Study of lithium iron phosphate positive electrode prepared by aqueous paste process for lithium ion battery -2nd report-

Study of lithium iron phosphate positive electrode prepared by aqueous paste process for lithium ion battery Hidetoshi Abe*1 Tomonori Suzuki*1 Takashi Eguro*1 Kiyoshi Kanamura*2 Kaoru Dokko*3 Mitsumasa Saito*4 Abstract
Recently, lithium ion battery with iron phosphate positive electrode having high temperature stability for safety has been extensively studied. In this study, the practical lithium iron phosphate electrode with aqueous paste process has been established through optimization of particle morphology, solid content of paste, and mixing conditions. The laminate type 3Ah Li-ion cell with developed electrodes has been confirmed to be realized excellent high-rate discharge performance and cycle life (more than 1000th) by the 1. Introduction
thermal runaway. We also formed electrodes using aqueous Recently, lithium ion batteries with medium or large pastes rather than conventional organic solvents in order to capacity have been rapidly developed as power sources for reduce the negative environmental impact when making hybrid electric vehicle or industrial equipment. On the other hand, as the increased size of batteries allows them to store Lithium iron phosphate is lower in conductivity than other more energy, they are expected to have a higher level of active materials and needs some improvement if it is used in an application where a high rate discharge is required. In Approaches to higher safety under study include (1) using addition, because of the large surface tension of water, which olivine-type lithium iron phosphate, spinel-type lithium is used as a dispersion medium when preparing a paste, we manganese oxide, or other positive active materials that anticipated difficulty in dispersing particles in comparison generate almost no oxygen in order to prevent the batteries with an organic solvent and reduced efficiency of current from catching fire or exploding during thermal runaway, (2) collection in the electrode (between the active material adding a fluorine-containing material to the electrolyte or particles and between the particles and the current collector). using an ionic liquid as the electrolyte to make the batteries To address these problems, we have worked to (1) develop flame-retardant and to prevent them from catching fire or lithium iron phosphate nano-particles that are highly exploding during thermal runaway, and (3) using PTC device, dispersive, (2) develop an appropriate dispersion technology, protection circuits, or separators with a shutdown effect to (3) optimize the conductors and binders to be mixed, (4) develop highly conductive lithium iron phosphate particles We have studied the electrode behavior of olivine-type (improve powder materials), and (5) elucidate interface lithium iron phosphate, which is effective for preventing reactions between the aqueous paste and the aluminum current collector and develop a corrosion protection technology that ensures the required conductivity. So far, a process for creating aqueous paste-based lithium *2 Faculty of Urban Environmental Sciences, Tokyo Metropolitan iron phosphate positive electrode has been established, and a *3 Faculty of Engineering, Yokohama National University *4 New Technology Research Laboratory, Sumitomo Osaka test lithium ion battery has been made using this process. By evaluating the capacity, discharge performance, battery life, considered to be a problem specific to aqueous paste and is a energy density, and other characteristics, it was demonstrated major issue for the practical application of an aqueous that the lithium ion battery has excellent battery performance. paste-based lithium iron phosphate positive electrode.
This paper reports the technology for preparing aqueous 2.1.2 Making Aqueous Paste-Based Positive
pastes and the evaluation results of a small cell (3 Ah) made Electrodes
To resolve this issue, we reviewed the morphology of lithium iron phosphate particles, the water content of the 2. Development of the technology for paste, and the dispersion method.
