| Li-ion batteries are widely used in portable electronic gadgets,electric vehicles,aerospace and other fields because of their high capacity,excellent cycle performance and little environmental pollution.With the rapid advance of consumer electronic products and new energy vehicles,the current technology cannot meet the increasing energy demand.Therefore,the development of high-capacity and fast charge-discharge batteries has become the focus of scientific research.Tin(Sn)with high theoretical capacity makes it a promising anode material for Li-ion batteries.However,Sn experiences huge volume deformation during lithiation and delithiation,which results in cracking and disintegration of active materials,exfoliation of active materials from current collectors,and repeated formation and fracture of the solid electrolyte membrane.This directly leads to capacity attenuation and the decay of cycle performance.To deeply understand the macroscopic failure behaviour of Sn anode materials and further make an optimization to electrode materials,it is imperative to investigate the evolution of mechanical properties upon lithiation.However,it is difficult to measure the variation of mechanical properties by using experimental methods,the reasons are that(1)the LixSn phases and solid electrolyte film formed during lithiation are metastable phases and they can easily react with oxygen and water when they are exposed in the air.Therefore,there are high requirements for the preparation of samples,experimental operations and equipment;(2)experimental results are easily affected by structures of electrode materials(such as,composite and thin film electrodes,different porosities,shapes and sizes of active particles)and humidity(such as,liquid and solid electrolyte).These make electrode materials show varied lithiation kinetics(e.g.different Li-ion diffusion rate),volume deformation and stress evolution,which directly results in marked variations of experimental results.Considering the difficulty of experimental measurement,by using first-principles calculation,this thesis systematically investigates the evolution of mechanical properties of active materials and interfacial mechanical properties of electrode-collector interfaces during the charge and discharge processes.The micromechanical failure mechanism of Sn anodes is given.Based on the obtained interface failure mechanism,we further perform optimization to interface properties of electrode-collector interface by using dopants.The main research contents of this thesis are as follows:(1)On the evolution of mechanical properties(such as bulk,shear and Young's moduli,Poisson's ratio,brittleness and ductility,anisotropy and ideal tensile strength)of Sn anodes during lithiation.Based on chemical bonding analysis,the microphysical mechanism of the change of mechanical properties during lithiation is provided.It is shown that the bulk moduli of LixSn alloys decrease almost linearly with Li content.While the shear and Young's moduli?Poisson's ratios and ideal tensile strengths of alloys fluctuate during the lithiation processes.The softened bulk moduli,large anisotropy and brittleness of alloys at high Li content make the surface of electrode materials prone to microcracks at the late stage of lithiation.Furthermore,due to large differences in crystal structures and mechanical properties of alloy phases during lithiation,high mismatch-induced internal stress is created in the lattices which will lead to microcracks and voids in electrode materials.(2)Based on the study of mechanical properties of active materials,the effect of lithiation on interfacial mechanical properties of electrode-collector is further explored.According to the surface energy tests of alloys and lattice mismatches between alloys and the current collector,stable electrode-collector interfaces are established.Then,the effects of lithiation on the interface strength are studied.Combining the analysis of interfacial chemical bonds,the microscale mechanism of interface failure is given.The results show that upon lithiation,the work of separation(Wsep)dereases from 1.59 J m-2 before lithiation to 0.45 J m-2 at the Li content of 0.78,showing a reduction of about 70%.Besides,the interfacial failure behaviour of electrode-collector interface is unraveled by using tensile simulation.Finally,based on first-principles calculations and linear elastic fracture mechanics,there is a relationship between the critical core and shell sizes and the state of charge for determining fracture and debonding of a Sn-Cu hollow core-shell structure.(3)On the influence of CuxSn alloys on interfacial mechanical properties of electrode-collector interface.The focus of this research is the influence of CuxSn alloys on the interface strength,interfacial chemical bonds and stress-strain behaviour of electrode-collector interface.The results show that CuxSn alloys can improve the interface strength of electrode-collector interface.The Wsep of Cu6Sn5/Cu and Cu3Sn/Cu interfaces are 1.73 and 1.74 J m-2,respectively,which are about 9%higher than that of Sn/Cu interface.In addition,CuxSn alloys enhance the deformation resistance of electrode materials at large strain.The fracture strain of Li2CuSn/Cu interface is much larger than that(0.16)of LiSn/Cu interface.The ductility of CuxSn alloys makes CuxSn/Cu interface display ductile fracture,which is different from the brittle fracture of LixSn/Cu interface.Through comparing the interfacial mechanical properties of the LixSn/Cu and CuxSn/Cu interfaces,the mechanical properties of the real electrode-collector interface are given.(4)Based on the obtained interfacial failure mechanism,Co doping has been used to improve interfacial properties of Sn electrode-collector.The effects of different interfacial doping sites on the structures,thermodynamic and electronic stabilities and interface strength of electrode-collector interface are investigated.It is shown that Co doping in the interface region can improve the interface strength of electrode-collector to different extents while Co doping in active materials and current collector decreases interface strength.Interfacial site is the best doping site,where Co atom tends to move to the first Sn layer near the interface region and forms strong chemical bonds with interfacial Sn,Cu and Li atoms.This reduces the accumulation of charges at interface and alleviates the attenuation of interface strength induced by lithiation.The Wsep of Sn/Cu and LiSn/Cu interfaces are increased by 9.4%and 17.7%,respectively.In addition,Co doping enhances the electronic stability of electrode-collector interface.According to the change of Wsep,electronic stability and formation heat with Li content,the optimum Co doping content is given.These findings are instructive to our understanding of the failure mechanism of Sn anodes and are also of great significance for clarifying their macroscopic fracture behaviours upon lithiation.It is expected that the results are helpful to the determination of a potential function in molecular dynamics and to the simulation of deformation and stress fields of electrodes at the mesoscopic scale.In addition,the study of doping modification of electrode-collector interface is helpful to improve capacity retention and cycle performance of batteries,which reduces the waste of materials and time caused by tedious experimental attempts and provides a theoretical basis for the further optimization of mechanical properties of Sn anode materials. |
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