The ESI Highly Cited Papers were frequently quoted by Thomson Reuters according to the ESI (Essential Science Indicators) Basic Scientific Indicators Database. The 22 subjects were frequently quoted according to the number of citations. The total number of citations was the same as the same year. Compared with the published papers, the paper ranks in the top 1% of the world's papers. From May 2005 to May 2016, SCI included 1 054 lithium-sulfur battery papers, among which 161 papers were cited by ESI. This paper selects some of the positive electrode materials for lithium-sulfur batteries. In particular, some technologies that can improve energy density, coulombic efficiency, and cycle and rate performance are described in detail in order to guide their industrialization.
At present, the demand for high-energy energy storage systems is getting higher and higher. Lithium-sulfur batteries are widely concerned because their theoretical energy density (2 567Wh/kg) is much higher than that of commercial lithium-ion batteries. Sulfur is rich in reserves, non-toxic and inexpensive. The electron conductivity of sulfur (S) is low, and the intermediate polysulfide and the final product lithium sulfide (Li2S) affect the sulfur utilization rate and the rate performance of the battery. Due to the different densities of sulfur (2.03g/cm3) and Li2S (1.66g/cm3), the volume change occurring during charge and discharge is as high as 80%, which causes electrode stress, destructs structural stability, and ultimately leads to capacity decay. In addition, the highly soluble polysulfide in the electrolyte shuttles between the positive and negative electrodes and deposits solid Li2S2/Li2S (shuttle effect) at the positive and negative electrodes, thereby causing irreversible loss of S, resulting in low coulombic efficiency and rapid capacity decay. Poor high rate performance and increased impedance. In recent years, scholars from various countries have focused their research on the direction of sulfur-based positive electrode composites. Mainly to select a material with a large specific surface and high conductivity as a base structure, physical and chemical methods are used to load sulfur onto the substrate to improve the conductivity of the sulfur-based positive electrode and to accommodate its volume change.
I. Carbon material composite sulfur cathode material
Carbon is the most easily available large surface area and highly conductive material. It is the most studied for the composite modification of matrix and sulfur. The spatial structure form is discussed.
1. One-dimensional carbon material composite sulfur positive electrode
Zhao et al.  used tube-in-tube structure as a carrier for sulfur positive electrode, and packed multi-walled carbon nanotubes (MWNTs) into hollow porous nanometers using a step-by-step implementation procedure. In the tube. This structure enhances electron conductivity, inhibits dissolution of lithium sulfide, and provides a large amount of pore space for sulfur immersion. A positive electrode material having a sulfur content of 71% exhibited high reversible capacity, cycle performance, and rate performance. When the current density is 500 mA/g, the discharge capacity is as high as 918 mAh/g, and the capacity after 50 cycles is 625 mAh/g.
Liang et al  improved the conductivity by synthesizing tubular polypyrrole (T-PPy) fiber as the conductive matrix of the cathode material of lithium-sulfur battery. The sulfur is sublimed by a co-heating process and combined with T-PPy, so that S is uniformly distributed in a nanometer size. The S loading and its degree of immobilization have a great influence on the electrochemical performance of the composition. With 30% sulfur loading, the reversible capacity of the S/T-PPy composite can reach 650 mAh/g after 80 cycles.
Li et al  prepared a hollow carbon nanofiber-supported sulfur composite (HCNF-S) as a positive electrode material for lithium-sulfur batteries by mixing solvents in an organic solvent. Due to the action of the hollow structure and the HCNF conductive network structure, it helps to disperse S, absorb polysulfide, and inhibit the formation of a Li2S layer. It exhibits exbatteryent cycle performance with an initial discharge capacity of 1 090 mAh/g and a capacity of 600 mAh/g after cycling 100 times at 1 C rate.
Yuan et al  prepared sulfur-coated multi-walled carbon nanotubes (S-Coated-MWCTs) by capillary action between sulfur and MWCNT, which has a distinct core-shell structure, and MWCNTs are uniformly distributed as shells. On the sulfur substrate. After 60 cycles, the specific discharge capacity was 670 mAh/g, and the cycle performance was significantly improved compared with the addition of the MWCNT battery to the simple S positive electrode material.
2. Two-dimensional carbon material composite sulfur positive electrode
In order to limit the sulfur/lithium sulfur compound to the positive electrode side and reuse it efficiently during the cycle to improve cycle stability and rate performance, Zhou et al.  designed A unique sandwich structure with a pure sulfur layer on the side of the graphene film 2. This structure provides fast ion and electron diffusion channels and also accommodates the volume expansion of sulfur during lithiation. It was cycled 300 times at 1.5 A/g, the specific discharge capacity was 680 mAh/g, the capacity loss rate was only 0.1%/cycle, and the Coulomb efficiency was maintained at 97%. Compared with aluminum foil current collectors and commercial diaphragms, it can reduce the contact resistance between components, reduce battery quality, and increase energy density.
Ding et al  prepared chemically cut graphene nanosheets to fix sulfur by modified chemically activated hydrothermal reduction of graphene oxide hydrogel. Its rate performance and cycle performance are both high, and the reversible discharge specific capacity at 1C is 1 379 mAh/g.
