Synthesis And Modification Of LiMn₂O₄ Cathode Materials For Lithium-ion Batteries

Spinel LiMn₂O₄ is one of the most potential materials owing to its merits of easypreparation, inexpensiveness, abundance of Mn resources, non-toxicity and environmentalfriendliness. However, capacity fade, especially at elevated temperatures (＞50℃), restricts itscommercial application. In this paper, optimizing the synthetic method, metal ion doping andsurface modification of spinel LiMn₂O₄, were used to stabilize the structure during the cycling,and to improve the electrochemical performance both at room temperature and hightemperature.Nano-LiMn₂O₄ cathode materials have been synthesized containing citric acid as achelating agent by a sol-gel method. Compared with LiMn₂O₄ synthesized by solid statemethod, crystal grains by sol-gel method, an average size of 107 run, have a higher degree ofcrystallization, similar orientation and dense arrangement. These structure advantages give theLiMn₂O₄ better electrochemistry characteristics. After 80 cycles with a discharge rate of 60mAh/g at room temperature, the spinel LiMn₂O₄ has a discharge capacity 108 mAh/g, andcapacity loss per cycle is 0.135 mAh/g. The LiMn₂O₄ has good cycling performance even at55℃, 99 mAh/g after 80 cycles.Spinel LiMn₂O₄ was improved by rare earth-doping. LiLa_0.01Mn_1.99O₄ andLiNd_0.01Mn_1.99O₄ were synthesized by a spray-drying method. They have a special structure,many particles together into a hollow ball of varying sizes, in which many nano-rods with thediameter of 60-100 nm interspersed. Nanorods formation is proportional to the amount of rareearth elements to enter into the lattice of the LiMn₂O₄, and more nanorods were found inLiNd_0.01Mn_1.99O₄, because Nd element can more easily enter into the lattice of LiMn₂O₄ thanLa. LiNd_0.01Mn_1.99O₄ have a improved rate capability. Even at higher current density (10C), itstill can deliver a capacity about 110 mAh/g, about 89% of the capacity under 0.1C. And ithave excellent cycle performance, 100 mAh/g remaining after 300 cycles at 1C. In a word,Rare-earth-doping can play an important role in stabilizing the LiMn₂O₄ structure so as toimprove its cycling stability and reduce polarization of the battery.Spinel LiMn₂O₄ was coated with fluorides. Compared with the common coatingmaterials such as the oxides, fluorides were more stable to electrolyte. In this work,MF3-coated LiMn₂O₄ (M=La, Y) has been successfully synthesized using coprecipitationmethod, and the effect of fluorides coating on the material structure and electrochemicalproperties was investigated. The results showed that the surface modification of LiMn₂O₄ by fluorides coating could obstruct the direct contact between LiMn₂O₄ particles and theelectrolyte, suppressing the surface reactions (decomposition of electrolyte and dissolution ofMn). In addition, the fluorides coating can also inhibit the re-deposition of the dissolvedmanganese ions on the surface of electrodes, ensuring good electrical contact and physicalcontact beween electrodes, improving the structure and electrochemical stability of theLiMn₂O₄. As a result, the cycling stability is improved. The sample coated with 0.5mol%LaF3 shows the best electrochemical performance.It delivered initial discharge capacity of122 mAh/g and remained at 113 mAh/g after 80 cycles. Besides, high-temperature cycleperformance has also been greatly improved, 98 mAh/g remaining after 80 cycles.Oxide Ca₃Co₄O₉ as anode materials was explored. In this paper, Ca₃Co₄O₉ andCa_2.95K_0.05Co₄O₉ were synthesized by the polyacrylamide gel method, and they ware exploredas anode materials. Electrochemical test showed that thermoelectric material Ca₃Co₄O₉ had agood cycling performance, and it had a theoretical capacity of 643 mAh/g. Not only the initialdischarge capacity was enhanced to about 1120 mAh/g, but also the cycling performance wasimproved by K-doping. After cycled for 50 cycles at 0.5 C, Ca_2.95K_0.05Co₄O₉ retained acapacity of 223 mAh/g, almost twice than Ca₃Co₄O₉.