With the rapid development of the electric vehicle industry and the field of new energy, and people's awareness of environmental protection, new electric vehicles have received widespread attention from the society. Traditional lithium-ion batteries are restricted by factors such as the theoretical specific capacity of the positive electrode material, and the energy density has reached the theoretical limit. In order to meet people's needs for electric vehicle driving range and battery energy density, researchers have turned their research directions to secondary battery systems other than lithium-ion batteries. Lithium-sulfur secondary battery is a new type of energy storage system with sulfur or sulfur-based composite material as the positive electrode and lithium as the negative electrode. The battery has a wide range of application prospects. But at the same time lithium-sulfur secondary batteries also have a series of problems: ①The conductivity of sulfur at room temperature is poor (the conductivity is 5×10-30S/cm), and a conductive agent needs to be added to the positive electrode material, but because the conductive agent does not participate in redox The reaction causes the specific capacity of the battery to decrease; ②During the charging and discharging process of the battery, the volume of the electrode continuously changes, the negative electrode shrinks and the positive electrode expands. The volume expansion of up to 79% will affect the physical structure of the sulfur electrode to a certain extent. As the cycle continues, the electrode is prone to pulverization, which affects the working cycle of charging and discharging; ③The intermediate product lithium polysulfide (Li2Sn, 1≤n≤8) generated in the reaction has poor conductivity and adheres to the electrode surface to affect oxidation The in-depth reduction reaction makes the cycle stability of the battery worse; ④The soluble high oxidation state long-chain lithium polysulfide generated during the charge and discharge process dissolves into the electrolyte, migrates and diffuses across the diaphragm to the negative electrode along the concentration gradient, and reacts with the negative electrode. , The reaction product short-chain lithium polysulfide and Li2S and Li2S2 that are insoluble in the electrolyte re-diffuse back to the positive electrode due to the concentration gradient, and are oxidized to the long-chain lithium polysulfide. The phenomenon of lithium polysulfide migration between the positive and negative electrodes of the battery is called the shuttle effect, which causes the consumption of the positive electrode active material, reduces the utilization rate of sulfur, and leads to the corrosion and passivation of the negative electrode, which affects the coulombic efficiency of the battery.

Based on the above reasons that affect the performance of lithium-sulfur batteries, current research hotspots are mainly in the design and modification of cathode materials, innovation in preparation processes, application of binders, improvement of electrolyte systems, and protection of lithium anodes.

1. Cathode material

1. Sulfur/carbon composite material based on conductive carbon
Carbon materials are ideal materials to improve electrical conductivity and increase the utilization of active materials. This is because carbon materials have high electrical conductivity, large surface area, abundant pores and narrow pore size distribution, as well as strong adsorption capacity with sulfur.

(1) Sulfur/carbon nanotube (S/CNT) composite material

Carbon nanotubes have good electrical conductivity, and their porous hollow structure can support a large amount of sulfur. The combination of sulfur and carbon nanotubes can significantly improve the performance of the electrode. Chen Junzheng [1] synthesized sulfur/multi-walled carbon nanotube (S/MWCNT) electrode materials with different tube diameters and sulfur contents using the segmented heating method, and screened out MWCNTs with a diameter of 10-20nm as the core through the comparison of comprehensive performance. 85% sulfur is a composite material under the optimal conditions of the shell.

Yuan uses the capillary application of MWCNT to make the elemental sulfur uniformly coated on the carbon nanotubes. The reversible discharge specific capacity of the prepared lithium-sulfur battery after 60 cycles is maintained at 670mAh/g. Geng's research group [3] used the direct precipitation method to prepare the S/MWCNT material, and the initial discharge specific capacity of the battery reached 1128mAh/g at a rate of 0.05C.

