| Recent years,the shortage of petrochemical resources and the environmental pollution caused by fuel car increasingly have been prominent problems,so the electric vehicles have been developed rapidly.However,limited by the endurance mileage,they cannot replace fuel vehicles fully in a short period of time.This problem is mainly attributed to the low energy density of current commercial lithium-ion batteries.Therefore,in order to meet the market demand for high-energy lithium-ion batteries,it is imperative to develop cathode materials with high energy density,safety and low cost for lithium-ion batteries.The energy density of lithium-rich manganese-based cathode materials are much higher than the current commercial cathode materials,and they possess great potential to become cathode materials for next-generation lithiumion batteries.However,this kind of high energy density also brings some shortcomings,such as low first-time coulombic efficiency,poor cycle performance,voltage attenuation and poor rate performance,etc.These problems also hinder the commercialization of lithium-rich manganese-based cathode materials.In order to promote the commercialization of lithium-rich manganese-based cathode materials,a variety of modification methods have been explored and adopted.Among many modification methods,element doping can make the forigen ion enters the crystal lattice of material,and the electrochemical performance can be improved by optimizing the crystal structure.Therefore,this paper aims to improve the structural stability and electrochemical performance of lithium-rich manganese-based cathode materials by ion doping modification.In this paper,the Li1.20Mn0.56Ni0.16Co0.08O2(LRMO)was prepared by co-precipitation-high temperature solid phase method,and the influence of anion and double ion doping on it was studied.PO43-polyanion and F’ anion are used as doping ions to explore the influence of doping content on the crystal structure,electrochemical performance and kinetic performance of LRMO.The PO43-polyanion doping adoptes a double precipitant method,which successfully achieves the doping during the preparation of the precursor.The XRD analysis confirms that the PO43-polyanion is doped into the crystal lattice and expand the interlayer spacing,which is beneficial for the improvement of the rate performance.The results show that 0.5 mol%PO43-doping increases the reversible capacity and the first coulombic efficiency(ICE),the first discharge capacity increased from 278 mAh g-1 to 290 mAh g-1 at 0.1 C,and the first coulombic efficiency also increase from 78.1%to 82.4%.Although the capacity of 1 mol%PO43-doped sample is low,it performes well in terms of capacity retention and voltage attenuation.It has a discharge capacity of 184 mAh g-1 at 1 C,and the retention ratio is as high as 93%after 90 cycles,the voltage drop is reduced from 0.314 V to 0.214 V.F-anion doping adoptes a wet chemical treatment combined with low-temperature sintering method.The organic solvent(NMP)used in the treatment process can reduce the adverse effect of proton exchange reaction on electrochemical performance.XRD analysis shows that the crystal structure of 2 mol%F-doped sample(2F-LRMO)is well-organized,and the peak ratio of I(003)/I(104)is as high as 1.50,which proves that F-doping is beneficial to reduce Li+/Ni2+mixing and improve Li+diffusion kinetics.Further electrochemical performance analysis shows that the 2F-LRMO sample has relatively excellent electrochemical performance,such as the high first discharge capacity(289 mAh g-1)and ICE(85.1%);it shows good long-cycle performance at 1 C,the first discharge capacity at 1 C reach 248 mAh g-1,and still maintain 194 mAh g-1 after 150 cycles with a retention ratio of 78.4%.The kinetic analysis shows that the 2F-LRMO sample has the highest Li+diffusion coefficient(7.22×10-14 cm2 s-1).Fe3+cations and PO43-polyanions are used as doping dual ions,and they are successfully doped into LRMO at the same time via a novel precursor doping method.This method can eliminate the uneven distribution caused by small dose doping in the traditional solid-phase doping method.The test results of ICP and EDS show that this doping method successfully realizes the uniform distribution of dual doping elements.Comprehensively analyzing the crystal structure and electrochemical performance of samples with different doping content,and determine Li1.20Mn0.543Ni0.16Co0.08Fe0.01P0.0067O2(FeP-LRMO)as the optimal co-doping sample.The FeP-LRMO and LRMO sample are investigated in terms of crystal structure,microscopic morphology and structure,element composition and valence state,and electrochemical properties.Thanks to its excellent structural stability,FeP-LRMO exhibits an excellent electrochemical performance,such as the first coulombic efficiency is as high as 88%;the first discharge capacity at 1 C is 242 mAh g-1,and it still maintains 214 mAh g-1 after 130 cycles with a capacity retention ratio of 88%;after 70 cycles at 5 C,the ultra-high discharge capacity of 154 mAh g-1 is still maintained,and the capacity retention ratio is 95.1%;the voltage attenuation is suppressed from 0.397 V to 0.276 V.In addition,in order to explore the synergistic mechanism of Fe3+and PO43-dual ion doping,we performed microscopic morphology,structure,element valence,EIS characterization and analysis on the electrode materials of FeP-LRMO and LRMO samples after cycling,and confirmed that the co-doping of Fe3+and PO43-has a significant effect on improving the structural stability of lithiumrich manganese-based cathode materials. |
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