Research On Synthesis And Electrochemical Performance Of Li₂FeSiO₄/Cathode Material For Lithium-ion Power Batteries

Lithium-ion batteries is the first choice of power supply due to its high theoreticalcapacity、high specific energy、no environmental pollution and no memory effect.Cathode material as one of the key materials of lithium-ion batteries, The limitation ofits performance has become a bottleneck for the performance of lithium batteries, therequisites for the cathode material of power battery are low cast and long cycle life,besides the good safety performance is the most important. Li2FeSiO4as a new type ofsilicate type anion material, its advantages contain high theoretical capacity, cheap andenvironmentally friendly source of raw materials, good thermal stability. Comparedwith layered (LiCoO2) and spinel (LiMn2O4) cathode materials, Li2FeSiO4is more safebecause of its stable structure, which make Li2FeSiO4become a new promising kind ofcathode materials in the field of power lithium ion battery. But the electronicconductivity and ion diffusion velocity is low, which hinder the large current dischargeability of Li2FeSiO4. Based on the above disadvantages of Li2FeSiO4, this paper adoptsthe ion doping and carbon coating method to enhance the conductivity properties ofLi2FeSiO4material. The microstructure, grain size and electrochemical performance ofmaterial were analyzed by the method of vanadium ion with different valence doped onFe sites and Si sites and the citric acid acts as carbon source for material. The specificwork is as follows:(1)Doped by vanadium at Fe sites influence on Li2FeSiO4In this paper, Li2Fe(1-x)VxSiO4lithium-ion cathode materials with V3+doped at Fesites was synthesized by machinery-solid-state method. The structure and morphologyof the Li2FeSiO4with V3+doped at Fe sites were analyzed by X-ray diffraction, scanning electron micrograph. The test results show that the main peak of materialphase is consistent with the XRD of Li2FeSiO4after doping, all are orthogonal crystalsystem(space group: Pmn21), but there are a few Li2SiO3and Fe3O4impurity peak. SEMtest results that: morphology of the samples is relatively uniform, good consistency ofsize distribution, the primary particle size between200nm and500nm. EIS test resultsshows that: with increasing vanadium content, the impedance of material increases, butthe constant current discharge test found that: Li2Fe(1-x)VxSiO4lithium-ion cathodematerials with V3+doped at Fe sites has two discharge platform, with increasingvanadium content, platform becomes flatness, and the second discharging platforminterval is longer. Doping V3+is failed to improve the conductivity of the material,however, the stable discharge voltage and stable discharge platform will be helpful tothe application of lithium-ion battery.(2)Doped by vanadium at Si sites influence on Li2FeSiO4In this paper, Li2FeSi(1-x)VxO4lithium-ion cathode materials with V5+doped at Sisites was synthesized by machinery-solid-state method. The structure and morphologyof the Li2FeSiO4with V5+doped at Si sites were analyzed by X-ray diffraction, scanningelectron micrograph. The test results show that the main peak of material phase isconsistent with the XRD of Li2FeSiO4after doping, all are orthogonal crystalsystem(space group: Pmn21), but there are a few Li2SiO3and Fe3O4impurity peak. SEMtest results shows that: morphology of the samples is relatively uniform, goodconsistency of size distribution, The primary particle size between200nm and500nm.EIS test results shows that: Compared with the undoped, the lithium ion diffusioncoefficient of doped samples are increased by an order of magnitude, the electricalconductivity of the Li2FeSiO4with V5+doped at Si sites is obvious improved. When thedoping amount is0.1, the lithium-ion diffusion coefficient of the Li2FeSi0.9V0.1O4samples is maximum (3.02x10-13cm2/s), and exhibited the best electrochemicalperformance, in the first discharge capacity is163.3mAh/g at0.1C, compared with theundoped increase23.5%, capacity remains at146.7mAh/g after10cycles, the dischargecapacity is113.3mAh/g under1C. This improvement mainly due to V5+doping in Si,the lithium-ion vacancy formed in the Li2FeSiO4lattice, which expand the lithium iondiffusion channel and debase the O stranglehold on Li, It is beneficial to the diffusion of Li+and improve the lithium ion diffusion rate.(3)Carbon coating influence on Li2FeSiO4Study on citric acid as carbon source, A series of Li2FeSiO4/C with different carboncontent was synthesized by machinery-solid-state method. The structure andmorphology of the Li2FeSiO4with different carbon content were analyzed by X-raydiffraction, scanning electron micrograph. The test results show that the main peak ofmaterial after carbon coated are consistent with uncoated, all are pure Li2FeSiO4,orthogonal crystal system(space group: Pmn21), and there are not impurity peak of Fe3+.SEM test results show that: the morphology of samples decreases with the increase ofcarbon content, The primary particle size between100nm and400nm, and the particlesize distribution is more concentrated, besides the consistency of particles is better, EIStest results shows that: Compared with the uncoated, the lithium ion diffusioncoefficient of coated samples is increased by an order of magnitude, which explain thatelectrical conductivity of the Li2FeSiO4with carbon coated is obvious improved. Whenthe coating amount is6wt%, the lithium-ion diffusion coefficient of the Li2FeSiO4/Csamples is maximum(4.26x10-13cm2/s), and exhibited the best electrochemicalperformance, in the first discharge capacity is165.2mAh/g at0.1C, compared withLi2FeSiO4increase50%, and close to the theoretical capacity of one lithium iondeintercalation, capacity remains at163.2mAh/g after10cycles, which has good cycleperformance.