(2) Sulfur/Mesoporous Carbon Composite Material

Mesoporous carbon (MC) materials can effectively increase the utilization rate of active materials and improve the performance of electrode materials by virtue of their excellent electrical conductivity, large specific surface area and pore volume. This is because the micropores and mesopores in the material are conducive to the transmission of electrons and ions, and can effectively adsorb elemental sulfur and redox reaction products, reducing the shuttle effect; the macropores in MC can increase the sulfur loading and facilitate electrolysis The full infiltration of the liquid also provides a containing space for the reaction product, which reduces the volume expansion and contraction damage.

In 2011, Nazar produced a carbon material with a double-layer pore structure. The specific surface area is as high as 2300m2/g, and the pore size is 2nm and 5.6nm, respectively. It is used as the carrier of elemental sulfur and the sulfur content can reach 50%. The specific discharge capacity in the first week at 1C is 995 mAh/g, and the specific discharge capacity after 100 cycles is maintained at 550 mAh/g, and the cycle performance is good. Subsequently, the research group further prepared ordered mesoporous carbon (CMK-3) with a pore volume of 2.1 cm3/g, and a composite material with a sulfur content of 70% was prepared by heat treatment. The performance was stable and the coulombic efficiency was close to 100%.

Many researchers have used the template method to prepare a variety of porous carbons with superior properties. Zhang Jing and Tang Qiong used polyvinyl alcohol and sucrose as carbon sources, and used nano-calcium carbonate with the hard template method to prepare layered mesoporous carbon. They studied the use of mesoporous carbon, conductive graphite and carbon nanotubes as conductive matrix. The electrochemical performance of lithium-sulfur battery, and the effects of specific surface area and pore volume on the performance of lithium-sulfur battery are analyzed in detail. The results show that the battery with S/MC composite material as the positive electrode has a first discharge specific capacity of 1389mAh/g at a discharge rate of 0.1C. After 100 cycles, the coulombic efficiency remains above 95%.

Strubel's research group used ZnO as a template to prepare porous carbon for use in lithium-sulfur batteries. Under the premise of sulfur content ≥3mg/cm2, a specific discharge capacity of >1200mAh/g was obtained. It can be seen that a lithium-sulfur battery made of a cathode material composed of mesoporous carbon and sulfur has significantly improved discharge specific capacity and battery cycle performance.

(3) Sulfur/carbon ball composite material

Compared with porous carbon materials, the density of carbon balls is higher, which helps to increase the volume specific energy of the sulfur cathode. The Archer team reported that the porous hollow carbon sphere material with a diameter of about 200nm is filled with elemental sulfur into the inner cavity of the carbon sphere, and the surface of the carbon sphere is covered with about 3nm micropores, and the sulfur loading can reach 70%. The discharge specific capacity after 100 cycles at 0.5C rate is as high as 974mAh/g. Gao et al. prepared porous carbon balls with uniform distribution by a simple method and used them to support sulfur element. Sucrose and sulfuric acid are mixed into a dilute solution, and then carbonized after heat treatment to obtain 200-300nm porous carbon balls, which are then fully compounded with sulfur element in the molten state and vapor state to obtain a carbon/sulfur composite material with a sulfur content of 42% . Relevant electrochemical results show that: at a lower discharge rate (200mA/g), the specific capacity of the electrode is 890mAh/g; at a higher discharge rate (1200mA/g), the specific capacity of the electrode is 730mAh/g, and the cycle stability very good. This may be due to the 0.7nm pore size inside the carbon ball, which makes it have a strong adsorption effect on sulfur.

(4) Sulfur/graphene cathode material
Graphene is composed of sp2 hybrid orbital carbon atoms, with special physical properties, excellent electrical conductivity and ultra-high theoretical specific surface area. In recent years, it has been widely used in energy systems such as battery materials and supercapacitors. Cui et al. used a chemical deposition method to wrap sulfur particles coated with a layer of polyethylene glycol (PEG) chains in graphene. The composite material contains 70% sulfur and the current density is 750mA/g. The specific capacity of the battery after 100 cycles can still be maintained above 600mAh/g. The graphene/sulfur composite material synthesized by Yuan et al. has increased the sulfur content to 80%, and is circulated at a current density of 210mA/g, and the coulombic efficiency is close to 100%. Tang reported that using calcium oxide (CaO) as a template to prepare graphene for use in lithium-sulfur batteries, a discharge specific capacity of 656mAh/g was obtained at 5.0C high-rate charge and discharge, and the performance was excellent.

