Preparation And Modification Of Spinel LiMn₂O₄ As The Cathode Material For Lithium Ion Batteries

The spinel LiMn₂O₄ cathode materials for lithium ion batteries has a high theoretical capacity(148 mAh g^-1),high working voltage,green environmental protection and other advantages.Besides,minimal volume change occurs in the process of ion insertion/extraction,resulting in a stable structure,which is beneficial to obtain excellent cycle stability.More importantly,spinel LiMn₂O₄ cathode materials occupy a part of the market.However,the Jahn-Teller effect seriously restricts the rate capability and cycle performance of the material,which leads to low energy density and can not further expand the market.Aiming at this scientific problem,we have prepared hollow LiMn₂O₄ microspheres and Mg and Na doped LiMn₂O₄ cathode materials by high temperature solid-state method,and systematically studied the electrochemical properties of the materials.We first synthesized LiMn₂O₄ hollow microspheres composed of nanoparticles by high temperature solid-state method.The hollow microspheres structure not only enhances the wettability of electrolyte,increasing the contact area of materials and electrolyte,but also shorten the diffusion distance of lithium ions in bulk material,improving the lithium ion diffusion coefficient.Furthermore,the microsphere structure relieves the volume change of the electrode during the charge/discharge process,and optimizes the the rate capability and cycle performance.The results showed that the specific discharge capacity reached 86.3 mAh g^-1 at the rate of 10 C,however,the ordinary LiMn₂O₄ powder shown only 73.9 mAh g^-1 at 10C.At the rate of 0.5 C,the capacity retention was 83.2%after 100 cycles,which was significantly better than that of the ordinary LiMn₂O₄ powder?68.5%?.Secondly,in order to alleviate the occurrence of Jahn-Teller effect in the charging/discharging process for LiMn₂O₄material and improve the capacity retention,we introduce the Mg and Na elements in the solid-state process to replace part of Mn elements and Li elements respectively in the lattice,preparing dual-doped LiMn₂O₄ cathode materials.The introduction of Mg element effectively alleviated the occurrence of Jahn-Teller effect and reduced the dissolution of Mn³⁺,meanwhile,the doping of Na element can increase and support the diffusion channels of lithium ions and accelerate the transmission of lithium ions,thus ensuring the stability of structure and improving the cycle performance of LiMn₂O₄.Therefore,Mg and Na doped LiMn₂O₄ cathode materials(Li_0.95Na_0.05Mg_0.1Mn_1.9O₄)exhibit excellent long cycle performance.At the rate of 0.5 C,the capacity retention is high up to 97.0%after 100 cycles.Finally,from the point of view of practical application,we prepared TiO₂ material as anode material,and adopted Li_0.95Na_0.05Mg_0.1Mn_1.9O₄ as the high voltage cathode material to assemble the full cell.The lithium ion battery with the operating voltage of about 2.8 V is obtained,and exhibit excellent electrochemical performance.The voltage range is close to the existing lithium ion batteries,and it is a very promising lithium ion battery system.