Synthesis And Properties Of High-capacity Layered Cathode For Lithium-ion Batteries

Lithium-ion batteries are widely used in consumer electronics,power tools,energy storage,aerospace and other fields owing to their core advantages of high energy density,high power density and high safety performance,simultaneously playing an important part of balancing energy and environmental issues and the carbon peaking and carbon neutrality goals.With the optimizing of technology and the increasing of energy demand,developing next generation of higher energy density lithium-ion batteries has become a top priority,of which the cathode material is the technical bottleneck that determines the energy density and accounts for more than one-third of the total cost.Therefore,developing cathode materials with lower cost and higher capacity is an important goal for current lithium-ion battery technology.Among the commercialized cathode materials,lithium cobalt oxide and ternary high-nickel materials（NCM）have the highest energy density,and are also the most competitive and possible ones to realize the next generation of higher energy density lithium-ion batteries.However,the harmful bulk phase transition and surface un-stability occurred during cycling are the key problems faced by both in basic research and industrial production.Specifically,there is still a gap between the actual discharge capacity（within 4.6 V）and the theoretical capacity for LCO materials.While lifting the cut-off voltage can effectively improve the discharge capacity,therefore,the failure analysis and modification technology of LCO at high voltage is a hot spot.In addition,for NCM materials,they process the advantages of lower cost and more capacity at low voltage range.In particular,the development of electric vehicles has significantly promoted the development of high nickel NCM materials.However,high nickelization also introduces serious phase transition and mechanical failure,which seriously limits its development.For this reason,single crystallization has become one new way out.In view of the above problems,this dissertation explores the failure analysis of high voltage LCO,the modification method of 4.6 V LCO materials,and the preparation process of high nickel NCM single crystals.In the second chapter,taking bare LCO and trace titanium doped（TLCO）as the research objects,spectroscopy and diffraction imaging technology are used to investigate lattice defect structure and the results show that trace titanium doping can introduce a large amount of lattice distortions including lattice twisting,bending,layer spacing changes,etc.In-situ XRD experiment in the first cycle shows that the harmful O₃-H_1-3 phase transition is significantly inhibited in TLCO.Furthermore,Ex-situ synchrotron diffraction imaging experiment demonstrate that the lattice defects introduced by trace doping can still stabilize the structure and cycling performance of TLCO during the cycling process.In the third chapter,two component coating strategy of Li₃PO₄（LPO）and different oxide composite is performed on the semi-finished LCO aiming at 4.6 V application.LPO/Sn O₂,LPO/La₂O₃,LPO/Sm₂O₃,LPO/H₃BO₃ are uniformly coated onto LCO with solid state method.As a result,the cycling performance at both room and high temperature together with rate performance are significantly enhanced.The 0.1C discharge capacities of LPO/La and LPO/Sm samples exceed 205 m Ah/g,the 0.5C discharge capacity can reach 198 m Ah/g,and the capacity retention rate of 100 cycles is above 96%.The LPO/La sample shows a discharge capacity above 210 m Ah/g at0.5C and 45°C,and a 50-cycle capacity retention of more than 85%.Further analysis demonstrate that composite coating can form stable interfacial protection on the surface,provide better interface kinetic conditions,reduce interfacial charge transfer impedance,inhibit surface cracks and reconstruction,thereby improving the electrochemical performance of 4.6 V LCO.In the fourth chapter,aiming at the synthesis of high-nickel NCM single crystal materials,the high-temperature solid phase sintering process is adopted.The effects of sintering temperature and Li/TM ratio are investigated in NCM622.The sintering temperature and Li/TM ratio were optimized as 960-970°C and 1.10,under which the0.1C discharge capacity can reach 216.56 m Ah/g（2.7-4.4V）,the 1C discharge is 198.62m Ah/g and remains 173.04 m Ah/g after 100 cycles.Subsequently,the effects of different cation additives on the morphology of NCM83 are explored,and the results show that Cs and K cations can significantly promote the growth of single crystals and lower the sintering temperature,while W and Sm cations has an inhibitory effect and Al cation shows no obvious effect.The electrochemical test results showed that Cs,W and Al cation additives can improve the cycle performance and rate performance of NCM83,and Cs addicted NCM83 shows the best performance with a discharge of202.57 m Ah/g（2.8-4.3 V）at 0.1C,177.63 m Ah/g at 1C,and 170.94 m Ah/g after 100cycles,with a high capacity retention rate of 96.2%tested at 30°C.Higher capacity is achieved at 45°C test condition,with a discharge capacity of 202.25 m Ah/g（2.8-4.3 V）at 0.1 C,194.26 m Ah/g at 1C,189.31 m Ah/g after 50 cycle,capacity retention rate of97.5%.In addition,the Cs addicted NCM83 has the best rate performance,with a discharge capacity of up to 162.8 m Ah/g at 3C rate.Further investigations reveal that the addition of Cs,W and Al cations can effectively inhibit the harmful H₂-H₃ phase transition of NCM83,stabilize the crystal structure,reduce cation mixing,and optimize the surface dynamics,thereby improving the cycle performance and rate performance of NCM83 material.In summary,this dissertation focuses on the high-capacity layered cathode materials of lithium-ion batteries,and conducts in-depth research on the defect structure and failure analysis of high-voltage LCO,the development of 4.6 V LCO,and the optimization of NCM622 and NCM83 single crystals.Firstly,the failure mechanism of4.6 V LCO was revealed from the perspective of lattice defects,and an effective modification strategy for the development of 4.6 V LCO was proposed,which achieved a high discharge capacity and cycling stability,and finally the sintering parameters for synthesizing NCM622 single crystal materials are optimized,different cation additives that can promote the growth of single crystals were successfully screened,and the cycling performance and rate performance of NCM83 single crystal materials were significantly enhanced.This work provides a certain technical support and experimental basis for promoting the development of high-voltage LCO and high-nickel NCM single crystal materials and their application in higher performance lithium-ion batteries.