Study On The Synthesis,modification And Electrochemical Properties Of Lithium-rich Layered Cathode Materials

The international community’s goal of’carbon neutrality’and concerns about energy and environment have accelerated the electrification in various countries,and thus put forward higher energy density requirements for lithium-ion batteries.Ascribed to the benefits of high specific capacity and high operating voltage,lithium-rich layered cathode materials（LRMs）have shown great potential to improve the energy density of lithium-ion batteries.However,LRMs suffer from many problems like severe capacity fading,voltage fading and poor rate performances during charge-discharge cycling,which degrade the electrochemical performances and impede commercial applications of the materials.Concentrating on cycle stability optimization of lithium-rich cathode materials,this dissertation is carried out from the aspects of bulk modification,surface modification and microstructure engineering:（1）Effect of lithium addition amount on structure and electrochemical properties of Li-rich layered materialsLi_1+xNi_0.45Co_0.1Mn_0.45O_2+x（0.05≤x≤0.4）with desired performances were prepared based on the Li content-crystal structure-electrochemical performance relationship.XRD refinement combined with electrochemical results showed that the crystal structure and the degree of Li/Ni disorder of Li_1+xNi_0.45Co_0.1Mn_0.45O_2+xwere regulated by modulating the Li content,which contributes to the formation of Li₂Mn O₃superlattice structure.The lithium-rich cathodes with x=0.3 exhibited the highest specific capacity(208 m Ah·g^-1),best rate performance(115 m Ah·g^-1at 10C)and excellent cycling performance（the retention of 91%after 120 cycles at 1C）.Such LRMs demonstrated stable crystal structure and low Li/Ni disorder,and thus contributed to the intercalation/de-intercalation process of Li⁺at high voltage,facilitating to suppress the voltage fading during charge-discharge cycling.At the same time,appropriate Li content brought about appropriate Li₂Mn O₃superlattice structure in LRMs,which is helpful to increase the specific capacity of LRMs.These results would enhance our understanding of the Li content-crystal structure-electrochemical performance relationship,and provide useful references for the design of LRMs with specific performances.（2）Synergistic effect of Al-Zr co-doping on electrochemical performance in lithium rich materialsWe prepared Al-Zr co-doped LRMs with different doping amounts by a solid-state method and investigated the effect of doping amount on structure and electrochemical properties.Consequently,the LRMs with 0.20%Zr⁴⁺and 0.10%Al³⁺in the lattice structure demonstrated excellent electrochemical performances.After500 cycles at 0.5C,the co-doped LRMs showed a higher capacity retention rate of67%,a higher discharge mid-voltage of 2.85 V and a lower voltage attenuation of0.78 V.Compare with undoped sample,the capacity retention is 55%,discharge mid-voltage is 2.75 V and voltage attenuation is 0.87 V.The discharge capacity of co-doped LRMs is 136 m Ah·g^-1at 5C,while that of undoped sample is only 101m Ah·g^-1.In-situ XRD analysis in combination with theoretical calculation results suggested Al-Zr co-doping effectively improved the cycling performances and alleviated voltage fading.It’s largely because that co-doping can enhance the binding energy between metal and oxygen and thus the structural stability of LRMs.As shown in GITT testing,Al-Zr co-doping improved the conductivity and diffusion coefficient of lithium ions,which is beneficial to promote the migration of lithium ions and enhance rate discharge performances.（3）Effect of Li₇La₃Zr₂O₁₂（LLZO）coating on structure and electrochemical properties of Li-rich layered materialsTo further improve the cycling performances and suppress the voltage fading,we prepared LLZO-coated Li-rich layered materials by a simple and efficient dry-coating method.Electrochemical results showed that the LRMs with 0.75 wt%LLZO coating exhibited enhanced rate performances and long-cycling performances,and delivered a discharge capacity of 150 m Ah·g^-1at 2C and a capacity retention of 63%after 400cycles.As confirmed by EIS results,the LLZO coating on the surface of the LRMs enhanced the diffusion of lithium ions at the electrode/electrolyte interface,and alleviated side reactions between the electrolyte and the lithium-rich materials.Furthermore,the cross section of the particles confirmed that LLZO coating effectively reduced the internal cracks caused by side reactions.As a result,LRM particle maintained a good internal structure during charge-discharge cycling,thus improving the long-cycling performances.（4）Study on structure and electrochemical properties in Li_1.2Ni_0.13Co_0.13Mn_0.54O₂single-crystal cathode materialSingle crystal Li_1.2Ni_0.13Co_0.13Mn_0.54O₂lithium-rich layered cathodes were successfully prepared for the first time.The influence of sintering temperatures and Li content on the morphology and the electrochemical properties of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂was studied in detail.By adjusting the sintering parameters and the x value in x Li₂Mn O₃·（1-x）Li Mn_1/3Ni_1/3Co_1/3O₂,we obtained the lithium-rich single crystal cathode materials with uniform morphology and grain size of about 2μm.Those single-crystal cathode materials exhibited enhanced cycling performances with a capacity retention of 69.6%after 500 cycles.As suggested by EIS and charge-discharge profiles,few grain boundaries and small surface areas suppressed the side reactions between single-crystal LRMs and electrolyte under high voltage,thus leading to slow impedance growth and stable structure during cycling.Meanwhile,SEM and TEM results showed that interlaminar slip occurred along the（003）crystal plane during charge-discharge cycling,and the stress accumulation caused by interlaminar slip led to cracks at the edge of the single crystal material,which aggravates the cyclic failure of the LRMs.These results would provide useful reference for the development of LRMs with long cycle and high performances.