2. Sulfur/oxide composite material
Nano-metal oxides are mostly used in sulfur/oxide materials. They use their large specific surface area and strong adsorption to improve the porosity of the cathode material, absorb polysulfide ions, reduce the shuttle effect, and catalyze the redox reaction. . Wei prepared S-TiO2 nanomaterials with a "yolk-shell" structure. The internal void structure can fully accommodate the volume expansion of sulfur during the reaction and minimize the dissolution of polysulfides. The initial discharge specific capacity is 1030mAh/g at a rate of 0.5C, and the coulombic efficiency remains at 98.4% after more than 1000 cycles. The most important thing is that after 1000 cycles, the capacity decay per cycle is only 0.033% on average, creating a new peak in the performance of long-period lithium-sulfur batteries.

3. Sulfur/polymer materials
High molecular conductive polymers have both the electrical properties and electrochemical redox activity of metals and semiconductors, and are extremely attractive in the research fields of electrochemical sensors, power supply systems, electrocatalysis, organic optoelectronic devices, and metal corrosion protection. Compounding polyacrylonitrile (PAN), polypyrrole (PPy), polyaniline (PAn), etc. with elemental sulfur to prepare electrode materials can improve the conductivity and stability of the electrode and improve battery performance. Xiao et al. prepared a three-dimensional cross-linked polyaniline carbon nanotube/sulfur composite material as a positive electrode at 280℃. Its structure is stable and easy to adapt to the volume change of the reaction product during the electrochemical reaction. The functional groups on the polyaniline carbon nanotube chain It can also absorb polysulfide ions with the help of electrostatic force to suppress the shuttle effect. After 100 cycles of this kind of battery at a charge-discharge rate of 0.1C, the specific discharge capacity is still 837mAh/g. Qiu et al. synthesized the pyrrole and aniline copolymer nanowire composite material by the template method, which has a rich porous network structure, good conductivity and strong adsorption. The first discharge specific capacity of the battery was as high as 1285mAh/g, and it remained at 860mAh/g after 40 cycles. Wu et al. [16] used a chemical oxidation polymerization method to coat the surface of elemental sulfur with a layer of polythiophene, which showed good performance in electrochemical cycles. The first discharge specific capacity was 1168mAh/g, and the discharge ratio after 50 weeks The capacity is 819.8mAh/g, indicating that polythiophene can effectively improve the conductivity of the electrode and alleviate the shuttle effect to a certain extent.

4. Binary metal sulfides
Most lithium-sulfur batteries use elemental sulfur as the active material. In addition, lithium-sulfur batteries with binary metal sulfides as the positive electrode have also attracted the attention of researchers due to their larger theoretical specific capacity and mature synthesis technology. Yufit uses a constant current to deposit a porous foamed FeSx film with a thickness of about 1μm on a Ni substrate. The single-cycle capacity loss after 650 cycles at a charge-discharge rate of 1C is less than 0.06%, with a long service life and stable performance. Han et al. prepared a composite material of metallic nickel wire and elemental sulfur by means of a ball milling method. The discharge specific capacity in the first week was 580mAh/g, and it remained at 550mAh/g after 200 cycles, and the attenuation rate was extremely low. It can be seen that the battery using binary metal sulfide as the cathode material has good cycle performance, but the actual specific capacity is significantly smaller than the battery made from the above three types of materials, and its disadvantages such as lower power density and active material utilization rate are still To be overcome.

