By starting from experimentally determined cross sections of rechargeable lithium-ion batteries, the effect of microstructure on the galvanostatic discharge of a LiCoO 2 ͉LiC 6 cell was numerically modeled. Results demonstrate that when small graphite particles are part of a population with large particle sizes, diminished macroscopic power densities develop and limit the response of the entirety of the cell. Small particle-size populations electrochemically interact with large particle-size populations and lead to a macroscopic capacity loss, compared to cells with uniform particle size. Such capacity loss is a result of the lithium exchange between small and large anode particle-size populations, instead of the lithium exchange between electrode particles of opposite polarity. The analysis suggests that graphite particles of size smaller than the average value dominate the macroscopic electrical response of the device, for the induced localized lithium depletion leads to an increase in the polarization losses of the anode. Lithium depletion in the anode starts in the small particles, is followed by particles of complex morphology and rough surfaces, and continues with the depletion of large particles embedded in a fast diffusion environment.Rechargeable lithium-ion batteries have undergone a great deal of progress through the introduction of materials and engineering approaches over the past 20 years. These improvements have boosted their reliability and power density 1-3 and have proven that the distribution of particle aggregates of active material within the space of the cell determines the performance of batteries in the limit of large discharge rates, high energy densities, and low salt concentrations. 4-13 For high power density applications, particleparticle electrochemical interactions dominate the response of the cell and promote the appearance of irreversible physical mechanisms. Microstructural processes such as interfacial side reactions, oxidation or reduction of the electrolyte, dissolution and replating of current collector metals, phase transformations within the electroactive materials, salt precipitation, and the formation of conductive dendrites within the separator, are several technologically relevant microphysical processes that diminish the lifetime of a rechargeable lithium-ion battery. 14,15 Recently developed cross-sectional characterization techniques in rocking-chair rechargeable lithium-ion batteries suggest that battery degradation mechanisms are localized microstructural events that are a consequence of particle configurations introduced during the processing of the composite electrodes. 16 The resulting particle configurations favor the occurrence of large localized variations in the cell's electrochemical behavior, leading to localized lithium accumulation or depletion. Moreover, available ex situ microstructural measurements of nanosized particulate electrode particles demonstrate that the benefits of using smaller lengthscales are lost when considering the sum of the contribut...

Lithiated FeS2 samples with various lithium contents were examined by a combination of ex situ X-ray powder diffraction and ex situ scanning electron microscopy to provide experimental evidence for intermediate phases and insights to lithium reaction mechanisms of pyrite at ambient temperature. Natural and synthetic FeS2 samples having distinctly different grain sizes were used in this study. Structural and morphological changes of these FeS2 samples were compared under low and high current densities. Structural and morphological evidence revealed the presence of intermediate phases upon two-voltage-step lithium reduction of FeS2 pyrite at ambient temperature. These intermediate phases were similar to some of those obtained from lithium reduction of FeS2 near 400°C. Of significance was X-ray powder diffraction evidence of a layered Li2+xFe1−xS2 false(0⩽x⩽0.33false) solid solution phase upon the 1.7 V lithium reduction of FeS2 at ambient temperature. A lithium reaction mechanism for two-voltage-step lithium reduction of FeS2 pyrite was proposed and compared to those proposed in the literature. In addition, it was found that one-voltage-step lithium reaction of FeS2 pyrite at 1.5 V or lower led to the formation of plate-like Li2S (plate thickness on the order of 20 nm) and Fe, which supported the reaction mechanism proposed by Fong et al. [J. Electrochem. Soc., 136, 3206 (1989).] © 2002 The Electrochemical Society. All rights reserved.

A novel experimental technique was developed to extract the KOH solution from the porous zinc electrode in an inert environment. This technique permits the ZnO/electrolyte interface, the ZnO/Zn particle interface, and the spatial distribution of ZnO in the porous anode in commercial alkaline Zn/MnO 2 AA batteries to be characterized for the first time by optical and scanning electron microscopy. Both type I and type II ZnO, previously observed in oxidation of planar zinc electrodes, were found in each completely discharged zinc particle in the porous electrode. The morphology, size, and location of these two ZnO forms strongly suggested that they were produced by a solution-precipitation route. The drain rate had a significant impact on the spatial distribution of ZnO in the anode. The segregation of ZnO near the separator became dominant at drain rates higher than 500 mA. The mechanisms by which the morphology and segregation of ZnO within the porous electrode could result in the reduced battery runtime are discussed.Growth and advances in the portable electronic industry require commercial alkaline Zn/MnO 2 AA batteries to function under high power and high drain conditions. Unfortunately, AA batteries operate inefficiently at high drain rates. 1 For example, the runtime of a AA battery at a 1000 mA continuous drain rate is typically less than 20% of that discharged at 10 mA. This inefficiency is a result of severe polarization across the Zn ͑anode͒ and MnO 2 ͑cathode͒ electrodes at high drain rates. The Zn/MnO 2 AA cell has a bobbin-type construction, where both electrodes are porous, as shown in Fig. 1. The anode consists of zinc powder that is usually suspended in a gelled electrolyte of concentrated KOH in water, and the cathode includes a physical mixture of electrolytic manganese dioxide and graphite, respectively. 1 It is accepted that the oxidation of zinc proceeds by a dissolution-precipitation process, while MnO 2 is reduced by a solid-state intercalation of H ϩ into the MnO 2 lattice, as followsRecent efforts in analyzing the AA battery discharge performance by mathematical modeling have suggested that the spatial distribution of the discharge product, ZnO, within the anode can have a significant effect on the electrode polarization and the battery performance under high drain rates. 2 In addition, the morphology and spatial distribution of ZnO may have a significant impact on the rechargeability of secondary alkaline systems that utilize a metallic zinc anode. Although the morphology and reaction mechanism of ZnO formation on planar zinc electrodes in concentrated KOH electrolytes is well studied, 3-12 limited information is available on porous zinc electrodes. Our study aims to understand the nucleation and growth process of ZnO and to investigate the effect of drain rates on the spatial distribution of ZnO in discharged porous zinc anodes.The morphology and formation mechanism of ZnO on the surface of polished zinc-plate electrodes in concentrated KOH electrolytes was first studied by Powers et al...

The rate capability of lithium cells containing a nano- FeS2 pyrite sample was compared to that of micrometer-sized FeS2 under an active material loading equivalent to that of commercial lithium battery electrodes. The nano- FeS2 sample had an average particle size of 0.5 μm and each particle consisted of nano- FeS2 crystals on the order of 50 nm. The rate capability of nano- FeS2 was superior to that of the FeS2 electrodes with an average crystal size of 10 μm but no improvement was found relative to the FeS2 electrodes having an average crystal size of 1 μm. The rate capability of commercial normalLi/FeS2 batteries may be limited by the ion conductivity of the electrolyte when FeS2 samples have crystal sizes smaller than 1 μm. © 2002 The Electrochemical Society. All rights reserved.

No abstract