Integrated Modification And Its Mechanism Study Of Ni-rich Layered Cathodes For Lithium-ion Batteries

Considering high specific capacity(>200 mAh g-1)and operating voltage(～3.8 V),Ni-rich layered oxide(LiNixCoyM1-x-yO2,M=Al/Mn,0.8≤x≤1)is one of the most promising cathodes for commercialization toward high-energy-density lithium-ion batteries.However,poor cycling stability and safe concerns limit wider applications of Ni-rich cathodes in electric vehicles(EV).These drawbacks can be attributed to high surface reactivity and intrinsic structural instability of Ni-rich cathodes during Li4 insertion/extraction.To address these problems,we proposed various facile modification strategies to achieve new breakthroughs in electrochemical performance and thennal stability of Ni-rich cathodes,and revealed their underlying mechanisms,providing feasible solutions and its vital theoretical foundations for further commercial applications of Ni-rich cathodes.First,by incorporating embedded type Y-doped ZrO2(YSZ)coating,a surface modified Ni-rich layered cathode LiNi0.88Co0.09Al0.03O2 was performed.YSZ coating prevents unfavorable surface side reactions with electrolytes and suppresses microcracks inside secondary particles for Ni-rich cathodes.Moreover,high concentrations of oxygen vacancies in YSZ relieve the hindrance of the coating for lithium-ion diffusion,and thus decrease surface impedance buildup.As a result,the YSZ coating strategy improves the rate capability and cycling stability of Ni-rich cathodes.Half-cell tests show that the 1 wt.%YSZ-modified sample provides a high initial capacity of 215.1 mAhg-1(2.5-4.3 V,0.1C)and a high capacity retention of 85.5%for 100 cycles at 1C compared with NCA(210.6 mAh g-1 for initial capacity,67.5%capacity retention for 100 cycles).Second,high-valence Ta5+ ions preferring spontaneous surface enrichment were introduced into LiNi0.88Co0.10Al0.02O2 Ni-rich cathode to improve the surface and bulk structural stability,achieving superior electrochemical performance and thermal stability.By incorporating spherical aberration-corrected transmission electron microscopy,synchrotron spectroscopy,differential electrochemical mass spectrometry,various in-situ coupling techniques,and first-principles calculations,an integrated picture of high-valence Ta5+ions doping mechanisms in Ni-rich cathodes is presented.In addition,as a case study,we proposed two new insights of high-valence ions doping mechanisms for Ni-rich cathodes,namely surface structural regulation and bulk charge regulation:1)Ta-induced surface structural regulation suppresses Ni reduction in deep delithiation,improving surface structural stability;2)Ta prompts bulk charge regulation that mainly reflects oxygen charge increase and localization,relieves severe anisotropic lattice expansion/shrinkage and excessive delocalization of oxygen charge in deep delithiation,suppressing microcracks and oxygen loss.To avoid the interference of Co and Al,Ta-doped LiNiO2 was synthesized and studied by using the same preparation method.Similar physicochemical and electrochemical properties for Ta-doped LiNiO2 are observed,which confirm the reliability of the above conclusions.Half-cell tests show that 0.4 mol.%Ta-doped sample provides an initial discharge capacity of 213.3 mAh g-1 at 0.1C and a superior capacity retention of 80%for 624 cycles with a high initial capacity of 191.3 mAh g-1 at 1C.2 Ah fullcell tests indicate the capacity retention for 1000 cycles is 85%at 1C vs.graphite.In addition,0.8 mol.%Ta-doped sample provides a capacity retention of 80%up to 768 cycles with an initial capacity of 183.1 mAh g-1 at 1C.Finally,to further improve the electrochemical performance and thermal stability of LiNi0.88Co0.10Al0.02O2 material,we combined the above two modification strategies,and the stabilizing effect of Ta for bulk and the protecting effect of YSZ for surface were integrated.Further,the coating process was optimized by a wet mixing method to improve the uniformity and decrease the thickness of the YSZ coating layer.The integrated modified sample provides a high initial capacity of 212.2 mAh g-1 at 0.1C and a capacity retention of 80%for 739 cycles with an initial capacity of 185 mAh g-1 at 1C.Differential scanning calorimetry(DSC)tests show that the exothermic peak temperature is 261℃.By surveying discharge capacity,cycling life,and thermal stability,the integrated modified sample presents better performance compared with reported literature on modification strategies,which exhibits a great potential for applications.Various analyses reveal the integrated modification mechanisms in stabilizing bulk and surface structure.Ta doping stabilizes the bulk structure and suppresses the microcracks.In addition,the dualstructural surface constructed by ultrathin YSZ coating and Ta-induced rock-salt phase further suppresses surface side reactions and structural degradation,improving the surface stability.Such synergistic effects boost the cyclability and thermal stability,which emphasizes the significance of the integrated modification strategy for the surface and bulk of Ni-rich layered cathodes.