Effects of Various Graphite Cathodes on Self-Discharge Behavior of Rechargeable Aluminum Batteries

Abstract

Renewable energy technologies, such as wind power and solar energy, have become great importance in modern society [1]. Unfortunately, the intermittent nature of these energies leads to a huge barrier in energy generation and stabilization in electricity supply [2]. The rechargeable batteries received a lot of attention due to high energy density and satisfactory cycle life. Rechargeable aluminum batteries (RABs) gain a great amount of research interest in recent times due to the low cost of aluminum compared to that of lithium. Aluminum, being the most abundant metal in the earth crust, has three-electron transfer characteristic, high volumetric capacity, and great safety, which make RABs suitable for large-scale energy storage [3]. However, problems like low discharge voltage, high corrosion of electrolyte, serious self-discharge impede the growth and development of RABs. Jiao’s group demonstrated self-discharge phenomena for Ah-level RABs, where self-discharge rate was about 6%/day [4]. Their research only presented the self-discharge phenomena, but not studied the factors affecting self-discharge behavior. Self-discharge is an important issue for practical applications. The self-discharge rate of RABs is much higher than that of commercial lithium-ion batteries, whose self-discharge rate is approximately 5%/month. It is thus necessary to investigate the factors affecting self-discharge properties of RABs. In this work, effects of various types of graphite cathodes on charge-discharge properties and self-discharge behavior in ionic liquid electrolyte are studied. The results demonstrate that the changes in d-spacing and morphology of the graphite are crucial. The phase transformation of the graphite cathode during charging/discharging and self-discharge is examined using in-situ X-ray diffraction (performed at National Synchrotron Radiation Research Centre, Taiwan). The reaction mechanism is discussed in details.