Heterogeneous phosphorus-doped WO3−x/nitrogen-doped carbon nanowires with high rate and long life for advanced lithium-ion capacitors

Abstract

Hybrid lithium-ion capacitors (LICs) are promising energy storage systems which could deliver higher energy density without compromising the high power and cycle life of supercapacitors. WO₃ is an appealing anode material for LICs due to its high theoretical capacity of 696 mA h g⁻¹ for Li⁺ storage. However, the practical implementation of WO₃ in LICs is limited by its low conductivity, sluggish kinetics and large volume changes during cycles. Herein, we report a “hitting three birds with one stone” strategy for the synthesis of heterogeneous phosphorus-doped WO_3−x/N-doped carbon (P-WO_3−x/NC) nanowires, which are prepared via the calcination of organic–inorganic hybrid WO₃/ethylenediamine(EDA) nanowires in Ar and subsequent thermal phosphorization. The EDA molecules are carbonized into a 1D NC matrix and WO₃ layers are converted into P-WO_3−x nanocrystals which are in situ embedded in the continuous 1D NC matrix forming a well-defined 0D-in-1D hybrid structure. The small P-WO_3−x nanocrystals offer short paths for Li⁺ diffusion and the continuous NC matrix facilitates electron transport and buffers the volume change of P-WO_3−x during discharging/charging. The oxygen vacancies and P heteroatoms with smaller electronegativity result in enhanced electrochemical activity due to the higher degree of covalent bonding and sharing electrons. The P-WO_3−x/NC nanowires are expected to act as high-performance anodes for LICs with high capacity, high rate and long life via the synergistic effect of P dopants and oxygen vacancies as well as the in situ formed conductive carbon matrix. Our results show that the P-WO_3−x/NC nanowire electrode delivers a high capacity of 490 mA h g⁻¹ at 1 A g⁻¹ and a high rate capacity of 230 mA h g⁻¹ at 3.2 A g⁻¹. Moreover, the P-WO_3−x/NC nanowires exhibit excellent cycle reversibility with 96.2% capacity retention over 2000 cycles at 1 A g⁻¹. A 4.3 V LIC prototype composed of the P-WO_3−x/NC anode with a commercial activated carbon cathode shows an ultra-high energy density of 195.58 W h kg⁻¹ at 597.2 W kg⁻¹ with 90.7% retention over 6000 cycles. These promising results provide fundamental insight into the synthesis of high-performance electrode materials for advanced electrochemical energy storage.