Preparation And Electrochemical Performance Of MoS₂ Supported By Carbon Derived From Apple Waste

Sodium is considered as one of the most potential materials to replace lithium resources in battery systems,due to its abundant natural resources,low exploitation cost and the similar properties with lithium.Recently,graphite,showing a theory capacity of 372.0 mAh g^-1,is employed as the commercial lithium ion battery anode.However,as an anode electrode for sodium ion battery,it shows the capacity below 30.0 mAh g^-1.This is mainly due to the large sodium ion radius and slower chemical reaction,so that the materials suitable for lithium-ion batteries are not necessarily suitable for sodium ion batteries.Amoung transition metal sulfides,MoS₂ is regarded as one of the most potential anodes for sodium ion battery due to its graphite-like laminar structure,large interlayer spacing（0.62nm）and abundant resource.The initial capacity of MoS₂ can up to 300.0 mAh g^-1.However,due to the weak van der Waals force between the layers of MoS₂,it is easy to stack in the charging and discharging process,and the volume expansion of the system will lead to the pulverization of material,resulting in a significant drop in capacity.In order to solve these problems,lots of work has been doing,mainly including the following two strategies.One is to reduce the stress by reducing the particle size of MoS₂ and increasing the interlayer space;the other is to compound molybdenum disulfide by using the flexible carbon nanomaterial as the substrate.In this thesis,an apple waste was used as carbon source.The MoS₂/carbon with several morphologies and structures as well as phases was obtained by prefabricated in-suit method and preform method.And the relationship between structure,composition and properties was studied.In order to explore the preform following by in suit method to synthesize the MoS₂/Apple pomace derived carbon,the graphite is used as a raw material to prepare the graphene oxide by Hummers method as the base to get MoS₂/rGO,so as to explore the prefabricated in-suit method.Futhermore,the S nanoparticles were employed to further stabilize the MoS₂/rGO structure.The apple pomace carbon was obtained by the hydrothermal carbonized method.In order to make MoS₂ grow more closely with the apple pomace carbon,the carbon is oxidized through the Hummers-like method.Then the prefabricated in-situ method was used to synthesize MoS₂/carbon composite.（1）When the graphite is used as a carbon source,the adsorption contribution was investigated.The morphology of products are strongly depended on the ratio of ammonium molybdate and GO.After 100 cycles,the S/MoS₂/rGO showed the discharge capacity of 504.6 mAh g^-1 at a current density of 100 mA g^-1.Even,after more than 400 cycles,the capacity can still reach 300.0 mAh g^-1,which is significantly higher than the MoS₂/rGO and the pure MoS₂.The analysis showed that the insertion of sulfur between MoS₂layers mainly plays two roles:first,during the intercalation of Na in-between the layers of MoS₂（above 1.0 V）,they function as blockers and inhibit the aggregation of MoS₂;then in the conversion reaction stage,the sulfur nanoparticles chemically participate in the Na storage process by forming Na₂S₅-rich compounds,which eventually improve the reversibility of the conversion reaction and thereafter the cycling performance.（2）The apple pomace collected from the juice concentration factory was used as the carbon source,and three dimensional hard carbon materials with two level structures were obtained by hydrothermal and carbonization methods for the sodium ion battery electrodes.The 3d network carbon with structure and morphology were abtained by three heating temperatures.The uniform morphology and a large number of conchiform macropores with a diameter of about 500 nm and a small number of micropores with a diameter of about 50nm were obtained under optimized 600 ^oC showing the best performace amoung three samples.At the high loading of 5.1 mg cm^-2,the area capacity can reach 1.91 mAh cm^-2 after 200 cycles with an average coulombic efficiency of 99.3%.Compared with normal mass loading of 0.9 mg cm^-2,the mass capacity can be as high as 208.5 mAh g^-1 after 200 cycles as well.The area capacity and mass capacity are both higher than most of the same kind of active carbon materials.This is mainly because that its unique three-dimensional network carbon structure increases the electrical conductivity of the material,providing a pathway for ion pathway.Its two-level pore structure can provide reaction microspace and alleviate volume expansion,which improved the electrochemical performance of electrode.（3）The MoS₂ composite was synthesized by controlling temperature and preparing process.By oxidizing the apple pomace carbon to improve the number of functional groups and active sites,the product was prepared with the promising performance by combining with MoS₂.When the carbonation temperature was 700 ^oC and the carbon was oxidized,the prepared material was uniformly covered with MoS₂ on the layer of the apple pomace carbon.At the current density of 1000 mA g^-1,the capacity within 100 cycles was stable at321.3 mAh g^-1,which is higher than untreat carbon,and at high current density of 1 A g^-1 the capacity can up to 286.0 mAh g^-1.