The DFT Studies Of Li/Na-graphene For The Application In Li-S Batteries And Na-ion Batteries

Lithium-ion batteries with superior performances,being widely used in portable devices,have been one of the cores of modern technologies and life.Now its range of applications is expected to extend to other high-tech areas,such as electric vehicles and aerospace.However,current lithium-ion batteries have failed to meet the needs of the market.People expect lithium-ion batteries have higher performances,such as energy power,energy density,and charge/discharge rate.Currently,lithium-sulfur batteries have attracted many researchers because of the large specific capacity of sulfur(1673 mAhg^-1)as the cathode.In addition,sulfur is rich,inexpensive,and environmentally friendly,though it also has some obvious shortcomings including the low specific conductivity of S₈(5x10^-30 S cm^-1 at 25?),and high dissolution rate of the short-chain polysulfides,which can make it easily dissolve in the electrolyte,resulting in the loss of active sulfur.These problems limit the cycle life of lithium-sulfur batteries.In order to prevent the dissolution of lithium-sulfur compounds,the current experimental researches mainly focus on carbon nano-composite materials as the cover layer of sulfur cathode,such as microporous and mesoporous carbon materials,graphene,graphene oxide,CNT,carbon fiber,hollow mesoporous phases,and so on.Though hard work has been done,the understanding of detailed micro-mechanism of the interaction between Li-S and carbon structures in carbon-sulfur composites still is limited.Since lithium is not abundant on earth,the availability of it has been limited.With the marketization of lithium-ion batteries,the price of raw material has increased,which leads to the high cost.Recently,sodium-ion battery has been regarded as a promising substitute of lithium-ion battery,since it have a similar principle with lithium-ion battery and sodium is rich in our earth.However,many researchers find that it is difficult to find the proper anode materials,because the radius of sodium ion?⁹8-102pm?is much larger,compared to that of lithium?76 pm?.Graphite,a stable nested layer compound in which Li ions can intercalate easily,is an excellent anode material of lithium-ion batteries.But for the Na ions,the most stable structure with the intercalation of Na ions is NaC₆₄,leading to the low electric capacity of³5mAh/g.At present,different layered carbon,such as hard carbon,soft carbon,expandable graphite and some other materials is being test in experiments.We look forward to find a new anode material which can absorb sodium ions effectively with high concentration.In this paper,in order to solve the two questions above,we studied?1?the interaction of lithium-sulfur clusters with graphene and defective graphene,?2?the interaction between sodium ions/clusters and graphene and the storage of sodium ions in defective graphene.The detailed introductions are as follows,1.With first-principles calculations under the existence of sulfur,we studied the interaction between lithium-sulfur clusters and graphene,and the adsorption of lithium clusters on defective graphene.We find that with the introduction of lithium ions,the interaction between sulfur and graphene is weakened,leading to the formation of lithium-sulfur clusters,which interact with graphene by charge transfer.We find that lithium ion can diffuse through double-vacancy graphene.However,sulfur can limit the diffusion of lithium.This is because of the formation of lithium-sulfur clusters near double-vacancy.With the increase of Li concentration near double-vacancy,lithium ions become to diffuse easily through double-vacancy graphene.In addition,with the analyzation of electronic properties,defective graphene with the adsorption of Li-S chains has good conductivity.2.With first-principles methods including van der Waals force,we analyzed the absorption of sodium ion and its clusters on graphene.It is found that the interaction between sodium and graphene is so weak that sodium ion and its cluster are difficult to absorb on graphene with high concentration.However,with double-vacancy on graphene,sodium ion and its cluster can easily absorb on the graphene.The sodium clusters are adsorbed near double-vacancy.With the increase of the cluster's size,the adsorption becomes to be weakened.In addition,we find that with the increase of the density of double-vacancy,sodium capacity is enhanced significantly.Under the high density of defects,the content of sodium on defective graphene is 10-30 times more than graphite.