Design,Preparation And Its Application For Lithium Ion Batteries With Three-dimensional Hierarchic Structures

To further improve the energy density and power density of lithium ion batteries,we introduce an optimized nano/micro hierarchical conducting network into the electrodes,which blocks up repeated nano porous units with a micro conducting network.The nano units guarantee negligible diffusion time and the micro conducting network provides faster electronic transfer,and hence boosts the electrochemical performance.On the basis of this concept,we have constructed three kinds of hierarchical porous conducting materials for lithium ion batteries:At first,by a solid-state reaction between cobalt acetate and melamine,we obtained a three dimensional nitrogen doped carbon nanotube–graphene hybrid structures with the aid of commercialized cobalt acetate and melamine as catalyst and carbon source,respectively.This hybrid structures not only show synergistic effects of carbon nanotube and graphene,but also can greatly shorten the diffusion lengths of lithium ions and store more active materials by the introduced mesopores with the nitrogen doping and carbon defects.What’s more,the metal catalyst is evenly enfolded in the carbon framework together with the nitrogen,which could strengthen the catalytic effect of the electrode.As cathode hosts for lithium–sulfur batteries,the hybrid structures deliver a reversible capacities of 800 mAh g-1 after 100 cycles at a current desity of 0.5C.Compared with traditional CVD process,our solid-state strategy is more efficient,operabler and economical.Secondly,we use cheap citrate and magnesium powder to successfully obtain a hierarchically porous graphene based aerogel by a solid-state magnesiothermic reduction,which replaced the time-consuming and expensive freeze-dry process.In this aerogel,the abundant macropores and mesopores are well interconnected with graphene layers,which enable rapid electron transfer and lithium ion diffusion.As cathode hosts for lithium–sulfur batteries,this hierarchical porous structure could hold a high sulfur loading(75 wt %)and uphold a capacity retention of 54% after 200 cycles at a current desity of 0.5C.At last,for the first time we employ an amorphous titanate based inorganic polymer and lithium salts to successfully synthesize a nitrogen doped carbon modified lithium titanate by a simple solid-state reaction.At the same time,the introduced conductive TiN remarkably improves the electronic conductivity and lithium ion diffusion coefficient.Compared with the conventional solid-state reaction,the capacity retention rate of our lithium titanate is pull high order of magnitude,and the reversible capacities retain more than double at a higher current density.