Study On The Synthesis And Modification Of LiFePO₄Cathode Materials For Lithium-ion Batteries By Wet-chemical Process

The olivine LiFePO₄cathode material for lithium-ion batteries has many advantagessuch as high specific capacity, excellent cycle performance, low cost, more safety and highlycompatible to environment. However, the low electronic and ionic conductivity lead to poorrate performance of LiFePO₄. In this paper, wet-chemical process was used to prepareLiFePO₄and the ways of modification including carbon coating, Na and Mo doping andadding a surfactant compound （CTAB） were also used to enhance the rate charge-dischargeperformance of LiFePO₄.Li_0.97Na_0.03FePO₄/C composite cathode material was prepared by a simple wetchemistry-carbothermal reduction method using Fe（NO₃）₃, LiNO₃, NH₄H₂PO₄and NaNO₃asstarting materials. The structure and rate charge-discharge properties of the samples werestudied by X-ray diffraction （XRD）, scanning electron microscope （SEM） and charge-discharge test. Furthermore, the kinetics of lithium-ion extraction and insertion at LiFePO₄/Cwere evaluated from cycLic voltammetry （CV） curves and electrochemical impedancespectroscopy （EIS）. The results indicate that the Li_0.97Na_0.03FePO₄solid-solution with oLivinestructure is obtained. Na-doping enlarges one-dimensional lithium-ion diffusion pathway inthe oLivine structure. Consequently, the materials exhibit a decrease in the charge transferresistance by67%and an increase in the Lithium-ion diffusion coefficient by3～4times. Theinitial discharge capacities of Li_0.97Na_0.03FePO₄/C sample at the rates of0.1C and2C are152mAh·g^-1and109mAh·g^-1, respectively, which increase4.83%and62.69%compared with thatof un-doped LiFePO₄/C.Similarly, LiFe_0.99Mo_0.01PO₄/C composite cathode material was synthesized via using（NH₄）₆Mo₇O₂₄·11H₂O as doping source. The crystal structure and properties of the sampleswere characterized through XRD, SEM, CV curves and charge-discharge test. The resultsshow that Mo-doping decreases the volume of unit cell for0.32%and shortens the diffusionroute of Li+, instead of destroying the crystal structure of LiFePO₄/C. As a result, theLiFe_0.99Mo_0.01PO₄/C sample heated for6hours owns better rate performance, which the initialdischarge capacity at1C is107mAh·g^-1. Furthermore, the LiFe_0.99Mo_0.01PO₄/C sample heatedfor8hours exhibits the best rate performance. The initial discharge capacities at the rates of 0.1C,1C and10C are141,111and74mAh·g^-1, respectively.LiFePO₄material was synthersized by hydrothermal method using LiOH·H₂O,FeSO₄·7H₂O and H₃PO₄as starting materials. Charging-discharging results indicate that bothof heat treating and carbon coating could increase the specific capacity of LiFePO₄. The initialdischarge capacities of LiFePO₄and LiFePO₄/C samples at0.2C are110and124mAh·g^-1,respectively. Adding0.32grams CTAB during hydrothermal synthesis will further improve therate performance of LiFePO₄/C. The initial discharge capacity at0.2C is133mAh·g^-1.Furthermore, LiFePO₄/C could be also prepared via hydrothermal method usingFe（NO₃）₃·9H₂O as iron source. The initial discharge capacity of the sample withCH₃COOLi·2H₂O as lithium source at0.05C is only96mAh·g^-1, which is lower than thesample using LiNO₃as ｌithium source with the initial discharge capacities of112and91mAh·g^-1at the rates of0.1C and1C.