Research On Preparation And Modification Of Li-rich Manganese Based Cathode Materials And Electrodes For Lithium Ion Batteries

The development of high-energy-density Li-ion batteries has been attracted many attentions from scholars due to the merits of energy crisis and environmental pollution.The cathode material is one of the most important limiting factors in the development of lithium-ion batteries.Among all the investigated cathode materials,Li-rich layered oxides(x Li2 Mn O3·(1–x)Li MO2,LLMO)are the most promising ones because of their extremely high capacity,low cost and toxicity,etc.However,the LLMO suffer from some significant challenges,such as low initial coulombic efficiency,long-term cycling instability and poor rate capability,which severely restrict them application.In this dissertation,lithium-rich layered oxide cathode materials were synthesized via a two-step synthesis method involving co-precipitation and high-temperature calcination,and ex-situ X-ray absorption spectroscopy(XAS)and high-resolution transmission electron microscope(HRTEM)measurements were used to reveal the fundamental voltage fade mechanism.Then,Se6+ and Mo6+ doping could significantly improve the rate capability and cycle performance of the LLMO,as well as decelerate the voltage decay.Finally,the surfaces of the LLMO were modified by nano-gold and conductive carbon which further improved the initial coulombic efficiency and cycle life.The manganese hydroxide precursors were synthesized by co-precipitation method.Meanwhile,the effects of p H value on structure,morphology,tap density and composition of Mn0.7Ni0.2Co0.1(OH)2 were investigated.The results show that Mn can complete sediment,and tap density of Mn0.7Ni0.2Co0.1(OH)2 is greater than 1.95 g cm-3,when p H value is at a range of 10.5-11.0.Li-rich layered oxide(Li1.2Mn0.56Ni0.16Co0.08O2)samples were prepared using a high-temperature solid state reaction method based on the manganese hydroxide precursors.The effects of the calcination temperature on the structural and electrochemical performances of the LLMO were investigated.Electrochemical experiments revealed that the initial discharge capacity of the LLMO-850 calcined at 850 oC is 266.8 m Ah g-1.The LLMO-900 did not show any attenuation after 100 cycles.Both ex-situ XAS and HRTEM revealed the fundamental mechanism of voltage fade in the LLMO is the changes in valence state and structural of Mn.In order to improve structure and electrochemical performances of the LLMO,Li1.2[Mn0.7Ni0.2Co0.1]0.8-xSex O2(x=0,0.07,0.14 and 0.21)and Li1.2[Mn0.7Ni0.2Co0.1]0.8-xMox O2(x=0,0.005,0.01 and 0.02)materials were synthesized in the presence of Na2 Se O3/NaOH and Na2 Mo O4/NaOH as the co-precipitation agent.The effects of Se6+ and Mo6+ doping on structure and electrochemical performances of the LLMO were investigated in detail.The results shown that the doped LLMO exhibited a long sloping region and short potential plateau in the initial charge curves,a large first coulombic efficiency(77 %),good rate capability(178 m Ah g-1 at a current density of 2000 m A g-1),high reversible capacity retention(93 %)and mid-point voltage retention(95 %)after 100 cycles when the doping ratio of Se is 0.14.The improved electrochemical performance of the Se-doped materials is attributed to the contribution of the Se ions in the bulk lattice,which could effectively enhance the structural stability and reduce the reaction of negative oxygen ions in the cathode materials.As for the Mo-doped LLMO,the first coulombic efficiency is 81.2 %,and the capacity retention is 93.9 % after 100 cycles when the doping ratio of Mo is 0.01,while the reversible capacity is 138 m Ah g-1 at a current density of 1000 m A g-1 when the doping ratio of Mo is 0.005.To further improve the discharge specific capacity and rate capability.The surfaces of LLMO were modified by nano-gold and conductive carbon.The LLMO was fist synthesized by nano-architecture of Li-rich manganese cathoded materials.Subsequent thermal spraying of gold or carbon onto the LLMO surface resulted in the formation of a new cathode material,e.g.Au@LLMO or C@LLMO.The investigation of the structure,morphology and electrochemical properties indicate that the fist discharge specific capacity of the Au@LLMO is 306 m Ah g-1 at a current density of 25 m A g-1,which is 12.5 % hight than that of the LLMO.The first coulombic efficiency of the Au@LLMO(85 %)is also higher than that of the LLMO(79 %).The rate capability of the Au@LLMO is 190 m Ah g-1 at a current density of 1250 m A g-1.The improved electrochemical property of the Au@LLMO is due to the fact that its hexagonal shaped nano-plates can shorten the path length for Li ion transport.In particular,the surface modification of Au can enhance the electronic conductivity,as well as catalyze the reduction reaction of oxygen and the oxidation reactions of its reduced products.All the factors can greatly contribute to the high specific capacity and high rate capability of Au@LLMO.While the C@LLMO has a high cycle and rate capability,and its capacity retention is 97.7 % after 100 cycles.Because the coating of conductive carbon on the electrode surface can improve the electrical conductivity of the electrode surface,which can avoid the corrosion of the electrolyte to the cathode material at a high voltage,thus improving the rate capability and cycle performances of the LLMO.