Optimization And Ion Separation Mechanism Of Scalable Electrochemically Switched Ion Film Electrode And Labyrinth Flow-type Separation Device

With the widespread application of lithium batteries in electric vehicles and portable electronic devices,market demands for lithium resources has remarkably increased in recent years.China has abundant reserves of lithium resources,in which about 80%of the total lithium reserves are stored in salt lake brines.However,the process of lithium extraction from salt lake brines presents a tremendous challenge because of the high Mg/Li ratios in brine lakes.The technologies for lithium ion extraction from salt lake brines mainly include adsorption,solvent extraction,precipitation,membrane separation and electrochemical methods.Among them,electrochemical method has become an ideal option due to its simple operation,high efficiency and less environmental impact.Electrochemically switched ion exchange（ESIX）,as a novel electrochemical ion separation technology,has recently attracted much attention in the field of lithium extraction from salt lake brines.The technology relies on the unique electrochemically switched ion exchange performance of electroactive functional materials,and can achieve selective uptake and release of target ions by regulating the redox state of electroactive materials deposited on the conductive substrate through an external electric driving force,which has the advantages of high selectivity,fast separation rate,good stability and environmental friendliness.The core of ESIX technology includes electroactive ion exchange material,scalable（i.e.industrial scale）electrochemically switched ion film electrode and ESIX separation device.Currently,in the field of lithium ion separation,spinel-type Li Mn₂O₄（or its de-lithiation productλ-Mn O₂）has received a lot of attention as an electroactive material with excellent selectivity for lithium ions.However,researches on scalable electronically switched ion film electrodes and ESIX separation devices are still in the initial stages.Scalable electrochemically switched ion film electrodes can be divided into two categories:unsupported and supported.Among them,unsupported film electrodes have high loading density and are not limited by the shape of the conductive substrate because they do not require the use of a conductive substrate.In this dissertation,a sodium alginate（Na-alg）binder was used to produce a tight bond between the electroactive lithium ion exchange material（λ-Mn O₂）and the conductive additive（r GO）,and an unsupportedλ-Mn O₂/r GO/Ca-alg film electrode was fabricated based on a vacuum freeze-drying technique coupled with a calcium chloride cross-linking strategy.However,it was found that the unsupported film electrode could result in galvanic corrosion at the solder joints and difficult to energize during the electrochemical test,which was not conducive to the later assembly of the device.Supported film electrodes have higher mechanical strength,better electrical conductivity,and more flexible in charging with the aid of a conductive substrate,which are convenient to assemble into separation devices.For the scaling up of the supported film electrodes,it is necessary to introduce adhesives to ensure sufficient adhesion between the electrode components.Polyvinylidene fluoride（PVDF）binder is widely used in supported film electrodes because of its good stability.However,the limited adhesion strength of PVDF could lead to the delamination or falling off of electroactive film from current collectors.Based on these considerations,a novel physically cross-linked electrochemically switched ion binder consisting of PVDF and functional binder polyacrylic acid（PAA）was developed in this dissertation for the fabrication of supported Li Mn₂O₄/C/PVDF-b-PAA film electrode.In addition,a labyrinth flow-type ESIX separation device was assembled using this supported film electrode,and the relevant parameters were optimized for efficient extraction and separation of lithium ions.The specific research contents and main conclusions are as follows:（1）The unsupportedλ-Mn O₂/r GO/Ca-alg film electrode was designed and fabricated by vacuum freeze-drying method,and the effective scaling up of the film electrode size was achieved via increasing of the spreading area of the electrode mixed solution.Based on the low ion diffusion resistance given by the three-dimensional（3D）porous network structure of the film electrode,it exhibited rapid lithium separation performance,and more than 90%of the equilibrium intercalation capacity was achieved in 60 min.When the electrochemical intercalation test was performed in simulated brine with high Mg/Li ratio（Mg/Li:～64.7）,the intercalation capacity of lithium ions on the film electrode was 15.0 mg·g^-1,and the separation factors of Li⁺/Mg²⁺reached 195.5.（2）The supported Li Mn₂O₄/C/PVDF-b-PAA film electrode was prepared on graphite plate by the coating method（Li Mn₂O₄ was directly used to prepare the film electrode in this study,because the acid treatment of Li Mn₂O₄ to synthesizeλ-Mn O₂would cause surface dissolution loss of the material）.Due to the excellent adhesive property of the PVDF-b-PAA electrochemically switched ion binder,the highly stable and scalable film electrode could be effectively fabricated.In addition,the ion-exchange reaction between the hydrogen protons on the carboxylic acid group（-COOH）in PVDF-b-PAA and lithium ions could promote the transfer of Li⁺ions at the electrode interface,which enhanced lithium extraction performance of the film electrode.For treating the actual brine（Mg/Li:～292.2）,the intercalation capacity of lithium ions on the film electrode was 18.3 mg·g^-1,the Mg/Li mass ratio in the recovery solution could decrease to 0.46,and the calculated Li⁺/Mg²⁺separation factor reached as high as631.08.（3）A labyrinth flow-type ESIX separation device was assembled by alternately and symmetrically stacking Li Mn₂O₄/C/PVDF-b-PAA and conductive carbon black/carbon fiber（C/CF）film electrodes,which was employed for selective extraction of lithium from simulated brine with high Mg/Li ratio（Mg/Li:～500）under dynamic conditions.Based on this bottom-up labyrinth flow field,increasing the number of cells in the device enabled more Li⁺ions to be captured in the simulated brine.Compared with the conventional driving mode of constant voltage,the collaborative driving mode of constant current-constant voltage could effectively improve the lithium extraction performance of the ESIX separation device.The extraction efficiency of Li⁺reached as high as 97.6%in 120 min with a coupled driving mode of 0.8 m A·cm^-2-1V.In addition,X-ray diffraction（XRD）test demonstrated that most of the adsorbed Mg²⁺ions could be removed by rinsing the surface of the film electrodes.