Controllable Synthesis And Modification Of One-dimensional Spinel LiMn₂O₄ Nanorod Cathode Materials

Much attention has been given to spinel LiMn₂O₄ as a promising positive material for advanced lithium-ion batteries(LIBs)for wide applications in portable devices,hybrid electric vehicles and smart grids due to its nontoxicity,lower price,and abundant manganese resource.Although LiMn₂O₄ anodes have the above advantages,the capacity of LiMn₂O₄ anodes will be seriously reduced due to the ginger Taylor effect and the dissolution of manganese in the long cycle charge/discharge process,especially in the high temperature environment.These disadvantages greatly limit the large-scale commercial application of LiMn₂O₄ anodes.In view of the above problems,the controlable synthesis and further modified by means of La element doping and TiO₂coating of spinel LiMn₂O₄ nanorods with high initial discharge capacity and promising cyclic performances were realized in this paper.In this work,the hydrothermal method combined with the solid-phase sintering process was used to achieve the controllable synthesis of LiMn₂O₄ nanorods with different diameters(30-40 nm,70-80 nm,110-120 nm,and 160-170 nm),and the effect of nanorod diameter on the surface chemical state and electrochemical performance of LiMn₂O₄ has been studied.Research shows that a large amount of Mn²⁺appears on the surface of LiMn₂O₄ nanorods with small diameters,which corresponds to the non-electrochemically active Mn²⁺[Mn³⁺Mn³⁺]O₄ phase(the electrochemically inactive surface).What's more,the LiMn₂O₄ nanorod with a critical diameter of approximately70-80 nm(LMO-2)displays the highest discharge capacity and promising cyclic performance.At 0.1 C rate,the first discharge capacity of LMO-2 at 0.1 C is 129m Ah·g^-1,even at a high rate of 20 C,it can still display a high discharge capacity of 62m m Ah·g^-1.In order to further improve the cycle stability,the La element is used to modify the LiMn₂O₄ nanorod cathode materials.The synthetic method,physical characteristics and electrochemical tests of the as-synthesized Li La_xMn_2-xO₄(x=0,0.01,0.03,0.05 and0.10,respectively)nanorods are investigated in detail.The electrochemical measurements show that the Li La_0.03Mn_1.97O₄sample(LMO-3)exhibits the best electrochemical performance,even at a high ambient temperature of 55?,it can still maintains a high capacity retentionof 67.8%after 500 cycles at 2 C rate.In addition,TiO₂-coated LiMn₂O₄ nanorods with different mass fractions(1 wt.%,3 wt.%and 5 wt.%,respectively)were synthesized by hydrolysis of different amounts of tetrabutyl titanate(TBOT).The TiO₂ coating layer can effectively inhibit the Mn dissolution caused by the disproportionation reaction of the LiMn₂O₄.However,the thicker TiO₂ coating layer could hinder the diffusion of Li⁺and affect the rate performance and cycle stability of LiMn₂O₄ cathodes.The LMO-T3 sample coated with3 wt.%of TiO₂ displays excellent electrochemical performance.The discharge capacity of LMO-T3 sample can exhibits a high discharge capacity of 69.3 m Ah·g^-1 at a current density of 20 C,and after 500 cycles at 2 C under a high temperature of 55°C,it can maintains a discharge capacity of 80 m Ah·g^-1.The manganese dissolution test indicates that TiO₂ coating can effectively inhibit the dissolution of Mn³⁺caused by the disproportionation reaction of LiMn₂O₄ electrode during charging and discharging,thus improving the rate performance and cyclic stability of LiMn₂O₄ cathodes.