(Invited) Investigation of the Reaction & Degradation Mechanism of Iron Based Cathodes for Sodium-Ion Batteries Using X-Ray Absorption and Time-Resolved X-Ray Diffraction Techniques

Abstract

Because of the growing level of energy consumption, numerous efforts have been made to develop high performance rechargeable batteries to use for large scale applications such as electrical energy storage systems (ESS). Lithium ion batteries have been the most popular and widely used batteries in portable devices like cell phones, laptops, etc. However, it has some constraints to be used for large scale applications as the cost for the raw materials for lithium is expensive. There have been ongoing studies searching for alternative shuttle ions and sodium can be one of the possible substitutes since it is more abundant and cheaper. The development of high performance cathode materials is required for the realization of sodium-ion batteries (SIBs) as the reduction potential of sodium is lower and its ionic radius is larger compare to lithium. Because of larger ionic radius, the de/intercalation of sodium causes to large volume contraction/expansion of the host materials which could affect the rate capability and long cycle life. The structural analysis during electrochemical reaction can provide better insight to understand the material’s behavior. Furthermore, thermal analysis is one of the critical issues for the electrodes and causes to the failure of the battery system. Here, we report the synthesis, electrochemical properties and investigate the reaction mechanism of iron fluoride hydrate and iron phosphate cathodes for SIBs. The crystal structure of iron fluoride hydrate is refined and the results show pyrochlore structure (with space group of Fd3m) with hydrate contents residing in zigzag positions. Thermal stability of the as prepared iron fluoride hydrate is determined using thermogravimetric analysis coupled with temperature-dependent TR-XRD. A nanocomposite of iron fluoride hydrate encapsulated within reduced graphene oxide sheets is prepared as cathode for the high performance SIBs. The nanocomposite delivers a high discharge capacity of ~260 mAh/g at current density of 0.05 C (1C=220 mAh/g) and it retains a discharge capacity of 230 mAh/g over 100 cycles, demonstrating a good cycle stability. The ex situ TEM and XANES have employed to determine the reaction mechanism. The surface of the olivine iron phosphate is modified with a polymer (thiophene) to improve the electrochemical performance. The coating with thiophene not only increases the electrical conductivity but also prevent the degradation of the material. The coated electrode exhibits high electrochemical performance and delivers a discharge capacity of 142 mAh g-1 with capacity retention of 94% after 100 cycles. The reaction mechanism of the coated electrode has been investigated using x-ray absorption spectroscopy. Thermal analysis of the cycled electrode is conducted using temperature-dependent TR-XRD.