Encapsulation of Selenium in Graphene Micro-Ball As a Cathode Material for Lithium-Selenium Secondary Battery Applications

Rapid developing of emerging electric vehicles and large-scale renewable energy storage has prompted the urgent need for lithium secondary batteries having high energy density, power density, long cycle life, and low cost. Lithium-ion battery (LIB) has been considered as a candidate for electrified automobiles such as plug-in hybrid electric vehicle (PHEV) and electrical vehicle (EV). Although extensive efforts have been made on the development of LIBs, the highest energy storage that LIBs can deliver is quite low to meet the demands of their successful deployment in electrified automobile applications.

In current research, the lithium secondary battery using the Group 6A elements such as sulfur and selenium elements as cathode materials and lithium metal as an anode material has been considerable attention because of their ultra-high energy storage abilities. Sulfur as a cathode material for lithium-sulfur (Li-S) battery delivers a theoretical specific capacity of 1675 mAh g-1 with a theoretical specific energy of 2600 Wh kg-1. Despite considerable advantages, there are still a number of challenges in Li-S battery applications as following; 1) the intrinsic insulating nature of sulfur, which leads to electrochemically low active material utilization, 2) the formation of electrolyte soluble polysulfide generating during the charging/discharging process, which provides  low coulombic efficiency, and short cycle life of sulfur electrode materials. To address these challenges, various carbon and conductive polymer materials have been employed to accommodate sulfur in order to get over its insulating nature and inhibit the dissolution of polysulfide into organic electrolyte.

Element selenium is thought of another candidate of a cathode material for high energy rechargeable lithium battery that research on lithium-selenium (Li-Se) battery is still at a very early stage. Although selenium yield a lower theoretical gravimetric capacity (675 mAh g^-1) compared to sulfur (1675 mAh g^-1), its higher density (4.82 g cm^-3; ca. 2.5 times higher than sulfur) countervails the low gravimetric capacity and provides a high volumetric capacity (3253 mAh cm^-3), which is comparable to that of sulfur (3467 mAh cm^-3). Moreover, its electronic conductivity (σ_Se = 1×10^-3 S m^-1) is considerably higher than that of sulfur (σ_S = 5×10^-28 S m^-1), which implies that selenium could provide electrochemically higher active materials utilization and fast reaction with lithium ion. However, selenium as a cathode material has still big challenges that selenium intermediate species generated during charging/discharging process are easily dissolved in organic electrolyte and are shuttled into anode side, leading to quite poor cycle stability. In order to overcome these issues, several approaches have been reported such as impregnation of selenium into porous carbon, adsorption of polyselenide in porous metal oxide and insertion of carbon layers between separator and cathode electrode.

Herein, we reported on encapsulation of the selenium in graphene micro-ball hybrid as a cathode material for Li-Se battery applications by employing the aerosol micro-droplet drying. The aerosol micro-droplet drying is a simple and scalable continuous manufacturing process for hybrid materials in a form of graphene micro-ball filled with selenium particles. Graphene micro-ball filled with selenium acted as a confinement media of polyselenide in micro-ball hybrid during the charging/discharging process and an electron conducting path for enhanced electrochemical reaction rate, leading to a high specific capacity, a good rate capability, and a long cycling stability of Li-Se batteries. More detailed on the synthetic procedure, morphology, electrochemical and structural properties of encapsulated selenium in graphene micro-ball hybrid materials will be discussed at the meeting.