Self-supported graphene oxide encapsulated chalcopyrite electrode for high-performance Li-ion capacitor

The combination of lithium-ion batteries and supercapacitors makes lithium-ion capacitors (LICs) the ideal energy storage device for commercial applications. However, the high performance of LICs is restricted due to various shortcomings such as low electrode specific capacity and sluggish reaction kinetics. Additionally, the undesirable ‘polysulfide-diffusion’ effect in the cost-effective metal sulfide-based electrodes results in high capacity degradation. Therefore, developing novel strategies to surmount these drawbacks is of prime importance for the development of high-performance LICs. Herein, a novel composite material with the unique architecture of graphene oxide (GO) encapsulation on the chalcopyrite (CuFeS2) microspheres is fabricated as an anode for LIC. Compared to the traditional binder-based approach for electrode fabrication, the anode is fabricated using a ‘binder-free’ approach to enable the exposure of high-density electroactive sites. As a result, the fabrication of a ‘self-standing’ flexible electrode is obtained that finds numerous applications in wearable technology systems. Detailed electrochemical analysis reveals the enhanced reaction kinetics of the composite electrode along with the improved specific capacities and rate capabilities. The unique architecture of the GO encapsulation not only prevents the structural collapse of the chalcopyrite microspheres but also suppresses the polysulfide diffusion, resulting in higher cyclic stability. The role of such carbon encapsulation is studied in detail using experimental studies and is strongly supported by theoretical calculations based on density functional theory (DFT) for the first time. The calculated binding energy and charge transfer confirmed the improved interactive features of Li ions with the composite electrode in terms of enhanced binding energy and higher valence charge transfer. Additionally, the favorable binding energy of Li2S and Li2S4 polysulfides to the composite electrode than the pristine CFS electrode reflects restricted polysulfide diffusion, resulting in outstanding cyclic stability. The assembled LIC device based on the composite electrode displays a large energy density of 192.33 Wh/kg, a high power density of 14,280 W/kg, and excellent cyclic stability of >90 % for 10,000 cycles, outperforming most of the reported metal sulfide-based LICs.