Research On Synthesis And Modification Of LiNi_0.5Mn_1.5O₄ High-voltage Cathode Material For Lithium-ion Battery

LiNi_0.5Mn_1.5O₄ material has become a promising high-energy density cathode material by virtue of its advantages such as high discharge voltage plateau at about 4.7 V?vs.Li/Li⁺?,abundant raw material and environmental friendliness.However,the LiNi_0.5Mn_1.5O₄ material is easy to have side reaction with organic electrolyte at high charge-discharge voltage,which causes Mn element dissolution and severe capacities fading.To address this shortcoming,a modification method which combines with bivalent and tetravalent ions co-doping was used to improve the electrochemical performances of LiNi_0.5Mn_1.5O₄ cathode material in this work.XRD,SEM,FTIR and electrochemical tests were adopted to characterize the prepared materials.The main contents and results were summarized as follows:?1?Spinel LiNi_0.5Mn_1.5O₄ materials doped with Mg and Si elements uniformly were successfully synthesized by sol-gel process.Firstly,the effects of Mg/Si single element doping and Mg-Si co-doping modifications on the LiNi_0.5Mn_1.5O₄ materials were compared.When cycled at 0.5 C in the voltage range of 3-5 V?vs.Li/Li⁺?,the undoped LiNi_0.5Mn_1.5O₄ material maintained 79.11%of initial capacities only.However,materials with superior cycling stability were obtained after Mg-doping,Si-doping and Mg-Si co-doping and the capacity retention were 94.76%,88.63%and 98.86%,respectively.It was obvious that the Mg-Si co-doping modification expressed the best effect on the cyclic stability of LiNi_0.5Mn_1.5O₄ material.Then,further explored the amount of Mg-Si co-doping on the influence of structure,morphology and electrochemical properties.Results showed that the most excellent comprehensive electrochemical properties occurred on the sample of LiNi_0.47Mg_0.03Mn_1.47Si_0.03O₄.This most excellent sample could release discharge capacity of 96.9 mAh g^-1 at 5 C rate,which was much superior than the undoped sample that delivered 53.3 mAh g^-1 capacities only at the same condition.Furthermore,the moderate Mg-Si co-doping could decrease the value of added resistance during cycling.?2?Combining with bivalent Cu²⁺and tetravalent Ti⁴⁺to modify the LiNi_0.5Mn_1.5O₄material.Results showed that a proper amount of Cu-Ti co-doping could inhibit the generation of rock salt phase of Li_xNi_1-xO to a certain extent.The results of electrochemical tests showed that Cu doped,Ti doped and Cu-Ti co-doped materials could exhibit the discharge capacity of 110.5 mAh g^-1,114.9 mAh g^-1 and 120.3 mAh g^-1after 100 cycles with a capacity retention of 88.90%,89.42%and 93.33%,respectively.The above results were superior to the undoped material.It was obvious that the effect of Cu-Ti co-doping was better than Cu doping and Ti doping.The influence of the amount of Cu-Ti co-doping on the LiNi_0.5Mn_1.5O₄ material was further explored.The LiNi_0.45Cu_0.05Mn_1.45Ti_0.05O₄ sample had the optimal cycling stability.This material could deliver a discharge capacity of 122.1 mAh g^-1 and with a capacity retention of 97.37%.?3?A series of LiNi_0.5-xMg_xMn_1.5-xTi_xO₄?x=0;0.01;0.03;0.05?materials were synthesized by sol-gel process.Mg-Ti co-doping made the particle size of LiNi_0.5Mn_1.5O₄material more uniform and exposed more crystal planes.These samples doped by Mg-Ti with amount of 0,0.01 0.03 and 0.05 could retain 79.11%,88.83%,99.26%and 86.35%of initial discharge capacity after 100 cycles at 0.5 C,respectively.Results showed that the proper amount of Mg-Ti co-doping could effectively improve the cyclic stability of LiNi_0.5Mn_1.5O₄ material and the sample with doping amount of 0.03 exhibited the optimal electrochemical properties.The optimal material could release a capacity of 99.7 mAh g^-1 when discharged at 5 C high rate,which was much superior to the unmodified material.