Molecular Simulation Of The Interaction Between Crack And Dopants In Silicon

During the charging/discharging cycle of silicon-based metal-ion batteries,cracks in the anode due to the insertion of active metal-ions will cause electrode failure,thus significantly impeding the development of metal-ion batteries.In this paper,the multi-scale coupled molecular simulation method is used to study the interaction between cracks and metal-ions and explains how cracks affect the storage,distribution,diffusion of metal-ions(lithium,sodium,magnesium)in the silicon anode at the atomic scale.It aims to provide practical theoretical support for further research on cracks in the silicon anode of metal-ion batteries.Specifically,the stable position,binding energy,dynamic properties of lithium,sodium,magnesium in the prefabricated cracks at the crack tip and surrounding area are studied.Compared with defect-free crystalline silicon,the crack tip area provides more space to accommodate lithium,sodium,magnesium.The crack tip provides the largest space due to stress concentration and the binding energy of this location is also the largest.The closer the other positions are to the crack tip,the greater the binding energy is and the more stable the conductive metal-ions can exist.Namely,there is a binding energy concentration consistent with the stress concentration and a binding energy gradient centered on the crack tip.The cracks can not only make lithium more stably stored in the silicon anode,but also change the storage properties of sodium and magnesium.Sodium and magnesium can not exist stably in silicon crystals.However,the appearance of cracks increases their binding energy from negative to positive,which makes them exist stably at the fracture tip and inside of the cracks,making it possible for silicon to be used as a negative electrode material for sodium-ion and magnesium-ion batteries.Additionally,cracks provide a fast path for metal-ions to diffuse to the crack tip along the crack propagation direction so that lithium,sodium and magnesium tend to diffuse from the nearby area to the crack tip both in dynamics and thermodynamics,and it is difficult for metal-ions to diffuse out in the opposite direction.In the loading direction perpendicular to the crack surface,there is no difference in the dynamics of lithium diffusing into the crack area or continuously moving in the defect-free area.And sodium and magnesium tend to move in the defect-free area in this direction.But as long as the metal-ions have entered the crack tip,it is difficult for the metal-ions to diffuse out of the crack tip because of the bigger energy barrier.As a result,lithium,sodium and magnesium will accumulate at the crack tip and be trapped in it.Eventually,the appearance of cracks reduces the number of active metal-ions participating in the battery cycle,thus hindering the charging/discharging of metal-ion batteries.Moreover,during the diffusion process,sodium and magnesium are also accompanied by reversible changes in the structure of the matrix silicon atoms.In addition,the accumulation of lithium,sodium,and magnesium at the crack tip can accelerate the amorphization of the silicon negative electrode,thereby reducing the fracture toughness of the crack and the driving force for diffusion.