Preparation Of Silicon/carbon Composites Anode And Research On Capacity Loss Mechanisms For Silicon Containing Li-ion Full Cell Battery

The rapid development of electric vehicles and consumer electronics has proposed great challenges for lithium ion batteries with high energy density and high cyclability.The anode electrode materials with high energy-density,long-life,high safety and reliability are urgently needed to substitute carbon anode materials.Among them,silicon carbon composite materials have become one of the most promising anode materials for the next generation lithium ion batteries,due to the high capacity and excellent cycling performance.In the present work,a series of micron scale silicon-based composite materials with high performance are prepared by a simple method that can be scaled-up.The electrochemical lithium intercalation performances and the relationship between electrochemical lithium storage and their structure were systematically explored.In addition,the preparation technology,degrade mechanism of silicon-based full batteries,are studied,and the optimization methods to achieve higher battery performance are discussed in depth.Micron silicon/graphite/carbon composites are prepared by a simple method through ball milling carbon and commercial micron-sized silicon powder.The results show that the composites exhibit improved electrochemical performance,including higher capacity,first-cycle reversible capacity,and excellent cycling performance,which are due to the reduction of silicon particle size and the uniform dispersion of silicon in composite materials.The electrochemical performance and the vibratory density of the material are improved by optimizing particle size.The mixing silicon/graphite/carbon with commercial graphite can promote the composite performance and at the same time retain the advantage of graphite density,presenting good prospects for large-sccale application.Using the commercial micron-sized oxidic silicon as raw materials,we obtain the sub silicon carbon composite material with excellent electrochemical performance through a simple preparation method.The stabilized lithium metal powders?SLMP?pre-lithiated method,can solve the problem of low initial coulombic efficiency.The promotive initial efficiency attributed to the formation of SEI layer through lithium inserted into the composite materials during the SLMP pre-lithiation step.Correspondingly,the capacity retention is 95%after 200 cycles.The micro-sized silicon@carbon@graphene spherical composites?Si@C@RGO?have been prepared by an scalable spray drying approach.The obtained Si@C@RGO anode exhibits a high initial reversible specific capacity of1599 mAh·g^-1,at current density of 100 mA·g^-1,and the capacity can be retain94.9%of the original charge capacity at a higher current density of 200 mA·g^-1.Moreover,the Si@C@RGO anode shows a high reversible specific capacity(951mAh·g^-1)even at a high current density of 2000 mA·g^-1.The excellent cycling stability and superior rate capability are attributed to the unique structural design of carbon coating and highly conductive graphene wrapping.The carbon shells and flexible graphene can effectively enhance the electrical conductivity of the composite and buffer significant volume changes of silicon during cycling.The presented spray drying strategy can be scale-up for industrial production of Si-based composite,and also,it can be extended to the design of other promising micro-sized electrode materials.The silicon carbon composite materials are used to prepare full cell.Our studies show that the electrochemical performance of Si@C/LiCoO₂ full batteries is pretty good in the initial cycles.However,gradual decay is mainly attributed to the repeated formation and decomposition of SEI film.After 30 cycles,the capacity of the battery began to speed up.Poor cycle performance and obvious capacity decay are mainly due to the loss of activate lithium,continuous formation of SEI film on the surface.It is well known that the continuous thickening of SEI will lead to the internal resistance of the battery,as a result,high cell polarization,fast battery capacity decay.We found that the capacity is only 501 mAh and only 36.7%capacity retention after 100 cycles.Analysis of the full battery,anode and cathode with attenuation modes,shows that the cathode capacity include the loss of active lithium,the degradation of the structure and polarization of the cathode material.Anode attenuation modes include anode structure and barrier effect?hindering lithium-ion deembedding?,anode polarization and degradation of anode materials.The study shows that addition of FEC can inhibit the decomposition of the electrolyte,reduce the internal resistance and improve the cycle performance of the battery.The study on the evolution of silicon anode in the cycle of the full cell and the attenuation mechanism of the full batteries will contribute to the practical application of silicon-based anode materials.The improvement in electrolyte can improve the cycle performance of the full batteries,point out a promising direction to realize the industrialization of silicon anode electrode materials.