Synthesis And Modification Of High Energy Density LiNi_0.9Co_0.1O₂ Cathode Materials

Cathode materials are considered as a key factor for restricting the improvement of the energy density of lithium-ion batteries,and the development of cathode materials with higher energy density is a prerequisite for the further commercial application of lithium-ion batteries,especially in the field of power batteries.High energy density LiNi_0.9Co_0.1O₂cathode materials show the huge development potential due to their high specific discharge capacity and energy density.Nonetheless,the practical commercial application of high energy density LiNi_0.9Co_0.1O₂ cathode materials has faced severe challenges due to critical issues such as cation mixing,surface residual alkali,irreversible phase transition,transition metal ion dissolution,and microcracks.Particularly,with the increment in nickel content,the synthesis becomes markedly more challenging,and the aforementioned critical issues intensify.Furthermore,developing LiNi_0.9Co_0.1O₂ cathode materials capable of enduring extreme conditions,such as high temperatures and high cut-off voltages（4.5 V）,holds substantial value and significance for their practical commercial deployment.Building on this premise,and grounded in the controlled synthesis process of LiNi_0.9Co_0.1O₂ cathode materials,this dissertation mainly focuses on the impact of surface coating,bulk phase co-doping,and doping-coating co-modification on the electrochemical properties of LiNi_0.9Co_0.1O₂ cathode materials as well as the internal modification mechanism via theoretical calculation,material characterization and electrochemical tests.The main innovation points and research contents are divided into the following five parts:（1）Controllable synthesis and electrochemical properties of LiNi_0.9Co_0.1O₂ cathode materials are studied.The effects of sintering temperature,sintering time,sintering process and lithium ratio on the structure,morphology,and electrochemical properties of LiNi_0.9Co_0.1O₂ cathode materials are meticulously examined through a controlled variable approach,leading to the recommendation of optimal synthesis conditions.Specifically,the ideal sintering parameters are identified as a pre-sintering temperature of 480oC,a final sintering temperature of 720oC,a pre-sintering time of 6 h,a final sintering time of20 h,two-stage sintering process,and a lithium ratio of 1.10:1.The study reveals that LiNi_0.9Co_0.1O₂ cathode materials synthesized under controlled preparation condition not only have better layered structure and spherical morphology,but also demonstrate a reduced degree of cation mixing.As a result,the LiNi_0.9Co_0.1O₂ cathode materials deliver a high first discharge specific capacity of 217.3 m Ah g^-1 at a rate of 0.1 C and still maintain a discharge specific capacity of 177.9 m Ah g^-1 at a rate of 1 C.（2）Preparation and electrochemical performance of boron and titanium compounds coated LiNi_0.9Co_0.1O₂ cathode materials are studied.Boron and titanium compounds coated LiNi_0.9Co_0.1O₂ cathode materials are successfully prepared by wet coating combined with solid-state sintering technology,and the influence mechanism of amorphous state coating layer on the interface performance of LiNi_0.9Co_0.1O₂ cathode materials is systematically studied.The research demonstrates that the amorphous state coating layer can prevent the direct contact between the cathode materials and the electrolyte,thus suppressing the interface side reactions and the dissolution of transition metal ions,and greatly enhancing the surface/interface stability of the cathode materials.Additionally,based on the structure of titanium diboride,the layered structure of boron atoms and the outer electrons of titanium can provide fast channels for the diffusion of electrons and ions,which is beneficial to improve the transfer rate of electrons and ions.As a result,the capacity retention rate of titanium diboride modified LiNi_0.9Co_0.1O₂cathode materials reache as high as 92.2%after 100 cycles at a rate of 0.5 C,whereas bare LiNi_0.9Co_0.1O₂ cathode materials only have a capacity retention rate of 79.9%.（3）Preparation and high temperature electrochemical performance of the derived phospholipid-like protective layer（YPO₄-Li₃PO₄）coated LiNi_0.9Co_0.1O₂ cathode materials based on yttrium metaphosphate are studied.YPO₄-Li₃PO₄ phospholipid-like protective layer is successfully constructed in situ on the surface of LiNi_0.9Co_0.1O₂cathode materials through wet coating combined with solid-state sintering technology,thereby improving the cycling performance of the materials at high temperature of 50oC.The derived phospholipid-like coating layer not only inhibits the continuous erosion of hydrofluoric acid on the interior of particles and the dissolution of transition metal ions,but also promotes the transport of lithium ions at the cathode materials/electrolyte interface.Concurrently,the in-situ construction process consumes residual lithium compounds,thereby diminishing surface residual alkalis and improving the processability of materials.As a result,the capacity retention rates of the modified LiNi_0.9Co_0.1O₂cathode materials after 100 cycles reache as high as 94.4%at 25oC（0.5 C）,and it remaines at 86.3%even at a high temperature of 50oC（0.5 C）,significantly higher than that of bare LiNi_0.9Co_0.1O₂ cathode materials（25oC/81.1%,50oC/58.2%,0.5 C）.（4）Preparation and electrochemical performance at high cut-off voltage of Mg/Si co-doped LiNi_0.9Co_0.1O₂ cathode materials are studied.Since LiNi_0.9Co_0.1O₂ cathode materials are prone to microcracks and particle breakage during cycling process at high cut-off voltage,leading to a decline in its electrochemical performance.Mg/Si co-doped LiNi_0.9Co_0.1O₂ cathode materials are successfully prepared by a one-step high temperature solid-phase method using co-doping strategy,and cycling performance is studied at a high cut-off voltage of 4.5 V.This strategy enhances the structural stability of the materials and effectively prevents the formation of microcracks and the collapse of lattice structure.The structure stability of Li-poor materials is enhanced significantly by the presence of a small number of Si-O bonds.As a result,the capacity retention rates of Mg/Si equimolar co-doped Li(Ni_0.9Co_0.1)_0.998Mg_0.001Si_0.001O₂ cathode materials after 100 cycles are 93.3%and 89.6%at 4.3 V and 4.5 V（0.5 C）,respectively,which are much higher than that of bare LiNi_0.9Co_0.1O₂ cathode materials（4.3 V/80.5%,4.5 V/66.6%,0.5 C）.（5）Preparation and electrochemical performance of Zr⁴⁺/PO₄^3-multifunctional integrated doping and derived Li₃PO₄ coating co-modified LiNi_0.9Co_0.1O₂ cathode materials are studied.Since LiNi_0.9Co_0.1O₂ cathode materials are prone to rapid capacity decay during cycling process at high rates,Zr⁴⁺/PO₄^3-multifunctional integrated doping and derived Li₃PO₄ coating co-modification strategy is adopted before lithiation.The materials are successfully prepared by a one-step high-temperature solid-phase method,and their long-term cycling performance is improved at high rates.The as-designed surface modification strategy significantly inhibits cation mixing and H2-H3 phase transition during the long-term cycling process,and reduces residual lithium compounds on the surface as well as suppressing surface side reactions,thereby enhancing structure/interface stability and lithium ion transport performance.Specifically,Zr⁴⁺tends to form a uniform doping,whereas PO₄^3-group presents a gradient distribution near the surface area and generates Li₃PO₄ coating layer.As a result,the capacity retention rate of the Zr⁴⁺/PO₄^3-multifunctional integrated doping and derived Li₃PO₄ coating co-modified LiNi_0.9Co_0.1O₂ cathode materials after 100 cycles is 93.7%at a rate of 0.5 C,and the capacity retention rate after 200 cycles is 80.4%at a rate of 1 C,which are much higher than that of bare LiNi_0.9Co_0.1O₂ cathode materials（0.5 C/81.7%、1 C/66.0%）.