Study On The Hydrothermal Synthesis Of Spinel-type Lithium Ion-sieve And Ni-Cr Doping

The spinel-type LiMn2O4 was directly synthesized via the hydrothermal synthesis with the raw materials of Mn(NO3)2, LiOH and H2O2. The ion-sieve with the selectivity to lithium ion was prepared further by extracting lithium ions from the Li-Mn-O lattice of the precursor with the acid. The characterizations of the precursors were carried out by XRD and TEM and the ion-exchange properties were measured; the precursor doped by nickel and chromium were synthesized.Spinel LiMn2O4 samples were prepared at different mole ratios of Li and Mn, different concentrations of H2O2, different heating temperature and reaction time via hydrothermal synthesis in order to study the effects of reaction conditions on the crystal constructures systematically. The results show that the precursor is almost pure cubic crystals when the mole ratio of Li and Mn is 2.5, the H2O2 concentration is 0.3 mol·L-1 and the reaction time is 12 hours at 120℃; the precursor crystal is one-dimensional line shape with a diameter of 20 run, between 300-500nm in length.The only 0.407% Li+ is remained in the sample and its crystal constructure has no change after treated with acide, indicating that the Mn-O framework of Li-Mn-O is not destroyed. So the sample meets the need of the structural stability of ion-sieve materials.By studying on the Li+ uptakes of ion-sieve in different solution pH, it was found that the increase of solution pH is favorable to the uptake of ion-sieve to Li+, especially in the higher value of solution pH, such as in 10-12; the adsorption capacity of ion-sieve reaches the maximum of 40.23mg·g-1 when the pure Li solution pH is 12; however, the uptake capacity decreases when the pH is more than 12. The decrease of Li+ uptake capacity may be caused by the destruction of the sieves due to the dissolution of manganese when solution pH is higher than 12. The adsorption capacity of ion-sieve is 10.46 mg·g-1 in the seawater. The equilibrium distribution coefficients (K4) of main metal ions in the seawater are in the order of Li+>>Ca2+> Mg2+> Na+> K+, indicating high selectivity for Li+ but much less for Na+, K+, Ca2+ and Mg2+ The precursors doped by nickel and chromium (LiMnCrO and LiMnNiO) were synthesized via directly hydrothermal method using Cr(NO3)3·9H2O and Ni(NO3)2·6H2O as doping ion-source. The effects of the concentrations of LiOH, Mn/Cr and Mn/Ni mole ratios, concentrations of H2O2 and heating temperature on the products were investigated. Finally the spinel LiMnCrO and LiMnNiO precursors were prepared.The experimental results showed that the ion-sieves of HMnCrO and HMnNiO had the same mechanism of ion exchange with the undoped ion-sieve. The exchange capacities of LiMnCrO and LiMnNiO to Li+ are 22.263 mg·g-1 and 20.467 mg·g-1 respectively when the solution pH is 12, and the exchange capacities decrease by 17.967 mg·g-1 (44.66%) and 19.963 mg·g-1 (49.12%) respectively compared with the undoped one.