Quantifying the effects of strains on the conductivity and porosity of LiFePO4 based Li-ion composite cathodes using a multi-scale approach

The polymer-composite binder used in lithium-ion battery electrodes must both hold the electrodes together and augment their electrical conductivity while subjected to mechanical stresses caused by active material volume changes due to lithiation and delithiation. We have discovered that cyclic mechanical stresses cause significant degradation in the binder electrical conductivity. After just 160 mechanical cycles, the conductivity of polyvinylidene fluoride (PVDF):carbon black binder dropped between 45-75%. This degradation in binder conductivity has been shown to be quite general, occurring over a range of carbon black concentrations, with and without absorbed electrolyte solvent and for different polymer manufacturers. Mechanical cycling of lithium cobalt oxide (LiCoO 2 ) cathodes caused a similar degradation, reducing the effective electrical conductivity by 30-40%. Mesoscale simulations on a reconstructed experimental cathode geometry predicted the binder conductivity degradation will have a proportional impact on cathode electrical conductivity, in qualitative agreement with the experimental measurements. Finally, ohmic resistance measurements were made on complete batteries. Direct comparisons between electrochemical cycling and mechanical cycling show consistent trends in the conductivity decline. This evidence supports a new mechanism for performance decline of rechargeable lithium-ion batteries during operation -electrochemically-induced mechanical stresses that degrade binder conductivity, increasing the internal resistance of the battery with cycling. Lithium-ion batteries (LIB) are an enabling energy storage technology for portable consumer electronics, electric vehicles and renewable power generation in part due to their high energy densities. The energy density is driven by not only the relatively large potential of lithium-ion chemistries, but also the ability of active materials to store large amounts of lithium.1 The most common graphitic carbon anode can absorb up to one lithium for every carbon atom. Recent research on higher capacity anodes such as silicon has highlighted an increased need for understanding the mechanics of lithium-ion batteries. As the lithium is shuttled between the anode and cathode, the active materials expand and contract to accommodate the lithium. The resulting volume changes are accentuated for high capacity materials such as silicon which can increase in volume by up to 400% during lithiation. 2Because most LIB electrodes are porous multicomponent composites, understanding the generation and impact of mechanical stresses on batteries can be difficult. The electrode is generally 50-75 vol% solid fraction with active material consisting of micron-sized particles held together by an active binder, which is itself a composite of conductive carbon particles and polymer. The performance of the battery is highly dependent on this complex structure which must allow efficient ion and electron transport through the electrode. The void space in the porous structure allows lith...

“…The elastic modulus is roughly 2 GPa at room temperature, in good agreement with published literature. 16,18 The modulus decreases gradually by 25% as the temperature is increased to 65…”

Section: Mechanical Properties Of Polyvinylidene Fluoride-carbon Blacmentioning

confidence: 99%

“…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…”

mentioning

confidence: 99%

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

Grillet

Humplik

Stirrup

et al. 2016

“…Malave et al 26 performed a similar analysis, but included the effect of anisotropic swelling with phase change, using the experimental data of Reimers and Dahn 7 . Still others have looked at particle fracture.5 On a larger length scale, Awarke et al 24 used representative volume elements to understand how strains and external loads affected particle properties using a spherical particle pack. Very recently, Chung et al 27 used 3D reconstructions from X-ray tomography of LiMn 2 O 4 to study the effect of particle size polydispersity and surface roughness on battery performance.…”

mentioning

confidence: 99%

A Framework for Three-Dimensional Mesoscale Modeling of Anisotropic Swelling and Mechanical Deformation in Lithium-Ion Electrodes

