Nanocrystalline LiFePO4 as cathode material for lithium battery applications S.C SIAH

Transcription

1 Nanocrystalline LiFePO as cathode material for lithium battery applications Abstract S.C SIAH Engineering Science Programme, National University of Singapore Kent Ridge, Singapore LiFePO was prepared by soft chemistry route. The synthesized LiFePO samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques prior to calcination at 600 for 6 h under inert Argon in a tube furnace to obtain well crystallized LiFePO. Calcined products were used to assemble lithium batteries using a Swagelok or Coin cell to investigate their electrochemical performance. The battery performance of the synthesized LiFePO shows reproducible charging and discharging cycles with a storage capacity of around 0mAh/g. 1. Introduction LiFePO has been the subject of intensive research in recent years as cathode material for rechargeable batteries [1, 2] due to its good electrochemical properties, high capacity of 170 mah/g, low cost and non-toxicity. Thus, LiFePO has a high potential of replacing the toxic and expensive LiCoO as the cathode material in commercial lithium-ion batteries. One of the drawbacks of LiFePO is its low conductivity [3]. This kinetic problem could be overcome by synthesizing LiFePO in nano-size with organic precursors such as sugar so that the thin layer of carbon obtained from the decomposition at high temperature can serve as a conductive coating for the LiFePO molecules. Another drawback is the ease of oxidation of Fe 2+ to Fe 3+ during synthesis and calcination. This could be resolved by adding a suitable reducing agent such as citric acid during synthesis and calcinating mixture under inert Argon atmosphere. In this paper, a soft chemistry synthesis route for LiFePO is described. LiFePO was characterized using XRD and TEM techniques. Electrochemical performances for the synthesized LiFePO samples were obtained using Swagelok and coin cells, and compared against the electrochemical performances reported in literature. 2. Experimental 2.1 Synthesis of LiFePO LiFePO was prepared from the following reaction: 0.5 / For conciseness, the waters of crystallization in these compounds are not expressed in the equation above. 1

2 LiFePO was prepared by mixing the precursors above with ethylene glycol as solvent and heat treated for 15 h at 270 C using an autoclave reactor. As mentioned above, the addition of 0.5 mol of Citric Acid as a precursor is crucial as it acts as a reducing agent and hence minimizing the amount of oxidizing into. The presence mol of sugar will ensure that at high temperature, the decomposed sugar molecules will provide an abundance of carbon traces which will be coated onto the molecules and hence improving the surface electronic conductivity of the synthesized. A total of 7 samples of were prepared in proportions as stated in Table 1. The precipitates obtained were washed and sonicated 3 times with Ethanol and dried using a vacuum oven. The photos displayed in Fig 1 refer to the observed color of the precipitates. Gluconic Citric Acid Colour Acid SC Black SC Grey SC Dark Green SC Light Green SC Grey SC Black SC Dark Green Table 1 : Table recording the ratio of precursors used for each sample. (a) (b) Fig 1: (a) photo of SC(L) and SC3(R), (b) photo of SC7(L) and SC5(R), It was found that the ratio of precursors used was found to have an effect on the color of the precipitate produced. It was observed that as more Gluconic Acid was added as precursor, the resultant precipitate will have a darker color. This is due to the fact that Gluconic Acid decomposes at high temperature and the carbon that is produced coat the formed. This layer of carbon coating serves to improve the surface electronic conductivity of the prepared crystals and results in a darker appearance of the precipitate. 2.2 Calcination of Samples SC1, SC, SC5 and SC6 were calcinated under inert Argon atmosphere at 600 for 6 h. Argon atmosphere was used so as to prevent from oxidizing into. However, due to mild air leak during operation of the tube furnace, these samples were calcinated under semi-inert atmosphere hence resulting in partial oxidization. Evidently, the precipitates turned reddish-brown, an indication that the grayish-green ions have been oxidized into ions. Fig 2 displays the photos of the calcinated samples. 2

