Research On Modification Of High Energy Density Nickel-Rich Ternary Cathode Materials

With the further commercial application of Electric Vehicles（EVs）,the market has put forward higher requirements for its endurance mileage.Moreover,“Made in China2025”has made specific requirement for the energy density of power batteries,which the energy density of single cell need to reach 300 Wh/kg in 2020 and 400 Wh/kg in 2025.Typical layered Ni-rich LiNi_xCo_yMn_1-x-yO₂ cathode materials have attracted extensive attention due to its high voltage platform,high compaction density and low cost.However,LiNi_xCo_yMn_1-x-yO₂ faces a series of key challenges,such as structural degradation,voltage platform decay and side reactions,which leads to poor cycling performance.Especially,when cycled at 4.5 V high cut-off voltage,the cycling stability of LiNi_xCo_yMn_1-x-yO₂ cathode materials trend to be severe.To overcome these issues,modification strategies such as surface coating,gradient doping are used to enhance the electrochemical performance of LiNi_xCo_yMn_1-x-yO₂ cathode material at 4.5 V high cut-off voltage.The main content includes the following aspects:1)The sol-gel method and in-situ chemical polymerization are successfully implemented to optimize the electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ at 4.5V high cut-off voltage.The Li₃VO₄-PPy coated LiNi_0.6Co_0.2Mn_0.2O₂ sample exhibits excellent cycling stability at 0.5 C/4.5 V with a retention rate of 93.7%after 100 cycles,whereas the pristine LiNi_0.6Co_0.2Mn_0.2O₂ only shows a capacity retention of 73.6%.Moreover,it also shows superior cycling performance at large current（2 C）with a retention rate of 93.8%,whereas the pristine LiNi_0.6Co_0.2Mn_0.2O₂ only shows low capacity retention of 61.6%.The dual-conductive layer effectively suppresses the side reactions on the surface of the cathode material and effectively optimizes the interfacial ionic/electronic transfer performance.2)The gradient phosphate polyanion doping and dual-conductive layer coating are applied to enhance the electrochemical properties of LiNi_0.6Co_0.2Mn_0.2O₂ cathode material at 4.5 V high cut-off voltage.Firstly,the gradient doping of PO₄^3-synergistically achieves the element doping and in-situ coating of Li₃PO₄ in the near-surface region.The result shows the P0.02-NCM sample exhibits the optimal electrochemical properties.Secondly,a wet-coating strategy is conducted to realize polyaniline（PANI）coating,which combines the gradient phosphate polyanion doping and dual-conductive layers（Li₃PO₄-PANI）coating.The phosphate polyanion gradient doping can be described as a“Support role”to optimize the crystal structure.Moreover,the dual-conductive（Li₃PO₄-PANI）layers can be described as a“Palisade role”to inhibit the side reactions and promote the ionic/electronic conductivity of the LiNi_0.6Co_0.2Mn_0.2O₂ cathode.As a result,the combination of gradient doping and dual-conductive layer coating can be utilized to improve electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ cathode at 4.5 V and 55℃.3)The core-shell structure induced by high-valent titanium ion(Ti⁴⁺)effectively enhances the surface structural stability of LiNi_0.8Co_0.1Mn_0.1O₂ cathode material.The thickened protective cation-mixed layer satisfies the key requirements for inhibiting surface structural degradation,lattice matching and volume shrinkage.As a result,the Ti-modified LiNi_0.8Co_0.1Mn_0.1O₂ cathode exhibits a superior cycling performance with a capacity retention of 92.3%after 100 cycles（0.5 C,2.7⁴.5 V）,whereas the pristine LiNi_0.8Co_0.1Mn_0.1O₂ cathode only shows a quite low capacity retention of 81.0%.The results of FESEM,HRTEM and electrochemical performance indicate that the epitaxial protective layer effectively inhibits the surface structural degradations,voltage platform decay,and secondary particle microcracking of LiNi_0.8Co_0.1Mn_0.1O₂ cathode material during the long-term cycling.4)A double-shell hybrid nanostructure induced by Si⁴⁺surface modification is applied to improve the electrochemical performance of LiNi_0.8Co_0.1Mn_0.1O₂ cathode material at 4.5 V high cut-off voltage.The double-shell nanostructure consists of a Li₂SiO₃ coating layer and a cation-mixed layer（Fm 3?m phase）,which realizes the effective integration of multiple functions.The analysis shows that the Si-modified NCM811 cathode exhibits outstanding cycling performance with a capacity retention of95.2%and 87.3%at 4.3 V and 4.5 V after 100 cycles,respectively.This performed double-shell hybrid nanostructure alleviates side reactions,structural degradation,and internal cracking,effectively enhancing surface structural stability.This efficient strategy provides a valuable step towards further commercial applications of high voltage LiNi_0.8Co_0.1Mn_0.1O₂ cathode and can be extended to other layered cathode materials.