Preparation Of Carbon-based Transition Metal Composites And Their Lithium-capacity Performance

Over the past decade,renewable energy has been a hot pursuit in science and industry due to the rapid consumption of fossil fuels and increasing concern for the environment.In order to utilize renewable energy effectively,high-performance electrochemical energy storage devices are urgently needed.As a result,tremendous effort has been made to this field with remarkable achievements.In addition,more and more low-value by-products,such as petroleum and petroleum coke,are produced during heavy oil refining.In this context,reconciling the deployment of renewable energy and the value-added use of the oil industry’s abundant and low-cost by-products is a major challenge.Lithium ion batteries（LIBs）are considered as one of the most important energy storage and conversion technologies,which has many advantages such as high energy and power density,long cycle life,low memory effects and strong adaptability in fields as portable electronic devices,stationary energy storage systems as well as rapidly developing electric vehicles.It is well known that the properties of anode materials are one of the important factors,which can determine the electrochemical performance of LIBs.So far,different kinds of LIBs anode materials have been developed,including carbon materials,transition metal oxides,conductive polymers,transition metal sulfides,and others.Here,the development of high-performance transition metal-based lithium anode materials are systematically studied,and the following meaningful results are achieved:（1）A template control strategy was developed to prepare transition metal oxide nanosheets with vertical array,which alleviated the volume expansion of anode materials.Vertically aligned mixed-phase CoO/Co₃O₄ nanosheets grown on wrinkled graphene（CoO/Co₃O₄/Gra）are controllably synthesized under the assistance of water-removable Na Cl-templates through facile freeze-drying and calcination processes.In-depth investigations reveal that the wrinkled graphene induces the bonding with nucleating precursors,and the nanosheets vertically grow along the crystal planes of multilevel-Na Cl.After the Na Cl template was washed away,a three-dimensional mesopores nanosheet network structure was formed.The as-formed mesoporous nanosheets-on-network structure endows the hybrids with efficient exposure of CoO/Co₃O₄ active sites and rapid charge-transfer channels for Li⁺and electron.And the multiple composites can integrate the advantages of their long complements and fully exert their synergistic effects to solve the problems of the single metal oxide electrode.Furthermore,the electrochemical performance of hierarchically mesoporous architecture is beyond that of the single individual structure on account of the sufficient electrode/electrolyte interfaces,rapid Li⁺diffusion and electron conduction,and stable structural integrity upon cycling.Serving as a LIBs anode material,the mesoporous CoO/Co₃O₄/Gra-4 hybrid yields superior reversible capacity(1390 mAh g^-1 at 0.1 A g^-1),high-rate performance(1130 mAh g^-1 at 0.2 A g^-1),and impressive cycling stability(811mAh g^-1 after 500 cycles at 1 A g^-1).This work may inspire the design of high-performance electrodes with advanced 3D hierarchical structures.（2）An interface engineering strategy was developed to construct transition metal base compounds with high-performance porous structure,which can solve the poor ion diffusion ability of transition metal base compounds with unsatisfactory microstructure.The strategy successfully constructed NiO/NiCo₂O₄/CoO/Gra heterostructures through a controlled salt-templated solvothermal synthesis followed by pyrolysis.The epitaxial porous heterostructures produced dual interfaces of NiO/NiCo₂O₄ and CoO/NiCo₂O₄.According to density functional theory,NiO/NiCo₂O₄and CoO/NiCo₂O₄ interfaces have higher Fermi energy levels,fast electron transfer and efficient Li⁺adsorption capacity.In addition,this unique porous interface structure can effectively alleviate the volume expansion during the lithiation/delithiation process,and greatly promote the electrochemical performances of hybrids.Serving as testing the performance for LIBs,the porous NiO/NiCo₂O₄/CoO/Gra heterostructure electrodes achieved a remarkable reversible capacity of 1932.8 mAh g^-1 at 0.1 A g^-1.Moreover,the anode also obtains impressive cycling stability of 1580.5 mAh g^-1 after 100 cycles at 0.1A g^-1,and even 1130 mAh g^-1 after 500 cycles at 0.5 A g^-1.This work provides a novel interface engineering strategy for developing multiple heterostructure anode materials for high-efficiency energy storage and conversion.（3）A gas/solid template method was developed,and a nitrogen/sulfur-codoped porous carbon（NSPC）anode material was successfully synthesized,which realized the high value-added utilization of petroleum asphalt.The synthetic strategy uses g-C₃N₄and NaHCO₃ as gaseous templates and Na Cl as solid template,which causes the formation of hierarchical PC with high specific surface area.The resultant porous structure and N-doping process can prevent the aggregation of nanosheets,maintain the structural stability upon cycling,and achieve rate-capable Li-storage.Serving as LIBs anode,reversible specific capacities of NSPC24 electrode reach 788 and 280 mAh g^-1 at0.1 and 1 A g^-1,respectively.Furthermore,its specific capacity still is maintained at 830mAh g^-1 after 115 cycles at 0.1 A g^-1.Even after 500 cycles,high specific capacities of727 mAh g^-1 at 0.5 A g^-1.The gas-solid bifunctional-template approach can guide the design of porous materials very well,meanwhile realizing the high value-added utilization of asphalt.（4）A sulfur-fixation strategy was developed to convert petroleum asphalt into a high-performance anode material,realizing its high value-added utilization.Nowadays,rational yet high value-added utilization of low-cost petroleum asphalt still faces a significant challenge.In light of its high carbon content and abundant sulfur atoms,here,an in-situ bonding sulfur strategy is proposed to fabricate transition metal（Mn,Mo,Fe,or Co）-based/carbon composites（TM-based/C）.This strategy successfully achieves the conversion from thiophenic-S of asphalt into transition metal sulfides.When tested as anodes for LIBs,the TM-based/C electrodes deliver high specific capacity and cycling performance.In addition,and the optimized MnO₂/MnS₂/C-2 electrode achieves low charging/discharging voltage platform of 1.2 V/0.58 V,and delivers long-term life of827 mAh g^-1 after 400th cycle at 0.5 A g^-1.After full battery performance test,LiFePO₄/（MnO₂/MnS₂/C-2）full cell exhibits a remarkably reversible capacity of 168mAh g^-1 at 0.8 C,and impressive cycling stability of 78%after 400 cycles at 1 C.This work may provide a new perspective for the high value-added utilization of low-cost petroleum asphalt for efficient anodes toward LIBs.（5）Based on high carbon content of asphalt,an S-doping porous carbon（SPC）framework structure was successfully prepared by using NaHCO₃ template.Then,a three-dimensional structure was prepared by in-situ growing MoS₂ nanosheets on the surface of the SPC using MoCl₅ as molybdenum source and sublimated sulfur as sulfur source.The as-formed material achieves good contact between MoS₂ and carbon substrate,and its interlaced structure can greatly improve the electron transfer rate and shorten the transmission path of Li⁺and e^-.In addition,its porous structure provides abundant reactive sites for Li⁺.When tested as lithium anode,the material obtains a high specific capacity of 1069 mAh g^-1 after 400 cycles.This work may provide a new idea for high value-added utilization of low-cost petroleum asphalt.