Preparation And Modification Of The High-voltage Spinel LiNi_0.5Mn_1.5O₄ Cathode Material

The electrochemical properties of high-voltage spinel LiNi_0.5Mn_1.5O₄ cathode material is mainly affected by the morphology,the content of Mn³⁺and the side reaction between the active material and the electrolyte.Therefore,in this paper,the control of spherical morphology,exposed crystal plane and the modification with doping and coating were respectively investigated to explore what effect they have on the electrochemical performance of LiNi_0.5Mn_1.5O₄ materials.The main research contents and conclusions are as follows:（1）The spherical LiNi_0.5Mn_1.5O₄ cathode materials were prepared by a rapid precipitation hydrothermal method using（NH₄）₂CO₃ and Na₂CO₃ as mixed precipitants.The molar ratio of（NH₄）₂CO₃ and Na₂CO₃ in the mixed precipitants has a great influence on the integrity of spherical morphology,the size of the primary particles and the ratio of Ni and Mn.When the molar ratio of（NH₄）₂CO₃/Na₂CO₃ was set at 1:2,the product exhibits a superior electrochemical performance due to its porous spherical morphology,smaller primary particle size and stoichiometric ratio of Ni and Mn.The highest specific discharge capacity and the capacity retention after 200 cycles of the sample is 129.4 mAh g^-1 and 75.3%at 0.5 C,respectively.The discharge specific capacities of the sample are 110.7 mAh g^-1 and 93.8 mAh g^-1 at 10 C and 20 C,respectively.（2）LiNi_0.5Mn_1.5O₄ cathode materials with different morphologies（different exposed crystal planes）were prepared by using rod-likeβ-MnO₂ as template followed by a calcination with lithium acetate and nickel acetate at different temperatures.As the calcination temperature increasing,the material experienced from rod to rod-like and then to truncated octahedral.During this process,the（100）crystal plane gradually exposed and expanded,and the particle size of the material gradually increased larger.In comparison with the（111）crystal plane,the（100）crystal plane improves the cycle and rate performance of the material greatly for its lower concentration of transition metal and higher concentration of lithium ions.When the calcination temperature was set at 775℃,the product exhibits the best electrochemical performance because it possesses both the（100）crystal face and the size of rod-like particle.The initial specific discharge capacity and the capacity retention after 100cycles of the sample are 130.1 mAh g^-1 and 89.3%at 0.5 C,respectively.The discharge specific capacities of the sample are 110.1 mAh g^-1 and 108.2 mAh g^-1 at 50 C and 100 C,respectively.The whole-cell using lithium intercalated graphite as anode displays a capacity retention of 97.6%after 100 cycles at 0.5 C.（3）Al co-doping Ni and Mn sites and Al₂O₃ coating were applied to modify the best sample in（1）by grinding it with nano-Al₂O₃ followed by a high-temperature calcination and stirring in liquid followed by heat treatment at different temperatures,respectively.Al doping was applied to stabilize the structure and increase the disorder degree of the material,so as to improve the cycle and rate performance of the material,respectively.When the doping content was set at 5%,the product has the best electrochemical performance.The initial specific discharge capacity of the sample is 128.4 mAh g^-1.The capacity retention after 200cycles at room temperature and high temperature are 85.6%and 88%at 0.5 C,respectively.The discharge specific capacities of the sample are 125.5 mAh g^-1 and 123.1 mAh g^-1 at 10 C and 20 C,respectively.Al₂O₃ coating was applied to hinder the direct contact between the material and electrolyte and prevent it from the erosion of the electrolyte,which will ease the dissolution of Mn and control the thickness of the SEI film reasonably.Therefore,the cycle performance of the material is greatly improved.When the heat treatment temperature was set at 500℃,the product has the best electrochemical performance.The initial specific discharge capacity of the sample is 128.8 mAh g^-1.The capacity retention after 200 cycles at room temperature and high temperature are 90.7%and 88%at 0.5 C,respectively.The discharge specific capacity of the sample is 106.6 mAh g^-11 at 20 C.