High Capacity Thick Cathode with a Porous Aluminum Current Collector for Various Rechargeable Lithium Batteries

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.