Preparation,Electrode Design And Modification Of Lithium-rich Manganese Based Cathode Materials

Recently,with the rapid development of advanced energy storage equipment such as electric vehicles and smart grid,the demand for high energy density of lithium-ion batteries is becoming more and more higher in human society.Therefore,the development of cathode materials for lithium-ion batteries with high discharge capacity and energy density is very urgent.Lithium-rich layered oxides have attracted wide attention due to their high discharge capacity,low cost and environmental friendliness.Although lithium-rich oxide cathode materials have many advantages,there are many problems to be solved,such as the low coulombic efficiency of initial charge and discharge,low rate performance,poor cyclic stability and the continuous decline of working voltage in electrochemical reaction.These shortcomings have hindered the commercialization process.Therefore,the purpose of this paper is to improve the electrochemical performance of lithium-rich cathode materials,and using combination of co-precipitation and high temperature solid-state method prepared lithium-rich oxide.Lithium-rich materials with optimum properties can be obtained by the selection of the appropriate lithium source and the best Ni-Co-Mn transition metal ratio.At the same time,the surface of lithium-rich materials was coated to improve their electrochemical performance.Using lithium-rich oxide as cathode material,the electrodes were designed to investigate the effect of different current collectors on the electrochemical performance of lithium-rich oxides.The main content is shown in the following aspects:Lithium-rich layered oxides were synthesized via co-precipitation using different lithium sources including LiOH,Li2CO3 and CH3COOLi.Scanning electron microscope(SEM),Thermo gravimetric analysis(TGA),Brunauer-Emmett-Teller(BET),Inductively coupled plasma atomic emission spectrometry(ICP-AES),X-ray diffraction(XRD)and electrochemical measurements were used to investigate the morphology,reaction process,specific surface area,composition,structure and electrochemical performance of the lithium-rich oxides,respectively.The use of different lithium sources mainly affects the primary particle size and secondary particle morphology of the final product.Using LiOH as the lithium source,the maximum discharge capacity of sample can reach to 272.1 mA h g-1 in the voltage range of 2.0-4.6 V at room temperature,even after 50 cycles,the retention rate is still reach 91.4%.The electrochemical impedance spectroscopy(EIS)results show that lithium-rich oxides using LiOH as the lithium source have the minimum value of impedance after 50 cycles.Therefore,the choice of appropriate lithium source is an effective way to improve the electrochemical properties of lithium-rich layered oxides.Each transition metal element in lithium-rich oxides has its exclusive function in the electrochemical reaction process.Therefore,the influence of transition metal Ni-Co-Mn ratio on the morphology,structure,surface valence and electrochemical properties of lithium-rich oxides was discussed in this paper.When Ni-Co-Mn has the best proportion,the secondary spherical carbonate precursor is composed of the primary tetrahedral nanoparticle,and this special precursor morphology leads to the porous and stable spherical lithium-rich oxide.Meanwhile,lithium-rich oxide 0.5LiNi0.29Co0.30Mn0.41O2 ·0.5Li2MnO3 with the optimal transition metal proportion has the best pore quality and the largest pore content.The results of XPS show that the content of Ni2+ in Ni gradually decreases with the increase of Mn content in lithium-rich oxides.The XRD refinement data show the sample 0.5LiNi0.29Co0.30Mn0.41O2·0.5Li2MnO3 has the largest cell volume and lattice parameter ratio c/a value.Lithium-rich oxide with the best transition metal ratio exhibits excellent electrochemical performances.The initial discharge capacity of this lithium-rich material at 0.1 C rate is 265.7 mA h g-1;and the discharge capacity can reach 210 mA h g-1 at 1 C rate,the capacity retention is up to 90.5%after 80 cycles.Lithium-rich layered oxide with different shell structure was synthesized by a simple wet-chemical surface deposition method.X-ray diffraction,scanning electron microscopy,transmission electron microscopy and X-ray photoelectron spectroscopy etc.techniques were applied to characterize crystal structure,morphology and micro-structure of samples.The surface of lithium-rich layered oxide can successively produce the island-like spinel,ultra-thin spinel and thick two-phase(spinel and amorphous manganese oxides)separation shell layers with the increase of coating amount.Formation process of different shell structure and the effect of the shell structure on lattice parameters were discussed.Different shell structure plays an important role in the electrochemical performances of lithium-rich oxide.Especially when the coating amount is 1 wt%,lithium-rich material with a uniform spinel Li4Mn5O12 shell layer has superior electrochemical performances,and the discharge capacity can maintain 209.9 mA h g-1 and 166.8 mA h g-1 even at 2 C and 5 C rates.This idea is of guiding significance for the study of the optimum coating in the future.The design of electrode is also a very important aspect to improve the performance of lithium-ion battery.The lithium-rich cathode material was synthesized via the combination of co-precipitation method and high-temperature solid-state reaction.Subsequently,the lithium-rich cathode material was coated on different current collectors to assemble a coin-type battery.After rate performance test,most of the lithium-rich oxide mixture separated from the aluminum foil current collector.Surprisingly,the mixture still can achieve intimate connection with the carbon nanotube(CNT)foil current collector,which significantly improves the high rate performance of lithium-rich cathode material.The discharge capacity of lithium-rich oxide using aluminum foil current collector is 89.3 mA h g-1 at 10 C rate.However,lithium-rich cathode material are coated on CNT sheet with 18 ?m thickness,which can respectively obtain discharge specific capacity of 143.7 mA h g-1 at 10C rate and 95.4 mA h g-1 even at 20 C rate.The electrochemical impedance spectroscopy(EIS)shows that the charge transfer resistance of lithium-rich cathode material on the CNT foil is smaller than coated on the aluminum foil.Meanwhile,we also investigated the instability of carbon nanotubes under continuous high voltage.Ultimately,replacing the heavier conventional current collector is an important direction to improve the gravimetric and volumetric energy density of Li-ion battery.