poly (3, 4-ethylenedioxythiophene) composites as cathode materials for rechargeable lithium batteries

poly (3, 4-ethylenedioxythiophene) composites as cathode materials for rechargeable lithium batteries

Solid State Ionics 310 (2017) 30–37 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi Synt...

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Solid State Ionics 310 (2017) 30–37

Contents lists available at ScienceDirect

Solid State Ionics journal homepage: www.elsevier.com/locate/ssi

Synthesis and electrochemical performances of LiV3O8/poly (3, 4ethylenedioxythiophene) composites as cathode materials for rechargeable lithium batteries

MARK

Limin Zhua,b, Wenjuan Lia,b, Zihenq Yuc, Lingling Xiea,b, Xiaoyu Caoa,b,⁎ a b c

College of Chemistry, Chemical and Environmental Engineering, Henan University of Technology, Zhengzhou 450001, PR China Key Laboratory of High Specific Energy Materials for Electrochemical Power Sources of Zhengzhou City, Henan University of Technology, Zhengzhou 450001, PR China School of Pharmacy, China Pharmaceutical University, Nanjing 211196, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Rechargeable lithium batteries LiV3O8/poly (3, 4-ethylenedioxythiophene) composites Cathode materials In-situ oxidative polymerization method Electrochemical performances

LiV3O8/poly (3, 4-ethylenedioxythiophene) (LVO/PEDOT) composites were synthesized via an in-situ oxidative polymerization process. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, galvanostatic discharge/charge tests, and electrochemical impedance spectroscopy techniques are used to characterize the as-prepared samples. The results demonstrated that the electrochemical performances of LVO/ PEDOT composites have greatly improved in comparison with bare LVO. The discharge capacities of 20 wt% LVO/PEDOT composite are 270, 265, 252, 240, and 229 mAh g− 1 and ˃95% capacity retention is maintained after the charge-discharge 50 cycles at the current densities of 60, 90, 120, 180, and 240 mA g− 1, respectively. A high reversible capacity of 176 mAh g− 1 (only 58 mAh g− 1 for the bare LVO) can be maintained after 50 cycles at a very high current rate of 2000 mA g− 1. Electrochemical impedance spectra results implied that the 20 wt% LVO/PEDOT composite revealed a decreased charge transfer resistance and increased Li+ ions diffusion ability. This noteworthy improvement is ascribed to the combination of PEDOT, which can act just as a defending layer to inhibit the LVO from direct contact with electrolyte and buffer volume change, and act just as a conductive network to improve the electronic conductivity, thus cycling stability and rate capability are improved.

1. Introduction Lithium-ion batteries (LIBs) have been considered to be the greatest predominant energy sources for applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their high power capability and long cycling life [1–4]. However, the limited electrochemical performance and high cost of cathode materials inhibit the further development of LIBs [5–7]. To date, the commonly used cathode materials, including LiCoO2, LiMn2O4, LiNixMnyCozO2 and LiFePO4, hardly meet energy and power demand of EVs and HEVs [8–11]. Therefore, it is very crucial to develop high performance cathode materials to replace the traditional cathodes for researchers nowadays. Among alternative cathode materials of LIBs, layered lithium trivanadate (LiV3O8, LVO) has grabbed remarkable attention owing to its high specific capacity, low cost, worthy thermal stability and safety [12–15]. However, the intrinsically low electronic conductivity, irreversible phase transformation and dissolution of minor quantity of vanadium element of LVO in the electrolyte result in poor rate capability and severe capacity fading during charge and discharge processes,



