Preparation And Electrochemical Properties Of High Nickel Cobalt-free Cathode Materials

Due to the high price of cobalt and the high specific capacity provided by high nickel,the research on reducing cobalt content and increasing nickel content is the key to solving the cost and capacity problems of layered transition metal oxides（NCM）.LiNiO₂,which has the highest nickel content and no cobalt,is promising.However,LiNiO₂ is unable to be commercialized due to a series of fatal problems,such as poor cycling performance,poor structural stability,and severe cationic mixing.In order to solve the above problems,this paper studied the modification of the layered LiNiO₂ cathode material from the aspects of coating modification and doping modification.The main contents are as follows:（1）This chapter was used commercial spherical Ni（OH）₂ and Li OH·H₂O as raw materials,the optimal synthesis temperature of LiNiO₂ was investigated.On this basis,the method of solid-phase mixing and low-temperature sintering was used to introduce about 35 nm thick selenium coating to modify LiNiO₂.At a cutoff voltage of 2.8-4.3 V and a current density of 1 C,the capacity retention rate of selenium-coated LiNiO₂（Se-LNO）cathode material reached 82.09%after 300 cycles,while that of unmodified LiNiO₂ was only 31.31%under the same conditions.In addition,selenium-coated improved the magnification performance of LiNiO₂,and the specific capacity of Se-LNO material is 168.7 and 149.6 m Ah·g^-1 at 2 C and 5 C current densities,respectively.XRD,SEM,TEM,XPS,and other characterization methods showed that the selenium-coated can enhance the structural stability of the material by inhibiting phase transformation,stabilizing the interface,enhancing the kinetic behavior of the electrode reaction and protecting the secondary particles from the corrosion of the electrolyte.（2）The Mg/Al co-doped LiNiO₂ cathode material was synthesized by in-situ introduction of Mg/Al on Ni（OH）₂ surface and lithium sintering,and the optimal doping gradient was studied.The XRD、SEM and electrochemical test results showed that 3%Mg/Al-LNO material has excellent high voltage ratio and cycling performance.At a high cutoff voltage of 2.8-4.6 V and a current density of 1 C,the capacity retention rate of 3%Mg/Al-LNO material for 500 cycles is 70%,and the specific capacity at 5 C and 10 C is 162.1 and 148.9 m Ah·g^-1,respectively.Both the cycle retention rate and the rate performance are higher than commercial NCM811.More than the unmodified LiNiO₂.The material characterization results show that the cathode material has stable Li⁺diffusion coefficient and good electronic conductivity.Mg/Al in Li layer and Ni layer,respectively,significantly inhibited the degree of Li⁺/Ni²⁺disorder and irreversible phase transition.In addition,the doping of Mg/Al elements refines the primary particles,increases the density of secondary particles,relieves the stress caused by repeated cycles,prevents material cracking,and avoids the erosion of electrolyte.Finally,the structural stability of LiNiO₂ was significantly improved.