Heteroatom-Doped Carbon-Shelled Iron Phosphide Microcuboids as Freestanding Negative Electrodes for Long Cycling Sodium-Ion Batteries

Abstract

Energy storage devices (ESDs) have become indispensable for maximum renewable energy penetration and progress towards the elimination of fossil fuels in electricity generation. Also, high-performance ESDs are urgently needed for wearable and flexible devices, electric vehicles, and internet of things (IOT) applications. Lithium-ion batteries (LIBs) presently occupy the largest share of the EES market, however the cost-per watt stored is too high and the finite supply of Li means that they cannot offer a long-term solution to the world’s energy challenge. On the other hand, sodium-ion batteries (SIBs) are regarded as complementary ESDs and potential replacements for LIBs due to their relatively lower cost, abundance, and uniform global distribution of Na.[1] Furthermore, SIBs can operate in a wide temperature range and inert with Al, benefiting the construction of low cost and safe ESDs. Nonetheless, the performance of SIBs is limited by sluggish diffusion of Na+ in Na-insertion materials owing to large size of Na+ (1.02 Å).[2]The major challenge to developing SIBs is the lack of suitable electrode materials capable of efficiently accommodating Na+. SIBs are rocking-chair cells like LIBs, however, Na-ion storage in traditional graphite is extremely poor (< 35 mAh g-1) due to insufficient interlayer spacing.[3] Phosphorous presents the highest theoretical capacity (2596 mAh g-1) for a SIB anode, however, it suffers from poor cycling and large volume expansion (490 %) during sodiation/desodiation.[4,5] Low cost and environmentally benign iron phosphides present slightly better volume expansion and a theoretical capacity of 926 mAh g-1, but its charge storage mechanism is unclear.[6] The lack of suitable SIB anode with high capacity and long cycle life remains a major hurdle to the full-scale development of SIBs.In this work, we synthesized core-shell FeP microcuboids on freestanding carbon cloths substrate as a high capacity (< 800 mAh g-1), long-cycling anode for SIBs. The synthesized FeP microcuboids exhibit distinct physical and morphological features for efficient sodium-ion storage. It possesses a porous core within a 10 nm carbon shell, which enhances the surface area, enable fast Na+ diffusion and buffers the FeP anode against large volume variations during cycling. The integration of different heteroatoms including fluorine and nitrogen successfully improved the electronic conductivity and structural stability. Finally, using techniques such as in-situ X-ray diffraction analysis and ex-situ TEM and electroanalytical methods, we probed the conversion reaction storage mechanism and uncovered a pseudocapacitive dominated storage process. Finally, our work provides crucial insights on the rational design of high-performing metal-based phosphide anodes for alternative-ion batteries.