Synthesis And Modification Of LiMn₂O₄Cathode Materials With Good High Temperature Performance And Long Life

Abstract:The serious problems such as energy crisis,environmental pollution,global warming have already threatened the human living and its development. To solve the problems above, governments have invested a lot of manpower and material resources in the development and utilization of electric cars. As the Li-ion battery has many advantages, such as high working-voltage, large capacity, long circle-life and non memory effect, it becomes the first choice of novel power battery. Both of LiFePO4and LiMn2O4were most likely to be used on power battery. When we studied on LiFePO4, manganese lithium ion battery cathode material was mostly produced in Korea and Japan. So it is significant to study LiMn2O4in that Korea and Japan block material and technology secrecy. This paper reports on the synthesis of spherical Mn3O4particles by a controlled crystallization oxidization method and the preparation spherical LiMn2O4material by a solid-state reaction. At the same time, in order to realize comprehensive utilization of resources, LiMn2O4was modified by liquid-phase doping method.Theoretical basis for controlled crystallization oxidization method synthesis of Mn3O4precursor was discussed?According to the principle of simultaneity balance and mass conservation, the cp-pH diagram of Mn-NH3-SO42--H2O was drawn. The crystal formation and growth mechanism was also discussed, which provided the theoretical foundation for synthesis of Mn3O4precursor with homo-morphology and uniform particle size distribution.The progress of controlled crystallization oxidization method for preparing Mn3O4precursor was studied systematically. The effect of reaction temperature,reaction time,stirring rate,molarity of MnSO4,molarity of NH3�H2O,molar ratio of NH3/Mn?feeding velocity of MnSO4on the physical chemical properties of Mn3O4were studied. The results show that under the condition of1.25mol�L-1MnSO4,2mol�L NH3�H2O,molar ratio of NH3/Mn2.4,feed MnSO4at600mL�h-1?stirring at500r�min-1,react for12h at70?, the purity,tap density and mean grain size of Mn3O4precursor are99.74%,2.28g-cm-j and11.20?m, respectively. According to the Raman result, all of the characteristic peaks match exactly with spinel Mn3O4.The progress of solid-state reaction at high temperature for prepare spherical LiMn2O4was studied systematically. The results show the optimum technical parameters as follows:the presintering temperature are500?and650?, the sintering temperature is800?and the sintering is10h. The results of electrochemical performance tests show that the first discharge specific capacity of LiMn2O4obtained under optimum conditions can reach125.5mAh�g-1at0.1C and119.9mAh�g-1at1C, the capacity retention ratio after300cycles is87.66%at room temperature, the first discharge specific capacity is114.9mAh�g-1at1C, the capacity retention ratio after200cycles is86.24%at55?. The CV spectrums indicated LiMn2O4cathode have two obvious redox peaks, which coincides with the charge and discharge curves of LiMn2O4.The progress of preparing Mg-doping lithium manganese oxide by uniting controlled crystallization oxidization and solid-state reaction has been studied. The results show that the content of Mg in Mg-doping spherical Mn3O4precursor that with large tap density can been controlled by adjusting molar ratio of NH3/Mn. XRD indicate that magnesium ionsentered into the lattice of LiMn2O4substituting some Mn. Mg doping improves the cycle performance of LiMn2O4. The electrochemical properties of LiMn2-xMgxO4which prepared from Mg-doping spherical Mn3O4in which the content of Mg in Mn3O4is about1.5%is the best. When it charges and discharges at148mA�g-1, the first discharge specific capacity of LiMn2-xMgxO4is113.1mAh�g-1at room temperature and121.4mAh�g-1at high temperature, after300cycles, the discharge, cycle performance can meet the standards of power battery.In order to further improve the cycle performance of LiMn2O4,cutting the raw material cost and comprehensive recovery of resource, LiMn2-xNixO4,LiMn2-xCoxO4and LiMn2-x-yNixCoyO4have been prepared by liquid-phase method. XRD indicate that Ni,Co ions substitute some Mn entered into the lattice of LiMn2O4that caused reduction of lattice constants. Co-doping lithium manganese oxide shows high discharge specific capacity and good performance. The first discharge specific capacity of LiMn2-xCoxO4which was prepared from Co-doping spherical Mn3O4in which the content of Co in Mn3O4is about8%is117.3mAh�g-1at1C, after200cycles, capacity retention ratio is95.82%; Ni-doping improve the cycle performance of lithium manganese oxide but its discharge specific capacity is too low; Ni,Co co-doping lithium manganese oxide shows high discharge specific capacity and good performance, LiMn2-xCox04prepared from the precursor in which both of Ni and Co content are1%shows the best electrochemical properties. When it charges and discharges at148mA�g-1, its initial discharge specific capacity is112.8mAh�g-1and its capacity retention ratio after500cycles is97.52%at room temperature; its initial discharge specific capacity is118.2mAh�g-1and its capacity retention ratio after300cycles is90.52%at high temperature. All of its physical and chemical indexes can meet the standards of power battery. The results of XPS show that valence state of Mn,Ni,Co in LiMn2-xCoxO4is+4,+2,+3, respectively.Finally, A pilot plant scale test of preparing spherical LiMn2O4and LiMn2-xCoxO4was carried out in a3mj reactor. The LiMn2O4prepared in pilot plant scale test shows good electrochemistry properties at room temperature. Its initial discharge specific capacity is115.8mAh�g-1and its capacity retention ratio after500cycles is89.29%; LiMn2-xCoxO4shows superior electrochemistry properties, its initial discharge specific capacity of is112.8mAh�g-1at room temperature and11.2mAh�g-1at high temperature, after500cycles, its capacity retention ratio is91.22%at room temperature and83.81%at high temperature. Cost analyses indicate that this progress shows good economic prospects.