Optimization of Metal Foam Cathode in Li Secondary Battery

Abstract

Li secondary batteries are used widely batteries in portable devices, electrical vehicles (EVs), and laptops, owing to their very high energy density, high operating voltage (about 3.6 V), small size, and lightweight compared to other secondary batteries. The commercialized batteries are to fabricate by coating the active material on the metal foil as a current collector to a thickness of about 40 ~ 100 μm. However, if the active material coated to a thickness of several hundred micrometers in order to manufacture a high capacity battery using a metal foil, peeling easily occurs between the active material and the metal foil, and the internal resistance of the electrode is increased. That is, since the active material has to be widened in area of the electrode coated with a thin active materials thickness, a large amount of separator and current collector are required regardless of the capacity, and the weight and volume of the battery are increased. In this paper, three-dimensional (3D) metal alloy foam was used as a current collector for battery instead of the conventional metal foil. The advantages of using metal foam are the much larger surface area, due to the many triple point (junctions of active material, metal frame, and liquid electrolyte) existing inside the electrode and shorter Li-ion diffusion length, therefore improving the electrical conductivity. In addition, metal foam as a current collector was enhanced the cycle-life due to the rough surface of metal frame enhanced adhesion properties and prevented separation of the active materials from the metal frames during the charge-discharge process. The electrochemical performances were compared for the carbon black content in metal foam cathode. The metal foam cathode with increased carbon black loadings enhanced the electrochemical performances and significantly decreased the charge transfer resistance. This is due to the large effective contact area between the conductive carbon material and the active material. However, high carbon black loading resulted in low cell capacity (mAh) at low current density, due to decreased amount of active materials. That is, high-capacity batteries used in low current are advantageous in reducing the content of carbon black, while high-power batteries are advantageous in increasing the content of carbon black in metal foam cathode. The electrochemical performances were also compared for the metal foam cathodes using different pore size of metal foam. In a case where the carbon black content was 15wt.%, the metal foam cathode of 450 μm-pore size showed the highest specific capacity and the lowest over-potential. This is because the small pore size has a short Li-ion diffusion length and provides a wide surface area for the oxidation/reduction reaction due to an increase in the triple point. However, in the case where carbon black content was 25 wt.%, 450 and 3000 μm-pore sizes showed similar specific capacity and over-potential. This is because a large pore size of metal foam cathode is greatly affected by carbon black. That is, light-weight and high capacity batteries are most efficient when fabricated by adding 15 wt.%-carbon black to a metal foam of 3000 μm-pore size. To improve the electrochemical performance of Li secondary batteries, it is important to optimize the electrode thickness and mass loading of active material. Thick electrodes exhibit higher cell capacity (mAh) and specific capacity (mAh g-1) compared to thin electrodes. This is due to the increase in the amount of active materials and longer time of Li redox reaction. However, to obtain the high-power performance at high current, it is advisable to stack of a thin metal foam cathode, due to the wide reaction surface area and the short Li-ion diffusion length. The electrochemical performances were compared to the electrode porosity in metal foam cathode. According to charge-discharge test and cyclic voltammetry analysis, metal foam cathodes with high intensity pressed after firing process lead to poor electrochemical and kinetic performances. This can be attributed to the reduced area of redox reaction. This means that the increases in the active material density caused a reduction in the efficiency of the battery because electrode porosity decreases and the diffusion limitation of Li-ion increases. This shows that the formation of the third pores into which the electrolyte can enter is very important factor when a battery is manufactured using the metal foam current collector. Therefore, the third pores should be maintained through a no press, pre-annealing process, and slightly press process. In addition, the large proportion of the metal frame enhanced the kinetic performance and decreased the charge transfer resistance due to the short diffusion length of Li-ion and large effective contact area between the metal frame and the LiFePO4/C particles. Recently, bendable Li secondary batteries have been studied due to the rapid development unique devices such as wearable, roll-up display and curved devices. However, bendable batteries have many limitations, including low electrochemical performances and low areal capacity (mAh cm-2). Therefore, we bendable batteries are fabricated using nickel-chromium (NiCr) alloy metal foam as the current collector of positive electrodes. As a result, we demonstrate the use of bendable lithium secondary batteries with a high areal capacity of about 3.5 mAh cm-2 at C-rate of 0.2, a high capacity retention rate in cycle life behavior, and a greatly low charge transfer resistance of around 6–8 Ω. Moreover, our bendable pouch batteries can be repeatedly bent to radii of 12.5 and 2.5 mm for several hundred times without fractures in the electrode and capacity fading. As described in this study, the three-dimensional metal alloy foam is the most promising current collector because possible to fabricate a high capacity and high power Li secondary battery compared to the foil-type current collector.