The requirement of energy-storage equipment needs to develop the lithium ion battery (LIB) with high electrochemical performance. The surface modification of commercial LiFePO4 (LFP) by utilizing zeolitic imidazolate frameworks-8 (ZIF-8) offers new possibilities for commercial LFP with high electrochemical performances. In this work, the carbonized ZIF-8 (CZIF-8) was coated on the surface of LFP particles by the in situ growth and carbonization of ZIF-8. Transmission electron microscopy indicates that there is an approximate 10 nm coating layer with metal zinc and graphite-like carbon on the surface of LFP/CZIF-8 sample. The N2 adsorption and desorption isotherm suggests that the coating layer has uniform and simple connecting mesopores. As cathode material, LFP/CZIF-8 cathode-active material delivers a discharge specific capacity of 159.3 mAh g−1 at 0.1C and a discharge specific energy of 141.7 mWh g−1 after 200 cycles at 5.0C (the retention rate is approximate 99%). These results are attributed to the synergy improvement of the conductivity, the lithium ion diffusion coefficient, and the degree of freedom for volume change of LFP/CZIF-8 cathode. This work will contribute to the improvement of the cathode materials of commercial LIB. Electronic supplementary materialThe online version of this article (doi:10.1007/s40820-017-0154-4) contains supplementary material, which is available to authorized users.

Sulfur reactivity in lithium−sulfur batteries highly depends on its distribution and morphology during cycling, which is of great significance to suppress the shuttle effect and promote conversion reaction. Herein, cobalt phosphide nanoflakes are prepared and used as a sulfur host. An improved redox kinetics from sulfur to lithium sulfide and the corresponding fast lithium-ion diffusion are observed to greatly promote the electrochemical performance of lithium−sulfur batteries. Meanwhile, for the first time, we propose "effective triple phase contact" and "insulated dead sulfur" to account for cycling performance differences of CoP@S and rGO@S batteries. The flower-like sulfur induced by CoP nanoflakes during cycling provides extra lithium-ion diffusion and electron transfer ways compared with agglomerated sulfur in the rGO@S cathode. The CoP@S battery shows good rate performance and delivers 520 mA h g −1 after 1000 cycles with an excellent Coulombic efficiency of 99%. In contrast, no conversion reaction happens after 600 cycles in the rGO@S battery, implying no existence of reactive sulfur. This research reveals the effect of morphological evolution of sulfur on the cycling performance and affords an insight for developing high-performance lithium−sulfur batteries.

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Numerous efforts have been devoted to addressing the abovementioned challenging issues and great progresses have been achieved during the past decades. [8][9][10][11] Nanostructured carbon-based materials with high electrical conductivities, large surface areas, and hierarchical pores are first applied as sulfur hosts to promote the performance of Li-S batteries. [12,13] Unfortunately, most carbon/sulfur electrodes show poor cycling performance and low Coulombic efficiency, especially under high areal sulfur loading condition. Up to now, more studies suggest that the weak interaction between carbon-based materials and lithium polysulfide results in the degradation of Li-S batteries. [14][15][16] Therefore, effective trapping of lithium polysulfide in cathodes has been considered to be a superior way to achieve high-performance Li-S batteries. Carbon-based hollow spheres, core-shell structure and heteroatom doping, which suppress polysulfide dissolution through physical and chemical strategies, are applied. [17][18][19] Recently, there is an increasing interest in polar transition metal composites which show stronger adsorption to lithium polysulfide than carbon-based materials. Numerous transition metal composites including metal oxide, [20,21] metal carbide, [22] metal sulfide, [23] metal nitride, [24] and metal phosphide, [25] all suffer from inferior electronic conductivity compared with carbon-based materials, and limited specific surface area for the adsorption and conversion reaction of sulfur species. In fact, in most cases, chemical confinement of lithium polysulfide is rather limited for promoting Li-S batteries performance because the trapped lithium polysulfide is hard to be effectively converted due to the sluggish redox kinetics. [26] Meanwhile, the abovementioned solutions can achieve good results under low areal sulfur loading condition, but are powerless with high areal sulfur loading. Therefore, the failure mechanism of Li-S batteries is still controversial and the new and effective solution is urgent to be find out.Recently, the conceptions of "dead sulfur," "dead sulfur layer," "inactive sulfur" have been proposed and show significant importance in Li-S batteries studies. [27][28][29][30] They all point out that the agglomeration and deposition of inactive sulfur relatedspecies during the discharging-charging cycles are main performance degradation factors for Li-S batteries. Our research also found out that the "dead sulfur layer" can even cause the complete failure of Li-S batteries. [28] Therefore, exploring the reasons An in-depth understanding of Li-S battery failure mechanisms is of significance for providing design guidance of promoting this class of batteries' electrochemical performance. During discharge, deposition of solid sulfur species on substrates is observed, leading to large contact resistance and sluggish redox kinetics. Then, the cumulative effect leads to the formation of isolated inactive sulfur species on low-dimensional substrates (0D, 1D, and 2D), which has been confirme...

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Lithium sulfur batteries hold great promise as the new generation post‐lithium‐ion batteries in virtue of high theoretical energy density (2600 W h/kg) and low cost of sulfur. However, the practical application of the batteries is being hindered by their poor cycling stability and low Coulombic efficiency, which is mainly attributed by the dissolution of polysulfides and low electronic conductivity of sulfur. Herein, a novel cathode host was designed by mixing the nanoflower‐like MnO2 with KB powder loaded on the commercial carbon paper, delivering excellent electrochemical performance even at a high sulfur mass loading of 2.1 mg/cm2. The nanoflower‐like MnO2 obtained from the typical hydrothermal method provides abundant active sites for adsorbing polysulfides chemically. The combination between this chemical capture effect and physical constraint introduced by the pores of nanoflower‐like MnO2 and KB powder suppresses the dissolution of lithium polysulfides out of cathode region. Consequently, the modified MnO2‐KB@carbon paper‐Li2S6 cathode exhibits an initial discharge capacity of 776.4 mAh/g at 0.5 C and retain 814.3 mAh/g after 100 cycles with nearly 100% Coulombic efficiency. Remarkably, self‐discharge rate has been restrained to a very level of 4.7% after 48 h resting, indicating the potential of the nanoflower‐like MnO2‐KB@carbon paper‐Li2S6 cathode for improved preserving ability of lithium sulfur battery.

Recently, transition-metal compounds (TMCs) with unique adsorptive and catalytic properties have shown great promise in lithium–sulfur (Li–S) batteries to inhibit the shuttle effect. However, current studies mostly focus on the morphology control of one specific TMC, while the relationship between the composition and performance is insufficiently revealed. Nevertheless, the polarity and catalytic activity are largely dependent on the components of TMCs, especially the anion species. Herein, we take Co–X (X = O, P, and S) compounds as example compounds and systematically investigate the compositional effects of Co–X compounds on their inhibition abilities for the shuttle effect. To conduct the investigation, CoS2, CoP, and Co3O4 flowers with identical morphologies and nanostructures were successfully grown on 3D conductive self-supporting carbon nanofibers (CNFs) via a facile electrospun method combined with post-heat treatment. When tested in Li–S batteries, the CoS2/CNF interlayer outperforms its phosphide and oxide counterparts, displaying the strongest adsorption ability and the highest catalytic activity toward polysulfides. Impressively, Li–S batteries coupled with CoS2/CNF interlayers exhibit outstanding electrochemical performance with a high specific capacity of 1115.2 mA h g–1 and enhanced cycling stability of 884.4 mA h g–1 after 200 cycles. We believe such an anion design strategy could open a new avenue for constructing high-performance Li–S batteries.