5. Improvement of preparation process
A lot of research work has proposed new methods on the basis of traditional processes to improve some of the electrochemical properties of materials. For example, a coating process is used to prepare the positive electrode active material to improve the working cycle capacity of the battery. Huang et al. coated one end of the ordered carbon nanotube array with a layer of PEG and composited with elemental sulfur to prepare a positive electrode. After 100 cycles at a rate of 0.1C, the capacity decay rate was as low as 0.38%. The Nazar research group [20] used the PEG solution dipping method to coat the surface of the CMK-3/S composite. The first discharge specific capacity of 1320mAh/g was obtained at a rate of 0.1C, and the coulombic efficiency was 99.9%, indicating the shuttle The effect is almost completely controlled.

A lot of research work has focused on the use of chemical methods to modify lithium-sulfur batteries. Recently, some scholars reported that physical vapor deposition methods, such as magnetron sputtering, were used to modify lithium-sulfur batteries and achieve stable cycle performance. Using activated carbon (AC) as the conductive substrate and elemental sulfur as the active material, the lithium-sulfur battery cathode material S/AC was prepared, and Al and Ti were respectively deposited on the surface of the S/AC electrode by the radio frequency magnetron sputtering method (Figure 1) , The electrode is modified to improve battery performance. Experiments show that at 0.5C charge-discharge rate, the initial discharge specific capacity of the lithium-sulfur battery with titanium and aluminum plating on the positive electrode surface is 1255mA/g and 1257mAh/g, respectively, and remains at 722mAh/g and 977mA/g after 100 cycles, Coulomb The efficiency is higher than 97%.

In addition, some researchers have proposed that inserting a conductive interlayer between the positive electrode and the separator can also effectively improve battery performance, such as multi-walled carbon nanotube interlayer, graphene interlayer and so on. Li Heqin's research group prepared a conductive carbon film with filter paper, and further deposited a metal aluminum film on the surface of the carbon film by magnetron sputtering (Figure 2). The performance of the battery containing aluminized carbon film has been greatly improved, the first discharge specific capacity at 1C is 1273mAh/g, after 100 cycles it still retains a reversible capacity of 924mAh/g, and the coulombic efficiency after 200 cycles is still maintained at 95% above.

In summary, sulfur-containing cathode materials are important aspects that determine the specific capacity and cycle performance of lithium-sulfur batteries. Using mesoporous carbon, polymers, oxides and other materials to compound with sulfur, the important purpose is to prevent the diffusion of polysulfides into the electrolyte, inhibit the shuttle effect, and improve the conductivity of sulfur to improve the overall performance of lithium-sulfur batteries. .

2. Binder

The binder with stable performance is helpful for the full compatibility of sulfur and the conductive agent, as well as the close contact between the positive electrode current collector and the active material. Therefore, it needs to meet the following characteristics: good adhesion, uniform pulping, and relatively high electronic and Ionic conductivity. The commonly used binders mainly include polyvinylidene fluoride, polyvinyl alcohol, gelatin, β-cyclodextrin and so on. RaoM et al. [23] studied polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), sodium carboxymethyl cellulose + styrene butadiene rubber (CMC + SBR) three binders to sulfur/carbon nanofiber composite The influence of the electrochemical performance of lithium-sulfur batteries with positive materials. Through comparison, it is found that when CMC+SBR is used as the lithium-sulfur battery binder, the battery performance is the best. Some researchers also compared the performance of cyclodextrin, gelatin, PVDF and polytetrafluoroethylene, and found that the overall electrochemical performance of the lithium-sulfur battery with cyclodextrin as the binder was the best.

3. Electrolyte system
1. Liquid organic solvent electrolyte

Carbonate and ether/polyether electrolytes are currently more mature commercial organic solvent electrolytes. Usually, appropriate additives are added to the electrolyte to improve the oxidation-reduction reaction activity. For example, taking 1mol/L lithium bistrifluoromethanesulfonate (LiTFSI)/ethylene glycol dimethyl ether (DME)+1,3-dioxolane (DOL) (volume ratio 1:1) as The electrolyte, adding 1% LiNO3 as an additive, can effectively improve the specific capacity and coulombic efficiency of the battery. Some researchers also use soluble polysulfides as additives to suppress the emergence of insoluble Li2S, which can also significantly improve the cycle stability of the battery.

