Study On Preparation Of Li-rich Layered Cathode Materials And Their Microstructural Modification

With the rapid development of digital electronic products and electric vehicles?EVs?,high energy and power density lithium-ion batteries are increasingly required.High-performance cathode materials for lithium-ion batteries have attracted much attention in recent years.Li-rich layered cathode material with a chemical formula of xLi₂MnO₃·?1-x?LiMO₂?0?x?1,M=the combination of Ni,Co,and Mn?has been considered as the potential alternative to the traditional cathode material LiCoO₂,due to its high capacity,nontoxicity,environmental friendly,low cost and high safety.However,Li-rich layered cathode material suffers some inherent disadvantages,such as lower 1st-cycle coulombic efficiency,poor rate capability and severe capacity fading during cycling,which have impeded its commercial applications.In this thesis,the approaches of chemical compositions optimization,surface coating and morphology modification were attempted to improve electrochemical performance of the Lirich layered cathode material,and the possible mechanisms were put forward.The analytical techniques of X-ray diffraction?XRD?,field-emission electronic scanning microscopy?FESEM?,transmission electronic microscope?TEM?,cyclic voltammetry?CV?,galvanostatic charge-discharge?GCD?and electrochemical impedance spectrometry?EIS?were employed to characterize the phase compositions,microstructures and electrochemical performance.The main contents and results were summarized as follows:?1?Li_1.2Mn_0.54Ni_0.26-xCo_xO₂?0.05?x?0.21?samples were synthesized by a solid-state coordination reaction,and the effect of the content of Co,Ni species on the phase compositions,microstructures and electrochemical performance have been investigated.The results show that the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ sample exhibits a well-developed layer structure and good electrochemical performance.It consists of highly dispersed uniform primary particles with an average size of 150-200 nm.Also,among the Li_1.2Mn_0.54Ni_0.26-xCo_xO₂?0.05?x?0.21?samples,Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ exhibits the highest degree of order of cation distribution,and the best electrochemical performance.Its first discharge capacity was determined to be 244.6 mAh·g-1,and its 1st-cycle coulombic efficiency to be 71.6% at a current rate of 0.1 C?1 C=200 mA·g-1?.It delivers a discharge capacity of 101.6 mAh·g-1 when increasing discharge rate to 5 C.After cycled for 100 cycles at 1 C,it still remains a discharge capacity of 123.7 mAh·g-1,and its capacity retention is as high as 84.4%,exhibiting excellent cycle stability.?2?Graphene,amorphous carbon?using sucrose as the carbon precursor?and AlPO₄ materials were coated on the surface of the above Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material,and the effect of the coating materials has been studied on the phase compositions,microstructures and electrochemical performance.The results show that all the coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ samples retain untouched layer structures.For the graphene-coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ sample(Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/graphene),the Li1.2Mn0.54Ni0.13 Co0.13O₂ nanoparticles were evenly wrapped in the wrinkled graphene sheets.As for the amorphous carbon-coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ sample(Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/sucrose),the amorphous carbon is evenly coated on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanoparticles.The AlPO₄-coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ sample(Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/AlPO₄)shows the uneven and incomplete coating layer on the surface of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanoparticles.The first discharge capacities of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/graphene,Li1.2Mn0.54Ni0.13 Co0.13O₂/sucrose and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/AlPO₄ were determined to be 249.9 mAh·g-1,205.5 mAh·g-1 and 197.1 mAh·g-1,respectively,and their 1st-cycled coulombic efficiency to be 77.3%,74.3%,82.0% at a current rate of 0.1 C,respectively.Compared with the bare Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ sample,all the coated samples exhibit obviously enhanced 1stcycled coulombic efficiency.The discharge capacities of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/sucrose and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/AlPO₄ samples were decreased to almost zero at 5 C,while Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/graphene sample still remains a high discharge capacity of 132.5 mAh·g-1.This demonstrates a superior rate capability of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/graphene sample.After cycled at 1 C for 100 cycles,their discharge capacities were determined to be 144.3 mAh·g-1,74.9 mAh·g-1,and 64.0 mAh·g-1,respectively,and their capacity retentions to be 84.2%,65.0%,49.9%,respectively.The Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/graphene sample exhibits the best cycling performance.?3?Finally,a pelletizing technology was used to modify the morphology of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ powder synthesized above,with the intention of improving its electrochemical performance.The results show that after the pelletizing process,the highly dispersed Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanoparticles are transformed into compact yet porous agglomerates with a near-spherical morphology.Among them,the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ sample pelletized at a pressure of 10 MPa exhibits the highest electrochemical performance.It shows a 1st-cycled coulombic efficiency of 77.6%.Its discharge capacity is 260.3 mAh·g-1 at 0.1 C,and decreases to 130.5 mAh·g-1 when increasing current rate to 5 C.It remains a discharge capacity of 256.6 mAh·g-1 when discharging current rate restored to 0.1 C,retaining 98.6% of its initial capacity.After cycled for 100 cycles at 1 C,it retains a discharge capacity of 153.8 mAh·g-1 with a capacity retention of 83.6%.The pelletizing process can facilely optimizes the morphology of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ powder,thus its discharge capacity,coulombic efficiency,rate capability and cycling stability have remarkably enhanced.