Metal Foam Cathode Filled with Active Material for High Power and High Capacity Li-ion Batteries

Abstract

the electrode density was increased from 0.79 g cm-3 to 1.19 g cm-3 and the electrode thickness was reduced from 700 μm to 500 μm. As a result, the voltage drop was reduced by 200 mV at 5 mA cm-2 without sacrificing the gravimetric capacity and the volumetric capacity was increased significantly due to the reduction of the electrode thickness. Considering the good electrochemical performance and easier process of insertion of active material into the metal foam, 800 μm cell size of metal foam is very promising for the thick cathode. Additionally, it is possible to reduce the weight of battery because the weight is much lighter compared with that of smaller cell size of metal foam. According to the result of fast charging test, the thick cathodes can be charged in less than one hour after controlling the electrode density. Due to the unique structure of metal foam, the mass loading of active material and the electrode thickness can be in the range of 10?60 mg cm-2 and 200?1000 μm respectively for high power and high capacity Li-ion batteries. Because the active material is not blocked by the metal foam current collector, some sides of metal foam cathodes are not necessarily stacked with an anode periodically (refer to Figure 3.5.1). When the battery is used at lower current rate, larger amount of anode current collector and separator can be saved. However, to obtain the highest power performance and the highest capacity at high current rate, every side of cathodes should be stacked with an anode. Based on the study, metal foam is one of the promising current collectors for commercial application of high power and high capacity Li-ion batteries after reducing the weight and cost.

one kind of cathodes were compressed after the slurry of active material in the metal foam was dried and then annealed at 140 °C for half a day and the other kind of cathodes were prepared without pressing. For the unpressed cathode, the charge transfer resistance is very sensitive to the content of carbon black compared with that of the pressed one. AC impedance analysis shows that the charge transfer resistance was 800 Ω when there was no addition of carbon black and it was decreased to 57 Ω after addition of 4 wt.% carbon black for the unpressed cathode. However, the charge transfer resistance was only 26 Ω for the pressed cathode even though there was no addition of carbon black. It is noteworthy that much higher capacity exhibited for the unpressed cathode between 0.5?5 mA cm-2 (0.12?1.25 C), even though the voltage drop at the plateau region was much higher when the addition of carbon black was 4 wt.%. The results illustrated that the redox reaction area was much larger for the unpressed cathode due to the less diffusion limitation of Li-ion. To improve the power performance for the unpressed cathode, the addition of carbon black was increased to 14 wt.% and as a result, there was almost no difference in voltage drop at plateau region at high current rate for the two kinds of cathodes. Obviously, the capacity of unpressed cathode exhibited much higher at high current rate because of the larger area of redox reaction. However, to increase the volumetric capacity, the metal foam cathode needs to be pressed and the electrode density should be optimized considering the diffusion limitation of Li-ion. A metal foam cathode containing 8 wt.% carbon black was pressed appropriately

Among rechargeable batteries, Li-ion batteries are more suitable for use in portable devices because of their high energy density and high power performance. With the development of industry and progress of science and technology, Li-ion batteries are being developed for large-scale energy storage. However, the energy density and electrochemical performances are still waiting to be further improved for the large-scale application. One of the major dilemmas is that the thickness of active material on the foil-type current collector is very thin. Generally, the thickness is around 50?100 μm for portable devices whereas the thickness is only 20?60 μm for hybrid electric vehicles to sustain high power performance. The battery is fairly heavy and large due to the increase in the use of inactive materials, such as current collectors and separator to obtain large-scale capacity. Additionally, the high production cost is one of the big issues with the increase of material and workload such as cutting, stacking, tap welding, sealing, and so on. To address the problem, three-dimensional metal foams have been used for ultra-thick Li-ion battery electrodes instead of commercial foil-type current collectors in this study. The electrochemical performances were compared for the cathodes using metal foam and foil-type current collectors at the condition of same amount of active material and same electrode density. According to the charge-discharge test at a high current rate and cyclic voltammetric (CV) analysis, better electrochemical performances were obtained for the metal foam cathode due to the large area of triple junction (junction of active material, metal frame, and electrolyte). Considering the better kinetic performance and the unique structure of metal foam, it is possible to increase the electrode thickness to enlarge the cell capacity and save the inactive materials and assembly time. Two kinds of thick metal foam cathodes were prepared