Study On Stable Surface Construction Of High Nickel Ternary Cathode Materials And Their Electrochemical Performance

The rapid development of electric vehicles and electronic devices has led to the development of lithium-ion batteries in the direction of higher energy density and cycle stability,of which high nickel ternary cathode material（NCM）is considered to be one of the most promising cathode materials due to its high discharge capacity and energy density.However,NCM are prone to severe surface structure degradation,irreversible oxygen release and interfacial side reactions during cycling,leading to rapid capacity and voltage decay,which hinders their commercial application.Without changing the structural characteristics of the electrode material itself,the polycrystalline Li_xCoO₂（LCO）,low content NaF and amorphous Li-La-Ti-O（LLTO）coatings are formed on the surface of Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）by surface coating modification method.It improves the surface stability of the electrode material while increasing the Li⁺diffusion rate at the surface and interface of the electrode material.The effects of process parameters such as heat treatment temperature and coating content on the electrochemical impedance,cyclic voltammetry,cyclic performance and multiplicative performance of the cathode material were systematically analyzed,and the mechanism of its electrochemical performance enhancement was analyzed.The specific results are as follows:（1）By constructing Li_xCoO₂ crystalline cladding layer on the surface of NCM811by wet chemical method,the effects of preparation process（heat treatment temperature and coating content）on the physical phase characteristics and electrochemical properties of NCM811 were investigated.The surface of NCM811 becomes increasingly rough with changes in heat treatment temperature and coating content,but its shape and structure remain unchanged;The electrochemical properties of Li_xCoO₂@NCM811 showed the best values at a heat treatment temperature of 500℃ and a coating content of 1.0 wt%:the capacity retention can be increased to 87%after 100 cycles at 1 C（2.8-4.5 V）,and the small potential difference of the CV curve also shows the high electrochemical reversibility and small electrode polarization of the material;Further electrochemical characterization showed that:Li_xCoO₂@NCM811 can achieve 71%cycle retention at 1C（2.8-4.5 V）for 300 cycles,77%capacity retention at a high charge cut-off voltage of 1C（2.8-4.8 V）,and 132.4 m Ah·g^-1 initial discharge capacity at 5 C（2.8-4.5 V）.The better recombination of CV curves at high voltages also shows that the material has good electrochemical reversibility.（2）A low-content NaF coating was generated in situ on the surface of NCM811 by surface hydrolysis,and the effect of the preparation process（heat treatment temperature and coating content）on the physical phase and energy storage characteristics of NCM811was explored.With the change of heat treatment temperature and coating content the surface of NCM811 gradually becomes rough,but its shape and structure do not change significantly;The electrochemical properties of NaF@NCM811 material reached the optimum value at a heat treatment temperature of 550℃ and a NaF content of 0.3 wt%:capacity retention increased from 53%to 96%after 100 cycles at 1 C（2.8-4.5 V）,the potential difference of CV curves decreased,the curve overlap increased significantly,and the electrochemical reversibility was higher;Further electrochemical characterization showed that:NaF@NCM811 can reach 81%cycle retention after 300 cycles at 1 C（2.8-4.5 V）,while the capacity retention of the material is also as high as 81%at a high charge cut-off voltage of 1 C（2.8-4.8 V）.The initial discharge capacity can reach 150 m Ah·g^-1at 5 C（2.8-4.5 V）.Moreover,the high recombination of CV curves and low potential difference values at a high charge cut-off voltage of 4.8 V also indicate the superior electrochemical reversibility and electrode polarization performance of the material.（3）Amorphous LLTO coating was deposited on the surface of NCM811 by co-deposition method,by analyzing the physical phase characteristics and electrochemical properties of LLTO@NCM811 materials with different coating contents,it was found that:the increase in coating content makes the surface of NCM811 increasingly rough,but does not affect the shape and structure of NCM811;the optimal cladding amount is1.0 wt%,the discharge capacity at 0.1 C is 232.4 m Ah·g^-1,the capacity retention after100 cycles at 1 C（2.8-4.5 V）is 93%,and the CV curves with high recombination and small potential difference also show excellent electrochemical reversibility and small electrode polarization;further electrochemical characterization showed that:LLTO@NCM811 can achieve 72%cycle retention after 300 cycles at 1 C（2.8-4.5 V）,its capacity retention can reach 73%at high charge cut-off voltage of 1 C（2.8-4.8 V）,and the initial discharge capacity can reach 154 m Ah·g^-1 at 5 C（2.8-4.5 V）.The CV curves at 4.8 V high charge cut-off voltage also show excellent curve overlap and Low potential difference value.