Synthesis And Polyelectrolyte Modification Study Of LiMn₂O₄ Through Combustion Method

With the development of lithium ion batteries, low cost electrode materials have attracted much attention. Spinel LiMn₂O₄ has been intensively investigated as a competitive cathode material for lithium ion batteries, because of its high security, abundant Mn resource, low cost and environmental benignity. Recently, LiMn₂O₄ has been widely used for large-scale lithium ion batteries in electric vehicles. However, spinel LiMn₂O₄ suffers from drawbacks like serious capacity fading, which might due to the Jahn-Teller distortion of Mn³⁺ ions. Lots of studies have demonstrated that the electrochemical properties of the material are affected by synthesis process, particle size and the surface morphology. In this work, we mainly focused on reducing particle size, surface lithium polyacrylate coating and functional binder to improve the electrochemical properties. Spinel LiMn₂O₄ was investigated by XRD, SEM, TEM and electrochemical test to research the structure, morphology and electrochemical performance. The main contents are as follows:（1） Ultrathin LiMn₂O₄ nanoparticles were successfully prepared by CNTs-assisted solution combustion method. Using LiNO₃ and 50%Mn（NO₃）₂ solution as the starting materials, CNTs as a fuel. The effects of CNTs on the structure and electrochemical performance of LiMn₂O₄ were investigated. Rate performance test demonstrates that CNTs dosage greatly affect the electrochemical behavior of LiMn₂O₄. LiMn₂O₄（C7%-LMO） prepared by choosing suitable CNTs dosage（7%） as a fuel shows high rate charge-discharge capability. XRD results indicate that C7%-LMO has smallest distortion degree in the crystal. SEM results reveal that C7%-LMO has the smallest average particle size is about 100 nm. This unique microscopic features guarantee C7%-LMO possesses the highest discharge capacity. At the rate of 0.2C, the second discharge capacity of C7%-LMO is 115.1 mAh·g^-1. Even at the rate of 10 C, the discharge capacity of C7%-LMO is still as high as 77.0 mAh·g^-1, after 100 cycles at 1 C, the capacity retentions of C7%-LMO is 94.8%. As a result, C7%-LMO shows the best electrochemical because LiMn₂O₄ samples with reduced particle size have short lithium diffuse distance.（2） The LiMn₂O₄ particles were successfully prepared via solution combustion synthesis. Using Li NO₃ and Mn（CH₃COO）₂·4H₂O and 50%Mn（NO₃）₂ solution as the starting materials. The effect of different amounts of PAALi coating on the structure and electrochemical performance of LiMn₂O₄ nanocomposite was investigated. The as-prepared material was characterized by XRD, TEM, ICP-OES and electrochemical test. The analysis results showed that LiMn₂O₄ particles are coated by PAALi, which prevent the Mn ion dissolving from LiMn₂O₄ and then enhance the stability of LiMn₂O₄ crystals in electrolyte, the 2% PAALi coating（It was designated as LMO@2%PAALi） possesse much better rate performance, higher discharge capacity and the best cycling performance. The initial discharge capacity of LMO@2%PAALi reaches up to 127.2 mAh g^-1 at 0.2 C. Even at the rate of 10 C, the discharge capacity of LMO@2%PAALi is still as high as 97.3 mAh·g^-1, The capacity retentions of LMO@2%PAALi is 94.8% after 100 cycles at 1 C.（3） The effect of different binders（polyving akohol（PVA）, lithium polyacrylate（PAALi） and LA132） on the electrochemical performance of LiMn₂O₄ material was investigated. The results showed the LiMn₂O₄ sample prepared by using PVA binder（LMO₃） possesses a better electrochemical performance. The initial discharge capacity of LMO₃ reaches up to 128.9 mAh g^-1 at 0.2 C, and the capacity retention of LMO₃ is 91.8% after 100 cycles at 1 C. On the basis of these, different mass ratios of PAALi/PVA（1:5, 1:7 and 1:9） were also used as binders to improve the electrochemical performance of LiMn₂O₄. Electrochemical tests indicate that the mass ratio of PAALi/PVA is 1:7（LMO5） exhabits the highest discharge capacity and best cycling performance. The initial discharge capacity of LMO5 is 129.1 mAh g^-1 at 0.2 C, after 100 cycles at 1 C, the capacity retentions of LMO5 is 94.1%.