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

Paiva, T. I.

Plancha, Maria João

Rangel, C. M.

Laboratório Nacional de Energia e Geologia Repository

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

Paiva Luís, Teresa

Repositório do LNEG

IV Iberian Symposium on Hydrogen, Fuel Cells and Advanced Batteries. Estoril, Portugal, June 26-28 2013 Characterisation and performance studies of a LiFePO4 cathode material synthesized by microwave heating  M.J. Plancha*, T.I. Paiva and C.M. Rangel  Laboratório Nacional de Energia e Geologia, Paço do Lumiar 22, 1649-038 Lisboa, Portugal  (*) e-mail corresponding author: mjoao.plancha@lneg.pt _____________________________________________________________________________________  Abstract 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 pro-cess 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.  ______________________________________________________________________________________  Keywords: LiFePO4 cathode, nanocomposites, Li-ion batteries, Microwave heating, Charge-discharge cycles   1 Introduction  Among various energy storage technologies, rechargeable lithium-ion batteries have been con-sidered as an effective solution to the increasing need for high-energy and power densities in elec-trochemical power sources. A battery cell with a LiCoO2 cathode was extensively used for portable applications. However, the toxicity and high cost of cobalt, the poor thermal stability that leads to safe-ty hazards associated with battery fires and the limited availability in nature of the LiCoO2 and oth-er lamellar compounds of the same family, have been the motivation for the research of novel cath-ode materials. Recently, due to its long lifespan, abundant resources, low toxicity and high thermal stability, the olivine lithium iron phosphate (LiFePO4) has emerged as the critical cathode material for applications such as large-scale power storage or electric transportation (e.g., electric ve-hicles and hybrid electric vehicles). This compound has also a high-lithium intercalation voltage (around 3.5 V vs. Li/Li+) and a large theoretical specific capacity (170 mAhg-1) [1]. As a result, there is a large demand for the production of high-performance LiFePO4 powders. The limited low conductivity of pure LiFePO4 can be addressed by modifying the powders structure using novel fabri-cation technologies. Advantages requirements for a good synthetic method are low energy consump-tion cost and reduced processing time. Microwave processing is a self-heating process that occurs through the absorption of electromagnetic energy. Since, the microwave energy is directly absorbed by the sample, uniform and rapid heating can be achieved within a short time period. The energy consumption and cost are lower than those re-quired for furnace heating. In addition, it is a low temperature process with good repeatability, which does not require a reductive gas for the synthesis of LiFePO4.  In the present work, a carbon LiFePO4 material obtained using a microwave assisted synthesis was incorporated into a two terminal Swagelok cell using a Li foil as anode and a Celgard membrane embedded in ionic liquid N-methyl-N-propylpyrro-lidinium bis(fluorosulfonyl)imide (Py13-FSI) togeth-er with the lithium bistrifluoromethane-sulfonamide salt (LiTFSI) as electrolyte. Characterisation and battery performance tests results are discussed.   2 Experimental The first step for the LiFePO4 synthesis with in-corporated carbon was the mixing of precursors (ferrous oxalate, ammonium dihydrogen phos-phate, lithium carbonate and dextrose - the source of carbon) by ball-milling with addition of a solvent (ethanol) for a generating dispersion, followed by solvent evaporation. It was used a value of 5.9 of BPR (ball-to-powder ratio) and a rotation speed of 300 rpm for 6 hours. Mixed precursors were then pelletized in several forms for the next step, the microwave heating, in order to get the carbon mod-ified nanocrystalline LiFePO4 (LiFePO4-C). It was IV Iberian Symposium on Hydrogen, Fuel Cells and Advanced Batteries. Estoril, Portugal, June 26-28 2013  used a modified domestic microwave oven for the heat treatment with an applied maximum power level of 500 W. The effects of microwave time and sample configuration were evaluated by the ac-quired results from X-ray diffraction analyses per-formed on each obtained product.  The product selected from the ones that origi-nated the best synthesis results was incorporated in a composite cathode material (80%) with 15% of carbon and 5% of PVdF. A lithium secondary bat-tery was then prepared with this cathode and with a metallic lithium foil as the anode. It was used as electrolyte, a membrane Celgard® 2400 with the ionic liquid N-methyl-N-propylpyrrolidinium bis (fluoro-sulfonyl)imide with the lithium bistrifluoro-methanesulfonamide salt, Py13-FSI/LiTFSI. This set, anode (Li foil) / Celgard with electrolyte / cath-ode (composite with synthesized LiFePO4-C), was built and placed in a two terminal Swagelok cell for characterisation and battery performance tests.  The synthesized LiFePO4-C/LiTFSI Py13-FSI interface was also characterised by Electrochemi-cal Impedance Spectroscopy (EIS). The EIS spec-tra were obtained by a Solartron 1250 Frequency Response Analyser coupled to a Solartron 1286 Electrochemical Interface in a 0.005-65000 Hz fre-quency range and with a sinusoidal excitation volt-age applied to the cell of 5 mV rms. The equivalent circuit parameters were estimated by using Com-plex Non-Linear Squares fitting.  