| In recent years, the spinel structure LiMn2O4has been considered as a mostpromising candidate for lithium ion batteries, due to their merits of abundantmanganese resources, environmentally friendliness, low cost, good thermal stability,proper redox potential and easy preparation. However, the spinel LiMn2O4suffersfrom a fast capacity fading during charge-discharge cycling, especially at elevatedtemperatures above55oC, which has limited its large scale industrial application.The phenomenon can be ascribed to the following reasons: Jahn-Teller distortion ofMn3+at deeply discharged state; manganese dissolution in the acidic electrolyte andlow purity of raw material for industry production. In order to overcome abovedisadvantages of the spinel LiMn2O4, we have made the following four researches inthis dissertation. Firstly, the high-purity manganese sulfate is prepared by usingFeF3·3H2O as agent for removing impurities from MnSO4·H2O; secondly, thehigh-purity spinel LiMn2O4with well-defined spherical morphology are prepared byco-precipitation method using highly pure MnSO4·H2O, Al2(SO4)3·18H2O andNa2CO3as precipitate agents; thirdly, spinel LiAlxMn2-xO4microspheres aresynthesized via co-precipitation method with the performance of spinel LiMn2O4enhanced by Al-doping; Finally, the samples of spherical spinel LiMn2O4withdifferent Li2ZrO3-coated quantity are synthesized via sol-gel method. The mainworks in this dissertation are as follows:(1)According to the following peculiarities of FeF3·3H2O: Ca2+, Mg2+can beprecipitated by F-; K+, Na+can form sulfate precipitation with Fe3+; the excess ofFe3+can hydrolyze to form Fe(OH)3precipitation in water solution, we chooseFeF3·3H2O as a new agent for removing impurities of MnSO4·H2O. Theconcentrations of Ca2+, Mg2+, K+and Na+in prepared high-purity manganese sulfateare less then100ppm, and which of Fe3+is even less then10ppm. The preparedhigh-purity manganese sulfate can be used as the raw material of anode material forlithium ion batteries.(2)By using high-purity MnSO4·H2O and NaCO3as precipitators, theammonium hydroxide as complex agent, the well-defined spherical MnCO3precursor is prepared via co-precipitation method. The effects of pH, temperature,stirring speed and the concentration of ammonium on the co-precipitation reaction are discussed. Then, Li2CO3and the precursor are uniformly mixed and heated toobtain the high-purity spherical spinel LiMn2O4. The test results show that the tapdensity and electrochemical performances of high-purity spherical spinel LiMn2O4are improved.(3)By using high-purity MnSO4·H2O, Al2(SO4)3·18H2O and MnCO3asprecipitators, the ammonium hydroxide as the complex agent, the spinelLiAlxMn2-xO4(x=0,0.02,0.06,0.1) microspheres are synthesized viaco-precipitation route, and their electrochemical performances are discussed.Compared with the other samples, the LiAl0.06Mn1.94O4sample exhibits the bestcycle performance especially at an elevated temperature (55oC). The initialdischarge capacity of LiAl0.06Mn1.94O4at1C after100cycles is113.9mAh g1at55oC, with the high discharge capacity retention of97.0%.(4)The Li2ZrO3coated spherical spinel LiMn2O4samples are successfullysynthesized. The research results show that due to the excellent lithium ionconductivity and chemical inertness of Li2ZrO3, the Li2ZrO3coating can minimizethe direct contact area of LiMn2O4/electrolyte interface and suppress dissolution ofMn3+, without affecting the lithium ion penetration through the coated layer. Theresults of electrochemical tests show that the3wt.%Li2ZrO3coated LiMn2O4sample exhibits the best cycle performance at both room temperature and elevatedtemperature (55oC). The initial discharge capacity of3wt.%Li2ZrO3coatedLiMn2O4sample at1C is126.7mAh g1with the capacity retention of99.0%after100cycles in25oC. The initial discharge capacity is129.5mAh g1, and thecapacity retention after100cycles is as high as90.2%at55oC, with the small Rctof34.2. The electrochemical performances are much better than those of thenon-coating spinel LiMn2O4sample. |
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