Controlled Syntheses And Lithium-ion Storage Performance Of Multi-dimensional Lithium Iron Orthosilicate Nano-materials

High performance energy storage and conversion systems have become a hotspot. Among them, lithium ion batteries with light weight and small size, highstorage capacity, long lifespan, high safety performance and environment friendlyhave a wide application prospect. However, the energy and power densities arelimited for the traditional cathode materials. Lithium ion battery is still difficult tomeet the requirements of next generation power battery devices. The novel silicatecathode material, with high theoretical specific capacity, stable cycle performance,abundant natural resources, cheap price and high safety performance, is growingconcern, showing a promising application. However, low conductivity and lithium iondiffusion kinetics of Li2FeSiO4cathode materials lead to a low specific capacity andlow power performance.In order to improve the performance of specific capacity, rate ability, life timeand to solve low conductivity and lithium ion diffusion kinetics of Li2FeSiO4cathodematerials, in this study, the nanostructural designing and controlling are carried out toprepare the multidimensional from0to3D Li2FeSiO4nanomaterials. The mainprogress is summarized as follows.(1) The P21/n structure nanocrystalline-Li2FeSiO4is prepared by a confinementeffect of three-dimensional conductive carbon frameworks which is formed through achelating reaction and subsequent pyrolysis. Benefit of enhanced electronicconductivity by carbon frameworks and Li-ion diffusion kinetics bynanocrystalline-Li2FeSiO4architectures, the novel nanocomposite shows1.28Li-ionstorage capacities (211.3mAh/g) at0.1C, corresponding to successive two steps ofoxidation and reduction of Fe2+/Fe3+/Fe4+. Furthermore, the discharge capacity is189.8,175.6,148.9,125.7and106.6mAh/g at a variable rate of0.5,1,2,5and10C,respectively, and then easily return to175mAh/g at1C. It is a surprise that the initialcapacity is90.9mAh/gat10C, and97.7%is retained after1000cycles.(2) The architecture of1D Li2FeSiO4nanorods, anchored on the graphene nanosheet (LFSNR@GNS), is designed and constructed through a simple PVP fixingmethod where1D Li2FeSiO4nanorods are achieved by ethylene glycol assistedhydrothermal method. The obtained LFSNR@GNS hybrids provide a rapid Li-ionand electron transport pathway, a very large surface to volume ratio to contact theelectrolyte, and facile strain relaxation to accommodate the volume variations duringlithiation/delithiation. When cycled between1.5-4.8V, the electrode based on thehybrid exhibits an optimal performance with high specific capacity (298mAh/g at0.01A/g), high rate and long lifetime, which is a very promising cathode material forcommercial applications that require high energy, long operating life and excellentabuse tolerance, such as electric vehicles. Astonishingly, when cycled between0-3V,as an anode material, it shows a high specific capacity (1530mAh/g at0.02A/g), highrate and long life performance.(3) A novel hierarchical shuttle-like Li2FeSiO4is synthesized using one stephydrothermal method with ethylene glycol assisted. The growth mechanism of theshuttle-like Li2FeSiO4constructed of single-crystal nanoplates is discussed and itselectrochemical performance as a cathode material for lithium ion battery isinvestigated. Note that the1stdischarge specific capacity of the new material withoutcarbon coating is180.6mAh/g at0.1C with a remarkably high coulombic efficiencyof97.5%, and an improved high-rate capability (71.0mAh/g at2C) is offered whichis comparable to the commonly carbon-coated Li2FeSiO4. Moreover, it exhibits a verystable discharge specific capacity at current densities from0.1to2C. Its excellentelectrochemical performance can be ascribed to the significantly improved diffusioncoefficient of lithium ions (5.6421*10-11cm2/s), which is greatly larger than thereported carbon-coated Li2FeSiO4nanocomposites and diatomic metallic ions (Ni2+,Cu2+, Zn2+) doped Li2FeSiO4.(4) A facile method is developed to obtain hierarchical flower-like LFS withsecondary nanopetals from hierarchical bud-like (shuttle-like) LFS. After activationby coating with2-3nm C@GNS layers, which facilitate both electron and iontransport, the new materials approach the two-Li+-ion-storage full capacity of327.2mAh/g (corresponding to a high specific energy of879Wh/kg), approaching the fulltheoretical capacity (332mAh/g) of LFS. Furthermore, the capacities at variousC-rates measured in this work are higher than those previously reported, and most importantly, stable long-term performance at high rates from5to20C within200cycles is observed.