Design And Performance Studies Of Structural Stability Of High Nickel-type Concentration Gradient Cathode Materials

The selection of positive electrode materials in lithium-ion batteries directly determines the quality of battery performance.High-nickel ternary cathode materials have become the research hotspot of future power battery cathode materials due to their high energy density.However,high nickel ternary cathode materials have shortcomings such as unstable surface structure and poor thermal stability,which limit their application in the market.The above problems can be improved to a certain extent through strategies such as bulk doping,surface coating and structural optimization design.In this paper,taking the high-nickel ternary material Li(Ni0.794Co0.11Mn0.096)O2 as the research object,by optimizing the structural design,a novel concentration gradient with high-manganese-nickel-rich shell,double transition layer with linear concentration change,and high-nickel core is obtained.The positive electrode material is Li(Ni0.9COO.05Mn0.05)0.6(Ni0.7Co0.2Mn0.1)0.3(Ni0.5Co0.2Mn0.3)0.07(Ni0.3Co0.2Mn0.5)0.03O2.The basic data of materials are obtained through parallel experiments and microscopic characterization methods,and based on this,first principles are used to calculate the density of states,energy bands,and charge differences of the materials,and then provide guidance for the performance testing of materials.The main contents are as follows:1.A multi-step co-precipitation method was used to synthesize a high nickel concentration gradient cathode material CG-NCM with a high manganese thin shell and a double transition layer,and parallel experiments were designed.After preliminary screening,it was found that the electrochemical performance of the CG-NCM material was the best.it is good.Based on this,the microscopic characterization of pure phase high nickel ternary material Li(Ni0.794Co0.11Mn0.096)O2(CC-NCM)and CG-NCM material was carried out.XRD results showed that the degree of Li+/Ni2+mixing of CG-NCM smaller.The ICP-AES results showed that the elemental ratios of both materials reached the expected values.The combined characterization results of FIB-SEM-EDS show that the CG-NCM samples form a structure in which the content of Ni gradually decreases from the center to the surface,and the content of Mn and Co increases gradually.The results of particle size analysis were confirmed with the results of XRD and SEM.XPS element valence analysis found that the molar ratio of Ni2+ in the CG-NCM sample was 28.84%,while the molar ratio of Ni2+in the CC-NCM sample was 35.59%,which was consistent with the XRD results.TEM,HRTEM characterization results further corroborate the XPS analysis conclusion.2.On the basis of microscopic characterization,CG-NCM and CC-NCM structures were constructed based on the CASTEP module in Material-Studio software.Through the optimization calculation,it is found that the final energy of the CG-NCM structure is-277338.7 eV,and the CC-NCM is-6983.94 eV,indicating that the CG-NCM structure is more stable.The calculation results of energy band and density of states show that the conductivity of CG-NCM is stronger.The calculated working voltage of CC-NCM is 3.6825 V and that of CG-NCM is 3.6931 V.It is predicted that the CG-NCM structure can withstand higher working voltage and has better electrochemical performance.The calculation of Gibbs free energy and entropy shows that the CG-NCM structure is insensitive to temperature and has better thermal stability.Through the comparison of charge differences,it is found that there is no significant difference in the charge accumulation near the O atoms and Ni atoms in CG-NCM and CC-NCM,and the structure does not affect the lattice of the material,which is confirmed by the XPS results.3.On the basis of the first-principles calculations,we adjusted the battery performance test conditions accordingly.The capacity retention rate of CG-NCM at 4.3 V and 1 C reached 96.25%;at 4.6 V and 1 C,the capacity retention rate was 89.92%,which were both higher than those of CC-NCM.During 200 charge-discharge cycles at 4.3 V,1 C,the average decayed capacity per cycle was 0.293 mAh g-1 for CC-NCM and 0.228 mAh g-1 for CG-NCM.After 500 cycles,the capacity retention rate of CG-NCM is still 61.44%.After 1000 charge-discharge cycles at 4.3 V,1 C,the capacity retention of CG-NCM was 24.17%,which was higher than that of CC-NCM,which was 11.80%.DSC test showed that the thermal stability of CG-NCM was better.CV tests under various conditions showed that the H2-H3 phase of CG-NCM was weaker.EIS analysis showed that the surface film resistance of CG-NCM was smaller and the electronic conductivity was better.The above results show that the high manganese thin shell on the surface of CG-NCM does not affect the capacity performance of the material,and it can effectively suppress the side reaction between the high nickel material and the electrolyte,and the double transition layer also plays a positive role in stabilizing the structure and electron transport,which will provide new research ideas for the development of highly stable lithium battery materials.