Preparation And Modification Of Co-Free Lithium-Rich Cathode Materials For Lithium-Ion Batteries

Cobalt-free lithium-rich cathode materials have attracted more and more researchers’attention because of their high working voltage,large specific capacity,environmental protection and low cost.However,its poor rate performance and cycle stability still hinder its application.In order to solve these problems,the electrochemical performance of cobalt-free lithium-rich cathode materials are improved based on morphology control,element doping and interface modification.The research contents are as follows:（1）The Co-free lithium-rich cathode materials(Li_1.2Ni_0.2Mn_0.6O₂,LRNMO)with a nano-micron combined morphology is prepared by oxalate co-precipitation route and further modified by an anion doping strategy.It has been found that the F^-anion doping can increase the content of Mn⁴⁺in LRNMO,reduce the influence of the Jahn-Teller effect,and further impede the transition from layered phase to spinel phase.As a result,the as-obtained Li_1.2Ni_0.2Mn_0.6O_2-xF_x（x=0.04,named as F4-LRNMO）shows an optimal electrochemical performance,high discharge capacity(243 m Ah g^-1)with a satisfactory initial coulombic efficiency of 84.37%at 0.1 C.Meanwhile,the F4-LRNMO sample also can display high capacity retention of 92.2%after 200 cycles at 1 C.（2）To enhance the interface stability of the material,we put forward a feasible interfacial modification strategy in virtue of Mn₃O₄ surface coating to construct protective shell and alleviate Jahn-Teller distortion during the charge/discharge process.It has been found that the increase of the oxygen vacancy on the surface of Li_1.2Ni_0.27Mn_0.54O₂ of（LRNMO）after Mn₃O₄ protective layer modification can effectively restrain the release of oxygen,and thus improving its comprehensive electrochemical performances.In particular,the initial coulombic efficiency of the optimized LRNMO-1 electrode is improved from 72.3%to 84.3%.The capacity retention rate is up to 95.6%at 250 m A g^-1（1 C）after 200 cycles.In addition,the LRNMO-1 electrode also has a good capacity retention rate（96.4%after 100 cycles）at a high current density of 5 C.Moreover,the designed LRNMO@Mn₃O₄ cathode exhibits higher ionic conductivity and better interface stability.（3）To enhance the stability of oxygen-localized structures in materials and increasing the diffusion rate of Li⁺ions,a surface modification strategy of Li_1.2Ni_0.32Mn_0.48O_2-δ（abbreviated as LRNMO-bare）doped with Ta⁵⁺ions are proposed,in which the Ta⁵⁺ions can enter the lithium sites of the lattice structure on the surface layer of LRNMO-bare and form a Ta₂O₅ coating layer.The modified electrode exhibited excellent rate performance and good cycling stability.Additionally,the surface doping significantly suppressed the drop in discharge potential and improved the initial coulomb efficiency,rate capability as well as cycling stability.Especially,the Co-free Li-rich cathode material with 2%Ta doping displayed excellent rate performance and good cycling stability.After 200 cycles at a rate of 1 C,the reversible capacity remains still 208 m Ah g^-1,the capacity retention rate and voltage retention rate are 94.9%and 85.5%,respectively.Even at 5 C,the 2%Ta-doped material can still reach a high discharge specific capacity of 160.5 m Ah.g^-1.Moreover,the initial coulomb efficiency（from 72.8%to 82.1%）and ionic conductivity of the 2%Ta-doped Co-free Li-rich cathode material are also properly improved.