Study On Electronic Structure And Surface Energy Of LiMn₂O₄ Cathode Materials

At present,the advantages of lithium-ion batteries are low cost,large specific capacity,safety and stability.It is widely used in new energy vehicles,digital products,power products,medical and security,etc.Since there are few nickel and cobalt reserves on earth with commercial exploitation value,it is more necessary to develop high performance lithium battery cathode materials with low cost and material saving.Li Mn₂O₄cathode material is more abundant than Li Co O₂cathode material.It is expected to succeed in replacing Li Co O₂as the most advanced and ideal energy material.The defect of Li Mn₂O₄cathode material is that the capacity is easy to decay at high temperature and the structure is difficult to maintain a stable state.Based on the above two problems,the doping and low exponential surface energy of Li Mn₂O₄cathode materials were studied.The structural mechanism is further explained by theoretical calculation.In order to improve the electrochemical performance of Li Mn₂O₄cathode material,a lithium battery material with high reversible specific capacity was designed.The relevant work is as follows:（1）The space group of Li Mg_0.5Mn_1.5O₄is P4332.The electronic structure,band structure and state density of the material were calculated by density functional method.A Li Mg_0.5Mn_1.5O₄unit cell with space group P4332 was established and the geometry was optimized.Compared with the experimental data,the error was less than 2%.According to the data analysis of the energy band structure and the density of states of each atom,it is further explained that doping will reduce the dissolution of Mn³⁺.The addition of transition metal magnesium ions makes the overall lattice volume of the cathode material Li Mn₂O₄shrink,and the structure is in a more balanced and stable state.（2）Based on first principles,the electrochemical properties of Li Mn₂O₄battery materials doped with Fe and Co were studied.The lattice parameters of Li Mn₂O₄cells decrease with Fe/Co doping,which reduces the possibility of Jahn-Teller distortion.For fe-doped spinel lithium manganate(Li₈Mn₁₅Fe O₃₂),the first discharge voltage increases to4.623V;For Co doped spinel lithium manganate(Li₈Mn₁₅Co O₃₂),the first discharge voltage is increased to 4.101V.The apparent increase in voltage is due to the fact that the doped Mn³⁺ions lose an electron and are oxidized during the first delithium process,but Fe and Co ions do not participate in this oxidation process and still remain in the ionic state.（3）Based on the first principles,the energy state of Li Mn₂O₄crystal on different low index surfaces is calculated.The lattice parameters of Li Mn₂O₄structure under GGA and GGA+U are compared.It is found that the lattice parameters of Li Mn₂O₄structure under GGA and GGA+U are obviously larger when the effective U value is selected for the D orbital of Mn.Calculate and analyze the five surfaces（100）,（010）,（001）,（110）and（111）,find the most stable exponential surface,and make the（111）surface become the most stable section of spinel structure by means of surface reconstruction.The results provide theoretical basis for the mitigation of capacity attenuation of Li Mn₂O₄materials and theoretical reference for the study of possible new structures under electrochemical conditions.