The Modification Study On High Voltage Cathode Material LiNi_0.5Mn_1.5O₄ By YPO₄ And YF₃

With the development of science and technology,plug-in hybrid electric vehicles and pure electric vehicles will play an increasingly important role in alleviating the energy crisis and environmental pollution.As a new type of energy storage device,lithium-ion battery has been widely used in various aspects,such as mobile phone,electric vehicle and aerospace,owing its many advantages such as high power density,high energy conversion efficiency,and environmental friendliness.However,traditional anode materials have not been able to meet the demands of high efficiency and high energy in terms of cost,safety and environment.In order to further improve the energy density of lithium ion battery,it is necessary to develop the anode material of lithium ion battery with high energy density and quick charge and discharge capacity.The energy density of the battery depends mainly on the energy density of the electrode material,which could increase the energy density of the battery by improving the working voltage of electrode materials.Therefore,the high voltage spinel material LiNi_0.5Mn_1.5O₄ has recently attracted the attention of researchers.The LiNi_0.5Mn_1.5O₄ cathode material is expected to be the anode material for the next generation of power batteries,which owing to theoretical specific capacity(147 mA h g^-1),high operating voltage?4.7 V?and fast three-dimensional lithium ion diffusion paths within the cubic lattice advantages of lithium ion channels.However,it suffers from a limited cycle life,the electrolyte is degraded and structure-related Mn/Ni ions are dissolved,which directly impacting on the electrochemical performance of active material and hindering its large-scale commercial application.In order to amend these defects,the surface modification and doping used to improve the electrochemical performance of cathode material.Elements of LiNi_0.5Mn_1.5O₄ were replaced with other metal cations or anions,which inhibiting the generation of impurities and improving the electrical conductivity and electrochemical performance.The greatest advantage of a surface coating is that it improves the electrochemical properties of the material without affecting the body phase structure of the material.In this work,LiNi_0.5Mn_1.5O₄ was synthesized by sol-gel method,then,our orientation is to mprove the electrochemical performance of material through a surface modification method.The main work is summarized as follows:1.The YPO₄ layer is successfully coated on the surface of LiNi_0.5Mn_1.5O₄ cathode by a wet hemical method,which enhancing its electrochemical performances.The crystal structures,electrochemical properties and thermal stabilities of the bare and coated materials are studied by X-ray diffraction?XRD?,field emission scanning electron microscopy?FESEM?,high resolution transmission electron microscopy?HRTEM?,electron diffraction spectroscopy?EDS?,inductively coupled plasma?ICP?,galvanostatic cycling,cyclic voltammetry?CV?,electrochemical impedance spectroscopy?EIS?and differential scanning calorimetry?DSC?.It has been found that the electrochemical performances of LiNi_0.5Mn_1.5O₄ surface modification.Especially,the 3 wt.%YPO₄-coated LiNi_0.5Mn_1.5O₄ material could still deliver a high specific capacity of 107 mA h g^-1 after 240 cycles,with a capacity retention of 77.5%,much higher than that of the pristine electrode.Further research shows that the YPO₄ coating layer inhibits the rapid increase of surface impedance,thus promoting the surface dynamics behavior.Through further material aging experiments,the improvement of electrochemical performances could be attributed to the formation of Lewis acid YF₃,converted from the YPO₄ coating layer in LiPF₆-based electrolyte,which not only scavenges the surface insulating alkaline species with high acidity,but also accelerates ion exchange on material surface and thus helps to generate the solid solution Li-Ni-Mn-Y-O on the surface of YPO₄-coated LiNi_0.5Mn_1.5O₄.2.Spinel LiNi_0.5Mn_1.5O₄ was successfully synthesized by a sol-gel method and is further coated ith YF₃?1 wt.%,3 wt.%and 5 wt.%?through a simple wet chemical strategy.The physical characterizations show that the YF₃ coating layers have little impact on cathode structure.Comparison of electrochemical performances indicate that 3 wt.%YF₃ modified electrode exhibits the highest reversible capacity(108 mA h g^-1)and best cycling performance?capacity retention of 79%?after 100 cycles at 0.1 C.Electrochemical impedance spectroscopy?EIS?analysis proves that the rapid increase of surface impedance could be suppressed by YF₃ coating layer,thus facilitates the surface kinetics behavior in repeated cycling.Additionally,differential scanning calorimetry?DSC?tests show that the YF₃ layer helps in enhancing the thermal stability.