Olivine electrode engineering impact on the electrochemical performance of lithium-ion batteries
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High energy and power density lithium iron phosphate was studied for hybrid electric vehicle applications. This work addresses the effects of porosity in a composite electrode using a four-point probe resistivity analyzer, galvanostatic cycling, and electrochemical impedance spectroscopy (EIS). The four-point probe result indicates that the porosity of composite electrode affects the electronic conductivity significantly. This effect is also observed from the cell’s pulse current discharge performance. Compared to the direct current (dc) methods used, the EIS data are more sensitive to electrode porosity, especially for electrodes with low porosity values. I. INTRODUCTION
Lithium iron phosphate, an emerging cathode material for lithium-ion batteries, has drawn great attention due to its good thermal stability, low cost, and environmental friendliness. The power characteristic of lithium ion phosphate, which suffers from poor electronic conductivity and sluggish lithium ion diffusion, has been improved by several approaches, such as carbon coating, doping, and particle size reduction.1–4 In addition to the active material physical and chemical properties, the performance of a lithium-ion electrode is highly dependent on the specifics of the fabrication process, which is often overlooked. During composite electrode preparation, any interactions of the active material with the solvent, polymer, and carbon additive during the mixing, coating, drying, and calendering can potentially have a significant influence on the electrode microstructure, which is critical to the electrode electrochemical performance. Previous work demonstrated how the composition of the electrode affects the electronic conductivity and cell performance.5,6 Electrochemical impedance spectroscopy (EIS) experiments and modeling work of the positive electrode also indicates that the number of parameters, including the active materials’ intrinsic physical properties, oxide particle structure, and oxide/carbon interface, have significant impact on the electrode’s performance.7,8 This study will focus on how the calendering process affects the electrode porosity, impedance, and even its electrochemical performance. II. EXPERIMENT
LiFePO4 powder with 5% carbon coating was received from Mitsui Engineering Shipbuilding Co. Ltd. (Tokyo, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0214
1656
http://journals.cambridge.org
Japan), and mixed with 4 wt% SFG-6 graphite, 4 wt% acetylene carbon black, and 8 wt% polyvinylidene fluoride (PVDF) binder. The mix was laminated on polyester (PET) film and aluminum (Al) foil for conductivity measurement and electrochemical testing, respectively. Each laminate was either used as is (no calendering) or calendered to two different thicknesses. The electrode porosity corresponding to each thickness was calculated using the theoretical density value of the individual components. The uncalendered electrodes have about 50% porosity. The lowest porosity of the calendered lamination i
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