Study On The Microwave Synthesis And Modification Of Manganese-basd Spinel Materials For Lithium Ion Storage

Cathod material has been a key factor for industry as an important part of lithium ion battery.Manganese-basedspinel cathode materials(LiMn₂O₄,LiNi_0.5Mn_1.5O₄)have attracted more attentions because of the abundant raw materials and good performance.In this paper,manganese-based spinel materials for lithium storage were successfully synthesized via different methods.Then,the optimum conditionof the method was selected.And the cathode materials were modified by doping,coating and doping-coating together.These methods could raise the electrochemical performance of materials.Firstly,the spinel cathode material LiMn₂O₄ was synthesized via solid-state method and microwave method,respectively.Subsequently,the presintering temperature and sintering temperature of microwave method were optimized.Based on the analysis of crystal structure,morphology and electrochemical properties,it was concluded that the optimal techuniques for preparation of LiMn₂O₄ by microwave method were presintering temperature at 500℃and sintering temperature at 780℃.To further study the spinel material LiMn₂O₄,the sample was doped by positive ion and negative ion.The results showed that the F-doped material LiMn₂O_4-xF_x（x=0.02）possessed higher first discharge specific capacity than the undoped sample.The capacity retention was up to 93.5%of its initial capacity at 1 C after 50 cycles.The first specific capacity of LiMn₂O_3.98F_0.02.02 via microwave method was 114 mAh/g at 0.2 C.And the capacity retention of doping material was up to 98.4%of initial capacity after 50 cycles.Secondly,the spinel cathode material LiNi_0.5Mn_1.5O₄ was prepared through the sol-gel,coprecipitation,hydrothermal assisted microwave method,respectively.The prepared spinel material LiNi_0.5Mn_1.5O₄ is a high-voltage cathode material,which existing P4₃32 structure and Fd3m structure.XRD results showed that different methods could synthesize single-crystal spinel.The crystal structures were between disorder structure and order structure.The first discharge specific capacities of LiNi_0.5Mn_1.5O₄synthesized by three methods were 123.3 mAh/g,117 mAh/g and 110.9 mAh/g,respectively.Among them,the spinel LiNi_0.5Mn_1.5O₄ material synthesized by sol-gel assisted microwave method had good rateperformance and cycle property.And the capacity retention after 50 cycles was up to 94.5%when charge-discharge cycle at rate of1 C.Then it was concluded that the optimal techuniques for preparation of LiNi_0.5Mn_1.5O₄ by sol-gel assisted microwave method were sintering temperature at820℃.The material showed higher discharge specific capacity and better electrochemical performance.Again,LiNi_0.5Mn_1.5O₄ was modified to study,including doping and coating modification study.The F-dope LiNi_0.5Mn_1.5O₄ materials were characterized by XRD,FTIR,charge-discharge cycle test.The results showed that F-dope LiNi_0.5Mn_1.5O₄materials tended to the order structure.The LiNi_0.5Mn_1.5O_3.98F_0.02.02 material had high discharge capacity of 131.7 mAh/g,and kept the capacity retention as high as 99.2%.The LiNi_0.5Mn_1.5O₄ material was further co-doping modified with the combination of Al/F.Al/F doping materialcan steady the crystal structure.The LiNi_0.49Mn_1.49Al_0.02O_3.98F_0.02 material had higher discharge specific capacity at the first cycle.It could maintain discharge capacity 95.9 mAh/g after 50 cycles.The LiNi_0.48Mn_1.48Al_0.02La_0.02O_3.98F_0.02.02 material had the higher first discharge specific capacity of 130.8 mAh/g at 0.2 C and the capacity attenuation was 3%of initial capacity after 50 cycles.MgO and activated carbon（C）surface coating LiNi_0.5Mn_1.5O₄ was detected by XRD,FTIR,charge-discharge cycle test.MgO-coated LiNi_0.5Mn_1.5O₄materials tended to the ordered structure.When MgO coating quantities were 2%and 5%,discharge specific capacity were 122 mAh/g and 120.3 mAh/g,respectively.Both 2%and 5%MgO coated materials maintained high discharge specific capacityafter 50 cycles.Surface modification could reduce the electrode polarization,and improve the electrochemical performance of LiNi_0.5Mn_1.5O₄ material.The C-coated LiNi_0.5Mn_1.5O₄materials were between disorder and order structure.The first discharge specific capacity of coating material（2%C）was 140.5 mAh/g.The capacity retention after 50 cycles was up to 94.5%when charge-discharge cycle at rate of 1 C.C surface coating can increase the conductivity,reduce the contact area of the electrolyte and the cathode material,and improve the electrochemical performance.Finally,combining the advantages of the doping and coating modification,the co-doping LiNi_0.48Mn_1.48Al_0.02La_0.02O_3.98F_0.02.02 material was coated by active carbon.XRD showed that combining of the doping and coating modificationcouldremain the same crystal structure.The modified materials had sharper peakes and less impurity phase compared with uncoating materials.When activecarbon amount was 2%,the LiNi_0.48Mn_1.48Al_0.02La_0.02O_3.98F_0.02.02 showed the optimal rate capability and reversibility.