Structural Regulation Of Carbon-based Composite Electrodes And Their Lithium/sodium Storage Mechanisms

High-performance secondary batteries technologies are highly dependent on the evolvement and application of advanced electrode materials.In particular,the anode materials will directly affect the first coulomb efficiency,cycle stability,and safety of the battery.Carbon materials with the advantages of low cost,low density,high abundance,high conductivity,and controllable structure have been widely used in commecial battery anode.However,the low storage capacity and slow kinetics limit their development in high-performance secondary batteries.Therefore,it is of great significance to explore novel carbon-based anode materials with a high specific capacity,high rate,and long life.In this paper,we committed to the structure modification of carbon materials by heteroatoms-doping,structural regulation,and compounding to achieve their excellent lithium/sodium storage properties.Moreover,their lithium/sodium storage mechanisms were deeply analyzed.The specific research contents are as follows:（1）Exploring the high-performance carbon anode is imminent due to the limited capacity of the graphite in lithium ion battery.Herein,combined with template method and heteroatom doping strategy,we synthesized the multivariate（nitrogen,phosphorus,and sulfur）doped porous soft carbon（NPSC）derived from the mesophase pitch.Such a NPSC anode shows excellent lithium storage capacity and stability.It exhibits a specific capacity of 500 m A h g^–1 after 500 cycles at a current density of 0.5 A g^–1.We found that its three-dimensional ion channels could accelerate the ion diffusion.Moreover,the abundant C-S bond induced by doped P provides more active sites and enlarges the carbon interlayer spacing.The synergistic effect instigated by multiple elements is benefit to reduce energy barrier for the magration of lithium ions,thus optimizes its electrochemical kinetics.This work indicates that three-dimensional ion channels and synergistic effect of multiple elements could effectively improve the lithium storage performance of carbon-based anodes.（2）Establishing the lithium metal anode with high specific capacity is of great significance to further improve the energy density of lithium secondary batteries.Aiming at the uneven mass and electron transfer in lithium metal,we propose to construct the silicon carbide/carbon（Si C/CC）composite,in which the Si C nanowhisker in situ grow on CC with gradient distribution.The three-dimensional gradient conductive topology is achieved due to the conductivity difference between Si C and CC.The results show that the Si C/CC framework provides rich channels for ion migration and optimizes electrochemical activity sites for Li deposition.Gradient conductive networks could guide the deposition of lithium metal from bottom to top,avoiding the accumulation of ions on the surface and reducing the dendrites formation.In addition,the Si C/CC network could decrease the size of deposited lithium,thus improving the utilization and reversibility of lithium metal.The symmetrical battery assembled with the Li@Si C/CC electrode can achieve a long cycle life of more than 1000 h at a current density of 1 m A cm^–2.The full cell assembled with lithium iron phosphate and Li@Si C/CC exhibits excellent cycling stability.The gradient conductive topology could be fabricated by structure modification to uniform the lithium deposition in carbon cloth.（3）To resolve the slow kinetics of carbon anode materials during ion de-insertion,the edge-rich sulfur-doped carbon nanorods（SCNs）were constructed by using the polymerization-pyrolysis method.The results show that the carbon layer of SCNs is arranged perpendicular to its axis,exposing a large number of edge active sites,which can shorten the ion transport path.In addition,the high content of C–S bond can optimize the chemical adsorption energy of the active site to the ions,as well as expand the carbon layer spacing.Thus,the reaction kinetics of sodium ions was improved.When used SCNs as the anode material for SIBs,a reversible specific capacity of 372 m A h g^–1 was obtained at a current density of 0.05 A g^–1,and can still be maintained at 175 m A h g^–1 after 1000 cycles at 1 A g^–1.It shows good rate performance and long cycle stability.By optimizing the carbon layer spacing and increasing the edge active site,the sodium storage performance and reaction kinetics of the carbon material were significantly enhanced.（4）Based on the previous chapter,to further improve the sodium storage capacity of the carbon anode,the carbon was composited with higher theoretical specific capacity of transition metal sulfide.The passion fruit-like carbon-coated Cu₂Zn Sn S₄ nanoparticles（CZTS@C）were prepared by simple hydrothermal method and subsequent dopamine coating.The carbon shell guarantees the electrochemical activity of the material,promotes the rapid transfer of electrons,and reduces the loss of intermediate products.In situ characterization results revealed the sodium storage mechanism of multi-electron transfer processes with intercalation,transformation,and alloying reactions for CZTS@C.DFT calculations confirmed that the heterogeneous interface formed after the first cycle enhanced the adsorption energy and intrinsic conductivity of the electrode,which significantly improved the electrochemical reaction kinetics.As the electrode for SIBs,CZTS@C exhibits a reversible specific capacity of 461 m A h g^–1 at a current density of 50 m A g^–1.After250 cycles at a current density of 1 A g^–1,it still maintains a reversible specific capacity of 303 m A h g^–1,showing good electrochemical stability and rate performance.In conclusion,compounding with the high-capacity compounds is an effective method for improving the electrochemical properties of carbon materials.