| The secondary lithium-ion battery(LIB)is regarded as one of the energy storage devices holding the maximum potential in present and in future.,ascribing to its advantages such as low costs,recyclability,and high energy density.Two-dimensional materials are characterized by large specific surface areas and high electron mobility,and some of them exhibiting superior electrochemical energy storage performances;they have received extensive and in-depth research since they were utilized as anode materials for various ion batteries.New two-dimensional materials name monolayer C2N of carbon nitrides(CxNy),it has graphene-like two-dimensional network lattice structures,and uniformly distributed nano-pores of specific sizesinside the network.Moreover,monolayer C2N contains a highly electronegative N element and the large pores with N atoms as the terminations,and which the researched properties have smaller about its realtions between of alkali metal atoms and the two-dimensional C2N monolayer material.First principles calculattions are adopted in this dissertation to systematically and thoroughly study the electrochemical energy storage properties of C2N monolayer materials in alkali metal ion batteries;boron doping and formation of graphene/C2N heterostructure to improve the electrochemical performance and micromechanism of anodes of alkali metal ion batteries of monolayer C2N two-dimensional materials.First-principles calculations are conducted to systematically study the saturated adsorption structure,theoretical capacity,open circuit voltage(OCV)and ion diffusion kinetics of alkali metal ions on the surface of monolayer C2N.In terms of the theoretical capacity,it is theoretically predicted that the theoretical capacities of monolayer C2N,as the anode materials of alkali metal ion batteries,are 1034.79(Li+),599.72(Na+),and 385.81(K+)mAh/g,respectively,superior to the capacity of current commercial graphite anode materials(～370 mAh/g).The open circuit voltages show excellent charging platforms,indicating that the monolayer C2N structure is suitable as an anode material for alkali metal ions.In terms of diffusion dynamics properties of alkali metal ions,the lowest diffusion energy barriers corresponding to the optimal paths for the diffusion of alkali metal ions on the surface of monolayer C2N are 0.23 eV(Li+),0.24 eV(Na+),and 0.10 eV(K+),respectively.The diffusion barrier of Li+is lower than that of graphite anode material(～0.40 eV)and graphene material(～0.30 eV).First-principles molecular dynamics(FPMD)indicates that the diffusion coefficients of Li+,Na+and K+on the monolayer monolayer C2N,at a temperature of 400 K,are 2.50×10-9 m2/s,5.33×10-9 m2/s,and 8.52×10-9 m2/s,respectively,showing that monolayer C2N is an excellent anode material for alkali metal ion(Li+,Na+and K+)batteries.In order to enhance the binding interactions and improving electrical conductivity of the alkali metal and the electrode material,comprehensive researches have been conducted on the structural stability and the electrochemical properties of C2N1-xBx(x=1/6,1/3,2/3 and 1)modified through substitution of N element in monolayer C2N with boron.Two-dimensional C2N1-xBx(x=1/6,1/3,2/3 and 1)structures can be formed by adjusting the doping concentration of boron;boron doping can proceed spontaneously due to the entropy-increasing effect caused by the release of N2 gas.For the electrochemical properties,the B-doped C2N shows a systematic improvement of theoretical capacities,OCVs and ion diffusion dynamics over those of monolayer C2N.In terms of the electrochemical performance of energy storage in alkali metal ion batteries,the C2N1-xBx(x=1/6,1/3,2/3 and 1)materials formed by B-doped monolayer C2N feature excellent theoretical capacity of battery electrode,OCVs,and diffusion coefficient of alkali metal ions(Li+,Na+,K+),which have been significantly improved and raised compared with those of monolayer C2N.In particular,the alkali metal ion(Li+,Na+,K+)batteries of two-dimensional monolayer C2B materials obtained through complete substitution of N with B show theoretical capacities of 1546 mAh/g(Li+),663 mAh/g(Na+)and 411 mAh/g(K+),respectively.The diffusion barrier further decreases with the increase of B content in C2N1-xBx composition.The lowest diffusion barriers of the monolayer two-dimensional monolayer C2B structure are Li+(0.75 eV),Na+(0.50 eV)and K+(0.25 eV),respectively.At 300K,the diffusion coefficients of Li+,Na+and K+on the surface of two-dimensional C2B monolayer material are 1.25×10-10m2/s,1.45×10-9 m2/s and 4.5×10-10 m2/s,respectively.The diffusion coefficient of Na+on the surface of monolayer C2B is 2 orders of magnitude higher than that of the commercial anode material graphite,indicating that monolayer C2B is a candidate material for fast-charging Na+ batteries.The heterostructure of graphene/C2N features a heterostructure synergistic reinforcement effect of van der Waals force between the layers,which may further improve the electrical conductivity and thermal conductivity of monolayer C2N electrode material and further increase the theoretical capacity of electrode.Graphene/C2N heterostructure has a 0.2 eV direct band gap;the Fermi surface of heterostructure retains the Dirac cone energy band dispersion relationship of graphene,so it has similar conductivity to that of graphene.Moreover,the graphene/C2N heterostructure shows excellent thermal conductivity of 1791.10 W/mK,which is 22 times higher than that of monolayer C2N(79.70 W/mK).In terms of electrochemical energy storage properties,the Li+storage capacity of the graphene/C2N heterostructure is 944.39 mAh/g the theoretical capacity of the heterostructure is improved compared with the capacity of bilayer graphene and bilayer C2N.The migration energy barrier corresponding to the optimal diffusion path is in the range of 0.2 to 0.5 eV,which is comparable to that of the commonly used anode materials of graphite.At 300 K,the planar diffusion coefficient of Li+in graphene/C2N heterostructure is 4.74×10-11 m2/s,which is comparable to those of Li+ in graphite anode materials at room temperature.Theoretical calculations further reveal that,for intercalation or deintercalation of Li+in the graphene/C2N heterostructure,not only the planar migration paths,but also diffusion channels perpendicular to the atomic planes of the two-dimensional materials exist.Overall,the results in this dissertation can provide necessary microscopic physical-chemical insights for conducting in-depth experimental studies into the electrochemical energy storage mechanism of anodes of monolayer C2N-based alkali metal ion batteries,and guide the development of new secondary alkali metal ion battery devices based on relevant two-dimensional materials and with high energy storage density and high power density.Boron doping,graphene heterostructure modification and two dimensions C2N materials outperform existing commercial graphite anode materials in terms of theoretical energy storage capacity and power density for alkali metal ion batteries from theoretical results.This dissertation provides new insights and new ways for developing battery anode materials with high energy storage density,fast charging capacity and excellent thermal conductivity. |
PDF Full Text Request