Sinteza, strukturna i elektrohemijska svojstva LiFePO4 i Li2FeSiO4 kao katodnih materijala za litijum-jonske baterije

Abstract

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. Utvrđeno je da se fluor ugrađuje u rešetku olivina u iznosu od 2 atom% isključivo na O2 mestu kiseonika što za posledicu ima smanjenu zapreminu jedinične ćelije i stabilizaciju strukture uz smanjenu defektnost, a povećanu kristaliničnost; ugradnja fluora nema bitnijeg uticaja na morfološke karakteristike praha; specifična električna provodljivost fluorom dopiranog LiFePO4 povećana je pet puta (oko pola reda veličine) u odnosu na čist LiFePO4; duži naponski platoi i veći specifični kapaciteti na svim brzinama punjenja i pražnjenja ukazuju na „dublju“ interkalaciju litijuma u olivinu dopiranom fluorom; katodna svojstva su dalje unapređena u kompozitu fluorom dopiranog LiFePO4/C zahvaljujući in situ dobijenom provodnom sloju ugljenika (5 tež%) koji oblaže čestice aktivnog materijala sprečavajući njihovu aglomeraciju i ukrupnjavanje. Kako bi se rast čestica dodatno ograničio ispitana je sinteza LiFePO4/C kompozita metodom kratkotrajnog termičkog tretmana prilikom čega je dobijena nanokristalna faza LiFePO4 (sred. veličine kristalita 35 nm) uz prisustvo minimalne količine defekata („antisite“, 2 at%) kao i amorfna faza C (40 tež%); dobijeni prah ostvaruje praktično teorijski kapacitet uz sjajnu reverzibilnost tokom 150 testiranih ciklusa. Ispitivanjem čistog Li2FeSiO4 i kompozita Li2FeSiO4/C izvedeni su sledeći zaključci: P21/n struktura Li2FeSiO4 podložna je nastanku „antisite“ defekta, i to isključivo na Li2 mestu kao rezultat međukatjonske elektrostatičke interakcije; na osnovu prostorne distribucije vrednosti sume valence veza litijuma izračunata je 3D mapa mogućih putanja litijumovog jona u okviru P21/n rešetke i utvrđeno da je transport litijuma dvodimenzionalan po ravnima (101); tokom cikliranja su zapažene promene strukture koje ukazuju na neuređenu Pmnb fazu; katodna svojstva unapređena su kod kompozita sa ugljenikom i utvrđeno je da sadržaj ugljenika bitno utiče na veličinu kristalita, ukupnu kristaliničnost, veličinu čestica i provodljivost kompozita, ali kod većih sadržaja ugljenika dalje poboljšanje katodnih karakteristika izostaje.

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. It was found that the fluorine is incorporated into the lattice of olivine LiFePO4 in the amount of 2 atom% exclusively on O2 sites of oxygen, resulting in a reduced unit cell volume and the structure stabilization (reduced concentration of defects and increased crystallinity); incorporation of a fluorine has no significant impact on the morphological characteristics of the powder; specific electric conductivity of the fluorine doped LiFePO4 is increased five times (about one order of magnitude) when compared to pure LiFePO4; longer voltage plateaus and higher specific capacity at all current densities indicate "deeper" intercalation of lithium in the fluorine doped olivine; the cathode performance was further enhanced in the composite of fluorine doped LiFePO4/C thanks to the in situ obtained conductive layer of carbon coating (5 wt%), which prevents agglomeration and particle growth. In order to further restrict grain growth, method of a short thermic treatment was employed for the synthesis of LiFePO4/C composite resulting in a nanocrystalline phase of LiFePO4 (average crystallite size of 35 nm) with minimal presence of defects (antisite, 2 atom%) and amorphous phase of carbon (40 wt%); as prepared powder achieves nearly theoretical capacity and reversible cycling during 150 tested cycles. The investigation of pristine Li2FeSiO4 and composite Li2FeSiO4/C resulted in following conclusions: P21/n structure of Li2FeSiO4 is prone to the antisite defect exclusively on Li2 sites of lithium as a consequence of electrostatic interactions between cations; using spatial distributions of lithium bond valence sum within P21/n structure, the map of possible lithium diffusion pathways was calculated on 3D grid and it was found that lithium transport is two dimensional in (101) planes; during cycling structural changes were observed leading, according to preliminary estimates, to the disordered Pmnb phase; cathode performance has been improved by preparing in situ composites with carbon and it was found that the content of carbon significantly affects the crystallite size, total crystallinity, grain size and conductivity of the composite, but if the carbon content is too high further improvement of cathode characteristics is absent (saturation occurs).