High-Valence Metal Ions Doping In Nickel-Rich Layered Cathode Material And Study On Their Modification Mechanism

Nickel-rich layered cathode materials have the advantages of high energy density and low price,which are suitable for automobile power batteries and have good commercial prospects.Compared with low-nickel layered cathode materials,nickel-rich LiNixCoyMe1-x-yO2(Me=Mn/Al,NCM/NCA,x?0.8)layered materials have poor structural stability,which leads to poor cycling performance,rate performance,and thermal stability and restricts the commercial development of nickel-rich layered materials.In this paper,commercial Ni0.88Co0.09Al0.03(OH)2,Ni0.91Co0.045Mn0.045(OH)2 precursor,LiOH·H2O,and nanoscale MnO2,MoO3,and WO3 are used as raw materials,Mn(3d),Mo(4d),and W(5d)doped nickel-rich layered materials are synthesized by high-shear mechanical mixing and high-temperature calcination process.The modification mechanism of high-valence metal ions(Mn/Mo/W)doping in nickel-rich cathode materials is studied in depth.(1)Different amounts of nano MnO2(1,and 2 wt%)are introduced into the surface of the Ni0.88Co0.09Al0.03(OH)2 precursor by high-shear mechanical mixing,and the Mn(3d)uniformly doped LiNi0.88Co0.09Al0.03O2 cathode material is obtained by high-temperature calcination with lithium hydroxide.The surface is reconstructed with a LixNiyMnzO-type rock salt layer of about 20 nm.The Mn-doped NCA material has elevated cyclic stability and thermal stability,at 2.5-4.3 V,the discharge capacity retention rate increases at 1 C from 64.6%to 82.2%after 200 cycles;at 2.7-4.5 V,the discharge capacity retention rate at 1 C increases from 77.7%to 86%after 100 cycles;the initial thermal decomposition temperature of the delithiated Mn-doped NCA material increases by about 25?.The reason for improved cycling stability and thermal stability lies in that,on the one hand,the surface rock salt layer is electrochemically inactive and alleviates the occurrence of surface side reactions;on the other hand,strong Mn(3d)-O bond(402.9 kJ mol-1)is formed between the doped Mn and O,which could inhibit the lattice change in the process of lithium intercalation/deintercalation,and reduces H2-H3 phase transition,thus slowing down the generation of microcracks between primary particles;at the same time,oxygen release is reduced during the thermal decomposition.(2)Different amounts of nano MoO3(1,3,and 5 wt%)are introduced into the surface of the Ni0.91Co0.045Mn0.045(OH)2 precursor by high-shear mechanical mixing,and the Mo(4d)gradient doped LiNi0.91Co0.045Mn0.045O2 cathode material is obtained by high-temperature calcination with lithium hydroxide.The surface is reconstructed with LixNiyMozO-type rock salt layer about 10 nm,and the primary particles are refined and radially distributed through the spherical particle.The 3 wt%Mo doped NCM material shows preferable electrochemical properties and thermal stability,at 2.7-4.5 V,the discharge capacity at 5 C increases from 156.8 mAh g-1 to 198.1 mAh g-1,and the discharge capacity retention rate at 1 C increases from 53.4%to 80%after 200 cycles;the spinel to rock salt phase transformation temperature of the delithiated Mo gradient doped NCM material increases by 50?.The improved high-voltage structure stability of Mo gradient doped NCM cathode material is due to that,strong Mo(4d)-O bond(560 kJ mol-1)is introduced to stabilize lattice oxygen,the H2-H3 phase transition under high voltage is inhibited,the generation of microcracks is reduced and the surface layered phase to inactive rock salt phase transformation is restrained;at the same time,oxygen release is reduced during the thermal decomposition.(3)1/2/5 wt%WO3 is coated on the surface of Ni0.91Co0.045Mn0.045(OH)2 precursor,and with following high-temperature calcination,the surface W(5d)doped LiNi0.91Co0.045Mn0.045O2 cathode material is synthesized.A LixNiyWzO-type rock salt layer of 5?10 nm is reconstructed on the surface and the primary particles become small and are radially distributed along the radial direction.The 2 wt%W doped NCM material shows excellent electrochemical properties and thermal stability,at 2.5-4.3 V,the discharge capacity retention rate at 1 C increases to 94.1%after 200 cycles;at 2.7-4.5 V,the discharge capacity retention rate at 1 C increases to 91.5%and the discharge capacity at 5 C increases to 199.5 mAh g-1;the spinel to rock salt phase transformation temperature of the delithiated surface W doped NCM material increases by 70?.The improved rate performance of surface W doped NCM material is due to the that the radial distribution of primary particles contributes to improving lithium-ion diffusivity during the cycling process.The enhanced cycling performance and thermal stability are due to the that the lattice oxygen is strengthened by introducing a strong bond of W(5d)-O(672 kJ mol-1),thus enhancing the surface structure stability,as a consequence,the generation of the surface inactive NiO phase and the occurrence of surface side reactions slow down during the long-term charge/discharge process;moreover,the incidence of microcracks between primary particles and the penetration of the electrolyte is alleviated;at the same time,oxygen release is greatly reduced during the thermal decomposition.