which restricts its practical application in LIBs [16–20]. To address these problems, various tactics have been exploited to ameliorate the electrochemical properties of LVO, such as nano-crystallizing [17], morphology tuning [18,20–22], heteroatom doping [23–26] and conductive layer coating [27–29]. Among these methods, conductive layer coating is a simple and efficient approach to maintain the structure stability and enhance electronic conductivity of LVO. Furthermore, the coating layers can effectively alleviate dissolution of vanadium element in the electrolyte. For instance, Huang et al. [30] have demonstrated that 0.5 wt% Al2O3 coating effectively improved the rate capability and cyclic stability of the LVO. Jiao et al. [27] reported that surface coating of LVO by 1.0 wt% AlPO4 effectively prevented capacity fade of LVO upon cycling. In addition to the aformentioned coating materials, other materials, such as AlF3 [31], Co0.58Ni0.42 oxide nanoneedles [32], are also employed for the same purpose. However, the above-mentioned oxides, fluorides, and phosphates coating layers are all nonconductive or electro-inactive. Therefore, conductive polymers applied as coating or compositing materials may be a good choice. As they can not only act as protective layers to avoid the direct contact of LVO with electrolyte

Corresponding author at: College of Chemistry, Chemical and Environmental Engineering, Henan University of Technology, Zhengzhou 450001, PR China. E-mail address: [email protected] (X. Cao).

http://dx.doi.org/10.1016/j.ssi.2017.08.002 Received 1 July 2017; Received in revised form 27 July 2017; Accepted 3 August 2017 0167-2738/ © 2017 Elsevier B.V. All rights reserved.

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the composites were described by Powder X-ray diffraction (XRD, Rigaku MiniFlex 600 with CuKα radiation, Rigaku Co., Tokyo, Japan), Fourier transform infrared spectroscopy (FT-IR, NICOLETAVATAR360 using KBr pellets, Nicolet Instrument Co., Madison, WI, USA), scanning electron microscopy (SEM, FEI-Quanta 250 FEG, FEI, Hillsboro, OR, USA) and transmission electron microscopy (TEM, HITACHI-HT7700, HITACHI, Tokyo, Japan). The cathode film was fabricated by mixing LVO/PEDOT composite, Ketjen Black (KB) and PTFE (microemulsion, 60 wt%) (70/20/10, wt%) into paste, then the paste was rolled into a slice and desiccated in vacuum drying chamber at 80 °C for 12 h. Finally, the slice was pressed onto an aluminum net to successfully fabricate cathode. CR2016 type testing cells were fitted in an argon-filled glove box (JMS-3, Nanjing Jiumen Automation technology Co., Ltd., Nanjing, China), Li metal disc as anode electrode, separated by commercial polyethylene separator and soaked with 1 mol L− 1 LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (1:1:1, v/v/v, provided by Zhangjiagang Guotai-Huarong New Chemical Materials Co., Ltd., Suzhou, China) as electrolyte. The assembled cells were charged and discharged by battery tester (LAND CT2001A, Wuhan Land Electronic Technology Co., Ltd. Wuhan, China). Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were performed on an electrochemical workstation (CHI 660D, Shanghai ChenHua Instruments Co., Shanghai, China). The amplitude of EIS signal was ± 5 mV and the frequency range from 100 kHz to 10 mHz under the open-circuit condition. To detect changes in the crystal structure and morphological of LVO and LVO/PEDOT composite electrodes during charge-discharge cycling, ex-situ XRD and SEM were performed on them after 50 cycles. The coin cells were disconnected in an argon filled glove box and the electrodes rinsed with DMC solvent in order to remove any residual salt. The recorded specific capacities (mAh g− 1) of LVO/PEDOT composites and the current density (mA g− 1) are based on the mass of the composite samples.