3 (a) (b) Fig 2: (a) photo of annealed SC 1(L) and SC3(R), (b) photo of annealed SC(L) and SC5(R) 2.3 Characterization of the synthesized X-ray diffraction The XRD patterns of annealed SC1 and unannealed SC2 and SC7 are shown in Fig 3. These patterns are compared against the standard pattern of pure Impurity Intensity Thetha sc1-annealed sc2 pure Fig 3: XRD patterns of annealed SC1, unannealed SC2 and pure phase. A comparison of the XRD patterns of these annealed and unannealed samples with that of the pure phase revealed many additional peaks. This suggested that the 2 samples were not entirely pure as the impurities account for the additional peaks in the XRD pattern. Few extra peaks were observed for annealed sample of SC1. Some of these peaks could be due to the oxidation of into. 3

4 2.3.2 Transmission electron microscopy (TEM) Sample SC2 was characterized using TEM and the following images were obtained. Images Observations A Fig is a overview of several LiFePO nanocrystals (SC2). It can be observed that crystals A and B have rectangular shapes which suggest that the arrangement of Li, Fe, P and O atoms within the crystals is highly ordered. B Fig C D It can be observed from the high resolution TEM image (Fig 5) that region C refers to the fringes characteristic of well crystalized phase and region D refers to the glassy background region, these two regions are separated by a clear boundary. No layer of carbon coating can be observed along this boundary. Also, since SC2 was prepared without the use of Gluconic Acid, it is expected that there will not be any layer of carbon coating along this boundary. Fig 5 Fig 6 is a selected area electron diffraction (SAED) image. It can be observed that the bright spots are arranged orderly in a systematic manner which suggests that the arrangement of Li, Fe, P and O atoms within the crystals is highly ordered. Fig 6

5 2.3.3 Electrochemical performance For potential battery application, the electrochemical performances of a selected LiFePO sample (SC2) was tested using Swagelok as well as Coin cells. The powder form of LiFePO samples were mixed with PVDF and Carbon in the volume ratio of 70:15:15 respectively and mixed thoroughly for 30 minutes. The resultant mixture was stirred using a magnetic stirrer for 8 h at room temperature and the slurry obtained was pasted onto Aluminum foils using Doctor blade and dried for 8 h in a vacuum oven at 80. The obtained cathode electrodes were then punched and pressed in the form of disk with a diameter of 10mm. The cathode disks were then assembled into Swagelok Cells and Coin Cells in a glove box under inert Argon atmosphere together with an electrolyte solution of 1M in a mixed solvent of ethylene carbonate and dimethyl carbonate (1: 1 volume ratio) and a polypropylene microporous separator membrane. Lithium foil served as the anode material. The cells were tested for their electrochemical performance by passing a constant current using Arbin BT2000 battery tester. In this electrochemical process, the following redox reactions take place: Charging at Cathode: LiFePO + xli + xe + Li1 x FePO Discharge at Cathode: Li x + 1 FePO + xli + xe LiFePO During the charging process, is extracted from LiFePO and is oxidized to. is transported to the anode through the electrolyte whereby it will be oxidized to become Li metal, while the electrons flow through the external circuit. In the reverse process, is produced at the anode and transported back to the cathode, thus reducing Fe 3+ to Fe 2+ to form. 1. Unannealed SC2 with a Swagelok Cell Voltage/ V Electrochemical Performance of SC2 with Swagelok Cell E Normalised capacity Fig. 7 Charge/ discharge curves of unannealed SC2 with a Swagelok Cell The Swagelok cell was cycled using a constant current of 30µA between 2.3 and.3 V. The voltage plateaus at 3.V on charging and 2. V on discharging corresponds to the lithium extraction and insertions reactions. For this experiment, a charging capacity of 100mA/g (0.59 Li + ) was observed in 5

