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Title:Synthesis and microstructural characterization of phosphate cathode materials prepared by a polymeric steric entrapment precursor route
Author(s):Ribero Rodriguez, Daniel
Advisor(s):Kriven, Waltraud M.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
low temperature
lithium oxide
Lithium iron phosphate
sodium iron phosphate
sodium titanium phosphate
aqueous batteries
polymeric steric entrapment precursor route
lithium ion batteries
Abstract:Due to its high energy density and cycling performance Li-ion batteries play an important role as energy storage technology in the future human development. Lithium ion batteries are appealing for applications that include portable electronics such as cellular phones and laptop computers. However, larger scale Li-ion battery system for vehicles and grid load leveling as well as complementary energy storage for renewable energy resources, such as solar and wind power seems to be the next target for metal-ion battery technology. In this work, a nanoscale and pure olivine structure LiFePO4 (triphylite) was synthesized at low temperature (since 300 ºC) using an organic–inorganic steric entrapment solution, from precursor chemicals of LiNO3, Fe(NO3)3•9H2O and (NH4)2HPO4 stoichiometrically dissolved in distilled water. A long-chain polymer such as polyvinyl alcohol (–[CH2–CHOH]-n or PVA) having a degree of polymerization corresponding to a molecular weight of 9,000 to 10,000 was used as the organic carrier for the precursors, which served for the physical entrapment of the metal ions in the dried network. Normally, when calcined and crystallized in air, this method leads to the synthesis of compounds where the cations are in their highest oxidation state. However, in this study we found a way to make compounds having lower oxidation states (e.g. Fe+2 versus Fe+3) which may have wider applications in the synthesis of other compounds having variable oxidation states, with potential applications in electronic ceramics of complex chemistry. LiFePO4 was selected as a model system to evaluate the influence of variables such as the amount of water, pH of the solution, drying procedure, HNO3 addition, amount of polymer, calcination/crystallization atmosphere and temperature on the synthesis. Then the variables were tuned to produce NaFePO4 and NaTi2(PO4)3 based on the concepts learned from the model system. NaFePO4 (maricite) was synthesized at low temperature (~ 300 ºC) using PVA as a polymer carrier and dissolving stoichiometric amounts of NaNO3, Fe(NO3)3•9H2O and (NH4)2HPO4 in water. For the NaTi2(PO4)3 system, a hybrid synthesis method was used because some reagents (NaNO3 and (NH4)2HPO4 sources) dissolved in water, but not in alcohol. Moreover, another reagent such as titanium (IV) isopropoxide (Ti[OCH(CH3)2]4, also called “TISO”) decomposed in water (forming isopropyl alcohol and a hydrated form of titania) but dissolve and remained stable in alcohol. However, the decomposition to titania could be hindered by adding excess isopropyl alcohol, so as to drive the equilibrium to the titanium (IV) isopropoxide side instead of the titania side. The titanium isopropoxide was therefore dissolved in isopropyl alcohol. For this hybrid method, an ethylene glycol (EG) monomer (HOCH2CH2OH) was chosen as a polymeric carrier. The resulting LiFePO4 or (Li2O•2FeO•P2O5), NaFePO4 or (Na2O•2FeO•P2O5) and NaTi2(PO4)3 (Na2O•2FeO•P2O5) powders were characterized by TG/DTA thermal analysis, X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) nitrogen absorption, inductively coupled plasma (ICP) emission spectroscopy and particle size analysis. In this work, has been demonstrated that by drying the solution at low temperature, release of organic or nitrates was avoided, which meant full availability of these two components (fuel and oxidizer) for the next step (calcination/crystallization). Fuel and oxidizer generated a strong exothermic reaction which could be used for crystallization of the desired compound at a lower temperature. In general, the morphology of the powders produced by the polymeric steric entrapment method was porous secondary particles formed from primary particles in the range of 20 nm – 10 microns, in the case of LiFePO4. These secondary particles were soft agglomerates that showed this particular microstructure, due to the violent exothermic decomposition reaction of organics reacting with nitrates. These porous structures have a higher specific surface area (40-50 m2/g) compared to the reference commercial LiFePO4 powder (17.92 m2/g), which it is desirable for ion and electron diffusion in lithium ion batteries. For the case of pure crystalline NaFePO4 phase, crystals of irregular shapes of about 100 nm - 200 nm were found in the powders crystallized at temperatures between 300 °C and 500 °C. Their specific surface area was around 28.92 m2/g. Moreover, for NaTi2(PO4)3, synthesized at 700 °C, the secondary particles were formed from primary crystals with no particular morphology in the range of 50 - 150 nm and showed specific surface areas of the amorphous and crystalline powders of 82.97 m2/g and 40.93 m2/g, respectively.
Issue Date:2015-01-21
Rights Information:Copyright 2014 Daniel Ribero Rodriguez
Date Available in IDEALS:2015-01-21
Date Deposited:2014-12

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