Study On The Characteristics Of Carbon Fiber-based Structure-energy Storage Integrated Composites

The development of electric vehicles such as BEV,PHEV and HV has further requirements for the lightweight design and functional integration of electric energy systems.Obviously,the simple combination of components with different functions can no longer meet the current demands.Through a reasonable process,combining the matrix with considerable mechanical properties and the functional ingredient with an ideal specific capacity to prepare a structured energy-storage integrated composite is an effective solution to the above problem.PAN-based carbon fiber has excellent tensile strength(>3500 MPa),elastic modulus(>195 GPa)and ideal electrical conductivity(2.5-10 S·cm^-1),which is the most promising matrix materials for structural energy-storage integrated composites.However,due to its low theoretical capacity(372 m Ah·g^-1),polyacrylonitrile-based carbon fiber cannot be directly used as the anode materials of the commercial lithium-ion battery.Therefore,it is particularly important to find a suitable material to be combined with the carbon fiber matrix to improve the cycle characteristics and rate characteristics.Transition metal oxides are considered to be ideal energy storage materials due to their high theoretical capacity(900-2000 m Ah·g^-1),low voltage working platform,abundant reserves and environmental friendliness.However,the volume expansion/contraction(200%-300%)during the lithium ion insertion/extraction process restricts the application of transition metal oxides as anode materials in the field of energy storage.In response to the above problems,this paper prepared three kinds of structural energy-storage integrated composites by combining M_n3O₄,nano-ZnCo₂O₄,and SnO₂-MOF with polyacrylonitrile-based carbon fibers,achieving the complementary advantages of the matrix and the functional body,and effectively overcoming their shortcomings.Advanced characterization methods such as XRD,TG,XPS,SEM,TEM were used to investigate the phase structure,microscopic morphology,and element valence states of the prepared composites;and the mechanical characteristics and electrochemical characteristics of the were tested by tensile experiments and constant current charge and discharge experiments.The effects of different components of structural energy-storage integrated composites and their synergy on the cycle characteristics,rate characteristics and mechanical characteristics of composites are deeply discussed.The main research results are as belows:The CF@Mn₃O₄ composite was successfully prepared by simple hydrothermal stirring,calcination annealing process,and the CF@Mn₃O₄ composite showed excellent cycle characteristics and rate characteristics.After 150 cycles,the specific capacity of 610.5 m Ah·g-1 can be maintained,which can be attributed to the synergistic effect of the carbon fiber substrate and Mn₃O₄.The carbon fiber acts as a buffer substrate to relieve the volume expansion of the deposited layer during charging and discharging process,at the same time,the trimanganese acts as the coating layer to provide more lithium ions during the charging and discharging process,which provided higher specific capacity.However,due to the high temperature treatment,the mechanical properties of CF@Mn₃O₄ composites are more severe than pure carbon fiber;at the same time,due to the larger size of Mn₃O₄,there is still irreversible capacity degradation during the cycle.In order to further improve the overall characteristics of the structured energy storage integrated composite,the ternary transition oxide ZnCo₂O₄ was introduced.The CF@nano-ZnCo₂O₄ composite was prepared by a reasonable process to reduce the grain size.The tensile strength,elastic modulus and elongation can reach 77.71%,77.81%and 77.78%of the pure carbon fiber.After 150 cycles,the discharge specific capacity can reach 782.7 m Ah·g^-1,and the charge specific capacity is 777.6 m Ah·g^-1,the Coulomb efficiency is 99.35%.Such excellent mechanical and electrochemical properties can be attributed to the synergistic effects of carbon fiber and nano-ZnCo₂O₄.The carbon fiber matrix can effectively buffer the volume expansion during charging and discharging process,and the nano-structure can effectively shorten the transmission distance of ions and provide larger specific surface area.Moreover,the stepwise reduction reaction of the ternary transition metal oxide during the lithium ion intercalation process can reduce the mechanical strain and stress.Due to the high discharge platform of ZnCo₂O₄,the working voltage requirements for lithium-ion batteries are relatively high.Besides,the specific capacity of CF@nano-ZnCo₂O₄ composites fluctuates greatly during charge and discharge.Therefore,we introduced SnO₂-MOF particles to further improve the cycle characteristics and rate characteristics of the structural-energy storage integrated composite.After 150 cycles,the discharge specific capacity of the CF@SnO₂-MOF composite is about 732.1 m Ah·g_-1,and the Coulombic efficiency is about 99.35%,which is 4.02 times that of pure carbon fiber.At the same time,the tensile experiment shows that the tensile strength,elastic modulus and elongation rate of the CF@SnO₂-MOF composite can reach 89.34%,87.05%and 88.89%of the original carbon fiber respectively.The above excellent comprehensive properties can be attributed to the unique layered structure of carbon fiber and SnO₂-MOF particles and the synergistic effect between the components.The close combination of discontinuous SnO₂-MOF particles and the carbon fiber matrix not only improves the cycle stability of the composite,but also retains the mechanical properties of the carbon fiber to a certain extent.The chemical process route in this experiment also provides a reference route for the synthesis of carbon fiber/transition metal oxide/MOFs structure with carbon fiber substrate and metal-organic framework as the framework.