Refining The Strategy And Mechanism Of Improving Lithium Ion Diffusion Behavior In Nickel-Rich Ternary Cathode Materials

LiNi_xCo_yMn_zO₂（NCM）has attracted extensive attention due to its high discharge-specific capacity.However,the rapid capacity degradation and poor rate capability hinder the application of Rich-Ni layered LiNi_xCo_yMn_zO₂（NCM）as cathode materials for high-energy lithium-ion batteries.In this paper,we focus on the multi-crystalline and single-crystalline NCM positive electrodes and investigate the mechanisms by which ion doping and magnetic field treatment can improve their ion transport behavior.High-temperature solid-phase method was used to successfully introduce Nb⁵⁺into the structure of layered LiNi_0.888Co_0.056Mn_0.056O₂（NCM88）cathode material,which inhibited the phase transition and further slowed down the attenuation of its Li⁺diffusion rate.Differential charge density calculations based on density functional theory（DFT）showed that besides Nb itself having a high charge transfer strength with oxygen atoms,it also enhanced the charge transfer between surrounding Ni,Co,Mn atoms and their respective surrounding oxygen atoms.This high charge transfer represents strong binding force between atoms,making the cathode material possess a more stable crystal structure.The stable crystal structure suppressed the H2-H3 phase transition during the charge and discharge process of the NCM88 cathode material,reducing the volume change of the unit cell（from 7.56%to 6.49%）,and improving the cycling performance.The Nb⁵⁺-doping-induced phase transition inhibition effect also slowed down the attenuation of Li⁺cation diffusion rate,thus improving the rate performance by enhancing the Li⁺ion conductivity.After discovering the inhibitory effect of Nb⁵⁺doping on the rate decay of lithium ion diffusion,we propose a new mechanism for improving the electrochemical properties of LiNi_0.92Mn_0.04Co_0.04O₂ by simultaneously enhancing both electronic and ionic conductivity through W⁶⁺doping.The bandgap of the cathode material is reduced to 1.1623 e V due to the increased number of electrons near the Fermi level after W intercalation.Such improved electronic conductivity subsequently leads to a suppressed polarization and reduced resistance,enabling an improved cycle life of up to 93.97%after 100 cycles at 0.5 C.Furthermore,the doping with W⁶⁺also introduced a strong W–O bond into the layered structure so that the thickness of the Li slab is expanded to 2.6476（?）,which reduces the energy barrier from 0.355to 0.308 e V for the migration of Li⁺within the Li slab,as confirmed by the DFT calculation.Consequently,the rate performance is greatly improved due to the reduced diffusion energy,with a specific capacity of 159.11 m Ag^-1 even at 5 C rate,indicating high potential for future applications.Finally,in order to achieve NCM cathode materials with excellent lithium-ion kinetics and further enhance their rate capability.A strategy to enhance the electrochemical performance of a single-crystal LiNi_xMn_yCo_zO₂（NCM）electrode is proposed herein based on a preferred orientation of a Li diffusion pathway along the working direction under an ordinary 0.4-T magnetic field.Hexagonal NCM622 single crystals can be aligned along their crystallographic c-axis parallel to the applied magnetic field because the crystal has an easy magnetization axis along the c-axis.The horizontal magnetic field–oriented NCM622 cathode（HMFC）shows inhibited polarization and increased lithium-ion conductivity.The suppressed polarization promotes cyclability,increasing the specific capacity from 157.93 to 168.75 m Ahg^-1 after the100^th cycle at 0.5 C.Furthermore,the increased diffusion coefficient(from 3.06×10^-10 cm²s^-1to 7.12×10^-10 cm²s^-1)causes substantial improvement in the rate performance,achieving a high specific capacity of 130.77 m Ahg^-1 at 10 C,while the electrode without magnetic treatment achieves a specific capacity of only 115.88 m Ahg^-1.Owing to such a superior electrochemical performance,the HMFC electrodes show high potential for lithium-ion batteries in the EV industry.Decreasing the magnetic field from 6 to 0.4 T for cathode alignment facilitates the applicability of the proposed strategy to production lines of battery plants using commercially available permanent magnets because a magnetic field of 0.4 T is easily achievable.