Research On Lithium Extraction From Seawater Based On Inorganic-Organic Composite Ion-Selective Membrane

Lithium and its compounds have a wide range of application in industry,especially in li-ion battery manufacture where there consumes over 7,000 tons of lithium a year.With the electrification process of society,many fields like electric vihicles,portable electronic devices and renewable energy storage will expand their demand for li-ion batteries,and the shortage of lithium resources will be more prominent.Although the average abundance of lithium in nature is relatively high,only a few ores(such as spodumene,lepidolite,etc.)and brines possess concentrated enough lithum for industrial mining.Moreover,most lithium deposits are located in a few countries such as Bolivia and Chile,which poses a huge threat to the global lithum supply.In response to the shortage of terrestirial lithium resources,some researchers suggest recovering lithium from waste lithium batteries.However,the proven lithium on terresitrial cannot meet the needs of renewable energy storage in the future.The total amount of lithium in ocean is 3,000 times more than that on land.If lithium can be extracted from ocean efficiently,the shortage of lithium resource will be completely resolved.Drawing on the process of extracting lithium form brines,researchers have developed solvent extraction,ion exchange and absorption,electrochemical intercalation,and various membrane separation techniques to enrich lithium from seawater.However,the yield and purity of lithium compounds extracted from seawater are limited due to the extremely low concentration of lithium ion(0.17 mg L-1)and the interference of other concentrated alkali and alkaline metal ions.In recent years,an electrolysis method based on seawater|lithium-selective ceramic membrane|organic electrolyte system has successfully extracted high-puirty lithium metal from seawater directly.Nevertheless,the high cost of noble metal catalyts for oxygen evolution reaction as well as thick and fragile ceramic separators constraint its practical application.The main porpose of this paper is to design and fabricate an inorganic-organic composite ion-selective membrane with flexibility and ultra-thin thickness,and to explore the feasibility of extracting lithium metal from seawater by electro-reduction method based on this flexible separator.This work developed a hot-pressing proccess which can be used to fabricate liion conducting membtrane consisting of a monolayered inorganic particles and polymer matrix,and Li1.5Al0.5Ge1.5(PO4)3-polyethylene membrane(LAGP-PE)with a thickness of about 55 ?m was successfully fabricated.LAGP particles in LAGPPE membrane would penetrate PE substrate during the hot pressing process.Benefit from the high li-ion conductivity and selectivity of LAGP,the penetrated LAGP particles could provide specific transmission channels for li-ions;PE is chemically stable and insoluble in common solvents at room temperature,and here it acted as a flexible base.Due to the heat shrinkage effect of PE,the interface of LAGP particles and PE matrix would retain a residual shrinkage stress,which ensured the tightness of this composite membrane.Finally,the LAGP-PE membrane achieved an ion conductivity of 5.5×10-5 S cm-1 and a selectivity of li-ion of 98.67% at room temperature,and could prevent water permeation for more than 120 h.Subsequently,a prototype composed of simulated seawater|LAGP-PE membrane|organic electrolyte was fabricated to extracte metallic lithium from the simulated seawater directly.XRD and XPS characterization confirmed that the deposit on cathode contained lithium metal and its compounds without sodium,which further proved the li-ion selectivity of LAGP-PE membrane.The lithium extraction device exhibited high coulombic efficiencies under constant currents from100 ?A cm-2 to 300 ?A cm-2,with the highest coulombic efficiency reaching 98.91%at 100 ?A cm-2.The energy consumption of this prototype was only 16.7 Wh g Li-1 at a current density of 100 ?A cm-2,and a photovoltaic solar cells with the same aera of LAGP-PE membrane was sufficienct to provide power for this device even if the lithium extraction current was increased to 300 ?A cm-2.Then,a flexible lithium extraction prototype was fabricated and sucessfully operated in simulated seawater,which further demostrated the advantage using LAGP-PE membrane.Finally,to avoid the usage of noble metal oxygen evolution catalyst,a thermal decomposition method was proposed to synthesis iron-doped nickel(Ni Fe/NC)and iron-doped cobalt(Co Fe/NC)alloy catalyst on nitrogen-doped carbon substitute.LSV results showed that doping iron into nickel and cobalt could reduce the polarization of simulated-seawater electrolysis.CV test showed that the electrochemically active specific surface area of the catalysts be doubled by iron doping,and the electrochemical impedance test implied that the kinetics of charge transport between catalysts and electrolyte could be improved.Unlike Electrolysis of freshwater,there is competition between chlorine evolution reaction and oxygen evolution reaction in seawater so lower overpotential can improve the selectivity of the oxygen evolution reaction,thereby increasing the calendar life of catalyst.Ni Fe/NC and Co Fe/NC could work stably for 60 h and 32 h at a current density of2 m A cm-2,resepectively,with the lifespan increased by 161% and 113% compared to the undoped ones.This work provides a simple technique to produce nomo-layerd inorganic particles-polymer composite membrane,and proves the feasibility of extracting lithium metal from seawater by electro-reduction method based on inorganic-organic composite ion-selective membrane quickly and efficiently.These results have certain enlightenment for the practical application of flexible devices for seawater lithium extraction.