6 the first cycle, this is about 55% of the theoretical capacity. After the first charging cycle, subsequent charging and discharging cycle yields an average capacity of 51mA/g. One abnormal observation was that the voltage plateaus for discharging occurs at 2. V which in a normal case should occur at around 3.V. This could be due to impurities present in the Swagelok cell. 2. Unannealed SC2 with Coin Cell Electrochemical Performance of SC2 with Coin Cell Voltage/ V Normalised capacity Fig 8 Charge/ discharge curves of unannealed SC2 with a Coin Cell The coin cell obtained was cycled 30 times under a constant current of 30µA between 2.3 and.3 V. The voltage plateaus at 3.5V on charging and 3.3 V on discharging corresponds to the lithium extraction and insertions reactions. A charging capacity of 6 mah/g (0.27 Li + ) was observed for the first cycle and the capacity subsequently settles down to an average value of 3 mah/g (0.2 Li + ) after the first cycle. The abnormal observations pertaining to the Swagelok cell were not observed in this case indicating that the expected electrochemical reaction occurs. The voltage plateaus at 3.5V on charging and 3.3V on discharging is in excellent agreement with results reported by Wang, Qiu and Li (2006) []. However, the observed capacity was only 20% of the theoretical observation. This could be due to the fact that SC2 was prepared without the use of Gluconic Acid and hence, LiFePO crystals were not coated by a layer of carbon which improves the conductivity [5]. 3. Conclusions Highly crystalline LiFePO materials were successfully synthesized by a soft chemistry route. The synthesized samples were characterized under XRD, TEM and electrochemical performance using a Swagelok and Coin cells. The charging and discharging capacity of SC2 were only 20% of the theoretical value and this could be attributed to the fact that the samples were not totally pure and the surfaces were not carbon coated. However, based on the images obtained from TEM, it could be observed that a highly crystalline structure is formed through this soft chemistry route. Hence, we expect that by incorporating carbon coating on LiFePO crystals a better electrochemical result can be obtained. 6

7 . Future Interests Due to time constraints and the lack of facility during the first 6 months of my research, the results I obtained were quite limited and elementary. However, I believe that the prospect of Lithium-ion batteries is immense and based on my short stint in UROP, I now have the confidence to prepare nanocrystalline LiFePO samples and characterize them. However, if given a chance, I feel that more research can be done in this area. Suggestions for Further Works: 1. I believe that by varying the amount of organic material used as precursors during the soft chemistry synthesis, the amount of carbon coating can be varied. This is important as carbon coating improves the conductivity of LiFePO crystals and hence will affect the overall electrochemical performance. By studying the relationship between the amount of organic material used and the electrochemical performance, the usage of organic material can be maximized and tailored to suit many different applications which requires high rate performances. 2. I believe that the amount of heat produced during the electrochemical performance can be studied in relation to the charging and discharging rate. A thermocouple lead can be inserted within the Swagelok cell and the temperature changes can be plotted against the charging and discharging curve at different charging rate. By varying parameters, heat loss can be reduced to the minimum which will lead to an overall increase in operating efficiency. 3. I believe that the ambient temperature will have an effect on the electrochemical performance. Experiments can be carried out in different temperatures ranging from 10 to 60 C and the performance be studied against the ambient temperature. This area of research will allow battery performances to be maximized under different operating temperature which is very relevant to real-life application. Current experiments were performed at room temperature in which the results might deviate at higher or lower operating temperatures. 5. Acknowledgement The author would like to thank Dr Palani Balaya for his guidance. The author also thank Mr. Joseph (M.E.), Saravanan (Chemistry) and Mr. Thomas, who have helped me at various stages. 6. References [1] A. K. Padhi, K. S. Nanjundaswamy, and J. B. Goodenough, J. Electrochem. Soc., 1, 1188 (1997) [2] S. Y. Chung, J. T. Bloking, and Y. M. Chiang, Nat. Mater., 1, 123 (2002) [3] R. Amin, P. Balaya and J. Maier, Electrochem. And Solid-State Letters, 10, A13- A16 (2007) [] Y. Wang, J. Wang, J. Yang, and Y. Nuli, Adv. Func. Mater., 16, (2006) [5] B. Wang, Y. Qiu, L. Yang, Electrochem. Comm., 8, (2006) 7