Jun Hui Jeong

Graphene–Selenium Hybrid Microballs as Cathode Materials for High-performance Lithium–Selenium Secondary Battery Applications

In this study, graphene–selenium hybrid microballs (G–SeHMs) are prepared in one step by aerosol microdroplet drying using a commercial spray dryer, which represents a simple, scalable continuous process, and the potential of the G–SeHMs thus prepared is investigated for use as cathode material in applications of lithium–selenium secondary batteries. These morphologically unique graphene microballs filled with Se particles exhibited good electrochemical properties, such as high initial specific capacity (642 mA h g−1 at 0.1 C, corresponding to Se electrochemical utilisation as high as 95.1%), good cycling stability (544 mA h g−1 after 100 cycles at 0.1 C; 84.5% retention) and high rate capability (specific capacity of 301 mA h g−1 at 5 C). These electrochemical properties are attributed to the fact that the G–SeHM structure acts as a confinement matrix for suppressing the dissolution of polyselenides in the organic electrolyte, as well as an electron conduction path for increasing the transport rate of electrons for electrochemical reactions. Notably, based on the weight of hybrid materials, electrochemical performance is considerably better than that of previously reported Se-based cathode materials, attributed to the high Se loading content (80 wt%) in hybrid materials.

Rational design of oxide/carbon composites to achieve superior rate-capability via enhanced lithium-ion transport across carbon to oxide

Coating oxides with conductive carbon is a widely used strategy to improve the rate capability of oxides by enhancing their electronic conductivity. However, there is a growing concern that a carbon layer may hinder lithium-ion transport to oxides, thus limiting the rate capability. Nonetheless, this issue has not yet been thoroughly investigated, and whether lithium-ion transport across a carbon layer does indeed limit the rate capability remains unclear. To single out the effect of lithium-ion transport across a carbon layer on the rate capability, we propose the rational design and synthesis of nano-perforated graphene (NPG)-wrapped oxide composites using commercial Li4Ti5O12 (LTO) and LiFePO4 (both with a particle diameter of ∼70 nm), wherein the NPG has nano-perforations on the basal plane of graphene. As the number of nano-perforations in the composites increases, the rate capability significantly increases. For example, NPG-wrapped LTO shows a specific capacity of 117.9 mA h g−1 at 100C and could be stably charged–discharged even at 300C. The excellent rate capability is mainly due to the enhancement of lithium-ion transport through the nano-perforations of NPG. Cyclic voltammetry and impedance analyses reveal that the improved rate capability of NPG-wrapped LTO is closely associated with an increase in the area of electrochemically active sites of LTO in the composite due to the enhanced lithium-ion transport through the nano-perforations of NPG, indicating that lithium-ion transport across a carbon layer could limit the rate capability of oxides coated with highly conductive carbon. These salient results will provide further impetus to the design and synthesis of novel high-rate carbon-coated oxides.