Studies On Surface Modification Of Spinel LiMn₂O₄and Its Compound

With the intensification of resource and environment crises, the lithium-ion battery cathode materialshave attracted wider attention, more and more researchers are committed to the synthesis andimprovements of cathode materials. Overall, the method of improving cathode materials mainly includetwo aspects. One is the chemical method, according doping or surface coating to decrease the direct contactbetween cathode and electrolyte, which could improve the discharge capacity rate capability and cycleperformance of the electrodes, especially at high temperature. The other is preparing mixed electrode byphysical methods, different types of cathode would compensate for their disadvantages each other,achieving the purpose of improving the electrochemical properties, the structure and thermal stability. Thedissertation mainly focused on modification of the two spinel materials LiMn2O4and LiNi0.5Mn1.5O4. It ismainly composed of the following aspects.In the first chapter: The cathode materials for lithium ion batteries were classified according to thedifferent types of metal in the materials. Then, the article focused on introducing the synthesis methods,structural characteristics, the capacity fading factors and disadvantage of the three main materials(LiNi0.5Mn1.5O4, LiMn2O4and LiNiO2) involved in the experiments, and summarized the modification ofcoating, doping and physical mixed oxide electrode made up of the three materials.In the second chapter: LiNi0.5Mn1.5O4was synthesis by co-precipitation via a two step drying method.Its surface was modified by the sol-gel method, which successfully provides a simple way to obtain theLiCoPO4coating layer for the LiNi0.5Mn1.5O4sample. Because the operating voltage of the LiCoPO4issimilar to the spinel LiNi0.5Mn1.5O4, we put forward the concept of using the material whose operatingvoltage are close to the host battery material as coating layer. The pristine and coated samples are characterized by a series of testing, such as XRD, SEM, TEM, EIS and so on. The results suggested thatthe presence of the coating layer significantly improved electrochemical properties and thermal stability ofthe battery material.In the third chapter: On the basis of theory in the previous chapter, we changed the method of coating.Firstly, we pretreated the Mn-Ni mixed hydroxide precursor with Co3(PO4)2coating layer, and then groundthe precursor with lithium hydroxide monohydrate and calcined in air. The TEM image showed that theLiCoPO4successfully coated on the surface of LiNi0.5Mn1.5O4, and the results of electrochemical test showthat the material modified showed better properties.In the fourth chapter: We prepared the layered LiNiO2and spinel LiMn2O4by sol-gel method, andblended the active materials with the mass ratio LiNiO2/LiMn2O4=1:9as the mixed electrode. Results ofXRD test showed the absence of undesirable peaks and confirmed the pure phase of LiNiO2and LiMn2O4.The electrochemical tests were carried out for single and blended electrode, the results showed that, withrespect to the single LiMn2O4electrode, the composite cathode material exhibited better specific capacity,rate properties and cycle stability.