and form conductive networks to improve electronic conductivity, but also perform as lithium electroactive materials to enhance the whole capacity of LVO/conducting polymer composites [33]. Up to now, LVO composited with polythiophene [33], polyaniline [34], and polypyrrole [35] have been successively synthesized and showed improved rate capability and cycling stability. Among the various conductive polymers, poly(3, 4-ethylenedioxythiophene) (PEDOT) has obtained tremendous attention due to its appropriate working potential, redox reactivity and extremely stable features [36]. These advantages enable PEDOT as a feasible composite material for LVO. However, to our best knowledge, PEDOT has not been applied to composite with LVO. Hence, in this study, LVO/PEDOT composites were synthesized via an in-situ oxidative polymerization method. The electrochemical performances of LVO/PEDOT composites as cathode for LIBs were systematically studied. 2. Experimental 2.1. Materials synthesis The bare LVO sample was synthesized by rheological phase reaction method in a similar manner to the reported reference [37]. First, the stoichiometrically LiOH·H2O, NH4VO3, and C6H8O7·6H2O were ground thoroughly. Then, the mixture was taken into a Teflon-lined container, and a few drops of deionized water were added under vigorous stirring. When the solid-liquid rheological state presented, the precursor was transferred to a Teflon-lined stainless autoclave and treated in an air oven at 80 °C for 8 h and then cooled. Then, precursor material was treated again at 100 °C for 12 h, and further calcined in an aerated muffle furnace at 350 °C for 10 h and then washed with deionized water. Lastly, reddish brown powders were obtained after dried in vacuum at 60 °C for 10 h. The synthesis procedure of LVO/PEDOT composites are illustrated in Fig. 1. Certain amounts of 3, 4-ethylenedioxythiophene (EDOT) monomer and LVO were dispersed into 50 mL chloroform (CHCl3) solution, and the suspension was magnetically stirred at 0 °C for 30 min in N2. Then, ferric trichloride (FeCl3) (4:1, FeCl3/EDOT mole ratio) was divided into two equal parts and added into the suspension at the interval of 1 h. The mixture was kept stirring at 0 °C for 1 h and then at 50 °C for 6 h in N2. Next, the obtained suspension was poured into methanol to precipitate, washed with deionized water and ethanol, and finally dried under vacuum at 50 °C overnight to obtain the darkslategray powder. The LVO/PEDOT composites with the contents of 10, 20, 30 and 40 wt% PEDOT were synthesized. Those composites were referred as to 10 wt%, 20 wt%, 30 wt% and 40 wt% LVO/PEDOT, respectively.

3. Results and discussion To determine the actual amount of PEDOT in the LVO/PEDOT composites, TGA were carried out from 28 °C to 800 °C with a ramping speed of 5 °C min− 1 in an aerated environment. The TGA curves of the LVO/PEDOT composites, bare LVO and PEDOT are shown in Fig. 2. It can be found that the weight loss is about 4% (the part of weight loss may be water) for the bare LVO to 800 °C, while PEDOT begins to decompose at around 240 °C. The main mass loss of LVO/PEDOT

2.2. Materials characterization The amount of PEDOT in the composite was determined by thermogravimetric analysis (TGA) via a Setaram 92 instrument (Setaram Instrumontation, Lyons, France). The structures and morphologies of

Fig. 1. Preparation processes of LVO/PEDOT composites including an in-situ polymerization reaction.

Fig. 2. TGA curves of poly(3, 4-ethylenedioxy thiophene) (PEDOT), the bare LVO and the LVO/PEDOT composites.

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Fig. 3. XRD patterns of bare LVO and various LVO/PEDOT composites.

Fig. 4. The FT-IR spectra of LVO and 20 wt% LVO/PEDOT composite. Fig. 6. CV curves of 20 wt% LVO/PEDOT composite at a scanning rate of 0.1 mV s− 1 (a) and at different sweep rates between 4.0 and 2.0 V (vs. Li+/Li) (b).

Fig. 5. SEM micrographs of LVO (a), the PEDOT (b), the 20 wt% LVO/PEDOT composite (c), and TEM micrograph of 20 wt% LVO/ PEDOT composite(d).

