Preparation And Modification Of High-Voltage Manganese-based Cathode Materials For Li-ion Battery

Spinel LiMn₂O₄which is abundant,free from contamination,safe and suitable forindustrialization has voltage up to3.8,It is considered to be one of the most promisingli-ion battery cathode material. But it has also presented many obvious shortcomingswhich mainly are poorly electrochemical stability and rapid capacity decay, these aremainly caused by the dissolution of Mn3+in LiMn₂O₄.Therefore,how to improve the struturestability of LiMn₂O₄came to be the research emphasis.LiNi_0.5Mn_1.5O₄with Spinel strutureis a kind of5V Li-ion battery cathode material.It has many advantages,for example,higher practical pecific capacity, higher power density, stable structure and good cycleperformance. However, in the actual process of use,LiNi_0.5Mn_1.5O₄shows poor cycleperformance,especially at elevated temperture.This phenomenon could be resultedfrom the existence of Mn3+in LiNi_0.5Mn_1.5O₄,Mn3+shall dissolve the electrolyte indischarge-charge cycling, resulting in the structure distortion of Li Ni0.5Mn1.5O4,even collapse.Therefore, synthetizing LiNi0.5Mn1.5O4with Mn3+as less as possibleis crucial to improve cyclability.In the thesis,we prepared a series of LiCo_yMn_2-yO₄（y=0,0.02,0.05,0.10,0.15） withsolid-state reactions and discussed the influence of Presintering temperature T1, Presintering time t1,Double-sintering temperature T2,Double-sintering time t2and Co-dopingcontent y on specific capacity and cycle performance of LiCo_yMn_2-yO₄. Research resultsshowed that the key factors influencing the discharge specific capacity of LiCo_yMn_2-yO₄by the main to the time are T2, T1, t1, t2and the y.When T1=450℃, t1=11h, T2=700℃, t2=12h, y=0, resulting LiMn₂O₄had relatively maximum initial discha rgespecific capacity which is up to127mAh/g （0.2C）. At the same time,the key factorsaffecting cycling stability of LiCo_yMn_2-yO₄by the main to the times are value y, T1, T2,t1, t2.When T1=450℃,t1=11h,T2=700℃,t2=12h,y=0.05, the resulting LiCo0.05Mn1.95O4has the best cycle performance. The capacity deterioration rate is6%during50 charge and discharge cycles at0.2C, but the deterioration rate of non-doped LiMn₂O₄reaches up to16%,the initial discharge specific capacity （123mAh/g） has no obviousdecay compared with LiMn₂O₄. Furthermore,the rate capacity had also been greatly improved.Same with LiMn₂O₄, a series of LiNi_0.5Mn_1.5O₄were prepared with solid-statereactions.XRD, SEM and charge-discharge tests are used to study the effects of anne-aling temperature and time,heat treatment method on crystallization,microtopogr aphyand electrochemical property.Research found that LiNi_0.5Mn_1.5O₄has the optimalelectrochemical performance when the annealing temperature of700℃,annealing timeof36h and threetimes of heat treatment are utilized.Its initial discharge specific capacityis130mAh/g （0.2C）, the capacity retention ratio is up to93%after97cycles at1C.Moreover, the rate capacity had also been improved.Also, carbon-coating can greatlyimproved electrochemical performance of LiNi_0.5Mn_1.5O₄.When the Carbon content is6wt%Sucrose, the resulting electrode material has a initial discharge specific capacity of138.6mAh/g （0.2C）. The capacity deterioration rate is only14%during97charge anddiacharge cycles at0.2C.However,the initial discharge specific capacity and deterioration rate of non-coated LiNi_0.5Mn_1.5O₄reaches up to138.6mAh/g and45%respectively.The results in this dissertation show that Surface modificatio n and Co-dopingcould greatly improve the electrochmical performance of LiMn₂O₄. Meanwhile, carbon coating,a kind of low-cost modification method, provided a simple but effective wayfor enhancing the performance of LiNi_0.5Mn_1.5O₄.