First-Principles Calculations Of The Cathode Material Of Aluminum-Ion Batteries

To improve energy efficiency and reduce environmental pollution,the develop-ment of electric vehicle and smart grid becomes an urgent need for human life.How-ever,the energy density,charging efficiency,and cost of current commercial lithium ion batteries(LIB)are insufficient to meet the great need created by the rapid developments of electric vehicles and smart grids.Developing new rechargeable battery systems has become a hot research field.In recent years,based on dual ion battery mechanism and the development of ionic liquid electrolytes,new aluminum/graphite cells have been proposed and investigated.Compared to previous aluminum ion batteries(AIBs),this system has achieved a huge breakthrough in cycle performance,voltage,and charging rate,and hence attracted extensive attention.Unfortunately,the usability of this AIB cell is limited by its energy density.For dual ion battery systems,the mass of the electrolyte needs to be taken into account when estimating the energy density.Furthermore,even if only considering the graphite cathode,the specific capacity does not exceed 150 mAh/g,because the AlC14 molecules occupy a large volume in graphite during charging.Studying the intercalation mechanism of AlCl4 in graphite will help to understand the intercalation process and to find strategies to increase the specific capacity in the graphite cathode.In addition,this study also has implications for graphite intercalation compounds(GICs)used as electrodes in rechargeable batteries in general.Studying the diffusion mechanism in the graphite cathode of aluminum ion batteries can not only help us un-derstand the rapid charging property,but also provide a reference for improving the charging rate in other batteries adopting graphite electrode.This thesis contains the following three parts of research work:1.The intercalation mechanism of graphite cathodes in AIBs.We identified the insertion morphology of AlCl4 in graphite through structural op-timization;the obtained structure is consistent with the experimental XRD results.The upper limit of theoretical specific capacity in this graphite cathode system is obtained by simulating the structural changes when different amounts of AlCl4 are intercalated into graphite.In addition,we also calculated the formation energy of AlCl4 GICs and ob-tained the theoretical voltage of this battery system in different specific capacity ranges.We found that the results of formation energy and therefore the theoretical voltage de-pend on the van der Waals correction schemes adopted in density functional calcula-tions.2.The diffusion mechanism of AlCl4 GICs.We obtained the diffusion coefficient of AlCl4 moving in graphite through ab initio molecular dynamics simulation and found that the relative motion between graphite layers cannot be ignored.Compared to molecular dynamics simulation restraining the movement of all C atoms,we found that there is a cooperative migration mechanism between AlCl4 and graphene layers.The diffusion of AlCl4 is not only affected by the energy barrier,but also promoted by the movement of the graphite layers.However,in long-time MD simulations,the interaction and correlation between AlCl4 and graphene layers is unstable.As a result,the weakened cooperative migration effect will cause AlCl4 to stagnate.Through semiempirical molecular dynamics up to nanoseconds,we found that the stagnation of AlCl4,which is caused by the weakening of correlation with the graphene layers,is reproducible on both time and space scales.And the frequecy of stagnation is more often when we present a stronger strain in GICs.3.Comparison of AlF4 graphite intercalation compounds.We found the AlF4 GICs support higher theoretical specific capacity and voltage compared to AlCl4 intercalation compounds.We also found that AlF4 has a higher dif-fusion coefficient in graphite than AlC14.Therefore,we proposed a theoretical protocol to replace Cl with F in aluminum/graphite cell.This element substitution can increase the energy density of aluminum ion batteries and reduce the volume expansion during charging,while maintaining the support of ultrafast charging rate in cathode.