Preparation, Properties And Applications Of Lithium Manganese Oxides And Their Lithium Ion-sieves

Interest in lithium resources has been increasing because of its wide applications in rechargeable lithium batteries and other related fields. But the present lithium mineral reserves in the world cannot meet the requirement of lithium in the near future. Therefore, to develope appropriate technology to recover lithium from liquid lithium resources, such as salt lake brine and sea water, is of great significance. The treatment of spinel lithium manganese oxide with acid causes the removal of nearly all the Li+from its tetrahedral sites and leaves the manganese oxide called "lithium ion-sieve". Lithium ion-sieve can adsorb lithium selectively in solution, so it has been studied extensively as the best inorganic lithium adsorbent, and began to be applied in lithium recovery area. Although the lithium ion-sieve has been researched in recent decades, the study is not comprehensive, thorough and systematic. There also lacks the discussion on the relationship between the structure of lithium manganese oxide and the performance of its lithium ion-sieve. To this end, it is necessary to widely study the crystal structure of various lithium manganese oxides and adsorption performances of their corresponding lithium ion-sieves.Firstly, spinel lithium manganese oxide of LiMn2O4 was prepared by solid method and liquid method at different temperatures, respectively. XRD and SEM analysis showed that the lithium ion-sieve of?-MnO2 obtained by acid treating LiMn2O4 maintained the spinel structure and morphology. During the acid treatment, the Li and the Mn extraction ratios of the sample prepared by liquid method at 550?were 97.25% and 15.47%. The resultant lithium ion-sieve could adsorb lithium in HCl-LiCl-LiOH solution, and the adsorption capacity increased with the increase of pH with the maximum adsorption capacity of 23.75 mg-g-1. The adsorption behavior of this lithium ion-sieve was modeled and fitted for Langmuir isotherm equation.To reduce the Mn extraction ratio of LiMn2O4 during acid treatment, Ni2+, Al3+, Ti4+and Sb5+ were used to substitute the Mn in LiMn2O4. TG-DSC, TG-DTA, XRD, SEM, EDS and other techniques were employed to characterize the relative materials. Various synthetic methods and conditions were adopted to prepare different substituted lithium manganese oxides with their theoretical chemical formula of LiMzMn2-zO4 (M=Ni, Al, Ti, Sb; 0?z?1). Ni, Al and Ti were fully integrated into the spinel lattice when z?0.5. The incorporated Ni and Al induced the lattice contraction, while the incorporated Ti caused the lattice expansion. LiNi0.5Mn1.5O4, LiAl0.5Mn1.5O4 and LiTi0.5Mn1.5O4 maintained their spinel structure and morphology after acid treatment. During acid treatment, the metallic element extraction ratios were w(Li)=28.12%, w(Mn)=7.08% and w(Ni)=10.14% for LiNi0.5Mn1.5.O4; w(Li)=59.04%, w(Mn)=12.70% and w(Al)= 14.40% for LiAl0.5Mn1.5O4; w(Li)=74.71%, w(Mn)=29.58% and w(Ti)=19.49% for LiTi0.5Mn1.5O4, respectively. Only the lithium ion-sieve derived from LiAl0.5Mn1.5O4 had a relatively high adsorption capacity of 20.21 mg·g-1 in HCl-LiCl-LiOH solution and its adorption behavior complied with Langmuir adsorption model. Due to the remarkable lattice contraction or expansion, other substituted spinels showed low Li extraction ratios or their lithium ion-sieves showed adsorption capacities. It was difficult for Sb to enter in the spinel LiMn2O4 to form Sb-substituted lithium manganese oxide. When z=0.5, the sample formed a combined structure of LiMn2O4 (spinel) and LiSbO3 (perovskite) in which manganese and antimony ions diffused mutually into perovskite and spinel to form a composite. The Li, Mn and Sb extraction ratios of sample z0.5 were 80.55%,34.20% and 4.34%, respectively, during acid treatment. After acid treatment, the resultant lithium ion-sieve exhibited the structural stability and morphology integrity, but the general performance of this lithium ion-sieve was not good.A series of spinel lithium-rich manganese oxide with theoretical chemical formula of Li2O·rMnO2 (1.75?r?3.0) was synthesized by citric acid complex method. XRD, SEM, XPS and IR were used to characterize the relative materials. The sample of Li2O·2.25MnO2 (LMO) prepared at 350?presented spinel structure, high Li extraction ratio of 95.45% and low Mn extraction ratio of 6.01% during acid treatment. After acid treatment, LMO transformed to lithium ion-sieve of MO, which maintained spinel structure and morphology. The average valences of Mn in LMO and MO were 3.82 and 3.91, respectively. The transformation from LMO to MO is consistent with the Li+-H+ion exchange mechanism. In addition, the adsorption properties of MO in HCl-LiCl-LiOH solution, LiCl-NH3-H2O-NH4Cl buffer system and the salt lake brine were comprehensively studied. In LiCl-NH3·H2O-NH4Cl buffer system, the lithium adsorption of MO was fit for Langmuir model. The calculated?G?was negative, which means the adsorption process occurred spontaneously;?H?was 3.319 kJ-mol-1 and?S?was 11.70 J-mol-1·K-1. The adsorption process obeyed the pseudo-second-order kinetic model and was controlled by intraparticle diffusion, boundary layer diffusion, etc. Lithium ion-sieve MO showed selectivity for Li+in mixed buffer solution, and the affinity order was Na+<K+<Mg2+Li+. The adsorption mechanism of MO was also H+-Li+ ion exchange mechanism. MO can be recycled at least 6 times in the brine with the adsorption capacity of about 10mg·g-1.MO foam, a foam-type lithium adsorbent, was prepared by polyurethane template method with pitch as binder. DTG, XRD, SEM, TEM, EDS and N2 adsorption-desorption test were employed to characterize the composition, structure, morphology, pore structure and other properties of the relative materials. The MO foam with three-dimensionally interpenetrating network consisted of the lithium ion-sieve of MO and oxygen-containing cross-linked pitch. The bulk of the MO foam presented meso-/microporous structure. The adsorption capacities of MO foam in HCl-LiCl-LiOH solution, LiCl-NH3·H2O-NH4Cl buffer system and salt lake brine were 8.73,3.83 and 1.49 mg(Li+)·g-1(MO foam), respectively. The affinity order for MO foam in mixed buffer solution was Na+<K+<Mg2+<<Li+. The combination of the pitch support and the MO particles in MO foam became loose after Li+ adsorption.Finally, the ion exchange mechanism and the redox mechanism for lithium extraction/insertion process in solution were reorganized according to Mn3+content and Mn defects in spinel lithium manganese oxide. The "Determine by Mn" model was proposed. A comprehensive discussion on the lithium extraction/insertion mechanism of the lithium manganese oxides involved in this thesis was made. A "Determine by Mn" mechanism map was created to visually evaluate the extraction/insertion mechanism of all kinds of spinel lithium manganese oxides.