Preparation And Lithium Uptake Properties Of Lithium Ion Sieves Derived From Spinel-type Manganese Oxide

The spinel-type manganese oxides(SMO) show the high selectivity to lithium ions in solution and can recover lithium selectively from solutions,so it is named as "Lithium Ion Sieve(LIS)" which can be used in lithium uptake or reclamation from brine,sweater and waste lithium batteries.The SMO-type lithium ion sieve is one of the most promising lithium adsorbents due to its high selectivity and adsorption capacity to Li⁺.An element of manganese has several oxidation states,therefore,how to control the reaction conditions to prepare lithium ion sieve with stable structure and good uptake performance is still the main problem of LIS synthesis.Secondly,investigation of the effect of various adsorption conditions on performance of lithium uptake by LIS and study of kinetics and thermodynamics of lithium uptake can provide the process parameters of lithium recovery and the theoretical description of Li⁺ extraction/insertion of LIS.It can provide useful information to guide the application in the field and promote the development of lithium recovery by LIS in industry.In this paper,the SMO-type lithium ion sieve was the main research object,the manganates and lithium salts were taken as the main raw materials.Two LISs with high uptake capacity,better selectivity and stable structure were successfully prepared by hydrothermal method and coprecipitation method based on the discussion of correlation between Li⁺ uptake performance and the preparation conditions of LIS precursors.The effects of preparation conditions on Li⁺ uptake were also investigated.Furthermore,the uptake performance with the prepared ion sieves were studied in depth,including the influences of solution pH,effect of lithium ion concentration,temperature,adsorption time and ionic strength on Li⁺ uptake.The Li⁺ uptake equilibrium,kinetics and thermodynamics were studied under different conditions,with the buffer system and without buffer system,and models were used to describe the experimental isotherms and kinetics.At last,the "Vacancy Effect" was presented to illuminate the lithium extraction/insertion mechanism of LIS,and some problems that exist in the process of lithium extraction/insertion were given helpful explanation basing on the analysis of XRD,XPS and calculated results.The main results were summarized as follows:First,the Li/Mn mole ratio of LIS precursor prepared with solid state method increased with the increase of Li/Mn mole ratio of raw material between 0.5～1,and all the Li-extracted materials also showed an ascending trend of lithium uptake with the increase of Li/Mn mole ration of reagent.The LIS synthesized with LiOH usually have higher Li⁺ uptake capacity than the one with Li₂CO₃.The LIS particles grow larger and the Mn₂O₃ impurities could be the primary reason for the decrease of Li⁺ uptake.The MnO₂·0.5H₂O LIS with higher uptake capacity can be prepared by hydrothermal technique.Similar material can also be obtained by coprecipitation technique,and it has smaller LIS particle size(nano-scale),larger specific surface area and higher H content,so it show the higher Li⁺ uptake capacity and better recycle performance even if the sample crystallinity is slightly inferior to LIS prepared by hydrothermal method.The Li⁺ uptake capacities of both ion sieves reached 34 mg·g^-1 which showed the excellent uptake performance,and the dissolving rates of both ion sieves were all less than 5%,which showed the good structure stability.Second,the results of solution pH decreasing in non-buffer system proved that the Li⁺-H⁺ ion-exchange played a major role in Li⁺ uptake by present SMO-type lithium ion sieves.Due to the decrease of solution pH,Li⁺ uptake cannot proceed completely.Moreover, it should be noted that the practical liquid lithium resources usually had weak basic or neutral buffer capacity,so it was very important to study Li⁺ uptake by ion sieve in weak basic buffer system.The results of Li⁺ uptake in pH 8.0 buffer solution revealed that the equilibrium can be reached in 24 hours and the equilibrium process can be well described by the Langmuir model.The processes of Li⁺ uptake by present SMO-type ion sieve were spontaneous,entropy increase and endothermic.The kinetic process obeyed pseudo-second-order kinetics model. The ionic strength only had a slight influence on Li⁺ uptake.The results of diffusion study indicated that both boundary layer and intraparticle diffusion may together control the uptake process,while the extent of the rate-controlled step was closely related with the crystal structure of ion sieves.At last,the results were integrated based on the discussions about the structure of LIS, precursor,relithiated LIS and the process of Li⁺-H⁺ exchange,respectively.Especially, through the discussion of the exchangeable H in LIS structure and the structure change of LIS in Li⁺ extraction/insertion,the "vacancy effect" was firstly proposed to interpret the process of Li⁺ extraction or insertion in SMO-type ion sieves.