Both the energy density and cycle stability are still challenges for lithium–sulfur (Li–S) batteries in future practical applications. Usually, light‐weight and nonpolar carbon materials are used as the hosts of sulfur, however they struggle on the cycle stability and undermine the volumetric energy density of Li–S batteries. Here, heavy NiCo2O4 nanofibers as carbon‐free sulfur immobilizers are introduced to fabricate sulfur‐based composites. NiCo2O4 can accelerate the catalytic conversion kinetics of soluble intermediate polysulfides by strong chemical interaction, leading to a good cycle stability of sulfur cathodes. Specifically, the S/NiCo2O4 composite presents a high gravimetric capacity of 1125 mAh g−1 at 0.1 C rate with the composite as active material, and a low fading rate of 0.039% per cycle over 1500 cycles at 1 C rate. In particular, the S/NiCo2O4 composite with the high tap density of 1.66 g cm−3 delivers large volumetric capacity of 1867 mAh cm−3, almost twice that of the conventional S/carbon composites.

ising candidates for the next generation of high energy storage system. Since proposed in the 1960s, [3] Li-S battery experienced an infancy stage in 1970-1990s, when researchers devoted to the fundamental redox reactions of sulfur in various electrolytes, [4] and a flourishing period after 2000 when high performance was achieved through sulfur/carbon (S/C) cathode and sulfurized-polyacrylonitrile (SPAN) cathode in ether-and carbonatebased electrolytes, respectively. [5] After 2009, great efforts have been made to further enahnce Li-S battery, including fabricating conductive cathode, [6] incorporating electrocatalyst, [7] modifying separator, [8] optimizing electrolyte, [9] and protecting lithium anodes. [10] The rational design of electrode structure with various carbon materials (1D, 2D, and 3D) greatly boosts the electrochemical performance of sulfur cathode. [11] Although the cycle stability is still struggling with 100 cycles, the gravimetric energy density (W G) of Li-S pouch cells has improved remarkably to promote the applications in which weight matters more than longevity. For example, Sion Power, a pioneer corporation in Li-S battery technology, has developed several prototypical Li-S cells with energy density of 350 Wh kg −1 /325 Wh L −1 for powering Airbus's Zephyr 7 drone for an 11-day nonstop flight in 2014. [12] Oxis Energy, another manufacture of Li-S battery, announced a new target of 500 Wh kg −1 in the near future after achieving 400 Wh kg −1 / 300 Wh L −1 for e-Buses, trucks, and marine applications. [13] Research institutions from China have also reported pouch Li-S cells with the energy density up to 400-600 Wh kg −1 for the potential application in unmanned aerial vehicle. [14] It is remarkable that the W G of Li-S battery has exceeded that of the best Li-ion batteries (250-300 Wh kg −1) with Ni-rich oxide cathode from Contemporary Amperex Technology Co., Ltd. (CATL), a giant manufacture of Li-ion batteries (Figure 1). With such great advantages, Li-S battery is possible to compete with commercial Li-ion batteries in specific field where high W G is the primary concern. Despite the attractive high W G , Li-S battery pales in comparison with Li-ion batteries in terms of volumetric energy density (W V). [18] Figure 1 compares W V and W G between Li-S and Li-ion batteries. With Ni-rich metal oxide as cathode, Li-ion batteries have already reached 700 Wh L −1 and can even exceed 1000 Wh L −1 for W V when coupling with high capacity Lithium-sulfur (Li-S) batteries hold the promise of the next generation energy storage system beyond state-of-the-art lithium-ion batteries. Despite the attractive gravimetric energy density (W G), the volumetric energy density (W V) still remains a great challenge for the practical application, based on the primary requirement of Small and Light for Li-S batteries. This review highlights the importance of cathode density, sulfur content, electroactivity in achieving high energy densities. In the first part, key factors are analyzed in a model on negative/positive ...

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.

Effective hosts of sulfur are essential for the application of lithium–sulfur batteries. However, various refined nanomaterials or carbon-based hosts possess low density, high surface area, and large porosity, leading to undesirable reduction on both gravimetric and volumetric energy densities. Herein, spherical metal oxides with high tap density are introduced as carbon-free hosts of sulfur for the first time. The ternary oxides show a superior synergistic effect of adsorption and electrocatalytic conversion of soluble intermediate polysulfides. Besides, oxide microspheres can build stable conductive frameworks and open channels in porous electrodes for fast transport of electrons and active diffusion of electrolyte. Such a synergistic effect and unique structural feature of porous electrodes are favorable for achieving good utilization and stable cycle performance of the sulfur cathode. Typically, the S/LiNi0.8Co0.1Mn0.1O2 composite exhibits good cycle stability with a low capacity decay rate (0.057% per cycle) during 500 cycles at 0.1 C. Importantly, due to the high tap density (1.81 g cm–3), the S/LiNi0.8Co0.1Mn0.1O2 composite delivers a larger volumetric capacity (1601.9 mAh cm–3 –composite), almost 2.3 times of S/carbon composite (689.4 mAh cm–3 –composite). Therefore, this work provides a feasible strategy to reach long life and high volumetric capacity of cathode based on metal oxides as sulfur hosts.

The lithiophilic gel polymer electrolyte is demonstrated to be feasible to stabilize the lithium anode in a quasi-solid-state Li–S battery.