Nanoporous biocarbon functionalised with crystalline FeS nanostructures as a high-performance anode for lithium-ion batteries and insights into its Li storage mechanism

Abstract

Low-cost hard carbon with nanoporosity derived from biomass-based precursors is an intriguing material for use as an LIB anode. Herein, iron sulfide functionalised porous carbon (PC) hybrids were synthesized through an ex situ solid-state mixing approach by utilising high surface area almond skin-derived PC and iron acetate and dithiooxamide as iron and sulphur sources, respectively. The amount of dithiooxamide was varied to yield FeS with different crystallite sizes. The optimized material FeS-PC-2 showed a high surface area (1941 m² g⁻¹), a large pore volume (1.15 cm³ g⁻¹), and a pore diameter in both micro (0.96 nm) and mesopore regions (2–3.5 nm). The average crystallite size of FeS was 20.95 nm, and NEXAFS studies revealed Fe and S bonding, which led to the formation of FeS within the macro channels created by the hierarchical porous structure. Owing to these characteristics, at a current density of 0.1 A g⁻¹, the FeS-PC-2 material delivered an initial high discharge capacity of 1784 mA h g⁻¹, which is maintained up to 1467 mA h g⁻¹ even after 100 cycles. First principles DFT level simulations implemented with van-der Waals corrections were performed to understand the enhanced specific capacity of materials. FeS is polarized, and the negatively charged S ions play a key role in accommodating a large number of Li-ions. The higher S to Fe concentration also plays a significant role in generating high Li storage. With 0.8% of FeS impurity in the PC, the predicted specific capacity is ∼702.3 mA h g⁻¹, which is 1.4 times higher than that of the bare PC. The current synthesis is low-cost, less time-consuming, and produces FeS-incorporated nanoporous carbons with high potential for LIB anodes.