Study On Doping Modification And Adsorption Performance Of Manganese-based Lithium Ion Adsorbent

This work focused on the comprehensive development and utilization of liquid lithium resources.We study both the adsorption in solution contain Li⁺and desorption behavior in hydrochloric acid solution by taking the spinel manganese lithium ion sieve(H_1.6Mn_1.6O₄)as the adsorbent with high adsorption selectivity and adsorption capacity,and using the doping modification method to reduce the Mn dissolution loss in the process of recycling.We have also explored the doping effect on the properties of adsorption and anti-dissolution mechanism by STEM and DFT theoretical calculation.This research work provides the theoretical basis for the exploitation of lithium resources in salt lakes.（1）The metal cation doped lithium manganese oxides precursor（LMO-R,R=Na,K,Mg,Ca,Fe,Co,Ni,Al）was prepared via combining hydrothermal and solid phase method and the lithium ion sieve HMO-R（R=Na,K,Mg,Ca,Fe,Co,Ni,Al）was obtained by acid leaching.The SEM morphology shown that doping and acid leaching have little effect on the morphology of samples.The precursor,adsorbent and samples after adsorption of Li⁺all maintained spinel structure（XRD）,indicating that the spinel structure was stable in acidic environment,which was conducive to the Li⁺adsorption and adsorbent recycling.FT-IR and XPS revealed that the contents of Li-O,-OH groups and Mn³⁺was changed after acid leaching,indicating the presence of Li⁺/H⁺ion exchange process and Mn³⁺dissolution loss during acid treatment.The results of adsorption and anti-dissolution loss imply that the doping of Na,Co and Al could both improved the adsorption capacity and reduce the dissolution loss of Mn.It was confirmed that Na replaced Li at the 16d position（Li⁺in this part was difficult to carry out ion exchange,mainly playing the role of stabilizing the skeleton）,and Al replaced Mn at the 16d position by STEM and DFT theoretical calculation.Since the bond energy of Na-O is stronger than that of Li-O bond,the structure stability is enhanced,and the dissolution loss of Mn is reduced.Al replaces Mn at the position of16d,and the bond energy of Al-O bond is stronger than Mn-O bond,which enhances the structural stability AND reduces the dissolution loss of Mn,which inhibits the irreversible capacity reduced caused by Jahn-Teller effect.Compare to LMO,the band gap of LMO-Na and LMO-Al was reduced showing the adsorption rate of Li⁺was increased by Na and Al doped.（2）F and S were used to replace the O position of Li_1.6Mn_1.6O₄.It was found that both the LMO-R（R=F,S）and HMO-R（R=F,S）were granular and spinel structure by SEM and XRD analysis.FT-IR and XPS analysis showed that F and S doping had little effect on it.F and S doping could improve the adsorption capacity of Li⁺,but had little effect on reducing the dissolution loss of Mn by a series of adsorption and anti-dissolution experiments.The O at the position of 32e on the surface was replaced by F and the band gap of the material was not reduced after F doping by DFT theory calculation,indicating that the adsorption rate of Li⁺was not increased by F doping.（3）The Li_1.6Mn_1.6O₄ of co-doped by cations（Na、Al）and anions（F、S）and the corresponding co-doped adsorbents were obtained by acid treatment.All the samples（LMO,HMO,r-LMO）were granular and spinel structure by SEM and XRD,which was help to the adsorption and recycling of Li⁺.The dissolution loss of Mn was reduced by co-doping compared to anion doping alone while the adsorption capacity was improved compared to the cationic doping alone.Both the adsorption capacity and structure stability were enhanced by co-doping compared to the undoped adsorbent.The co-doped adsorbents also have a good selectivity and cyclic stability.Metal cation doping can reduce the Mn dissolution loss,while anion doping can improve Li⁺adsorption capacity.Anion and cation co-doping combines the advantages of cation and anion doping.