A simple graphical sketch of the Na–S cell proposed and the advantages of the 3N-NMCN cathode are shown in Fig. The robust hierarchical cathode design in this work demonstrates effectiveness in developing a stable cathode with high mass loading and stable kinetics, which can be generalized to other batteries systems. Importantly, the 3N-NMCN cathode delivers satisfactory electrochemical performance in RT Na–S batteries, in terms of specific capacity (1165.9 mA h g −1 at 167.5 mA g −1), rate capability (658.4 mA h g −1 at 16750 mA g −1), and cycling stability (almost no capacity attenuation after 2800 cycles at 8375 mA g −1). Benefiting from these structural and compositional advantages, a high-sulfur loading of 85wt.% (2.6 mg cm −2) is achieved. Fe 3N shows prominent affinity to NaPSs via Na–N and Fe–S bonding, leading to efficient NaPSs dissociation.
Both the pores within nanofibers and the interstice between layers can provide adequate space for sulfur loading. The as-prepared 3N-NMCN cathode can be directly used as a freestanding electrode without any other binders, conductive additives and current collectors, which significantly reduces the time and cost for assembling the battery. As far as we know, there has been no previous study employing nitrides in cathode hosts for Na–S batteries. The NMCN featured with a hierarchical pore structure can accommodate massive sulfur, suppress volume expansion, and stabilize the cycling process. In this research work, we propose a highly efficient Fe 3N catalyst on N-doped multilayered carbon networks (NMCN) that enables strong adsorption and fast dissociation of NaPSs for advanced Na–S batteries. Therefore, it is an urgent need to explore the rational design of hierarchical cathodes, which can synergistically inhibit the shuttle of NaPSs and enhance the electrochemical reactivity of sulfur with sodium under high-sulfur content. What’s worse, the notorious shuttle effect of NaPSs will be exacerbated and result in worse cycling stability. However, without proper cathode design, the massive load of sulfur will form a thick insulating layer, which eventually reduces the utilization rate and reaction kinetics. Being on a par with the energy density of Li–S batteries, a sulfur loading more than 70% in Na–S batteries is indispensable due to the larger atomic mass of Na. It has been acknowledged that 70% sulfur loading is critical to achieve high-energy-density Li–S batteries 16, 17, 18, 19. As Na–S batteries development is in its infancy, the attention to increase sulfur loading of Na–S cathodes is still insufficient. Metal nitrides, featured with low cost, excellent chemical stability, good conductivity, and strong adhesion to polysulfides, have been widely investigated in Li–S batteries 13, 14, 15 however, such metal nitrides have been rarely applied in RT Na–S batteries and their effectiveness and associated mechanisms in RT Na–S batteries remain unclear.Ī further issue for most reported studies is the low sulfur content which could hinder the practical application. Recently, electrocatalysts (including metals, metal oxides, metal sulfides, etc.) are proposed to accelerate the conversion kinetics of polysulfides and suppress the shuttling of polysulfides based on the strong catalytic activity 10, 11, 12. Subsequently, polar materials with intrinsic affinity to sulfur/polysulfides are introduced to anchor polysulfides chemically and thus enhance the cycling stability. However, the nonpolar feature of these carbon materials signifies insufficient mutual interaction to completely eliminate the shuttling of polysulfides. Nanocarbons 7, 8, 9 with excellent electronic conductivity and large surface areas are employed to balance the inferior electronic conductivity of sulfur and physically confine sodium polysulfides (NaPSs). In attempting to address these problems, constructing hierarchical sulfur hosts is highly desirable. These problems are typically triggered by the following intractable issues, which include the insulating nature of sulfur and its discharged products, the sluggish electrochemical reactivity of solid sulfur with sodium, the high solubility of polysulfides in the electrolyte 4, 5, 6 and the huge volume expansion during the discharge/charge process. Similar to Li–S batteries, Na–S batteries are also facing several challenges, such as low sulfur loading, unsatisfactory reversible capacity, inadequate rate capability, and rapid capacity fading 1, 2, 3. Room-temperature sodium–sulfur (RT Na-S) batteries hold significant promise for large-scale application due to the abundance, nontoxicity, low cost and high theoretical capacity of both electrodes (Na anode and S cathode) 1, 2, 3.
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