Facile synthesis of hollow Zn-Co-S and Zn-Co-P nanostructures supported on Ti3C2Tx MXene nanosheets for high-performance lithium-ion capacitors

Abstract

Lithium-ion capacitors (LICs) represent a novel energy storage solution, merging the high energy storage capacity of lithium-ion batteries with the adequate power density of supercapacitors. However, the electrochemical performance of lithium-ion capacitors is notably constrained by the inherent imbalance between the charge storage mechanisms and electrochemical reaction dynamics of their anodes and cathodes. In this study, a dual-shell hollow structured ZnCo2S4 was prepared via a two-step hydrothermal method and compared with spherical ZnCo2P4 prepared by calcination. Subsequently, they were respectively anchored onto two-dimensional Ti3C2Tx nanosheets, which possess abundant surface functional groups, through electrostatic adsorption, forming layered semi-encapsulated framework structures. The introduction of MXene significantly enhances the conductivity of the material, restricts the expansion of ZnCo2S4 and ZnCo2P4 during the Li+ insertion/extraction process, and inhibits their aggregation. Under a three-electrode configuration, ZnCo2S4/Ti3C2Tx exhibits excellent rate performance (1004 C·g−1 at 10 A·g−1) and cycling stability (95 % capacity retention after 5000 cycles at 2 A·g−1). The discharge capacity of the ZnCo2S4/Ti3C2Tx LIB is up to 1360 mAh·g−1 at 0.1 A·g−1, and the discharge capacity of the ZnCo2P4/Ti3C2Tx LIB is 1121 mAh·g−1 at 0.1 A·g−1. As an anode material for LIC, ZnCo2S4/Ti3C2Tx demonstrates excellent specific capacitance and rate performance (105 F·g−1 at 0.5 A·g−1). At a power density of 337.10 W·kg−1, ZnCo2S4/Ti3C2Tx//AC LIC exhibits a significant energy density of 136.25 Wh·kg−1. At a current density of 2 A·g−1, ZnCo2S4/Ti3C2Tx//AC LIC retains 89 % of its capacity after 5000 charge-discharge cycles, demonstrating excellent stability for practical applications. ZnCo2P4/Ti3C2Tx//AC LIC exhibits an energy density of 45.20 Wh·kg−1 at a power density of 623.07 W·kg−1, and after 5000 cycles at 2 A·g−1, its capacity retention rate reaches 80 %. These findings suggest that ZnCo2S4/Ti3C2Tx and ZnCo2P4/Ti3C2Tx composites are promising anode materials for LICs.