Structural Degradation And Doping Modification Of Nickel-based Layered Cathode Materials For Lithium/Sodium Ion Batteries

The cathode material is a short board that affects the key performance of the secondary ion battery,such as the life,charge/discharge speed,endurance and so on.The structural change during charge/discharge process is the main factor leading to the degradation of battery capacity and rate performance.In this paper,the Nickel-based cathode material is taken as the research object,combined with the experimental results,the first principles calculation method is used.Aiming at the problems of transition metal(TM)diffusion,P2-O2 phase transition and doping of Nickel-based cathode materials in Lithium ion battery(LIB)and Sodium ion battery(NIB),the effects of structural stability on Li/Na ion diffusion were studied.The structure evolution and performance degradation mechanism of cathode materials during charge/discharge were revealed,which provided strategies for the research and development of high-performance cathode materials.The main contents of this paper are as follows:In LIB,experiments have found that the segregation of TM in LiNi_xMn_yCo_zO₂ternary cathode materials causes phase changes on the surface,which is the main reason for its performance degradation.By studying the diffusion mode and energy barrier of TM in LiNi_0.33Mn_0.33Co_0.33O₂(NMC333)cathode material,we have revealed the structure evolution mechanism of cathode material and the results on Li ion diffusion.It is found that the TM diffusion mechanisms of the cathode material NMC333 are different.In the bulk TM diffuses through the mechanism of forming cation disorder with Li,while on the near surface TM diffuses through the vacancy mechanism.By calculating the diffusion barriers in different directions,it is found that TM diffuses into the Li layer first,and then diffuses and segregates in the Li layer.Among them,Ni diffuses more easily than Co and Mn ions.The formation of these segregations causes local phase changes and blocks the diffusion channels of Li ions.In NIB,the P2 phase in the Nickel-based layered cathode material Na_xTMO₂provides a wide diffusion channel for Na ions.During the charge/discharge process,experiments have found that the P2-O2 phase transition is the main reason for the decline of electrochemical performance.We choose the phase transition of P2-O2 and the diffusion of Na ions in Na_xNi O₂ Nickel-rich cathode material for research.Through the calculation of formation energy,it is found that the structure evolution of Na_xNi O₂ is related to the concentration of Na ions.The phase transition of P2-O2occurs at 0.16<x<0.22;when the concentration of Na ions is lower,the O2 phase will be more stable.The octahedral channel in the O2 structure is narrower than the prism channel in the P2 structure.The local compressive strain caused by the phase change further reduces the Na ion diffusion channel;Oxygen vacancies are more easily formed in O2 phase,which can induce Ni ions to diffuse to octahedral sites of Na layer while reducing the distance between Na layers,thus blocking the diffusion channel of Na ions.Therefore,the phase change of P2-O2 inhibits the diffusion of Na ions,resulting in the degradation of the rate performance of the battery.Considering that the TM diffusion in LiNi_xMn_yCo_zO₂ and the P2-O2 phase transition of Na_xTMO₂ will cause the structure of the cathode material to be unstable and hinder the diffusion of Li/Na ions,we adopt the doping method to stabilize the layered structure:?)Aiming at the formation of cationic disorder(Ni/Li)in LiNi_0.6Mn_0.2Co_0.2O₂(NMC622),which results in structural instability,we find that Na/F co-doping can improve structural stability and promote Li ion diffusion.The results show that doping of F promotes cation disorder and reduces the barrier of Ni diffusion into the Li layer,doping of Na inhibits cation disorder and increases the barrier of Ni diffusion into the Li layer,while Na/F co-doping can greatly inhibit the disorder of cations and Ni diffusion.The synergy between Na and F increases the distance between Li layers and reduces the Li ion diffusion barrier(42.8%).Therefore,the co-doping strategy is an effective method to inhibit the diffusion of TM,stabilize the layered structure and promote the diffusion of Li ions.?)As for Na_0.67Ni_0.33Mn_0.67O₂,the P2-O2 phase transition is prone to occur at the end of charging.We found that Mg doping,especially after it forms enrichment,can inhibit the phase transition and stabilize the layered structure.At the begin of charging,Mg stays in the TM layer steadily,and then Mg starts to diffuse into the Na layer when the Na content is about 0.25,and then forms Mg enrichment in the area far from the Ni vacancies in the Na layer.After Mg is enriched,it can effectively prevent the generation of cracks by suppressing the volume change,and at the same time reduce the phase transition of P2-O2 at the end of charging.The diffusion of Na ions is related to the position of Mg.At the begin of charging,Mg doping promotes the diffusion of Na ions by increasing the distance between the Na layers.At the end of charging,the reduction of the distance between the Na layers inhibits the diffusion of Na ions.Therefore,Mg doping can not only regulate the diffusion of Na ions,but also suppress the generation of phase transitions and cracks,thereby stabilizing the layered structure of the system.