Roberts

Brunini

Long

et al. 2014

Lithium-ion battery electrodes rely on a percolated network of solid particles and binder that must maintain a high electronic conductivity in order to function. Coupled mechanical and electrochemical simulations may be able to elucidate the mechanisms for capacity fade. We present a framework for coupled simulations of electrode mechanics that includes swelling, deformation, and stress generation driven by lithium intercalation. These simulations are performed at the mesoscale, which requires 3D reconstruction of the electrode microstructure from experimental imaging or particle size distributions. We present a novel approach for utilizing these complex reconstructions within a finite element code. A mechanical model that involves anisotropic swelling in response to lithium intercalation drives the deformation. Stresses arise from small-scale particle features and lithium concentration gradients. However, we demonstrate, for the first time, that the largest stresses arise from particle-to-particle contacts, making it important to accurately represent the electrode microstructure on the multi-particle scale. Including anisotropy in the swelling mechanics adds considerably more complexity to the stresses and can significantly enhance peak particle stresses. Shear forces arise at contacts due to the misorientation of the lattice structure. These simulations will be used to study mechanical degradation of the electrode structure through charge/discharge cycles. Capacity fade in lithium-ion batteries (LIB) is potentially influenced by a large number of mechanisms, 1 one of which is mechanical degradation of the electrode microstructure. Both electrodes experience mechanical deformation, and in this paper, we focus on the cathode of the lithium-cobalt-oxide (LiCoO 2 , or LCO) system.2 Cathodes are made up of a three-dimensional (3D), percolating, bicontinuous network consisting of solid, electroactive particles, a polymer binder, and electrolyte. The bicontinuous nature of this network is critical, as Li + ions must be able to transport from the anode, through the separator, to any particle in the cathode via the electrolyte. At the same time, electrons must be able to transport through the solid particle network to the current collector. Any particle that becomes physically isolated from, or poorly connected to, its network or from the electrolyte does not contribute to the electrochemical reactions, resulting in capacity loss.One way in which particles may become disconnected from their network is by the swelling, shrinking, and fracture mechanisms that may occur through many charge-discharge cycles.3-6 As lithium intercalates into the electrode particles, the crystal lattice spacing may change either isotropically or anisotropically. For LiCoO 2 cathodes in particular, the crystal lattice shrinks anisotropically upon lithium intercalation 7,8 As was measured by Reimers and Dahn, 7 and later in more detail by Amatucci et al., 8 the lattice shrinkage upon lithiation can be quite significant and is anisotropic in n...

“…The spatial non-uniformity notwithstanding, porosity induced by lithium intercalation/de-intercalation is often described in terms of the cell-level quantity of SOC. 24 To state the obvious, such approaches ignore spatial variations and, therefore, the results of this non-uniformity. In contrast, our work has taken a step toward a framework in which the dynamics of lithium intercalation/de-intercalation drive the evolution of spatially non-uniform porosity expressed in terms of spatially non-uniform volume fraction fields.…”

Section: Discussionmentioning

confidence: 99%

“…al. 23 Awarke et al 24 studied the variation of porosity in microscale computations on representative volume elements by assuming a porosity expression involving the local state of charge and volumetric strain. Then, with a homogenized model at the macroscopic, electrode scale, they imposed spatial profiles for the state of charge and studied how the porosity and conductivity varied.…”

Section: Introductionmentioning

confidence: 99%

Intercalation Driven Porosity Effects in Coupled Continuum Models for the Electrical, Chemical, Thermal and Mechanical Response of Battery Electrode Materials

Wang

Siegel

Garikipati

2017

We present a coupled continuum formulation for the electrostatic, chemical, thermal and mechanical processes in battery materials. Our treatment applies on the macroscopic scale, at which electrodes can be modelled as porous materials made up of active particles held together by binders and perfused by the electrolyte. Starting with the description common to the field, in terms of reaction-transport partial differential equations for ions, variants of the classical Poisson equation for electrostatics, and the heat equation, we add mechanics to the problem. Our main contribution is to model the evolution of porosity as a consequence of strains induced by intercalation, thermal expansion and mechanical stresses. Recognizing the potential for large local deformations, we have settled on the finite strain framework. We present a detailed computational study of the influence of the dynamically evolving porosity, upon ion distribution, electrostatic potential fields, charge-discharge cycles and mechanical force generated in the cell.