As one of the methods to improve the energy density of lithium secondary battery, the use of high capacity anode and cathode materials has been focused. Although metals and alloys are mentioned as anode material candidates with high capacity, on the other hand, the candidates for high capacity cathode are insufficient currently. Therefore, a large amount of active material has to be accumulated and compressed to the cathode to achieve high energy density in consideration of the capacity balance with anode (Fig. 1). However, good performance cannot be obtained in the conventional thick coating electrode with a large amount of active material. The thick coating electrode has many problems. The ohmic resistance increases as the distance between a current collector and active material is made longer. In addition, many cracks, dropouts, and exfoliations are caused by volume changes of the coated layer during charge and discharge. In this study, the high capacity thick cathode has been realized by using a porous aluminum as a current collector as a practical method to improve the energy density of lithium secondary battery (Fig. 2). The combination of the thick cathode with a very high capacity per unit area and a high capacity anode can omit the amount of supporting materials such as a separator in batteries, resulting in the high energy density. The various thick cathodes were prepared using lithium iron phosphate, lithium cobalt oxide, and lithium nickel-cobalt-manganese oxide, respectively, and their electrochemical characteristics were evaluated by half-cell test. Though the cathodes had about several times larger capacity than conventional coating cathodes, they exhibited excellent rate performance. For example, the cell comprised of the thick cathode using lithium iron phosphate with a porous aluminum and two pieces of graphite anodes showed approximately 4-5 times capacity of the cell using conventional coating electrode at the same effective electrode area, and no large difference was observed for a in the polarization of charge-discharge at 1.0CA (Fig. 3). In a cycle-life test, the cell exhibited an excellent charge-discharge cycle performance, and retained approximately 80% of initial capacity even at 2,000th cycle (Fig.4). Such excellent performance is expected to be due to the easy electrolyte permeation from one side to the other side of the thick cathode, which is achieved by a porous aluminum current collector in addition to three-dimensional current collection. Of course, the lithium-ion diffusion in the cathode is expected to be insufficient for charge and discharge reactions when the cathode and C rate become thicker and larger, respectively. Thus, further optimization is needed for the cathode as well as for the electrolyte solution and separator to improve the ion supply in the cell. Such battery design will be also mentioned in this study. Figure 1
Lithium ion battery has a high energy density. Therefore, it has been used for various applications. However, electric power consumption of portable devices increases due to multifunction of devices, year by year. Higher energy density battery is strongly required. One of the methods to improve the energy density is utilization of lithium metal for the anode material. Lithium metal has the lowest electrode potential and the highest capacity per unit weight in all elements. However, the electrochemical reaction of lithium metal, which is deposition/dissolution reaction, has the problem such as an occurrence of the internal short circuit between anode and cathode due to a formation of the dendritic lithium metal growth during charging process. Particularly, the internal short circuit by the lithium dendrite is known to cause significant capacity loss and the serious accidents such as burst and firing of cells. It may be said that these phenomena are accelerated by non-uniform reduction of Li + ion at the surface of lithium metal anode. It is important to realize a highly uniform reduction at surface of lithium metal anode to suppress the dendrite growth that is the biggest problem of the lithium metal rechargeable battery. For this purpose, the formation of the large number of uniform reaction site and the lithium ion movement between anode and cathode are seriously required. The test cells with three dimensional ordered macro porous separator (3DOM separator) which has highly homogeneous pores and a high porosity were prepared. These cells can start from the first discharge process. Though, the cathode material for conventional lithium ion battery cannot start from the first discharge. In order to enable the partially first discharge, manganese dioxide, which did not contain lithium, was added as the second cathode material to lithium cobalt oxide as the first cathode material. At the first partial discharge process, lithium metal dissolved into electrolyte to form many holes at the surface of lithium metal anode. The 3DOM separator provides uniform current distribution to the lithium metal surface during the first discharge. A large number of pores formed on the anode surface can be used as a reaction site of the deposition/dissolution reaction of lithium metal, and it is expected that a local growth of large lithium dendrite is suppressed. This conceptual scheme is shown in Figure 1. Furthermore, the 3DOM separator can hold a lot of electrolyte due to approximately 2 times higher porosity than that of the conventional separator. This enables a smooth lithium ion movement between cathode and anode. By these effects, lithium dendrite growth is suppressed. Lithium metal anode rechargeable cells were assembled by using lithium cobalt oxide including manganese dioxide additive with 0 %, 10 %, and 20 % in weight, respectively as a cathode material, lithium metal as a anode material, 3DOM separator as a separator, and 1.3 mol dm -3 lithium hexafluorophosphate dissolved into the solvent which contained ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate with a volume ratio of 2:5:3. 1 weight % vinylene carbonate was added into the electrolyte. Firstly, the cells were discharged to 2.0 V at 0.1 CA. After that, the cycle performance was evaluated. The cells were charged by 0.5 CA constant current followed by constant voltage charge at 4.3 V until the current reduced to 0.05 CA. Then the cells were discharged to 2.0 V at 0.5 CA. The temperature was at 25 °C. The cycle performance was shown in Figure 2. It was confirmed that cycle performance improved as increasing added manganese dioxide percentage in the cathode material. Many reaction sites may be formed on the lithium metal surface of anode with increasing first discharge capacity. The battery performance was not still sufficient for a practical battery, optimization of kinds and quantity, composition of second cathode material, and adjustment of the electrolyte composition and concentration of lithium salt are necessary. In addition, the safety of lithium metal rechargeable cell should be tested. In the future, we will demonstrate a high energy density battery by combining lithium metal anode. Figure 1
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