![]() ![]() Herein we have successfully explored a spatially confined sulfuration strategy for the preparation of uniform yolk-shell spheres. To the best of our knowledge, however, no yolk-shell structure has been studied as yet due to the challenges of its synthesis. More importantly, the suitable void space is able to buffer the large volume variations of FeS during sodiation/desodiation processes, which could maintain the original nanoparticle morphology of the and gain prolonged cycling stability 33. Second, the nanosized FeS cores can offer large electrode/electrolyte contact areas and short diffusion paths for electron and ions, which are favourable to improve the sodium reaction rate, alleviating structural degradation and shortening Na + diffusion paths. First, the porous carbon shells could enhance conductivity of the active materials, leading to a high reversible capacity. To solve these problems, we designed yolk-shell nanospheres as cathode materials. It is obvious that achieving high capacity is essential and imperative for practical application of the Na/FeS battery. The Na/FeS battery, however, shows inferior electrochemical properties due to the detrimental challenges of low conductivity, sluggish kinetics and severe volume changes in the FeS cathode during sodiation/desodiation processes 25. Among all the reported metal sulfides, FeS has risen to prominence, owing to its high theoretical capacity ( ∼610 mAh g −1), high voltage plateau, cost effectiveness, environmental benignity and abundance in nature. On the other hand, the emerging sodium–metal sulfide battery has drawn extensive attention owing to its high energy and power densities the typical cathodes includes iron sulfide (FeS) (ref. Thus, innovation leading to sodium-based technologies with high energy and power densities is urgently needed. Even though essential progress has been achieved on the ambient Na/S battery 23, 24, the best result can merely reach the energy density of 191 Wh kg −1 over 200 cycles. Compared with the Li/S battery, however, operation of the Na/S battery at ambient temperature faces a greater critical challenge, because the shuttle effect of sodium polysulfides is much exacerbated, leading to low efficiency and rapid capacity decay on cycling 22. Among these sodium-based energy storage systems, the RT-Na/S battery is predicted to deliver high energy density (theoretical value: 760 Wh kg −1). ![]() For Na-O 2 batteries, the exploration is only in its initial stage 20, 21. For NIBs, there is still a long way to go to achieve a sodium-ion full cell system with satisfying energy density and cycling life, as the current research on NIBs has been mainly focused on the search for suitable cathodes 6, 7, 8, 9, 10, 11, 12, 13, 14 and anodes 15, 16, 17, 18, 19. Therefore, research on sodium-based technologies, including Na-ion batteries (NIBs), room-temperature sodium-sulfur batteries (RT-Na/S), and novel Na-O 2 batteries, has gained momentum due to the overwhelming advantages with regards to the low cost and abundance of sodium resources 3, 4, 5. Nevertheless, it should be pointed out that concerns about LIBs have arisen both in terms of the high cost and the limitations of lithium resources 1, 2. ![]() Lithium-ion batteries (LIBs) have successfully been applied in EV and plug-in hybrid EV trials. Electric vehicles (EVs) and plug-in hybrid EVs are emerging, to reduce our energy dependence on fossil fuels for transportation systems in the future. Owing to the increased demand for energy and the need to reduce carbon emissions, energy storage innovation has been a constant global concern over the past decade. Furthermore, this spatially confined sulfuration strategy offers a general method for other yolk-shell metal sulfide–carbon composites. This sustainable sodium–iron sulfide battery is a promising candidate for stationary energy storage. Nanostructural design, including of nanosized iron sulfide yolks ( ∼170 nm) with porous carbon shells ( ∼30 nm) and extra void space ( ∼20 nm) in between, has been used to achieve excellent cycling performance without sacrificing capacity. The proven conversion reaction between sodium and iron sulfide results in high capacity but severe volume changes. Here, uniform yolk-shell iron sulfide–carbon nanospheres have been synthesized as cathode materials for the emerging sodium sulfide battery to achieve remarkable capacity of ∼545 mA h g −1 over 100 cycles at 0.2 C (100 mA g −1), delivering ultrahigh energy density of ∼438 Wh kg −1. Nevertheless, achieving high capacity and cycling stability remains a great challenge. Sodium–metal sulfide battery holds great promise for sustainable and cost-effective applications. ![]()
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