3 Results For most experimental conditions tested, struc-tural characterisation by X-ray diffraction proved that the olivine phase LiFePO4 was synthesized with a crystallisation degree depending on the mi-crowave irradiation time, even though other crystal-line phosphates were also obtained as sub-products. The materials synthesized with higher microwave times showed higher crystallisation de-grees. Scanning electron microscopy (SEM) showed an uneven surface and, overall, a particle-agglomerated morphology with inhomogeneous particle size distribution. Smaller submicron parti-cles with a size between 50 and 150 nm are bounded on the surface of larger ones (600 nm - 3 µm). Distributed in the sample are few fragments with sizes as big as 15 µm, also with smaller parti-cles on their surface.   Just after the Li / LiFePO4 cell preparation, EIS characterisation took place. Impedance spectra typically exhibited a depressed semicircle in the high frequency region, attributed to the elec-trode/electrolyte charge transfer process. A con-stant phase element was used instead of the dou-ble layer capacitance to account for the porous nature of the cathode. At lower frequencies, a Warburg element was fitted corresponding to the solid-state diffusion of Li+ on the cathode.  Cyclic voltammetry was also applied to the Swagelok cell in the 2.5 - 3.9 V potential range. The existence of two redox processes (Fig.1) was revealed and confirmed by galvanostatic charge and discharge tests. It is associated to Li ions in-tercalation to/extraction from two different com-pounds in the synthesized cathode product. One of them, the LiFePO4 with its expected two-phase electrochemical process between FePO4 and LiFePO4, was evident in the 3.3-3.5 V potential range.                               Fig. 1. Cyclic voltammograms of a synthesized LiFePO4-C electrode measured with a scan rate of 5mVs-1 in two different voltage potential ranges. E=2.5-3.6 V (a); E=3.1-3.9 V (b).    The good reversibility and stability of the LiFePO4 redox processes were established by results from continuous cycles of potentiodynamic tests.  The electrochemical performance of the LiFePO4 was evaluated, between 3.1 and 3.9 V, at room temperature, by galvanostatic charge and discharge measurements. Typical curves for differ-ent current values applied on cell cycling are pre-sented in Fig. 2.     (a) (b) IV Iberian Symposium on Hydrogen, Fuel Cells and Advanced Batteries. Estoril, Portugal, June 26-28 2013              Fig. 2. Charge/discharge curves of the synthesized LiFePO4-C electrode at different applied currents.  The capacity loss occurring during the first few cycles performed at 2 A was attributed to possible losses of cycling lithium due to the growth of a SEI film that can be formed on the cathode’s surface and consumes lithium. EIS analysis, done after cell cycling, confirmed the potential presence of this surface passivation layer. After about 9 cycles, the remaining reversible capacity was maintained up to the final performed cycle at that current. The cou-lombic efficiency presented, throughout all used discharge and charge rates, a value between 0.9 and 1.0, indicating good reversibility between the intercalation and extraction processes for all per-formed cycles. It was then inferred that the capaci-ty decline with the current rate (or applied current) increase is not the result of damage, but rather reflects the lithium transport limitation in the LiFePO4 compound. Optimisation conditions for getting a better cathode material are put forward.  4 Final Remarks A LiFePO4-C composite cathode material was successfully synthesized using the microwave heating process, preceded by mixture and activa-tion of precursors by ball-milling. SEM observations showed a surface presenting a particle-agglomerated morphology with inhomogeneous particle size distribution. The cathode electrochemical characterisation, performed between 2.5 and 3.9 V, revealed the existence of two redox processes in two different potential ranges, one of them belonging to the two-phase transformation between FePO4 and LiFePO4. Good reversibility was evidenced by the charge/discharge behaviour at different current rates applied to the test cell, with the coulombic efficiency presenting a value very close to unity. Good capacity retention after 9 cycles per-formed at 2 A, with no noticeable fading after that, expresses the stability of the redox processes oc-curring in the battery cell. The cell impedance is almost constant thereafter and it is presumed that the formed SEI film stops growing. Modifying the synthesis conditions, namely more rigorous atmosphere control minimizing oxy-gen and the quantity of carbon used for decreasing the particle size, while at the same time obtaining a LiFePO4 compound with more homogeneous sur-face, are features to be taken into account in future work in order to get higher cell capacity values and rate capability improvement. 5 Acknowledgements  This work has been partially supported by FAI under the REIVE project – Redes Eléctricas Inteli-gentes com Veículos Eléctricos. 6 References [1] Padhi, A.K., Nanjundaswamy, K.S., Goodnough, J.B., J. Electrochem. Soc. 144, pp. 1188-1194, 1997 [2] Song, M.-S., Kang, Y.-M., Kim, J.-H., Kim, H.-S., Kim, D.-Y., Kwon, H.-S., Lee, J.-Y., J. Power Sources, 166, pp. 260-265, 2007 [3] Lewandowski, A.P., Hollenkamp, A.F., Donne, S.W., Best, A.S., J. Power Sources, 195, pp. 2029-2035, 2010   1 A 2 A 5 A 

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

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