Inverstigation Of Synthesis And Modification Of Nickel Rich Cathode Material With Layered Structure For Lithium Ion Battery

As the fast growing of commercial electronics and electric vehicles markets,high-capacity-cathode&anode materials which are the key technologies to be developed by the large economy communities and multi-national companies.High-nickel cathodes are considered as the most important cathode materials to construct the next-generation lithium ion batteries due to its high capacity,high rolling density and relative maturity of its manufacture system.However,its practical application is seriously restricted by the difficulty of synthesis and strong activity of surface.In this work,we systematically investigate the synthetic technique,analyze the influence of the generated Li₂CO₃ on its electrochemical performance,design and prepare a novel protective structure to address its inherent surface instability.Our developed materials present the good structural entirety and excellent electrochemical stability.In this dissertation,the following works were investigated:（1）The influence of the synthetic parametes on the structure and electrochemical properties of LiNi_0.8Co_0.1Mn_0.1O₂ cathodeThe optimiztion of oxygen content is thoroughly investigated in this chapter.Although the crystal defects and surface decomposing layer exist in all samples,the materials synthesized at pure oxygen atomsphere have the best crystal entirety,possessing the lowest oxygen defects,cation disorder ratio,and only 1 nm thickness of decomposed layer contained Li₂CO₃in the surface.The discharage capacity is 192.9mAhg^-1 at 0.1C rate and 160.1 mAhg^-1 at 5C rate.（2）Stability of Li₂CO₃ and its influence on electrochemical performanceLi₂CO₃ is easy to generate in surface reaction due to the surface sensitivity of nickel rich cathode.Although it possessed the good electrochemical inertness,lithium carbonate tends to react with electrolyte to generate Li F,release CO₂ and POF₃ gas.To identify its influence on electrochemical performance,Li₂CO₃ layer was generated on LiNi_0.8Co_0.1Mn_0.1O₂ surface and then trnasformed to porous nano-LiF coating with electrolyte’s attack.Although discharge capacity and rate capability are similar for both samples,the transformation to LiF significantly improves the cyclic stability,from from37.1%to 91.9%after 200 cycles.（3）Surface modification by Li₆Zr₂O₇ solid state electrolyte coatingLi₆Zr₂O₇ coating layered was prepared from Ni_0.8Co_0.1Mn_0.1（OH）₂ with ZrO（OH）₂coating layer.Zr element is distributed in surface.Li₆Zr₂O₇ coating material on nickel rich cathode was confirmed by XRD,TEM and SAED.Only 1 mol%Li₆Zr₂O₇ coating presents the similar electrochemcial performance to the control sample.（4）Nickel rich cathode with novle core shell structureNi_0.8Co_0.1Mn_0.1（OH）₂ precursors were coated by Co（OH）₂ and ZrO（OH）₂-Co（OH）₂ respectively.Cathode that prepared by Co（OH）₂ coated precursor would form Co substituted material.Lattice parameters and cation disorder are reduced by Co substitution.10%Co（OH）₂ presents the best electrochemcial before and after storage in 55℃stream.While novle core shell structure could be sysnthesized by ZrO（OH）₂-Co（OH）₂coated precursor.EDS and SIMS results are proved that double coating layer cathode is core shell structure.Lattice parameters of core shell cathode is similar with pure LiNi_0.8Co_0.1Mn_0.1O₂.XPS results show that nickel ratio in surface is decreased by core shell structure.Soft X-ray XAS proved that Ni³⁺is highest and Ni²⁺is lowest in core shell cathode surface which is more stable.The shell part is composed with Co rich microcrysttal,which could constrain the discahge voltage deacy and charge transfer resistance.This structure cathode discharges 180.4 mAhg^-11 at 0.1C below 4.3V cutoff and keeps 89.5%capacity after 1000^th cycle at 1C rate.It also has good high temperature stored performance.The 200^th cycle capacity retention ratio is high to 82%after sotred in 55℃stream for one month.