Synthesis, structural and electrochemical properties of LiFePo4 and Li2FeSiO4 as cathode materials for lithium-ion batteries : doctoral dissertation

Predmet istraživanja ove doktorske disertacije su litijum gvožđe(II) fosfat (LiFePO4) i litijum gvožđe(II) silikat (Li2FeSiO4), koji predstavljaju novu generaciju polianjonskih interkalarnih katodnih materijala za litijum-jonske baterije. Cilj istraživanja je sinteza LiFePO4 i Li2FeSiO4 različitim metodama, kao i ispitivanje njihovih strukturnih i elektrohemijskih svojstava. U cilju unapređenja katodnih svojstava ova dva materijala, koja su ograničena njihovim svojstvenim jonsko-elektronskim transportnim osobinama, prahovi LiFePO4 i Li2FeSiO4 su modifikovani korišćenjem različitih načina sinteze putem formiranja in situ kompozita sa ugljenikom, regulacijom veličine zrna ili anjonskim dopiranjem korišćenjem jona fluora F- kao dopanta. Metodom precipitacije i naknadnim odgrevanjem na 700°C u inertnoj, blago redukcionoj atmosferi sintetisani su prahovi čistog LiFePO4, zatim LiFePO4 dopiranog fluorom, kao i kompozita fluorom dopiranog LiFePO4 i ugljenika, gde je LiF korišćen je kao izvor fluora, a stearinska kiselina kao izvor ugljenika. Kompozit LiFePO4 i ugljenika (LiFePO4/C) je sintetisan originalnim metodom kratkog odgrevanja (5-10 min) prekursora u celuloznoj matrici, u istim uslovima tempe-rature i atmosfere, i naknadnim naglim hlađenjem. Prah Li2FeSiO4 sintetisan je reakcijom u čvrstom stanju na 750°C takođe u inertnoj, blago redukcionoj atmosferi. Kompozit Li2FeSiO4 i ugljenika (Li2FeSiO4/C) je sintetisan metodom kratkog odgrevanja (5-10 min) prekursora u metilceluloznoj matrici, u istim uslovima temperature i atmosfere, i naknadnim kvenčovanjem. Dobijeni prahovi su ispitivani rendgenostrukturnom analizom, termogravimetrijskom analizom, merenjem raspodele veličine zrna, merenjem specifične električne provodljivosti, elektronskom mikroskopijom, mesbauerovom spektroskopijom i galvanostatskim merenjem.The research topic of this doctoral thesis are lithium iron(II) phosphate (LiFePO4) and lithium iron(II) silicate (Li2FeSiO4), representatives of the new generation of intercalation polyanionic cathode materials for lithium-ion batteries and alternatives to the old materials based on oxides. The aim of the research is the synthesis of LiFePO4 and Li2FeSiO4 with different methods, as well as the investigation of their structural and electrochemical properties. In order to improve cathode performance of these two materials, which is limited with their intrinsic ionic-electronic transport properties, the powders of Li2FeSiO4 and LiFePO4 were modified using different methods of synthesis by creating in situ composites with carbon, by grain size control or by anion doping using fluorine ion F- as dopant. Using precipitation method and subsequent annealing at 700°C in an inert, slightly reductive atmosphere, powders of a pristine LiFePO4, fluorine doped LiFePO4 and fluorine doped LiFePO4/carbon composite, wherein LiF is used as the fluorine source and stearic acid as the carbon source. The composite of the undoped LiFePO4 and carbon (LiFePO4/C) was synthesized by the original method of a short termic treatment (5-10 min) of a precursor in the cellulose matrix under the same temperature and atmosphere conditions, and subsequent quenching. Li2FeSiO4 powder was synthesized by solid state reaction at 750°C also in an inert, slightly reductive atmosphere. The composite of the Li2FeSiO4 and a carbon (Li2FeSiO4/C) was synthesized by the short thermic treatment (5-10 min) of a precursor in methylcellulose matrix, under the same temperature and atmosphere conditions and subsequent quenching. The obtained powders were examined by X-ray diffraction (XRD), thermogravimetric analysis (TGA), particle size analysis (PSA), specific electrical conductivity measurements, electron microscopy (EM), Mössbauer spectroscopy and galvanostatic testing