Studies On The Construction Of Carbon Conductive Network In Sulfides/Phospide Compounds For The Improvement Of Sodium Storage Performance

With the increasing demand for lithium-ion batteries but the limited lithium resources,it is urgent to develop new low cost and high energy density batteries.Sodium ion batteries have become one of the most promising alternatives.However,it is found that graphite materials used in commercial lithium ion batteries are not suitable for sodium ion batteries,so it is very important to develop new anode materials for sodium ion batteries.Conversion and alloy materials have attracted wide attention due to their high theoretical capacity.Metal sulfides in conversion materials have a good prospect of application.The low price and high theoretical specific capacity of metal sulfides have attracted the attention and have been studied.Alloy materials,such as phospides,have high theoretical specific capacity,so they have been paid more and more attention.However,there is a severe problem in both of them where violent volume expansion will occur during charge and discharge.It will lead to electrode cracking,losting electrical contact between active material and current collection.So the electrochemical performance of the material will decline sharply,and the cycle performance will be poor.At present,it has become a research hotspot to alleviate the volume expansion to improve electrochemical performance.Among all the strategies,nano-scales and the construction of carbon conductive network are considered to be more effective methods.In this work,a high N-doped carbon coated NiS_x is successfully prepared by a simple large-scale method.The NiS_x/HNCNs-400 demonstrates the best electrochemical performance with excellent rate performance and cycle stability.At the large current density of 5.0 A g^-1,the NiS_x/HNCNs-400 delivers a high capacity of 230.5 mA h g^-1,which is much better than the other three NiS_x/HNCNs with the capacity of 147.9,167.6 and 115.2mA h g^-1 at the temperature of 350,450 and 500?respectively.And it exhibits the best cycle stability with the capacity of 238.3 mA h g^-1 after 50 cycles.The excellent electrochemical performances contribute to two reasense.The ultra-small size can shorten the diffusion pathway of the Na⁺,improving the rate performance and cycle stability of the materials.And the N-doped conductive carbon layer can not only provide more transport pathways for electron,but also relax the stress of the volume expansion during the sodiation/desodiation progress.In order to further improve the rate and cycling performance of NiS_x/HNCNs-400composites,a good graphene carbon-conducting network is added by in-situ synthesis.Dual-carbon conductive network modified NiSx nanocomposites?NiSx/HNCNs/GO?are synthesized.NiS_x/HNCNs/GO-2 anode material shows excellent electrochemical performance as anode materials for sodium ion batteries.At the current density of 0.1 A g^-1,the reversible capacity of the first cycle is up to 488 mA h g^-1.And the capacity of 351.5 mA h g^-1 is still high at the high current density of 5.0 A g^-1,showing excellent rate performance.This shows that the dual-carbon conductive network greatly improves the electrochemical performance of the materials.In order to further improve the capacity of anode materials,the conversion reaction mechanism can be changed into the conversion alloying reaction mechanism,which can effectively increase the capacity of the materials.In chapter 5,GeP₅ is used as anode material of sodium ion battery.However,due to the serious volume expansion during charging and discharging,it is necessary to introduce conductive carbon network to synthesize GeP5/acetylene black/partially reduced graphene oxide?GeP5/AB/p-rGO?by simple ball milling method.As the anode material for SIBs,it exhibits excellent rate performance and deliver a large reversible capacity of 175 mA h g^-1 at the current density of5.0 A g^-1.Remarkably,the GeP₅/AB/p-rGO composite shows the reversible capacity of 400mA h g^-11 after 50 cycles at the current density of 0.5 A g^-1.The material shows excellent electrochemical performance and has a good prospect of application.The construction of the dual-carbon conductive network greatly improves the sodium storage performance of the sulfur/phospide compounds anode materials.It can be found that this method can also be applied to other electrode materials through the study on its action mechanism.Therefore,it has far-reaching significance to study the mechanism of dual-carbon conductive network to improve the properties of sodium storage materials.