| Non-renewable fossils fuels such as coal and oil are about to mined out, the exploitation and use ofwhich has caused serious environmental pollution, so they will gradually be replaced by renewable energy.However, the development of the solar energy, wind energy, tidal energy and other renewable energy notonly costs too much but is also limited by geographical location and environmental conditions. Theseintermittent energy supplies can hardly be connected to electric grid for people to use directly; thereforethere need to be energy storage devices to adjust. Since Lithium-ion secondary battery has the high energydensity, long repeat service life and is environmentally friendly, it has received much attention fromscholars in recent years. While the cathode material Li2FeSiO4of Lithium-ion secondary battery has severaladvantages such as high theoretical capacity, voltage stability, high safety, good cycle performance, etc.Moreover, the raw material of Li2FeSiO4is rich in resources, inexpensive and environmentally friendly,which makes it a very promising cathode material. However, synthesizing high purity Li2FeSiO4is difficultand the electric conductivity of the material is not good, which make it difficult to play an excellentperformance. Therefore, this paper makes some research on the synthetic conditions and the effect ofdoping on the electrochemical performance of Li2FeSiO4. Details are as follows:The first chapter of this article reviews the advantages and disadvantages of the anode and cathodematerials used in Lithium-ion secondary battery, and the structure, synthesis and modification of Li2FeSiO4material.In the second chapter, based on the comparison of the XRD patterns, we made some research on theoptimizing experimental condition for synthesizing Li2FeSiO4by high temperature solid-state method andsol-gel method, and the crystallinity of the samples roasting under the experimental conditions of differentprecursors, different sintering temperature, with or without reducing gas protection. By comparing theresults under different experimental conditions, the optimum synthesis process undergone is as follows:water is not added during synthesis precursor materials, no N2protection, Li: Fe: Si molar ratio of2.1:1:1in the synthesis of precursor which is calcined at700℃for10hours in the furnace tube with protectivegas N2, in this way the crystallinity for the sample is much higher. On this basis, add certain amount of sugar to synthesize Li2FeSiO4/C composites, XRD patterns shows the crystal structure of the material doesnot change, and the degree of crystallinity is high; SEM photographs show a uniform dispersion of thematerial particles, the size of the material particles is smaller, and the agglomeration is significantlyimproved. Under optimal conditions the discharge capacity of the synthesized material is not high,compared with the theoretical capacity of the material166mAh g-1, under1/32C conditions initialdischarge capacity of81.7mAh g-1is much lower.In order to further improve the performance of the material, the third chapter explores the impact of tindoping and doping of the material on the electrochemical properties of the material under the optimumsynthesis conditions. The Sn2+/Sn4+during the process of charge-discharge can theoretically provide twoelectrons. Thus, it is possible that the capacity of material can realize partially embed in/extrude of twolithium ions. XRD pattern shows that the structure of doped sample Li2Fe1-xSnxSiO4/C (x=0.005、0.01、0.03、0.05) has not changed, only when x=0.10, the sample reaches an impurity peak of tin simplesubstance in the XRD pattern. With the increasing of Sn content, the first discharge capacity of dopedsample with a tendency of increasing and then decreasing, among which Li2Fe0.95Sn0.05SiO4/C has the maxdischarge capacity (114.3mAhg-1). Through in-depth experimental study on Li2Fe0.95Sn0.05SiO4/C material,the author finds that it has standard morphology, uniform particles, smaller size, and lower agglomerationthan non-doped samples. What is more, through cyclic voltammetry and AC impedance tester, it is foundthat the cycle performance and conductivity of the material have been greatly improved.The last chapter of this article preliminary explores material Li2FeSiO4/C modified with the thirdperiod transition metal (Ti, V, Cr, Mn, Co, Ni, Cu, Zn). Experimental results show that the morphology ofdoped material has been effectively improved than that of updoped one, the material particles disperseuniformly, and the agglomeration reduces. By comparing the initial discharge capacity of the material, wefind out the discharge capacity increased after the material has been doped, Mn-doped material improvesthe most, followed by the V-doped material. After in-depth study of Li2Fe1-xMnxSiO4/C(x=0.005、0.01、0.05)doped with Mn, we find out the crystal structure did not change, while the conductivity and the firstdischarge capacity of the material have been effectively improved, but the cycle performance is still notideal enough. |
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