The Effects of Defects on Localized Plating in Lithium-Ion Batteries

Abstract

This work investigates how local cell defects can induce local lithium deposition and dendrite growth in a lithium-ion cell that appears to otherwise be performing correctly. Using local pore closure in the battery separator as a model defect, we experimentally demonstrate the occurrence of local lithium deposition during cycling in coin cells containing deliberately manufactured local regions of separator pore closure. We further investigate the local plating phenomena observed in these experiments using an axisymmetric finite element model of the defect-containing coin cell geometry. Our simulations show that the pore closure acts as an "electrochemical concentrator," creating locally high currents and overpotentials in the adjacent electrodes. This leads to lithium plating if the local overpotential exceeds equilibrium potential in the negative electrode. We examine the sensitivity of the local plating behavior to various materials, geometric, and operating parameters to identify mitigation strategies. The results of this work can be generalized to any defect that creates spatially non-uniform current distributions. The formation of metallic lithium, or lithium plating, is a wellknown and potentially dangerous degradation mechanism in lithiumion batteries.1 Lithium plating directly leads to capacity loss through corrosion with the cell's electrolyte 2 and in the worst case scenarios can lead to catastrophic failure by creating an internal short circuit. 1 Recent work shows a link between mechanics and lithium plating such that lithium plating is aggravated at higher levels of internal mechanical stress 3 and generally occurs in a periodic structure indicative of the cell's mechanical design. 3,4,5 However, the underlying physical mechanisms governing the relationship between lithium plating and mechanical stress remain unexplained. In this work we explain the observed link by experimentally demonstrating that local mechanical deformation of the battery separator can cause local lithium plating in an otherwise well-functioning cell. We support these experimental results with numerical simulations and an analytical analysis, which show that local separator deformation creates "hot spots" of locally high electrochemical activity that can cause local plating. While this work uses separator deformation as a model mechanical defect, the results are generally applicable to any defect that results in non-uniform ionic currents. Our results demonstrate the essential role of nonuniformities in causing failure in a seemingly well-functioning and welldesigned battery cell, suggesting a new paradigm in which batteries are designed to be resistant to failure in the presence of an assumed pre-existing defect.There are many known lithium-ion cell defects in addition to separator deformation that can result in spatially non-uniform internal operation.6 Some examples of defects in lithium-ion cells include local electrolyte drying, 7 current collector delamination, 7 electrode/separator interface separation, 8 copper plating f...

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