The practical application of lithium-sulfur (Li-S) batteries remains remote because of rapid capacity fade caused by the low conductivity of sulfur, dissolution of intermediate lithium polysulfides, severe volumetric expansion, and slow redox kinetics of polysulfide intermediates. Here, to address these obstacles, a new sulfiphilic and highly conductive honeycomb-like spherical cathode host constructed from hollow metallic and polar Co 9 S 8 tubes is designed. Co 9 S 8 can effectively bind polar polysulfides for prolonged cycle life, due to the strong chemisorptive capability for immobilizing the polysulfide species. The hollow structure, as the sulfur host, can further prevent polysulfide dissolution and offer sufficient space to accommodate the necessary volume expansion. Well-aligned tubular arrays provide a conduit for rapid conduction of electrons and Li-ions. More importantly, the experimental results and theoretical calculations show that Co 9 S 8 plays an important catalytic role in improving the electrochemical reaction kinetics. When used as cathode materials for Li-S batteries, the S@Co 9 S 8 composite cathode exhibits high capacity and an exceptional stable cycling life demonstrated by tests of 600 cycles at 1 C with a very low capacity decay rate of only ≈0.026% per cycle.

Novel KV(PO) and three-dimensional conductive network KV(PO)/C nanocomposites are successfully fabricated and further evaluated as cathode materials for potassium-ion batteries for the first time. The KV(PO)/C nanocomposite exhibits a high-potential platform of 3.6-3.9 V and a good capacity retention of at least 100 cycles. This work may provide new insight into developing cathode materials for potassium-ion batteries.

To tackle aforementioned challenges, intensive efforts have been carried out to design optimized carbon/sulfur cathodes, such as compositing sulfur with mesoporous carbon, [5] carbon nanofibers, [6] carbon nanotubes, [7] carbon spheres, [8] and graphene. [9] Although these carbon-based materials greatly improve the electric conductivity of electrode thereby providing high capacities, [10] the weak interaction between nonpolar carbon and polar polysulfides is not sufficient to limit the dissolution of LiPSs. To further boost the polysulfides adsorption capability and promote the redox reaction kinetics, various polar sulfur host, including metal, metal oxides, metal sulfides, and perovskite [11][12][13][14] are engineered as ideal candidates to not only possess good bonds between polar material and LiPSs, but also accelerate the conversion of LiPSs to solid Li 2 S 2 /Li 2 S, giving rise to a good cycling stability.Heterostructures constructed from coupling nanocrystals with different bandgaps have attracted extensive attention and widely used in photocatalysis, sensor, and energy storage. [15] Benefiting from the internal electric field at heterointerfaces, heterostructures can facilitate charge transport and enhance the surface reaction kinetics. [16] Inspired by the unique advantages of heterostructures, we propose and construct a doubleshelled NiO-NiCo 2 O 4 heterostructure@C hollow nanocages as an efficient sulfur host for advanced Li-S batteries. NiO has strong adsorption for LiPSs that can remarkably immobilize LiPSs through physical and chemical interactions at molecular level. [17] However, the low electrical conductivity makes it difficult for the immobilized LiPSs to fully involve in the electrochemical reactions, thus slowing the redox kinetics of LiPSs conversion reactions. Introducing NiCo 2 O 4 that possesses much better electrical conductivity and higher redox activity into nickel oxides can promote electron transfer for LiPSs conversion reactions. [18] The synthetic approach to the S/NiO-NiCo 2 O 4 @C composite is schematically shown in Figure 1a (for experimental details, see the Supporting Information). Uniform Ni-Co prussian blue analogue (PBA) nanocube precursor was first prepared by a facile coprecipitation strategy. Afterward, double-shelled NiO-NiCo 2 O 4 heterostructure@C nanocages were obtained through a facile calcination treatment coupled with a simple hydrothermal carbon-coating process. After a melt-diffusion process, Double-shelled NiO-NiCo 2 O 4 heterostructure@carbon hollow nanocages as efficient sulfur hosts are synthesized to overcome the barriers of lithiumsulfur (Li-S) batteries simultaneously. The double-shelled nanocages can prevent the diffusion of lithium polysulfides (LiPSs) effectively. NiO-NiCo 2 O 4 heterostructure is able to promote polysulfide conversion reactions. Furthermore, the thin carbon layer outside can improve the electrical conductivity during cycling. Besides, such unique double-shelled hollow nanocage architecture can also accommodate the volumetric eff...

The construction of single-atom catalysts (SACs) with high single atom densities, favorable electronic structures and fast mass transfer is highly desired. We have utilized metaltriazolate (MET) frameworks, a subclass of metal-organic frameworks (MOFs) with high N content, as precursors since they can enhance the density and regulate the electronic structure of single-atom sites, as well as generate abundant mesopores simultaneously. Fe single atoms dispersed in a hierarchically porous N-doped carbon matrix with high metal content (2.78 wt %) and a FeN 4 Cl 1 configuration (FeN 4 Cl 1 /NC), as well as mesopores with a pore:volume ratio of 0.92, were obtained via the pyrolysis of a Zn/Fe-bimetallic MET modified with 4,5-dichloroimidazole. FeN 4 Cl 1 /NC exhibits excellent oxygen reduction reaction (ORR) activity in both alkaline and acidic electrolytes. Density functional theory calculations confirm that Cl can optimize the adsorption free energy of Fe sites to *OH, thereby promoting the ORR process. The catalyst demonstrates great potential in zinc-air batteries. This strategy selects, designs, and adjusts MOFs as precursors for high-performance SACs.

The emerging flexible electronic devices have stimulated the development of flexible batteries, in which flexible electrodes are indispensable components. Graphene, known for its excellent electrical conductivity and mechanical stability, can be used as an ideal flexible substrate. Recently, many efforts have been devoted to graphene-based electrodes for flexible batteries. Herein, this review summarizes recent advances in the development of graphene-based electrodes for various flexible batteries, including metal-ion batteries (ions of Li, Na, Zn, Al, etc.), lithiumsulfur batteries, and metal-air batteries (Li-and Zn-air batteries). Besides, major challenges and future developments of flexible batteries are also discussed. K E Y W O R D S flexible batteries, flexible electrodes, graphene