| Electrolyte is an integral part of the lithium-ion battery and plays an important role forthe performance, safety and stability of lithium-ion battery. In recent years, with the emerge ofhigh voltage cathode materials and high voltage batteries, the development of high voltagetolerance electrolyte has becoming a urgent issue in the field of lithium-ion battery due to thecarbonate-based electrolyte can not tolerance the high voltage over4.5V. Although there are afew reports for the new high-voltage electrolyte solvents and additives, it is still far fromcommercial application.In this thesis, we attempted to prepare high voltage tolerance electrolyte through addingadditives to the carbonate based electrolyte. We investigated the effect of additives on theelectrochemical stability window and ionic conductivity of the electrolyte at roomtemperature, it was found that the electrolyte with5wt%addition of succinonitrile oradiponitrile exhibited best performance and stability. Further addition of LiBOB additivecould improve the the cycling performance of the electrolyte in Li/LiNi0.5Mn1.5O4batterysignificantly. The capacity retention was high up to89%at2C after200cycles.The effect of added succinic anhydride on the performance of the carbonate-basedelectrolyte in a high-voltage lithium ion battery has been investigated. It was found that theaddition of succinic anhydride significantly improves the electrolyte’s performance, especiallyits stability, for a5V LIB with high-potential LiNi0.5Mn1.5O4as the cathode material. Theinitial discharge capacity is124.5mAh g-1at the current density of29.4mA g-1(0.2C). Thecells with the electrolyte containing3wt%succinic anhydride exhibits quite good cyclingstability, the capacity retention up to200cycles at1C and2C are90%and92%respectively,compared with only48%and47%for a cell with normal carbonate electrolyte withoutaddition of additives.The probably mechanism of succinic anhydride is suggested. We believe theimrpovement of the addition of succinic anhydride may result from the inhibition of succinicanhydride to the oxidation of the carbonate-based electrolyte. Actually, the addition ofsuccinic anhydride can delay the onset potential of oxidation by ca.0.5V, as succinicanhydride has lower oxidative stability than the carbonate and thus decomposes first.Secondly, succinic anhydride may be participated in the formation of the protective surfacelayer on the LiNi0.5Mn1.5O4electrode. On the one hand, the surface layer reduces the electroderedox reactions between the cathode and electrolyte; on the other hand, it can improve the conductivity of the material, and the cells also shows better reversibility of lithium and kineticreactions. This is the leading factor for the enhanced the cycling performance ofLi/LiNi0.5Mn1.5O4cells upon cycling at high voltage at room temperature.We also examine the other three additives in the high-voltage electrolytes. It was foundthat the addition of o-phthalic anhydride will result to good cycling stability. The capacity ofLi/LiNi0.5Mn1.5O4cell using electrolytes with the addition of1wt%o-phthalic anhydride canbe maintained after200cycles at5C and has a little fade. The reason for the goodperformance is not clear currently, need further investigation. Ethylene carbonate is replacedby fluorinated ethylene carbonate, we mixed succinonitrile and the fluorinated ethylenecarbonate-based solvents, the electrolyte showed a good anodic electrochemical stability athigh potential (about5.0V vs. Li+/Li), but unfortunately fluorinated ethylene carbonate didnot improve the poor compatibility between the LiNi0.5Mn1.5O4cathode and thesuccinonitrile-mixed electrolyte, resulting in the poor cycling performance ofLi/LiNi0.5Mn1.5O4cells. The amount of fluorinated ethylene carbonate addition may be yet tobe further optimized. |
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