preparing aqueous pastes
A new method for preparing lithium iron phosphate was 2.1 Experiment
employed to make carbon-coated particles with a new 2.1.1 Issues for Practical Application of Electrodes
morphology. To inhibit the volume change during drying, a Based on the previous study results, it has been found that paste with a lower water content was prepared. In addition, a conditions when an aqueous paste is prepared (materials, new dispersion technique that allows uniform dispersion and dispersion methods, etc.) alter the arrangement of particles in advanced control of particles was employed. Figure 1 shows the electrode after it is applied and have a significant the flowchart for preparing pastes. The conventional influence on various characteristics, including the charge and dispersion method using a medium was also used with the discharge characteristics at different charge and discharge same materials to create a paste for comparison. The same rates and the cycle life characteristics. materials (conductor and viscosity improver) as the previous By using a beads mill or other dispersion methods using a time were used, except that an aqueous binder from Zeon medium with nano-particles of lithium iron phosphate in particular, good results could be obtained due to uniform This time, lithium iron phosphate and graphite were used for the positive electrode and for the negative electrode, respectively, to form a small lithium ion cell, which was then evaluated. To achieve the proper capacity ratio of the positive electrode to the negative electrode, the positive active material had to be applied twice as thick as had been New mixing method
First, an aqueous paste was prepared using the same method and the same materials the previous time, and applied to a 20 µm thick aluminum foil with a film applicator. The solid content was increased from 50 g/m2 to 100 g/m2. After that, it was dried in a hot air drier kept at 100°C for 10 minutes, but the dried electrode had numerous cracks in the coated film, which easily fell off. This is probably because Fig. 1 Flowchart of new aqueous paste preparing the increased thickness of the paste caused a great volume change (shrinkage of the coated film), and the coated film could no longer bind to the aluminum. This phenomenon, which is not observed with organic solvent paste, is These pastes were applied and dried under the same 2.1.4 Observation of Changes in the Surface
conditions as described in the previous section, the rolled by a Shape of the Aluminum Current Collector
roll press to the set thickness, and stamped into 20 mm Before and After Charge and Discharge
diameter disks, which were for evaluation as electrodes. After 29 charge and discharge cycles were repeated under Evaluation
of Positive Electrode the conditions in Table , the cells were dismantled and the
Characteristics
positive electrodes were removed, washed, and dried. Next, The C values*5 of the electrodes for evaluation were the coated film layers were removed by a film removal calculated based on the mass of the active material per method using concentrated sulfuric acid, and the surface of the aluminum current collectors was exposed for observation. The electrodes for evaluation were used as working Similarly, specimens for comparison were removed from electrodes, and an ethylene carbonate (EC)-based mixed pressed and unused electrodes, and the surface of these solvent in which lithium hexafluorophosphate (LiPF6) was current collectors was observed by a scanning electron microscope (JEOL JSM-5310LV, hereinafter called the reference electrodes of lithium metal were used, and a porous “SEM”) for morphological changes resulting from corrosion polyethylene membrane was used as a separator. These were combined with polypropylene battery cases to make three-electrode cells, which were then subjected to 2.2 Results
electrochemical evaluation. Table 1 shows the conditions of a 2.2.1 Aqueous Paste-Based Positive Electrode
charge and discharge test. The test environment was kept at Made by the New Method
Cracks were not found in the positive electrode made by the new method after the paste was dried or pressed, and it was found that good electrodes can be made by this method. The positive electrode made by the conventional method developed cracks under the same drying conditions. Thus the drying conditions were changed to 40 minutes at 50°C to Changing the particle morphology and the water content alone could not adequately inhibit the development of cracks, but there was a difference in the level of crack inhibition Evaluation of Positive Electrode
Characteristics
Figure 2 shows discharge results at different discharge rates for these electrodes (under conditions for Cycles 6 to 9 in Table 1). According to the test results, there were no large differences in discharge capacity between these electrodes, *5 The charge/discharge rate is expressed as a multiple of the value (C) when the capacity of the battery is expressed in Ah (ampere but the electrode made by the new method had better hours). In the electrode test in this paper, however, the theoretical capacity was used as the C value. polarization characteristics and smaller electrode resistance.
Fig. 3 Change of discharge capacity during cycling for two types of paste preparation Conventional method for aqueous paste preparation Fig. 2 Discharge rate characteristics for Figure 3 shows the changes in capacity in a charge and discharge cycle test conducted in the 10th to 29th cycles, and Figure 4 shows 0.5 CA charge and discharge characteristics In the cycle test, the electrode made by the new method showed no decrease in discharge capacity while the electrode made by the conventional method showed gradual decreases in discharge capacity. In addition, the electrode made by the new method showed no change in discharge characteristics with an increasing number of cycles while the electrode made by the conventional method showed increases in polarization with an increasing number of cycles. characteristics for two types of paste preparation at 11th, 20th, and 29th 2.2.3 Changes in the Surface of the Aluminum
An ethylene carbonate (EC)-based mixed solvent in which Current Collector Between Before and After
lithium hexafluorophosphate (LiPF6) was dissolved was used Charge and Discharge Cycles
as an electrolyte. Table 2 summarizes the cell composition Figure 5 shows SEM photographs of the surface of the and Figure 6 shows a photograph of the cell. aluminum current collector of the electrode made by the new method both before and after the charge and discharge cycle Cathode LiFePO4 electrode with new method Fig. 5 SEM photos of Al current collector at After the first charge and discharge cycle, at a current of Observation of these photographs shows that there were no 0.2 CA, the cell was charged to reach 3.6 V and subjected to corrosion or other changes in the surface of the aluminum a constant-voltage charge for a total of 7 hours, then current collector under the evaluation conditions used this discharged to 2.0 V. This cycle was repeated 5 times for initial activation. The environment was kept at 25°C±3°C. After that, a discharge test in a discharge range between Trial Production and Evaluation of a Small Cell
0.2 CA and 10 CA was conducted. The charge conditions 3.1 Experiment
were used same as for initial activation were used.
