Synthesis And Modification Of High Capacity Lithium-rich Manganese-based Layered Cathode Materials

Lithium-ion energy storage device,a green renewable energy with wide applicability and high efficiency and reliability,is one of the most effective choice to solve problems of global warming and ecological environmental pollution.With the rapid development of lithium-ion batteries in the fields of robots,electric vehicles and energy storage,higher requirements and challenges have been put forward on the development of cathode materials with high energy density,high power density,high safety and long life.Therefore,extensive attentions have been paid to lithium-rich manganese-based layered cathode materials x Li₂Mn O₃·(1-x)Li MO₂(M=Cr,Mn,Co or Ni),which possess advantages of high capacity,high safety,environmental friendly and low cost.In addition to the above advantages,there are some problems limited the large-scale application of these materials,such as large irreversible capacity loss in the first cycle,poor cycling performances and severe capacity or voltage fade.In this thesis,we take the research status of lithium-rich manganese-based layered cathode materials as background,take lithium-rich manganese-based layered cathode materials as research objects,to improve their electrochemical performance as the research purposes,we have carried out the exploratory research and analysis on component design and synthesis,voltage regulation and surface modification:1.A series of lithium-rich manganese-based layered cathode materials x Li₂Mn O₃(1-x)Li Mn_0.33Ni_0.49Co_0.18O₂(x=0.3,0.4 and 0.5)with varied x value were prepared by sol-gel method.Structure and morphology of all samples were characterized by XRD and SEM,and the effects of x value on electrochemical performance have been studied as well.These results showed that with the increasing of x value,the initial coulombic efficiency and the average discharge voltage decreased,while the specific discharge capacity increased.The component of 0.4Li₂Mn O₃·0.6Li Mn_0.33Ni_0.49Co_0.18O₂ material achieved the best comprehensive performance of a reversible capacity of 202.2 m Ah·g^-1,a capacity retention of 93.3%and a high average discharge voltage of 3.58 V after50 cycles at 0.1 C,.The electrochemical impedance spectroscopy(EIS)demonstrated that both the stable SEI films and the fast electrochemical kinetics were contributed to the performance.Thus,the 0.4Li₂Mn O₃·0.6Li Mn_0.33Ni_0.49Co_0.18O₂ material is considered to be a cathode material with high voltage,high specific capacity and excellent cyclability.This work has a significance guiding for the component design of lithium-rich manganese-based cathode materials with high specific energy,high voltage and high cycle stability.2.Li_1.17Mn_0.50Ni_0.24Co_0.09O₂ cathode materials were successfully synthesized by carbonate co-precipitation method.The effect of p H value during co-precipitation process on morphology,structure and electrochemical properties of materials were investigated.It was found that LMNCO-8.50 showed the best reversible specific capacity,capacity and voltage stability,and high resistance of high rate during cycling.LMNCO-8.50 exhibited the highest discharge specific capacity(263.6 m Ah g^-1)andcoulombic efficiency(79.0%),as well as the smallest irreversible capacity(70 m Ah g^-1)in the first cycle.Moreover,the LMNCO-8.50 had an obviously suppressed voltage fade with the average discharge voltage of 3.65 V,and owned a capacity retention up to93.7%after 100 cycles at 1 C.As p H value increased from 7.50 to 8.50,the electrochemical performance of materials gradually increased,which was ascribed to the gradually decreasing of Li/M value,the gradually diminishing lattice structure defects,the increasing stability of surface films and the decreasing of charge transfer impedance with the increasing of p H value.3.Layered-spinel composite Li_1.2Mn_0.6Ni_0.2O₂ materials were synthesized successfully by KCl molten salt method.The effect of temperature on crystal structure,surface morphology and electrochemical performance of materials were investigated by XRD,SEM,Raman and TEM.The results showed that layered-spinel composites can be formed by KCl molten salt method at 750-900?.The