Preparation And Modification Of High-voltage Cathode Materials For Lithium Ion Batteries And Their Electrochemical Performance

Lithium ion batteries have been widely recognized as the most important energy storage devices and power sources for portable electronics.Nowadays,battery performance requirements have become more stringent,especially in terms of energy density and cycle life.LiNi_0.5Mn_1.5O₄ is considered one of the most promising high voltage cathode materials for lithium ion batteries,which has a discharge platform of 4.7V,with theoretical capacity of 147 mAh·g^-1,also,environmentally friendly and low cost are both its advantages.However,LNMO suffers from lots of problems,such as the low coulombic efficiency of the fisrt cycle,poor rate capability,irreversible structural change due to the Jahn-Teller effect etc.In view of the above problems,we have done the follwing two works,the main conclusions are as follows:1.nanosized LiNi_0.5Mn_1.5O₄ was successfully synthesized by sol-gel using polyvinylpyrrolidone（PVP）as dispersant and ethylene glycol（EG）as size-controlled additive,and the influence of the partical size and crystal on electrochemical performance of the material were investigated.The exprimental results show that the partical size of the prepared cathode material was approximately 200nm in diameter,and has an average particle size distribution.As for the parallel sample,which partical size was about 1μm,both the two samples are disorderd spinel stucture.Reducing the partical size is benificial to promote the electrochemical performance of the LNMO cathode material.The nanosized material achieved the discharge specific capacity of 128.1mAh·g^-1 at 0.2C rate and maintained 119 mAh·g^-1after 100 cycles with capacity retention of 92.9%.The material also yielded discharge capacity of 128.1 mAh·g^-1,123.5 mAh·g^-1,124.4 mAh·g^-1,116.9 mAh·g^-1 and 101.9 mAh·g^-11 at 0.2C,0.5C,1C,2C and 5C rate,respectively,and it was better than the microsized LNMO material.2.Two types of spherical LNMO particals were successfully systhesised through a one-step template-dissolve salt dipping method using Mn₂O₃ and MnO₂ as the templates respectively.The inffluence of template selection to this method which can systhesis spherical LNMO particals were investigated.Additionally,we compared the performance of the two materials systhesised by the sol-gel and the template-dissolve salt dipping method seperately so that we can understand more about the advantages and disadvantages of the two method.Also,the inffluence of the crystal structure to the electrochemical performance were investigated.The exprimental results show that the two types of the prepared materials both has good shape of sphere,and with an average distribution of partical size at about 1μm.Moreover,both two materials are ordered spinel structure.The performance of the material which inherited MnO₂ is unsatisfactory,not as well as the Mn₂O₃ ones.The latter material delivered a discharge specific capacity of 127.8 mAh·g^-1 at 0.2C rate,and matained 118.3 mAh·g^-1 after 30cycles,with a capacity retention of 92.6%.When the rate changes to 2C,the material shows a discharge capacity of 127.4 mAh·g^-1,and matained 108.2 mAh·g^-1 after 30cycles,with a capacity rentention of 84.9%.