Theoretical Study On The Micro-mechanism Of Lithium Extraction From Brine By Electrodialysis With Carboxyl Modified Nanopores

Lithium-ion batteries are widely used in electronic equipment,new energy vehicles and other industries,and thus the exploitation of lithium resource is very important.At present,the technology of extracting lithium from lithium ore is relatively mature,but lithium resources mainly exist in salt lakes and seawater.The membrane separation method can continuously separate and purify Li⁺in brine with almost no pollution and has excellent application prospects.Since the physicochemical properties of Mg²⁺and Na⁺,which are abundant in brine solution,are very similar to Li⁺,it is challenging to purify Li⁺by membrane technology.The modification of nanopores can effectively improve or reduce the energy barrier of ion permeation,which is beneficial to improve ion selectivity and ion permeation efficiency.At present,the vast majority of researches were carried out on chloride-type salt lakes,but the reality is that most salt lakes in China are sulfate-type.And thus,it is necessary to explore the microscopic factors of the separation of Mg²⁺and Li⁺in sulfuric acid salt lakes.Molecular dynamics simulation can explore the ion association,ion hydration and migration properties of ions near nanopores,and then reveal the ion selectivity and microscopic mechanism of nanopores.Therefore,molecular dynamics simulations were adopted to investigate the effects of the size of functionalized graphene nanopores,the intensity of electric fields,ion concentration on the Mg/Li separation of various sulfate-type brines,and to gain insight into multiple mechanisms of Li⁺extraction,and also to explore new methods of extraction Li⁺from sea and the related mechanisms.This paper focuses on the effects of nano-pores and electric field,as well as the effects of ion association and dissociation near nano-pores on Li⁺selectivity.The results of this thesis are as follows:For the Mg²⁺/Li⁺separation of salute brines by nanopores,nanopores can promote ion association near nanopores under a strong electric field,and ions transporting through small nanopores are free from ion association.That is,dissociation of the associated species needs to occur before ions transporting through smaller nanopores.Both processes of ion association and dissociation result in nanopores playing an important role in regulating ion selectivity.The association between Mg²⁺/Li⁺and SO₄^2-,as well as the association between Mg²⁺and COO^-groups near the nanopore,can be adjusted by electric fields,thereby improving the selectivity of Li⁺.The strong association of Mg²⁺with nanopore COO^-groups renders the Li⁺selectivity(S_Li/Mg⁺²⁺=3.10,SD=1.04)of smaller nanopores（1.2 nm）for dilute solutions and weaker electric fields(0.6 V nm^-1).Under a strong electric field(1.4 V nm^-1),the association degree between Mg²⁺and SO₄^2-is higher than that between Li⁺and SO₄^2-,and most Li⁺ions are in a non-associative state,thereby resulting in the excellent Li⁺selectivity for 1.2 nm nanopore(S_Li/Mg⁺²⁺=3.61,SD=0.43)and high ion permeation efficiency.At a large pore size of 3.5 nm,the separation efficiency of Mg²⁺/Li⁺ions is also excellent(S_Li/Mg⁺²⁺=6.47,SD=0.58),which can be attributed to the reverse migration of negatively charged Mg²⁺ion clusters and the dissociation of Li⁺from ion associated species.For the Li⁺/Na⁺separation from simulated sea water by nanopores,the association degree between Na⁺/Li⁺and Cl^-can be significantly affected by adjusting the electric field strength,thereby affecting the migration characteristics of ions,resulting in Li⁺selectivity.An excellent selectivity of Li⁺(S_Li/Na⁺⁺=3.15,SD=0.24)of 1.6 nm can be achieved when the ion concentration C=1.5 mol L^-1under an electric field of 0.5 V nm^-1.Simulation results of high ion concentration solutions showed that high concentration and electric field can promote ion association in solutions,but do not lead to an obvious ion selectivity.Dynamics of ion association characteristics near nanopores showed that nanopores play an unique role in improving ion selectivity and ion flux,by promoting ion association near nanopores and the followed dissociation process.The transport of Na⁺is slowed down by the promoted the association of Na⁺-Cl^-near nanopores,whereas the transport of Li⁺is accelerated by the dissociation process near nanopores,and thus,resulting in high Li⁺selectivity and high ion flux of nanopores under certain conditions.The association degree of Na⁺-Cl^-is not obviously affected by the change of electric field,while the association degree of Li⁺-Cl^-increases with the increasing electric field.Therefore,an optimal Li⁺selectivity can only be obtained for a nanopore under a relatively weak electric field.Our results also showed that dehydration of ions may not be an important factor of ion selectivity for nanopores with the size of>1 nm.In addition,the results also showed that the ion selectivity of nanopores is not so sensitive to ion concentration,except for high concentrations.In this dissertation,the synergistic effect of nanopore and applied electric field in regulating lithium ion selectivity and multiple mechanisms of directly extracting lithium from brines were explored.Our results showed that strong electric field is suitable for the Mg²⁺/Li⁺separation in sulfate solution,while lower electric field is suitable for Li⁺/Na⁺separation in chloride solution.This work provided insight into the economical and efficient extraction of Li⁺from sulfate type salt lake and sea water.