2. All solid electrolyte

The density and structural characteristics of the solid electrolyte can allow more charged ions to gather at one end, conduct more current, and then increase the battery capacity. Compared with liquid electrolytes, solid electrolytes have more obvious advantages, including: inhibiting lithium dendrites, stable circulation, good safety, long service life, and high energy density. All-solid-state electrolytes are usually doped with lithium salts into polymers to achieve ion conductivity. Fisher [24] synthesized a solid polymer electrolyte film based on LiTFSI and PEO. The ionic conductivity at 0°C and 25°C was 0.117mS/cm and 1.20mS/cm, respectively, which is suitable for use as an electrolyte for lithium-sulfur batteries. The Nagao research group [25] used ordered mesoporous carbon CMK-3 as the conductive matrix to prepare the cathode material, and Li3.25Ge0.25P0.75S4 as the solid electrolyte. Under 500MPa pressure, the two were stacked on the lithium anode and pressed into a solid state. The reversible capacity of the battery after 50 cycles is higher than 1000mAh/g.

At present, there are two important problems that plague the industrialization of all-solid-state batteries: one is that the ionic conductivity of the solid electrolyte at room temperature is not high; the other is that the interface impedance between the solid electrolyte and the positive and negative electrodes is relatively large. In recent years, some research institutions have made breakthroughs in these areas. For example, Qingdao Energy Storage Industry Technology Research Institute ("Qingdao Energy Storage Institute") proposed the design concept of "rigid and flexible" solid electrolytes, and leveraged the advantages of different materials. , Innovatively composite "rigid" porous framework materials and "flexible" polymer ion transport materials. In order to effectively reduce the interface impedance, they proposed the "in-situ self-formation" mechanism, which firstly infiltrates the liquid monomer molecules into the electrode interface, and then polymerizes in-situ into a high-molecular-weight solid electrolyte. This "in-situ self-forming" system effectively solves the problem of ion conduction at the solid-solid interface, and at the same time effectively improves the distribution of lithium ions at the interface to inhibit lithium dendrites; in order to solve the inevitable extrusion in the practical application of solid-state batteries For the failure of solid-solid interface caused by phenomena such as puncture and puncture, Qingdao Energy Storage Institute uses the temperature response gelation process of thermoreversible polymers to construct a solid-state battery system with a "cooling recovery" function. After being strongly squeezed or folded, although the contact between the electrolyte and the electrode is broken, and the battery performance drops sharply, the effective solid-solid interface can be reshaped through a simple low-temperature cooling step to achieve efficient recovery of battery performance.

3. Gel polymer electrolyte
The lithium salt and polymer are added to the plasticizer, and they are mutually soluble in a suitable organic solvent to form a gel polymer network with stable structure, strong plasticity and excellent ion transport ability. Wang et al. used PVDF and hexafluoropropylene to immerse in a specific electrolyte to prepare a gel electrolyte with an ionic conductivity of 1.2 mS/cm. Rao et al. prepared high-porosity polymer electrolyte membranes by electrospinning and combined with different ionic liquids to prepare new gel polymer electrolytes. Carbon nanofiber-sulfur composite as positive electrode and PAN/PMMA polymer membrane and electrolyte N-methyl-N-butylpiperidine bis(trifluoromethylsulfonyl)imide (PPR14TFSI): polyethylene glycol The gel polymer electrolyte composed of dimethyl ether (PEGDME) (1:1) constitutes a new type of lithium-sulfur battery system, and the capacity remains at 760mAh/g after 50 cycles at 0.1C.

4. Ionic liquid electrolyte
Ionic liquid refers to a liquid composed entirely of ions. It has the characteristics of low vapor pressure, good ionic conductivity and thermal conductivity, wide temperature range in liquid state, hard to volatilize, non-combustion, and electrochemical stable potential window much larger than other aqueous electrolyte solutions. Therefore, applying ionic liquids to battery electrolytes can reduce self-discharge, improve system stability and safety, and have broad application prospects in research fields such as new high-performance batteries, solar cells, and capacitors.

