of 2600 W h kg −1 . [2] However, the development of Li-S battery is plagued by several challenges that must be addressed. First of all, sulfur is both electrically and ionically insulating, as well as its lithiated product Li 2 S, which necessitates the incorporation of sulfur into a conductive matrix. [3] In addition, quite different from traditional Li intercalation compounds, sulfur suffers electrochemical dissolution and deposition reactions and generates a series of polysulfide (PS n ) species, of which highorder PS n (4 ≤ n ≤ 8) are soluble in etherbased electrolyte and prone to diffuse to the anode side for reducing deeply to the insoluble Li 2 S 2 and Li 2 S. Correspondingly, Li 2 S 2 and Li 2 S could migrate back to the cathode and be oxidized. [4] The so-called "shuttle effect" leads to an irreversible loss of active sulfur and a fast degradation of cycle stability. Consequently, during the redox reaction, the repetitive dissolution and deposition reactions of the PS n passivate both cathode and anode gradually, resulting in a considerable increase of the electrode impedance. [5] Moreover, the density difference between sulfur (2.07 g cm −3 ) and Li 2 S (1.66 g cm −3 ) leads to a significant volume expansion, which is adverse to the mechanical strength of sulfur cathode. [2c,6] All of these factors restrict severely the electrochemical performance of sulfur cathode.To date, upsurge of attention has been paid in fabricating high-efficiency and stable sulfur cathode, the vital component of the Li-S battery. These efforts focus on developing novel nanocomposites by incorporating sulfur into various host materials, such as carbonaceous materials (including porous carbon, [7] hollow carbon spheres, [8] carbon nanotube/fibers, [9] graphene and its derivatives, [10] or hybrid carbon hosts, [11] ) conducting polymers, [12] metal oxides, [13] and metal or covalent organic frameworks. [14] These host materials are expected to promote the electron transfer, accommodate the volumetric expansion, and trap the soluble PS n . In this respect, carbonaceous materials are proven to be a promising option owing to their excellent electrical conductivity, outstanding mechanical strength, and multiple architectures. Typically, the members with higherdimensional contrast structure are endowed with exclusive superiority in compositing with sulfur. In detail, carbon nanotubes (CNTs) possess classic 1D structure and exhibit a self-weaving Carbon materials have attracted extensive attention as the host materials of sulfur for lithium-sulfur battery, especially those with 3D architectural structure. Here, a novel 3D graphene nanosheet-carbon nanotube (GN-CNT) matrix is obtained through a simple one-pot pyrolysis process. The length and density of CNTs can be readily tuned by altering the additive amount of carbon source (urea). Specifically, CNTs are in situ introduced onto the surface of the graphene nanosheets (GN) and show a stable covalent interaction with GN. Besides, in the GN-CNT matrix, cobalt nanoparticles with...

A lithium anode protected with a porous Al2O3layer is beneficial for improving the cycle stability and capacity retention of a lithium–sulfur battery.

For high-energy lithium-sulfur batteries, the poor volumetric energy density is a bottleneck as compared with lithium-ion batteries, due to the low density of both the sulfur active material and sulfur host. Herein, in order to enhance the volumetric energy density of sulfur cathode, a universal approach is proposed to fabricate a compact sulfur cathode with dense materials as sulfur host, instead of the old-fashioned lightweight carbon nanomaterials. Based on this strategy, heavy lanthanum strontium manganese oxide (La 0.8 Sr 0.2 MnO 3), with a high theoretical density of up to 6.5 g cm −3 , is introduced as sulfur host. Meanwhile, the La 0.8 Sr 0.2 MnO 3 host also acts as an efficient electrocatalyst to accelerate the diffusion, adsorption, and redox dynamics of lithium polysulfides in the charge-discharge processes. As a result, such S/La 0.8 Sr 0.2 MnO 3 cathode presents high gravimetric/volumetric capacity and outstanding cycling stability. Moreover, an ultra-high volumetric energy density of 2727 Wh L −1-cathode is achieved based on the densification effect with higher density (1.69 g cm −3), which is competitive to the Ni-rich oxide cathode (1800-2160 Wh L −1) of lithium-ion batteries. The current study opens up a path for constructing high volumetric capacity sulfur cathode with heavy and catalytic host toward practical applications of lithium-sulfur batteries. Achieving higher energy density is the continuous driving force for the development of secondary batteries. Among all the commercial secondary batteries, lithium-ion batteries (LIBs) possess high gravimetric and volumetric energy densities, almost approaching their limitation of energy densities based on the inherent intercalation chemistry. [1] Beyond LIBs, lithium-sulfur (Li-S) battery has attracted considerable attention due to the high theoretical gravimetric and volumetric energy densities of 2600 Wh kg −1 and 2800 Wh L −1 , respectively.

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