| Braught to the commercial market by Sony in 1991,lithium ion batteries have been widely used in 3 C products,such as cell phone,laptop,etc.The application of litium-ion batteries as power batteries in Tesla's electric sports cars and BYD's electric buses in recent years helps them draw wide attention.High-power 3 C products like smart phones,pure electric veicles are requiring higher energy and power density as well as battery safety.The performance of lithium-ion battery is mostly related to cathode materials.At present,the discharge capacity of tranditional cathode materials is under 170 mAh g-1,which is becoming ever more outdated.Ni-rich layered cathode and Li-rich layered cathode,on the contrary,are drawing extensive attention for they feature feature up to 200 mAh g-1 discharge capacity(Ni at 80%with low rate current)and more than 250 mAh g-1,respectively.But they are not free from problems.For instances,the structure change during the first charge process or in high rate current of Ni-rich cathode material,and the voltage decay of Li-rich cathode material.This article focuses on improving the electrochemical performance of the cathode materials by adopting synthesis,coating,and element tuning.The in situ X-ray differaction(in situ XRD)and OEMS(online continuous flow differential electrochemical mass spectrometry)were developed to study the specific structure change and/or gas evolution during the charge/discharge or CV process.The research content is summarized as follows:1.Spherical-like Li-rich layered cathode materials were synthesized by carbonate co-precipitation method with nickel sulfate,cobalt sulfate,and manganese sulfate as metal sources,sodium carbonate as a precipitant,anmmonium bicarbonate as a complexing angent.Li-La-Zr-O solid electrolyte has been successfully coated on the surface of Li-rich cathode materials.The coated and uncoated cathode materials were characterized and analysized by SEM.TEM,XRD,EIS,and charge-discharge test.Electrochemical results demonstrate that the Li-La-Zr-O coated cathode remaines 196.1 mAh g-1 discharge capacity with 84.5%capacity retention after 455 cycles at the current density of 200 mA g-1 and remaines 180.5 mAh g-1 discharge capacity with 77.7%capacity retention after 605 cycles at the current density of 200 mA g-1.2.The voltage decay of Li-rich layered cathode materials was suppressed by adjusting Co/Ni ratios and the corresponding mechanism was investigated.The cathodes and precursors with different Co/Ni ratios were synthesized by carbonate co-precipitation and were characterized by SEM,TEM,in situ/ex situ XRD,OEMS,and charge-discharge test.Electrochemical results demonstrate that the Li1.13Ni0.275Mn0.580O2 cathode only exhibit 0.255 V medium discharge voltage decay after 100 cycles at the current density of 200 mA g-1.OEMS and in situ XRD results indicate that the voltage decay of Li-rich layered cathode is closely related to the discharge process of the three regions which includes the reduction of oxygen ions,the cobalt and nickel ions,and the manganese ions.The large amount of oxygen loss at the end of the first charge process will accelerate the voltage decay of subsequent cycles.3.The voltage decay of Li-rich layered cathode materials was suppressed by adjusting Mn/(Co+Ni)ratios and the corresponding mechanism was investigated.The cathodes and precursors with different Mn/(Co+Ni)ratios were synthesized by carbonate co-precipitation and were characterized by SEM,TEM,in situ XRD,OEMS,and charge-discharge test.Electrochemical results demonstrate that the Mn/(Co+Ni)raito has a major impact on the voltage decay.When controlling the Ni/Co ratio at 1.95,if the ratio of Mn/(Co+Ni)is less than 1.75,the discharge medium voltage decay of the cathode materials will be less than 0.178 V after 100 cycles at 1 C rate.If the ratio of Mn/(Co+Ni)is 1.10,the discharge medium voltage decay of the cathode materials will only be 0.054 V after 100 cycles at 1 C rate.OEMS and in situ XRD results indicate that the oxygen loss of Li-rich layered cathode mainly occurs on the surface of the primary particles.The oxygen loss at the end of the first charging process is an important cause of the voltage decay but no only reason.4.Spherical Ni-rich layered cathode materials were synthesized by hydroxide co-precipitation method.The evolution of the spherical precursors and structure of product were characterized and analysized by SEM,in situ XRD,CV,and charge-discharge test.The current peaks in the CV process of NCM 811 layered cathode material was assigned to the corresponding phase transition process by in situ XRD characterization.The in situ XRD results of different polarized NCM 811 cathode in the first charging process indicates that the phase separation in the first charge process is related to the polarization of the first circle.The greater the polarization,the more serious the phase separation phenomenon will be observed.The in situ XRD results of NCM 811 cathode at 2 C rate indicates that phase separation will occur at the end of the discharge process at high discharge current rate.It is speculated that this phenomenon may be caused by the different rate of lithium ion migration on the surface and internal of the primary particle of the Li-rich layered cathode under high discharge current rate.In summary,this work obtains spherical Li-rich manganese layered cathode and Ni-rich layered cathode materials by co-precipitation method.The voltage decay of Li-rich manganese layered cathode was successfully tuned and suppressed by adjusting the transition metal ions of nickel,cobalt,and manganese.In situ XRD and OEMS methods were applied to investigate the mechanism of Li-rich manganese layered cathode and Ni-rich layered cathode during the electrochemical process.The study of this work is of great importance to provide the possible causes and suppression methods for the voltage decay of Li-rich manganese layered cathode materials,and the structural transformation mechanism of Ni-rich layered cathode in the first charging process by different polarization and discharge process at high rate current density. |
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