Study On The Structure,Oxygen Defects,and Thermodynamic Stability Of Cathode Material LiMn₂O₄ For Li-ion Batteries

Li-ion batteries are considered to be ideal energy storage devices due to environmental friendliness and high specific capacity.Since commercialization,Li-ion batteries have dominated the consumer market for portable electronic devices,especially notebook computers,mobile phones and many medical devices.With the development of Li-ion batteries technology,the application field is shifting from portable electronic devices to longer life,safer and lower cost applications,such as electric vehicles,energy storage,etc.,and these fields have put forward higher requirements on the performance and cost of Li-ion batteries.The cathode provides Li source,and the cost and quality occupy a large part of the entire battery,so the performance and cost of the cathode material have a greater impact on the battery.It is more urgent to develop cathode materials of low-cost,high-safety,high-capacity,environmentally friendly.Spinel Li Mn₂O₄has become a hot candidate for cathode materials due to its high capacity,good rate performance,abundant Mn reserves and low price.However,its severe capacity degradation at high temperatures limits its commercial application.Recent studies have shown that the failure of Li Mn₂O₄is closely related to the Jahn-Teller distortion and disproportionation reaction of Mn³⁺ions,but the mechanism is still unclear.Whether the serious capacity failure of Li Mn₂O₄during charge/discharge process at high temperature is related to the thermodynamic instability of oxygen has not been clearly reported.In order to further grasp the effect of the side reaction of Mn³⁺ions on the performance of Li Mn₂O₄batteries,and understand the thermodynamic stability of oxygen during charge/discharge process,the structure,oxygen defects and thermodynamic stability of Li Mn₂O₄are studied using the first principles calculation method based on density functional theory in this work,which will promote the application of Li Mn₂O₄in the electrode materials of Li-ion batteries,and make Li-ion batteries develop rapidly towards the direction of low cost,high safety and long life.In order to understand the influence of the Jahn-Teller distortion of Mn³⁺ions on the structure,the Gibbs free energy and configuration entropy of the cubic and tetragonal phases with different configurations is studied.It is concluded that the internal force of phase transition of Li Mn₂O₄at low temperature comes from the configuration entropy.At low temperature,the charge distribution of the system becomes more ordered,the contribution of configuration entropy decreases,and the energy of the system decreases.In order to reduce the disproportionation reaction and stabilize the Mn³⁺ions on the surface of Li Mn₂O₄,the adsorption of organic small molecules propylene carbonate(PC)and ethylene carbonate(EC)on(100)surface is studied.By calculating the adsorption energy,it is concluded that the adsorption process is exothermic.EC molecules are inclined to interact with Li⁺ions via carbonyl oxygen(O_c)on the Li Mn₂O₄(100)surface with an adsorption energy of-0.76 e V,while PC molecules are inclined to interact with Mn³⁺ions via carbonyl oxygen(O_c)on the Li Mn₂O₄(100)surface with an adsorption energy of-1.16 e V.It is also found that the charge transfer between EC and the(100)surface is very small when EC molecules are adsorbed on the Li Mn₂O₄(100)surface,and the charge transfer is mainly the redistribution of charge within EC molecules at this time.However,when PC molecules are adsorbed on the Li Mn₂O₄(100)surface,a large amount of charge transfer occurs between Mn and O_c.At this point,Mn is the electron donor and O_cis the electron acceptor,which leads to the oxidation of Mn³⁺ions on the Li Mn₂O₄(100)surface,reducing the Jahn-Teller distortion and disproportionation reaction of Mn on the surface,thus stabilizing the Li Mn₂O₄(100)surface and helping to improve the capacity and cycling performance of Li Mn₂O₄battery.In order to understand the thermodynamic stability of oxygen at high temperature,the formation energy of oxygen vacancy is studied in bulk and on its low-index stable surface of Li Mn₂O₄,and compared with other typical cathode materials.Due to the formation of the?-Mn O₂phase after delithiation,Li Mn₂O₄is more difficult to form oxygen vacancies in the delithiated(charged)state,while other cathode materials are more likely to form oxygen vacancies.The delithiated Li₂Mn O₃is very unstable both in the bulk and on the surface.The bulk and surface of Li Mn₂O₄are stable in the ground state.For layered Li MO₂(M represents transition metal ion),the(003)surface is more stable than the(104)surface,and Mn can stabilize the O atoms on the NCM-333(104)surface.Temperature has a significant effect on the formation of O-vacancy.When the temperature rises to a certain critical point,the formation energy of O-vacancy changes from positive to negative.The oxygen on the Li Mn₂O₄(100)surface is thermodynamically unstable when the temperature is higher than 765 K,and the bulk becomes unstable above1093 K.The formation energy of O-vacancy also increases with the increase of oxygen partial pressure,but it is not as significant as the effect of temperature.So,in the experiment,increasing the partial pressure of oxygen can be used as a practical treatment method to suppress the formation of O-vacancy at high temperature.