Controllable Synthesis, Structural Evolution, And Modification Of Ultra-high Nickel Lithium Cathode Materials

With China’s favorable policies for promoting the development of new energy vehicles and achieving“carbon peaking goals”,the lithium-ion battery has been developed rapidly.As the core component of lithium-ion battery materials,cathode materials determine the energy density,cycle life and cost of batteries.Among the cathode material systems of Li-ion batteries,the ultra-high nickel materials have become the primary choice for high energy density batteries.And with the increasingly stringent requirements for cost and sustainability of lithium batteries,the trend of low cobalt or even no cobalt has been favored by the power battery industry.However,with the increase of nickel content and the reduction of Cobalt content in cathode materials,the cycle performance of materials is accelerated to decay and the safety performance is worse,which hinders the commercial application of ultra-high nickel materials.Based on this,this paper selected the ultra-high nickel cathode materials（Ni%≥90%）as the research objects,and studied the evolution law of microstructure of ultra-high nickel cobalt-free material during the sintering process.And the influence on material structure and properties by elements doping and oxide coating was researched systematically.Finally,the influence of microstructure（single crystal,polycrystal）on material properties was researched.The main research contents and results are as follows:（1）The first part is trying to analyze the structural evolution during sintering process.Li Ni_0.94Mn_0.04Al_0.02O₂ was selected as the object to study the structure transformation,valence change and real-time evolution of impurity phase during the sintering process,and then the high temperature solid phase synthesis mechanism of layered transition metal oxides was described.The results show that the Co-free precursor mixed lithium sintering process consists of three stages,namely,the lithium embedding stage at low temperature（<500℃）,lithium ion insertion and cationic ordering stage（500-800℃）,the structure decomposition stage（>800℃）.The results show that the order degree of metal cations,the concentration ratio of Ni³⁺ions and the formation of rock salt phase were constantly changing during the whole sintering process,and tend to the optimal structural state at 800℃,and the corresponding electrochemical performance of the material was relatively better.In further research,the influence of Co content on the evolution of material structure,such as the influence of Co on the formation temperature and decomposition temperature of layered structures,and the inhibitory effect on cation mixing have been systematically studied.This division of labor provides theoretical reference for in-depth understanding of the evolution mechanism of cathode material structure.（2）The 2^nd part of the work focuses on the dopant to modify the crystal structure.The relationship between Li/Ni mixed arrangement and structural stability of Li Ni_0.90Co_0.06Mn_0.04O₂ cathode material was studied.The results showed that the Mo doped samples had the best structural and cyclic stability.Due to the charge compensation effect,the doped high state elements would lead to the increase of Li/Ni mixing degree.Combined with the electrochemical performance and in situ XRD analysis,it was concluded that the Ni-O-Ni structure formed by Li/Ni mixed row had a positive effect on the stability of the cathode material structure.（3）The 3^rd part of the work focuses on the interface modification.The oxides containing B,Al,Mg,Sr,Cs and Co were coated on the surface of the cathode material(Li Ni_0.90Co_0.06Mn_0.04O₂)after washing.The influence of metal oxide coating on the structure and properties of the cathode material after washing was studied.The results showed that the electrochemical performance of the boron oxide coated cathode material was obviously better than other samples.The specific discharge capacity was 220.5 m Ah/g at 0.1 C under 25℃,and the capacity retention rate was95.3%after 50 cycles at 1 C.The capacity retention rate was 94.2%after50 cycles at 0.5 C under 45℃.The analysis showed that boron can form a lithium boro-oxygen coating on the surface of the materials,which was conducive benefit for the rapid transfer of lithium ions.The coating temperature and amount of boron oxide compound were further optimized.（4）From the perspective of commercial application,the 4^th part of the work prepared polycrystalline and single crystal with same composition of Li Ni_0.93Co_0.04Mn_0.03O₂and studied the influence of micromorphology and structure of ultra-high nickel cathode materials on their electrochemical properties were studied.The results showed that the primary particle size of polycrystalline materials was smaller,which led to the short transmission path of lithium ions,so the discharge capacity,rate and low temperature performance were better under the same test conditions.At the same voltage,high-nickel polycrystalline is 6 m Ah/g higher than SC-NCM.With the increase of charge-discharge rate,high-nickel polycrystalline can still maintain a capacity retention rate of nearly 59.3%at 10 C rate,while SC-NMC materials are only 39.4%.The larger particle size and better mechanical strength of single crystal materials were conducive to inhibiting the physical structure of the material from volume changes in the process of charge and discharge,slowing down the physical and chemical attenuation of the material,and showing higher cycle retention rate,less storage gas production and better thermal stability under high temperature and high voltage.Tested at 45°C in a voltage range of 2.8-4.2 V,ultra-high nickel polycrystalline in full cell can only run 652 cycles with a cut-off condition of 80%capacity retention,while monocrystalline materials can run 1186 cycles.This work provides a theoretical reference for a deeper understanding of the evolution mechanism of ultra-high nickel cobalt-less/free cathode material structure.The crystal structure、interface modification methods and micro-morphology explored in this paper also provide new ideas for the industrial application of ultra-high nickel cathode materials.