Modification And Electrochemical Properties Of Layered Ni-Rich LiNi_0.8Co_0.1Mn_0.1O₂ Cathode Materials

The development of renewable energy has become significantly urgent because of increased consumption of fossil energy and induced environmental pollution.Compared to conventional lead-acid batteries,lithium-ions batteries are advantageous in terms of high-power density,long lifespan and environmentally friendly.However,the density of lithium-ions batteries must be enhanced due to the rapid development of electric vehicles.Among numerous cathode materials,Ni-rich Li Ni_xCo_yMn_zO₂（x+y+z=1,x>0.6）layered oxides are the most promising due to their high capacity,environmentally friendly and low cost.However,the inferior cycling stability and microcracks due to the significant volume changes in unit cell during charging and discharging impede its large-scale application.In this thesis,in order to optimize the electrochemical performance of Li Ni_0.8Co_0.1Mn_0.1O₂,based on the summary of the research status and the in-depth understanding of the reason of defects for Ni-rich cathode materials,the optimizations are carried out from the aspects of morphology adjustment,crystal structure modulation and cation mixing majorization.The main contents and results were summarized as followes:1.Li Ni_0.8Co_0.1Mn_0.1O₂ hollow nano-micro hierarchical microspheres（H-LNCM）were synthesized using multishelled hollow structured Ni O microspheres as precursor with Co source and Mn source.The traditional synthetic methods of hollow structured Ni O microspheres generally require complicated steps.Herein,a general and straightforward strategy is developed to synthesize multishelled porous hollow microspheres by hydrothermal method and calcination at high temperature.The structure of materials was characterized by XRD,SEM and HRTEM.Compared to conventional cathode materials,The H-LNCM cathode materials exhibit excellent cycle stability.After 100 charge-discharge cycles,the capacity retention at 1 C is 93.3%.Notably the capacity retention is 87.0%after 300 cycles at 5 C compared to 72.0%for the conventional NCM.The high performance can be attributed to the distinctive hollow microspheres structure can provide a buffer zone during the Li ion intercalation/deintercalation process and mitigate the generation of microcracks.2.The Ni-rich cathode material Li Ni_0.8Co_0.1Mn_0.1O₂ with primary particles arranged radially was synthesized by precisely controlling the p H value of precursor synthesis process for 11.4.Compared to the structure of primary particles randomly distributed in conventional Ni-rich materials.The migration path of Li⁺can be decreased with the arranged radially primary particles,and the consistent crystal orientation might alleviate the mechanical stress induced by volume variation inside particle during charging and discharging.The LNCM-11.4 delivers high reversible capacity(228.8 m Ah·g^-1 at 0.1 C)and excellent cycle stability（92.9%capacity retention after 100 cycles under 1 C）.The superior electrochemical performance may attribute to the arranged radially primary particles that not only promote the migration of Li⁺,but also alleviate the anisotropic pressure during cycling.This special morphology modulation provides a simple way to promote the performance of Li Ni_0.8Co_0.1Mn_0.1O₂ for practical application.3.single crystal Li Ni_0.8Co_0.1Mn_0.1O₂cathode materials was synthesized via a single crystal Ni_0.89Co_0.11C₂O₄·2H₂O precursor that synthesized with solvothermal method and calcined with excessive lithium source and Mn source at a relative low temperature.The structure of materials was characterized by XRD,SEM and HRTEM.Excessive lithium source not only reduce the sintering temperature but also ensure the mono dispersed micrometers scaled particle distribution.The synthesized cathode material via 50%excessive lithium source calcination showed an initial discharge capacity of 226.9 m Ah g^-1 at 0.1 C with a Coulombic efficiencies 91.2%and 95.1%capacity retention after 100 cycles under 1 C at room temperature.Meanwhile the sample also has an outstanding cycling stability at a high cut-off voltage.The enhanced properties of as-prepared Li Ni_0.8Co_0.1Mn_0.1O₂ are due to the mono dispersed micron scaled morphology decrease the contact area between electrode and electrolyte and mitigate the formation of microcracks.The mono dispersed micron scaled precursor and calcined with excessive lithium provide a feasible method for the synthesis of single crystal Ni-rich cathode materialsIt is a great challenge to ameliorate the cation mixing and optimize the structural stability for Ni-rich cathode materials.Herein,the Li Ni_0.8Co_0.1Mn_0.1O₂ was synthesized by sintering Ni_0.8Co_0.1Mn_0.1（OH）₂ precursor with different ratios of excessive lithium source.The structure of materials was characterized by XRD,SEM,XPS and STEM-EELS.It was found that a proper excessive lithium contents contributes to increase the proportion of Ni³⁺in the surface and then optimize cation mixing.Moreover,the space of Li slab also can expand by moderate increasing the lithium source and promote the mobility of Li⁺.The Li Ni_0.8Co_0.1Mn_0.1O₂ with excessive lithium source of 10%exhibits excellent electrochemical performance.It delivers a high discharge capacity of 211.5m Ah g^-1 at 0.1 C with a Coulombic efficiencies 88.6%and 93.3%capacity retention after 100 cycles under 1C.