A positive electrode using an aqueous paste prepared by Next, a cycle life test was conducted. In this test, the cell the new method, a graphite negative electrode, and a was subjected to a constant current charge at 1.0 CA to reach separator made of a porous polyethylene membrane were a voltage of 3.6 V, to a constant voltage charge at 3.6 V to combined into a electrode assembly, which was packed with reach a current of 0.05 CA, and then to a constant current a laminated aluminum film to make a cell with a capacity of discharge at 1 CA to reach 2.0 V (100% DOD*6). 3.2 Results
Figure 7 shows the results of the discharge at various Fig. 8 Change of discharge capacity retention during The capacity did not decrease until a discharge rate of 2 CA, and the capacity retention rate at a discharge rate of 10 CA when compared with a discharge rate of 0.2 CA was greater than 90%. The energy density was 97 Wh/kg. Fig. 9-1 Comparison of 1CA charge and discharge Figure 8 shows changes in discharge capacity in the charge characteristics at various cycle at 25°C and discharge cycle test and Figure 9 shows charge and discharge curves in the 2nd, 500th, and 1000th cycles at different temperatures (9-1: 25°C, 9-2: 45°C). The discharge capacity retention rates (1 CA) in the 1000th cycle at 25°C and 45°C were 90% and 85%, respectively, and it was found that the cell showed excellent charge and discharge cycle characteristics. In addition, the charge and discharge capacities decrease with the increasing number of charge and discharge cycles, but no changes in charge and discharge plateau voltage due to increased resistance Fig. 9-2 Comparison of 1CA charge and discharge characteristics at various cycle at 45°C cycle life test These cells have not reached the end of their life and are 4. Discussion
By reviewing the morphology of lithium iron phosphate • The 3 Ah cell made by the aqueous paste-based lithium particles, the water content of the paste, and the dispersion iron phosphate positive electrode obtained in this study method, a practical aqueous paste-based lithium iron was found to have good high rate discharge phosphate positive electrode that inhibits the development of characteristics (10 CA) and a long cycle life (1000 or cracks when dried could be obtained. This is probably because volume reduction (shrinkage of the coated film) • It was confirmed that an aqueous paste of lithium iron during drying could be inhibited, and the optimum phosphate almost never causes corrosion to aluminum arrangement of active material particles, the conductor, the current collectors when a positive electrode is made or The dispersion method had a significant influence on the Based on the results of this study, we will address lifetime electrode characteristics, and improving the dispersion and cost issues and issues related to mass production method resulted in better initial polarization characteristics technologies for practical application in the future. and greatly inhibited increases in polarization with increasing number of cycles. This is probably because the optimized 6. Acknowledgments
arrangement of the active material and the conductor This study was conducted under a contract from the improved current collection performance, structural Innovative Technology Development Research Project of the maintenance characteristics, and electrolyte diffusivity. Japan Science and Technology Agency, and we deeply The 3 Ah cell was found to have good high-rate discharge appreciate the help of everyone involved. characteristics and a long charge and discharge cycle life. In particular, increases in polarization with increasing number of charge and discharge cycles were not observed with this cell, This technical report is an English translation of Japanese and it is expected to have stable output characteristics over a long period of time. However, in the charge and discharge cycle life test, the capacity reduction was larger at a high temperature (45°C) than at a low temperature (25°C). Thus, the float charge characteristics and self discharge characteristics at high temperatures, along with the application where the cell is used, should be examined. We will work to further improve the characteristics of this cell and review the materials, parts, and structure not only of the aqueous paste-based lithium iron phosphate positive electrode developed this time but also of negative electrode 5. Conclusion
• A new method for making positive electrode was developed, and a practical aqueous paste-based lithium iron phosphate positive electrode that can inhibit the development of cracks when the applied paste is dried

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