The Doping Modification Of LiMn₂O₄ Cathode Materials For Lithium Ion Battery

Spinel LiMn2O4 is one of the most promising cathode for lithium ion battery for its high working voltage,high safty,low pollution,low cost,abundant resources.The low initial capacity and its rapid decrease of the spinel haven't been improved up to now,However the commercial applications of LiMn2O4 are badly blocked by its poor cycling performance.Through the most widely adopted method to overcome the problems was cation doping,anion doping or co-doping.This dissertation is intending to investigate the modification of spinel lithium manganese cathode material for lithium-ion batteries by ion doping, so as to improve its electrochemical performance. Spinel LiMn2O4 were synthesized by a melting-impregnation method with different source of lithium .Cation(Mg,Al )and anion (F) co-doping methods were adopted to modify the spinel LiMn2O4 cathode. Their effects on the crystal structure and electrochemical properties of the result spinel lithium manganese were studied respectively. Firstly, Spinel LiMn2O4 were prepared by a melting-impregnation method with lithium nitrate and lithium hydroxide respectively. with the calcination temperature increased , when used lithium nitrate as the source of the lithium , the cell parameters of spinel samples increased gradually. Its initial capacity is gradually increased,also.when used lithium hydroxide as the source of the lithium, the lattice constants of spinel samples decreased Its initial capacity is gradually reduced. In contrast, the electrochemical properties of LiMn2O4 spinel with lithium hydroxide as the source of the lithium is better. F-Mg co-doped spinel LiMn1.9Mg0.1O4-yFy were synthesized by melting-impregnation methods as above-mentioned. Their microstructure were examined by powder XRD analysis , All samples were identified as a pure spinel phase with a space group Fd3m where lithiumions occupy the tetrahedral (8a) sites, Mn3+, Mn4+ andAl3+ ions reside at the octahedral (16d) sites, and O2? and F? ions are located at the 32e sites. the crystal degree, morphology and cycling performance are greatly improved , When the same amount of magnesium in spinel,the effect of fluorine-doping on samples is the most obvious ,with the increase of fluorine, the initial discharge capacity of LiMn1.9Mg0.1O4-yFy(y=0.03,0.05,0.1)gradually increase. In contrast, when used lithium hydroxide as the source of the lithium, the electrochemical performance of LiMn1.9Mg0.1O4-yFy (y = 0.03,0.05,0.1) is better, It is the same as LiMn2O4 spinel. when used lithium hydroxide as the source of the lithium, the electrochemical performance of LiMn1.9Mg0.1O4-yFy (y = 0.03,0.05,0.1) is better,also.However,electrochemical property of LiMn1.9Al0.1O3.97F0.03 were prepared by melting-impregnation methods as above-mentioned. Al and F dual-doped LiMn2O4 could gain worse electrochemical properties and have lower initial capacity than only the Mg and F co-doped LiMn2O4.The modified spinel cathode material with some cations and anions multiplex doped were synthesized for the first time. By choosing a different lithium source and controlling the doping quantum, perfect spinel lithium manganese cathode were produced which have high initial capacity as well as good cycle performances at room temperature. The causes of the capacity fading of spinel lithium manganese were discussed. The action mechanism of ion doping was analyzed in terms of the crystal structure and electrochemical properties.