calcination temperature had an important effect on phase compositions,ratio of phase compositions,degree of cation-mixing and particle sizes of products.With the increasing of calcination temperature,the integrated spinel phase changed from Li Mn₂O₄ to Li Mn_1.5Ni_0.5O₄.Furthermore,electrochemical reaction mechanisms among samples prepared at varied temperatures were different.Electrochemical reaction kinetics of Ni²⁺/Ni⁴⁺and Mn³⁺/Mn⁴⁺in LMNO-850 were the fastest.LMNO-850 sample possessed the best cyclability and rate performance which exhibited a discharge specific capacity of 263.0m Ah g^-1,a high specific capacity(203.3 m Ah g^-1)and capacity retention(95.6%)after1C cycling,and a high rate capacity of 142.8 m Ah g^-1 at 5 C.The excellent electrochemical performance of LMNO-850 was mainly contributed to its lower degree of Li/Ni mixing,layered-spinel surface integrated heterostructure,lower charge transfer impedance and higher lithium ion diffusion coefficient.4.Li_1.2Mn_0.6Ni_0.2O₂ samples were coated with a uniform amorphous carbon layer(2-3 nm thickness)by acrylamide surface modification.After surface modification,the cationic mixing degree of the material decreased,and the covalence of M-O bond on the surface increased.At a lower cut-off voltage(4.6 V),electrochemical activation of Li₂Mn O₃ was inhibited,and voltage fade was significantly mitigated.LMNO-modified materials exhibited superior capacity retention,cyclability,average discharge voltage stability and rate performance.The discharge specific capacities of LMNO-modified samples at 0.1 C and 1 C were 240.3 m Ah g^-1 and 145.1 m Ah g^-1,respectively,with corresponding capacity retentions of 117.9%and 103.8%.The average discharge voltage increased to 3.501 V by 17 m V.The capacity of LMNO-modified samples increased to 242.1,222.6,195.6,170.5,143.9 and 108.3 m Ah.g^-1 at 0.1,0.2,0.5,1,2and 5 C,respectively.The excellent electrochemical performance of LMNO-modified materials was related to the surface coated amorphous carbon layer,the enhancement of both%O^n-and M-O bond on the surface.5.The effect of excess-lithium content,calcination temperature and aluminum isopropoxide surface modification on structure,morphology and electrochemical properties of Li_1.2Mn_0.6Ni_0.2O₂ were studied.Studies on different amounts of excess-lithium content have shown that with the increasing of excess-lithium content,primary particles of materials increased,electrochemical activation of Li₂Mn O₃ decreased,and electrochemical reaction kinetics slowed down,the first-cycle discharge capacity and initial columbic efficiency of materials also decreased gradually.When excess-lithium content was 1%,the initial coulombic efficiency of the material was 77.1%,and the discharge specific capacities at 0.1 C and 1 C were 233.7 m Ah·g^-1 and 157.8 m Ah·g^-1respectively.The capacity retention and coulombic efficiency after 100 cycles at 1 C current density were 94%and 99.9%,respectively.Research on different calcination temperature have revealed that with the increasing of calcination temperature,particle sizes and crystallinity of materials increased,as well as the reaction activity of Li₂Mn O₃and Li Ni_0.5Mn_0.5O₂.When the calcination temperature was 900?,the material exhibited the best cycling performance(225.5 m Ah·g^-1 for 0.1 C and 169.3 m Ah·g^-1 for1 C)and capacity retention(102.6%for 0.1 C and 104.7%for 1 C).Studies on aluminum isopropoxide surface modification of Li_1.2Mn_0.6Ni_0.2O₂ have shown that an amorphous Al₂O₃ layer about 20 nm thickness was uniformly coated on the surface of Li_1.2Mn_0.6Ni_0.2O₂,which was controlled by the hydrolysis process of aluminum isopropoxide.The specific discharge capacity and coulombic efficiency of the modified LMNO-Al sample were 274.6 m Ah·g^-1 and 81.1%,respectively,while the capacity retention was 92.2%after 50 cycles at 0.1 C.In general,the discharge specific capacity and initial coulombic efficiency can be improved by aluminum isopropoxide modification.The improvement of electrochemical properties of surface modified sample LMNO-Al was related to the following factors:1.uniform Al₂O₃ coating acted as the protective layer;2.maintains more oxygen vacancies;3.the decreasing of interface charge transfer impedance.