Study On Synthesis And Properties Of LiFePO₄ And LiFePO₄ Matrix Composites

In this paper,LiFePO₄/C and xLiFePO₄-yLi₃V₂?PO₄?₃ were synthesized by carbon-thermal reduction,and the effect of their choices of raw materials,optimization of various process parameters,ion doping on physical and electrochemical properties were studied.Crystal structure,surface morphology and electrochemical properties of the materials were systematically studied by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,different rate charge and discharge test,AC impedance measurements,cyclic voltammetry tests and other testing methods.Using carbon thermal reduction method to synthesize LiFePO₄/C with LiOH·H₂O as lithium source,NH₄H₂PO₄ as phosphorus source,different iron sources and carbon sources regarded as raw materials,and the impact on the performance of LiFePO₄ / C were studied.The results showed LiFePO₄ cathode material synthesized with Fe₂O₃ and tartaric acid has smaller particle size,uniform distribution,complete crystalline particles,high purity,a higher first charge and discharge specific capacity?115.342 mAh·g-1?,a stable charge and discharge platform and a smaller degree of polarization.Different calcining time and the addition of carbon were studied to determine the optimum reaction conditions for preparing LiFePO₄/C: pre-calcined at 350? for 4 h,calcined at 750? for 7 h and added 9.5wt% carbon.The first discharge specific capacity was 136.385 mAh·g-1 at 0.1C rate,and after 20 cycles,the specific capacity still kept over 99.8%,which exhibits excellent cycle performance of the material.Using carbon-thermal reduction to synthesize xLiFePO₄-yLi₃V₂?PO₄?₃ composites,and explores the effect of temperature,time,the composite ratio and other parameters on material performance.Optimization conditions of preparing a composite material: the composite ratio of 9:1,calcination temperature 750? and calcination time 7 h.The composite material has two phases,including olivine-type structure LiFePO₄ and monoclinic structure Li3V2?PO4?3.There were four voltage platforms on the charge-discharge curves,including a voltage platform of LiFePO₄ at about 3.4 V and three characteristic voltage platforms of Li3V2?PO4?3 from 3.6 V to 4.2 V.All of charge-discharge platforms are clear and smooth,and the electrochemical properties of two materials.Using Nb³⁺ to dope LiFePO₄ and 9LiFePO₄-Li₃V₂?PO₄?₃ composite material,and studing the Nb³⁺ doping amount impacted on two material properties.The results show LiFe_0.98Nb_0.02PO₄/C and 9LiFe_0.99Nb_0.01PO₄-Li₃V₂?PO₄?₃ exhibit more excellent electrochemical performances with fine powder particles,uniform,reduced reunion and unchanged structure.Addition of Nb³⁺ did not change crystal structures of LiFePO₄ and 9LiFePO₄-Li₃V₂?PO₄?₃,but the ion entered into the lattice and changed the lattice constant.Compared with the un-doped LiFePO₄/C,the particles were small,uniform and small agglomeration phenomenon.The first discharge capacity of LiFe_0.98V_0.02PO₄/C was 146.881 mAh·g-1 at 0.1C,than the first discharge capacity of un-doped sample of 136.385 mAh·g-1 increased by 7.7%.After 20 cycles,discharge specific capacity still kept over 99.6%.The discharge capacity was 105 mAh·g-1 at 1C.Compared with un-doped material,the first discharge capacity of 9LiFe_0.99Nb_0.01PO₄-Li₃V₂?PO₄?₃ of 147.146 mAh·g-1 at 0.2C increased by 11.4%,and the discharge specific capacity was 107.905 mAh·g-1 at 5C.It explains the addition of Li₃V₂?PO₄?₃ helps improve the charge-discharge performance of LiFePO₄/C at high rate and cycle performance.