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.