Synthesis Of LiMn₂O₄ As Cathode For Lithium Ion Battery And Its Properties

Nowadays,under the great pressure of deprivation of traditional energy,searching for new clean energy and developing new low-cost energy saving facilities has become an inevitable trend,and the development of new energy storage materials has become a significant topic.Lithium ion battery,as an important part of new clean energy,has drawn wide attention over the whole society.Among all the lithium battery related materials,cathode materials remain the key material of lithium ion battery given its high-cost and the fact that it is the short slab that determines the overall performance of lithium ion battery.In this thesis,we explore the approaches of enhancing the electrochemical property of LiMn₂O₄ cathode materials through doping and introduction of porous structure in the materials.After analyzing the stability of the sample through computation and simulation method,further modifications are conducted via carbon coating and graphite composition.Hence,the overall performance of the material is further enhanced.Firstly,we explored the synthesis method of preparing Fe doped LiMn₂O₄,as well as the choice of template.Porous LiFe_0.2Mn_1.8O₄ is prepared based on applying a nitrate decomposition method and introducing surfactant as soft template to form porous structure.Through the characterization of XRD,XPS,we can confirm that the phase of the prepared sample is spinel,and Fe element exists as Fe³⁺.The morphology and structure of the sample were measured by SEM,TEM and BET,indicating the porous structure of the resultant materials.The nano-sized crystallines are inter connected to each other and provide foundation for a favorable capacity and cycle performance.Subsequent electrochemical tests are carried out,including discharge capacity,cycle performance,rate capability and resistance measurements.The results show that the sample synthesized with soft template shows better electrochemical performances.The polarization of sample prepared with soft template is much less than that of the sample prepared with hard template.Possibly,this can be explained by the larger pore diameter of the sample synthesized with soft template.Next,we determined the stability of the sample through simulation and calculation.With the VASP software,formation energies of Li_0.5Fe_xMn_1-xO₂system with different x value,i.e.,Fe doping concentration,are calculated.This energy is compared with the energy of the compound with the same chemical formula but a disordered phase,as well as that of the compounds that are already in the OQMD database.The calculation results show that the formation energy of ordered Li_0.5Fe_xMn_1-xO₂ is lower than that of disordered phase of the structures in OQMD.Hence,the samples that can stably exist should be ordered phase.Also,the concentration of Fe in the prepared Li_0.5Fe_xMn_1-xO₂ is 0.1,lies between two extreme points on the convex hull consisted of data points represent the compounds with compositions that are stable.These evidences illustrate that the synthesized structure is stable ordered phase,which agrees well with our previous experiment results.Finally,with the simulation results at hand,it is safe to conduct further modification on porous LiFe_0.2Mn_1.8O₄.With glucose as carbon source,carbon coated LiFe_0.2Mn_1.8O₄ was synthesized.Graphene composited sample was fabricated by mixing and dispersing porous LiFe_0.2Mn_1.8O₄ and graphene evenly in solutions,followed by firing of the dried mixture.Carbon coating and graphene composite would not affect the composition of LiFe_0.2Mn_1.8O₄ through the characterizations of XRD and XPS.Also,SEM,TEM and BET tests prove that the porous morphology won't be affected by the coating of carbon,and the LiFe_0.2Mn_1.8O₄ crystalline disperse evenly on the surface of graphene.Then the electrochemical property of the material such as discharge capacity,cycle performance,rate performance as well as resistance are tested,and the results show that the carbon coating and graphene composition have only slight effect on the discharge capacity of the material,but the rate performance and cycle capability are greatly improved.Between them,graphene composition has a better improvement than carbon coating,which is probably due to the dispersion of crystalline and the improvement of electricity conductivity.Since the content of graphene is larger than carbon coating,the resistance of graphene composited LiFe_0.2Mn_1.8O₄ is smaller than carbon coated LiFe_0.2Mn_1.8O₄,thus explains the favorable cycle capability and rate performance.