First Principle Studies Onadsorption And Diffusion Behavior Of Li On Graphene, BC_x And C_xN Surfaces

Lithium-ion battery is not only used in various types of portable electronic devices,but also the potential selection of electric vehicles and new energy storage in power.Numerous experimental and theoretical calculations show that boron, nitrogen-dopedgraphene nanomaterials as lithium battery anode material has a high specific capacity, andexcellent charge and discharge characteristics. According to reports and research scholarsfound that boron-carbon compounds in different proportion show different lithium storagecapacity. Electrochemical character and the carrying capacity of lithium-ion of graphenewill be significantly affected because of the different contents of nitrogen atoms.In order to explain the reason of boron-carbon compound with high capacity oflithium storage and nitrogen-carbon compounds have good charge-discharge cyclecharacteristics. The adsorption and diffusion behavior of Li particle on pristine graphene,BCxand CxN surfaces have been systematically calculated through the first principlemethod. Mainly, we carried out the following research work: Firstly, the adsorption anddiffusion behavior of Li particle on pristine graphene in different size have been studied.Then the adsorption and diffusion behavior of Li particle on derivatives system ofboron, nitrogen-doped graphene (C3N, C7N, C5N and BC3, BC7, BC5) surfaces have beencalculated. After calculations the main conclusions are as follows:（1）Single Li particle adsorbed on graphene of coverage for1/30ML and higher than1/30ML can be stable. The diffusion energy barrier of Li on4×4and（33×33）R30°graphene is0.336eV and0.344eV, respectively.（2）The adsorption energy of Li on the most stable adsorption sites of BC3、BC7、BC5surfaces is3.086eV,3.564eV,3.756eV（3.706eV）, respectively. And thecorresponding adsorption energy on C3N、C7N、C5N surfaces is0.978eV,1.283eV,1.070eV（1.022eV）. Compared with pristine graphene, boron content for25at.%，12.5at.%，16.67at.%are electron-deficien system that significantly enhances the adsorption energyof Li, whereas nitrogen content for25at.%,12.5at.%,16.67at.%are electron-rich systemthat the adsorption energy of Li is slightly weakened. The sequence of Li-binding capacityof different compound with different boron (nitrogen)-carbon ratio is in BC5>BC7>BC3 and C7N>C5N>C3N order.（3）The diffusion energy barrier of Li on BC3surface forward and reversealong the most likely path are0.336eV and0.344eV, respectively. which are only0.191eV and0.348eV when Li on C3N surface. The results revealed that compared to pristinegraphene, boron content for25at.%weakened the diffusion of Li, nitrogen content for25at.%promoted the diffusion of Li.（4）The diffusion energy barrier of Li on BC7surface forward and reversealong the most likely path are0.375eV,0.358eV,0.318eV and0.385eV, respectively.which are only0.053eV,0.189eV,0.121eV and0.248eV when Li on C7N surface. Theresults revealed that boron content for12.5at.%weakened the diffusion of Li, nitrogencontent for12.5at.%promoted the diffusion of Li.（5）The diffusion energy barrier of Li on BC5surface forward and reversealong the most likely path is0.345eV. It is only0.074eV when Li on C5N surface. Theresults revealed that migratory aptitude of Li on graphene which boron content for16.67and the pristine graphene are substantially the same, nitrogen content for16.67at.%promoted the diffusion of Li.The adsorption energy of Li on BC3, BC7, BC5surfaces are much higher than thepristine graphene surface, it provide the necessary conditions for adsorb more Li atoms,which also explains why these compounds has a higher Lithium storage capacity.Compared to BC7and BC5, adsorption energy of Li on BC3surface is the lowest, that isnot to say the more boron content the higher lithium storage capacity, there is a suitablecontent of boron makes the Lithium storage capacity reached the maximum. such as BC5compound. For the single C3N, C7N, C5N compounds, the higher nitrogen content the lessstable Li adsorbed. And it is disadvantageous to store more Li atoms. The diffusion energybarrier of Li on C3N, C7N, C5N surfaces are much lower than the pristine graphene surface.This makes diffusion easier. It is also explains why the nitrogen-doped graphene has agood charge-discharge cycle characteristics.