in a glove box with H2O level <10 ppm. Cyclic voltammetry (CV) was done at a scan rate of 0.3 mV/s between 1.5 and 4.0 V. The electrochemical performance tests were carried out using an automatic galvanostatic chargedischarge unit, WBCS3000 battery cycler (WonA Tech. Co.), between 1.5 and 4.0 V, and at a current density corresponding to 0.1, 0.2 and 0.3 C at room temperature. 3. Results and discussion FTIR spectra of PANI and doped PANI samples are presented in Fig. 1. The spectra have characteristic peaks from where the effect of doping on PANI with lithium salts can be evaluated. The changes in the spectrum of PANI can be observed from the position, intensity and shape of the peaks in the spectra of the doped PANI. On comparison with the undoped PANI, the characteristic peaks in the spectra of doped PANI are shifted to lower wave numbers. On doping delocalization of the ring electrons takes place that decreases ring electron density, hence absorption bands appear at the lower wave numbers. The significant lowering of the wave numbers is a measure of the effectiveness of doping [17,18]. The bands of PANI that are present at 1585 [N-Quinoid ring(Q)–N stretching (str.)] are shifted lower to 1575 cm 1 for PANI doped with LiPF6. Other noticeable shifts in bands are from 1498 cm 1 [Nbenzenoid ring (B)–N str.] to 1492 cm 1; 1307 cm 1 [C–N str. in QBQ, QBB, BBQ] to 1301 cm 1; 1242 [C–N str. in BBB] to 1240 cm 1; 1164 cm 1 [a mode of QNH+B] to 1145, and 831 cm 1 [C–H out of plane bending] to 821 cm 1 for PANI doped with LiPF6. UV–vis spectra of PANI and PANI-LiPF6 are compared in Fig. 2. The spectrum of undoped PANI is characterized by the electronic

Fig. 1. FTIR spectra of PANI and PANI-LiPF6.

transition at about 330 and 635 nm. Transition at 635 nm corresponds to the p–p* transition of quinoid ring, and transition at 330 nm corresponds to p–p* transition of benzenoid ring. After doping, the peak at 635 nm completely vanished and two new peaks formed at 410 and 841 nm. These two new peaks are assigned to be the polaron band formation followed by LiPF6 doping which reveals the transition of PANI from intrinsic to conducting one. Surface morphology of ES-PANI is represented in Fig. 3. It is clear from SEM image that dispersion of particles is uniform and it can enhance the electrical conductivity through the conducting phase of the cathode. It may be due to the application of high voltage, preventing the aggregation of conducting carbon during electrospraying process. Presence of fiber forming PVdF assists the formation of fiber/rod-like structure to PANI. It is observed that the pore size of the cathode is ranging from nano- to micro-level. It provides large surface area which enhances the intercalation reaction due to higher level of available sites. Cyclic voltammetry (CV) provides both qualitative and quantitative information on electrode process. Fig. 4 shows the first, third and fifth CV curves of the cell at room temperature between 1.5 and 4.0 V with a scan rate of 0.3 mV/s. A multi-step redox process seems to be operating for ES-PANI electrode with anodic peaks at 3.5 V and cathodic peak at 3.0 V. Upper peak at 3.5 V corresponds to the oxidation (Li+ or PF6 doping) and lower peak at 3.0 V corresponds to the reduction (Li+ or PF6 de-doping). The CV curves follow same pattern with slight difference in path, showing there is some difficulty in effective doping and de-doping process of Li+ or

Fig. 2. UV–vis spectra of PANI and PANI-LiPF6.

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Fig. 3. SEM image of ES-PANI composite cathode. Fig. 6. Cycle life of ES-PANI at different C-rate (25 8C, 1.5–4.0 V).

Fig. 4. CV curves during cycling of ES-PANI (25 8C, 0.1 C-rate, 1.5–4.0 V).

PF6 during cycling which may be due to less amount of cathode material used in the cell. The performance of the cell using ES-PANI on repeated charge– discharge cycling at 0.1 C-rate is shown in Fig. 5. The cell delivers an initial discharge capacity of 133.5 mAh/g which is about 93% of the theoretical capacity of the material. The cell delivered a high discharge capacity of 142.5 mAh/g at fourth cycle which is about 100% of theoretical capacity of PANI. The porous structure of the ES-PANI provides a large surface area for the efficient intercalation

reaction inside the cell and thereby maximum utilization of the active material which leads to maximum discharge capacity. The porous structure makes the ions free to move, allowing them to reach maximum active sites of the cathode. After 4th cycle a slow fading in discharge capacity is visible and the discharge capacity reaches a minimum value of 86.5 at 50th cycle which is about 61% of theoretical value which may be due to the irreversible loss of active material. Charge–discharge performance of the cell cycling at different Crate is shown in Fig. 6. At 0.2 C-rate, the cell delivers an initial discharge capacity of 74 mAh/g and remains constant at a value of 79 mAh/g which is about 55% of theoretical value. At 0.3 C-rate, the cell shows an initial discharge capacity of 55 mAh/g and reached a stable value. Compared to 0.1 C-rate, 0.2 C-rate and 0.3 C-rate show better stability in cycle performance with lower discharge capacity. The results show that the ES-PANI can be also used as active material for supercapacitors where fast Faradaic charge transfer takes place at the electrode material. 4. Conclusions PANI was doped with lithium ionic salt, LiPF6 efficiently. The doping level of the system is confirmed by FTIR and UV–vis spectroscopy. From FTIR comparison, the characteristic peaks of undoped PANI are shifted to lower wave numbers which indicate the effect of doping process. Complete disappearance of peak at 635 nm and the formation of new peaks at 410 and 841 nm as a result of doping from PANI to PANI-LiPF6 in UV–vis ensure the doping effect. Cathode was prepared using electrospraying technique to make porous/fibrous morphology. From SEM image, it is clear that the surface of cathode is porous and the pore size ranges from nano- to micro-level. The CV reveals that the doping and de-doping processes of Li+ or PF6 ions are reversible. The highest discharge value of 142.5 mAh/g was obtained which is about 100% of theoretical value. There is slow capacity fading during cycling and the value reached 86.5 mAh/g at 50th cycle. At higher C-rates, the cell delivers a stable discharge capacity. The results have proved that ES-PANI is a good candidate as cathode material in lithium ion batteries. Acknowledgements

Fig. 5. Cycle life of ES-PANI (25 8C, 0.1 C-rate, 1.5–4.0 V).

This research was supported by the Ministry of Knowledge Economy (MKE) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation and also by Pioneer Research Center for Nano-morphic Biological Energy Conversion and Storage.

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