| A series of environmental problems caused by excessive exploitation of fossil fuels have aroused great attention.It is imperative to develop green and renewable energy.Since the commercialization in the 1990 s,lithium-ion batteries(LIBs)have been widely used in portable electronic products,automotive equipment and other fields.However,with the increasing demand of energy density and capacity for energy storage devices,traditional LIBs have reached the energy limit,which cannot meet the requirements of practical applications in large-scale energy storage,electric vehicles,aerospace and other fields.It is especially necessary and important to develop novel high-performance battery systems to surpass traditional LIBs.Among many emerging rechargeable secondary battery systems,lithium-sulfur(Li-S)batteries and lithium-selenium(Li-Se)batteries have attracted special attention of researchers.This is mainly because sulfur has the advantages of abundant reserves in the earth's crust,wide range of existence,low cost,high safety.It is a very attractive and competitive lithium cathode candidate.Moreover,Li-S batteries can provide a high specific capacity of 1675 m Ah g-1 with an average voltage of 2.15 V.The theoretical gravimetric and volumetric energy densities are 2567 Wh kg-1 and 3467 m Ah cm-3,respectively,which are about 3-5 times of the traditional LIBs.Elemental selenium,in the same elemental group as sulfur in the periodic table,has similar electrochemical properties to sulfur.In addition,due to its semiconductor properties,the electronic conductivity(1 × 10-3 S m-1)is about 20 orders of magnitude higher than that of sulfur(5 × 10-28 S m-1),and it has higher electrochemical activity and utilization.Moreover,the Li-Se batteries can provide a theoretical capacity of 675 m Ah g-1 and a high theoretical volume energy density(3254 m Ah cm-3)comparable to that of a Li-S batteries due to the higher density of selenium(4.8 g cm-3).So it is considered as a potential new electrode material.However,many obstacles prevent their commercialization application,such as the insulating properties of sulfur and Li2S2/Li2 S,severe shuttle effect caused by the solubility of intermediate polysulfide and the high volume expansion of selenium during the charging and discharging process.In view of the existing problems of Li-S and Li-Se batteries,we proposed two different strategies for modification research.The research results are shown as follows.Firstly,we prepared a three-dimensional composite material(Fe2N/N-r GO)in which Fe2 N nanoparticles were grown in situ on nitrogen-doped graphene via hydrothermal and high temperature nitridation under ammonia atmosphere route.This is the first time that Fe2N/N-r GO has been used to modify the separator for Li-S batteries.Benefit from the in situ growth,Fe2 N nanoparticles are evenly distributed on the inner and outer surfaces of the graphene sheets.The composite material has high conductivity and good structural stability.In particular,it combines the chemisorption of polar material Fe2 N and physical barrier of graphene,which effectively inhibits the shuttle effect of polysulfides.In addition,Fe2 N can effectively promote the conversion of soluble polysulfides to insoluble product Li2S2/Li2 S by catalysis.Therefore,the cell with a modified separator delivers good cycle stability and excellent rate performance.The first discharge specific capacity is as high as 1343.7 m Ah g-1 at 0.5 C(1C = 1675 m Ah g-1).After 200 cycles,there is still a reversible capacity of 847.7 m Ah g-1,and the coulombic efficiency is close to 100% during the cycle.Next,we use the cheap and environmental friendly agar as a carbon source to prepare porous carbon material through high temperature carbonization and KOH activation etching using a mass ratio of KOH to agar of 1:1.The prepared carbon matrix(3D-ODPC)with large specific surface area and abundant pore structure is used as a carrier of selenium through melt-diffusion strategy.The composite material(Se/3D-ODPC)as Li-Se batteries cathode material shows excellent electrochemical performance.At a current density of 0.2 C(1 C = 675 m Ah g-1),the first discharge specific capacity is 983.7 m Ah g-1,and the second cycle has a reversible specific capacity of 539 m Ah g-1.After 300 long cycles,the reversible discharge capacity is 509.9 m Ah g-1,which is equivalent to 94.6% of the second cycle discharge specific capacity.In this paper,in view of the many challenges faced by Li-S and Li-Se batteries,we prepared Fe2N/N-r GO composite material to modify the separator for Li-S batteries and 3D-ODPC as the carrier of selenium to effectively alleviate volume expansion in Li-Se batteries.These strategies have achieved improved the electrochemical properties of Li-S and Li-Se batteries,and taken a step forward in their commercialization process.Additionally,it also provides novel ideas for the preparation of high-performance materials in the future. |
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