The Effect of Microstructure on the Galvanostatic Discharge of Graphite Anode Electrodes in LiCoO[sub 2]-Based Rocking-Chair Rechargeable Batteries

Abstract

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...

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