Electrólitos poliméricos para sistemas electroquímicos de energia

Tese apresentada no Instituto Superior Técnico para obtenção do Grau de Doutor em Engenharia QuímicaNa primeira parte do trabalho são sintetizados filmes poliméricos de poli(óxido de etileno) dopado com cloreto de zinco, iodeto de zinco, cloreto de níquel ou iodeto de níquel. Os filmes, com espessuras na gama 50-100μm, são caracterizados morfológica e microestruturalmente, procedendo-se também à análise do comportamento térmico dos mesmos. O estabelecimento de diagramas de equilíbrio de fases permite prever termodinamicamente a melhor relação estrutura/condução iónica. Esta propriedade é estudada na gama 20-155ºC, determinando-se a influência da temperatura e da concentração salina na condutividade dos electrólitos. A caracterização electroquímica prossegue com a determinação dos números de transferência catiónica, que permitem identificar o tipo de transportadores de carga. Factores como a possibilidade de existência de associação iónica nos electrólitos PEO-MX2, são considerados nos estudos electroquímicos. Os domínios de estabilidade electroquímica são analisados em função da temperatura, determinando-se tensões de decomposição e tensões de descargas catódica e anódica. Na última fase deste trabalho é escolhido o electrólito polimérico que apresenta melhores propriedades para incorporação numa bateria totalmente sólida. As minicélulas, com cátodos de inserção baseados em Nb2O5 e o PEO4ZnCl2 como electrólito, são estudados a 55ºC. A capacidade de retenção de carga e o comportamento das baterias a ciclos de descarga/carga são analisados

Electrochemical characterization of poly(ethylene oxide)- zinc chloride system and its application in rechargeable batteries

The system PEOnZnCl2 with n=4-16 was studied in view of its potential application in a solid state rechargeable zinc battery. A.c. conductivity and cationic transference number measurements, in the temperature range 20-150ºC, were performed and the electrochemical stability window was established for the polymer electrolyte with n=4 composition. The ionic conductivity, σ, of this film, follows a VTF behaviour, with an activation energy of 3.3 ± 0.2 kJ mol-1 and σ values were found between 2.50x10-7 S cm-1 at 24ºC and 4.81x10-4 S cm-1 at 145ºC. Acceptable zinc ion transference numbers of 0.36 (medium value) and decomposition voltage values between 3.19V (20ºC) and 1.44V (150ºC) were estimated. Cyclic voltammetric studies using Zn/PEO4ZnCl2/Zn cell indicated reversibility of the Zn/Zn2+ couple at the electrode/electrolyte interface. Several cells Zn(-)/PEO4ZnCl2/Nb2O5(+) were assembled and studied at 55ºC, with several discharge current densities. Results of cell’s discharge profiles, capacity values, charge-discharge cycles behaviour and stability are reported

Characterisation and performance studies of a LiFePO4 cathode material synthesized by microwave heating

Lithium iron phosphate with incorporated carbon, LiFePO4-C, was synthesized by the microwave-assisted method. X-ray diffraction analyses showed higher crystallization degrees for samples submitted to higher irradiation times. A particle-agglomerated morphology was associated as revealed by scanning electron microscopy. The electrochemical character-istics of a composite cathode containing the synthesized product were evaluated. The two-phase electrochemical process between FePO4 and LiFePO4 was evidenced in the cycling voltammogram profile and its reversibility and stability were demonstrated. An additional redox reversible reaction was revealed and assigned to another phosphate present in the synthesized product. The charge/discharge performance study indicated a good capacity retention after the initial cy-cles where capacity fading was associated to the resistance of a SEI film that forms and grows on the cathode’s surface. Results obtained by electrochemical impedance analysis before and after cell’s cycling are discussed

