A Research Study On The Modification Of Layered Oxide Cathode Materials For Lithium/Sodium-ion Batteries Using High-valence Metal Ions

In China,the rapid development of electric vehicles and energy storage devices is demanding more and more power and energy storage batteries,which requires new lithium/sodium-ion batteries with high energy density,cycling life and low cost.In this context,the cathode material is the core component in lithium/sodium-ion batteries that largely determines their cycling life and cost.Layered oxide cathode materials,with advantages of high specific capacity and ease of synthesis,are the most promising cathode materials for lithium/sodium-ion batteries.However,layered oxide cathode materials exhibit poor cycling performance and rate capability,which limits their commercial applications.To address these issues,this study attempts to dope high-valence 4d/5d metal ions into high-nickel layered cathode materials（for lithium-ion batteries）and low-nickel layered cathode materials（for sodium-ion batteries）to improve the electrochemical performance of these materials.（1）The high-nickel LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）cathode material has the advantages of high capacity and low cost,making it one of the most important cathode materials for power batteries.However,the rapid capacity decay due to unstable structures during NCM811 charge and discharge limits its further applications.To address this issue,we adopted a modification method of W（5d）doping to stabilize the layered structure.In principle,on one hand,W-O bonds are stronger than TM-O bonds（TM:Ni,Co,and Mn）,which can reduce the production of oxygen during charging and stabilize the surface structure of the NCM811 cathode.On the other hand,the high-valence W⁶⁺induces the formation of more Ni²⁺and thus appropriately increases the Li⁺/Ni²⁺exchange,which can further stabilize the layered structure during its charge/discharge processes.Our first-principles calculations show that W doping can increase the formation energy of oxygen vacancies in the cathode material,change the electronic density of states,and reduce the total energy of the layered structure,making the structure more stable.In addition,W doping can widen the lithium ion diffusion channels,lower the energy barriers for lithium ion migration,and improve the rate performance.Therefore,we used a co-precipitation method to in-situ dope the precursor with W and then synthesized the cathode material by a solid-phase method.Electrochemical tests show that the W-doped cathode material exhibits excellent electrochemical performance,with a capacity retention rate of 95.06%and a remaining discharge specific capacity of 143.38 m Ah g^-1 after 200 cycles at 1 C,which is much higher than the pristine material’s 81.5 m Ah g^-1.（2）The high-nickel,cobalt-free cathode material LiNi_0.9Mn_0.1O₂（NM91）is a promising candidate for future power batteries due to its high energy density and low cost.However,the absence of cobalt results in poor rate and cycling performance.To address this issue,we developed an Nb-doping method based on the fact that surface enrichment of Nb（4d）on the secondary particle during solid-state sintering process can effectively inhibit particle growth（confine the particle size）.This method aims for shortening the diffusion path of lithium ions and preventing strain accumulation and micro-crack formation during cycling,thus improving the rate performance and cycling stability of NM91 material.Our experimental results display that 0.5 mol%Nb-doped NM91 material exhibits a discharge capacity of 111.2 m Ah g^-1 at 5 C,and a discharge capacity of 159.4 m Ah g^-1 at 2 C.After 100 cycles,the capacity retention rate is 86%,which is much higher than the 59.3%of pristine material.At 0.1 C,the capacity retention rate after 80 cycles is 85.7%,which is much higher than pristine material.SEM results show that the undoped cathode material exhibits severe cracking and secondary particle pulverization after cycling,while the Nb-doped cathode material remains dense and spherical.（3）The NaNi_1/3Fe_1/3Mn_1/3O₂（NFM111）cathode material for sodium-ion batteries has a high theoretical specific capacity,and has additional advantages of high abundance and low cost.However,NFM111 suffers from low initial efficiency and practical capacity during cycling.We used high-valence Mo ions and a surface doping strategy to modify the NFM111 material,trying to improve its specific discharge capacity through charge regulation.After surface doping with Mo ions,more Ni³⁺has transformed into Ni²⁺,which resulted in more Ni²⁺participation in the charge/discharge process and improved the reversible capacity of NFM111 material.The results showed that the surface Ni²⁺content of the cathode material doped with 0.66 wt%Mo increased from 67.8%（pristine NFM111）to 72%.The discharge specific capacity of the doped material increased from 94.59（pristine NFM111）to 127.03 m Ah g^-1 at 0.1 C,and the initial efficiency of the doped material increased from 73.9%（pristine NFM111）to 96.5%.