Synthesis And Modification Study On LiFePO₄Cathode Material

Olivine-type lithium iron phosphate, LiFePO4, has been considered as one of the most promising cathode materials for lithium ion batteries due to its low cost, nontoxic, high lithium intercalation voltage, high theoretical specific capacity and greater thermal stability. Especially with the development of the new energy vehicle industry and the national policy support. LiFePO4, as one of the power cell cathode materials will have wide application prospects. Nevertheless, it has suffered with its inherent low electronic conductivity and lower lithium diffusion kinetics, which have seriously restricted its popularization and application.Recently, synthesizing nano-size LiFePO4powders has considered as an effective way to overcome these troubles, and wet chemical preparation methods are the classic approach to realize the processes. However, the physicochemical properties of the wet chemical preparation system are complicated and it is hard for us to have a deep understanding, which results in chosing the experimental conditions blindly to some extent. Aiming at this actuality, the synthesis processes of LiFePO4via wet chemical method were studied theoretically and experimentally. Optimum conditions were predicted and then verified by the subsequent experimental results. Furthermove, considering the unclearness of the calcination mechanism in the traditional high-temperature solid-state synthesis processes, TG-DSC coupling technology was used to research the reaction kinetics of the calcination processes for LiFePO4synthesis, and the reaction mechanism were revealed. Based on this study, LiFePO4powers were prepared using carbothermal reduction method, and the optimum conditions were obtained.(1) Thermodynamics study of LiFePO4synthesis via wet chemical methodTaken the LiOH-FeSO4-H3PO4-H2O system as research subject, the distribution of the ions in solution and the predominance-area diagram of each precipitate unit were studied systematacially. The results showed that LiFePO4can be directly synthesized at298K and pH value0～11.3, with its optimum precipitation pH value8-10.5. Considering the low phase transformation kinetics of the LiFePO4formation, metastable Li-Fe(Ⅱ)-P-H2O precipitation system was also studied and the results indicated that Li+-excess was essential for LiFePO4prepared.(2) Synthesis of LiFePO4by hydrothermal and co-precipitation methodsBased on the theoretical analysis results, LiFePO4were synthesized via hydrothermal and co-precipitation method, respectively. For the hydrothermal processes, the pH value, reaction temperature had strongly influence on the LiFePO4formation, and well-crystallized LiFePO4powders can be obtained under pH value7-9, temperature over150℃conditions. For co-precipitation processes, pH value, ion concentration, precipitation temperature and the mole ratio of Li: Fe(Ⅱ):P had greater impact on the compositions of LiFePO4precursor. Controlling the conditions of pH valuethe7-9, reaction temperature65℃, initial Li+concentration1mol/L, Li:Fe(Ⅱ): P=3:1:1, stoichiometric ratio of the LiFePO4precursor were obtained, and it was consistent with the thermodynamics analysis results. In addition, well-distributed of LiFe0.96Mg0.04PO4/C powders were synthesized under the obtained optimum synthesis conditions by using the above-prepared precursor as raw material. The size of LiFe0.96Mg0.04PO4/C powders was about1um, and its specific discharge capacity at0.1,0.2,0.5C charge/discharge rates were160,150,138mAh/g, respectively. Besides, the material showed good cycle performance.(3) Thermal kinetics of LiFePO4synthesis reaction in the calcinations processesTG-DSC coupling technology was used to research the reaction kinetics of the calcination processes for LiFePO4synthesis. The results indicated that the low temperature dehydration reaction corresponded with the Avrami-Erofeev(n=2/3) model, i.e. G(a)=[-1n(1-α)]2/3,/(α)=1.5(1-α)[-1n(1-α)]1/3. For the high temperature synthesis reaction, its mechanism fitted the Avrami-Erofeev(n=1/4, m=4) mode, i.e. G(α)= [-ln(1-α)]1/4,f(a)=4(l-α)[-1n(1-α)]3/4. Additionally, the non-isothermal kinetic equations of each reaction were shown as the following equations: Low temperature dehydration reaction: High temperature synthesis reaction:(4) Synthesis of LiFePO4synthesis via carbothermal reduction methodWell electrochemical performance of LiFe0.96Mg0.04PO4/C powders were synthesized through carbothermal reduction method by using the Li2CO3、FePO4·2H2O as raw material, C6H12O6·H2O as reduction agent and carbon source,self-prepared magnesium salt (MgNH4PO4·H2O/MgHPO4·3H2O mixture) as doping agent. The initial specific discharge capacity of LiFe0.96Mg0.04PO4/C at0.1,0.2,0.5C rates were162,156,150,140mAh/g, respectively.20cycles after, its discharge capacity were162,157,154,140mAh/g, matching with the capacity-keeping rates100,101,103,100%, respectively. It demonstrated good charge/discharge properties and cycle performance.