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Fig. 7. Charge-discharge curves of LVO electrode (a) and 20 wt% LVO/PEDOT composite at the current rate of 60 mA g− 1 (b), the cyclic performance of LVO and LVO/PEDOT composites at the current rate of 60 mA g− 1 (c), rate capacities of LVO and 20 wt% LVO/PEDOT composite at the various current densities (d).

composites from 240 to 400 °C is ascribed to the degradation of PEDOT, and the approximate weight contents of PEDOT in the 10 wt%, 20 wt%, 30 wt%, and 40 wt% LVO/PEDOT composites are confirmed to be 8 wt %, 18 wt%, 28 wt%, and 37 wt%, respectively. Fig. 3 shows the XRD patterns of the bare LVO and LVO/PEDOT composites. It can be seen that all the prepared materials fit well to the standard diffraction peaks of LVO (JCPDS 72–1193) and the PEDOT phase was not observed in this patterns. Moreover, the peak positions of the LVO/PEDOT composites are similar to LVO, signifying that the PEDOT does not change the basic crystal structure of LVO. It is worth noting that the diffraction peaks of the LVO/PEDOT composites became broader as the content of the polymer increasing. Apparently, the 20 wt % LVO/PEDOT composite showed the widest X-ray diffraction peaks, which suggests that this composite has the lowest crystallization levels [38]. FT-IR spectra of the bare LVO and 20 wt% LVO/PEDOT composite are shown in Fig. 4. FT-IR spectrum of the bare LVO showed three strong infrared absorption characteristic peaks at 957, 746 and 588 cm− 1, which are assigned to V]O stretching vibration, the symmetric V-O-V stretching vibration and the asymmetric V-O-V stretching vibration, respectively [39–41]. However, the FT-IR spectrum of LVO/ PEDOT composite showed new peaks at approximately 1525, 1393, and

Fig. 8. Cyclic performances of the LVO and 20 wt% LVO/PEDOT composite electrodes at high current densities between 2.0 and 4.0 V.

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credited to the formation of solid electrolyte interfaces during the first scan [27,45]. However, CV curves were mostly overlapped in the following cycles and the shapes were well remained, indicating excellent electrochemical reversibility and cycling stability of LVO/PEDOT composites. Fig. 6b showed the CV curves measured at different scanning rates from 0.1 to 0.4 mV s− 1 in the second cycle. With the scanning rate increasing, the height of the redox peaks increased and the shape of the anodic and cathodic peaks became somewhat unsymmetrical, which was attributed to an increased electrode polarization and diffusion-limited intercalation/deintercalation processes of Li+ ions. Fig. 7a and b showed the discharge/charge curves of the bare LVO and 20 wt% LVO/PEDOT composite over a voltage range of 2.0–4.0 V at a current density of 60 mA g− 1. It is obvious that the 20 wt% LVO/ PEDOT composite exhibited different charge and discharge potential plateaus compared to the bare LVO. From the beginning, the composite electrode demonstrates more evident and symmetrical plateaus, declaring lower electrochemical polarization. The main charge-discharge plateaus located at about 2.8, 2.5, and 2.2 V, which corresponded to the CVs results. Moreover, the 20 wt% LVO/PEDOT composite showed better electrochemical performance than the bare LVO. As shown in Fig. 7a, the bare LVO displayed the second discharge capacity of 264.5 mAh g− 1, and then rapidly declined to 184.5 mAh g− 1 after 50 cycles. However, the 20 wt% LVO/PEDOT composite delivered a discharge capacity of 270 mAh g− 1 during the second cycle and the reversible capacity still maintained at 260 mAh g− 1 even after 50 cycles, which manifested PEDOT compositing ameliorated the electrochemical performance of LVO remarkably. Besides, as shown in Figs. 7c and 10 wt% and 30 wt% LVO/PEDOT composites shown better electrochemical performances than the bare LVO but poorer than 20 wt% LVO/PEDOT composite. The 40 wt% LVO/PEDOT composite displayed the worst electrochemical performances under the same condition, which was probably owing to that the over-thick coating layer hindered Li+ ions diffusion into/from the electrode [33]. The results suggested that a proper amount of PEDOT can both act as a conducting layer to promote the transport of electrons and Li+ ions among the LVO particles as well as a guard to inhibit direct contact of LVO with electrolyte. Fig. 7d displayed the rate capabilities of the bare LVO and 20 wt% LVO/PEDOT composite. At a low current density of 30 mA g− 1, the composite electrode provided a specific discharge capacity of 272 mAh g− 1. Satisfactorily, it still possessed a capacity of 235 mAh g− 1 as upon increasing the discharge rate to 240 mA g− 1, which was much higher than that of the bare LVO (153 mAh g− 1) at the same current density. Finally, when decreasing the rate again to 30 mA g− 1, the composite can almost recover its primary capacity, revealing its excellent electrochemical reversibility. As shown in Fig. 8, the cycleability of the bare LVO and 20 wt% LVO/PEDOT composite at the high current rate of 2000 mA g− 1 was compared. Only 58 mAh g− 1 of the discharge capacity of the bare LVO were attained from the initial discharge capacities of 122 mAh g− 1, and the capacity retention was merely 47.5%. Nevertheless, The 20 wt% LVO/PEDOT composite electrode can reach a high specific discharge capacity of 195 mAh g− 1 at the second cycle at 2000 mA g− 1 and retained 176 mAh g− 1 (90.3% of the initial discharge capacity) after 50 cycles. Therefore, it was safe to say that PEDOT improved structural stability of the LVO significantly and the LVO/PEDOT interface was conducive to the electrochemical performance at high current density. To further understand the outstanding electrochemical performance of 20 wt% LVO/PEDOT composite, EIS of the bare LVO and 20 wt% LVO/PEDOT composite were carried out. As displayed in Fig. 9, the Nyquist plots were mainly consisted of two semicircles in the high to medium frequency range. The medium frequency region described the charge-transfer resistance (Rct), and a sloping line in the low frequency region, relating to the Li+ ion diffusion resistance in the electrodes (ZW). It was clear that the LVO/PEDOT composite electrode showed a much lower Rct (151 Ω) than the bare LVO (829 Ω), evidencing that the

