Synthesis Of Porous Nanofibers Based On Ti-O Lithium Ion Sieves And Their Application In Lithium Ion Extraction

Lithium is rewarded that the metal is advancing the world and driving the development of science and technology.However,the supply does not seriously meet the demand for lithium.Lithium resources in salt lakes account for 2/3 of the world’s lithium reserves.Recovery of lithium from brine is drawing more and more attention.However,salt-lake brine contains a variety of metal ions,such as sodium（Na）,magnesium（Mg）,calcium（Ca）and potassium（K）.Highly selective separation of lithium from other metal ions in salt-lake brine is a great challenge.Here,a general strategy is developed to fabricate a series of porous titanium-oxygen based lithium ion sieves（LIS）nanofibers which were investigated for lithium recovery from brines.The details are summarized as follows:（1）The lithium ion sieves（LIS）have gained great interest in lithium recovery.Several spinel-type manganese-based LIS like lithium manganese oxides（LMOs）have been widely applied for Li⁺recovery.However,serious Mn loss for LMOs during acid treatment is still a problem.Compared with the manganese-type counterpart,Ti-O based ion sieves are more chemically stable due to the stronger Ti-O bond.Here,a simple strategy was developed to synthesis porous spinel Li₄Ti₅O₁₂nanofibers（P-LTO-NF）and porous H₄Ti₅O₁₂nanofibers（P-HTO-NF）.P-HTO-NF transformed from the electrospun porous Li₄Ti₅O₁₂nanofibers after acid treatment（H?Li）were systematically investigated in lithium recovery from brine water.P-HTO-NF possesses a superior adsorption capacity（59.1 mg/g）,which is nearly close to the theoretical value（63.77 mg/g）.The Freundlich isotherm model can well describe the adsorption isotherm data.The adsorption equilibrium can reach within 30 min（C₀=300 mg/L,p H=11,S/L=60 mg/60 m L）.The equilibrium distribution coefficient（K_d,m L/g）for Li⁺（232）is extremely higher than that for competing ions(1.41 for Na⁺,1.17 for K⁺,0.88for Mg²⁺,0.58 for Ca²⁺)（C₀=40 mg/L for Li⁺,p H=8）,indicative of a highly selective recovery of lithium from brine water.The LIS show excellent stability with a low Ti dissolution and the adsorption capacity for Li⁺remains 86.5%after 6 cycles.Moreover,P-HTO-NF could be readily regenerated and reused for Li⁺adsorption through an ion exchange mechanism.These results indicate that P-LTO-NF/P-HTO-NF is a promising lithium ion sieves for Li⁺adsorption from brines（Chapter 2）.（2）Generally,the specific functions of the adsorbent such as magnetic separation can be added by doping the adsorbent with other special substances.Based on the lattice vacancies existed in spinel structures of lithium titanium oxides(Li₄Ti₅O₁₂),metal ion or metal oxide could be doped into Li₄Ti₅O₁₂to prepare doped lithium ion-sieves adsorbents.Titanium-based lithium ion sieves from conventional solid phase synthesis generally form tiny particles which are easy to aggregate,resulting in a reduction in the adsorption capacity for Li.Thus,it is still challenging to synthesize Ti-O based LIS with high specific surface area and low mass transfer resistance.Porous materials are of great interest in the fields of catalysis,adsorption and host/guest supports owing to their high specific surface areas.Electrospinning is a simple and cost-effective strategy to prepare nanofibers for lithium storage.However,there is no report about electrospinning synthesis of lithium ion sieves.In this study,a series of spinel titanium oxides and doping titanium-oxides-type nanofibers with porous structure were fabricated by electrospinning and calcination technique,including Li₄Ti₅O₁₂（P-LTO-NF）,Li₄Ti₅O₁₂/Fe₂O₃（P-LFTO-NF）and Li₄Ti₅O₁₂/Zr O₂（P-LZTO-NF）.The microstructure of all products are porous nanofibers,which significantly accelerated the deintercalation and intercalation of Li⁺from and into the vacancies in the framework.All of them have high adsorption capacity and high selectivity for Li⁺as compared to other competitive ions.Li₄Ti₅O₁₂/Fe₂O₃can be magnetically separated and Li₄Ti₅O₁₂/Zr O₂has a more stable crystal structure than spinel Li₄Ti₅O₁₂.The developed synthesis strategy will advance such materials toward the high-performance applications such as lithium ion battery and lithium recovery（Chapter 3）.