Sun et al. used 0.5M LiTFSI/methylpropylpyridine bis(trifluoromethanesulfonimide) as the electrolyte for lithium-sulfur batteries, tested at room temperature at a rate of 0.05C, and the initial specific capacity was as high as 1420mAh/g; When it reaches 50℃, the specific capacity of the first release is still 1350mAh/g, after 10 cycles it is 782mAh/g, and the high temperature stability is good. It has also been reported that adding 5% to 10% of imidazole-based ionic liquid to the liquid electrolyte greatly improves the electrochemical performance and low-temperature stability of the battery.

In a lithium-sulfur battery with a liquid electrolyte, as the polysulfide continues to dissolve, the viscosity of the electrolyte will gradually increase, thereby affecting the battery's discharge capacity. In contrast, the solid electrolyte prevents the dissolution of sulfur and polysulfides in the battery, but its transmission rate is significantly lower. Ionic liquid electrolytes can effectively improve such problems and have a good industrialization prospect.

Fourth, lithium negative electrode protection

Due to the shuttle effect of the charge and discharge process, lithium polysulfide, an intermediate product of the oxidation-reduction reaction, will diffuse to and react with the lithium negative electrode, leading to corrosion and passivation of the negative electrode and degradation of battery performance. Generally, on-site protection and off-site protection are used to form a protective film on the surface of the negative electrode to prevent the formation of lithium dendrites, improve the cycle life and stability of the battery, and reduce potential safety hazards.

On-site protection refers to the formation of a protective film on the surface of the negative electrode of the battery by means of a specific chemical reaction method. For example, adding an additive capable of reacting with lithium into the liquid electrolyte, or by ultraviolet curing polymerization, etc., to form a stable SEI film on the surface of the negative electrode, thereby reducing the degree of passivation of the negative electrode, improving the cycle stability of the lithium-sulfur battery, and further Play the purpose of preventing overcharging.

Another way is to modify the lithium metal sheet first, and then assemble the battery, that is, off-site protection. Skotheim [29] synthesized an alloy transition layer on the surface of the lithium sheet to resist the reaction between lithium and the electrolyte and maintain the stability of the lithium negative electrode during the cycle.

Affinito uses low-temperature evaporation and radiation methods under vacuum to deposit polymer protective films of different thicknesses on the surface of the lithium negative electrode to improve the stability of the negative electrode. Another idea is to use sulfur dioxide (SO2), sulfuryl chloride (SO2Cl2), thionyl chloride (SOCl2) and other oxidants or phosphoric acid (H3PO4), phosphorous acid (H3PO3) and other inorganic acids to chemically treat lithium to form a passivation layer. , Prevent the electrolyte from corroding the lithium negative electrode, and improve the service life and safety performance of the battery.

V. Conclusion

Compared with traditional lithium-ion batteries, lithium-sulfur batteries have the advantages of high theoretical specific capacity, good safety, low storage cost, and environmental friendliness, and have broad research and development prospects. At present, the purpose of research on lithium-sulfur batteries is to improve their actual specific capacity and cycle performance. The research direction is important to achieve the overall performance of the battery through the modification of the cathode material, electrolyte, lithium anode, binder, and process optimization. improve. The performance of the positive electrode material is an important factor that determines the specific discharge capacity and cycle performance of the battery. From the current research status, the battery assembled by the composite positive electrode of various porous conductive carbon and sulfur has better overall performance. The porous structure and Better conductivity is beneficial to improve the utilization rate of sulfur.

At present, researchers are trying to explore new modification directions: the treatment and improvement of the positive and negative electrode spacer films; the modification and protection of electrodes and diaphragms through methods such as coating and coating. With the continuous deepening of research and the improvement of technology, the performance of lithium-sulfur batteries has gradually improved. Based on its broad market demand and development space, lithium-sulfur secondary batteries will become an important research direction in this field in the future.

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