Theoretical Investigation On Performance Of Graphyne As Ion Battery Anode

The sharp decline of fossil fuel has attracted attention to the renewable energy sources,because they can not only alleviate environmental pollution but also reduce the dependence on fossil fuels.Affected by season and climate,the supply of various renewable energy sources is unstable that makes efficient storage and conversion devices be highly desired.In this regard,ion batteries turn out to be the most promising energy storage and conversion equipment at current stage owing to high energy density,limited self-discharging and low maintenance costs.Anode materials is an important part of lithium-ion batteries（LIBs）and sodium-ion batteries（SIBs）,which mainly determines the barriers capacities.Although having larger specific capacity than other anode materials,the lithium and sodium metal are difficult to apply as anodes because of safety concerns and poor rechargeability.At present,carbon group materials,like silicon,germanium,and tin,are potential candidates because of their high specific capacity.Nevertheless,the massive volume changes during lithiation and delithiation causes their cycling performance degradation.Graphite,carbon nanotubes and graphene although can exhibit high stability,their development is restricted by their small specific capacities.Graphyne（Gy）is one type of carbon material that is composed of sp-and sp²-hybridized carbon atoms which has large conjugated system,large specific surface area and excellent chemical stability.Gy can provide potential space for Li/Na adsorption.When two-dimensional Gy is seamlessly rolled into graphyne nanotubes（GyNTs）,GyNTs has both structural characteristics of Gy and adjustable curvature.It has been confirmed in previous reports that carbon nanotubes have stronger binding affinities to Li atoms than zero-curvature graphene which shows that GyNTs has the potential to become ion-batteries anode.Most of the researches are onγ-Graphyne（γGy）,and there are few reports on other Gy and GyNTs.Therefore,this thesis adopts the Density Functional Theory（DFT）method to systematically explore the related properties and mechanisms ofα-Graphyne（αGy）,β-Graphyne（βGy）,γ-Graphyne（γGy）and their seamlessly rolled toα-Graphyne nanotube（αGyNTs）,β-Graphyne nanotube（βGyNTs）,γ-Graphyne nanotube（γGyNTs）as LIBs/SIBs anode materials,and provide theoretical support for experimental workers.In the first chapter,the background of this thesis form working principle,development process,types of anodes materials and the current development status of LIBs and SIBs.Additionally,the purpose and significance were expounded.In the second chapter,the theoretical foundation,calculation software,calculation methods and model construction are introduced in detail.In the third chapter,the performance ofαGy,βGy,γGy,αGyNTs,βGyNTs,γGyNTs as LIBs anode materials were mainly explored.The results show that graphyne materials can provide more active sites for Li,and make the maximum lithium storage specific capacities much higher than other carbon materials.The maximum specific capacities ofαGy,βGy,γGy,αGyNTs,βGyNTs,γGyNTs are 698,807,744,3488,3069,2232 m Ah·g^-1,respectively.Moreover,Gy and GyNTs can provide a low energy barrier for Li diffusion via a new channel.The maximum lithium storage specific capacity of GyNTs is 3～5 times of the graphene’s theoretical specific capacity.Especially,the lithium storage maximum specific capacities are forαGyNTs andβGyNTs close to that of lithium metal anode(3860 m Ah·g^-1).In the fourth chapter,the performance ofαGy,βGy,αGyNTs andβGyNTs as SIBs anode materials were investigated.Previous studies have demonstrated that when most materials such as hard carbon and graphene used as SIBs anode,there are still suffer from the low capacity and poor cycling stability.Graphyne with large conjugated structure and specific surface area can effectively solve the problem of difficulty in intercalation/de-intercalation of Na.We calculated the Na storage maximum specific capacities ofαGy,βGy,αGyNTs,βGyNTs were558,372,1544,1302 m Ah·g^-1,respectively.The sodium storage capacity of the above materials is lower than that of lithium storage,which is also caused by low binding energy of Na adsorption on graphyne materials.Gy and GyNTs can also provide a lower migration barrier for the diffusion of Na and they exhibit the good rate performance.In comparsion,the GyNTs show better performance of SIBs anode materials than Gy.The maximum storage Na specific capacities of GyNTs exceed that of carbon nanotubes and Graphdiyne,which prove GyNTs are excellent SIBs anode materials.In the fifth chapter,this thesis has been summarized and proposes future work.In summary,by performing density functional theory calculations,we investigated the performance of graphyne materials as LIBs/SIBs anode.Our results revealed that graphyne materials are excellent LIBs/SIBs anode materials with considerable specific capacity,excellent structural stability and good rate performance.The curvature effect of GyNTs not only show higher specific capacities than two-dimensional Gy,but also further promote the diffusion and migration of Li/Na.Moreover,the good structural stability of GyNTs has improved the cyclability.Thus,we believe GyNTs are better ion-batteries anode materials and our results suggest a new strategy to design novel high-capacity ion-batteries anodes.