Li et al  designed and synthesized a carbon-sulfur nanocomposite coated with reduced graphene oxide (RGO), which played a good role in sulfur fixation. The thermally exfoliated graphene sheet (TG) has a large specific surface area, a large pore volume, a high electrical conductivity, and a uniform pore distribution, and is used to form a TG-S superimposed nanocomposite with S. Then, RGO is coated with TG-S through a liquid phase process, and the preparation process is shown in FIG. The RGO-coated TG-S nanocomposite (RGO-TG-S) is more effective than TG-S in preventing the diffusion of polysulfides. It exhibits exbatteryent electrochemical performance, and after 200 cycles at 0.95 C (1.6 A/g), the reversible discharge capacity is close to 667 mAh/g, and the coulombic efficiency is as high as 96%.
Nitrogen-doped graphene (NG) as an ideal conductive matrix is ??an ideal material for high-performance lithium-sulfur batteries. Qiu et al.  prepared non-added nanocomposite cathode materials by coating nano-sulfur particles with NG sheets. (S@NG). The sulfur loading rate of this material is 60%, and the electrochemical performance is outstanding. The discharge specific capacities at 0.2C, 0.5C, 1C, 2C, and 5C are 1 167 mAh/g, 1 058 mAh/g, and 971 mAh/g, respectively. 802mAh/g, 606mAh/g. During the 2 000 cycles, the capacity loss rate was 0.028% per cycle, and the coulombic efficiency remained 97% after the cycle.
Zhang et al  prepared two graphene-S composites containing 50% (mass fraction) sulfur by hydrothermal method and thermal mixing method, and passed transmission electron microscopy and energy dispersive X-ray spectroscopy. It was found that in the hydrothermally prepared sample (NanoS@G), the sulfur nanocrystals were about 5 nm in size and uniformly distributed on the graphene sheets. However, in the graphene-S composite (S-G) produced by the hot mixing method, the sulfur content is even 50 to 200 nm. X-ray photoelectron spectroscopy reveals a strong chemical bond between sulfur nanocrystals and graphene. Compared with the S-G composite, the nano S@G composite exhibits superior electrochemical performance as a positive electrode. The NanoS@G composite has an initial discharge capacity of 1 400 mAh/g and a sulfur utilization rate of 83.7% at a current density of 335 mA/g, and the capacity remains above 720 mAh/g in 100 cycles. The strong attachment of sulfur nanocrystals to graphene limits the migration of sulfur and polysulfides, inhibiting the "shuttle effect" and thus exhibiting higher coulombic efficiency and capacity retention. Electrochemical impedance also shows that strong bonding improves the electrochemical kinetics, allowing electrons/ions to migrate quickly, resulting in good rate performance.
3. Three-dimensional carbon material composite sulfur positive electrode
Yuan et al  prepared a layered self-supporting carbon nanotube (CNT)-S paper electrode with a sulfur loading of 6.3 mg/cm2. In CNT-S, short CNTs act as short-range electronically conductive frames to accommodate sulfur, and long CNTs act as long-range conductive networks and interdigitated mechanical frames. The initial discharge capacity was 995 mAh/g, and the sulfur utilization rate was 60%. At a magnification of 0.05 C, the capacity decay rate of the first 150 cycles was only 0.2%/cycle.
Zhou et al  synthesized graphene coated mesoporous carbon/sulfur (RGO@CMK-3/S) composite by combining the advantages of CMK-3 collective and RGO coating in a simple way. This modification greatly improves cycle stability and discharge specific capacity. MK-3 has a rich mesoporous structure, which can provide buffer space for sulfur volume change and channel for lithium ion diffusion. At the same time, RGO can physically and chemically prevent the diffusion and dissolution of polysulfides. The sulfur loading of RGO@CMK-3/S is 53.14%. Due to the special structure of the material, the discharge specific capacity still reaches 734 mAh/g after 100 cycles of 0.5 C.
Xu et al  used a one-step pyrolysis of metal organic framework (MOF-5) to encapsulate sulfur into porous carbon nanosheets (HPCN). HPCN has a three-dimensional layered porous nanostructure with an average thickness of about 50 nm and a high specific surface area (1 645 m2/g) and a large pore volume (1.18 cm3/g). At 0.1C rate, the initial discharge capacity of HPCN-S reached 1 177 mAh/g; the cycle was cycled 50 times at 0.5 C rate, the discharge capacity reached 730 mAh/g, and the Coulomb efficiency remained at 97%.
Su et al  significantly reduce the charge transfer impedance by placing a conductive porous WMCNT layer between the positive electrode material and the separator, and regionalizing and inhibiting the active material Dissolution during the cycle (see Figure 3 for details). The porous structure of WMCNT paper can be used as a current collector to accommodate a readily soluble polysulfide intermediate, thereby exhibiting exbatteryent cycle performance, and the discharge specific capacity is maintained at 962 mAh/g after 50 cycles of 0.2 C.
Ding et al.  reported for the first time a novel method for synthesizing sulfur-carbon egg yolk-shell particles - sulfur is completely coated in a conductive carbon shell structure, in which the sulfur content of the filling can be well controlled And adjustment. In the egg yolk shell structure, the sulfur spherical particles occupy only a part of the inner space of the highly conductive carbon, which makes the high conductive carbon can accommodate the expansion of the volume of sulfur during the battery cycle. The three-dimensional ordered nanostructured positive electrode material based on sulfur-carbon egg yolk-shell particles has an initial discharge capacity of 560 mAh/g and exbatteryent cycle performance.
 Zhou Xiangyang, Xie Jing, Yang Juan, et al.Improving the performance of lithium–sulfur batteries by graphene coating [J].Journal of Power Sources, 2013, 243:993-1000.
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