Hydrothermal Growth And Characterization Of LiFePO₄ Crystals

Olivine structure of LiFePO₄ as a kind of very important advanced lithium-ion battery cathode material has been widely applied to high energy,high power charging system such as plug-in hybrid electric vehicles?PHEV?^[1,2].Despite its showed excellent properties such as electrochemical activity,high heat stability,environmental friendly^[3-6],but the bottleneck of electronic/low ionic conductivity is still difficult to overcome,so it is difficult to be used widely in high power devices proposal^[3,7,8].The past few decades,a large number of attempts to focus on improve the electrical conductivity to the application level,one of the most successful method is the preparation of LiFePO₄-C composites:the LiFePO₄ particles with carbon network,system of electric conductivity can be achieve LiCoO₂ level.Another has proved feasible method is in Li of LiFePO₄ lattice?M₁?or Fe?M₂?with bivalent(Mg²⁺,Mn²⁺and Ni²⁺,Cu²⁺)or super valence ion(Al³⁺,Cr³⁺,Zr⁴⁺,Nb⁵⁺),but until now to explain its mechanism is still controversial^[9-12].Preparation of submicrometer or nanometer scale LiFePO4 has been proved that can improve the electrochemical performance for shortening the lithium ions and electrons migration path,to decrease the size of the lattice also is particularly important for electrode,electrolyte interface contact and can reduce the lithium ion insertion?out?the lattice mechanical stres.But then also can appear many problems to be solved,for example by nanometer LiFePO₄,and then enhance the activity of his,so the nano FePO₄ on the surface of the LiFePO₄ferrous ion is easily oxidized,which formed the trivalent iron ions,the activity of such impurities were introduced and material loss ever^[10-12].In the nanoscale LiFePO₄structure,due to the non-high crystallization of the LiFePO₄ lattice,the stable voltage cannot be obtained.In the case of nanometer size,the batches produced by LiFePO₄are often unstable and inconsistent.The single-crystal LiFePO4 was synthesized by Guilin hydrothermal method,and the single crystal of lithium iron phosphate was prepared.The single crystal has a six-prism structure or a crystal with a round shape,isometric appearance.XRD spectra of the single-crystal LiFePO4 is obtained by the experiment.In 10°to40°strong within the scope of the main peak,compared with nm level lithium iron phosphate,mm leve crystal has one more?020?surface.It is indicated that in the later period of crystal growth,mainly controlled by?020?surface,the main six prismatic properties are ending at?010?,?200?,?101?surfaces of three,these advantages face this is expected to be in the exchange of electrochemistry and surface properties play an important role.Based on the analysis of LiFePO₄ crystal structure,the geometrical environment of lithium ion diffusion in lithium iron phosphate was analyzed.The lithium ion is diffused in the direction of?010?,so it is possible to propose the design synthesis to reduce the axis of the b axis.To expand the2-dimensional flaky structure of the ac direction of LiFePO₄.It is expected to increase the diffusion rate of lithium ions,thus improving the conductivity,multiplier performance and low temperature performance of LiFePO₄ materials.Selected by the single crystal of LiFePO₄ in ultraviolet-visible absorption spectrum wavelength range,known LiFePO₄ large absorption in the uv area.There is a wide band in the visible area,with a smaller absorption value,and the majority of the light passes through,which is why the color of LiFePO₄ single crystal is not deep.The hydrothermal growth of LiFePO₄ crystals has a good permeability.The high symmetry absorption and transmission of LiFePO₄ crystals 1000 nm³000 nm wavelength will make a breakthrough in the application of near-infrared band window devices.It provides a feasible growth scheme for the growth of hydrothermal method of the crystal and a certain experience of the synthesis of LiM(⁺²)PO₄ crystals.The morphology,structure and spectral characteristics of the single crystal were studied.