Synthesis And Performance Study Of Spinel Lithium Manganese Cathode Materials For Lithium-ion Batteries

Spinel lithium manganese oxides have been recognized as one of the mostpromising cathode materials for lithium ion batteries because of their abundantresources, low cost, environmental friendliness, high discharge voltage plateau, goodsafety and facilitation of synthesis. However, the poor cyclability and capacity fadeimpede their wider range of applications, especially at elevated temperatures. thereasons for above problems could be contributed to Jahn-Teller distortion, manganesedissolution caused by disproportionated reaction of Mn3+and electrolytedecomposition. In this dissertation, we focus on the following three contents: firstly,based on exploring the effects of crystal structure of the precursor MnO₂onelectrochemical properties of spinel LiMn₂O₄, high stability of LiMn₂O₄was prepared;secondly, the spherical LiMn_1.5Ni_0.5O₄powder with excellent cycle performance wassynthesized from the carbonate coprecipitated route; thirdly, a novel concentrationgradient material which possesses a synergetic effect from both the cycling stability ofLiMn_1.5Ni_0.5O₄and the safety of traditional LiMn₂O₄was prepared, and this novelmaterial improves effectively the electrochemical performance of traditional LiMn₂O₄even at high temperature. The main contents in this dissertation are as follow:（1） Spinel-phase LiMn₂O₄samples for lithium-ion battery were synthesized byfour different polymorphs of MnO₂（α-, β-, γ-, δ-MnO₂） through one-step solid statemethod. The effects of crystal structure of the precursor MnO₂on structural andphysical chemistry properties of the spinel LiMn₂O₄were discussed. Results showthat the particle size and surface morphology of the LiMn₂O₄sample obtained fromβ-MnO₂are well-distributed and more regular. The initial discharge capacity is126mAh g^-1at_0.5 C (74mA g^-1) between3.0and4.4V. After100cycles, its capacityretention is still83.3%, the results are much better than the other three samples.Hence, β-MnO₂is the much better Mn-source to prepare spinel LiMn₂O₄.（2） Spherical Mn_0.75Ni_0.25CO₃precursor powder was successfully synthesized bythe carbonate coprecipitated route. The effects of two synthesis processes wereexamined, the results indicate that LiMn_1.5Ni_0.5O₄sample synthesized from mixtureof (Mn_0.75Ni_0.25)xOyand Li2CO₃shows more excellent electrochemical properties,delivers an initial discharge capacity of14_1.5 mAh g^-1at the rate of_0.5 C in thevoltage range of3.0-4.9V, and its capacity is still13_1.5 mAh g^-1after100cycles. （3） A novel LiMn_1.87Ni_0.13O₄with a concentration-gradient structure wassuccessfully synthesized and characterized. In cell testing at55oC, theconcentration-gradient LiMn_1.87Ni_0.13O₄sample exhibited relatively high dischargecapacity of108.2mAh g^-1in the range of3.0-4.4V and excellent cyclability with acapacity retention of90.2%after200cycles, which is obviously better than thecommercial spinel LiMn₂O₄. What is more, in the range of3.0-4.9V, theLiMn_1.87Ni_0.13O₄sample showed an exceptional capacity of129.1mAh g^-1and aretention of91.9%after100cycles at55oC. Based on the results from CV, XRD andSEM, the concentration-gradient LiMn_1.87Ni_0.13O₄showed a more signicant structuralstability during cycling, indicating this novel spinel lithium mangese oxide materialhas a promising prospect as the cathode material for advanced lithium ion batteries.