Study On Dual Surface Modification And Mechanism Of Li-excess Rocksalt-structural Li_1.2Ni_0.3Ti_0.3Nb_0.2O₂ Cathode Materials

At present,the design and development of high-energy-density cathodes have become the key to the further development of lithium-ion batteries.Li-excess rocksalt-structural materials with high capacity(>200 m Ah g^-1)and high energy density attract more attention recently.However,there are still some intrinsic defects for these materials,such as serious voltage attenuation and poor high-rate performance.In order to overcome the above shortcomings,domestic and foreign scholars mostly use Mn-based materials as the matrix to improve performances through element substitution and surface coating,but there are few studies on Ni-based materials.Considering that Ni-based materials involve two-electron reactions ensuring higher theoretical capacity and the problem of dissolution for Ni ions is smaller than that for Mn ions,this dissertation sets the Ni-based material Li_1.2Ni_0.3Ti_0.3Nb_0.2O₂（LNTNO）as the matrix.On the basis of studying the physical and chemical properties and deficiencies of LNTNO,different coating materials（carbon,Al₂O₃and carbon+Al₂O₃）are used to modify the performance of LNTNO.The effects of these materials on the performance of LNTNO with relating mechanism are investigated thoroughly.The main contents are as follows:First,using the sol-gel method to synthesize LNTNO cathode material and studying physical and chemical properties.The results show that LNTNO possesses a cationic disordered rocksalt structure and the particles are within 150 nm.The first discharge specific capacity of LNTNO at20 m A g^-1 is 206.8 m Ah g^-1 with a coulombic efficiency of only 77.2%.After 50 cycles at 40 m A g^-1,the capacity is only 122.5 m Ah g^-1 with the capacity retention rate of only 76.5%.Insitu-XRD result indicates that the irreversible oxygen release of LNTNO is less than Li-rich Mn-based materials.Nevertheless,XANES and XPS tests show that the activities of Ni²⁺/Ni⁴⁺and O^-/O^2-in LNTNO have been limited,and there is still irreversible oxygen release.Secondly,the LNTNO surface is coated with different contents（LNTNO:sucrose=5:1,6:1,7:1,8:1）of carbon by the liquid phase method combined with sintering at different temperatures（350,400,450,500℃）.The lithium diffusion coefficient of LNTNO@C,synthesized by appropriate sintering temperature（450℃）and carbon source ratio（6:1）,is 5.568×10^-13 cm² s^-1（twice as much as that of bare LNTNO）.The first discharge specific capacity of that material is268.2 m Ah g^-1,and the capacity remains 187.4 m Ah g^-1 after 50 cycles at 40 m A g^-1with a capacity retention of 81.6%.Additionally,LNTNO@C（450,6:1）exhibits better high rate performance with a capacity of 176.4 m Ah g^-1 at a current density of 400 m A g^-1.Carbon coating improves the capacity by promoting the activity of O^-/O^2-,and improves the rate performance by increasing the electronic and ionic conductivity.Then the surface of LNTNO is coated with different contents（0.5wt%,1wt%,2wt%,3wt%）of Al₂O₃ by the liquid-phase method combined with solid-phase sintering.The lithium diffusion coefficient of LNTNO@Al₂O₃,obtained by the coating amount of 1wt%,is 9.339×10^-13 cm² s^-1（three times as much as that of bare LNTNO）.The first discharge specific capacity of the material is 208.1 m Ah g^-1 and the capacity retention is 93.4%after 50 cycles at 40 m A g^-1.The Al₂O₃ layer results in a decrease of the lattice,which increases the Ni²⁺/Ni⁴⁺activity and the main voltage.Besides,the Al₂O₃ layer elongates the cycle life of the material by reducing side reactions,the interface,and charge transfer impedance.Finally,the liquid phase method combined with the solid-phase sintering is used to carry out the dual coating of carbon and Al₂O₃ on the surface of the LNTNO.The lithium diffusion coefficient of LNTNO@C-Al₂O₃ is 1.74×10^-12 cm² s^-1,one order of magnitude higher than that of the bare LNTNO.And the first discharge capacity of LNTNO@C-Al₂O₃ is 224.9 m Ah g^-1.The capacity maintains 161.2 m Ah g^-1 after 100 cycles at 40 m A g^-1 with the retention of 81.5%.XPS,XAS,and DFT results indicate that the double-coated LNTNO@C-Al₂O₃ possesses more O vacancy and the narrower overlap between Ni 3d and O 2p bands,corresponding to higher O^-/O^2-and Ni²⁺/Ni⁴⁺activity.The carbon-doped layer improves the capacity and high rate performance,while the Al₂O₃ layer reduces resistance and increases cycle life.