Role of Plastic Deformation of Binder on Stress Evolution during Charging and Discharging in Lithium-Ion Battery Negative Electrodes

Abstract

We studied the mechanical damage within a lithium-ion graphite-based porous electrode during electrochemical cycling. The effects of charging-discharging rate and the variation in graphite diffusivity on average stress in the electrode cell were investigated. In particular, differences between spatial and average stress evolution in graphite particles were explored. We considered two different microstructures: a) graphite particles connected together with binder bridges and b) graphite particles encased in binder shells. Electrochemical charging-discharging in a composite electrode was simulated by spatially resolving the electrode and electrolyte phases. As indicated by experimental measurements, the binder is assumed to follow an elastic-plastic stress-strain relation. Average stress developed in the electrode was calculated for different binder yield-stress levels and an appropriate yield-stress value was chosen on the basis of experimental findings of the literature. We find the stress in the particles can be of the order of 43 MPa, and can be particularly large in regions where the particles come in close contact with their neighbors. The average stress in the electrodes, however, is the range of 10 MPa and is largely determined by the mechanical properties, in particular the yield stress of the binder. Computed stress profiles were compared qualitatively with experimental measurements using the wafer-curvature method. Elastic stresses and plastic strains predicted by 3D models are shown to be close to those predicted using simpler 2D models of the microstructure.Lithium-ion batteries have gained prominence in electronic devices such as cell phones and laptop computers owing to their high energy density that reduces their weight by half and their volume by 20% compared to other batteries, such as nickel-cadmium and nickel-metal-hydride. Lithium-ion batteries also present an attractive solution as renewable energy sources for the transportation industry. Lithium diffuses into the electrode, and hence they undergo volumetric expansion resulting in stress and large plastic deformations, which can potentially lead to the failure of electrodes and of the binder that holds the particles together. [1][2][3][4][5] To address the stability of lithium-ion batteries and to study the stress response during the electrochemical intercalation and deintercalation processes, theoretical and numerical investigations were conducted at particle and electrode levels.The mechanical properties of different binder types have been extensively studied. 6-10 Experimental investigation shows that in a lithium-ion battery electrode, binders form thin non-continuous layers on the particle surface. 11,12 Modeling the realistic morphology of binders is an extremely difficult task, which requires using image processing techniques. 13,14 To avoid this difficulty, Awarke et al. 11 assumed that the electrode pore volumes are fully occupied by binders that could overstate the stiffness of the electrode. They presented a 3D mesoscale model of th...

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…”

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

“…The model system is a spherical graphite particle covered with a uniform layer of "binder," as drawn in Figure 1; this and all subsequent references to "binder" should be understood to describe the composite of PVDF and conductive additive that surrounds graphite particles in real batteries. Similar structures were used by Rahani and Shenoy 20 in simulations of multiple particles. A study by Hao and Feng 21 used a similar model geometry, but Figure 1.…”

Measuring the coating adhesion strength of electrodes for lithium-ion batteries

“…Furthermore, the coating has to withstand the mechanical demands during cell operation. This includes the mechanical stress during lithium intercalation and de-intercalation due to the expansion and shrinkage of the electrochemically active material particles [1][2][3]. This swelling especially induces mechanical tension at the particle-binder-substrate interface.…”