Fig. 9. EIS of the bare LVO sample and 20 wt% LVO/PEDOT composite under open-circuit condition (a), the relationship curves between ZRe and ω− 1/2 in the low frequency range (b).

1335 cm− 1, which are attributed to the C]C and CeC stretching in the thiophene ring [42]. The peaks at approximately 1200 and 1080 cm− 1 are probably ascribed to the stretching of the C-O-C [43]. The band at approximately 980 cm− 1 could be ascribed to vibration of CeS bond in the thiophene ring [44]. The FT-IR result analysis suggested that the LVO/PEDOT composite has been successfully achieved in this work. The SEM and TEM images of the bare LVO, PEDOT and 20 wt% LVO/PEDOT composite are shown in Fig. 5. The bare LVO sample revealed a nanorod aggregate, PEDOT appeared as flakiness with a coralloid surface, while 20 wt% LVO/PEDOT composite displayed an accumulation of flakiness and the average particle size is ascend to 200–300 nm, which indicated that LVO/PEDOT composite has been successfully achieved. The TEM image showed that the surfaces of the LVO nanorods were non-uniformly coated with a layer of PEDOT and the average thickness of the polymer was around 40 nm. The electrochemical performances of the LVO/PEDOT composites were studied to examine the effectiveness of PEDOT in improving the electrochemical properties of LVO electrode. Fig. 6a displayed the initial three cycles of CV curves for the 20 wt% LVO/PEDOT composite electrode at a scan rate of 0.1 mV s− 1 between 4.0 and 2.0 V (vs. Li+/ Li). Normally, there were five pairs of reversible redox peaks, and three main reduction and oxidation peaks of which appeared at potentials of around 2.2, 2.5, and 2.8 V. They can be ascribed to the Li+ insertion and extraction processes for the electrode. In addition, there was a little difference between the initial CV curves and the rest, which may be 34

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Fig. 10. Ex-situ XRD patterns of pristine LVO powder and electrode at the 50th cycle (a), pristine 20 wt% LVO/PEDOT composite powder and electrode at the 50th cycle (b), XRD patterns of Al net (c), and KB (d). Fig. 11. SEM images of electrode surfaces of (a) LVO before cycle, (b) LVO/PEDOT composite before cycle, (c) LVO after 50 cycles, and (d) LVO/PEDOT composite after 50 cycles.

