VOID FORMATION AND SURFACE ROUGHENING IN AMORPHOUS LI-ALLOY ELECTRODES
Lithium alloys with metallic or semi-metallic elements are attractive candidate materials for the next generation negative electrodes of lithium ion batteries due to their large specific and volumetric capacities.The key challenge with lithium alloy electrodes,especially Li/Si,has been their large volume changes during insertion and extraction of lithium.These volume changes can lead to pulverization and debonding.Recent efforts to improve the eye lability of Li-alioy based electrodes are highlighted by the utilization of nano-structured materials,including Si nanowires (NWs),crystallineamorphous Si core-sheil NWs,seaied Si nanotubes and nano-structured carbon/silicon composites.It is widely accepted that nano-sized materials provide better accommodation for diffusion-induced stress (DIS) and thus improve a batterys cyclability. The response of lithium alloy electrode materials is dominated by two-way coupling between stress development and diffusion,large inelastic flow and amorphization.In this work,diffusion-induced stress (DIS) development and stress-enhanced diffusion (SED) in amorphous lithium alloy nanowire battery electrodes are investigated using a finite deformation model,accounting for full two-way coupling between diffusion and stress evolution.Analytical solutions are derived using a perturbation method.The analyses reveal significant contributions to the driving force for diffusion by stress gradient,an effect much stronger than those seen in cathode lattices but so far has been neglected for alloy-based anodes.The contribution of stress to diffusion is small at low lithium concentrations,this lack of stress-enhanced diffusion (SED) leads to significantly higher DIS levels in early stages of a charging cycle.As lithium concentration increases,SED becomes more pronounced,leading to lower DIS levels.The long-term DIS level in the material scales with charging rate,nanowire radius,and the mobility of Li ions as modulated by the effect of stress.The solutions obtained provide guidance for lowering stresses during charging.In particular,lower charging rates should be used during the initial stages of charging cycles. The above analysis is based on the assumption that Si atoms are much more immobile than Li atoms.In reality,Si has covalent bonds that prevent free migration,while Li does not.At low Li concentrations,the diffusion of Si is highly insignificant compared with the diffusion of Li,because the diffusivity of the smaller Li ions is much higher than that of the larger Si atoms.At higher Li concentrations,the diffusion of Si may be significant.To investigate the effect of Li/Si interdiffusion,numerical analysis based on finite element method with mixed forms has been carried out.The results show significant contribution of stress gradient to the driving force for silicon migration in high-concentration regime,an effect that causes significant stress relaxation even when conditions are such that no inelastic deformation occurs.This effect has hitherto been neglected.The intensity of the interdiffusion-induced stress relaxation depends on the relative strength of the driving forces for diffusion due to stress-free chemical potential and mechanical stressing.Relaxation occurs when the mechanical driving force is dominant and stress intensification occurs when the cnemical driving force dominates.The time rate of stress relaxation or intensification,on the other hand,depends on the ratio between the diffusivities of silicon and lithium.Specifically,the larger the ratio,the faster the interdiffusive stress relaxation or intensification.Even at a low diffusivity ratio of 0.01,the diffusion of silicon is sufficient to produce a significant effect when the electrode is charged at a rate of 1℃.The results obtained suggest that interdiffusion and inelastic deformation are the two primary mechanisms for stress relaxation in Li/Si electrodes. Because interdiffusion and inelastic deformation are coupled processes,the failure mechanisms in nanosized lithium ion electrode materials can be distinctly different from those in larger electrodes.In particular,void formation and surface roughening have been observed 1 and found to affect the integrity and electric connectivity of the anode. Here, a model accounting for electromechanical coupling and elastoplasticity is developed and used to analyze relevant issues in Li/Si electrodes. The analysis of the growth of nanovoids is based on the balance of stress work rate and the rate of surface energy change. For anodes consisting of a carbon nanotube inner core and a Si outer shell, factors affecting the critical void size for unstable growth or spontaneous collapse are identified and quantified. The analysis of surface roughening focuses on the interplay between diffusion and surface kinetics under the constraining effect of a solid-electrolyte interphase (SEI), revealing the role of charge-discharge response asymmetry and providing an explanation for the more rapid development of roughness observed at very high Li concentrations.
Lithium ion batteries diffusion void surface roughening
Y.F.Gao M.Zhou
The George W.Woodruff School of Mechanical Engineering,Georgia Institute of Technology,Atlanta,GA 30332-0405,USA
国际会议
The Third International Conference on Heterogeneous Material Mechanics(第三届国际非均匀材料力学会议)
上海
英文
324-325
2011-05-22（万方平台首次上网日期，不代表论文的发表时间）