EIS diagnostics of aged Li-ion batteries

High-power batteries with long cycling life and adequate storage behaviour are needed as energy sources devices for (hybrid) electric vehicles and lithium-ion rechargeable cells are the most promising candidates. In this work, Li-ion cells with a nominal capacity of 10 Ah were studied. Electrochemical impedance spectroscopy (EIS) was used for studying the cycling ageing effect on discharge capacity fade. EIS measurements were conducted in a galvanostatic mode, by means of a Solartron Electrochemical Interface 1286 and a Solartron FRA 1250 controlled by Zplot from Scribner Associates. Scanning frequency ranged from 600 Hz to 0.005 Hz and the ac amplitude was set to 100 mA. An equivalent circuit complex non-linear least squares fitting procedure was used for spectra analyses. Cell charge transfer and film resistances were estimated at several cycle life stages and its evolution on cycle number was analysed. Capacity losses were estimated after 300, 600 and 1200 cycles at C/1 discharge rate and were found to be 5.8, 7.5 and 16.8% of the initial capacity, respectively. EIS data revealed that the major factor responsible for the observed capacity fade was the cell’s charge transfer resistance (Rct) increase following the opposite tendency of the discharge capacity values with cycle number. Very little change can be attributed to film resistances as a result of ageing by cycling. The SEI layer thickness appeared to increase from 0 to 300 cycles remaining almost constant up to 1200 cycles. The effect of a high storage temperature on the performance of the sealed commercial batteries was evaluated by means of discharge capacity measurements and impedance behaviour. The thermal ageing conditions were applied over time and the results were interpreted taking into account the cell’s state of charge (SOC). Before storage at 45 oC, in an open circuit state, cells were fully charged (100% SOC). At defined intervals, cells were cooled to ambient temperature and capacities were determined by a discharge step. As the cells aged up to 25 weeks at 45 oC, the C/1 capacity slowly fades, at first, with a capacity retention of about 95% after 16 weeks of storage. However, the value of discharge capacity loss reached 10.9%, after 25 weeks (Fig. 1). Impedance data were fit to an equivalent circuit with a high frequency arc associated to the SEI film resistance (R sei) and a second arc at medium frequencies to Rct

Degradation of lithium Iron phosphate-based cathode in lithium-ion batteries: a post-mortem analysis

Commercial Li-ion batteries were studied in view to investigate the degradation of the positive electrode in an end-of-life battery condition. Post-mortem analyses were performed by using SEM and DRX techniques; structural and morphological changes after prolonged cycling were evaluated comparatively to a fresh cathode sample. The cycling procedure based on a constant current (CC)/constant voltage (CV) charge and CC discharge was executed, being the condition of end-of-life battery achieved after submitting the Li-ion battery to nearly 2000 charge/discharge cycles. EDS analysis revealed zirconium element as the dopant of a LiFePO4-based cathode of the battery under study. According to X-ray diffraction results for the fresh (charged condition) cathode, the positive electrode includes in its constitution a mixture of crystalline compounds, LiFePO4 and FePO4. SEM images displayed and DRX patterns obtained for the cycled cathode showed modifications compared to the fresh cathode results, evidencing the degradation of the battery at the end-of-life: decrease in the density of microparticles associated to areas where the insertion/de-insertion occurs; decrease of the LiFePO4/FePO4 ratio; both results pointed out to the occurrence of battery loss capacity with the imposed charge/discharge cycles

The effect of chloride as catalyst layer contaminant on the degradation of PEMFCs

In this work, the effect of chloride as a catalyst contaminant was studied on the performance and durability of a low power open-cathode fuel cell, intended for passive management of water. In an ex-situ study, cyclic voltammetry was used to assess the redox behaviour of platinum in chloride contaminated solutions at room temperature.The cell was operated integrating a range of relative humidity (RH) from ~30 to 80% and temperatures from 5 to 55 ºC. Results indicated that 60% RH is associated to maximum fuel cell performance over the studied temperature range. An examination of the fuel cell components after 100 h of operation revealed that chloride contamination has produced cathode failure associated to catalyst migration favored by operation conditions that allowed platinum particles to break free from their carbon backing and migrate toward the polymer electrolyte. Migration resulted in precipitation with larger mean particle size distribution within the solid electrolyte when compared to the original catalyst layer, rendering a very significant loss of thickness in the cathode material