Studies On Preparation And Modification Of Spinel LiNi_0.5Mn_1.5O₄cathode Materials

Today, new-typed electronic vehicles are being developed worldwide to reducethe the rapid depletion of fossil fuels and increasingly worsened environmentalpollution. To meet the requirements of widespread applications of hybrid and electricvehicles, lithium-ion batteries with higher energy densities, power densities and workpotentials are continuously explored these years.In this work, nano-sized spinel LiNi0.5Mn1.5O4cathode material was firstlyprepared by a resorcinol-formaldehyde polymer sol-gel method. Two ways ofmodification were carried out subsequently to improve the electrochemicalperformance of LiNi0.5Mn1.5O4, including metal ions doping by Mg(LiMgxNi0.5-xMn1.5O4) and Zn (LiZnxNi0.5-xMn1.5O4) and surface coating by Al2O3.Finally, the effects of synthesis method, different dopants and surface coating on theperformance of LiNi0.5Mn1.5O4were discussed.First of all, nano-sized spinel LiNi0.5Mn1.5O4cathode material was prepared bysol-gel methods, using metal acetates, resorcinol and formaldehyde as precursors.XRD, SEM tests were utilized to characterize the structure, morphology, and particlesize of LiNi0.5Mn1.5O4.Galvanostatic charge/discharge and cyclic valtammetrymeasurements were used to study the electrochemical performancesIn the process of preparing the nano-sized spinel LiNi0.5Mn1.5O4cathode material,the crystalline purity and the size distribution can be controlled by preheating the gelprecursor. The content of phenolic aldehyde, heating temperature and sintering timeaffected the structure and performance greatly. The optimized synthesis condition issummarized as below: R/L ratio is4:1, preheated the gel precursor at350°C, reheatedat700°C for12h. The nano-sized spinel LiNi0.5Mn1.5O4powder prepared at abovedoptimized condition has an average crystal size of60.6nm, with initial specificdischarge capacity and coulombic efficiency of132mAh/g and94.3%, respectively,retaning84.65%of its initial capacity after50cycles.The mechanism of discharge capacity decay was also discussed as follows: thcrystal structure changes and increased content of impurity, which were caused byby-reaction between cathode material and the decomposition of electrolyte and thedissolution of transition metal ion are the main criminals for the deterioration of theperformance.Mg and Zn ions were adopted in the spinel LiNi0.5Mn1.5O4doping. When the cathode was doped with Mg ion, the discharge plateau of LiMgxNi0.5-xMn1.5O4changes evidently with the increasing content of Mg ion as compared with undopedLiNi0.5Mn1.5O4and no improvement in capacity was observed. When doped with Znion, the as-prepared LiZnxNi0.5-xMn1.5O4has high purity and crystalline. The dopedsample shows similar discharge pleatus and better cycling stablity compared withundoped LiNi0.5Mn1.5O4, indicating that Zn ion could be an effective dopants toimprove the electrochemical performance of LiNi0.5Mn1.5O4.Furthermore, Al2O3was used to coat LiNi0.5Mn1.5O4. The electrochemical testsshow that the capacity retention of coated LiNi0.5Mn1.5O4with1%,3%and5%Al2O3after100cycles are80.63%、83.87%and88.6%, respectively, exceeding that ofuncoated LiNi0.5Mn1.5O4, demonstrating that amorphous coated Al2O3can restrain thereaction between cathode material and decomposition of electrolyte, and thus improvethe cycling performance of uncoated LiNi0.5Mn1.5O4.