Preparation And Properties Of Nickel-rich Ternary And Lithium-rich Manganese-based Cathode Materials For Lithium-ion Batteries

With the rapid development and application of electric vehicles,portable appliances,and smart grid storage,environmental pressures and energy issues are becoming increasingly prominent.Reducing environmental pollution while meeting human energy needs is a major challenge.Nowadays,lithium-ion batteries have higher power density,higher energy density,and better safety and stability than other secondary batteries,and they have attracted much attention.The various aspects of the performance of the cathode material are one of the keys to lithium-ion batteries.The existing cathode materials,such as lithium cobaltate and lithium iron phosphate,can no longer meet the increasing demand.Therefore,researchers are devoted to the development of positive electrode materials with high specific capacity and rate performance.The high specific capacity of nickel-cobalt lithium manganese lithium and manganese-based lithium-rich cathode materials for lithium-ion batteries has caused extensive research by researchers.The main purpose of this paper is to synthesize high-performance lithium-ion battery cathode materials.Ternary LiNi_0.6Co_0.2Mn_0.2O₂ and 0.5Li₂MnO₃·0.5LiCo_0.5Mn_0.5O₂ are selected as research objects.The research on the preparation,characterization and performance regulation provides experimental foundation and theoretical basis for the research and preparation of high-performance lithium-ion battery cathode materials.Nickel-rich layered materials(LiNi_1-x-yCo_xMn_yO₂,x?0.5)have attracted considerable interests as promising cathode materials for lithium ion batteries owing to their higher capacities and lower cost.Nevertheless,nickel-rich electrode materials usually suffer from poor cyclability due to the unavoidable by-reactions between Ni4+ on the surface and electrolytes.The design of gradient-concentration particles with Ni-rich inside and poor outside is proved to be an efficient way to address this issue.The material design of the inner nickel-rich outer layer and the manganese-rich material can effectively alleviate the side reactions to Ni4+ and the electrolyte,improve the structural stability of the material,and improve the rate performance and cycle performance of the material.Through the rational design of the hydroxide co-precipitation process,the raw material solution and the feeding method in the preparation process were accurately designed,the metal ion concentration of different reaction time was adjusted,and the same raw material and reaction time were used to prepare a successful synthesis.For LiNi_0.6Co_0.2Mn_0.2O₂?NCM622?materials with different gradient concentrations,the atomic ratio of nickel and manganese decreases linearly from the inner core to the outer shell.NCM622 microspheres with four different gradient-concentration slopes?the atomic ratio of Ni/Mn decreasing linearly from the core to the outer layer?are produced to investigate and compare their microstructure characteristics and electrochemical performance.It is found that different gradient concentrations of NCM622 materials have a significant effect on their microstructure and electrochemical properties.The as-prepared electrode materials GC3.5 type NCM622?the final atomic ratio of Mn is 3.5?with optimal gradient concentrationachieve excellent electrochemical performances,delivering a discharge capacity of over 176 m Ah g-1 at 0.2 C rate and exhibiting capacity retention of up to 94% after 100 cycles at 1 C.Using co-precipitation preparation method,by optimizing the parameters and process in the precursor preparation process and controlling the precipitation process and preparation conditions,a quasi-spherical nickel-cobalt-manganese GC3.5 type NCM622 ternary cathode material with uniform size,morphology and structure has been prepared.With the help of the orthogonal test design,the test results were analyzed using the range method.The primary and secondary order of the influence of each factor on the tap density results was reaction time,ammonia concentration,stirring speed,and p H value,which affect the particle size and distribution of the product particles and the tightness of the primary particle arrangement on the surface.The optimal level was determined as a reaction time of 16 h,ammonia concentration of 0.5 mol L-1,stirring speed of 600 r min-1,and p H value of 11.The GC3.5 spherical precursor prepared by the best level group has good sphericity,an average particle size of 6.6 ?m,a flat surface,less secondary particle agglomeration,excellent processing performance,and a tap density of 2.08 g cm-3.At a discharge rate of 0.2 C,the specific discharge capacity of the material is 175 m Ah g-1;the specific discharge capacity of 20 C/0.2 C of the material is 70%;after 200 cycles,the capacity retention rate is 87.5%.According to the optimized preparation process of the gradient nickel-cobalt-manganate-preparation,the experimental study on the scale up preparation of lithium-ion battery ternary cathode material?Li Ni1-x-y Cox Mny O2?was completed.The batch yield was more than 5 kg.The prepared ternary material with high specific capacity was used to manufacture different specifications of experimental soft battery cell with good electrochemical performance.A simple stepwise co-precipitation method of oxalate,based on the principle of the non-equilibrium mass diffusion,has been used to prepare Co0.5Mn0.5C2O4,which was then used as a template to synthesize Ni0.6Co0.2Mn0.2C2O4 precursor.Due to the effect of the Oswald ripening process,the diameter of the rod-like particles becomes bigger once and is converted into a hollow structure.After mixing with lithium source and sintering,a spherical LiNi_0.6Co_0.2Mn_0.2O₂ cathode material for lithium ion batteries is obtained.The material is a secondary regular spherical particle composed of uniformly dispersed primary rod-shaped particles.There is a gap between the primary rod-shaped particles,and the particle size is about 6-8 ?m.The gap between the thin particles of the primary particles of the material increases the specific surface area of the material,which increases the contact area between the material and the electrolyte,shortens the migration distance of lithium ions inside the material particles,and is conducive to rapid charge and discharge and rate performance.The gap between the primary particles of the material and the hollow structure can buffer the volume change of the material during the charge and discharge process,reduce the structural deformation,avoid the micro-cracks in the particles,etc.,thereby improve the cycle performance.At a discharge rate of 0.2 C,the specific discharge capacity of the material is 180.6 m Ah g-1;the discharge specific capacity ratio of 2 C /0.2 C is 81.6%,and the discharge specific capacity ratio of 10 C / 0.2 C is 77.2%;after 100 cycles,the capacity is still maintained at 150.6 m Ah g-1,and the capacity retention rate was 94.2%.Using a simple oxalate co-precipitation method,by adjusting the reaction conditions of the stirring speed and the effect of the solution shear force on the sample morphology,combined with the subsequent mixed lithium roasting,various morphological Li-rich manganese-based 0.5Li2 Mn O3·0.5Li Co0.5Mn0.5O2 cathode materials for lithium-ion batteries were prepared respectively.The results show that the sample prepared by coprecipitation at a low stirring speed has a solid spherical morphology,which is assembly of massiverod-like primary particles.There are many gaps between the rod-shaped particles at one time,which provides a convenient channel for the electrolyte diffusion,and helps the rapid transport and diffusion of lithium ions inside the material,thereby exhibiting excellent electrochemical performance.The as-prepared solid sphere sample exhibits a greater tap density of 1.7 g cm-3 and better electrochemical performances?i.e.,discharge capacity of 233.8 m Ah g-1 at 0.2 C rate,2 C/0.2 C specific discharge capacity ratio of 62.2% and capacity retention up to 90.8% at 0.5 C rate after 100 cycles?,compared with the dispersed microrod sample.