composited PEDOT can facilitate faster Li+ transport and high-speed electron transportation. The Li+ diffusion coefficient (DLi+) can be figured up according to the following equation [46]:

DLi+ = 0.5 (RT An2 F 2σω C ) 2 In the equation, R is the gas constant, T is the temperature, A is the effective contact area between the electrode and the electrolyte, F is 35

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Faraday's constant and C is the concentration of the Li+ ions in the cathode calculated based on the crystallographic cell parameter of LVO. The slope (σω) can be obtained from Fig. 8b which showed the fitting linear of ZRe vs. ω− 1/2. The DLi+ value of the LVO and 20 wt% LVO/ PEDOT composite were calculated to be 2.58 × 10− 16 and 2.34 × 10− 14 cm2 s− 1, respectively, demonstrated the quicker Li+ ions diffusion ability of LVO/PEDOT composite. In order to further investigate the structure evolution of LVO and LVO/PEDOT composite, ex-situ XRD patterns of both the electrodes disassembled from CR2016 cells at 50th cycle are performed in Fig. 10. It is found that the XRD pattern of LVO/PEDOT composite cycled electrode almost unchanged except from the diffraction pattern of aluminum net and KB, the main crystal structure can still maintain after 50 cycles, suggesting good structural stability. However, from the Fig. 10a, we can see that the XRD pattern of LVO cycled electrode changed a lot, the crystal structure of LVO may be destroyed during charge and discharge process. The morphological changes of LVO and LVO/PEDOT composite electrodes after 50 charge-discharge cycles were characterized by exsitu SEM. Figs. 11a and b are SEM images of electrode membranes of the bare LVO and LVO/PEDOT composite before cycle, which all display the smooth surface. Fig. 11c are SEM images showing the surface of the LVO after 50 cycles, where large cracks can be clearly observed on the surface of the electrode, and the structures of LVO are pulverized and agglomerated, which caused partially electrode material deactivation, and then resulted in a significant drop in capacity. As shown in Fig. 11d, LVO/PEDOT composite electrode still exhibit a flat surface with no obvious change after 50 cycles, suggesting a good retention ability of morphological integrity of electrode, which may be attributed to the fact that the volume changing of the LVO induced by structural changes during charge-discharge could be buffered and accommodated by PEDOT. Hence, LVO cathode achieved high cycle stability and rate performance in combination with PEDOT.

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4. Conclusion LVO/PEDOT composites have been successfully synthesized via an in-situ oxidative polymerization process. In the different proportions of LVO/PEDOT composites, the 20 wt% LVO/PEDOT composite displayed the best cycling stability, high discharge capacity and rate capability than that of bare LVO and other LVO/PEDOT composites. The discharge capacity of 20 wt% LVO/PEDOT composite delivered as high as 260 mAh g− 1 after 50 charge-discharge cycles at 60 mA g− 1 and still remained a capacity of 235 mAh g− 1 even at 240 mA g− 1. The excellent electrochemical performances of 20 wt% LVO/PEDOT composite are probably contributed by the cooperating action of PEDOT which acts as a defensive layer to avoid direct contact with electrolyte and volume change leading to the pulverization of LVO, and a conductive network to enhance electronic conductivity. The composited PEDOT effectively reduced the electrochemical reaction resistance, and increased the lithium ion diffusivity of LVO/PEDOT electrodes, demonstrating that LVO/PEDOT composites are the promising cathode materials for LIBs. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 21403057), Program for Innovative Team (in Science and Technology) in University of Henan Province, China (17IRTSTHN003), Program for Science and Technology Innovation Talents in Universities of Henan Province, China (No. 18HASTIT008), Fundamental Research Funds for the Henan Provincial Colleges and Universities, China (No. 2014YWQN03, 2015RCJH10), Program for Henan Science and Technology Open and Cooperation Projects, China (No. 172106000060), Natural Science Foundation of Henan Province, China (No. 162300410050), International Science and Technology 36

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