Mechanical Degradation of Graphite/PVDF Composite Electrodes: A Model-Experimental Study

Abstract

Mechanical failure modes of a graphite/polyvinylidene difluoride (PVDF) composite electrode for lithium-ion batteries were investigated by combining realistic stress-stain tests and mathematical model predictions. Samples of PVDF mixed with conductive additive were prepared in a similar way to graphite electrodes and tested while submerged in electrolyte solution. Young's modulus and tensile strength values of wet samples were found to be approximately one-fifth and one-half of those measured for dry samples. Simulations of graphite particles surrounded by binder layers given the measured material property values suggest that the particles are unlikely to experience mechanical damage during cycling, but that the fate of the surrounding composite of PVDF and conductive additive depends completely upon the conditions under which its mechanical properties were obtained. Simulations using realistic property values produced results that were consistent with earlier experimental observations. © The Author An understanding of mechanical degradation in battery electrodes will support the design of batteries with longer cycle life. There are numerous studies that explore lithium-ion battery degradation mechanisms through both experimental and modeling work.1-9 Since commercial lithium-ion battery electrodes are typically composites consisting of active material, binder, and conductive agent, mechanical degradation of the electrode depends on the individual properties of the electrode components as well as their interactions. For instance, crack generation in anode particles (such as graphite and silicon) might accelerate self-discharge due to the exposure of new surface to electrolyte solvents, which leads to further solid electrolyte interphase (SEI) film growth. 4,7 As another example, damage to surrounding material might disrupt electrically conductive paths to the active material particles, resulting in battery capacity fade.Graphite is known to undergo a volume change of approximately 10% during cycling, 10 and previous modeling work 3,11 has explored the possibility of mechanical damage in isolated graphite particles due to such volume changes. However, in typical commercial graphite electrodes, active material particles are not isolated, but instead bound into a porous structure with polymer binder and conductive agent. This surrounding material experiences stress due to the expansion and contraction of active material, and in turn acts as a mechanical constraint on the active material particles.12-14 Moreover, the mechanical strength of a binder such as PVDF changes with exposure to electrolyte solution. 13,15 Hence, in predicting stress generation in these composite systems, one needs to consider volume changes and lithium concentration gradients within active material particles, as well as the influence of the surrounding material.This work combines, for the first time, experimental determination of mechanical properties of the binder and conductive agent composite under realistic conditions, with simulations t...

Conductivity Degradation of Polyvinylidene Fluoride Composite Binder during Cycling: Measurements and Simulations for Lithium-Ion Batteries

“…This method created a uniform coating of binder on the outside of all particlesa morphology proposed and used by several other researchers. 17,19,25 This approach was consistent with the mechanics calculations described by Mendoza et al 29 Multiple binder thicknesses were explored, from 10-100 nm. Mendoza et al 29 examined the impact of binder on the effective modulus of a cathode.…”

“…5,15 The most common polymer for battery applications is polyvinylidene fluoride, a fairly stiff semi-crystalline polymer with a Young's modulus in the range of 1.5-2.5 GPa. [16][17][18][19] The glass transition temperature of PVDF is −30…”

Computational and Experimental Observation of Li‐Ion Concentration Distribution and Diffusion‐Induced Stress in Porous Battery Electrodes

“…Therefore, the fracture of porous electrodes would begin in larger particles. This phenomenon has been verified by many researchers who have studied the fracture of electrodes by numerical simulations or experimental observations …”

“…Stress fields in electrode particles can lead to fractures, a phenomenon that has been investigated recently. Takahashi et al . investigated the failure modes of graphite particles surrounded by a binder layer.…”

Effects of Cycling Ranges on Stress and Capacity Fade in Lithium-Ion Pouch Cells

“…6,[29][30][31] Mechanisms that cause stress to decrease include stress relaxation, polymer deformation (binder or separator), and electrode fatigue. [32][33][34][35] In this work, it is assumed that under normal cycling conditions, film growth is the dominant mechanism causing stress to increase, and the film may be composed of SEI layers, lithium plating, or a combination of both. 26 Starting with a simple model, we explore the effect of the film layer growth on the stress change, and then add a term for stress decrease to obtain a generalized relationship.…”