Ionothermal Synthesis And Structural Characterization Of LiMPO₄（M=Fe,Mn） Cathode Material And Its Electrochemical Performance

Ionothermal synthesis is a novel method that uses ionic liquids simultaneouslyas both the solvent and potential structure directing agent in the formation ofinorganic materials. Use of ionic liquids provides an inherent advantage toionothermal method lies in the feasibility of carrying out the reaction atatmospheric pressure.The LiFePO₄and LiMnPO₄powders were synthesized in hydroxyl ammoniumionic liquids under mild condition at low temperature and ambient pressure via anionothermal synthesis route. Four types of ionic liquids, including2-hydroxyethyl-ammonium lactate（EAL）,2-hydroxyethylammoniumacetate（EAA）, bis-（2-hydroxy-ethyl）ammonium acetate（DEA） and tri-（2-hydroxyethyl）ammonium lactate（TEL）are selected as reaction medium. The dissertation investigates the impacts of the ionicliquids types, reaction time, synthesis temperature, surfactant, and heat treatment on thepreparation of the cathode materials and their properties. The products werecharacterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM）,transmission electron microscopy （TEM）, and galvanostatic charge/dischargemethod, respectively.The results reveal that:（1） Olivine LiMPO₄（M=Fe, Mn） could be obtained inEAL, EAA, DEA and TEL ionic liquids under the mild condition of lowtemperature and ambient pressure via an ionothermal synthesis method.（2） Themorphologies of LiFePO₄samples obtained in different ionic liquids were uniformolive-shaped but their particle sizes varied, with the length of400nm in EAA, DEA,and TEL while2.5μm in EAL; however, the LiMnPO₄samples had variousmorphologies of rod-like, flake-like or platelet-like with particle sizes ranging from100to300nm.（3） The possible formation mechanisms of olive-shaped LiFePO₄particles in ionic liquids might comprise the crystal formation and particle growthprocesses, in which ionic liquids mainly functioned as the solvent and structuredirecting agent, respectively.（4） The addition of sodium dodecyl sulfonate （SDS） orhexadecyl trimethyl ammonium bromide（CTAB） in ionothermal synthesis could controlthe morphologies and particle sizes of LiFePO₄.（5） The LiFePO₄cathode material obtained in hydroxyl ammonium ionic liquids exhibited the best electrochemicalperformance with a specific discharge capacity of122.9mA h/g at0.1C, and with asuperior performance of145.1mA h/g at0.1C after a subsequent heat treatment.Without further processing, the pure LiMnPO₄obtained in ionic liquids nearly had novoltage plateaus in its discharge profiles, while LiMnPO₄/C obtained by asubsequent heat treatment with sucrose exhibited a good performance with aspecific discharge capacity up to115.3mA h/g at0.05C. It was also indicated thatthe ionothermal synthesis in hydroxyl ammonium ionic liquids could be apromising route to prepare LiMPO₄（M=Fe, Mn） cathode materials for lithium-ionbatteries.