Theoretical Study On The Ion Selectivity And Mg²⁺/Li⁺ Separation Mechanism Of Functional Modified Graphene Nanochannel

Lithium-ion batteries are widely used in electric vehicles and electronic equipment,and are abundant in salt-lake brines and seawater.It is challenging to exclude coexisting magnesium ions from these water resources to extract Li⁺ions efficiently,because Li⁺has similar ionic property to Mg²⁺in aqueous phases since their sizes are nearly identical.The one-atom-thick and planar structure of graphene can minimize the resistance of ion transmission,and ensure high permeability and efficiency ion screening.Therefore,the use of graphene nanoporous membranes for high-efficiency Mg²⁺/Li⁺separation from salt-lake brines is feasible and promising.However,the separation mechanism is still unclear due to the complex microscopic factors that will affect the Mg²⁺/Li⁺membrane separation.Thus the efficiency method of Mg²⁺/Li⁺separation still needs to be researched,and the microscopic mechanism of ion selective transportation is essential to be further investigated.Based on molecular dynamics simulation and dynamics analyses,the functional modified graphene nanopore for the selective Mg²⁺/Li⁺separation has been studied in this work,and the micro-mechanism of the selective transportation of ions in confined spaces has been discussed in detail.The specific research contents are as follows:The efficiency separation of Mg²⁺/Li⁺by carboxylate graphene nanopore adsorbed with bivalent cations was investigated.The results show that carboxyl modified graphene nanopores can adsorb cations(Ca²⁺or Mg²⁺)stably,and these graphene nanopores exhibit higher preference to Li⁺than to Mg²⁺even when the pristine hole diameter is larger than 2 nm,with the Mg²⁺/Li⁺ratio down to 0.2-0.5.The Li⁺selectivity can be attributed to two aspects.On the one hand,the looser hydration layers of Li⁺make it be less blocked by the hydration layers of Ca²⁺on the edge of the nanopore,resulting in the larger available area in the nanopore for Li⁺to transport.On the other hand,the ion association of Mg²⁺or Li⁺with Cl^-can be enhanced near nanopores,the stronger ion association of Mg²⁺with Cl^-leads to a stronger permeating resistance for Mg²⁺.This work displays the possibility of Mg²⁺/Li⁺separation in large graphene nanopores,which can lower the cost of graphene perforation and also achieve high lithium-ion selectivity.The selective Mg²⁺/Li⁺separation of phosphonic acid modified graphene nanopores under an electric field was also investigated.The results show that ion hydration stability and the interaction of ions with functional groups are two main factors for Li⁺selectivity of the graphene nanopore at an electric field of a certain magnitude.Compared to Mg²⁺,the weaker hydration stability of Li⁺enables it pass through nanopore more easily by“contact association-dissociation”with phosphonic acid groups.As the pore size and the driving force of ion transportation increase,the influence of those two factors on ion transportation will be less effective,thus the Li⁺selectivity declines.The relatively faster migrate of Mg²⁺under an electric field and the more difficult penetration of Mg²⁺may lead to the block effect of nanopores,which could impede the conduction of Li⁺.By applying a proper electric field perpendicular to the direction of ion penetration(E_Y),the blockage effect can be eliminated.At the same time,applying a small driving force parallel to the transportation direction of ions to promote the ion penetration.The strong driving effect of E_Yand the relatively insufficient driving effect in Z direction on Mg²⁺can result in a extremely high Li⁺selectivity of the nanopore,the Mg²⁺/Li⁺ratio has reached 0.074.This work shows a new idea of efficient extraction of Li⁺by the rational utilization of functional groups and electric fields.This research provides effective methods for Mg²⁺/Li⁺separation by graphene nanoporous membrane,and reveals the effects of microscopic factors such as ion hydration,ion association,and functional groups,as well as electric field on ion migration in confined space.This work brings new method and inspiration to realize low-cost or high-efficiency ion screening of nanoporous membrane materials.